Three-dimensional titanium dioxide nanomaterials.

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Three-Dimensional Titanium Dioxide Nanomaterials Dina Fattakhova-Rohlfing,*,† Adriana Zaleska,‡ and Thomas Bein*,† †

Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5-13 (E), 81377 Munich, Germany ‡ Department of Environmental Technology, Faculty of Chemistry, University of Gdansk, Wita Stwosza 63, 80-308 Gdansk, Poland 3.3. Transformation Approaches 3.3.1. Hydrothermal/Solvothermal Transformations of Amorphous Titania Particles 3.3.2. Titanate−Titania Transformations 3.3.3. Sacrificial Templates 3.4. Templating Methods 3.4.1. Nanocasting of Spherical Templates 3.4.2. Evaporation-Induced Self-Assembly (EISA) 4. Hierarchical Hollow Spheres 4.1. Nontemplating Approaches 4.1.1. Solvothermal and Hydrothermal Processes 4.1.2. Transformation Approaches 4.2. Templating Approaches 4.2.1. Coating of Spherical Templates 4.2.2. Template-Directed Growth of Hollow Spheres 5. Porous Fibers 5.1. Hydrothermal and Solvothermal Synthesis 5.2. Electrospinning 5.3. Formation of Hollow Structures via the Kirkendall Effect 6. 3D-Titania Materials with Hierarchical Morphology 6.1. Hierarchical Titania Materials with Multiscale Porosity 6.2. Hierarchical Titania Formed by Porous Sphere Arrays 6.3. 3D-Hierarchical Titania Morphologies Organized over Multiple Length Scales 6.4. Fabrication of 3D-Hierarchical Titania Morphologies via Biotemplating 7. Conclusions and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References Note Added after ASAP Publication

CONTENTS 1. Introduction 1.1. Characteristics of Porosity 1.2. Basic Tools/Synthetic Methods for the Generation of Porous Titania 1.3. Specific Morphologies of 3D Titania Materials 2. Porous Films 2.1. Template-Free Approaches 2.1.1. Assembly of Preformed Titania Particles 2.1.2. Films Prepared via Sol−Gel Approach 2.2. Film Growth Approaches 2.2.1. Solvothermal/Hydrothermal Growth 2.2.2. Glancing Angle Deposition (GLAD) 2.2.3. Electrochemical Methods 2.3. Templated Approaches 2.3.1. Evaporation-Induced Self-Assembly (EISA) 2.3.2. Colloidal Crystal Templating 2.3.3. Nanocasting and Hard Templating 2.4. More Complex and Hierarchical Porous Morphologies 3. Porous Spheres 3.1. Spherical Particle Agglomerates 3.1.1. Formation of Spherical Particle Agglomerates in Sol−Gel Reactions 3.1.2. Formation of Porous Particle Agglomerates in Hydrothermal/Solvothermal Conditions 3.1.3. Assembly of Preformed Nanoparticles 3.2. Formation of Porous Titania Spheres via Hydrothermal and Solvothermal Methods 3.2.1. Fluoride-Assisted Hydrothermal/Solvothermal Reactions 3.2.2. Hierarchical Spheres Assembled from Rutile Nanowires © 2014 American Chemical Society

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Special Issue: 2014 Titanium Dioxide Nanomaterials

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Received: April 10, 2014 Published: August 19, 2014 9487

dx.doi.org/10.1021/cr500201c | Chem. Rev. 2014, 114, 9487−9558

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Figure 1. Examples of TiO2 with different pore size: (a) microporous TiO2 (pore size: ca. 1.5 nm) (Reprinted with permission from ref 65. Copyright 2008 Elsevier); (b) mesoporous TiO2 (pore size: ca. 30 nm) (Reprinted with permission from ref 66. Copyright 2011 Royal Society of Chemistry); and (c) macroporous TiO2 (pore size: ca. 130 μm) (Reprinted with permission from ref 67. Copyright 2013 Elsevier).

Figure 2. Examples of porous titania materials with different porous structures: (a) worm-like TiO2 structure (Reprinted with permission from ref 70. Copyright 2009 American Chemical Society); (b) cubic structure (Reprinted with permission from ref 71. Copyright 2003 American Chemical Society); (c) 2D hexagonal structure of TiO2 (Reprinted with permission from ref 72. Copyright 2004 John Wiley and Sons); and (d) co-continuous cubic 3D double gyroid network (Reprinted with permission from ref 73. Copyright 2003 American Chemical Society).

as porous titania materials,6−16 porous spheres,17,18 shells,19 nanosheets,20 nanorods,21 fibers,22 nanotubes,23,24 protonated titanates,25 or highlighting the possibilities of specific synthesis approaches such as atomic layer deposition (ALD),26 sol−gel synthesis,27 green synthesis,28 electrochemical synthesis,29−31 or facet control.32 Several reviews deal with various applications of 3D-titania materials in solar cells,18,33−40 photocatalysis and photoelectrochemistry,41−51 electrochemical energy storage,16,52−59 self-cleaning coatings,22,60 antifogging coatings,61 or molecular separation.62 In this Review, we will describe the numerous ways that have been recently developed for creating three-dimensional titania nanomaterials. Naturally, all titania materials are three-dimensional, but it helps to structure this vast field according to the dominant dimension of interest. Accordingly, complementary reviews in this Special Issue will cover other titania morphologies as well as different fields of their application. A focus on threedimensional nanomaterials implies a discussion of synthetic strategies for controlling the titania nanostructures in all three dimensions. Very often, such materials will be characterized by a particular porosity, with attention to pore size distribution, nature of the pores, and pore volume. After an introduction to porosity and the fundamental synthetic methods for the generation of porous titania, we will discuss the specific morphologies of titania materials to be treated in this Review. In section 2, we treat the synthesis of porous films, emphasizing a distinction between template-free spontaneous and templated approaches. Numerous different precursors and deposition techniques have been developed for the creation of porous titania films. In section 3, we discuss the formation of porous spheres, with strategies ranging from agglomeration methods to solvothermal/hydrothermal techniques and stepwise

1. INTRODUCTION The enormous interest in the synthesis, physical properties, and applications of the various forms of titanium oxide materials is based on many factors. Being nontoxic, abundant, and easily available, its applications range from paint pigments to photocatalysis, photovoltaics, and electrical energy storage. The performance in many of these applications depends critically upon mass transfer to active surface sites, charge transfer at the titania surface, and charge and/or ion transport in the bulk of the material. These processes are controlled to a large degree by the nanomorphology of titania materials due to the strongly increasing role of the interface-related processes caused by the large surface area, as well as the effects brought about by the diminishing size of the titania bulk (including crystallinity and crystal domain size, defect concentration, nature, and density of grain boundaries) on its electron and ion transport properties. As compared to other titania morphologies such as zero-dimensional (nanoparticles), one-dimensional (nanowires, rods, and tubes), or two-dimensional (layers and sheets), the 3D-titania structures feature high accessible surface areas in combination with the interconnected network of a bulk titania phase. Moreover, the spatial organization of the solid and empty components in 3D-titania structures opens the way to further develop unique functionalities such as tunable anisotropy in transport properties, size-selective properties, or the possibility to control interactions with light through photonic effects. Considering the great practical relevance of 3D-titania nanomorphologies, there is already a rich literature on different aspects of 3D-titania materials. Several earlier reviews deal with the general approaches toward the fabrication of 3D-titania morphologies,1−5 with a focus on particular morphologies such 9488


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Figure 3. Different 3D-titania morphologies illustrated schematically with representative electron micrographs.

Figure 4. Overview of different synthetic strategies for porous titania films. Casting of preformed titania (A1) species, electrostatic layer-by-layer (ELBL) deposition (A2), electrostatic spray technique (A3), electrophoretic deposition (A4), low-temperature processing (A5), sol−gel process (A6), evaporation-induced self-assembly (EISA) (A7), colloidal crystal templating (A8), and electrochemical techniques (A9).

1.1. Characteristics of Porosity

transformation concepts. More complex morphologies are found in hierarchical hollow spheres, which are covered in section 4. Again, we note the value of solvothermal and transformation methods, in addition to templating. In section 5, we extend the morphology to porous fibers, obtained by solvothermal and electrospinning techniques. This Review culminates in a treatment of three-dimensional titania materials with hierarchical morphology (section 6), where several of the above-mentioned strategies have been successfully combined to create even more complex structures. In many cases, the development of these more complex structures is motivated by further optimization of mass or charge transport for the diverse applications of titania.

For most applications of porous solids, the diameter of their pores is a key parameter. On the basis of their porosity, the bulk materials can be classified into three main categories: (a) microporous (50 nm) (Figure 1). Regarding porous threedimensional titania, the structural parameters such as pore size, ordering, and even pore shape (regular or irregular) are highly dependent on the preparation route. Pores that have a continuous channel of communication with the external surface of the body are described as open pores. Some may be open only 9489


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from porous films and spherical morphologies to complex hierarchical 3D structures. Hydrothermal methods can be combined with other methods, thus extending the scope of available hierarchical 3D morphologies even further. Additional control over the porosity of the resulting materials can be gained by the use of porogens, which can either be formed spontaneously during the fabrication process (typical examples include phase separation in the particle dispersions or formation of gas bubbles in a chemical reaction) or added intentionally (examples include carbon black or sucrose acting as sacrificial templates for disordered porous films). Chemical transformation of primary titania structures is another possibility to fabricate porous titania. This group of processes includes solvothermal treatment of titania species, such as transformation of amorphous titania species to their porous crystalline counterparts, or topotactic temperature-induced transformation of layered titanates to crystalline titania with similar morphology. Diverse growth techniques, where the formation of titania starts at the surface of appropriate substrates, can also be classified as nontemplated methods. Titania can be grown chemically in hydrothermal or solvothermal reactions, which typically requires the presence of titania seeds or layers acting as nucleation sites. These processes can be performed on planar substrates resulting in the formation of porous films, as well as on the substrates with more complex geometry such as spheres, rods, or wires, enabling the fabrication of hierarchical titania morphologies. In the glancing angle deposition (GLAD) technique, source material (e.g., TiO2) is vacuum-deposited onto a substrate that is inclined at an oblique angle to the impinging vapor molecules, resulting in a self-shadowing film growth and a porous thin film with columnar morphology. Electrochemical techniques offer a convenient and versatile route to create TiO2 thin films in aqueous or organic solvent-based electrolytes via anodic oxidation of a Ti surface, or via electrochemically triggered reactions of titanium species on a conducting substrate used as a working electrode. Growth methods are particularly suitable for the fabrication of oriented titania morphologies such as nanowires, branched nanowires, aligned nanotubes, or for oriented crystal growth of titania materials. Another nontemplated approach enabling formation of porous titania with a defined morphology is electrospinning, which produces porous titania fibers by passing titaniacontaining solutions through an electrically conductive nozzle tip while applying a high voltage. The diameter of the porous fibers can be varied from the micrometer to the nanoscale depending on the surface tension, molecular weight, and viscosity of the solutions, respectively. Templated approaches provide much higher control over the porous titania structure by using objects with a well-defined shape and 3D organization to guide the evolution of structure and morphology. The templated approaches involve selfassembly methods, in which the periodically ordered template is formed in situ (soft templating), and nanocasting or hard templating methods, in which a preformed shape-persistent template is infiltrated with a titania precursor. Soft-templating (and one of its variations called evaporation-induced selfassembly, EISA) is a common way to fabricate periodic mesoporous titania materials with an unprecedented degree of control over the size, shape, and spatial arrangement of the pores. The surfactant-based templating method usually proceeds via the coassembly of the titania precursors and supramolecular aggregates of a surfactant acting as porosity template. The types of the surfactant and the titania precursor, and the

at one end, and they can be described as blind (dead-end or saccate) pores. Others may be open at two ends (through pores).63 Regular pores may also be classified according to their shape: (a) 0D-pore shape (spherical, close to spherical), (b) 1Dpore shape (channels, slit-like, cylindrical), (c) 2D-pore shape (sheets, lamellar), and (d) 3D-pore shape (cubic, ellipsoidal, more complex shapes).16,64 The types of pore order in different porous materials have been reviewed in detail.9,12,68,69 Examples of titania materials exhibiting different degrees and types of pore ordering are presented in Figure 2. 1.2. Basic Tools/Synthetic Methods for the Generation of Porous Titania

Three-dimensional and more specifically porous titania materials can be obtained by a broad range of chemical and physical methods (see Figures 3 and 4). In general, these methods can be roughly divided into spontaneous (template-free) and templated approaches. The nontemplated methods enable the facile and straightforward fabrication of porous titania with various pore sizes, pore distributions, and specific surface areas, which can be tuned by the choice of composition and processing conditions. However, the porosity obtained by the nontemplated methods is usually irregular, featuring either a broad distribution of pore sizes or pore shapes, or a random orientation of uniform pores. Much better control over the porosity and texture is achieved with templated approaches, although usually at the expense of higher fabrication costs. Spontaneous approaches involve a large group of very different techniques that can be further divided into assembly and growth methods, both physical and chemical. One of the common nontemplated techniques is the assembly of preformed titania particles, which can be made from particle dispersions or pastes by casting, dip- or spin-coating, different spraying techniques, electrostatic layer-by-layer deposition, or electrophoretic deposition. The resulting porosity is largely determined by the properties of the particles used for the assembly and by their packing, which enables manufacturing a large variety of different porous titania morphologies through the choice of suitable particles used as building blocks. Alternatively, porous titania can be formed via chemical transformations of molecular titania precursors. Sol−gel reactions, in which titania is formed in a series of hydrolysis and polycondensation steps of molecular titania precursors, provide numerous examples of such transformations. Although the sol−gel chemistry of titania is rather complex as compared to the assembly of preformed titania species, it is more versatile for controlling the properties of the porous morphology. Titania with different phase composition, crystallinity, morphology, pore architecture, and shape can be obtained from the same type of precursors by varying the conditions of titania sol formation and by postsynthesis treatments. Solvothermal and hydrothermal processes comprise another large group of nontemplated reactions enabling the formation of a broad variety of porous titania morphologies with different pore size, crystallinity, and porosity. In contrast to the sol−gel processes where the primary reaction products are usually amorphous, hydrothermal or solvothermal reactions often lead to the direct formation of crystalline porous titania. Variation of the type of titania precursor, solution pH, reaction conditions (temperature and time), and the presence of additives drastically influences the course of reaction and the structure of the resulting products, hence providing a versatile tool for the fabrication of a wide diversity of 3D titania architectures ranging 9490


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This section will give an overview of the basic chemical and physical methods used for the fabrication of various porous titania films, which are summarized in Figure 4.

surfactant-to-titania ratio, largely determine the properties of the resulting porous titania structure. The pore size obtained by surfactant templating typically is in the mesopore range and beyond, from about 4 to about 100 nm, although the formation of the larger pore sizes requires the use of special polymer surfactants and can become more challenging. Periodic porous titania architectures with much larger pore sizes of several hundred nanometers can be prepared by colloidal crystal templating, which is based on the ability of uniform spherical colloidal particles to self-assemble into crystalline arrays with a periodic cubic packing motif (so-called opals). Filling the interstitial space of the opals with a titania precursor followed by formation of titania and the removal of template results in porous titania replicates, so-called titania inverse opals, which closely reproduce the size and arrangement of the colloidal template. Replication of preformed porous templates (nanocasting) is a universal method to fabricate porous titania with diverse types of porosity that are determined by the structure of the templates (molds). One of the examples of nanocasting is biotemplating, which enables the fabrication of 3D-hierarchical titania morphologies with sophisticated structure and ordering via the use of native or pyrolized biological objects.

2.1. Template-Free Approaches

2.1.1. Assembly of Preformed Titania Particles. Assembly of preformed titania nanoparticles is one of the common methods to fabricate porous titania films. In this approach, the processes of particle synthesis and the film fabrication are decoupled from each other. Independent of the particle synthesis, which can require harsh conditions and high temperatures, the film processing can be performed in mild conditions and at moderate temperatures, which makes this approach compatible with processing condition for various devices. This enables the use of many types of particles prepared by various methods, which can differ in phase composition, crystallinity, size, and shape. The porosity of the obtained films is largely determined by the properties of the particles used for the assembly and by their packing, which enables manufacturing a large variety of different porous titania films by selection of the suitable particles used as building blocks. The chapters below provide a short overview of the most common techniques used for the particle assembly into porous titania films. 2.1.1.1. Casting of Titania Particles. Casting of the preformed titania species on various substrates is one of the simplest ways to prepare porous titania films.74−77 In this method, the particles are dispersed in suitable solvents, and the dispersion is deposited on a substrate using different deposition techniques such as drop-casting,74 doctor-blading,75 spin-coating,76 or different spraying techniques.55,78−83 Casting of dispersed titania particles does not require any additives, templates, or structure-directing agents; the thickness of the films obtained in this way can reach several tens of micrometers, and the crystalline films can be obtained even at room temperature using already crystalline nanoparticles. The quality of the particle dispersions used for the coating and the absence of agglomerates are crucial for the formation of homogeneous films showing low light scattering. Stable particle dispersions can be prepared by selecting suitable solvents where the particles are dispersible without agglomeration, or by using surface-capped particles with high colloidal stability.84 More frequently, however, the homogeneous dispersions are obtained by a prolonged sonication or magnetic stirring of particles in different dispersing solvents (water or organic solvent, typically alcohols).75 Solvents with low surface tension such as alcohols (e.g., ethanol) typically result in smoother films after deposition. A critical point of this method is the rather limited control over the film porosity, which corresponds to the textural porosity between the assembled particles. Finally, the connectivity between the particles deposited at low temperatures is usually poor. This feature can lead to a low mechanical stability of the deposited films; it can also be disadvantageous for applications requiring good charge transport properties of porous titania layers. Usually, an additional postsynthesis treatment (such as mechanical pressing, thermal treatment, or chemical treatment) is required to improve the particle connectivity in the cast films, which will be addressed in more detail later in this section. 2.1.1.2. Electrostatic Layer-by-Layer Deposition (ELBL). An improved connection between titania particles and enhanced control over the film thickness and the film structure in zdirection (normal to the substrate) can be achieved by electrostatic layer-by-layer (ELBL) deposition. In the ELBL deposition approach, alternating layers of charged nanoparticles

1.3. Specific Morphologies of 3D Titania Materials

Nanostructured titania can take different forms (macroscopic morphologies) such as 0D morphologies (porous spheres), 1D morphologies (porous fibers), 2D morphologies (porous films), and even more complex hierarchical morphologies combining different types of porous structure and/or different types of macroscopic shape. The characteristics of both microscopic and macroscopic morphologies have a great impact on the physical properties of 3D titania materials and their performance in different applications. It is therefore of great interest to dispose of synthetic techniques for the control of structure and morphology of 3D titania materials on different size scales, and these subjects will be the focus of this review. Other issues to be addressed include control of crystallinity and phase, that is, type of crystallinity (amorphous, polycrystalline, degree of crystallinity, crystal size), phase control, and phase composition. Moreover, we will discuss ways to achieve and control crystallinity without collapse of the porous morphology. The most common techniques will be described more in detail in this Review in relation to the specific titania morphologies (Figure 3).

2. POROUS FILMS Porous titania films are used in numerous applications such as dye-sensitized solar cells (DSCs), electrochromic devices, antifogging, antibacterial, and self-cleaning coatings, and many others. Therefore, the possibility to fabricate different porous titania materials as films on suitable substrates has been extensively explored. The formation of porous films requires strategies to control the nature of porosity, as well as additional control over the parameters specific for film morphology such as thickness, adhesion to the substrate, homogeneity, roughness, and mechanical stability. For applications in semiconductor devices, the control over the charge transport properties of the films becomes extremely important. The latter depend on the crystallinity of the titania framework (including phase composition and size of the crystalline domains), the continuity of the titania framework, and the connectivity between the titania domains. Finally, processability and the scalability are often important issues that have to be taken into account. 9491


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spherical titania aggregates,89 nanoribbons,90 or nanotubes.91 Polar solvents such as water or water−alcohol mixtures are typically used due to their better conductivity. The titania particles in this approach have to be surface-charged, which can be achieved by adjusting the pH or by surface modification. Application of higher voltages leads to a better adhesion of the charged titania particles to the substrate and a better quality of the films. However, in aqueous solutions, the possible applied voltage is limited by the water electrolysis reaction. The latter can be overcome by the addition of a cosolvent to suppress the water electrolysis reaction, working at a voltage below the threshold for water electrolysis, or by applying pulsed potentials or an AC electric field.92 Electrophoretic deposition enables the fabrication of films at room temperature, which is especially advantageous for the preparation of films on flexible and temperature-sensitive substrates.89,93,94 This technique provides homogeneous and smooth layers. Moreover, it does not pose any restrictions concerning the substrate geometry and gives access to titania coatings both on planar and on complex shaped substrates, including patterned electrodes95 and porous titania layers.96 However, electrophoretic deposition has some limitations that are partially similar to the binder-free casting of titania particles described above. Thick electrophoretically deposited layers tend to crack due to contraction after evaporation of the solvents. It is possible to overcome this problem by applying an AC voltage92 (Figure 6) or by multistep electrophoretic deposition with

and oppositely charged layers are sequentially deposited on a substrate by dipping in the corresponding solutions; one dipping sequence leads to the formation of roughly a monolayer of the particles. As oppositely charged layers, polyelectrolytes are typically used, including negatively charged poly(acrylic acid)85 or positively charged polydiallyldimethylammonium chloride,86 resulting in the formation of composite titania−polymer layers after deposition. The polymer can be removed by heating in air (usually temperatures of ca. 400 °C are required to combust the polymer completely). For small colloidal titania nanoparticles, the ELBL deposition can be applied to prepare extremely thin crystalline titania coatings with excellent control over the film thickness.87 The ELBL approach can also be applied for deposition of dispersible particles of different size and different shape including porous spheres, hollow spheres,85 and nanorods. Furthermore, it is possible to obtain hierarchical layers combining different porous titania morphologies using different types of titania particles. Agrios et al.86 have fabricated hierarchical titania films composed of a mesoporous layer deposited from about 16 nm titania nanoparticles and a top scattering layer of large 250−400 nm particles (Figure 5).86

Figure 6. Top view (a) and cross-section (b) of a titania film obtained by electrophoretic deposition of P25 titania particles in an AC electric field (Reprinted with permission from ref 92. Copyright 2012 Elsevier).

intermediate drying steps.97,98 Deposition at low voltages can lead to a poor adhesion of films to the substrate, while higher voltages may result in undesirable electrolysis of the electrolyte. Similar to the other binder-free low-temperature techniques, the interparticle connections in the films prepared by electrophoretic deposition are weak. Means to improve the connectivity include sintering, mechanical compression,93,88 postdeposition CVD,99 or sol−gel treatment.100 An interesting approach to improve the connectivity between titania particles was proposed by Benehkohal et al.101 who used chemically assisted electrophoresis of titania particles in the presence of Zn(NO3)2. During electrophoretic deposition, Zn(NO3)2 is transformed to Zn(OH)2 acting as a chemical glue between the titania particles, resulting in porous interconnected TiO2−ZnO composites after annealing. Electrophoretic deposition itself can also be used to improve the interparticle connectivity of the low-temperature processed titania films. Thus, Grinis et al.96 have described a way to fabricate interconnected porous crystalline titania films at low temperatures without additional annealing. They used electrophoresis of amorphous titania sols to deposit an ultrathin conformal amorphous titania coating on crystalline mesoporous

Figure 5. SEM images of (a,b) top views of nanoparticulate films deposited by electrostatic layer-by-layer (ELBL) technique from two diferent types of titania particles, (c) top view of scattering particles over nanoparticulate film, and (d) cross-section of a nanoparticulate film topped with a scattering layer (Reprinted with permission from ref 86. Copyright 2006 American Chemical Society).

Although the films obtained after one coating are very thin, rather thick uniform coatings can be obtained by a multiple coating procedure (50 deposition cycles of 16 nm particles result in a 1.5 μm thick titania layer after calcination). 2.1.1.3. Electrophoretic Deposition of Titania Particles. Electrophoretic deposition of titania nanoparticles is another possibility to obtain homogeneous porous titania films on conducting substrates. Here, two electrodes are immersed in a dispersion of charged titania particles, causing their movement toward one of the electrodes after application of an electric field. This technique can be applied to any titania powder that can be suspended in a suitable solvent, including commercial titania particles88 as well as more complex titania morphologies such as 9492


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Figure 7. Schematic diagram of (a) electrostatic spray and formation of hierarchically structured TiO2 nanospheres; (b,c) low magnification SEM image of a crack-free photoelectrode composed of hierarchically structured TiO2 nanospheres: top view (b) and cross-section (c); (d) high magnification cross-section SEM image of a photoelectrode composed of hierarchically structured TiO2 nanospheres with diameter of 260 nm; and (e) an enlarged image of hierarchically structured TiO2 nanospheres composed of nanoclusters of P25 titania (Adapted with permission from ref 83. Copyright 2011 American Chemical Society).

titania films (prepared by electrophoretic deposition of larger crystalline titania particles). The uniform titania coating obtained in this way connects the larger particles together, resulting in improved electron transport properties. 2.1.1.4. Spray Coating of Preformed Titania Particles. The electrostatic spray technique83,102 is a cheap and simple process enabling a direct deposition of thin films from their colloidal solutions. The electric field develops an electric charge on the liquid surface, and the charge is carried away by the droplets detaching from the jet. The deposition efficiency of the charged droplets is usually much higher than that of the uncharged droplets, which improves the adhesion between the materials and substrates and enables a binder-free deposition of different dispersible titania particles on conducting substrates. The electrostatic spray technique can also be used for the deposition of titania nanoclusters, whose size can be controlled by the flow rate, the applied voltage, and the concentration of particle dispersions. This technique enables a room-temperature deposition of thick (ca. 17 μm) crack-free crystalline films with different morphologies, which can be changed from smooth to hierarchical porosity composed of micropores and mesopores (Figure 7). 2.1.1.5. Low-Temperature Processing of Crystalline Titania Films. The films assembled from preformed titania particles at low-temperature techniques described above usually suffer from a poor connectivity between the particles, which limits their mechanical stability and electronic properties. The contact between the particles can be significantly improved by annealing the films at elevated temperatures leading to particle sintering. The temperature required for particle sintering decreases with the decreasing particle size, but even for very small particles temperatures of at least 300−400 °C should be applied to sinter the particles, leading to the formation of an interconnected porous network. High temperature treatment is the conventional and frequently used way to improve the interparticle connectivity. However, many applications, especially those involving temperature-sensitive components and polymer substrates, require low-temperature approaches for improving the interparticle connections.

Mechanical compression is a simple and efficient way to improve the connection between the deposited particles and to obtain mechanically stable and electrically conducting nanostructured films at room temperature. For that purpose, a pressure from 10 to 60 MPa is applied on the deposited film;103 there is an optimum pressure that can be applied to the films assembled from different particles. The increased pressure leads to a monotonous decrease in the film porosity (e.g., for a film deposited from Degussa P25 nanoparticles, the porosity decreases from an initial 69% to about 50% after compression103), with a simultaneous increase in the mechanical stability. The compression of thick films leads to a significant reduction of cracks104 and improved transparency due to removal of larger agglomerates. Porosity, mechanical stability and transparency can be controlled by the variation of the applied pressure. Moreover, using this method, it is possible to pattern the films by applying the pressure only at certain areas, or make profiles in the films using the patterned pressure masks. Even better particle connectivity can be achieved by a combination of heating and pressure, which can be done either by a subsequent calcination of compressed films, or by simultaneous heating and compressing the films.105 The electrical conductivity of the cast films can be substantially improved by the use of agglomerates with already interconnected particles as building blocks for the film assembly instead of single particles.106,107 The building blocks such as porous titania spheres can be calcined at high temperature before the film assembly to achieve a good contact between the titania crystallites, and subsequently they can be assembled into films using the low-temperature approaches described above (Figure 8).106 Even better results are achieved by the use of monocrystalline porous titania particles reported by Snaith et al. exhibiting superior electronic properties.108 The as-cast low-temperature films assembled from such mesoporous crystals demonstrate higher electronic conductivity and electron mobility than the sintered nanoparticle films due to the absence of interparticle boundaries in the monocrystalline scaffold. Besides annealing at high temperatures, there are also several low-temperature sintering methods that can be used to improve 9493


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and condensation rates of molecular titanium compounds are greatly accelerated under hydrothermal conditions of enhanced temperature, humidity, and pressure. The most spectacular feature of this method is that the formation of crystalline titania phases takes place already at very moderate temperatures such as 100 °C.115 In a typical procedure, the coated films are placed into an autoclave or a desiccator, and a small amount of water is added at the bottom of the reactor so that the sample is not in direct contact with water but with steam during the reaction. Saturated solutions of different salts can be used for a better control of the relative humidity inside the reactor.11 The reactor is placed in an oven at a certain temperature; usually, heating at 100 °C for 12 h is sufficient to obtain a crystalline titania phase. The hydrothermal treatment of chloride- or sulfate-containing precursors cannot be applied with acid-sensitive substrates such as indium tin oxide (ITO), due to the chemical etching of the substrate by the acids produced during hydrolysis. A very elegant and efficient way to improve the interconnectivity of the titania networks is the “chemical sintering” of the particles, which can be achieved either by post-treatment of the deposited films, or can be performed during the film deposition by modification of the coating solution. One of the most popular methods of chemical sintering is a postsynthesis treatment of deposited titania films with titanium tetrachloride (TiCl4). TiCl4 post-treatment is commonly performed by soaking the deposited films in a 40 mM TiCl4 aqueous solution at 70 °C for 30 min. This treatment can be followed by low-temperature or high-temperature annealing, depending on the application requirements. The TiCl4 treatment not only improves connection between the particles by “gluing” them with a hydrolyzed titania phase, but also helps to eliminate surface defects acting as electron traps on the particle surface in the titania films. Instead of titanium tetrachloride, prehydrolyzed titania sols can be used for the postsynthesis treatment of the deposited films. In a typical procedure, the films are immersed for 1 min in a sol prepared from tetrabutyl titanate, diethanolamine, ethanol, and water.109 ALD coating116 and electrophoretic coating96 are other possibilities to improve connection between the titania particles by a conformal deposition of a very thin layer of amorphous titania. Addition of some amount of molecular titania precursor such as TiOSO4,114 alkoxides,117,118 titanium(IV) bis(ammonium lactato)dihydroxide,119 or titania sol to crystalline titania particles is another way of in situ chemical modification, which greatly improves the interconnectivity of the titania network after a suitable low-temperature or high-temperature sintering. Addition of small amounts (typically no more than 20 wt %) of the amorphous titania component to the crystalline particles is greatly beneficial for the improved interparticle connections and improved adhesion to the substrate. 2.1.1.6. Porous Titania Films Coated from Titania Pastes. One of the most common methods to prepare porous titania films from preformed particles involves their processing into viscous pastes using a small amount of solvents and some dispersing agents (such as acetylacetone or acids). The pastes can be made from any type of titania powders including nanoparticles, spherical aggregates, mesoporous powders,81,107,120,121 nanorods,122 etc., as well as mixtures of different titania morphologies.123,124 This approach also enables the fabrication of porous titania films with any desirable phase composition (anatase, rutile, or brookite) by selection of corresponding titania particles for the paste fabrication.125 The pastes can be spread on

Figure 8. SEM and TEM micrographs of a control film fabricated with commercial TiO2 paste and the same film coated with titania sol to cement the particles together (sol-modified film): (a,b) cross-sectional micrographs of surface structure for the control film and the solmodified film, respectively; (c,d) SEM micrographs of cross-sectional structure for control film and sol-modified film, respectively; the inset in (c) shows the TEM micrograph of the nanoporous structure of the control film; (e) TEM micrograph of sol-modified structure; and (f) TEM micrograph of the compact layer represented in (e) (Reprinted with permission from ref 109. Copyright 2009 American Chemical Society).

connection between the nanoparticles. One of such methods is UV-irradiation in air, which is usually performed by illumination of deposited films with a 125 W UV lamp for 3 h.110 Although the electronic properties of the films sintered in this way are worse as compared to the films sintered by calcination, it enables fabrication of organic-free porous films on temperature-sensitive substrates and without densification. Another advantageous feature of the UV sintering method is that the formation of interconnected porous networks takes place without densification and decrease in porosity. Enhanced connection between the particles can be achieved via raster scanning laser sintering;111 the electronic properties of the films sintered in this way are comparable to those obtained via high-temperature calcination. Sintering is achieved by rastering the laser beam over the entire titania film with successive parallel offset scans; the use of a UV scanning laser system with strong light absorption in the TiO2 ensures a rapid increase of the local temperatures in the illuminated areas. Hydrothermal treatment (also called delayed humidity treatment11,112) is a very efficient way of chemical sintering of materials containing hydrolyzable titanium compounds such as titanium chlorides, alkoxides, or TiOSO4.113,114 The hydrolysis 9494


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Figure 9. Sintered titania films made of pastes prepared from linear and branched titania nanorods whose shape is shown in the insets (Reprinted with permission from ref 122. Copyright 2013 American Chemical Society).

Further control over the porosity of the resulting films can be gained by addition of sacrificial templates acting as porogens in the paste composition. Combustion of the templates from the coated films by calcination leads to porous films with pore sizes approximately corresponding to the size of the templates. Examples of such porogens include carbon black acting as a cheap porogen for disordered macroporous films140, or polystyrene (PS) spheres of submicrometer size resulting in macroporous titania films with submicrometer cavities created by the combustion of polystyrene141 (Figure 10).

the substrates using different techniques. Doctor blading, screen printing,126−128 and roll-to-roll printing are the most common techniques providing uniformly thick coatings on the planar substrates. The films prepared in this way are usually calcined (the typical temperature range is 300−550 °C) to remove the volatile components and to sinter the particles. One of the simplest ways to make titania pastes relies on the change of rheological properties of titania particle dispersions. This is achieved by the surface modification of titania particles upon prolonged stirring with acids such as acetic acid or HCl, leading to the formation of thick viscous pastes.129−131 The porosity of such films results mainly from the interparticle voids (textural porosity), with the pore size being dependent on the size of the particles used for the paste fabrication. More control over the porosity and morphology of the films can be achieved by using organic additives acting as adhesives (such as Triton X-100132), binders, thickeners,133 or porositycontrolling agents. Ito et al.134,135 have introduced a very popular protocol for the fabrication of titania pastes, which are made from titania particles, polyethylene glycol, hydroxypropyl cellulose, and water. Another conventional method for paste preparation utilizes terpineol and ethanol as solvents and ethyl cellulose as a thickener.121 Calcination of the coatings prepared from this paste results in the formation of highly porous crystalline titania films possessing both meso-sized (2−50 nm) and macro-sized (over 50 nm) pores arising from the phase separation between the titania particles and the polymers.136,137 The coating of pastes enables the fabrication of rather thick (several micrometer) films that are smooth, homogeneous, and crack-free. The thickness and the quality of the films depend on the nature and the quantity of the organic additives used.138 Larger amounts of organics give thinner pastes that can be more easily distributed on a substrate, but cracks can be formed during film drying and calcination. The typical film thickness obtained from pastes is about 5−10 μm, but much thinner films (below 1 μm) can also be made by using less viscous pastes, and much thicker films can be prepared by repetitive coating. Using different types of titania particles, it is possible to prepare a broad variety of porous films with different pore morphologies. Assembly of spherical particles usually results in the formation of mesoporous films with a nonperiodic textural porosity. More ordered mesoporous films with a narrow pore size distribution can be obtained from ground mesoporous powders prepared by surfactant templating.139 Titania nanorods, nanofibers, and dendrimers after calcination create 3D-titania networks with large open pores122 (Figure 9). Assembly of secondary titania units, such as mesoporous titania spheres or hollow spheres,106 enables one to obtain even more complex hierarchichal morphologies combining different types of porosity.

Figure 10. SEM images of films made from titania pastes containing different weight ratios of polystyrene (PS) spheres: (a) TiO2:PS = 10:1 and (b) TiO2:PS = 20:1 pastes. Panels (c) and (d) show the crosssectional SEM images of PS-templated TiO2 films. The magnified image (d) reveals ellipsoidal pores (Adapted with permission from ref 141. Copyright 2012 Royal Society of Chemistry).

The pastes prepared from different types of titania particles can be sequentially coated onto each other in different order, enabling fabrication of even more complex architectures combining different porous titania morphologies.126 Figure 11 shows an example of a hierarchical double layer film obtained by this technique, which is composed of a layer of octahedral titania nanocrystals and a top layer of agglutinated mesoporous titania microspheres.142 2.1.2. Films Prepared via Sol−Gel Approach. Fabrication of porous films from preformed titania nanoparticles described in section 2.1 does not involve chemical transformations; the properties of the films obtained in this way are largely determined by the properties of titania particles used for the film assembly. Alternatively, porous titania films can be formed via chemical transformations of molecular titania precursors. Sol−gel reactions provide numerous examples of such transformations. 9495


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Figure 11. Double layer titania film made of pastes prepared from different types of titania particles, octahedral nanocrystals and agglutinated mesoporous microspheres: (a) cross-sectional SEM image; (b) schematic illustration. Red ellipse contours denote agglutination between mesoporous hierarchical TiO2 microspheres by 3D necking. (c) Enlarged SEM image showing 3D necking between microspheres in top view. The white rectangle contours enclose the necking areas (Reprinted with permission from ref 142. Copyright 1998 American Chemical Society).

solvent leads to the transition from the liquid sol into a solid gel phase.1 In the final step, wet gels could be dried by evaporation, producing so-called xerogels, or using other techniques (e.g., supercritical or freeze-drying) forming so-called aerogels having lower density and higher surface area. The structure and the morphology of the resulting network strongly depend on the nature of the precursors, the water content, the pH, the temperature, the solvent, and the relative contribution of hydrolysis and polycondensation reactions during the allowed reaction time.10 Different reaction times (aging) lead to different compositions and properties of the resulting titania species.146 A careful control of all of the parameters of the sol−gel reaction enables fine-tuning the structure and the size of the resulting sols and, subsequently, the morphology of the resulting films. Furthermore, any additives or ligands that can specifically bind to the precursor titanium compound or the reaction products influence the rates of hydrolysis and condensation steps leading to a different composition of the resulting titania sols. The use of coordinating ligands (hydrolysis inhibitors) such as triethanolamine or acetylacetone is a common approach to gain more control over the course of the sol−gel reaction, by slowing the rates of hydrolysis and condensation steps.147 The colloidal solutions of the titania sols can be deposited on any wettable substrates using different coating techniques such as spin-coating, dip-coating, drop-casting, or different spraying techniques148 providing very uniform and conformal coatings. The thickness of the obtained coatings can be easily varied from very thin (subnanometer) to a few hundred nanometers by variation of the composition and concentration of the coating solutions, as well as the processing of the films. The latter is controlled by a variation of the dip- or spin-coating rate and by an adjustment of the temperature and the relative humidity in the coating chamber. Although the sol−gel chemistry of titania is rather complex as compared to the assembly of preformed titania species, it is more versatile for controlling the properties of the obtained films. Titania films with different phase composition, crystallinity, morphology, pore architecture and shape can be obtained from

The chemistry involved in the sol−gel process is based on inorganic polymerization reactions occurring in aqueous or nonaqueous systems. Precursors are usually organic titanium compounds such as alkoxides (Ti(OR)4 (OR = OCnH2n+1)) or titanium salts such as chlorides, sulfates, etc.10 The conventional sol−gel processes are based on the formation of titanium oxopolymers via hydrolysis and polydendensation of molecular precursors, while the “non-hydrolytic sol gel” reactions are based on the condensation in nonaqueous media of molecular titania precursors with oxygen donors other than water (e.g., alkoxides, ethers, alcohols, etc.).41 The titanium oxo-polymers were studied in detail by J. Blanchard, C. Sanchez et al.143−145 The growth of the titanium oxo-polymers was controlled to produce sols and gels by adjusting two main parameters: the hydrolysis ratio H = [H2O]/[Ti] and the inhibitor ratio p = [H=]/[Ti] or a = [acacH]/[Ti] (acacH = acetylacetone). The proposed model for the structure of these oxo-polymers was based on the condensation of subunits with a mean composition Ti(OZ)x(OH)yO2−(x+y)2x (OZ = Acac + OBun) and having a gyration radius of about 2.0−2.5 nm. Blanchard et al. proposed that these subunits are made with a titanium-oxo core built with μ2-O, μ3-O, μ4-O bridges and that their surface is capped by the residual ligands OZ.143 Increasing the hydrolysis ratio H or decreasing the inhibitor ratio (p or a) mainly results in an increase of the size and the fractal dimension of the oxopolymers, while the size and the mean composition of titanium oxo-cores constituting the subunits are apparently not modified. In the conventional sol−gel process, the development of Ti−O− Ti chains is favored at low water content, low hydrolysis rates, and excess titanium alkoxide in the reaction mixture, resulting in three-dimensional polymeric skeletons.1 The formation of Ti(OH)4 is favored at high hydrolysis rates and a medium amount of water, and leads to an insufficient development of a three-dimensional polymeric skeleton and loosely packed particles.1 In a typical sol−gel process employed for TiO2 formation, a colloidal suspension, or a sol, is formed through the acid-catalyzed hydrolysis of the precursors followed by a polymerization reaction.1 Complete polymerization and loss of 9496


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Figure 12. Titania film prepared by a sol−gel approach after calcination at 500 °C: without surfactant treatment (left) and after immersion in cetyltrimethylammonium chloride (CTAC) (right) (Reprinted with permission from ref 154. Copyright 2002 Springer).

Figure 13. SEM images of (a) surface and (b) cross-section of TiO2 aerogel film (Reprinted with permission from ref 150. Copyright 2002 Springer).

the same type of precursors by varying the conditions of titania sol formation before coating and postsynthesis treatments of the coated titania films. The pore size of the sol−gel films prepared from titania sols without any additives can be tuned in a very broad range from a few nanometers149 to a few hundred of nanometers by variation of the sol composition and the aging conditions.146,150 The range of available porous morphologies and pore size can be extended even further by the use of different additives and templating agents to the precursor solution,149,151,152 which will be addressed more in detail later. The as-deposited titania gels are typically amorphous, not fully condensed, and contain large amounts of organic substances such as residual ligands, nonevaporated solvents, or additives. Transformation of these coatings into a rigid titania film requires postdeposition processing, which is a very important step leading to significant changes in the film properties and morphology and providing an additional possibility for the tuning of morphology. Heat treatment is the most conventional method to convert the gel film into a dense oxide film. However, evaporation and decomposition of organic substances during thermal treatment lead to significant volume changes and shrinkage, inducing considerable stress in the resultant films. When the stress is too large to be compensated by the plastic deformations, macroscopic cracks can be created. Not only the macroscopic morphology but also the microscopic structure such as crystallinity and phase are largely affected by heat treatment conditions. Usually, amorphous titania crystallizes to the anatase phase at temperatures around 400 °C, resulting in nanostructured films with a disordered uniform porosity and a narrow distribution of particle size; the pore size and the particle size strongly depend on the calcination temperature and time. Higher temperatures lead to a rapid growth of titania crystalline domains, from a few nanometers at moderate temperatures to several tens of nanometers at temperatures beyond 500 °C, accompanied by changes in the porous morphology.

Before the condensation is completed with the formation of a rigid titania network, the composition of deposited films is still tunable and prone to changes, enabling the further modification of the morphology and crystallinity of the sol−gel titania coatings. One example of such postdeposition modification is a surfactant treatment of the deposited films. Immersion of titania gel coatings in a solution of cetyltrimethylammonium chloride (CTAC) at room temperature followed by drying and calcination at 500 °C results in the formation of crystalline titania films consisting of a periodic arrangement of columnar grains of about 30 nm in diameter and pores of about 10 nm in size (Figure 12).153,154 The morphology of the films after surfactant immersion is different from that of the untreated coatings, which is composed of uniform spherical grains about 30 nm in diameter separated by pores of about 3 nm. One of the interesting techniques enabling fabrication of highly porous crystalline titania films at low temperatures is supercritical extraction. Using supercritical drying in CO2, Sung et al.150 prepared thick (ca. 1 μm) and highly porous titania films with about 76% porosity without any porogens. The films crystallize already during supercritical extraction at 250 °C with formation of the porous anatase network, which is also retained without significant changes in morphology after calcination at 400 °C (Figure 13). 2.1.2.1. Macroporous Films via Phase Separation. The range of available porous morphologies and pore size distributions obtained by the sol−gel approach can be extended further by the addition of polymers such as polyethylene glycol,155−161 poly(vinyl alcohol),151 or hydroxypropyl cellulose152 to the precursor solution. Coating the polymercontaining precursor solution on a substrate using conventional coating techniques (spin-coating, dip-coating, or doctor blading) followed by removal of polymer via calcination or extraction produces macroporous titania films with controllable pore size. Formation of the macroporous morphology in this process relies 9497


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Figure 14. SEM images of the surface morphology of TiO2 films prepared from a precursor sol−gel titania solution containing poly(ethylene glycol) (PEG) with different molecular mass and different concentration: PEG with an average molecular weight Mav = 1000 and concentrations of 2.0 g/100 mL (a) and 4.0 g/100 mL (b), and PEG with a Mav = 2000 and concentration of 2.0 g/100 mL (c) (Reprinted with permission from ref 162. Copyright 2004 Springer).

condensation rate is strongly affected by the conditions of the film deposition such as the deposition rate, temperature, and the relative humidity in the deposition chamber.156−158 The polymers generally increase the viscosity of the sol−gel solutions, enabling deposition of thicker films in one coating. Even thicker films can be fabricated by addition of viscous solvents such as terpineol to the precursor solution, which greatly increases the films thickness proportional to the terpineol amount. Addition of ca. 10% terpineol to the ethanolic sol−gel solutions enables the coating of about 2.5 μm thick crack-free macropous films.163 Larger amounts of terpineol hinder the formation of the porous structure and lead to the formation of cracks after drying and calcination. Another method for a surfactant-free fabrication of macroporous titania films is a photopolymerization-induced phaseseparation (PIPS).164,165 In this process, the precursors and solvents are miscible with each other, and the phase separation is driven by the photopolymerization, which provides better control over the deposition and porosity formation. The solutions for the film coating typically contain a small molecular organic monomer (such as dipentaerythritol pentaacrylate), a polymerization initiator (2,2′-azobis(isobutyronitrile)), and titanium alkoxide in ethanol. Irradiation of the coated layer induces polymerization of the organic monomer, leading to phase separation after reaching the miscibility gap. Removal of the polymer by calcination leads to the formation of macroporous TiO2 films containing interconnected macropores with a size of ca. 1 μm. The porosity of the titania films and the density of pores can be easily controlled by the adjustment of the polymerization conditions, the initial monomer to titania ratios, and the type of monomers (Figure 15).

on spontaneous macroscopic phase segregation between the polymer and the titania precursor. The properties of the obtained films are determined by a complex interplay between different processes taking place during film formation such as the macroscopic phase segregation, the condensation of the titania precursor, and the interaction between the polymer and the titania species. The rates of all of these processes and thus the properties of the resulting morphology are strongly affected by the composition of the coating solution as well as conditions of film processing, which enables one to tune the film characteristics by a careful adjustment of the deposition conditions. The type of polymer (its molecular weight,159 solubility, and the hydrophilic−hydrophobic contrast), the concentration of the polymer in solution, the polymer to titania ratio, the type of solvent, as well as the nature of the titania species formed in the precursor solution as a result of sol−gel transformations are decisive for the formation of a specific morphology. The variation of those parameters allows one to change significantly the morphology of the titania films, which can be varied from dense layers, arrays of particle agglomerates, and porous sponge-like structures to macroporous films with different pore size. A higher molecular weight of the polymer typically leads to a larger pore size of the macroporous film, while increasing the amount of the polymer results in a higher pore density, increased surface area, and a more ordered pore arrangement. Thus, variation of the molecular weight and the polymer concentration in the poly(ethylene glycol)−titania systems enables one to tune the pore size of the macroporous films from 10 to 500 nm and the specific surface area from 51 to 72 m2 g−1 (Figure 14).162 There is an optimum value for the weight percentage of polymer, which is typically in the range between 2% and 10%. The morphology formed during deposition of the polymercontaining precursor solution results from the competition between the phase separation and the condensation of the titania sol. Formation of a porous morphology requires that the rate of the phase separation is faster than the solidification of the titania precursor; otherwise, dense films are formed before the phase separation can take place. Ideally, the macroscopic structure should be formed first as a result of the phase separation, and then “frozen” by solidification of the titania sol. The phase separation is enhanced by decreasing the compatibility between the polymer and the precursor solution, which can be tuned by the choice of the polymer and by variation of the solvent polarity.160,161 The hydrolysis and condensation rates are typically slowed by the use of less reactive titanium compounds, by addition of strongly complexating ligands, and by decreasing the water content in the precursor solution. Additionally, the

2.2. Film Growth Approaches

2.2.1. Solvothermal/Hydrothermal Growth. Hydrothermal synthesis is a simple and effective method to produce a wide diversity of hierarchical titania architectures. The heterogeneous growth in hydrothermal reactions typically requires the presence of a titania seed layer, which acts as a nucleation site. The structure and morphology of the seed layer, as well as conditions of the hydrothermal process (type and concentration of reactants and additives, temperature, and pH), play an important role in the structure and morphology of the resulting titania layers. Variation of these parameters and modification of the reaction conditions enable the fabrication of even more complex titania layers with a hierarchical 3D-architecture. Hydrothermal or solvothermal growth of 3D-titania films usually proceeds in two steps. The first step includes formation of 9498


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a primary titania scaffold, which can have different dimensions and geometry and can be prepared by different, not only hydrothermal, methods. In the second step, this scaffold either transforms to another titania morphology under hydrothermal reaction conditions, or it acts as a nucleation site for the hydrothermal growth of the secondary “derived” titania morphology. Among other parameters, the pH of the reaction is of primary importance for the structure and composition of the resulting titania layer. Hydrothermal reaction in acidic conditions usually promotes the direct formation of a crystalline rutile phase, while reaction in basic conditions usually results in the formation of layered titanates or amorphous titania phases. 2.2.1.1. Acidic Conditions. Using two sequential hydrothermal reactions in acidic aqueous solution of TiCl4, Lee et al.166 fabricated a hierarchical 3D-porous titania film containing different scales of crystalline (rutile) titania particles (Figure 16). The film growth rate, the size, and the shape of the titania particles are governed by the concentration, pH, and temperature of the TiCl4 precursor solution. In the first step, a porous macroscale titania film was grown hydrothermally on a dense titania seed layer. The higher concentration of TiCl4 (0.5 M) and the reaction temperature of 80 °C used in the first step promote a fast growth rate, yielding a 20 μm thick film formed by aggregation of elliptical particles 50 nm in length and 25 nm in width. The film obtained in the first step was used as a scaffold for a subsequent hydrothermal deposition of titania particles from the same precursor. In the second step, a lower precursor concentration (0.3 M) and lower reaction temperature of 70 °C

Figure 15. SEM images of porous films prepared via photopolymerization-induced phase separation (PIPS) process from solutions containing Ti(OC3H7)4 and different amounts of organic monomer (dipentaerythritol pentaacrylate) with monomer:Ti molar ratios of (a) 0.0025, (b) 0.005, (c) 0.0075, and (d) 0.01 (Reprinted with permission from ref 164. Copyright 2009 American Chemical Society).

Figure 16. Hierarchical 3D-porous titania film grown by hydrothermal reactions in acidic aqueous solution of TiCl4 in two sequential steps: (a,b) SEM images of particulate TiO2 films on an FTO substrate after the first growth stage at higher TiCl4 concentration and higher reaction temperature promoting a fast growth rate; the film cross-section is also shown (a). (d) SEM image of a TiO2 particulate film after the second stage of deposition at lower temperature and lower TiCl4 concentration leading to the formation of smaller titania particles. TEM images of TiO2 particles produced by the first growth stage (c) and after the second deposition stage (e); the arrows indicate the direction perpendicular to the lattice plane (Reprinted with permission from ref 166. Copyright 2012 American Chemical Society). 9499


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dendrites sediment on their surface, forming a top layer. The final nanostructured layer is obtained after calcination and sintering. 2.2.1.2. Basic Conditions. Generally, 3D-titania networks can be formed from disordered interconnected arrays of titania nanowires or nanotapes, which are easily grown in the hydrothermal conditions on seed layers without special orientation. Using cast anatase nanoparticles as a seed layer,170 thick porous layers composed of interwoven anatase nanotapes can be grown. The hydrothermal growth proceeds in strongly alkaline conditions using 10 M NaOH solution in a sealed autoclave at 150−170 °C for 20 h. The formed layer, calcined after reaction at 450 °C, consists of disordered array of crystalline anatase nanotapes with an average width of about 60 nm and a height of about 7 μm. The nanotapes are interconnected to a 3Dnetwork-like structure throughout the whole film featuring high porosity with a large pore volume (Figure 18).

were used to decrease the reaction rate, resulting in the formation of smaller rod-like particles on the surface of the larger ones and increasing the specific surface area of the porous films. In a similar way, numerous hierarchical 3D-titania layers can be fabricated by using different types of primary titania scaffolds. Hydrothermally grown titania nanorod arrays can be further decorated with titania nanobranches in a subsequent hydrothermal reaction, leading to the formation of interconnected 3Dtitania layers.167,168 Defects and high energy sites of the primary titania nanorods act as nucleation sites for the second hydrothermal reaction, leading to the growth of a large number of smaller titania wires protruding from the surface of the larger ones. After prolonged reaction time, the nanobranches crossed and interconnected with each other in a 3D-network (Figure 17).

Figure 17. Morphologies of TiO2 nanobranched arrays: typical top view SEM images of TiO2 nanobranched arrays at (a) low and (b) high magnifications; a cross-sectional view of the TiO2 nanobranched arrays at (c) low and (d) high magnifications; (e) and (f) are typical TEM and HRTEM images, respectively, of the branched TiO2 nanorods (Reprinted with permission from ref 167. Copyright 2011 Royal Society of Chemistry). Figure 18. Plan-view (a) and cross-sectional view (b) SEM images of a titania nanotape array obtained by a hydrothermal growth in alkaline conditions on anatase nanoparticles used as a seed layer (Reprinted with permission from ref 170. Copyright 2008 Elsevier).

By combination of the substrate-induced growth and a homogeneous nucleation, it is also possible to obtain hierarchical titania structures in one step in the same hydrothermal process. Sun et al.169 reported the fabrication of a hierarchical bilayer titania film consisting of 1D-nanowire bottom arrays and a 3Ddendritic microsphere top layer. To make such layers, the FTO glass was placed in an autoclave together with an aqueous precursor solution composed of titanium tetraisopropoxide, HCl, cetyltrimethylammonium bromide, and ethylene glycol. During the reaction, titania seeds are formed both on the FTO surface and in the bulk, but at different rates. With the continuing synthesis reaction, oriented 1D titania nanowire arrays grow on the FTO substrate as the bottom layer, and the 3D titania

By changing the morphology of the seed layer and modifying the conditions of the hydrothermal reaction, it is also possible to fabricate truly connected titanate networks. Zhang et al.171 used as seeding layer macroporous anatase films prepared via a phase separation sol−gel approach (see section 2.1.2). The starting porous film with a thickness of 1.2 μm and pore size ranging from 80 to 100 nm was hydrothermally treated at 130 °C in 10 M NaOH solution for 6 h. The initial hydrothermal process is 9500


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Figure 19. Titanate networks (b,c) prepared by a hydrothermal treatment in basic conditions of a macroporous anatase film (a): FESEM and TEM images of the starting titania film (a) and the film obtained after hydrothermal reaction (b,c) (Reprinted with permission from ref 171. Copyright 2010 American Chemical Society).

Figure 20. Morphology of TiO2 samples obtained by the GLAD technique: (a) tilted columnar microstructure obtained at a zenithal angle of 80° (Adapted with permission from ref 178. Copyright 2014 Elsevier), (b) zigzag stacked multilayers obtained by changing the azimuthal orientation of substrate between 0° and 180° (Adapted with permission from ref 178. Copyright 2014 Elsevier), (c) columnar nanostructures obtained by normal evaporation (0°) (Adapted with permission from ref 178. Copyright 2008 Elsevier), (d) helical columnar structures deposited at an angle of 86° with substrate rotation rate of 0.5 rpm (Adapted with permission from ref 179 Copyright 2010 Elsevier), (e) nanopillar film of Nd-doped TiO2 deposited at a deposition angle of 85° (Adapted with permission from ref 180. Copyright 2012 Elsevier), and (f) tilted nanopillars obtained at flux angle of 75° (Adapted with permission from ref 181. Copyright 2007 Elsevier).

dominated by the dissolution of porous titania film, providing the building blocks for the titania network growth. The resulting 3D network is constructed from titanate nanotubes directly linked together, with the distances between the joint centers ranging from 80 to 200 nm (Figure 19). Each nanotube has an inner and outer diameter of about 6 and 12 nm, respectively. The pore distribution of the network structure was found to be the same in all directions in the nanotube film. The formation of branched nanotubes is explained by the oriented crystal growth mechanism with a continuous seed formation occurring throughout the growth process. Another common technique to prepare porous titania films is hydrothermal oxidation of titanium. Titanium foil as well as thin titanium films deposited on different substrates can be used for this purpose. Hydrothermal oxidation of titanium in 15−30% H2O2 solution at temperatures of 80−90 °C results in the formation of porous layers with an average pore size of about 100 nm.172,173 The walls of the titania scaffold are largely amorphous; the crystallinity can be enhanced by a postdeposition hydrothermal treatment in water,173 or by calcination at 300−400 °C in air with formation of a crystalline porous anatase network.172

2.2.2. Glancing Angle Deposition (GLAD). Glancing angle deposition (GLAD) is a single step physical vapor deposition technique (PVD) that can be used to produce a variety of porous thin film morphologies including straight pores, posts, zigzag, and helical structures,174,175 as shown in Figure 20. Source material (e.g., TiO2) is vacuum-deposited onto a substrate that is inclined at an oblique angle to the impinging vapor molecules, resulting in a self-shadowing film growth and a porous thin film with a columnar morphology.176 The rate and incident angle of vapor flux as well as the substrate rotation speed during deposition were found to critically affect pillar microstructure.177 Van Popta et al. stated that during the initial stages of film growth, nucleation sites shadow portions of the substrate surface and build up sloped columns instead of coalescing to create a continuous thin layer.176 These columns fan out laterally due to a lack of atomic shadowing perpendicular to the deposition plane, forming a second structural anisotropy, which is responsible for the biaxial nature of columnar thin films (Figure 21a and b). Columnar upgrowth always occurs in a direction favoring the incident vapor flux. As a result, intricate thin film nanostructures can be 9501


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Figure 21. GLAD technique: (a and b) A schematic illustration of deposition kinetics of oblique-angle deposition including the shadowed regions that result in the growth of an array of directed nanorods (Adapted with permission from ref 182. Copyright 2009 Elsevier); (c) a schematic diagram of the GLAD setup. A second stepper motor rotates the substrate about an axis normal to the page to control the incident flux angle (θ). Computer control of the stepper motor is accomplished using feedback of the deposition rate from a crystal oscillator thickness monitor (Adapted with permission from ref 174. Copyright 1997 AIP Publishing LLC).

Figure 22. Thresholded images in (a), (c), (e), (g), and (i) of top-of-film planar view SEMs of high deposition angle (α) GLAD vertical post TiO2 samples: (b) α = 65°, (d) α = 70°, (f) α = 75°, (h) α = 80°, and (j) α = 85° (Adapted with permission from ref 184. Copyright 2011 Elsevier).

deposited at oblique angles exhibited intercolumn porosity, with roughness along the sides of the columns. As the deposition angle decreases, these films become denser, with less spacing between columns, as shown in Figure 22. The krypton gas adsorption data for these TiO2 films revealed that they exhibited significant mesoporosity in the 2−10 nm range, while examination of SEM images provided an estimate of their macroporosity in the 10 to over 100 nm range. The authors showed that the mean void distance between columns is greater than 100 nm for films deposited at an oblique angle of 85°, with spacing decreasing to tens of nanometers for angles from 70° to 80°. For a deposition angle of 65°, the films are quite dense and mesoporosity dominates. The pore size distribution of the titania films was found to have a logarithmic normal distribution with peak pore diameters from 2−3 nm at 50° to 4−5 nm at 65° deposition angle. The specific pore volumes of the TiO2 films were calculated from the krypton adsorption isotherms, and up to 65° the rise in pore volume was linear with deposition angle, with a slope of 0.01 cm3 g−1 deg−1.184

accomplished by dynamically changing the orientation of the deposition plane using in situ substrate motion176 as schematically shown in Figure 21c. Thus, a helical column can be obtained by holding the deposition angle constant while rotating the substrate at a constant speed, relative to the deposition rate.176 Increasing rotation speed resulted in the structure degeneration into a vertical post because the helical pitch became less than the column diameter,177 whereas chevron or zigzag structures could be created by holding the substrate fixed for one arm of the zigzag before quickly rotating the substrate 180° to form the second arm.176 Furthermore, the pore size of structures obtained by the GLAD technique can be varied from less than one up to hundreds of nanometers in width and up to a few microns in length. Morphology control depends on self-shadowing and limited adatom diffusion, and is achieved by precisely positioning the substrate relative to the incoming vapor flux.183 The mesoporosity and pore volume of TiO2 films deposited at angles from 45° to 85° using the GLAD technique were investigated by Krause et al.184 They observed that films 9502


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oxide as soluble fluoride complexes (Figure 23). The nanotube growth rates are also determined by a balance of water and

Li et al. reported the preparation of TiO2 films with a glancing angle equal to 0°, 60°, 70°, and 84°.185 With the angle increasing, the nanostructures of TiO2 films changed from nanoparticle to separated vertical columns. Films formed with α = 70° and 84° consisted of separated vertical columns with a diameter of about 30−60 nm, and showed thickness variations from 400 to 500 nm, respectively.185 The impact of substrate temperature on cracking in TiO2 GLAD capping layers was investigated by Kupsta et al.186 Standard vertical post films were grown, followed by deposition of a capping layer at low deposition angles. Crack area, density, branching, and length distribution were quantified as a function of substrate deposition temperature from 50 to 400 °C. The crack density and area decreased with increasing substrate temperature, without adversely affecting the underlying vertical post nanostructure deposited at low temperatures.186 The effect of substrate rotation as well as post-deposition heat treatment on the structure of titania films was systematically investigated by Wong et al.179 The first series with constant film thickness of 4 μm was prepared with various substrate rotation rates of 0.17, 0.5, 1.0, and 7.0 rpm under the fixed incident angle of 86°. FESEM analysis revealed orderly, packed helical columnar structures with significant differences between films deposited at different rotation speeds. With the increasing substrate rotation rate, the number of turns of each helical column increases proportionally, resulting in changes in the film morphology and the film porosity due to the self-shadowing effect. In a next step, two series of various film thicknesses from 6 to 10 μm were deposited at a constant rotation rate of 0.17 rpm with two different incident angles at 86° and 73°. When the films were grown thicker, the pores on the series deposited at angle 86° remained about the same. In case of films deposited at 73°, the increase in film thickness from 6 to 10 μm resulted in the pores shrinking or even closing.179 2.2.3. Electrochemical Methods. Electrochemical techniques offer a convenient and versatile route to deposit TiO2 thin films from aqueous or organic solvent-based electrolytes. Advantages of electrochemical techniques as compared to other techniques include: (a) low process temperature, (b) low cost of raw materials and equipment, (c) simplicity, (d) tight control of film thickness, uniformity, and deposition rate, and (e) possibility of deposition on substrates of complex shape. The electrochemical methods used for TiO2 preparation include: anodic oxidation of Ti in electrolytes such as aqueous solution containing fluorides,187,188 organic-based electrolytes containing fluorides,24,31,189,191 or hot phosphate/glycerol mixtures;192−196 cathodic electrodeposition using solutions of TiO2+ and electrochemical formation of OH− for the formation of TiO2 films;197−200 and anodic electrodeposition of TiO2 from TiCl3 solution201−203 or Ti(IV)-alkoxide stabilized solutions.204−206 Anodization of titanium in aqueous or organic-based electrolytes containing fluorides leads either to the formation of compact oxide layers or to porous oxides in the form of nanopores or nanotubes (ordered or disordered), depending on preparation conditions such as anodization time, applied voltage, temperature, Ti foil roughness, electrolyte composition, and calcination parameters. 31,207−209 It was shown that the morphology and structure of nanotubular/nanoporous TiO2 layers obtained by anodic oxidation are strongly affected by the solution parameters, including fluoride ion concentration, pH, and water content in the electrolyte.29,210 As reported by Macak et al.,208 the anodic growth of compact oxides on metal surfaces and the formation of tubes are governed by a competition between anodic oxide formation and chemical dissolution of the

Figure 23. Mechanism of TiO2 nanotube evolution during anodic oxidation of Ti in the presence of fluoride ions: (I) formation of barrier oxide, (b) local activation of surface and pore formation, (III) growing of tree-like structures (competition between ability to form water-soluble TiF62− complexes and precipitation of TiO2) (Reprinted with permission from ref 210. Copyright 2007 Elsevier).

fluoride concentration. High fluoride concentration enhances the dissolution of TiO2 (TiO2 + 6F− + 4H+ → [TiF6]2− + 2H2O), while addition of water allows for a sufficient rate of titanium oxidation (Ti + O2 → TiO2). Usually ordered titania nanotubes, showing thickness not exceeding 500−600 nm, are grown in HF electrolytes at low voltages, while in case of higher voltage, compact oxide layers are formed instead of ordered nanotubes. It was reported that upon increasing the applied voltage to 30− 40 V, the nanotubular structure is destroyed and the nanotube surface layer changes to a porous sponge-like structure.191,211,212 Usually, more uniform and longer nanotubes are fabricated in organic-based electrolytes such as solutions based on ethylene glycol189,207,213−217 or glycerol.218−221 Using viscous solvents for anodization resulted in nanotubes with smooth walls191,209 (Figure 24). Solvents with high viscosity reduce the mobility of fluoride ions and other ionic species, thus reducing the growth rate, but they also reduce current fluctuations and therefore assist in the formation of smoother nanotube walls.209 In general, the pore or tube diameter is reported to be linearly dependent on the applied anodic potential during growth.222,223 It was found that 9503


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Figure 24. Effect of electrolyte type on the morphology of TiO2 nanotube arrays fabricated by anodic oxidation: (a) smooth nanotubes formed in ethylene glycol-based electrolyte with low water content (2 vol % H2O); (b) nanotubes consisting of uneven rings stacked upon each other formed in glycerol-based electrolyte with high water content (45 vol % H2O); and (c) porous sponge-like structure obtained in HF water-based electrolyte and at relatively high voltage (30 V) (Reprinted with permission from ref 191. Copyright 2014 Elsevier).

Figure 25. Examples of TiO2 structures obtained by electrochemical deposition: (a) FESEM image of TiO2 nanotube arrays obtained by two-step anodization on electropolished Ti foils (Reprinted with permission from ref 233. Copyright 2013 Elsevier), (b) SEM image of TiO2 thin film fabricated by cathodic deposition at ITO surface followed by calcination at 300 °C (Reprinted with permission from ref 200. Copyright 2006 Elsevier), and (c) SEM image of TiO2 nanotubes deposited during an anodic electrodeposition process at the surface of the through-hole template serving as working electrode (Reprinted with permission from ref 234. Copyright 2013 Elsevier).

anodization. It was found that the pore diameter is not significantly affected by the formation voltage in this system, being ∼6 nm and 10−12 nm in the outer and inner regions of the films, respectively.193 Few reports are available on the cathodic and anodic electrodeposition (in contrast to oxidation) of TiO2 films. Natarajan and Nogami225 and Zhitomirsky197,226,227 have reported on the cathodic deposition of TiO2 films. The electrochemical synthesis of TiO2 thin films via cathodic deposition led to the formation of TiO(OH)2·xH2O gel films

in H3PO4/HF electrolytes at potentials between 1 and 25 V tubes could be grown with any desired diameter ranging from 15 to 120 nm, at tube lengths from 20 to 1 μm.222 Relatively thick TiO2 mesoporous films were also fabricated by titanium anodization in a hot glycerol electrolyte (160−180 °C) containing K3PO4 and K2HPO4 with reduced water content.193,224 A 12 μm thick anodic film was formed by anodizing at 5 V for 3.6 ks, in contrast to a thickness of 2.1 μm obtained at 20 V. This effect was associated with the crystallization of the anodic oxide, which induces generation of gas during 9504


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featuring either a broad distribution of pore sizes or pore shapes, or a random orientation of uniform pores. Much better control over the porosity characteristics is achieved via the use of porosity-generating templates or structure-directing agents. Different templated approaches for the fabrication of 3D-titania films will be briefly described in the following chapters. 2.3.1. Evaporation-Induced Self-Assembly (EISA). Surfactant templating (often called true liquid crystal templating) is a common way to fabricate periodic porous titania materials with a high control over the size, shape, and mutual arrangement of the pores. The soft-templating method usually proceeds via a coassembly process of the titania precursors (titanium alkoxides, titanium tetrachloride, etc.) with supramolecular aggregates of a surfactant acting as template (sol−gel process67,235−243), at elevated temperature (hydrothermal244−246 or solvothermal route247), or combining both types of process (sol−gel + solvothermal/hydrothermal248−251). The evaporation-induced self-assembly (EISA) method is a special variation of the surfactant-templated approach aimed at achieving control over the macroscopic morphology in addition to porosity control. The conventional liquid crystal templating methods developed for bulk porous materials contain surfactant molecules in concentrations above their critical micelle concentrations (CMC), such that the surfactant molecules form supramolecular liquid-crystalline assemblies of the surfactant micelles acting as the porosity templates. Because of the high concentration of the surfactant, such solutions are normally rather viscous, thus restricting their processing toward different macroscopic morphologies. In contrast, EISA starts with much less viscous solutions containing surfactant molecules at concentrations far below their CMC, which enables their processing toward different morphologies. The EISA method is especially suitable for the fabrication of thin porous films. However, other morphologies such as porous spheres and porous fibers as well as more complex hierarchical porous morphologies can also be prepared via the EISA approach. Because of the unprecedented degree of porosity control, the EISA method became a very popular technique for the fabrication of different periodic porous materials.9,252−257 Applications of the EISA method for the fabrication of porous morphologies and mechanistic details are described in several comprehensive reviews (refs 2, 6, 9, 12−14, 16, 69, and 258−260). In this chapter, we briefly describe the main features of the EISA process regarding the fabrication of porous titania films, and discuss recent developments in this area. Formation of titania films in the EISA process starts with the coating of the precursor solution on a desirable substrate using different coating techniques, typically spin-coating, dip-coating, doctor blading, inkjet printing, or spray coating.261−263 By variation of the deposition rate and the composition of the coating solutions, films with various thickness ranging from extremely thin (a few nm) patterned surfaces264 to several hundreds of nanometers can be obtained. The film deposition in the EISA process does not have any specific requirements regarding the type of the substrate except wettability of the substrate in the coating solution, although the substrate surface can influence porosity formation. The films can also be conformally coated on any type of shaped substrates with a complex geometry, including porous substrates. The coating solutions are composed of a suitable surfactant and a precursor for the titania phase in a volatile solvent (typically alcohols, water−alcohol mixtures, or THF). It is possible to use different titania precursors in the EISA process, but major requirements include good dispersibility in the coating solution and

on the cathodic substrates, resulting from aqueous peroxotitanium solutions. An aqueous solution of TiOSO4 in the presence of NO3− ions was usually used as a precursor. When the substrate is cathodized at potentials below ca. −0.9 V, nitrate is reduced to generate OH− ions.199 NO3− + H 2O + 2e− → NO2− + 2OH−

The electrochemically generated base traps soluble Ti species to form a Ti hydroxide gel film on the electrode: TiO2+ + 2OH− + x H 2O → TiO(OH)2 ·x H 2O

The deposited film is then heated to induce crystallization of TiO2: TiO(OH)2 → TiO2 + H 2O

In a different approach, the redox reaction between TiCl3 and NaNO3 to form Ti(IV) and NO2− prior to cathodic deposition was proposed by Hu et al.228 to be the key step for promoting the TiO2 deposition. The continuing reduction of NO2− to N2 and NH3 generates even more OH− ions, effectively enhancing the deposition of TiO2. The advantages of the cathodic and anodic electrodeposition as compared to anodic oxidation are as follows: porous TiO2 films can be deposited on various substrates such as ITO glass198−200,225,229,230 (Figure 25b), FTO glass, Pt foil,197 platinized silicon wafers,197 Al2O3 membranes, or Ti substrates. In most electrodeposition experiments, the solution of titanium salts was used in the acidic pH range of 1−3 only.197−199,226,227 Alternatively, thin films of titanium dioxide have been deposited on ITO substrates by cathodic deposition from an aqueous alkaline solution containing titanium complexed with EDTA by Lokhande et al.231 These reaction conditions led to the formation of amorphous, compact, pinhole free, and adherent titanium oxide film in contrast to open porous structures observed for cathodically deposited TiO2 films made in an acidic bath, as reported by Karuppuchamy et al.199 Anodic oxidative hydrolysis of acidic TiCl3 has been employed to deposit TiO2 films on FTO glass,201 metallic (Pt, Au, Ti) electrodes,201 and aluminate.232 Wessels et al. have reported the anodic electrodeposition of crystalline porous TiO2 films from aqueous TiCl3 under the influence of the surfactant SDS (sodium dodecyl sulfate) at temperatures between 60 and 80 °C.202 Very small pores with a diameter of only 1−2 nm were found for this material. The authors also investigated the codeposition of TiO2 with different organic dye molecules as structure-directing agents.203 The best results were obtained for the pH-indicator bromothymol blue, which led to the formation of thick, crystalline, and porous films. Highly porous TiO2 films were prepared by anodic electrodeposition from titanium alkoxide solutions containing benzoquinone, 2-methylbenzoquinone, or the dye 2,9,16,23-tetrasulfophthalocyaninatonickel(II) followed by calcination at 450 °C.205 Samples obtained by using benzoquinone during electrodeposition showed the highest porosity, having a large fraction of pores smaller than ca. 8 nm (pore volume related to the total pore volume equaled 0.263 cm3/cm3).205 2.3. Templated Approaches

The nontemplated methods described above enable the facile and straightforward fabrication of porous titania films with various pore sizes, pore distribution, and specific surface areas, which can be tuned by the selection of composition and processing conditions. However, the porosity of the films obtained by the nontemplated methods is usually irregular, 9505


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Figure 26. Scheme illustrating the formation of porous titania films via evaporation-induced self-assembly (EISA) process.

burn out the organics. It can be also carried on in nonoxidizing atmosphere, or different heating procedures can be applied. Thermal treatment is a very important and critical step in the EISA deposition of the porous titania films, leading to significant changes in the crystallinity of the titania scaffold and also significant changes in the macroscopic morphology. The asdeposited films typically contain large amounts (around 50 vol %) of organics including surfactant and the residual solvent. Removal of the organics during thermal treatment results in large volume contraction of the surfactant-derived films. Furthermore, condensation and crystallization of the titania scaffold results in large changes in density, which is more pronounced for the sol− gel-derived titania material. As a consequence, heating of asdeposited surfactant-templated films leads to volume shrinkage and can result in crack formation and even delamination for the thicker films. Therefore, the thickness of the films obtained after a single coating usually does not exceed a few hundred nanometers (typical thickness of the films obtained from conventional coating solutions can be tuned between about 100 and 500 nm). Because of the stabilizing and supporting role of the substrate, the shrinkage of the porous films is usually uniaxial and takes place normal to the substrate, resulting in the distortion of the pore shape. Porosity generation in the titania films can be achieved using different ionic and nonionic surfactants. Commercially available nonionic triblock copolymers of the Pluronic family with the general formula PEOxPPOyPEOx (where PEO is poly(ethylene oxide) and PPO is poly(propylene oxide)) are the most common templates for the fabrication of periodic mesoporous titania films.72,269−276 The Pluronic polymers are soluble in different polar solvents including alcohols and THF and can self-organize into different liquid-crystalline phases with lamellar, hexagonal, or cubic structure.277−280 The hydrophilic part of these polymers is composed of the PEO groups, while the PPO block is more hydrophobic, so that the micelles formed in polar solvents usually have an exposed hydrophilic PEO shell and a PPO core. The size of the mesopores is determined to a large extent by the volume of the hydrophobic core and is only slightly influenced by the size of the PEO part. The typical pore size that can be obtained with Pluronic polymers can range from about 5 to 14 nm281 depending on the type of the polymer, type of the solvent, presence of swelling agents, ionic strength of the solution, interaction with inorganic precursors, as well as processing conditions (humidity and temperature).282,283 The pore size and pore shape are strongly affected by the densification and crystallization of the titania lattice, typically resulting in the

compatibility with the surfactant without agglomeration or flocculation. The most typical titania precursors are amorphous titania species formed via sol−gel reactions of the titanium compounds, but titania clusters, preformed titania nanoparticles, or mixtures of different titania precursors can also be used. The types of the surfactant and the titania precursor and the surfactant to titania ratio largely determine the properties of the resulting titania films and the porosity characteristics. In addition, the processing conditions also play a very important role in the formation of the porous structure. Formation of porous morphologies in the EISA process follows a complex mechanism, which proceeds via several steps9,69,71,265−267 (Figure 26). Because of the low concentration of the surfactant in the starting coating solutions, self-assembly of the surfactant molecules and the titania precursor into a composite liquid-crystalline phase starts only after evaporation of some amount of solvent (giving the name to the technique). This implies the dynamic character of the surfactant-directed self-assembly process, which is influenced at this step by the rate of evaporation, the relative humidity, and the film thickness. The degree of complexity becomes even higher when the sol−gel transformations of the titanium compounds are used for the formation of the titania framework, which is most commonly the case. Self-assembly of the sol−gel-derived titania precursors proceeds simultaneously with their continuous hydrolysis and condensation. The rates of these processes are influenced both by the solution composition (such as catalysts or inhibitors of hydrolysis) and by the water content in the films and in the coating chamber. Exceedingly fast condensation typically results in the formation of disordered structures. Ideally, the selfassembly with the formation of a desirable structure should be finished before the titania phase is solidified. In some cases, the self-assembly proceeds with the formation of a so-called tunable steady state, where the inorganic system is not completely condensed and the film composition is still interacting with the environment through the vapor phase. The mesostructure of this state is still flexible and can be potentially modified by a postdeposition treatment. The last steps of the film deposition involve complete solidification or crystallization of the titania framework and the removal of the surfactant. These two steps can be performed separately, for example, by drying the film first followed by a subsequent extraction of the surfactant in organic solvents.268 Typically, however, the solidification of titania and the removal of the surfactant are performed simultaneously by heating the films at temperatures above the surfactant decomposition temperature. The heating is usually performed in air (calcination) to 9506


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unilateral pore shrinkage in the direction perpendicular to the substrate.284 The rapidly growing number of applications based on porous titania films has initiated a search for surfactants providing even larger pore size and pore volumes. The emerging classes of novel surfactants have a larger molecular weight than Pluronic polymers and a higher amphiphilic contrast between the hydrophilic and hydrophobic blocks. This results in lower critical micelle concentrations and higher stability of the micelles, making them less susceptible to the influence of their environment and enabling the formation of porous morphology in a broader range of processing conditions. We note that in addition to surfactants, whose assembly is strongly affected by the solution composition resulting in complex phase diagrams of possible porous morphologies, shape-persistent templates can also be used in the EISA process enabling reliable formation of porous films. Recently, we have demonstrated the use of nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films with a well-defined, narrow pore size distribution.115 The hydrophilic block of the majority of the polymers is poly(ethylene oxide) (PEO), while hydrophobic blocks include block-poly(ethylene-co-butylene) (PHB), polyisoprene (PI), polybutadiene (PB), polystyrene (PS), poly(methyl acrylate) (PMA), poly(vinyl chloride) (PVC), and polyisobutylene (PIB). Block copolymers with the structure PEO−PHB referred to as KLE,69,267,285,286 diblock copolymers of the PEO−PIB type,287 and triblock copolymers of PEO−PB−PEO type288 are able to form a variety of lyotropic liquid-crystalline structures, such as cubic, hexagonal, and lamellar phases, and have good templating properties, such as a strong tendency to form ordered structures in a broad range of solvents, and they offer high thermal stability. For these polymers, the size of templated mesopores increases almost linearly with the molar mass of the hydrophobic middle block of the polymer, and can range from about 7 nm to about 55 nm depending on the type of the polymer (Figure 27). PS−PEO-type polymers containing an aromatic polystyrene block with a very high hydrophobicity represent another important class of surfactants enabling fabrication of periodic porous titania films with a large pore size and different porous architectures. The large and tunable molecular weight of the hydrophobic PS blocks can generate pore sizes from about 10 nm up to 250 nm. Diblock PS−PEO copolymers289−300 undergo a good−poor solvent pair induced phase separation in a mixed solution containing a good solvent for both PEO and PS blocks (such as 1,4-dioxane or dimethylformamide) and a poor solvent for the PS block (such as water, HCl, and titanium alkoxide). By adjusting the weight fractions of those components, a variety of titania films with different morphologies including nanoparticles, nanovesicles, nanowires, nanogranules, worm-like aggregates, foams, flakes, gyroids, or cubic porous morphologies can be obtained.293 Triblock-copolymers PEO−PS−PEO,301 which have very high hydrophobic/hydrophilic contrast, assemble into large spherical micelles in the solution, which leads to the formation of highly ordered macroporous TiO2 thin films with pore size of ca. 280 nm301 (Figure 28). Triblock terpolymers of PI−PS−PEO type (poly(isoprene-b-styrene-b-ethylene oxide)302 enable the assembly of a continuous gyroid-like titania network with 20−30 nm mesopores, resulting in films with extremely high porosity and internal surface area. Other systems include polystyrene-block-polybutadiene-block-polystyrenebased triblock copolymers.303

Figure 27. Porous titania films prepared via the evaporation-induced self-assembly (EISA) method using PEO−PB−PEO polymers with different molecular weight of PB blocks (top-view SEM images of calcined films): (a) 3200 g mol−1; (b) 5000 g mol−1; and (c,d) 10 000 g mol−1. Fast Fourier transformations (FFT) of the SEM pictures are shown in the inset (Reprinted with permission from ref 288. Copyright 2012 John Wiley and Sons).

Figure 28. Macroporous titania films prepared via evaporation-induced self-assembly (EISA) method using PEO−PS−PEO polymers. SEM images of the films calcined at (a,b) 550 and (c,d) 1000 °C, respectively. Scale bar: 400 nm for (a,c); 200 nm for (b,d) (Reprinted with permission from ref 301. Copyright 2008 American Chemical Society).

Graft copolymers are another group of templating agents enabling fabrication of porous titania layers with a large pore size of about 30−70 nm.304−306 Graft copolymers contain hydrophilic poly(oxyethylene methacrylate) (POEM) side chains that selectively incorporate inorganic TiO2 precursors, and a hydrophobic poly(vinyl chloride) (PVC) backbone, which can produce a mesopore upon calcination. The porous morphology of titania films templated with these polymers can be tuned by variation of the amount of titania precursor. The pore size decreases and the wall thickness increases with increasing amounts of titania precursor in the solution.304 9507


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Figure 29. Morphological changes accompanying crystallization of titania films prepared via evaporation-induced self-assembly (EISA) method: (a) wide-angle X-ray scattering (WAXS) and (b) small-angle X-ray scattering (SAXS) raw data obtained during thermal treatment of a titania film deposited from a solution containing TiCl4/H2O/EtOH/Pluronic F127 at 40% relative humidity during evaporation and 60% during the following hours. No diffraction related to the presence of anatase particles is recorded below the critical temperature of crystallization (Tc). Above Tc, the (101) anatase peak starts to appear and increases in intensity. Nanoparticles start to form above 450 °C and grow progressively in size with increasing temperatures. (c) Scheme representing the structural variation during the various steps of heat treatment. (d) HRTEM images of TiO2-based films treated at 100 °C for 12 h ([111] zone axis); (e) treated up to 500 °C (Tc) for 6 h ([110] zone axis); and (f) treated at 730 °C for 20 min (Adapted with permission from ref 71. Copyright 2003 American Chemical Society).

2.3.1.1. Approaches toward the Fabrication of Crystalline Porous Titania Films in the EISA process. One of the important and challenging issues in the EISA processing of titania films is the control of crystallinity and phase composition of the titania scaffold. The films deposited from the conventional sol−gelderived titania precursors are typically amorphous. They can be transformed into a crystalline phase by a thermal treatment at 400 °C, corresponding to the typical nucleation temperature of anatase, which is however higher than the decomposition temperature of the majority of surfactants. As a result, formation and growth of the titania crystals are not confined by the organic template, leading to a partial collapse of the porous structure during crystallization. The fabrication of titania films combining a defined, ordered porous morphology with a highly crystalline titania scaffold using the EISA technique was a subject of intensive research activity.71,307−313 Grosso et al.71 have developed a special heating treatment, so-called Delayed Rapid Crystallization (DRC), to produce fully crystalline titania films with periodic porosity and high thermal stability. They found that the complete condensation of the amorphous titania scaffold before crystallization is crucial for its transformation into its crystalline counterpart without collapse. In the DRC treatment, the asdeposited dry films are slowly heated to a temperature just below that of the anatase formation (400 °C), followed by a long treatment at this temperature. This step leads to a complete condensation of the amorphous network in the confined space of the organic template. A following rapid increase of temperature up to 500−700 °C induces rapid nucleation of the amorphous phase with the formation of a large amount of monodisperse

crystalline nanoparticles, which progressively grow in size by sintering with increasing temperatures (Figure 29). The titania films deposited from TiCl4 as precursor and Pluronic F127 as the template exhibit organized mesoporosity with a cubic Im3m pore structure, 35% volume porosity, more than 100 m2 g−1 surface area, and a fully nanocrystalline anatase framework after calcination at 700 °C. Although this technique was initially developed for the Pluronic surfactants, it can be also applied to other surfactants.267,285 Mesoporous titania films typically crystallize into the anatase phase, although formation of other phases can be achieved by changing the composition of the titania precursor. Prochazka et al. have obtained mesoporous films of crystalline TiO2(B) by adding small amounts of H3PO4 to the precursor titania sol hydrolyzed in strongly acidic conditions.313 Besides thermal stabilization, the condensation of the amorphous titania scaffold prior to calcination can be achieved by different post-treatment approaches, which include chemical stabilization with bases, supercritical drying, and hydrothermal treatment. Condensation of the titania oligomers is greatly accelerated in basic conditions, leading to the solidification of templated mesostructured titania scaffolds at low temperatures. For stabilization of the titania frameworks, the deposited films can be treated prior to calcination with bases such as ethylenediamine,309 an aqueous solution of ammonium hydroxide,314 sodium hydroxide,315 or ammonia vapor.268 The amount of base has an effect on the titania mesostructure due to the change in pH. For ethylenediamine, the amount necessary for reaching pH 11 was found to be sufficient to efficiently stabilize the 9508


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Figure 30. Schematic representation of the CASH method. In situ-formed carbon acts as a rigid support and enables the synthesis of highly crystalline mesoporous transition-metal oxides with large, uniform pores. TEM image of (a) as-made TiO2, (b) heat-treated under argon, and (c) HRTEM image of titania prepared by CASH method after calcination in air. Inset: Image showing d101 spacing, d101 = 3.48 Å, of the anatase structure (Adapted with permission from ref 311. Copyright 2008 Nature Publishing Group).

carbonization of the surfactant. In this method, some sulfuric acid is added to a conventional ethanolic solution for deposition of the titania films containing titanium isopropoxide as a precursor and Pluronic-type polymers as surfactants. The sulfuric acid acts as the acidic catalyst for carbonization of the Pluronic polymer to form an amorphous carbon scaffold. The mesoporous titania films obtained via heat treatment at 550 °C in N2 and then at 450 °C in air feature a completely crystalline anatase framework with high porosity having a specific surface area of 193 m2 g−1 and a pore size of 5.1 nm. 2.3.1.3. EISA: Assembly of Nanoparticles. The amorphous character of the titania scaffold and its significant shrinkage during crystallization are critical issues of mesoporous titania films obtained from molecular precursors in the sol−gel process. The shortcomings of the conventional EISA process have stimulated the search for other types of titania precursors for the assembly of porous films. An attractive solution is the replacement of amorphous titania sols by crystalline building blocks. Crystalline nanoparticles have a defined phase composition, which is much less affected by the assembly conditions; they are more robust, less prone to density changes, and could potentially form the mesoporous crystalline network at milder conditions. For the successful incorporation into a mesoporous structure, the crystalline building blocks should meet certain requirements. The particles have to be rather small for being compatible with the wall thickness of the mesoporous material and with the micelle size of the structuredirecting templates. The particles must not be agglomerated and have to be perfectly dispersible in the coating solution. Moreover, the surface properties of the crystalline particles are expected to be of a great importance, as the crystals should be able to interact with the templating agent. On the other hand, the ability of the particles to form a continuous network should be retained. To increase crystallinity of the precursor titania species, Ozin et al.317 pretreated a coating solution before film deposition under solvothermal conditions. The stock solution containing titanium butoxide, Pluronic P123, conc. HCl, and 1-butanol was heated in sealed autoclaves at 100 °C for 1−2 h, resulting in

mesoporous structure against collapsing during the subsequent calcination. Supercritical drying of deposited films in supercritical carbon dioxide prior to calcination308,316 is another strategy to enhance the thermal stability of mesoporous films. The ordered mesoporous structure is preserved without unidirectional pore shrinkage even at calcination temperatures above 750 °C. 2.3.1.2. Carbon Stabilization. An alternative strategy to avoid the crystallization-induced collapse of the mesostructure is to stabilize the template, which could in principle act as a rigid support confining and directing the crystal growth. This can be achieved by carbonization of the organic template. Carbon is thermally stable in an inert atmosphere and can be later removed by calcination in air, such that the carbon stabilization method combines the advantages of soft structure-directing assembly and hard-templating techniques. For example, the carbon template can be prepared in two steps: by removing first the surfactant from the amorphous titania film and then backfilling the pores with an appropriate precursor for carbon such as furfuryl alcohol.310 A more straightforward way to create a carbon template is to generate it in situ during the EISA process. A general method for the in situ template carbonization, the socalled CASH method (combined assembly by soft and hard chemistries),16,311 is applicable to polymer surfactants containing sp2-hybridized-carbon blocks, such as diblock copolymers of PIb-PEO type (poly(isoprene-block-ethylene oxide)).300 To obtain highly crystalline materials, the as-deposited films are heated in argon to 700 °C. Polyethylene is easily decomposed on heating, whereas the more thermally stable polyisoprene is converted to an amorphous carbon when heat-treated under an inert atmosphere. At the same time, the metal oxide crystals nucleate, grow, and sinter into wall material. The carbon is subsequently removed by heating the material in air to 450 °C, leaving a wellorganized, highly crystalline mesoporous titania retaining the original mesoporous structure, even after heat treatment to temperatures as high as 1000 °C (Figure 30). Zhao et al.312 have extended the CASH method also to aliphatic surfactants by exploiting the sulfuric acid-catalyzed 9509


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Figure 31. SEM images of mesoporous TiO2 thin films prepared via an EISA process using KLE polymer by the sol−gel route (a,b) and by the nanoparticle route (c,d) (Reprinted with permission from ref 319. Copyright 2010 American Chemical Society).

Figure 32. (a) Scheme of formation of crystalline mesoporous titania films (right side) via the “brick and mortar” approach. The nanocrystalline titania “bricks” (light blue, left side) are dispersed in amorphous titania “mortar” (gray), which is periodically self-assembled around the micelles of the polymer template (magenta). (b) A diagram showing the development of crystallinity in samples from cast solutions after calcination at 300 °C as a function of the fraction of nanoparticles in the precursor solution. This is correlated to the formation of different mesoporous structures: distorted cubic mesophase (I), channeled structure (II), and particulate phase (III). The dotted line shows the theoretical crystallinity of the films before calcination. (c) SEM images on silicon substrate (first column), HRTEM images (third column), and the corresponding SAED patterns (second column) of composite titania films containing 0% (c), 15% (d), and 100% (e) of titania nanoparticles (Reprinted with permission from ref 70. Copyright 2009 American Chemical Society).

partial crystallization of the titania species. The films deposited from this solution exhibit a well-defined hexagonal mesostructure with 6 nm-sized anatase nanocrystallites embedded in the inorganic framework. The amount of the crystalline phase is 10− 23% even after aging at temperatures as low as 120 °C, which opens the way to low-temperature fabrication of partially crystalline titania films even on temperature-sensitive substrates. Longer times of solvothermal treatment resulted in a significant increase of the crystallinity of the films, but also led to precipitation of titania in the sol, thus hindering the formation of the mesostructure. Brezesinski et al. demonstrated that even completely crystalline preformed nanoparticles can be assembled into periodic porous structures using the EISA approach.288,318,319 The suitable particles with a sufficient redispersibility (up to 20

mg/mL) in ethanol were prepared in a solvothermal reaction with benzyl alcohol, yielding crystalline anatase particles with an average size of 4−5 nm. For the synthesis of porous films, the nanoparticles were dispersed in ethanol together with KLE polymer used as a template. Coating of this solution followed by calcination resulted in disordered, but macroscopically homogeneous, porous architectures with a pore size of 17−19 or 20−25 nm, depending on the type of the KLE polymer (Figure 31). The nanoparticulate films are crystalline directly after deposition and keep their integrity without fusing up to 600 °C, while the films prepared in a similar way from conventional sol−gel precursors start to crystallize only at around 500 °C with the formation of 13−14 nm large crystals. The difference in crystallization behavior explains the observation that despite the better crystallinity the films assembled from nanoparticles show rather 9510


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Figure 33. (a) Scheme of assembly of ligand-stripped nanocrystals with poly(N,N-dimethylacrylamide)-b-polystyrene (PDMA-b-PS) micelles; (b) SEM images of mesoporous films of 3 × 20 nm TiO2 nanorods (inset: TEM of the nanorods alone); and (c) cross-sectional SEM of 200 and 600 nm thick TiO2 nanorod-based mesoporous films (Adapted with permission from ref 322. Copyright 2012 American Chemical Society).

poor interparticle connectivity and rather low carrier mobility in the crystalline framework.319 Aiming at improving crystallinity and interconnectivity of periodic mesoporous films, some of us introduced a so-called “brick and mortar” strategy that combines the favorable features offered by crystalline and amorphous precursors.70 The formation of fully crystalline interconnected porous titania scaffolds with a defined porous architecture was achieved by the fusion of preformed titania nanocrystals (prepared in a way similar to that described above) with surfactant-templated sol− gel titania, which acts as a structure-directing matrix and as a chemical glue. The similar chemical composition of both bricks and mortar leads to a striking synergy in the interaction of crystalline and amorphous components, such that crystallization is enhanced upon thermal treatment and highly porous and highly crystalline structures are formed at mild conditions. Addition of titania nanoparticles also changes the character of porosity, and induces an increase in pore size and pore volume and an opening of the pore system (Figure 32). The films containing 70−80 wt % of nanoparticles show the highest performance in photocatalysis and photovoltaics due to an advantageous combination of crystallinity, good connectivity between the crystals, and high porosity, while the films containing only nanoparticles suffer from compromised pore periodicity and especially from inferior electrical connectivity between the nanocrystals. The applicability of this approach to other types of polymer templates and nanocrystalline titania seeds was also demonstrated by Kohn et al.320

The crystallinity and the electronic properties of porous titania scaffolds are strongly affected by the type of the nanoparticles and by their crystallization behavior. In another approach developed by some of us, ultrasmall titania nanoparticles prepared by a solvothermal reaction in tert-butanol were used as the building blocks for mesostructure assembly.321 A solution of particles after synthesis can be directly used for the preparation of mesoporous films using commercial Pluronic templates. The minute size of the ultrasmall nanoparticles (ca. 3 nm) leads to better sintering upon calcination and therefore to enhanced electronic properties of the resulting mesoporous films. Moreover, the films show less shrinkage during calcination because of the crystalline nature of the titania particles, thus enabling fabrication of thicker films. Milliron et al.322 have developed a generally applicable and elegant approach toward the assembly of colloidal nanocrystals of various shapes into ordered mesoporous materials. This approach is based on the use of ligand-stripped nanocrystals (NOBF4 is used as the stripping agent for titania nanoparticles) in combination with a new class of block copolymers acting as a template. The polymers contain a porogenic polystyrene domain alongside with a poly(N,N-dimethylacrylamide) domain that strongly adsorbs to the naked surface of the nanocrystals, thus driving their ordered assembly. Using this approach, nanocrystals of diverse size, shape, and composition can be assembled into ordered mesoporous architectures (Figure 33). 2.3.1.4. Generation of Thick Films with the EISA Process. As mentioned before, the films prepared in an EISA process with amorphous sol−gel precursors are strongly limited in their 9511


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Figure 34. Cross-sectional SEM images of a hierarchical “brick and mortar” titania film with bimodal porosity prepared from solutions containing Pluronic P127 and ethyl cellulose (a,b). The ethyl cellulose-derived pores are visible in the inset. Film thickness is 8 μm after a sequential deposition of 4 layers and calcination at 425 °C (Reprinted with permission from ref 339. Copyright 2014 Royal Society of Chemistry).

Figure 35. Examples of the titania films with vertically oriented pore channels obtained via the structural transformation of porosity upon calcination: (a) HRTEM images of grid-like titania films (illustrated in (b)) obtained by treatment of titania film with a cubic Im3m structure at 730 °C; the white stars indicate the pore positions in the initial structure.71 (c) Cross-sectional HR-TEM images of titania film consisting of crystalline nanopillars with inverse mesospace (illustrated in (d)), which was obtained by calcination of titania film with a 3D hexagonal structure (P63/mmc) at 400 °C (Reprinted with permission from ref 342. Copyright 2006 American Chemical Society).

thickness due to cracking. This severe cracking or even delamination of films deposited as thicker coatings is caused by the large changes in density due to removal of the template and the crystallization and densification of the initially amorphous sol−gel precursors. Methods to overcome this limitation include multiple coating approaches where individual thin layers are coated on top of each other with intermediate calcinations to reduce the stress in the individual layers.300,323−334 Still, this approach is rather time-consuming, and it only allows for depositing films with about several hundred nanometer thickness per cycle. Moreover, the repetitive heating leads to crystallization and densification of the underlying layers, thus deteriorating the porosity.323,326,327,335

Addition of larger crystalline particles (such as commercially available P25 particles) to the sol−gel coating solutions helps to increase the film thickness and to prevent formation of cracks. Although the large particles do not participate in the mesostructure formation, they act as rigid binders to prevent delamination. Hence, incorporation of about 5 wt % of P25 nanoparticles enables fabrication of crack-free films up to 10 μm thick in the EISA process.304,336 An efficient way to minimize the volume changes in the sol− gel-derived films involves solidification of the amorphous scaffold prior to calcination. Chen et al.337 have employed a liquid paraffin treatment of the deposited films to promote extended condensation of the titania scaffold and to prevent the thick mesoporous films from excessive shrinking. Thick mesoporous 9512


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Figure 36. (a) Illustration of the process used to generate vertically aligned mesochannels in an air flow method. Effect of air flow rate and incident angle on mesochannel alignment: cross-sectional TEM images of mesostructured titania films deposited on silicon substrates with horizontal air flow rates of 8 m/s (b), and with air flow incident angle of 50° at a flow rate of 5.5 m/s (c). The insets correspond to the FFT “diffractograms”. All samples were sliced along the air-flow direction with the air flowing from right to left. Scale bar = 100 nm (Reprinted with permission from ref 349. Copyright 2012 American Chemical Society).

TiO2 films can also be synthesized by a one-step dip-coating method. The films with a thickness ranging from 3 to 12 μm can be obtained by one-step dip-coating, depending on the withdrawal rates and the viscosities of the sols. The volume and density changes upon heating are drastically reduced when nanoparticle precursors are used for the film assembly, due to their intrinsic crystallinity. The thickness of the “brick and mortar” films described above is usually about 1 μm after a single coating and can reach 1.8 μm without cracking and delamination.338,339 Even thicker films can be obtained by multiple coating using a small number of steps. In contrast to the sol−gel films whose porous morphology deteriorates after a few subsequent coatings, the porosity and surface area of the “brick and mortar” films scale linearly with the number of coatings without deterioration of the bottom layers. The number of coatings required to obtain a thicker film can be further reduced by increasing the viscosity of the coating solutions due to addition of thickeners such as ethyl cellulose.339 In this way, approximately 2 μm thick layers are obtained after calcination in a single coating step. The preparation of multilayer films by a sequential spin-coating and calcination procedure enables the production of films with an overall thickness of up to 10 μm in only five steps. Along with increasing film thickness, the ethyl cellulose introduces an interpenetrating macropore network into the films, leading to the formation of hierarchical porous films with bimodal porosity, with the smaller mesopores resulting from the structure-directing agent, Pluronic F127 (Figure 34). 2.3.1.5. Fabrication of Films with Vertical Porosity in the EISA Process. Although EISA is a very versatile method for fabricating different types of ordered porous structures with tunable porosity parameters, it offers only a rather low degree of control over the orientation of the pores with respect to the substrate. In anisotropic pore structures such as hexagonal ones, the pores are typically oriented parallel to the substrate due to the

strong interaction of its surface with the surfactant micelles, which makes it generally difficult to obtain films with vertically oriented mesopores (more details can be found in the review articles340,341). One of the simple and straightforward approaches to fabricate mesoporous titiania films with vertically oriented channels is based on uniaxial shrinkage of the pores in a direction normal to the substrate as was described in the part 2.3.1.1 (Figure 29), leading to structural transformation of porosity upon calcination. Using an optimized heating protocol, nanopillars with perpendicular porosity can be obtained from a hexagonal pore structure,342,343 and grid-like crystalline titania structures can be derived from 3D cubic structures71,344 (Figure 35). Another possibility to tune the orientation of the pores during the EISA is changing the interaction of surfactant with the substrate, which can be achieved by modification of its surface. Rankin et al.345−347 demonstrated that a chemically neutral sacrificial copolymer layer (such as Pluronic block copolymer) deposited on a substrate controls the orientation of the pores, inducing the channels to tilt away from the substrate plane. This effect is however limited to a critical thickness below 100 nm. Tolbert et al.348 have shown that patterning of the surface strongly influences the development of mesostructure and that it can lead to vertical orientation of the pores provided that the mesostructure of the deposited film precisely matches that of the surface pattern. This approach, called nanometer-scale epitaxy, was used to produce a mesoporous silica film with a vertically oriented hexagonal honeycomb pore structure, which was deposited on a cubic mesoporous titania film with the same lattice parameters acting as a pattern.348 Shan et al.349 introduced an air flow method to control the orientation of titania pores in the mesoporous films. In this simple method, a surfactant-containing titania sol is dried after deposition in a jet of hot air, with the effect that the cylindrical 9513


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Figure 37. Three methods of preparing periodic macroporous structures by colloidal crystal templating. Top: A preformed colloidal crystal is infiltrated with precursor material that is processed to form the 3-D structure after removal of the template. Middle: Uniform templating spheres and titania precursor species are codeposited to form a 3D-structure after template removal. Bottom: Core−shell structures are assembled into periodic arrays, forming close packed hollow shells (Reprinted with permission from ref 8. Copyright 2007 American Chemical Society).

filling, while excessive amounts lead to the formation of a dense titania overlayer. The methods to control these issues include multiple infiltration steps370 and the application of vacuum371 (Figure 38). Other techniques for the filling of the interstitial voids of the colloidal crystal template with the titania precursor include electrochemical deposition354 and RF sputtering.372

micelles align along the direction of the shear force. The flow rate and the incident angle of the air flow were shown to be the key factors controlling orientation of the mesochannels (parallel, perpendicular, or oblique) during the EISA process (Figure 36). 2.3.2. Colloidal Crystal Templating. Colloidal crystal templating provides a straightforward and effective route toward porous titania films with controlled pore sizes and pore structure.350 This approach is based on the ability of uniform spherical colloidal particles to self-assemble into crystalline arrays with a periodic cubic packing motif (so-called opals) (Figure 37). Molding of the opals with a titania precursor followed by the removal of template results in porous titania replicates, so-called titania inverse opals, which closely reproduce the size and arrangement of the colloidal template. Most commonly, polymer (such as polystyrene or poly methyl methacrylate) or silica spheres351 are used as the templates; the use of other types of homodispersed colloidal particles such as alumina beads has also been reported.352 The spherical particles of these materials are available in different sizes, which can be varied from about 50 nm up to a micrometer, and a narrow particle size distribution, which is replicated in the pore size of the titania inverse opals. The most common route to the fabrication of macroporous titania films with an inverse opal structure is infiltration of a preformed colloidal crystal template with a precursor titania solution. The infiltration method involves two main steps: the bead assembly and their subsequent impregnation with suitable titania precursors. Sol−gel titania precursors are the most frequently used,352−367 but preformed colloidal titania particles can also be applied for this purpose.368,369 Typically, the infiltration is performed via drop casting, spin-coating, or dipcoating. The amount of the titania precursor should be controlled very carefully, as lack of precursor leads to incomplete

Figure 38. Top view (a) and cross-sectional view (b) of TiO2 inverse opals fabricated using a sandwich-vacuum infiltration technique (Reprinted with permission from ref 371. Copyright 2011 American Chemical Society).

The two-step infiltration method offers the advantage of good control over the film morphology as well as universality. As the deposition of the colloidal template and its replication with the titania precursor are separated from each other, this method can be applied to practically any type of colloidal template and any titania precursor. The macroporous films obtained in this way have a very high degree of pore ordering, as the processes of colloidal template deposition and the infiltration with the titania phase can be optimized separately. By variation of the deposition 9514


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protocol, films with a thickness ranging from a submonolayer to several tens of micrometers can be prepared. Another approach toward the fabrication of 3D-porous titania films is the direct coassembly of the titania precursors and the colloidal beads (the so-called codeposition method).263,373−375 The codeposition method offers several advantages such as better control over the precursor-to-template ratios, simplicity, and shorter fabrication times. One of the main prerequisites of this approach is compatibility of the colloidal dispersions of the titania precursor species and the (latex) beads without flocculation and/or phase separation, which is however not always possible and thus limits the scope of this method. After solidification of the titania phase, the colloidal template can be removed leaving a highly ordered periodic titania (inverse) replica of the opal. Silica templates are usually removed by etching with hydrofluoric acid or a strong base such as sodium hydroxide.376 Polymer spheres can be easily removed by thermal decomposition or extraction, which leads to the final macroporous structure. The crystallinity of the resulting macroporous titania morphologies can differ widely for the various synthesis approaches. Key properties such as phase composition, crystal shape, crystal size, and interconnectivity of the crystals strongly depend on the choice of the titania precursors and/or the subsequent thermal treatment. Some of us377 have investigated several titania precursors including colloidal sol−gel titania and titania nanoparticles with different crystallinity and crystal sizes, to compare their influence on the properties of the resulting macroporous films. The amorphous sol−gel-derived titania species are most capable of replicating the template bead arrays, resulting in the best periodicity of the macroscopic pore system. Moreover, the films assembled from the amorphous precursors are smooth, and feature good mechanical stability and good adhesion to the substrate. However, the sol−gel-derived films are prone to strong volume changes and shrinkage due to densification upon crystallization of the titania scaffold during the high-temperature calcination. Hence, only relatively thin films can be prepared without cracking and delamination of the titania coating. This undesired shrinkage is greatly reduced when larger and highly crystalline titania nanoparticles are used for the film assembly. However, the films prepared in this way have a lower order of the pore packing, a lower mechanical stability, and inferior adhesion to the substrate, which becomes more pronounced for larger crystalline building blocks. The advantages of both strategies can be combined by blending amorphous and crystalline titania species for the film assembly in a “brick and mortar” approach.70,377 An addition of about 30 wt % of amorphous precursor to the crystalline particles is sufficient to obtain rather regular macroporous films with good adhesion to the substrate, largely reduced shrinkage, and an increased thickness. Moreover, the specific crystallinity parameters such as the size of the crystalline domains, packing density of the crystallites in the macroporous walls, and interconnectivity between the crystals strongly depend on the type of the precursor. The larger the size and the higher the crystallinity of the building blocks used for the film assembly, the lower the packing density of the titania crystals in the macroporous titania scaffold after calcination. The walls of the films obtained from the amorphous precursor are smooth and dense. In contrast, the walls of the nanoparticulate samples feature additional textural porosity due to the crystalline solid building blocks (Figure 39). An alternative approach for the fabrication of 3D-materials is the deposition of composite core−shell particles.358 This method

Figure 39. SEM top view images of calcined (500 °C) macroporous TiO2 films assembled from poly(methyl methacrylate) (PMMA) beads and different titania precursors: sol−gel titania (a), and anatase nanoparticles with the size of about 4 nm (b), about 6 nm (c), and about 20 nm (d) (Reprinted with permission from ref 377. Copyright 2014 Royal Society of Chemistry).

provides good control over the wall thickness in the resulting scaffold by tuning the thickness of the shells. However, depending on the material of the shell, this method involves the risk of generating isolated air spheres within the bulk after template removal, if the density of the shell is high or if its walls become too thick. 2.3.3. Nanocasting and Hard Templating. Replication of preformed shape-persistent porous templates (nanocasting or hard templating) is a universal method to fabricate porous titania with many types of porosity. In contrast to the soft templating methods where the template self-assembly and thus titania formation is strongly influenced by the processing conditions, the 3D morphology of the titania obtained by hard templating is solely determined by the structure of the templates (molds). Decoupling of the template formation from that of the formation of the titania phase enables the use of practically any possible techniques for the fabrication of desirable templates, from bottom-up approaches such as self-assembly, to top-down approaches such as different lithographic methods, as well as a combination of different approaches. Besides offering the desired structure and morphology, the template should be removable after the titania formation. Therefore, the typical template materials are either silica or alumina that can be selectively etched using either alkali solutions or HF, or polymer templates that can be either dissolved in organic solvents or, more commonly, burned away by calcination. Formation of titania inside the template is typically achieved via three general ways: (i) casting (impregnation) of the template with titania precursor, (ii) coating of the template walls using different techniques, or (iii) template-confined growth of the titania phase. Wettability of the template voids with the titania precursor is an important prerequisite for hard templating, which can be improved by a chemical modification of the template voids. (i) Casting or impregnation of the template voids with a suitable titania precursor is one of the most common methods of hard templating (the details can be found in a review by Caruso et al.378), with sol−gel titania or titania salts being the most typical titania precursors. The examples of reported processes include 9515


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Figure 40. (a) Scanning electron microscope images of titania nanotube arrays produced within 0.2 μm AAO membranes after supercritical CO2 drying following the dissolution of AAO, and (b) side view of 450 °C calcined nanotubes showing the hexagonally ordered mesoporous structure (Reprinted with permission from ref 394. Copyright 2007 John Wiley and Sons).

impregnation with titania sol−gel precursor of polymer gels,379−381 cellulose and polymer membranes,382−387 highly ordered polymer templates with formation of mesoporous gyroid titania structures,289,388 highly ordered mesoporous silica templates,107,389−392 or sol−gel impregnation of porous carbon templates.393 Hierarchical morphologies can be obtained by a combination of hard and soft templating; in this case, titania precursor cast in the larger pores of the hard template usually contains amphiphilic molecules acting as soft template for the smaller pores. An interesting example of such a process is a combination of evaporation-induced self-assembly (EISA) with highly ordered hard templates, enabling a high level of control over the formation of 3D titania morphologies in a broad size range. Using this approach, which can be actually considered as EISA process in a confined space, Wang et al.394 have fabricated well-aligned titania nanotubes with mesoporous walls via casting of anodic aluminum oxide (AAO) membranes with sol−gel titania solution containing Pluronic P123 polymer. By variation of the precursor dilution, the thickness of the mesoporous walls formed after evaporation of the solvent can be tuned from several tens of nanometers to the complete pore filling with formation of mesoporous rods (Figure 40). (ii) Coating of the template walls with titania layers is especially beneficial for controlling the thickness of the titania walls in the 3D titania materials formed by hard templating.378 Atomic layer deposition (ALD) is one of the investigated coating techniques in this context, due to an unprecedented degree of control over the thickness of the titania walls and the possibility of conformal coating of templates with any type of complex geometry;395−400 the reader is referred to a review about ALD coating of any rigid 3D materials for more details.26 Titania can also be coated on the walls of different templates using other techniques such as layer-by-layer deposition (an example includes formation of titania nanotubes by the layer-bylayer coating of anodic aluminum oxide (AAO) membranes401,402) or sol−gel coating of different templates such as disordered glass,403 ordered alumina membranes,404 polymer inverse opals,405 or ZnO nanorods with the formation of titania nanotubes.406 An example of titania coating in combination with highly ordered hard templates is the fabrication of holographically defined TiO2 films.407 In this process, holographically defined polymeric template produced by multibeam interference lithography was spin-coated with dilute TiCl4 precursor solution, resulting in a macroporous triply periodic bicontinuous titania network after removal of the template (Figure 41).

Figure 41. (a) Fabrication of an inverted TiO2 structure from a holographically patterned photoresist template: coating of the TiO2 shell and subsequent removal of the template. (b) SEM image of macroporous polymer patterns on an FTO substrate and an inset image of the magnified surface. (c) SEM image of inverted TiO2 patterns and an inset image of the magnified surface. A and B indicate triply periodic, bicontinuous air networks formed after removal of the polymer template and after coating with TiO2 (scale bars for (b) and (c) indicate 3 μm) (Reprinted with permission from ref 407. Copyright 2010 American Chemical Society).

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(iii) In addition to impregnation and coating, titania can also be grown inside the porous templates using different growth techniques introduced in the previous parts. The strong point of this approach is the possibility to control the crystallinity and crystal orientation of the titania phase in addition to its morphology. Solvothermal and hydrothermal processes are among the most intensively used growth techniques. The examples of 3D titania materials obtained in this way include highly crystalline mesoporous rutile obtained by a hydrothermal growth inside porous polymer membranes,408 mesoporous polycrystalline titania with a common orientation of the titania nanocrystals (so-called “single-crystal-like mesocages”) obtained by a hydrothermal titania growth in the pores of mesoporous silica particles,409 ordered mesoporous titania with exposed (001) facets obtained by a solvothermal growth inside mesoporous silica,410 or growth of a mesh of titania nanofibers via hydrolysis of TiCl4 filled in the pores of mesoporous silica films.411 Using solvothermal growth inside a mesoporous silica template, Snaith et al.108 have obtained truly monocrystalline titania particles with a highly ordered mesoporous structure (mesoporous TiO2 single crystals) featuring superior electronic properties and electron mobility due to the absence of grain boundaries. Formation of titania monocrystals was achieved by hydrothermal growth in a dilute TiF4 solution at reduced temperatures, thus lowering the rate of nucleation and growth. The critical issue of confining growth to the template was solved by pretreatment of the silica template in a solution of TiCl4 to form microscopic “seeds” for the crystal growth (Figure 42). Besides using chemical growth, titania can also be deposited by electrochemical methods such as electrophoretic deposition inside hard templates such as alumina membranes,412 or it can be grown by electrochemical deposition. One of the reported examples describes the formation of porous titania films with vertically oriented pores, as well as titania films with an unusual gyroid architecture, via replicating aligned polymer templates.413 The polymers used in this process are cylinder-forming PFS-bPLA (poly(4-fluorostyrene)-b-poly(D,L-lactide) polymers, which can be aligned in an electric field and selectively etched to remove the PLA block. Electrochemical deposition of titania in the pores of the polymer layers resulted in vertical titania nanowires with a diameter of 12 nm, or in a continuous interwoven titania gyroid network with an approximate strut diameter of 12 nm73,414 (Figure 43).

Figure 42. (a) Schematic visualization of mesoporous TiO2 singlecrystal nucleation and growth within a mesoporous template. Transmission electron micrograph (b) and electron diffraction Laue pattern (c) collected from a complete mesoporous crystal (synthesized at 190 °C, 40 mM TiF4) assigned to anatase TiO2 with [001] beam incidence. (d and e) Equivalent image and Laue pattern for crystals synthesized at 130 °C in 120 mM TiF4, indexed with [010] or [100] incidence (Adapted with permission from ref 108. Copyright 2013 Nature Publishing Group).

2.4. More Complex and Hierarchical Porous Morphologies

3. POROUS SPHERES Spherical morphologies of porous titania have been shown to be beneficial for various applications such as electrochemical energy storage, dye-sensitized solar cells (DSCs), photocatalysis, and chromatography, just to name a few, which has stimulated intensive research aimed at the fabrication of porous titania spheres with a defined shape and tunable parameters of porosity. The progress in this field is highlighted in comprehensive reviews by Chen and Caruso,415 Xu et al.,18 Dai et al.,416 and Piquemal et al.417 Spherical morphologies of 3D-titania comprise diverse architectures such as spherical particle agglomerates, porous spheres with periodic porosity, hierarchical porous spheres containing several types of porosity, flower-like spheres, spherical flaky assemblies, spherical bundles of nanorods and nanowires, or urchin-like structures. Important characteristics of spherical 3Dtitania morphologies include bulk properties such as phase composition and crystallinity, porosity parameters such as the type of porosity, the pore size, and the pore structure, as well as

In addition to the 3D-titania films described in section 2, other porous titania morphologies such as porous spheres, porous fibers, and different hierarchical morphologies are of great interest for titania-based applications. Complementary to the character of porosity that can be achieved with the approaches introduced in section 2, the formation of more complex porous morphologies requires additional control over their specific macroscopic shape. In the following chapters, the fabrication of different porous titania morphologies will be presented in the order of the increasing degree of complexity of their geometric shape, from porous 0D-morphologies such as porous spheres (section 3) and hollow spheres (section 4) to porous 1Dmorphologies such as porous fibers (section 5), and to even more complex hierarchical porous morphologies combining different types and scales in their 3D-structure (section 6). A schematic overview of possible preparation routes of hierarchical porous TiO2 structures is given in Figure 44. 9517


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ation, and dispersibility in solvents. The following section gives an overview of basic techniques enabling fabrication of various spherical titania morphologies. 3.1. Spherical Particle Agglomerates

A large group of porous titania spheres is formed by aggregated titania particles exhibiting disordered textural porosity. This type of spherical morphology is usually formed via a spontaneous assembly of nanoparticles, which can be achieved by different synthesis methods. To get uniformly sized spherical agglomerates with a narrow particle size distribution, the coagulation of the nanoparticles should occur in a controlled way. This can be achieved by tuning the colloidal stability of nanoparticles due to steric or electrostatic stabilization. 3.1.1. Formation of Spherical Particle Agglomerates in Sol−Gel Reactions. A large group of methods to fabricate uniformly shaped particle agglomerates relies on in situ formation and agglomeration of the nanoparticles. Sol−gel reactions are broadly used for this purpose, as they enable good control over the rate of particle growth and agglomeration.418 The particle formation conditions can be tuned by the selection of titania precursor compound, type of the solvent, reaction pH, addition of hydrolysis and condensation regulators such as urea, ammonia, or dodecylamine,419,420 as well as addition of electrolytes such as KCl for tuning the electrostatic repulsion between the nanoparticles.421,422 Moreover, these reactions can be performed in the presence of different structure-directing agents and surfactants influencing the colloidal stability of the titania nanoparticles and controlling the character of porosity, such as alkylamines,423−426 Pluronic-type polymers,427−429

Figure 43. SEM of nanostructured array morphologies obtained by hard templating via electrochemical deposition of titania in the pores of the PFS-b-PLA polymer layers, viewed from the top surface and crosssection perspectives. (a,c) Standing TiO2 cylinders. In (c), the wire stability is improved by a thin, partially continuous layer of electrochemical overgrowth at the array surface, which hinders lateral collapse. (b,d) Gyroid network (Reprinted with permission from ref 73. Copyright 2009 American Chemical Society).

morphology-specific characteristics such as the size of spheres, homodispersity and the size distribution, degree of agglomer-

Figure 44. Schemes detailing possible preparation routes of hierarchical porous TiO2 structures: sol−gel method (B1), hydrothermal/solvothermal route (B2), surfactant-assisted hydrothermal/solvothermal route (B3), combining of hard template with hydrothermal/solvothermal route (B4), bubbling of surfactant-containing mesoporous precursor (air−liquid foam strategy) (B5), and electrospinning (B6). 9518


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Figure 45. Fragmentation and compression during spray-drying for the formation of porous titania spheres. When salt is added to the sols, the nanoparticles are preaggregated in the initial colloidal droplets, while no preaggregation occurs in the absence of added salt. SEM images of spray-dried particles produced from TiO2 sols, in the absence of added salt (first row) and with addition of 0.15 M Al(NO3)3 (second row) (Reprinted with permission from ref 434. Copyright 2002 American Chemical Society).

Figure 46. Titania porous spheres obtained by a hydrothermal treatment of titanium sulfate in the presence of ammonium chloride: SEM images at (a) low and (b) high magnifications and (c) XRD pattern (Reprinted with permission from ref 440. Copyright 2012 American Chemical Society).

biopolymers such as sodium alginate430 and aspartic acid,431 graft copolymers,432 and crown-ethers.433 Sol−gel reactions usually lead to the formation of spherical agglomerates composed of smaller (a few nm) primary nanoparticles. The size of the porous spheres can by varied from about 30 to 700 nm by changing the reaction conditions. Spherical titania agglomerates can be fabricated by spraydrying of concentrated sols. The morphology of mesoporous

titania gel microspheres obtained in this process is strongly affected by the ionic strength of the titania sol.434 Addition of an appropriate salt to the titania sol results in the formation of welldefined spherical particles during drying; otherwise, the droplets can fragment during drying with the formation of fine powders (Figure 45). Porous aggregates formed in the sol−gel reaction are usually amorphous and have to be calcined to obtain crystalline titania 9519


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Figure 47. (A) Schematic illustration of the self-assembly procedure for the fabrication of mesoporous TiO2 nanocrystal clusters (Reprinted with permission from ref 450. Copyright 2010 American Chemical Society). (B) TEM images of corresponding clusters assembled from different titania nanocrystals: (a,b) 5.1 nm nanodots; (c,d) 6.6 nm nanodots; and (e,f) 3 nm × 28 nm nanorods. Insets show TEM images at high magnification (Reprinted with permission from ref 448. Copyright 2010 John Wiley and Sons).

scaffolds, which often leads to the agglomeration of porous spheres and a decrease in the specific surface area. This can be avoided by a combination of sol−gel and solvothermal or hydrothermal processes, in which the amorphous aggregates formed in the sol−gel reaction are treated under solvothermal or hydrothermal conditions, as described more in detail in the following section. 3.1.2. Formation of Porous Particle Agglomerates in Hydrothermal/Solvothermal Conditions. Formation of porous spherical agglomerates via in situ growth and agglomeration of titania nanoparticles can also be performed in hydrothermal or solvothermal conditions. In contrast to the sol− gel processes, where amorphous titania primary particles are formed during the reaction, the titania particles obtained in hydrothermal or solvothermal reactions are usually already crystalline, which enables the direct formation of crystalline porous spheres, without subsequent calcination. Composition, crystallinity, and shape of the titania particles formed in these processes are very sensitive to the type of precursor, type of the solvent, acidity or basicity of solution, reaction temperature, and the reaction time. Similar to the case of the sol−gel reactions, the presence of additives can greatly influence the process of particle formation. Variation of the reaction parameters enables

fabrication of a large variety of porous hierarchical titania spheres with different crystalline structure and different porosity. Urea-assisted hydrothermal treatment of TiCl4 as a titania source results in the formation of crystalline anatase titania spheres with high surface area and mesoporous structure. The spheres with a diameter of about 0.4−1.3 μm are composed of anatase nanoparticles of about 10 nm, resulting in the disordered mesoporosity and a high surface area of up to ca. 120 m2 g−1.435−437 Particles with a similar morphology can be obtained using TiCl3 as precursor and polyethylene glycol as a structuredirecting agent.438 Monodisperse spherical agglomerates with tunable size of 200−650 nm composed of crystalline rutile nanoparticles are formed in a low-temperature solvothermal reaction of TiCl4 in the presence of 1,2-ethanediol.439 Hydrothermal treatment of titanium sulfate in the presence of ammonium chloride440,441 or urea442 leads to the direct formation of crystalline porous spheres with an average size of 840 nm. The spheres exhibit disordered porosity with a surface area of 166 m2 g−1 and a pore size of 8.9 nm resulting from the packing of the nanocrystals (Figure 46). Spherical particles with a similar morphology can also be obtained in a microwave-assisted solvothermal reaction of titanium butoxide in alcohols such as isopropanol, ethanol, and butanol,443 and in a boric acid-assisted solvothermal reaction of tetra-n-butyl titanate in n-butanol.444 9520


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Figure 48. SEM (a−e) and TEM (f) images of the precipitates obtained in a solvothermal reaction of titanium n-butoxide and acetic acid at 200 °C at different reaction times: (a) 1 h, (b,c) 2 h, and (d−f) 5 h. The upper right inset in (f) shows the related SAED pattern, and the lower right inset is an enlarged TEM image (Adapted with permission from ref 455. Copyright 2011 American Chemical Society).

tion448,450,452 (Figure 47). After washing, the porous spheres can be dispersed in water without aggregation. An alternative approach to low-temperature sintering of particles in porous spheres was proposed by Tartaj et al.449 In this synthesis, the porous agglomerates were assembled from amorphous and crystalline precursors acting as nanoseeds for crystallization in a surfactant-stabilized microemulsion process, resulting in the formation of mesoporous anatase spheres with sizes between 50 and 70 nm.

Formation of the porous agglomerates in the latter reaction was explained by a confinement effect of the water−butanol interface in a reaction-limited aggregation of the generated titania nanocrystals. Ionic liquids emerged in the past years as important medium for the solvothermal fabrication of porous titania architectures.416,445 Antonietti et al.446 have reported the synthesis of porous titania microspheres composed of 2−3 nm titania nanocrystals forming a 3D porous network with a very large surface area of 554 m2 g−1. These spheres are obtained by a hydrolysis of TiCl4 at 80 °C in 1-butyl-3-methylimidazolium tetrafluoroborate acting as solvent. The reaction is believed to proceed via reaction-limited aggregation of titania nanoparticles. Hydrothermal synthesis can be also performed in microemulsion droplets, which confine agglomeration of the in situ generated nuclei.447 Hydrothermal reaction of titanium sulfate in a water−cyclohexane emulsion stabilized with cetyltrimethylammonium bromide leads to the formation of crystalline spherical aggregates composed of 4−5 nm anatase nanocrystals with a wormhole-like mesoporous structure and a pore size of about 3 nm. 3.1.3. Assembly of Preformed Nanoparticles. Besides the controlled agglomeration of in situ generated titania particles, porous spheres can also be obtained from preformed titania nanoparticles. The use of preformed crystals offers the advantage of better control over the morphology and porosity of the spherical agglomerates via changing the size, shape, crystallinity, and composition of the titania building blocks employed for their assembly. The controlled assembly of particles is usually performed in a surfactant-assisted microemulsion448−450 or miniemulsion451 process. The presence of surfactant is usually required to disperse and to stabilize the nanoparticles as well as the emulsion droplets acting as size-confining templates for particle agglomeration. The formed aggregates can be stabilized by calcination via sintering of the nanoparticles. To prevent the collapse of porosity due to particle growth and to maintain the spherical morphology, the particle agglomerates can be impregnated in solution with a thin layer of silica, which is etched away in NaOH aqueous solution after calcina-

3.2. Formation of Porous Titania Spheres via Hydrothermal and Solvothermal Methods

Solvothermal and hydrothermal processes build a very large group of reactions enabling the formation of a broad variety of porous titania spherical morphologies with different size, crystallinity, and porosity. Formation of porous titania spheres can take place spontaneously via sol−gel transformations of molecular titania precursors under solvothermal conditions, which is achieved by carrying out the reactions in autoclaves at enhanced pressures and temperatures. Spontaneous formation of spherical morphologies in this type of processes is usually possible only at a certain set of the reaction conditions and requires a judicious optimization of the reaction parameters, which include the type and concentrations of the reacting components, the type of solvent, solution pH, and the reaction temperature and time. A large number of solvothermal and hydrothermal processes leading to porous titania spheres has already been developed.453 The formation of porous spherical titania morphologies in solvothermal/hydrothermal conditions can follow different mechanisms, which include the controlled agglomeration of in situ generated particles (examples of such reactions were given above), the reaction-limited oriented growth of the nanocrystals with different crystalline structure, the chemical transformation via dissolution−precipitation (dissolution−recrystallization) mechanisms, or topotactic transformations. The most typical reactions leading to formation of porous spheres are reviewed below. Solvothermal reactions of titanium alkoxides and acetic acid form an important group of processes for the spontaneous formation of porous titania morphologies.454−457 Depending on 9521


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reservoir for soluble titanium-species for the further formation of other products. Moreover, the esterification reaction between acetic acid and alcohols triggers a slow release of water, controlling the rate of precursor hydrolysis. Further variation of the morphologies of porous titania spheres can be achieved by addition of surfactants directing the crystal growth, as well as by manipulation of the growth kinetics by changing the type of the titania precursor, or by addition of water. Zhong et al.457 have investigated the formation of porous titania spheres in a solvothermal reaction of titanium tetraisopropoxide in the presence of acetic acid, 2-propanol, and organic amines (octylamine, aniline, and isobutylamine). The amines guide the unidirectional crystal growth, promoting formation of flower-like spheres composed of sheet-like crystals or fiber bundles. The quantity of water was found to play a crucial role in the morphology and porous structure of the final titania products. When water is supplied slowly from the esterification reaction between acetic acid and alcohol, titania nanoparticles and porous titania spheres are obtained. Addition of water in the H2O:TiO2 molar ratio of 1:1 to 60:1 results in the formation of macroporous titania microspheres with a large surface area. At higher water ratios, porous titania with an irregular shape was formed. Flower-like porous spheres composed of thin titania nanosheets are formed in a diethylenetriamine-assisted hydrothermal reaction of titanocene dichloride at 120 °C,459 as well as in a solvothermal reaction of titanium(IV) isopropoxide and diethylenetriamine in isopropyl alcohol at 200 °C.460 For the last process, the flower-like spheres are comprised of nearly 100% high energy (001) anatase facets. The fraction of the (001) facets can be decreased by addition of hydrofluoric acid to the reaction mixture.461 Flower-like titania spheres are also formed in a solvothermal reaction of tetrabutyl titanate and glycerol in ethanol at 180 °C.462 This process involves formation of an intermediate titanium glycerolate precursor, which reacts further with the formation of flower-like structures via a nuclei growth− dissolution−recrystallization mechanism. In all of those reactions, the as-synthesized products are amorphous and can be transformed to the crystalline anatase phase by calcination. 3.2.1. Fluoride-Assisted Hydrothermal/Solvothermal Reactions. Fluoride-assisted hydrothermal and solvothermal reactions are broadly applied for the fabrication of porous titania spheres.463−468 The presence of fluoride facilitates crystallization of anatase crystals, which enables the fabrication of crystalline porous titania spheres directly after the synthesis.469,470 Moreover, the fluoride-ion is widely accepted as the main species to stabilize the highly reactive (001) facets, resulting in an increased fraction of those facets in the formed spheres. Single-crystalline anatase porous spheres dominated by (001) facets are obtained in a solvothermal reaction of titanium butoxide and HF in an aqueous isobutyl alcohol at 180−200 °C.465,466 Flower-like hierarchical spheres formed in this process are assembled from anatase nanosheets with a thickness of around 10−20 nm and a length of about 1.2 μm, acting as the basic building units. A similar morphology with a high ratio of exposed (001) facets is also formed in a solvothermal reaction of tetrabutyl titanate in diglycol in the presence of HF and acetic acid.468 The as-prepared hierarchical spheres of ca. 200 nm in size are assembled from highly crystalline anatase nanosheets with a thickness of ca. 6 nm, resulting in disordered mesoporosity with a pore size of about 3.5 nm and a high surface area of ca. 250 m2 g−1.

the reaction conditions, this simple system can provide a surprisingly large variety of titania nanostructures with different crystalline structure, porosity, and size. Solvothermal treatment of titanium n-butoxide and acetic acid without any other solvent leads to different morphologies with tunable size ranging from nanoparticles, nanofibers and hierarchical spheres to ellipsoids, depending on the reaction temperature, reaction time, and the concentration of the precursors.454 The lower reaction temperatures of 120−150 °C result in the formation of amorphous phase and titanium complexes, while increase of the reaction temperature to 160−200 °C leads to the formation of crystalline anatase phases.454,455 Additionally, the reaction time greatly influences the development of morphology, from nanoparticles and fibers at shorter reaction time, to spherical and spindleshaped porous particles at longer reaction times454,455 (Figure 48). The solvothermal reactions of this system in the presence of ethanol and sulfuric acid result in the formation of amorphous spherical particles that are converted to crystalline porous spheres with anatase structure after calcination.458 The acetic acid is believed to play a key role in these processes. Reaction of acetic acid with titanium alkoxides leads to the formation of a titanium−acetate complex, which can serve as a slow-release

Figure 49. SEM images at different magnifications (a−d) of porous Fdoped TiO2 microspheres prepared by a hydrothermal treatment of TiF4 powder; (e) an individual single microsphere showing detailed texture and porosity (ca. 1 μm in diameter); and (f) EDX spectrum of porous F-doped TiO2 microspheres (Reprinted with permission from ref 467. Copyright 2006 Royal Society of Chemistry). 9522


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Figure 50. Schematic illustration of the formation process of submicrometer rutile spheres in a hydrothermal reaction of TiCl3 in the presence of NaCl, and FE-SEM images of the corresponding stages (Reprinted with permission from ref 472. Copyright 2012 Royal Society of Chemistry).

Hierarchical nanoporous F-doped titania spheres467 can be prepared by a low-temperature hydrothermal treatment of TiF4 powder in 1 M HCl solution at 180 °C. This reaction leads to the direct formation of crystalline spherical particles with about 1 μm in diameter composed of agglomerated anatase crystals with an average size of ca. 11−15 nm, forming about 3 nm large mesopores (Figure 49). 3.2.2. Hierarchical Spheres Assembled from Rutile Nanowires. Urchin-like spherical morphologies composed of radially assembled rutile nanorods or nanowires are typically obtained in hydrothermal reactions in strongly acidic conditions in the presence of Cl− ions, which are favorable for the formation of rutile. The presence of Cl− ions is also known to suppress the growth of the (110) planes, resulting in anisotropic growth with formation of nanorod arrays.471−473 Typical reactions include hydrothermal treatment of TiCl4 in water474,475 or hydrochloric acid476 at temperatures of 150−180 °C. The initial concentration of TiCl4, reaction temperature, and reaction time have a strong impact on the phase purity and the morphology of the formed particles: higher concentration and higher reaction temperatures usually result in better size uniformity and phase purity of the particles. Rutile titania morphologies are also formed in a hydrothermal reaction of titanium n-butoxide in concentrated hydrochloric acid,477 or in a mixture of oleic and hydrochloric acids478,479 at 180 °C. HCl plays a key role in guiding the structures and morphologies of the formed titania particles, which can be cuboid, flower, cauliflower, and ball-shaped depending on the volume of concentrated acid. The diameter of the rutile spheres can vary from several hundreds of nanometers to several micrometers depending on the reaction conditions. Submicron urchin-like rutile spheres are formed in a hydrothermal reaction without hydrochloric acid using oxidative hydrolysis of TiCl3 in the presence of NaCl as a source of Cl− ions.472 The reaction takes place at the relatively low temperature of 90 °C for 12 h. The reaction starts with the formation of dense ball-like spheres assembled from nanoparticles, which gradually transform to urchin-like morphologies assembled from numerous rutile TiO2 nanorods (Figure 50). The BET surface area increases from ca. 35 m2 g−1 for the initial particle agglomerates to ca. 60 m2 g−1 for the urchin spheres obtained after 12 h.

treatment of amorphous titania spheres provides a very convenient way toward fabrication of nonagglomerated porous titania morphologies with a high surface area.422,453,480−487 In this process, the amorphous titania either solidifies or transforms into a crystalline phase via a solution−recrystallization mechanism. The crystallization process can be directed by changing the conditions of the solvothermal reaction such as solvent, pH, temperature, and time, as well as by addition of specifically adsorbing species, surfactants, or condensation catalysts. Crystalline porous spheres can be obtained by transformation of porous amorphous agglomerates,480 as well as of nonporous amorphous particles. Morphology and the degree of condensation of the amorphous particles have a strong impact on the morphology of particles formed after solvothermal transformation. The solidification of the titania scaffold in the hydrothermal or solvothermal reactions can be accelerated by the addition of condensation catalysts such as ammonia. In a process developed by Chen et al.,480 amorphous spherical agglomerates were treated in autoclaves at 160 °C for 16 h in an ethanol−water mixture containing different amounts of ammonia. This treatment leads to the crystallization of an amorphous titania network with the formation of an anatase phase, resulting in crystalline, mesoporous titania beads with surface areas up to 108 m2 g−1 and tunable pore sizes from about 14 to 23 nm423 (Figure 51). Kim et al.488 obtained highly porous crystalline spheres by a solvothermal treatment in ethanol at 240 °C of two types of amorphous spherical particles obtained by controlled hydrolysis of titanium isopropoxide. Transformation of monodispersed spherical particles results in the formation of well-defined porous spheres consisting of tightly interconnected titania nanocrystals of about 13 nm in size, while transformation of deformed spherical particles leads to crystalline porous structures with diverse sizes and shapes. An alternative approach toward the fabrication of nonagglomerated mesoporous crystalline spheres was developed by Yang et al.489,490 This process involves synthesis of dispersible titanium diglycolate spheres in a sol−gel reaction of tetrabutyl titanate using ethylene glycol and acetone as solvents, followed by boiling in water with the formation of porous spheres composed of anatase nanocrystals. 3.3.2. Titanate−Titania Transformations. In addition to phases such as anatase, rutile, and brookite, titanium dioxide can also exist in layered structures, which are derived from those of protonated titanates. Titanate morphologies are generally

3.3. Transformation Approaches

3.3.1. Hydrothermal/Solvothermal Transformations of Amorphous Titania Particles. Solvothermal or hydrothermal 9523


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of hierarchical flower-like spheres constructed from interconnected ultrathin nanosheets with several layers or even monolayer titanate. Because of the very thin structural units, the titanate spheres can exhibit very high surface areas of up to 450 m2 g−1.491 An important feature of the layered titanates is their facile conversion to crystalline titania by calcination at temperatures above 300 °C via topotactic transformations492 (Figure 52a). The calcination is accompanied by shrinkage of the structure due to dehydration and transformation of nanosheets to small titania particles, but the general macroscopic structure remains preserved. This makes the layered titanates important precursors for the fabrication of hierarchical porous titania spheres with a very high surface area. Hierarchical spheres of various titanate morphologies can be obtained in a variety of solvothermal and hydrothermal reactions under basic conditions. The size and morphology of the hierarchical titanate spheres are easily tunable by varying the synthesis conditions. High surface area microspheres with mesoporous structure composed of titania nanosheets can be obtained by a hydrothermal or solvothermal reaction of titanium alkoxide in basic conditions in the presence of amines491,493−496 (Figure 53). Another common way of generating layered titanate spheres is a hydrogen peroxide-assisted hydrothermal reaction of titanium metal powder in NaOH.497,498 Takezawa et al.499 have developed a procedure for the fabrication of titanate spheres with controlled architectures via slow hydrolysis of TiF4 immobilized in an agar matrix. The layered titanate spheres can be also produced by hydrothermal transformation of titania powders in alkaline condition, presumably via a dissolution−recrystallization mechanism. The morphology of precursor titania particles has a large influence on the structure, size, and morphology of the formed titanates, which opens a way to a large variety of hierarchical porous spheres. The reported procedures utilize, for example, commercial P25 titania powders, which are transformed to urchin-like titanate spheres in a hydrogen peroxide-assisted hydrothermal reaction in NaOH.500,501 The titania colloids can also be obtained by an alkaline hydrothermal precipitation,502 liquid-phase laser ablation of a Ti plate leading to the formation

Figure 51. Mesoporous TiO2 beads obtained through a combined sol− gel and solvothermal process in the presence of hexadecylamine (HDA) as a structure-directing agent: (a,b) SEM images of the spheres after solvothermal treatment and calcination, (c) TEM images of an ultramicrotomed sample, and (d) HRTEM image of an elongated anatase crystal. The sample was prepared with an HDA:H2O:Ti molar ratio of 0.5:3:1 during precursor bead formation and with 0.45 M ammonia during the solvothermal process (Reprinted with permission from ref 423. Copyright 2010 American Chemical Society).

formed in alkaline conditions via hydrothermal treatment of titanium compounds, followed by the substitution of alkaline cations with protons in acid washing. The use of different precursors in hydrothermal processes results in different evolution and growth processes and great differences in the final titanate structures. Upon optimization of synthesis conditions, the layered titanates can be produced in the form

Figure 52. (a) Different schemes of phase transformations upon heat treatment of layered orthorhombic and monoclinic titanate materials. (1) Formation of tetragonal anatase TiO2 by delamination of layered orthorhombic titanate splitting along the ⟨b⟩ axis. (2) The layered monoclinic titanate undergoes topotactic structural condensation (step 1), forming a TiO2(B)-like intermediate, and then monoclinic TiO2(B) is obtained via breaking the intermediate product along the connected corners (green line, step 2) of three TiO6 units of one chain. (b) Schematic illustration of morphology evolution during calcination of titanate spheres with increasing temperatures. Depending on the calcination temperature, the microspheric surface can have a branch-like structure composed of nanoflakes and nanoparticles, a heterogeneous assembly of small crystallites, or larger and more irregularly shaped crystals derived from the coalescence of smaller grains (Reprinted with permission from ref 492. Copyright 2012 American Chemical Society). 9524


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of amorphous TiO2 species,503 or electrochemical oxidation of Ti foil in alkaline media in an ultrafast electrochemical spark discharge spallation process.492,504 3.3.3. Sacrificial Templates. Formation of composite titania spheres containing a second component in different synthetic processes, followed by the removal of the second component acting as a sacrificial template, is an alternative way to afford hierarchical spherical titania morphologies. Nanostructured titania microspheres with a mesoporous structure were prepared by a solvothermal reaction of titanium tetraisopropoxide and sucrose.505 During the reaction, the sucrose is carbonized in the micrometer-sized spheres with incorporated titania particles, which transform to the porous titania spheres after calcination. A similar morphology is also formed in a microwave-assisted solvothermal reaction of titanium isopropoxide in the presence of resorcinol and formaldehyde, which are carbonized during the reaction.506 Calcination of the composite spheres produces crystalline nanostructured titania spheres with specific surface areas as high as 130 m2 g−1. The use of TiCl3 as precursor in a similar process leads to monodisperse single

Figure 53. (c) SEM and (d) TEM images of the flower-like titania spheres obtained by calcination at 500 °C for 2 h of the layered protonated titanate hierarchical microspheres synthesized in solvothermal reaction in the presence of hexamethylenetetramine (Reprinted with permission from ref 491. Copyright 2012 Royal Society of Chemistry).

Figure 54. FESEM and TEM images showing the evolution of the TiO2 nanorod spheres prepared by a resorcinol-formaldehyde route. (a) FESEM image of the TiO2 nanorod spheres before calcination, (b) enlarged FESEM image of (a) reveals that the tips of some TiO2 nanorods can be seen on the surface of the sphere, (c) TEM image of a TiO2 nanorod sphere before calcination, (d) FESEM image of a TiO2 nanorod sphere after calcination, (e) enlarged FESEM image of (d) reveals the individual TiO2 nanorods in the sphere, (f) TEM image of the TiO2 nanorod sphere after calcination, and (g and h) FESEM of several monodisperse TiO2 nanorod spheres after calcination (Reprinted with permission from ref 507. Copyright 2012 Royal Society of Chemistry). 9525


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Figure 55. Macroporous particle formation by spray-drying and colloidal templating of brookite nanoparticles in the aerosol-assisted EISA process (top), and SEM images of porous titania particles prepared with precursor solutions consisting of brookite nanoparticles only (a) and with solutions containing different ratios of brookite nanoparticles and polystyrene latex (PSL) particles of different size: (b) 1:1 brookite nanoparticles to 300 nm PSL particles, (c) 1:2 brookite to 300 nm PSL particles, (d) 1:3 brookite to 300 nm PSL particles, (e) 1:2 brookite to 200 nm PSL particles, and (f) 1:2 brookite to 400 nm PSL particles. Reprinted with permission from ref 521. Copyright 2007 Wiley.

procedure is the presence of pyridyl moieties, which allow interaction with the titania precursors. 3.4.2. Evaporation-Induced Self-Assembly (EISA). Spherical titania beads with a periodic porosity can be fabricated by a self-assembly approach via templated assembly of titania precursors in the EISA process, which was introduced in section 2.3.1 for the porous films. The shaping of the templatecontaining titania precursor into spherical morphology is typically achieved via aerosol processing, by spray-drying of precursor solutions in tubular ovens with a gas flow. As templates in the aerosol-assisted EISA process, surfactants (usually block copolymers providing larger pore size),514−516 colloidal particles of polymers517 or silica,518,519 or a mixture of both can be used,514,520 enabling fabrication of periodically organized porous titania spheres with different pore size and pore hierarchy. Titania precursors in this process are typically titanium alkoxides or titanium salts; moreover, the aerosol-assisted EISA formation of porous spheres using preformed titania nanoparticles was also reported. By using preformed nanoparticles, crystalline titania phases different from the typically formed anatase can be obtained. Using this approach, Iskandar et al.521 was able to produce spherical brookite particles with the tunable size of 200−400 nm and a highly organized hierarchical macro-/ mesoporosity (Figure 55). Templated self-assembly of porous spheres can also be performed in emulsion synthesis522 by encapsulating polymer colloidal particles in emulsion droplets of hexane in which a titanium alkoxide precursor is dissolved. Calcination of the condensed particles after removal of the solvent leads to spherical

crystal rutile titania nanorod spheres with a large percentage of (110) facets507,508 (Figure 54). 3.4. Templating Methods

3.4.1. Nanocasting of Spherical Templates. Porous titania spheres can be made by a hard templating approach via nanocasting of porous beads of different materials, such as polymers,509,510 ion exchange resins,511 silica, or carbon.512 The obtained titania materials replicate the shape of the spherical templates, enabling fabrication of porous titania with a very welldefined shape and a narrow particle size distribution. Using different template beads, porous titania spheres with a size ranging from several hundred nanometers to several millimeters can be obtained. The replication is usually performed by impregnating the dispersed template beads with the titania precursors, typically titanium sulfate,511,512 titanium oxysulfate,509 or titanium alkoxides.512 The key to the successful replication of the porous structure is a high loading of the titania precursor in the pores. This can be enhanced by a chemical modification of the pores for a better adsorption of the titanium ions, as well as by multiple impregnation. The molecular titania precursor is hydrolyzed after loading followed by formation of an amorphous titania framework, and afterward the template can be removed by calcination yielding crystalline porous titania particles, or the template can be etched in case of silica. This technique can also be used for nanocasting of spherical poly(styrene)-block-poly(2-vinylpyridine) copolymer particles, resulting in spherical titania particles with a tunable porous morphology and controlled size.513 A key feature of this 9526


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Figure 56. SEM and TEM (insets) images of titania spheres prepared by a template-free solvothermal reaction of TiOSO4, showing their structural evolution with reaction time: solid spherical particles (a) were obtained after 1 h reaction time. Small surface platelets (b) were formed after 12 h and grow further with time, forming an urchin-like prickly surface (c−f). The platelets formed shells on the solid cores, creating a core−shell structure (c) after 1 day reaction time. The cores shrink with time, forming a sphere-in-sphere structure with continuously reducing inner sphere size (d−f). The inner spheres finally vanished after 14 days of reaction and created a hollow spiny structure (f). Scale bar is 1 μm for all images (Reprinted with permission from ref 523. Copyright 2007 American Chemical Society).

particles of 1−50 μm in size and an inverse porous structure with close-packed monodisperse pores.

types of reactions leading to the formation of hollow porous spheres are largely the same as for the solid ones. Very often, both solid and hollow spheres can be obtained in the same type of process by a slight variation of the reaction conditions and the reaction time.523−526 Despite the apparent differences of these reactions utilizing different titania precursors and different synthesis protocols, the formation of hollow spheres follows a similar mechanism. In the first step of the reaction, a molecular titanium precursor hydrolyzes with formation of primary titania species, which then assemble into secondary spherical aggregates to minimize their surface energy. The spherical aggregates undergo subsequent transformations involving a further condensation or crystallization of the primary titania species. Because of a higher concentration of the hydrolyzing agents and the higher chemical potential of the surface titania species, these transformations usually start from the surface of the secondary spherical agglomerates exposed to the reaction medium. As a result, core−shell spherical agglomerates with a more solidified

4. HIERARCHICAL HOLLOW SPHERES 4.1. Nontemplating Approaches

A variety of titania hollow spheres and hierarchical hollow spheres with different size, crystallinity, and porosity can be fabricated in spontaneous processes without any shape-directing templates. Important types of such processes are hydrothermal and solvothermal reactions, chemical and thermal transformations of porous spheres, spray- or aerosol-assisted processes, and emulsion syntheses, which are described in more detail in the next sections. 4.1.1. Solvothermal and Hydrothermal Processes. Similar to the porous spheres described in the previous section, a broad variety of hollow porous spheres with different morphology, phase composition, and character of porosity can be prepared via hydrothermal and solvothermal approaches. The 9527


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hydrolysis was generated in a high-temperature (350 °C) solvothermal reaction of titanium n-butoxide and ethanol. The formation of hollow porous morphology in this process takes place only in a certain range of volume ratios of ethanol:titanium n-butoxide of 3:1 to 1:1, yielding mesoporous particles with a textural porosity. At higher ratios, smooth nonporous particles are formed, while at lower ratios the hollow spheres shrink and collapse into irregular shapes. A slow hydrolysis and condensation rate at enhanced temperature leads to the formation of crystalline anatase particles already during the solvothermal process, without the need for a further calcination step. Formation of hollow spheres in solvothermal reactions is also possible under conditions of base catalysis. Urea is among the most typical catalysts; the use of other compounds such as triethanolamine was also reported. The amino bases play a multiple role in these processes: they control the rate of hydrolysis, influence agglomeration behavior of the primary titania species via changing the reaction pH and the surface charge of titania particle, and affect the crystallization behavior of the titania spheres. Guo et al.529 have obtained mesoporous hierarchical titania spheres with a yolk−shell morphology in a solvothermal reaction using TiCl4, urea, and ammonium sulfate in a water−ethanol mixture with a molar ratio of TiCl4:H2O:EtOH:(NH4)2SO4:urea of 1:86:24:1:46. The reaction in autoclaves at 95 °C for 5 h followed by the calcination of the collected precipitate at 500 °C in air for 3 h produces nonagglomerated yolk−shell spherical particles with an average diameter of about 3 μm, mesoporous 60−80 nm thick walls composed of 8−10 nm crystalline anatase nanoparticles, and BET surface area of ca. 140 m2 g−1. The presence of ethanol as a cosolvent and the use of ammonium sulfate as a dispersing agent were found to be essential for the formation of the hollow titania particles. Pan et al.530 have used a diethylenetriamine-assisted solvothermal synthesis to prepare hierarchical titania hollow spheres consisting of nanothorns with exposed anatase (101) facets. The reaction mixture for this process contains titanium isopropoxide, diethylenetriamine (DETA), and water in 2propanol (molar ratio of Ti/H2O/DETA = 1/0.5/0.16). The precipitate obtained after the solvothermal reaction at 200 °C for 24 h and calcination at 450 °C consists of hollow spherical particles with a size of 1.5−3.0 μm that can include additional inner spheres (the presence of the inner spheres depends on the water content and the reaction time). The shells are porous, being composed of ca. 30−60 nm vertically oriented nanothorns with exposed anatase (101) facets. The water plays a critical role in the hollowing process, as only heavily aggregated TiO2 solid microspheres with nanoflakes were obtained without water addition. In this reaction, diethylenetriamine acts as a hydrolysis agent and, in combination with 2-propanol, as a crystal growth stabilizer leading to the formation of exposed anatase (101) facets. A slight modification of the reaction protocol with a decreased amount of water changes the crystallization conditions, leading to the formation of a similar morphology but with exposed anatase (001) facets.531 Hollow anatase titania spheres with mesoporous shells can be easily fabricated via fluoride-mediated hydrothermal reactions.532−536 Fluoride induces the hollowing process of TiO2 microspheres, and the rate of the process can be readily tuned by changing the fluoride to titanium ratio. Moreover, the addition of fluoride promotes the crystallization and crystallite growth of anatase-phase TiO2 primary nanocrystals, which leads to the

outer shell are formed. The further growth of this shell takes place in the confined space of the spherical assemblies at the expense of the core species, which dissolve, diffuse to the outer shell, and deposit there until all of the core material is consumed. This process can be described as inside-out Ostwald ripening driven by minimization of the total surface energy. Depending on the diffusion conditions and reaction time, this process can lead to the formation of spheres with a tunable morphology, ranging from solid spheres, porous spheres, yolk−shell spheres, spherein-sphere morphologies, and hollow spheres. Given the complexity of the hollow sphere formation mechanism, it is not surprising that alteration of each of the reaction steps via modification of the reaction conditions can affect the morphology of the final product. The rate of hydrolysis is strongly influenced by the type and concentration of the titania precursor, the type and the relative amount of the solvent, the amount of water in the reaction mixture, the hydrolysis conditions (acidic or basic), the presence of coordination or stabilizing agents, and the reaction temperature. All of those parameters will also affect the aggregation behavior of the primary titania particles, as well as solidification and crystallization behavior of the secondary spherical agglomerates. The possibility of different combinations of all of these parameters explains the large number of reactions leading to the formation of hollow spherical titania morphologies. A very simple approach toward fabrication of hollow titania spheres in a solvothermal reaction without any templates and additives was proposed by Li et al.527 In this process, the crystalline hollow spheres are obtained after solvothermal reaction of a mixture of tetra-n-butyl titanate (TBOT) in ethanol containing small amounts of concentrated sulfuric acid and water. The reaction at 180 °C for 4 h leads to the formation of well-defined crystalline titania hollow spheres with a size of ca. 6−10 μm. The molar ratios of water to alkoxide and sulfuric acid to alkoxide were found to play a key role in the formation of hollow microspheres. The hollow microspheres are formed only when the H2O:TBOT molar ratio is below 3, and the H2SO4:TBOT molar ratio is between 0.2 and 0.8. The shell of the hollow particles obtained in this process is formed by small (several nm in size) titania nanoparticles, which are already crystalline corresponding to the anatase phase. Another acid-triggered solvothermal reaction leading to the formation of hollow titania spheres utilizes titanium oxosulfate, TiOSO4, as a titania precursor.523 Solvothermal reaction at 110 °C of a solution containing TiOSO4, glycerol, alcohol (methanol, ethanol, or propanol), and ethyl ether in a molar ratio = 1:16:40:11 leads to the formation of titania spheres with a diameter ranging from 1.6 to 3.1 μm and tunable morphology that can be adjusted from solid, sphere-in-sphere, to hollow sphere by a choice of the alcohol and the reaction time (Figure 56). The water required for the initial hydrolysis of the titania precursor is initially absent in the reaction mixture. However, the required water is slowly released via a sulfuric acid-catalyzed esterification reaction between alcohol and glycerol leading to a controlled hydrolysis rate. The as-synthesized spheres are amorphous; they crystallize to anatase after calcination at 550 °C for 3 h while retaining their morphology. Slow release of water via esterification reaction for a controlled hydrolysis of a titania precursor can also be achieved without addition of catalysts. Lu et al.528 have used this reaction to fabricate highly crystalline hierarchical anatase hollow spheres in a nonaqueous solvothermal method in the absence of water, templates, or additives. In this process, the water needed for 9528


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Figure 57. SEM (a and b) and TEM (c−e) images of hierarchical hollow spheres prepared by a solvothermal reaction of peroxotitanium complex in the water−ethanol system. The inset in (a) gives the corresponding diameter distribution histogram, and (f) the schematic presentation of the shape evolution from amorphous spheres to hierarchical hollow spheres (Reprinted with permission from ref 543. Copyright 2012 Royal Society of Chemistry).

In a synthesis developed by Yu et al.,542 hollow anatase microspheres were prepared by a hydrogen peroxide-assisted hydrothermal reaction of Ti powder in 30% H2O2 and 0.1 M hydrofluoric acid. The reaction starts with the formation of peroxotitanium acid precursor obtained via reaction of titanium fluoride complexes with H2O2. Hydrolysis of this precursor in hydrothermal conditions followed by the inside-out Ostwald ripening leads to the formation of crystalline titania hollow spheres composed of anatase nanoparticles with exposed (001) facets. The spheres with an average diameter of around 1 μm exhibit bimodal pore size distributions in the mesoporous and macroporous regions with a BET surface area of 43 m2 g−1. Another type of peroxotitanium precursor, the peroxotitanium complex [Ti(H2O2)(OH)4−n]n+(OH)n, is formed via dissolution of titania P25 particles in a mixture of hydrogen peroxide and ammonia. Solvothermal reaction of this precursor in a water− ethanol mixture at 160 °C produces hierarchical hollow spheres with a diameter of 750 nm composed of crystalline anatase nanospindles.543 Growth of this unique structure is explained by an initial formation of amorphous spherical particles due to hydrolytic and nonhydrolytic condensation of the peroxotitanium complex in the water−ethanol system (Figure 57f, step 1), followed by ammonia-directed growth of anatase crystals along the ⟨001⟩ direction to form spindle-like particles in the Ostwald ripening process (Figure 57f, steps 2−4). Hydrogen peroxide-assisted hydrothermal synthesis of potassium titanium oxalate precursor in acidic conditions544

formation of crystalline porous titania spheres directly after hydrothermal synthesis. The pH value of the reaction system greatly affects the progress of the hydrolysis and condensation process. The hydrolysis rate is much slower in acid conditions and rapidly increases in basic conditions, enabling tuning the reaction rate and the aggregation behavior of the primary titania particles by selecting appropriate catalysts. The fluoride-assisted synthesis of hollow spheres was initially developed for acidic conditions using titanium sulfate or titanium oxosulfate TiO(SO4) as a precursor.537−540 This process can be also performed in the presence of urea as a basic catalyst for hydrolysis.541 One example of the fluoride-assisted hydrothermal reactions for the fabrication of hollow titania particles was described by Liu et al.534,541 In this synthesis, a solution of Ti(SO4)2 and NH4F in water is heated in autoclaves at 160 °C for 6 h with the formation of hollow spheres and chain-like hollow titania aggregates. The as-prepared noncalcined particles are crystalline anatase. The spheres with an average size of 500−800 nm are composed of agglomerated anatase nanocrystals, and exhibit mesoporosity with an average pore size of 12 nm and BET surface area of 168 m2 g−1. Hydrogen peroxide-assisted hydrothermal reactions comprise another large group of processes enabling the spontaneous formation of porous titania hollow spheres. Depending on the reaction mixture, various hollow titania morphologies with different phase compositions can be obtained. 9529


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Figure 58. Schematic representation of the formation of hollow titania spheres via a surfactant-assisted solvothermal reaction. (a) Formation of Ti3+− PEG−urea composites. (b) Hydrolysis of the Ti3+−PEG−urea composites outside to form the shell of the microspheres. (c) More Ti3+−PEG−urea composites hydrolyze to form TiO2 nanoparticles. (d) All Ti3+ species hydrolyze; TiO2 nanoparticles with low crystallinity redissolve and recrystallize to from particles with high crystallinity. For the legend of TiO2 nanoparticles, gray solid circles represent the TiO2 nanoparticles with low crystallinity; black hollow circles represent the TiO2 nanoparticles with high crystallinity. PEG = polyethylene glycol (Reprinted with permission from ref 524. Copyright 2011 IOP Publishing Limited).

sacrificial template, complexing agent, and water supplier. A solvothermal reaction at 200 °C for 20 h of an amino acid and titanium n-butoxide in anhydrous ethanol results in the formation of hierarchical hollow titania spheres, whose morphology and porosity are strongly dependent on the choice of the amino acid. Amino acids with a larger number of amino groups (corresponding to a higher isoelectric point) have better structure-directing abilities to generate nanosheet-assembled hollow spheres and yolk/shell structures. The specific morphologies and the mesopore size of hollow structures can also be tuned from 3.4 to 10.7 nm by adjusting the titanium precursor concentration and the size of the amino acid molecule. Amino acids play a multiple role in the growth mechanism of the hollow spheres. At the temperature of the solvothermal synthesis, the amino acids melt with the formation of conglobates acting as a template for the titania growth. The esterification reaction between the amino acid and the alcohol produces water, leading to the in situ hydrolysis of the titanium n-butoxide on the template surface and formation of the titania shell. The asprepared structures are calcined in air to form porous nanoparticle-assembled hollow titania. Another approach toward fabrication of hollow spheres involves the hydrothermal reaction of hydrolyzable titanium compounds such as titanium tetrachloride550 or ammonium hexafluorotitanate551,552 in the presence of glucose or polysaccharides.553 The course of the reaction is sensitive to the pH of the solution, which can be controlled by addition of urea to the reaction mixture.550 The reaction is carried on in autoclaves at 180 °C for 24 h. The products of this reaction are spherical particles composed of a mixture of amorphous titania and carbon, which are formed as a result of hydrolysis of the titanium compound accompanied by a simultaneous carbonization of the glucose. Formation of the hollow spheres takes place during calcination of the as-prepared particles in air at temperatures beyond 400 °C. In this process, the amorphous titania is crystallized, and at the same time the carbon is combusted with formation of CO2 diffusing toward the outside and presumably compressing the TiO2 particles at the surface region inside a shell. The progress of calcination leads to the formation of the hollow particles composed of aggregates of titania particles exhibiting textural porosity with an average pore size of 8 nm and a BET surface area of 131 m2 g−1.550

leads to the formation of well-crystallized rutile hollow spheres with a three-dimensional hierarchical architecture. Besides participating in the hydrolysis and crystallization process via coordination to the titania precursor, in this reaction hydrogen peroxide decomposes with the formation of oxygen bubbles acting as a template for the formation of hollow spheres. Agglomerated hollow spheres can be also prepared in the hydrogen peroxide-assisted hydrothermal reaction of titanium sulfate.545,546 Addition of surfactants to the reaction mixture is another possibility to form porous hollow titania spheres in a solvothermal process. The reagents for the surfactant-assisted solvothermal reactions typically include titanium alkoxide547 or titanium chloride524 as the titania precursors, a surfactant such as polyethylene glycol524 or decaoxyethylene cetyl ether,547 and sometimes a hydrolysis catalyst such as urea,524 acetic acid, or hydrochloric acid, which are dissolved in ethanol or water− ethanol mixtures. Heating of the mixture in autoclaves at temperatures from 60547 to 180 °C525 results in a direct formation of titania hollow spheres that can be separated by centrifugation and washed to remove the surfactant. The morphology and the crystallinity of the titania particles are strongly influenced by the reaction conditions: hollow spheres547 and yolk−shell particles525 have been obtained in different reaction conditions. The initial step of this process is presumably a slow hydrolysis of a surfactant-coordinated titanium compound followed by the formation of surfactant-stabilized particles or smaller particle agglomerates. The primary particles assemble further with the formation of larger mesoporous spherical agglomerates stabilized by a surfactant on the outer interface (Figure 58).524 The shell of the hollow particles obtained in a solvothermal process is formed by small (several nm in size) titania nanoparticles, which are agglomerated in a disordered wormhole-like mesoporous structure with high BET surface areas (BET areas from 105 to 378 m2 g−1 were reported for different particles). The as-prepared particles are already crystalline, corresponding to anatase547 or anatase/rutile mixed phases.524 The phase composition and the crystal size depend on the composition of the reaction mixture and the reaction time. Huang et al.548,549 have developed a solvothermal synthesis of mesoporous titania hollow spheres using amino acids as 9530


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4.1.2. Transformation Approaches. Hydrothermal or solvothermal treatment can be used to transform solid titania spheres into their hollow counterparts (named chemically induced self-transformation, CIST). Yu et al.526 have employed fluoride-assisted crystallization of titania in hydrothermal reactions for the fabrication of hollow spheres from preformed solid titania spheres. Hydrothermal treatment at 180 °C of amorphous titania solid spheres in an aqueous solution of NH4F leads to their crystallization and transformation to hollow spheres. The size of the crystalline domains increases with the reaction time from 6.6 nm after 30 min to 18.8 nm after 12 h. The increase in the crystallite size is accompanied by an increasing diameter of the hollow cavity. The walls formed by aggregated titania particles exhibit a textural porosity with a pore size of about 10 nm. The molar ratio of NH4F to TiO2 has a significant influence on the morphology of the TiO2 samples, leading to increasing crystallite sizes with increasing fluoride content (Figure 59).

in air transforms hydrogen titanate into the hierarchical anatase spheres with the same morphology, such as sea-urchin-like assemblies555 or hollow microspheres composed of anatase nanotubes.556 A topotactic transformation of titanium oxidifluoride (TiOF2) to anatase was used by Wang et al. to fabricate hierarchical titania nanoboxes.557,558 The TiOF2 nanocrystals with a defined cubic shape with edge lengths of about 300−500 nm can be easily prepared in a solvothermal reaction using tetrabutyl titanate, hydrofluoric acid, and acetic acid as precursors. The hydrothermal treatment of the TiOF2 nanocubes in aqueous solutions with different pH values (pH = 2−13) at 80 °C results in the formation of fully crystalline box-like hollow structures of the same shape. The nanobox walls consist of aligned anatase nanorods of about 80 nm in length and 5 nm in diameter, arranged perpendicularly to the surfaces of the nanobox (Figure 60). In this process, the TiOF2 acts as a self-limiting sacrificial template. Because of a close match of crystalline structures, the topotactic transformation of TiOF2 to anatase in hydrothermal conditions takes place with a minimal spatial reorganization, which ultimately leads to the ordered growth of aligned anatase nanorods. 4.2. Templating Approaches

4.2.1. Coating of Spherical Templates. Coating of titania precursors on the surface of polymer beads is one of the most widespread techniques for the fabrication of hollow titania spheres. Coating is typically performed in solution by dispersing the polymer beads in a suitable titania precursor. Colloidal titania sols obtained by hydrolysis of molecular titanium compounds are among the most frequently used precursors; however, titanium salts or preformed titania nanoparticles can also be applied. The key requirement is a good affinity of the precursor titania species to the surface of the template bead. This is usually achieved by surface modification of the polymer beads via different techniques such as plasma treatment or coating with polyelectrolytes or surfactants. Li et al.559 have fabricated hollow titania spheres with a mesoporous shell using polystyrene beads as the templates. An important step in this approach is the surface modification of the hydrophobic polystyrene beads with hydroxyl groups achieved by plasma treatment. The surface-modified polystyrene spheres were immersed in a solution containing ethanol, ammonia, and titanium(IV) isopropoxide, resulting in the formation of amorphous titania shells in a sol−gel process. The polystyrene can be easily removed at room temperature by dissolution in tetrahydrofuran resulting in hollow titania spheres with a shell thickness of around 25 nm (Figure 61). The titania shell obtained in these conditions is composed of amorphous titania nanoparticles demonstrating textural mesoporosity with a pore size around 3.7 nm and a BET surface area of 68 m2 g−1. More control over the porosity of the titania shell can be achieved by the use of surfactant-containing titania sols for bead templating.560,561 Guo et al.561 have developed a procedure for the fabrication of hollow titania spheres with thick mesoporous walls by coating poly(methyl methacrylate) (PMMA) beads with a titania sol containing n-hexadecyltrimethylammonium bromide surfactant. Before deposition, the surface of the PMMA beads was modified with ionic liquids to control the interfacial properties for promoting the deposition of titania sol. After deposition, the polymer-titania beads were treated with an ammonia solution to solidify the mesoporous sol into a gel; afterward, the polymer was removed by calcination at 650 °C.

Figure 59. Schematic illustration of the formation of anatase TiO2 hollow spheres via hydrothermal treatment and self-transformation of amorphous TiO2 solid spheres in an NH4F aqueous solution at an F/Ti ratio = 1. The left and right panels, respectively, show the TEM images and corresponding XRD patterns of the intermediate products prepared at 180 °C for 0 (a,b), 30 (c,d), 60 (e,f), 120 (g,h), and 720 min (i,j), respectively. The scale bar is 100 nm (Reprinted with permission from ref 526. Copyright 2010 Royal Society of Chemistry).

Another example of the CIST method was described by Bian et al.554 who have prepared Au-encapsulating hollow titania spheres in a two-step process. In the first step, Au3+-containing titania spheres were prepared via a solvothermal reaction; the solid spheres were transformed to the mesoporous hollow spheres by a consecutive treatment under hydrothermal conditions. Titanate−titania transformation is another possibility to fabricate hollow titania spheres composed of 1D-titania structures (nanowires and nanotubes). Different spherical titanate morphologies can be easily synthesized via hydrogen peroxide-assisted hydrothermal reactions of titanium metal555 or titania precursors556 in alkali metal hydroxide solutions (see section on porous spheres for more details). Annealing at 400 °C 9531


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Figure 60. Formation of 3D hierarchical TiO2 nanoboxes via a template-engaged topotactic transformation of single-crystalline TiOF2 nanocubes: (a) TEM image of an as-prepared hierarchical TiO2 nanobox; (b) TEM image of the shell of a hierarchical TiO2 nanobox consisting of oriented aligned TiO2 nanorods; (c) HRTEM image taken from the marked area in (a); and (d) the corresponding SAED patterns for (a) (Reprinted with permission from ref 557. Copyright 2012 Royal Society of Chemistry).

Figure 61. (a) Schematic illustration of the fabrication of hollow oxides using plasma-treated polystyrene spheres as sacrificial templates; SEM (b) and TEM (c) images of hollow titania microspheres. The scale bars are 400 nm, and the scale bars in the insets are 200 nm (Reprinted with permission from ref 559. Copyright 2008 American Chemical Society).

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Figure 62. (a) Schematic formation of TiO2 hollow spheres composed of anatase nanosheets with exposed (001) facets via in situ growth in the sulfonated gel matrix of polystyrene hollow spheres (PHS). (b,c) SEM and TEM images of TiO2 hollow spheres treated in air. (d) TEM images recorded around the edge of a TiO2 sphere with nanosheets. (e) HRTEM image of a single nanosheet; the lattice fringe spacing in both directions is 0.19 nm (Reprinted with permission from ref 565. Copyright 2011 Royal Society of Chemistry).

This procedure provides uniform hollow spheres with a disordered mesoporous titania shell exhibiting a very high specific surface area of 256 m2 g−1. The size of the hollow spheres can be tuned from 2 to 60 μm by the choice of PMMA beads used for the templating. The range of available hierarchical porous titania morphologies can be further extended by variation of the morphology of the polymer templates. Yang et al.562 have used hollow latex beads to fabricate a series of porous titania hollow spheres with a pillared surface and double-shell composite hollow spheres. The hollow latex cage used as template consists of two layers of polymers: an external hydrophobic polystyrene shell connected via hydrophilic channels to an interior hydrophilic shell of poly(methyl methacrylate) that can be loaded either with water or tetrabutyl titanate. By changing the loading sequence, composite spheres of different morphologies can be obtained. Immersion of water-preloaded latex cages in tetrabutyl titanate produces hollow spheres with titania pillars protruding from the surface. Alternatively, when TBOT-preloaded cages are immersed in water, titania forms only on the interior hydrophilic

surface and not within the channels. The composite spheres can be further used as templates to grow material on the outer surface to create double-shell hollow spheres. The latex cages can be removed by calcination at 450 °C in air with formation of porous hollow titania morphologies exhibiting textural porosity. Not only titania sols (which have to be transformed to a desirable titania phase by some suitable treatment such as condensation or heating), but also preformed titania particles can be coated on the surface of polymer templates. The use of preformed particles often enables a better control over the phase composition and crystallinity of the obtained hollow titania morphologies. As an example of such an approach, the direct formation of crystalline porous hollow spheres with a rutile phase from colloidal dispersions of crystalline rutile nanoparticles was demonstrated.563 The key factor in this process is a strong electrostatic interaction between the polymer template and the inorganic precursor, which was achieved by using anionic polymer brushes and positively charged crystalline rutile nanoparticles. The polymer−rutile composite was transformed to the porous crystalline rutile balls via sintering of the crystals, 9533


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which was performed in a two-step heating procedure. The first heating step performed in argon atmosphere converts the polymer brushes to amorphous carbon acting as a rigid support and preventing the mesostructure from collapse. 4.2.2. Template-Directed Growth of Hollow Spheres. Direct formation of crystalline titania shells was achieved by Lei et al.564 via growing titania in a fluoride-assisted hydrothermal process. Prior to coating with the titania precursor, the polystyrene beads were functionalized by adsorption of polyelectrolytes using a layer-by-layer technique to form a bilayer consisting of a polycationic inner layer and a polyanionic outer layer. The polyelectrolyte-coated PS beads were incubated in a solution of (NH4)2[TiF6] and H3BO3 at pH 2.9 at 50 °C for 8 h. The polymer was removed by etching in toluene, leaving titania hollow spheres. In contrast to other deposition methods resulting in randomly assembled titania particles, the fluorideassisted hydrothermal deposition used in this reaction leads to the formation of oriented anatase nanocones with exposed (101) facets. The oriented growth is induced by the polyions, and the orientation of crystal growth is influenced by the HF generated in the reaction. The spherical shells of about 300 nm in thickness are highly porous with a BET surface area of 92 m2 g−1 and an average pore size of 11 nm. Another procedure for the fabrication of highly crystalline titania spheres with a preferential orientation of crystals facets was developed by Ding et al.565 The formation of anatase nanospheres with a large fraction of exposed (001) facets was achieved by the use of polystyrene hollow spheres acting as templates for the titania growth. Sulfonated polystyrene hollow spheres were dispersed in isopropyl alcohol containing diethylenetriamine and titanium(IV) isopropoxide and heated in an autoclave at 200 °C for 24 h. Because of the presence of hydrophilic functional groups within the polymeric gel shell, the growth of titania takes place within the polymer gel matrix, forming sheets. Calcination of the composite product at 400 °C in air produces highly crystalline hollow spheres with a hierarchical structure. The titania shells built from anatase nanosheets give rise to disordered mesoporosity with a narrow pore size distribution, an average pore size of 5−7 nm, and a BET surface area of 135 m2 g−1 (Figure 62). Besides polymer beads, silica beads are frequently used as templates for the fabrication of porous titania hollow spheres. The silica beads of various size, shape, and morphology can be easily prepared by different approaches, which make them attractive templates for the fabrication of different hollow titania morphologies. As compared to polymers that can swell or dissolve in organic solvents, or shrink during thermal treatment, the silica beads are more robust. This makes them compatible with a larger variety of coating solution compositions and reaction temperatures, which broadens the scope of titania coating techniques. On the other hand, the stability of silica toward calcination as well as its stability in organic solvents make its removal more difficult as compared to polymer templates, as the latter can be simply dissolved in certain organic solvents or burned out by calcination. Silica can be removed by etching in hydrofluoric acid or in hydroxide solution. However, the etching process has to be carefully controlled, as it can eventually damage the titania material. Similar to the polymer bead templating, silica-templated titania hollow structures are conventionally prepared by coating silica beads with colloidal solutions of titania sols that can also contain surfactants for porosity control.566 The coating procedure produces composite particles with a titania shell,

whose thickness can be conveniently increased by a repetitive coating. The titania shells obtained by conventional sol−gel methods are amorphous and have to be further treated to produce a crystalline titania phase, which is typically done by calcination at 450−550 °C. Because of their high thermal stability, silica templates are the most advantageous for such type of high-temperature processes as they practically do not change their shape during calcination. An additional protection of the titania layer during calcination can be also performed to prevent the collapse of porosity, resulting in highly crystalline hollow titania morphologies with a high surface area and defined pore size. Some of the strategies involve sol−gel deposition of a thin protective silica layer566,567 or a double carbon-silica coating568 over the titania shell preventing the coalescence and growth of the titania crystals during calcination. The protective silica coating can be removed together with the silica template core during the final etching with alkali bases, thus producing mesoporous anatase hollow spheres with a high surface area (Figure 63).

Figure 63. Schematic illustration of a silica-protected calcination procedure developed for the fabrication of mesoporous anatase TiO2 hollow spheres. (Right) Corresponding TEM images of the products at each step: (a) SiO2, (b) SiO2@TiO2, (c) SiO2@TiO2@SiO2 after calcination, and (d) final hollow TiO2 after the removal of the SiO2. In this case, the TiO2 coating was repeated three times to achieve a shell thickness of 50 nm (Reprinted with permission from ref 566. Copyright 2012 John Wiley and Sons). 9534


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Additional possibilities to fabricate hierarchical porous 3Dtitania materials are provided by the use of silica molecular sieves instead of solid spheres.571,572 The pores of the molecular sieves can be filled with a titanium precursor, which can subsequently be transformed into a titania phase with unusual morphology. Xiong et al.572 developed a method to generate hollow fibrous titania networks. In this process, mesoporous silica particles were filled with TiCl4 at 90−150 °C under pressure, enabling a high loading of the titanium compound and complete pore filling. Subsequent hydrolysis of the impregnated mesoporous beads leads to the growth of titania nanofibers toward the outside of the mesoporous molecular sieve (Figure 65). Various titania

In addition to the conventional procedure involving chemical etching of the silica beads in the final step, several methods are known where the etching of silica template takes place simultaneously with the formation of titania material. These strategies are very attractive as they enable a facile fabrication of hollow porous titania spheres in one step. One of such processes, based on template-directed deposition and in situ template-sacrificial dissolution, uses TiF4 as a precursor for the titania porous shells.569 A dispersion of silica beads in a 0.01−0.1 molar solution of TiF4 in water is kept at 60 °C. During the reaction, TiF4 is hydrolyzed resulting in the formation of crystalline anatase particles on the surface of the silica beads, and a simultaneous release of hydrofluoric acid gradually dissolving the silica template. After 12 h of reaction, hollow titania spheres with an average size of 2.2 μm and a narrow size distribution are formed. The wall thickness and size of the TiO2 hollow spheres can be controlled by adjusting the concentration of the TiF4 precursor and the size of the silica beads. The 300−350 nm thick walls of the hollow spheres consist of spherical aggregates of about 15 nm primary particles. Agglomeration of particle aggregates on different scales results in hierarchical porosity containing micropores due to the aggregation of primary particles, small mesopores (4 nm), and large mesopores and macropores (from 10 to 110 nm) due to packing of secondary aggregates, and about 1.5 μm large macropores due to internal cavity space (Figure 64).

Figure 65. Scheme showing the formation process of TiO2 fibers on mesoporous spheres (a); SEM images of fibrous hollow titania spheres templated by SBA-15 after removal of silica (b) (Reprinted with permission from ref 572. Copyright 2006 John Wiley and Sons).

morphologies composed of different types of fibers (leaf-like, rods, or flower-like) can be obtained in this way by the choice of molecular sieve and by changing the hydrolysis conditions. Beads of different materials can be used as hard templates for the fabrication of hollow titania spheres. Carbon combines advantages of silica templates such as chemical stability in organic solvents and shape persistence, but in contrast to silica it can be easily removed by calcination in air. Formation of titania layers on carbon templates can be achieved via strategies similar to those used with polymer and silica templates, which include layer-by-layer deposition of titania precursor, sol−gel coating, or hydrothermal reactions.573 The surface of carbon beads is usually rich with functional groups acting as nucleation sites for the titania growth, such that additional surface functionalization is usually not required. Carbon beads are available with a large variety of size, shape, and porosity, which broadens the scope of available templated titania morphologies. As an example, hollow carbon beads with mesoporous walls were used as template for the fabrication of crystalline mesoporous metal oxides including titania.574 The formation and crystallization of the titania takes place within the channels of the mesoporous carbon template, resulting in porous titania after calcination. In this way, the

Figure 64. (A) TEM images of TiO2 hollow spheres obtained from silica particles in a 0.02 M TiF4 aqueous solution at 60 °C reacting for 0, 1, 5, and 12 h. (B) Schematic illustration of the formation of hierarchically nanoporous TiO2 hollow spheres. The scale bar is 200 nm (Reprinted with permission from ref 569. Copyright 2008 American Chemical Society).

In another procedure developed by Leshuk et al.,570 formation of the crystalline hollow titania spheres was achieved in two steps. In the first step, silica beads were coated by a layer of amorphous titania sol in a sol−gel reaction. The titania−silica core−shell particles were afterward dispersed in water (solution pH was adjusted to 12) and heated in autoclaves at 140 °C. During hydrothermal reaction, amorphous titania transforms into a crystalline anatase phase via a dissolution−precipitation mechanism with the formation of a disordered porous titania shell composed of anatase crystals. At the same time, the amorphous silica beads gradually dissolve under hydrothermal conditions at pH 12. This process leads to the formation of crystalline porous titania hollow spheres with a specific surface area of about 300 m2 g−1, and simultaneous selective etching of the silica templates. 9535


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Figure 66. Synthesis of hollow titania structures from iron oxide templates. (a−c) TEM images and (d) SAED pattern of TiO2 nanococoons; (e−g) TEM images of TiO2 nanoboxes; (h) XRD profiles of the TiO2 hollow structures (scale bars: (a) 500 nm; (b) 200 nm; (c) 50 nm; (e) 1 μm; (f) 500 nm; (g) 100 nm) (Reprinted with permission from ref 575. Copyright 2012 John Wiley and Sons).

5.1. Hydrothermal and Solvothermal Synthesis

hollow sphere morphology can be replicated with generation of significant porosity in crystalline titania. Particles and spherical morphologies of metals and metal oxides represent another emerging class of sacrificial hard templates that can be used for the synthesis of hollow porous titania morphologies.17,85,575−577 A versatile one-step method for the fabrication of hollow morphologies of titania and other oxides using iron oxide beads as a sacrificial template was introduced by Wang et al.575 This method involves the hydrothermal growth of titania shells from TiF4 on α-Fe2O3 beads, which are simultaneously dissolved by the hydrofluoric acid released during the reaction. A variety of uniform titania hollow nanostructures, such as nanococoons, nanoboxes, hollow nanorings, and nanospheres, can be readily generated by using beads with different geometries (Figure 66). In a similar way, cadmium hydroxide can be used as a self-etching sacrificial template for the titania formation. Anisotropic cadmium hydroxide crystals with a rod-like or plate-like morphology can be easily grown by a hydrothermal route. The use of these anisotropic crystals as templates enables fabrication of hollow titania particles with a defined geometric shape.578

Hydrothermal and solvothermal routes described in sections 2.2.1, 3.1, and 3.2 for the fabrication of porous films and spherical morphologies can be also used to obtain porous 1D-titania morphologies such as nanotubes,579−581 nanorods,582−585 nanowires,586,587 nanobelts,588 or porous nanoflakes.589 Although the complete synthesis route is straightforward, every single factor including the choice of TiO2 precursors, temperature of hydrothermal/solvothermal synthesis, type and concentration of additives, duration of the thermal process, the subsequent washing procedure, and subsequent calcination plays a crucial role in controlling the properties of the final TiO2 products (crystal structure, surface area, porosity, morphology, optical properties, and photocatalytic activity). Generally speaking, basic solutions favor the formation of nanotube-like structures, while acidic reaction mixtures, containing stabilizing agents, promote the formation of nano- or microsphere-like structures, as shown in Figure 67. In a typical hydrothermal synthesis of TiO2 nanotubes (TNTs), TiO2 or a precursor is dissolved in a concentrated aqueous solution of NaOH and autoclaved in a temperature range of 110−150 °C to convert the reaction mixture into nanosize crystallized tubular (titanate) structures.23 TNTs obtained by the hydrothermal route can exhibit unique features, including a large specific area (up to 478 m2/g) and pore volume (up to 1.25 cm3/g).590−595 However, extensive application of the hydrothermal method for TNT formation could be limited due to long reaction times, the required highly concentrated NaOH

5. POROUS FIBERS Fabrication of diverse one-dimensional titania morphologies such as nanowires, rods, tubes, and fibers is reviewed in detail in complementary reviews in this Special Issue. In this section, only the selected techniques enabling fabrication of porous onedimensional titania are covered. 9536


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Figure 67. Schematic diagram showing typical products obtained from the synthesis of porous TiO2 structures via hydrothermal routes.

Figure 68. Schematic illustration of the growth mechanism of a mesoporous TiO2 nanosheet-based hierarchical tubular superstructure (a). SEM and TEM of starting materials TiOSO4·H2O (b) and samples solvothermally treated for 3 h (c), 12 h (d), and 48 h (e) (Reprinted with permission from ref 596. Copyright 2013 Elsevier).

A mesoporous TiO2 nanosheet-based hierarchical tubular structure has been successfully synthesized on a large scale via a facile solvothermal approach using TiOSO4·2H2O as a starting

solution used during synthesis, and difficulties in achieving uniform size.23 9537


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Figure 69. Schematic diagram of the electrospinning technique used to prepare TiO2 nanofibers (a) (Reprinted with permission from ref 598. Copyright 2012 Elsevier), and images of different types of TiO2 nanofibers (b) (Reprinted with permission from ref 602. Copyright 2011 American Chemical Society); (c) (Reprinted with permission from ref 603. Copyright 2010 American Chemical Society); (d) (Reprinted with permission from ref 597. Copyright 2011 Royal Society of Chemistry); (e) (Reprinted with permission from ref 598. Copyright 2012 Elsevier); (f−h) (Reprinted with permission from ref 600. Copyright 2010 John Wiley and Sons).

hydrolysis of TiOSO4 takes place, resulting in hydroxylated titanium dioxide. The hydroxylated titanium dioxide gradually reacts with glycerol to form titanium glycerolate and crystallizes on the surface of hydroxylated titanium oxide fibers through heterogeneous nucleation, accompanied by the growth of titanium glycerolate nanosheets forming hollow tubular nanostructures.596 Finally, the titanium glycerolate is converted into anatase TiO2 by calcining at 600 °C, as schematically shown in Figure 68.

material in a solution containing glycerol, ethanol, and ethyl ether (volume ratio = 1:2:1).596 The obtained tubular structure was 1−2 μm in diameter and 6−10 μm in length, consisting of interleaved nanosheets with a thickness of about 24 nm. The BET surface area and the average pore size were calculated to be 93 m2 g−1 and 30 nm, respectively. The authors found that glycerol and ethanol were crucial for the formation of the hierarchical tubular structure. If glycerol and ethanol were absent, only TiO2 spheres were formed, while in the absence of glycerol irregular aggregated particles were obtained. The optimized volume ratio of glycerol to ethanol was found to be 1:2. A mixture of tubes and spheres or tubes with nanosheets was observed for a glycerol to ethanol ratio less than 1:2 or higher than 1:2, respectively. The starting material TiOSO4·2H2O was converted into an amorphous phase after 3 h of solvothermal reaction. At the first stage of reaction, under high pressure and high temperature, a small amount of water is generated from the decomposition of TiOSO4·2H2O and the ether formation between ethanol and glycerol. In the presence of trace water,

5.2. Electrospinning

Electrospinning is an effective tool for the fabrication of microfibers, nanowires, nanofibers, and other structures. Electrospinning can produce anatase in the form of a fabric by passing a sol−gel solution through an electrically conductive nozzle tip where a solution droplet is formed. Upon applying a high voltage, the droplet gets charged and changes its shape into a cone, which is then elongated into a fiber of micrometer- or nano-size depending on the surface tension, molecular weight, and viscosity of the solution (Figure 69).597 Most studies on TiO2 9538


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nanofibers have focused on electrospinning from a titania precursor solution mixed with polymer, such as poly(vinylpyrrolidone) (PVP),592,597−601 poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc),602 poly(acrylo nitrile) (PAN),603 poly(ethylene glycol) (PEG), and poly(ethylene oxide) (PEO)604 to achieve a certain viscosity. Hierarchically structured TiO2 nanofibers reported by Hwang et al. had a nanorod-in-nanofiber morphology and were obtained by electrospinning a precursor composite solution (8 wt % PVAc in dimethylformamide, TIP, and acetic acid) directly onto a glass substrate, followed by calcination to remove polymers and develop the anatase phase (Figure 69b).83 The resulting layers present pores in two distinct size domains: macropores (100− 200 nm) and mesopores (∼20 nm). TiO2 nanofibers of ca. 500 nm in diameter and a few micrometers in length consisting of densely packed spherical nanoparticles of ca. 20 nm in size and having mesopores of 3−4 nm were fabricated by electrospinning a TiO2 dispersion in a 10 wt % viscous solution of PAN (Figure 69c).603 Vertically aligned anatase TiO2 nanowires (Figure 69d) were fabricated by electrospinning a solution containing PVP, TIP, acetic acid, and ethanol onto the surface of a copper strip rolled over a rotating disc.597 A unique hierarchical cancellousbone-like structure, composed of a TiO2 nanofiber network of trabeculae forming rod- and plate-shaped scaffolds (Figure 69e), was fabricated by combining the electrospinning technique with a hard templating method using silica as a template.598 The threedimensional network exhibited the anatase structure and a BET surface area of 226 m2 g−1. The resulting material showed a mesoporous structure with an average pore size of 5−6 nm. A hierarchical heterostructure fabric (TiO2/NiO, TiO2/ZnO, and TiO2/SnO2) was obtained by combining the electrospinning technique with a hydrothermal method.600 Close examination of the obtained nanofibers revealed that secondary NiO nanorods (Figure 69f), SnO nanofibers (Figure 69g), or ZnO nanorods (Figure 69h) stand nearly perpendicular on the surface of the primary TiO2 substrate. Among the nanorods, many pores with different diameter were found in these hierarchical structures. Using a combination of electrospinning with a hydrothermal reaction, Sun et al. fabricated cedar-like hierarchical nanostructured titania films with an anatase/rutile composite phase.605 In this process, the electrospun titania film with the anatase structure was used as a substrate for the growth of titania rutile nanorods in the hydrothermal reaction. The length and diameter of the TiO2 nanorods can be controlled by changing reaction time, solution concentration, and reaction temperature. During the first hour of the reaction, small titania nanoparticles are formed on the fibers acting as crystal seeds to guide the subsequent growth of the titania nanorods. The length and diameter of the nanorods proportionally increase with the reaction time and reach up to 50−100 nm diameter and several micrometers in length (Figure 70). Nanocrystalline fibrous anatase prepared by electrostatic spinning from ethanolic solution of Ti(IV)butoxide, acetylacetone, and poly(vinylpyrrolidone) employing the Nanospider industrial process was reported by Zukalova et al.606 In this manufacturing process, fibers were grown from the surface of a rotating cylinder covered with a thin layer of the polymer solution instead of the conventional nozzle tip. These titania fibers were smoothly converted into cubic titanium oxynitride, TiOxNy fibers during thermal treatment in ammonia atmosphere (600 °C for 4 h). The obtained TiOxNy fibers were convertible back into TiO2 fibers by heat treatment in air at 500 °C, and they contained anatase as a main phase.

Figure 70. SEM image of (a) TiO 2 fiber fabricated by the electrospinning method, then calcined at 500 °C; (b) hierarchical brushy TiO2 film synthesized by the hydrothermal approach; (c) high magnification image of highly branched TiO2 nanostructures; and (d) TEM image of a single branch of hierarchical TiO2 nanostructure (inset is the electron diffraction pattern) (Reprinted with permission from ref 605. Copyright 2008 Royal Society of Chemistry).

5.3. Formation of Hollow Structures via the Kirkendall Effect

The Kirkendall effect has been used as a fabrication route to design hollow nano-objects. It basically refers to a nonreciprocal mutual diffusion process through an interface of two phases such that vacancy diffusion occurs to compensate for the unequality of the material flow, resulting in a migration of the initial interface. The Kirkendall effect is a consequence of the different diffusivities of atoms in a diffusion couple causing a supersaturation of lattice vacancies. This supersaturation may lead to a condensation of extra vacancies in the form of so-called “Kirkendall voids” close to the interface.607 It was proposed that two main stages might be involved in the development of the hollow interior, as shown in Figure 71.608,609 A solid-state reaction occurring during the sintering of ceramics involves various types of diffusion processes (bulk, surface, or grain-boundary diffusion) and mechanisms (cation counter diffusion, or cation−anion joint diffusion). Implementa-

Figure 71. Different diffusion processes during the growth of hollow nanostructures induced by the Kirkendall effect. A represents the core component and AB the product phase: (a) at an early stage, small Kirkendall voids are generated via bulk diffusion at the interface; (b) at a later stage, when the voids contact the inner surface of the product layer, the enlargement of the hollow core can be caused by surface diffusion of atoms of the core material along the skeletal bridges (Reprinted with permission from ref 608. Copyright 2007 American Chemical Society). 9539


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Figure 72. Microscopic study of TiO2 nanotube evolution. (a) STEM image and EELS spectrum imaging of the Ti, Zn, and O elements on a two-cycle sample. The ZnO core residue is highlighted by a white dashed box. (b) TEM images of a TiO2 nanotube formed via 10 cycles of TiCl4 pulse without introducing any H2O precursor. Inset on (b-i) is the corresponding cross-sectional SEM image showing the hollow feature; (b-ii) higher magnification TEM images; (c-i,ii) TEM images of amorphous TiO2 coated ZnO nanowire after 10 h annealing. Voids at the core/shell interface are marked with white dashed lines. Inset on (c-i) is the corresponding cross-sectional SEM image. (d) Schematic illustration of the Kirkendall effect combined with cation exchange process in the TiO2 nanotube evolution (Reprinted with permission from ref 611. Copyright 2014 American Chemical Society).

tion of the solid-state reaction using nanowires as one reactant has been previously used to obtain compound oxide nanotubes, such as spinel MgAl2O4 (ZnFe2O4) nanotubes using singlecrystal MgO (ZnO) nanowires as the substrates. The interfacial solid-phase reaction took place upon annealing the core/shell nanowires at 700−800 °C, during which cation pairs (Mg2+ and Al3+, or Zn2+ and Fe3+) diffuse in opposite directions while the oxygen sublattice is fixed.610 Recently, mesoscale hollow TiO2 nanostructures were formed via cation exchange reactions, showing likely a manifestation of the Kirkendall effect in a vapor−solid deposition system. This process involved the cation exchange reaction between TiCl4 vapor and ZnO solid, forming TiO2 and causing diffusion of the reactants and products through the TiO2 shell. An adapted atomic layer deposition (ALD) system was used to provide pulsed TiCl4 and H2O vapor to ZnO nanowires at a temperature of 600 °C for structure replication. The growth followed typical ALD operation cycles where TiCl4 and H2O pulses were separated by 60 s N2 purges. Upon introducing TiCl4 and H2O, the ZnO phase shrunk and completely disappeared from the samples after 10 cycles of deposition, due to the cation exchange reaction:

The average inner diameter of the as-received TiO2 nanotubes was about 40 nm, which was 67% of the original ZnO nanowire size (averaging about 60 nm in diameter). In the initial reaction step, the TiO2 layer on the ZnO surface was formed. When a completed TiO2 shell was formed, solid-state diffusion had to take place to form a center-hollowed structure, which was beyond the operational range of the usual cation exchange mechanism, as shown in Figure 72.611

6. 3D-TITANIA MATERIALS WITH HIERARCHICAL MORPHOLOGY 3D-Titania materials with hierarchical porous architecture are of great interest in processes involving interface- and transportrelated phenomena, such as adsorption, catalysis, energy conversion, and storage,37,612 or biomaterials.613 Implementing a hierarchical organization of different scales of porous frameworks and tuning the morphology of the pore network provides a flexible means to optimize the performance of titania materials in various applications (see also reviews about hierarchical titania: hierarchical porous films,14,15 hierarchical titania structures for dye-sensitized solar cells,37,612 spontaneously formed porous and composite materials,614 and diatom biotemplating613).

TiCl4 + 2ZnO → TiO2 + 2ZnCl 2 9540


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Figure 73. (a) Scheme of formation of hierarchically porous thin film using a combination of supramolecular templating and phase separation. (b−e) FE-SEM micrographs of samples prepared in an EISA process from a sol−gel titania solution containing Pluronic P127 as the template for mesoporosity and poly(propylene glycol) (PPG) as a pore enhancing agent as a function of PPG/Ti ratio: (b) 0; (c) 6.25 × 10−3; (d) 1.5 × 10−2. (e) FE-SEM cross section of a 80 nm thick film. The white arrows indicate macropores along the film thickness (Reprinted with permission from ref 627 Copyright 2009 American Chemical Society).

6.1. Hierarchical Titania Materials with Multiscale Porosity

Another strategy is the combination of two or more types of self-assembly processes resulting in different types of porosity, such as surfactant-templating for the fabrication of periodic mesopores, and phase separation leading to disordered macropores.339,625−629 Hierarchical macro/mesoporous materials can be easily prepared in this way by addition of large molecular weight polymers such as poly(propylene glycol)627,628 or ethylcellulose339 to a surfactant-containing titania sol. Figure 73627 shows a range of hierarchical films with tunable bimodal porosity combining small mesopores and large macropores with 13−18 and 20−150 nm size, respectively. The films were obtained in an EISA process from titania sols containing Pluronic F127 and different amounts of poly(propylene glycol). Further increase in the degree of complexity of the hierarchical porous architectures and even higher control over the parameters of porosity can be achieved via templating approaches. In this way, independent optimization of different porosity types and design of tailored porous materials is possible. A successful strategy for the fabrication of highly organized hierarchical macro-/mesoporous titania materials with a periodic organization and uniform parameters of porosity both on mesoporous and on macroporous scales is a combination of surfactant templating with colloidal crystal templating approaches520,630−633 (Figure 74). This can be realized in a onepot procedure, by dispersing colloidal latex spheres in a surfactant-containing titania sol and shaping it into the desired morphology using a suitable method described in previous sections. Another approach is the successive combination of different templating approaches. One of the commonly used strategies involves impregnation of a colloidal crystal assembly with a surfactant-containing titania sol, leading to a perfectly periodic macroporous titania with the walls composed of mesoporous titania material634−636 (Figure 75).

The majority of reports on the fabrication of hierarchical porous titania deal with systems containing different scales of pores. Such materials, which combine large pores with walls that consist of the small ones, are of interest for applications involving molecular adsorption and diffusion phenomena, and whose functionality is therefore largely governed by the properties of the pores. One of the facile ways toward fabrication of hierarchical titania materials with multiscale porosity is heat treatment of porous titania materials derived from a sol−gel process. Heat-induced transformations of amorphous titania frameworks typically lead to the generation of textural mesoporosity in the titania walls formed by closely packed crystallites. Calcination of porous titania materials obtained in spontaneous sol−gel processes (described in section 3.1) provides a simple route toward the fabrication of hierarchical porous titania materials without the use of templates. In this way, macro-/mesoporous titania can be obtained by calcination of macroporous titania spontaneously formed due to phase separation of a titania matrix and a poorly miscible organic phase including polymers,247,615 or during sol− gel assembly.616−621 More control over the porous architecture in hierarchical materials can be achieved by combination of different strategies for porosity generation.622 One example is a combination of methods for macroporous foam formation with the surfactanttemplating methods for mesoporous materials. Macrocellular titania foams with well-defined mesoporous walls can be obtained by bubbling of surfactant-containing mesoporous precursors.623,624 At the macroscopic length scale, an air−liquid foam strategy provides the control over the open-cell morphologies. At the meso- or nanoscopic length scales, various mesogenic templates or latex colloids can be used to promote porosity. 9541


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Figure 74. Top: Schematic procedure for the preparation of highly organized periodic TiO2 in a one-pot self-assembly of colloidal poly(styrene-coacrylic acid) (PSA) polymer spheres and triblock copolymer Pluronic P123, serving as the macro- and mesoporous structure-directing agents in the EISA process, respectively. Typical SEM images of (a) top surface view, (b) cross-sectional view, (c) TEM, and (d) HRTEM images of the assynthesized titania films. (Reprinted with permission from ref 632. Copyright 2012 American Chemical Society).

6.2. Hierarchical Titania Formed by Porous Sphere Arrays

little control over the periodicity of the particle ordering, resulting in disordered assemblies of spherical particles. Highly periodic arrays of porous particles can be prepared by templating approaches, which tend to be more elaborate than those for the fabrication of titania inverse opal structures. Chen et al.637 have proposed a double-replication procedure of silica opal layers to obtain three-dimensionally ordered arrays of submicrometer-sized mesoporous titania spheres with high surface area and high crystallinity. This multistep procedure involves surfactant templating within a polymer inverse opal matrix, which is manufactured by the polymerization of methyl methacrylate monomer in the voids of silica opal. A simplified variation of this procedure uses a lithographically patterned holographic macroporous polymer template instead of silica opal, which is filled with colloidal titania particles to form a periodic array of mesoporous titania opals (Figure 76).638

Another type of 3D-hierarchical titania materials is actually a replica of the materials described in the previous section; it can be considered as an array of porous particles separated by the voids between the packed particles. Such materials can be easily formed by an assembly of porous spheres described in section 3 or porous hollow spheres described in section 4. Assembly of the porous particles can be performed by different ways introduced in section 2.1 (including particle casting, electrophoresis, pastes, etc.) to give multiscale porous materials with uniformly distributed pores provided by spherical building units, as well as the pores due to the voids between the close-packed spherical particles. This approach has several advantages: it can be applied to practically any type of porous particles, which enables tuning the characteristics of hierarchical morphology in a broad range by a corresponding modification of the building blocks taken for the assembly. The method is simple and scalable, which makes it attractive for large-scale applications such as catalysis, adsorption, batteries, or solar cells. The hierarchical morphologies formed by assembly of the porous particles can also be formed spontaneously during deposition/growth processes, as was described in the sections 3.1 and 3.2. However, typically the assembly of porous particles or spontaneous growth provides

6.3. 3D-Hierarchical Titania Morphologies Organized over Multiple Length Scales

Another type of 3D-titania systems involves hierarchical ordering over several length scales. This type of hierarchical morphologies is generally produced by a combination of top-down and bottomup approaches. Self-assembly approaches introduced in the previous chapters are compatible with various top-down techniques such as molding, patterning, or lithography, which 9542


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Figure 75. Top: Illustration for the preparation of hierarchical macro-/mesoporous titania films via nanocasting of titania sol−gel solution containing Pluronic P123 polymer into the regular voids of a three-dimensional periodic colloidal crystal. Typical SEM (a) and TEM (b,c) micrographs of the macro-/mesoporous titania films. The black arrows in (a) indicate the interconnected channels between macropores in the films, whereas the white and black arrows in (c) indicate the mesopores in the macroporous walls (Reprinted with permission from ref 634. Copyright 2011 American Chemical Society).

Deep X-ray lithography can be directly used for the patterning of titania films without the use of molds. Selective exposure of still wet sol−gel titania coatings to X-ray radiation leads to local densification of the titania framework in the exposed areas and permits the selective etching of the unexposed layer. Deep X-ray lithography can be easily combined with diverse bottom-up processing strategies that include evaporation-induced selfassembly (EISA), nanostructured sol−gel coatings, and nanocasting. Selective X-ray treatment of the EISA deposited films enables the fabrication of ultrathin nanoperforated titania membranes.641 An example of even more sophisticated hierarchical titania nanostructures is shown in Figure 78, which illustrates the fabrication of patterned titania layers for nanofluidics composed of mesoporous titanosilicate pillared layers supporting a roof of the same material.642

allows for numerous possibilities for the fabrication of complex hierarchical titania structures. Periodically ordered arrays of porous titania can be prepared by a combination of surfactant or colloidal crystal templating and lithography. One of the first techniques reported for the fabrication of patterned films periodically ordered over different length scales, from several nanometers to several centimeters, was proposed by Stucky et al.639 This approach is based on confining the solidification of template-containing sol−gel titania in the pores of a lithographically prepared mold. The mold can be either pressed on the cast solution, or it can be filled by capillary flow of the titania solution into the channels. Calcination of the solidified film (to remove the template) results in ordered patterns of porous titania arrays with periodic porosity. Alternatively, the titania precursor solution for the EISA process can be cast on lithographically patterned molds prepared directly on a substrate, where the mold can later be removed by calcination.640 This technique is more facile and compatible with almost any type of coating solutions. The patterning of the mold can be performed via electron beam lithography, which enables fabrication of very small patterns (Figure 77).

6.4. Fabrication of 3D-Hierarchical Titania Morphologies via Biotemplating

Biotemplating of naturally grown “preforms” into ceramic structures represents another possible approach for creating microstructurally designed materials. “Wood petrification” is a well-known natural phenomenon that has inspired chemists to use cellulosic derivatives as matrix for shaping silica and other 9543


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Figure 76. Preparation of hierarchical titania through lithographic patterning. (a) Scheme for formation of four-beam interference and the fabrication of the macroporous polymer structures, (b) filling of the holographic patterns with mesoscale colloidal particles, and (c) coating of precursors and removal of dual templates. Scanning electron microscopy (SEM) images of (d) the surface and (e) the cross-section of the macroporous polymer template, and the hierarchical titania structure obtained with 60 nm (f) and 110 nm (g) colloidal titania particles. Scale bar: 1 μm (Reprinted with permission from ref 638. Copyright 2011 John Wiley and Sons).

Figure 77. Left panel: Schematic diagram of the patterning of mesoporous thin films in the shape of pillars. Right panel: (A) An FE-SEM image of the patterned mesoporous TiO2 thin film with a pillar period of 1 μm. (B) A top-view and (C) a cross-sectional view of the enlarged image of one mesoporous pillar. (D) An HR-TEM image showing the TiO2 anatase nanocrystallites embedded in the mesoporous pore wall of the pillar (Reprinted with permission from ref 640. Copyright 2004 IOP Publishing Limited).

inorganic oxides.643−645 In general, a substitution process has been applied to convert the biological preforms into oxide ceramics. Native or pyrolyzed biological preforms are internally coated with salts or titanium-containing organic precursors and subjected to oxidation to remove the carbon structure.646

Biotemplating could be used to obtain TiO2 in the form of fibers,647 foam-like structures, or porous particles.648−650 Bioinspired morphosynthesis strategies were used for fabricating nanomaterials from the nanoscopic to the macroscopic scale, thus creating sophisticated structure and ordering. Different 9544


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Figure 78. Fabrication of patterned titania layers for nanofluidics. First row: Scheme illustrating the various steps that can be successively applied to prepare the pillared planar nanochannel (PPN) layers. (I) Fabrication of a polymer template layer by dip-coating of PS−PLA block copolymer solution on different substrates followed by solvent annealing to achieve the vertical orientation of the PLA domains; (II) selective etching of the PLA blocks followed by UV-curing to obtain a polymer hard template with vertically oriented pores; (III) nanocasting of the polymer template with a titania precursor containing amphiphilic polymer (EISA process); (IV) micro- or nanopatterning with deep X-ray lithography; and (V) elimination of sacrificial domains via solution etching and thermal treatment. Second row: Electron and optical microscopy images of typical PPNs layers with various morphologies and different thickness of the roofs. (a) SEM (profile cut) and (b) TEM (top view) images of nonporous TiO2 PPN, and (c) SEM image (profile cut) of PPN layers with the mesoporous titania pillars and the roof fabricated with the Pluronic F127-templated titania precursor. Scale bars = 50 nm for (a,b) and 100 nm for (c) (Adapted with permission from ref 642. Copyright 2010 American Chemical Society).

Figure 79. SEM micrographs of bimorphic rutile TiO2-ceramics derived from (a) pine wood, (b) rattan, (c) cellulose fiber felts, and (d) cardboard after sintering at 1200 °C for 1 h (Reprinted with permission from ref 646. Copyright 2004 Elsevier).

of the biomineralization process is that besides gaining some control over the structure, size, aggregation, morphology, and crystallographic orientation of the oxide phase, this method yields advanced synthetic materials in an environmentally benign approach.669 Porous TiO2 with cellular morphology was manufactured from biological preforms via sol−gel processing using cellulose fiber

biological species were used for the synthesis of hierarchical functional TiO2, including eggshell membranes,650−652 bamboo membranes,653 pollen grains,654−656 petals,657 plant skins,658 cotton fibers,582,659,660 wool fibers,661 paper,651 wood,646,662 peptides,663,664 silica-forming proteins,665 diatoms (single-celled algae),613,665−668 yeast cells,669 viruses,670−672 native starch,673 etc. As compared to other preparation processes, the advantage 9545


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Figure 80. Schematic mechanism for the formation of fibrous titania microspheres using chitosan hydrocolloid as template (top) and SEM evidence of replication (“transcription”) of the fibers: (a) the secondary structure of native chitosan M0, (b) the fibers replicated by inorganic oxides, and (c) pure inorganic oxide with filamentary structure after chitosan removal (bottom) (Reprinted with permission from ref 674. Copyright 2011 Elsevier).

extracellular proteins and polysaccharides was produced on the surface of the cells. These biosurface active macromolecules contain some hydrophilic anionic groups including carboxyl and −OPO32−.669 Thus, the hydrophilic anionic groups could serve the two following purposes: (1) to provide nucleation sites for targeted cations, and (2) to accumulate negative charges on the biotemplate surface. When titanium cations were added into the bioemulsion, they were attracted to the cell surface by electrostatic interactions with the negatively charged −COO− and −OPO32− groups. This process induced the formation of a titania layer on the surface of the yeast cell wall and resulted in the creation of a hierarchically ordered mesoporous TiO2 structure. Pollen grains served to develop both hollow TiO2 microspheres654 as well as hierarchical mesoporous titania with ellipsoidal shape and an open pore network on a reticular shell,656 depending on the preparation route used. Copper− titanium dioxide hollow microspheres were fabricated by direct coating of rape pollen through an improved sol−gel process.654,655 The average diameter of the reported hollow spheres was 15−20 μm, and the thickness of the hollow sphere shells was approximately 0.6 μm. The specific surface area ranged from 40 to 228 m2 g−1 and was affected by the calcination temperature (400−600 °C) and copper loading (1−6 wt %). Ellipsoidal mesoporous TiO2 with a surface area of 96 m2 g−1 was prepared by a facile two-step soaking process followed by calcination at 450 °C.656 Nitrogen adsorption−desorption measurements suggested the coexistence of large mesopores and macropores in the obtained structure. The size of mesopores ranged from 2 to 40 nm, while the open pore networks on reticular shells had a size of about 600−800 nm. This kind of open network, extending from the surface to the inner side and

felts, pine woods, rattan plants, as well as corrugated cardboard structures as templates.646 Biological materials were vacuum infiltrated with a low viscosity titania sol, which was prepared from titanium isopropoxide and modified with acetic acid. Pyrolysis in inert atmosphere at 800 °C and subsequent annealing in air at 1200 °C resulted in the formation of highly porous, bimorphic rutile TiO2 with complex micromorphology, as shown in Figure 79. The features of the initial cellular anatomy of the biological preforms were well reproduced in the TiO2reaction product after annealing at 1200 °C. Structures derived from rattan exhibited a homogeneous microstructure in which large pores of about 200−300 μm were surrounded by smaller pores of about 50 μm (Figure 79b). The porosity of these materials varied from 79% to 89% for cardboard, pine, cellulose fibers, and rattan, respectively.646 Highly porous and nanofibrous titania was obtained using chitosan polysaccharide microspheres as nanoassembling system in a sol−gel mineralization of monomeric alkoxides.674 Supercritical CO2 drying avoided the collapse of the transient hybrid material network and resulted in the creation of fibrous filaments featuring a dual meso- and macroporous network with surface areas ranging from 110 to 310 m2 g−1. It was found that aminometal coordination (−NH2 → Ti) and hydrogen bonding between hydroxyl groups and oligomeric metal oxide species play a pivotal role during the mineralization process, as schematically shown in Figure 80. Interactions between titanium cations and a biotemplate surface were also observed when Baker’s yeast cells, S. cerevisiae, were used as templates in the TiO2 preparation.669 During the cultivation process of yeast cells (fermentation), a bioemulsifier with acidic matrix macromolecule metabolites including 9546


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Figure 81. SEM image of hierarchically structured rutile formed through dehydration of rSilC titania: (a) part of the fractured hemisphere, (b) details from the region near the external surface region (the arrowheads indicate some of pores inside microspheres), and (c) detail of the external surface. Scale bars: 5 μm (a), 1 μm (b), and 500 nm (c) (Reprinted with permission from ref 665. Copyright 2006 John Wiley and Sons).

titania particles or through the very flexible sol−gel processing of molecular precursors. In addition, direct growth and deposition techniques have also attracted attention, for example, using hydrothermal reactions, glancing angle deposition, or electrochemical techniques. The use of various types of templates has been highly successful for the design of nanomaterials including titania films and other morphologies. Here, a “molecularly programmed” aggregate of surfactant molecules (soft templating) or a pre-existing shape (hard templating) is made to interact with appropriate titania building blocks followed by polymerization and consolidation, and ultimately removal of the template to open the pore system. For the design of porous thin films, both soft templating methods with surfactants and hard templating methods with colloidal crystals and through nanocasting have been developed. The challenge of creating vertically oriented mesopore systems has attracted much interest, and is discussed in this context as well. Porous titania spheres can also be made in numerous ways, including the formation of agglomerates of nanoparticles (premade and through sol−gel or hydrothermal processing), the direct hydrothermal and solvothermal growth, or through creative chemical transformation strategies. Even for the synthesis of porous spheres, hard- and soft-templating strategies have been used successfully. Extending the field of titania spheres further, we also discussed the synthesis of hierarchical hollow spheres, where similar strategies can be used (nontemplated synthesis via hydrothermal growth and chemical transformation, as well as templating with spheres). Hydrothermal and solvothermal methods can also be used to form porous fibers. On the other hand, the highly versatile electrospinning technique has proven its power to generate porous titania fibers in a quasi-continuous process. Finally, we have explored examples of the near endless possibilities to combine any of the above methods for the synthesis of hierarchical three-dimensional titania nanostructures. These include materials with multiscale porosity, hierarchical titania materials built from arrays of porous spheres, materials organized over multiple length scales, and examples of biotemplating making use of the great diversity of natural nanoand micromorphologies. It appears likely that future research efforts, building upon the large accumulated experience on the synthesis of titania nanomaterials, will be directed toward achieving specific desired physical properties such as surfaces comprised of specific crystal structures and/or crystal facet populations, more control over the “internal quality” of the crystalline titania in different 3Dmorphologies, extremely high surface areas, hierarchical path-

forming barrel-shape networks, is anticipated to facilitate mass transport. One of the most prominent sources of inspiration for inorganic materials synthesis has been the diatom. Diatoms are a major group of unicellular eukaryotic algae that produce cell walls composed of hydrated, amorphous silica.7,663 A two-stage photobioreactor cultivation process was used by Jeffryes et al. to metabolically insert titanium into the patterned biosilica of the diatom Pinnularia sp.666 Titanium was preferentially deposited as a nanophase, lining the base of each frustule pore, with an estimated local TiO2 content of nearly 80 wt %. Thermal annealing in air at 720 °C converted the biogenic titanate to anatase TiO2 with an average crystal size of 32 nm.666 Recently, mesoporous TiO2 assembled inside the macropores of diatom frustules by sonochemical condensation of titania precursors followed by thermal treatment was reported by Mao et al.675 The amount of TiO2 inside the macropores of the diatoms was controlled by varying the sonication time, and it differed from 11 to 30 wt % for 6 and 12 h treatment times, respectively. Pioneering work of Kröger and co-workers demonstrated that silica-forming proteins (named silaffins) have the ability to control the crystal structure of a nonbiogenic material.665 Strikingly, the TiO2 initially formed from titanium(IV) bis(ammonium lactato)dihydroxide (TiBALDH) in the presence of recombinant silaffins, rSilC, was transformed into highly crystalline rutile at room temperature and neutral pH when exposed to dehydrating conditions (Figure 81).665 Conventional syntheses of microcrystalline rutile require high temperature (e.g., 600−800 °C); the formation of nanocrystalline rutile has only been achieved at extremely acidic pH values (pH < 1). Kröger et al. proposed that the rutile-forming activity of rSilC may result from the primary protein structure. The sequence of rSilC contains many lysine residues that may enable this silaffin to act as an acid−base catalyst in the condensation of TiO6 octaedra.665 Thus, they supposed that the highly repetitive primary structure of rSilC may serve to guide the rearrangement of TiO6 octaedra into the rutile crystal lattice.

7. CONCLUSIONS AND OUTLOOK This Review aims to recognize the impressive research efforts that have been devoted to the synthesis and study of threedimensional titania nanomaterials. With important and exciting applications acting as a driving force, a vast space of successful synthetic strategies has been mapped out already, and we expect that additional techniques will be developed in the future. We have discussed how porous films can be made using template-free approaches through the assembly of preformed 9547


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ways for efficient long distance charge carrier transport, or very short pathways for ion insertion. The lessons learned on this way should also be very useful for the deliberate design of other metal oxide nanostructures with interesting properties.

Adriana Zaleska is a Professor in Chemical Technology at the University of Gdansk and head of the Department of Environmental Technology. She obtained a Ph.D. in 2000 in the field of chemical technology at Gdansk University of Technology (Poland). Following appointments as a Visiting Scientist at the University of Utah and California Institute of Technology, she was appointed as adjunct at the Faculty of Chemistry at Gdansk University of Technology. During this time she started to study the correlation between surface properties and visible light-induced photoactivity of TiO2-based materials. She completed her D.Sc. (habilitation) in 2009, and in 2012 she became the head of a new research group at University of Gdansk followed by promotion to Full Professor at the same university in 2014. Her current interests concern functional materials synthesis and characterization, heterogeneous photocatalysis, environmental technology, air treatment, and nanotechnology.

AUTHOR INFORMATION Corresponding Authors

*Phone: +49 89 218077604. Fax: +49 89 218077605. E-mail: [email protected]. *Phone: +49 89 218077 623. Fax: +49 89 218077622. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Thomas Bein studied chemistry at the University of Hamburg (Germany). Subsequently he completed a joint Ph.D. program at the University of Hamburg and the Catholic University of Leuven (Belgium), focusing on zeolite inclusion chemistry and heterogeneous catalysis. Following an appointment as Visiting Scientist at the DuPont Central Research and Development Department in Wilmington (DE), he joined the University of New Mexico as an Assistant Professor. In 1991 he was appointed Associate Professor of Chemistry at Purdue University in West Lafayette (IN), followed by promotion to Full Professor of Chemistry at the same university. In 1999 Professor Bein assumed a position as Chair of Physical Chemistry at the LudwigMaximilians University in Munich, Germany. Professor Bein’s research focuses on the creation of functional porous nanostructures such as mesoporous materials, metal−organic frameworks and zeolites, the physical chemistry of interfaces, chemical sensors, targeted drug delivery, and energy conversion.

Dina Fattakhova-Rohlfing has obtained her education in chemistry at the Kazan State University, and has completed her Ph.D. degree in electrochemistry at the A. Arbuzov Institute of Organic and Physical Chemistry in Kazan, Russian Federation in 1994. She worked afterward as a research scientist in academic institutes in Prague (J. Heyrovsky Institute of Physical Chemistry), Bordeaux (National School of Chemistry and Physics), and Hannover (University of Hannover). Since 2006 she works as a lecturer and since 2012 as a professor for the international master’s program Advanced Materials Science (AMS) at the Ludwig-Maximilians University Munich (LMU), Germany. She works in an interdisciplinary field combining electrochemistry and materials chemistry. Her present research is devoted to the development of functional nanostructured materials for electrochemical and photoelectrochemical applications. The developed systems include novel morphologies of transparent conducting oxides, synthesis approaches to fabricate ultrasmall metal oxide nanoparticles for efficient energy conversion and storage, and hierarchical titania morphologies for solar cells, photocatalysis, and electrochemical energy storage.

ACKNOWLEDGMENTS D.F.-R. and T.B. acknowledge funding from the DFG Excellence Cluster “Nanosystems Initiative Munich (NIM)” and from the Bavarian Networks “Solar Technologies Go Hybrid” and UMWELTnanoTECH for work described here. The important contributions of our coworkers and collaborators are gratefully acknowledged. A.Z. acknowledges funding from the Polish National Centre for Research and Development (Small Grant Scheme)/Polish-Norwegian Research Programme (PHOTOAIR), for work described here. REFERENCES (1) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (2) Yu, C. Z.; Tian, B. Z.; Zhao, D. Y. Curr. Opin. Solid State Mater. 2003, 7, 191. 9548


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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on August 19, 2014 without all of the authors’ final corrections applied. The corrected version was reposted on August 28, 2014.

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