One-dimension-based spatially ordered architectures for solar energy conversion.

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One-dimension-based spatially ordered architectures for solar energy conversion Siqi Liu,ab Zi-Rong Tang,b Yugang Sun,c Juan Carlos Colmenares*d and Yi-Jun Xu*ab The severe consequences of fossil fuel consumption have resulted in a need for alternative sustainable sources of energy. Conversion and storage of solar energy via a renewable method, such as photocatalysis, holds great promise as such an alternative. One-dimensional (1D) nanostructures have gained attention in solar energy conversion because they have a long axis to absorb incident sunlight yet a short radial distance for separation of photogenerated charge carriers. In particular, well-ordered spatially high dimensional architectures based on 1D nanostructures with well-defined facets or anisotropic shapes offer an exciting opportunity for bridging the gap between 1D nanostructures and the micro and macro world, providing a platform for integration of nanostructures on a larger and more manageable scale into high-performance solar energy conversion applications. In this review, we focus on the progress of photocatalytic solar energy conversion over controlled one-dimension-based

Received 21st November 2014

spatially ordered architecture hybrids. Assembly and classification of these novel architectures are

DOI: 10.1039/c4cs00408f

summarized, and we discuss the opportunity and future direction of integration of 1D materials into high-dimensional, spatially organized architectures, with a perspective toward improved collective

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performance in various artificial photoredox applications.

1. Introduction Modern civilization is dependent on fossil fuels, a nonrenewable energy source, for storage and distribution of solar energy.1 As a form of ‘‘preserved solar energy,’’ fossil fuels are being consumed at a rate 500 000 times faster than the Earth produces them.2,3 Combustion of fossil fuels causes environmental pollution and enormous CO2 emission, with drastic implications for human life.4,5 Thus, human society must look towards alternative renewable, carbon-neutral energy sources for storage and conversion of solar energy.3 Semiconductor-based heterogeneous photocatalysis, which allows direct conversion of solar energy into chemical energy via a renewable route, could be the most viable long-term solution with potential to address these environmental and energy issues.6–8 Since the discovery of water splitting obtained under UV radiation with a TiO2 electrode,9 various photocatalysts have been developed and studied for diverse a

State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350002, P. R. China. E-mail: [email protected]; Fax: +86 591 83779326; Tel: +86 591 83779326 b College of Chemistry, Fuzhou University, New Campus, Fuzhou, 350108, P. R. China c Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illonis 60439, USA d Institute of Physical Chemistry of the Polish Academy of Sciences, ul. Kasprzaka 44/52, 01-224, Warsaw, Poland. E-mail: [email protected]

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photocatalytic processes.10–12 Although significant progress have been made in this area, challenges remain, such as high recombination rate of electron–hole pairs, low quantum efficiency, and inferior utilization efficiency of solar light.13 One-dimension-based semiconductor nanostructures have the possibility to overcome these challenges and realize large-scale implementation of solar energy conversion.14 One-dimensional (1D) nanostructures such as nanowires (NWs), nanotubes (NTs), nanorods (NRs) and nanoblets (NBs), have attracted attention in photocatalysis because of their unique geometrical and electronic characteristics, which can provide large aspect-ratio, direct pathways for charge transport and decouple the direction of charge carrier collection, and their low reflectance induced by light scattering and trapping.15,16 The length and diameter dimensions at the nanoscale lead to fascinating properties associated with 1D nanostructured materials.17 Subtle control of physical parameters and material compositions, including density of states and the transport of electrons and photons of 1D nanostructures, can tailor the properties to meet specific requirements of photoredox processes.17–20 In particular, longitudinal growth of 1D nanostructures to form spatial architectures has potential for photocatalytic solar energy conversion because of unique merits such as lower carrier recombination loss and vectorial charge-carrier transport perpendicular to the charge collecting substrates.18–20 Recently, the capability of nanoscopic materials to self-organize into large-scale assembly structures that exhibit unique

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collective properties has opened up new and exciting opportunities in the field of nanotechnology.21,22 The integration of 1D materials into two-dimensional (2D) and three-dimensional (3D) higher-order architectures with rational design and biomimetic principles is a feasible and highly efficient strategy to obtain collective, or enhanced, optical, electrical, and mechanical properties for photocatalytic solar energy conversion applications.23,24 Growth and alignment of 1D nanostructures into 3D spatially high-dimensional architectures, bridges the gap between the nano- and the micro- and macro-scales, and forms the basis for an assembly of 1D materials into higher hierarchy domains.17,25 Micro- and macro-sized architectures based on 1D nanomaterials with well-defined facets, size, morphology, or anisotropic shape, provide a versatile platform to integrate nanostructures at a larger and thus manageable scale into high performance in diverse photocatalytic applications.24,26–28 More specifically, precise maneuver of 1D nanostructures over larger areas, thereby resulting in 2D or 3D higher-order architectures, could reduce or even eliminate device malfunction because of mechanical strain, minimize alignment or fabrication errors, and form flexible or bendable device arrays when substrates are soft.17,29 In addition to the organization issue, original 1D nanostructures can be selectively functionalized to obtain an ordered alignment by self-organization, vapor–liquid–solid (VLS), electrodeposition (ED), or solvothermal methods, etc.30 The highdimensional, spatially ordered architectures achieved, which retain the structural integrity of the 1D nanostructures, would improve the ‘‘composite system’’ function by combining or even magnifying the advantages of each component, thus having great potential for practical advanced nanoscale applications in photocatalytic solar energy conversion and storage.31 In this review, we have summarized the progress of a variety of one-dimension-based spatially ordered architectures in terms of dimension of composition unit, and highlighted their photocatalytic applications in solar energy conversion with selected typical examples, including nonselective pollutant degradations, selective organic transformations, and artificial photosynthesis. These one-dimension-based spatially ordered hybrids are classified into two categories: (1) 1D nanostructure arrays grown on substrates with different dimensions, thus forming 1D–1D branched nanostructures, 1D–2D carpet-like arrays, and 1D–3D urchin-like architectures; and (2) 2D or 3D units grown on 1D individual substrates, forming spatially ordered architectures. The review concludes with a concise perspective on the current status, opportunity, and future directions of these one-dimension-based spatially ordered architectures with a view toward improved performance in different photocatalytic redox applications.

2. Fundamentals of one-dimensionbased spatially ordered architectures in solar energy 2.1

Basic principle for semiconductor-based photocatalysis

A semiconductor is characterized by an electronic band structure in which the highest occupied energy band, the valence band (VB),

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Scheme 1 Schematic illustration showing photoexcitation in a semiconductor followed by de-excitation events.

and the lowest empty band, the conduction band (CB), are separated by a band-gap, that is a region of forbidden energies in a perfect crystal.32 Photoexcitation of the semiconductor particles creates mobile electrons and holes in the CB and VB, creating pathways as depicted in Scheme 1. Photogenerated electrons and holes can recombine on the surface (pathway A) or in the bulk (pathway B) of the semiconductor particle within a few nanoseconds, with the energy dissipated as heat, which is detrimental to the efficiency of a semiconductor photocatalyst.33 Simultaneously, the photogenerated electrons and holes can transfer to the semiconductor surface by spontaneous migration. The photogenerated electrons can further transfer to adsorbed organic or inorganic species or the solvent, which is more efficient when the species are pre-adsorbed on the semiconductor surface.34 Thus, the semiconductor can donate electrons to reduce an electron acceptor (pathway C); in turn, a hole can migrate to the surface and an electron from a donor species can then combine with this ‘‘surface’’ hole, thus oxidizing the donor species (pathway D).35–38 Thereby, subsequent anodic and cathodic redox reactions can be initiated. These constitute the basic principle of semiconductorbased photocatalysis toward solar energy conversion. 2.2

Fundamental properties of 1D nanostructures

It is widely accepted that for a structure to be termed a nanostructure, the threshold is 1–100 nm in at least one dimension, hence the name 1D nanostructures for systems with the lateral dimension in the nanometer scale.39 In 1D nanostructures, the electronic wave-functions are constrained by quantum effects in nanoscale directions, resulting in the quantum confinement effect.15 When the diameter of a 1D nanostructure is below its critical value such as the exciton Bohr radius, wavelength of light, phonon mean free path, critical size of magnetic domains, exciton diffusion length, and others, the physical properties of the material could be significantly altered within the confines of the 1D nanostructure surfaces.40,41 In addition, 2D confinement which is based on large surface-tovolume ratio and large aspect ratio endows 1D nanostructures


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with unique properties in comparison with irregular or otherdimensional nanostructures.42 The 1D nanostructures could serve as an expressway in the axial direction for direct conduction of quantum particles such as electrons, phonons, and photons.43 This fast and long-distance photogenerated electron transport and energy transport gives 1D nanostructures great potential for use in advanced solid-state devices in photocatalytic solar energy conversion applications.44,45 Additionally, high length-to-diameter of 1D nanostructures could enhance the light absorption and scattering, which benefits the photocatalysis.46–49 As a result, 1D nanostructures provide a convenient platform for testing and understanding fundamental concepts about the roles of dimensionality and size in optical, electrical, and mechanical properties, and for photocatalytic solar energy conversion applications.45,50 2.3 Benefits for integration of 1D nanostructures in spatially ordered architectures Using integrated systems is expected to yield qualitative and quantitative improvements in properties resulting from the collective physical or chemical properties that depend on components, size, spacing, and higher-order structure.51,52 Vertically oriented 1D nanostructures offer an excellent material architecture for photocatalytic solar energy conversion because of their unique merits including efficient carrier separation, decoupling directions of light absorption and charge-carrier collection as well as directional charge carrier transport.19,20 Therefore, integration of vertical 1D nanostructures onto different substrates to form high-dimensional micro or macro architectures is of great importance in development of one-dimension-based spatially ordered architectures in diverse solar energy conversion applications.17,21,53 For example, 1D nanostructure arrays grown on 1D nanostructure substrates to form branch architectures, which can be either homogeneous or heterogeneous, have been widely used for solar-to-fuel conversion because of their excellent properties such as enhanced specific surface area, high charge-carrier collection efficiency and easy nanoscale-integration of different functional materials. Construction of secondary 1D branches on pristine 1D nanostructure backbone has been demonstrated to be an efficient way to increase the roughness factor (defined as the total surface area per unit substrate area) and simultaneously maintain the outstanding charge-carrier collection properties of singlecrystalline 1D arrays.54 Such one-dimension-based spatially ordered architectures could improve composite systems by combining or even magnifying the advantages of each component, as both the individual parts and the interfaces between them can determine the performance of the entire system.26,31

3. Strategies for fabricating onedimension-based spatially ordered architectures 3.1

Synthesis of 1D nanostructures

There has been much growth in nanotechnology recently, and it is now regarded as one of the major technologies for the future.


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The fabrication of 1D nanostructures can generally be divided into two approaches, ‘‘top-down’’ and ‘‘bottom-up,’’ on the basis of the direction of size development of the implemented materials.55 Top-down approaches usually start with patterns made on a large scale, then reduce the lateral dimensions before forming nanostructures.56 Bottom-up approaches do the reverse, by assembling small particles like atoms or molecules into ordered nanostructures.57 3.1.1 Top-down. Top-down strategies for nanomachining can be subdivided into two categories as illustrated in Fig. 1: (1) transfer of a computer-generated pattern onto a larger piece of bulk material by macroscopic tools, and then ‘‘sculpting’’ a nanostructure by physically removing material, for example by wet/dry etching; or (2) directly ‘‘writing’’ (add or rearrange) materials onto a substrate using macroscopic tools.58 In the first category, optical lithography (OL) is the most common technique. Recent technological advancements in the field of OL have demonstrated the suitability of these techniques to fabricate ordered 1D nanostructure arrays with excellent control over placement and feature-size. Lateral NWs can be fabricated from layered substrates or epitaxial thin films, prepared by molecular beam epitaxy (MBE) or metal organic vapor phase epitaxy (MOVPE), respectively.59 Similarly, vertical NWs can be prepared from bulk or layered substrates by using a resist mask with lateral dimensions below 100 nm and transferring the mask pattern deep into the substrate via an appropriate anisotropic etch process.60 Other lithography processes including electron beam lithography (EBL),61 extreme ultraviolet (EUV),62 X-ray lithography (XRL),63 ion beam lithography (IBL),64 and electron beam induced deposition (EBID) lithography,65 are being considered to extend lithography scaling beyond current UV lithography capabilities for 1D semiconductor nanostructure manufacturing. The second category of top-down approaches include nanoimprint lithography (NIL)66 and scanning probe lithography (SPL) (e.g. dip-pen nanolithography (DPN) and scanning tunneling microscopy (STM)).67 In NIL, a rigid master (mold) is pressed into a thin resist heated above its glass transition temperature. After cooling the resist below its glass transition temperature, the mold

Fig. 1 Overview of various growth techniques for 1D nanomaterials using top-down approaches. OL, optical lithography; XRL, X-ray lithography; EUV, extreme ultraviolet; EBL, electron beam lithography; EBID, electron beam induced deposition; IBL, ion beam lithography; NIL, nanoimprint lithography; SPL, scanning probe lithography.

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is removed and an anisotropic etching process is employed to remove the residual resist in the compressed area.56 Standard NIL allows formation of nanostructures down to 20 nm.68 However, NIL has certain limitations in replicating large-scale (in micrometer range or larger) and nanoscale patterns simultaneously.56 Currently, patterns can be as small as 15 nm using DPN, whereas STM offers the unparalleled capability to position individual atoms to pattern structures with ultrahigh, subnanometer precision. However, DPN and STM are also serial techniques and are therefore not suitable for high-volume manufacturing technologies.58 Top-down approaches offer a wide range of structures of high quality and yield, and have dominated the area of 1D semiconductor nanostructure device definition and placement for decades. However, as the density of devices on a chip continues to scale, significant difficulties are encountered.59 Methods are generally neither cost- nor time-effective, and for some, resolution below the 100 nm range is not easily achievable.58 3.1.2 Bottom-up. The essence of 1D nanostructure formation by bottom-up approaches is crystallization, a process which has been under investigation for a long time.69 Nucleation and growth are the two fundamental steps for evolution of a solid from a vapor, liquid, or solid phase.69 While the concentration of building blocks such as atoms, ions, and molecules of a solid is sufficiently high, they can aggregate into small clusters (or nuclei) through homogenous nucleation. Continual supply of the building blocks allows the nuclei to serve as seeds for further growth to form larger 1D nanostructures, classifying this as a bottom-up method.56,69,70 One benefit of bottom-up 1D nanostructure growth over topdown processing is that 1D nanostructures grown by bottom-up methods may be doped in situ during crystal growth by incorporating dopant precursors in the 1D nanostructure synthesis procedure. Bottom-up growth does not require destructive techniques to generate additional charge carriers,59 and some self-assembled nanostructures have been found to exhibit unique collective properties that differ from the properties displayed by the constituent building blocks.25 Over the past decade, a number of chemical methods have been demonstrated as bottom-up approaches for generating 1D nanostructures with different levels of control over parameters such as dimensions, morphology, and monodispersity.69 Some of these synthetic strategies are schematically illustrated in Fig. 2, including (i) growth of 1D nanostructures using the intrinsically anisotropic crystallographic structure of the solid (Fig. 2a); (ii) reduction of symmetry of a seed by introducing a liquid–solid interface (Fig. 2b); (iii) formation of 1D nanostructures using different templates with 1D morphologies (Fig. 2c); (iv) modification of the growth habit of a seed using super-saturation; (v) kinetic control of growth rates of various facets of a seed using an appropriate capping reagent (Fig. 2d); (vi) growth of 1D nanostructures by self-assembly of zero-dimensional (0D) nanostructures (Fig. 2e); and (vii) reduction of the size of 1D microstructures (Fig. 2f).69 Although bottom-up approaches are generally unsuitable for formation of nano-fluidic devices at this stage of development because of the random positioning of the obtained nanostructures,

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Fig. 2 Schematic illustrations of various strategies to induce 1D growth to obtain 1D nanostructures: (a) use of the anisotropic crystallographic structure of a solid; (b) confinement by a liquid droplet; (c) confinement by a template with 1D morphology; (d) use of appropriate capping reagent; (e) self-assembly of 0D nanostructures; and (f) size reduction of a 1D nanostructure.

they take advantage of physicochemical interactions for hierarchical synthesis of ordered nanoscale structures toward various solar energy conversion applications.56 Nevertheless, the requirement for longer, better controlled, and interconnected 1D nanostructures could be achieved by combining bottom-up and top-down approaches. 3.2 Preparation of one-dimension-based spatially ordered architectures Most nanostructures, including one-dimension-based spatially ordered architectures, can be prepared via top-down and bottom-up approaches. However, the synthesis of one-dimensionbased spatially ordered architectures could be made more specific and characteristic. In many cases, fabrication of these novel architectures involves a secondary growth mechanism. First, nanostructure substrates with different dimensions are fabricated via various methods. Then, 1D nanostructure arrays or other units are grown onto substrates through solution growth, phase transition growth, VLS, or solution–liquid–solid (SLS) growth approaches. Notably, these methods always comprise oriented or selective seeds/precursors adhesion. One-dimensionbased spatially ordered architectures can also be realized in one step via a ‘‘self-catalytic’’ process. In the following section, some typical synthesis techniques are discussed and summarized. 3.2.1 Solution growth of one-dimension-based spatially ordered architectures. Considering the ease of manipulation, a solution method might be the most facile way to assemble colloidal 1D nanostructures into spatially organized architectures.21 Solution methods such as hydrothermal, solvothermal, electrodeposition (ED), and chemical bath deposition (CBD), have been widely applied for growth of 1D nanostructures onto diverse nanosubstrates. Using different types of 1D, 2D, or even


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3D nanostructures as the growth substrates, solution growth can be straightforwardly applied to produce hierarchical one-dimensionbased spatial architectures with a slight modification.24 A wide variety of these novel architectures could be obtained by rationally combining different solution methods. In addition, solution methods for synthesis of one-dimension-based spatial architectures can achieve high yield and large-scale homogeneity. The only limit to the yield is the amount of precursor, which determines the homogeneity of the achieved architectures.24 Additionally, the easy control of length and density of onedimension-based spatial architectures also benefits the study of growth kinetics and mechanisms such as hetero-epitaxy and Ostwald ripening in crystal growth. 3.2.2 Metal catalyst-assisted growth of one-dimension-based spatially ordered architectures. The growth of unidirectional 1D nanostructures such as NWs by VLS requires a metal nanocluster as catalyst. Similarly, sequential catalyst-assisted growth of onedimension-based spatial architectures, for example growth of secondary 1D structures or even hyper-branches, can take place by introduction of new metal catalyst onto the primary surfaces of backbones. Therefore, early reports on one-dimension-based spatially ordered architectures usually involved depositions of metal catalyst, followed by VLS or SLS growth.71,72 Typically, there are three steps in this sequential catalyst-assisted growth process: (1) fabrication of various dimensional nanostructures as the backbone; (2) deposition of metal cluster catalyst onto the primary nanosubstrates; (3) secondary growth of the 1D structures onto the primary backbones via VLS or SLS approaches. Using this metal catalyst-assisted method, good control of the density and length of the 1D nanostructures can be obtained by adjusting the amount of the metal particles deposited on the primary nanostructure backbones, and the growth time, respectively. The use of different types or sizes of metal catalyst for formation of alternate 1D nanostructures provides an additional control.71 3.2.3 Phase transition induced branching. Phase transition during growth without adding intentional branching catalyst, is another interesting approach for fabricating one-dimensionbased spatially ordered architectures.73,74 However, this approach is always limited to the materials which can crystallize in wurtzite, zinc blende close-pack crystal structure or mixed poly-type under specific conditions.24 For instance, CdTe in zinc blende structure is selective at a smaller nanoscale size while wurtzite is more stable in bulk phase. The CdTe is first manifest as a nucleate, then branched nanowires (BNWs) are realized to form a tetrapod geometry after the CdTe reaches a certain critical size.74 Crystal phase transition can be modified by chemical solvents during solution growth.75 3.2.4 One-step self-catalyzed growth of one-dimensionbased spatially ordered architectures. Compared with multistep secondary growth, a ‘‘self-catalytic’’ process such as vapor oxidation, laser ablation, vapor transport and deposition, as well as their combinations, realizes one-dimension-based spatially ordered architectures in a single step. For example, ZnO and ZnS with wurtzite crystals can exhibit a branched nanostructure, by simple oxidation of the metal Zn.76 In such


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self-catalytic mechanisms, reactants might have multiple rather than single roles.77 3.2.5 Other branching mechanism induced growth of onedimension-based spatially ordered architectures. Despite elucidation of the growth techniques already mentioned, better understanding of formation mechanisms of one-dimension-based spatially ordered architectures could further assist precise control and intelligent design of one-dimension-based spatially ordered heterostructures. For example, as most fabrication strategies of one-dimension-based spatially ordered heterostructures involve a secondary growth approach, investigation of the interfacial interactions between vertically aligned 1D nanostructure arrays and substrates may provide more inspiration. Recently, an electrostatic self-assembly approach has been proven to be efficient for introduction of secondary component seeds.78 Although the secondary 1D nanostructure arrays are grown via a solution approach, this electrostatic interaction in the interface still provides another choice in fabrication of one-dimensionbased spatially ordered architectures. The crystallographic lattice structure at the interface has been found to be important in defining the structural characteristics of grown 1D nanostructures. The interface prefers to take the least lattice mismatch, which can lower the heteronucleation energy barrier, thus making this a key factor in determining growth behavior of the secondary phase.79–82 For instance, the crystalline orientation of the metal catalyst particle may determine growth direction and side surfaces of the secondary vertically aligned 1D nanostructure arrays.79–82 The role of screw dislocation defects in promoting one-dimension-based spatially ordered architecture growth has been demonstrated in a study of pine-tree-like PbS NWs, which were synthesized by chemical vapor deposition (CVD) methods.83 By finding continuous dislocation lines at the centers of the NW trunks, a screw-dislocation-driven NW growth mechanism has been proposed. However, the NW branches are grown via the VLS mechanism. Additionally, some one-dimension-based spatially ordered architectures such as FeSi84 and TiSi285 BNWs have been fabricated by a chemical vapor transport approach, but the branching mechanism is not well understood.

4. Classification of one-dimensionbased spatially ordered architectures Generally, one-dimension-based spatially ordered architectures are briefly mentioned and discussed sporadically as hierarchical structures or functional nanostructures. However, as novel multifunctional architectures, no systematic classification is as yet available. Herein, we systematically classify one-dimension-based spatially ordered architectures with emphasis on the dimension of the component units. The one-dimension-based spatially ordered hybrids can be classified into two categories: (1) 1D nanostructure arrays grown on substrates with different dimensions, thus forming 1D–1D branched nanostructures, 1D–2D carpet-like arrays, and 1D–3D urchin-like architectures; and (2) 2D or 3D units grown on 1D individual substrate, forming spatially ordered architectures.

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4.1 1D nanostructure arrays grown on substrates with different dimensions 4.1.1 1D nanostructure arrays grown on 1D substrates forming branched architectures. The 1D nanostructure arrays, grown on another 1D substrate forming nanotree-like architectures, are usually called branched architectures. The high surface area and direct transport pathway for charge carriers make branched architectures especially attractive in solar energy conversion applications.86,87 Just as trees in the natural world expose large surfaces for effective photosynthesis, the extensively branched architectures have large areas to harvest solar light and cause increased scattering that improves light absorption.87 The high surface area can also increase the number of interactions between surface catalytic centers in a solar energy conversion system. The dendritic-like semiconductor nanostructures provide a direct charge carrier transport pathway in both the trunks and branches, thus optimizing charge collection efficiency, similar to the efficient transport of water and carbohydrates in natural trees resulting from their fractal geometry. Thus, the integration of multiple materials into one nanostructure to form 3D branched architectures with diverse functions is potentially significant.24 Herein, the branched architectures are subdivided into four categories taking into account the components of the 1D unit: single component branched architectures, carbon-based branched heterostructures, metal-based branched heterostructures, and semiconductor-based branched heterostructures are discussed with selected typical examples. 4.1.1.1 Single component branched architectures (a) Single semiconductor branched architectures. Many single semiconductor branched architectures such as TiO2,88 CdS,89 ZnO,90 SnO2,91 Si,71 GaN,71 PbS,92 and GaP72 have been fabricated via various approaches. For example, Jaramillo et al. synthesized TiO2 branched nanorods (BNRs) consisting of a NR trunk and branches.88 The NR trunks were synthesized on TiO2-seeded F-doped tin oxide (FTO) substrates using a hydrothermal method and the branches were obtained by a solution phase method in a subsequent growth step. Although integrated branched structures incorporate multiple components for their various properties and applications, single component branched semiconductors provide a more direct model for investigating the structural advantages (Fig. 3a). BNRs have better charge transport and light absorption properties than nanoparticle (NP) films and larger surface areas for more efficient carrier collection than bare NRs. Thus, TiO2 BNRs show the largest photocurrent and best incident photon-to-current conversion efficiencies (IPCE) when compared with TiO2 NP film and TiO2 bare NR film, as shown in Fig. 3b and c. The branches improve efficiency by means of (i) improved charge separation and transport within the branches because of their small diameters, and (ii) a four-fold increase in surface area which facilitates hole transfer at the TiO2 interface. (b) Single carbon branched architectures. Carbon-based nanostructures with sp2-hybridized atoms such as carbon nanotubes (CNTs) and carbon fibers have been demonstrated to show exclusive properties including large active surface area,

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Fig. 3 (a) Schematic description and corresponding SEM images, (b) J–V curves under illumination from a class AAA solar simulator (100 mW cm 2) and (c) IPCE spectra measured at an applied bias of 0.6 V versus RHE (c) of TiO2 BNRs, NRs, and NPs. Reprinted with permission from ref. 88. Copyright 2011 American Chemical Society.

high current density, high thermal and electrical conductivity, and chemical stability.93 Branched junctions, which provide intra-molecular connection and in situ self-assembly, can make integration of 1D carbon materials to a functional system for realistic application with improved properties simpler and more reliable.94 Jin’s group reported the growth of aligned arrays of CNTs by a plasma enhanced chemical vapor deposition (PECVD) process using Ni catalyst particles with a tip-growth mechanism (Fig. 4).95 By increasing the plasma power, the degree of sputtering from the initial Ni catalyst particle can be enhanced, which can control density of the branches. The positions of the branches also can be controlled by patterning a mask over parts of the initial CNT surfaces or directly patterning the catalyst particles for each branch using electron beam lithography. Although the potential applications have not been tested, this new multibranching CNT structure still exhibits superior physical properties compared with common CNTs as a scaffold for the support of Pt nanoparticles. (c) Single metal branched architectures. Shape-controlled synthesis of metal nanostructures has achieved great success in recent years for size- and shape-dependent properties. Among these, metal dendrites are particularly popular, as they have long main trunks and parallel secondary branches with sharp edges or tips, as well as nanoscale junctions. Qi et al. synthesized

Fig. 4 Schematic illustration of the sequence of events leading to formation of a multibranched CNT structure. Reprinted with permission from ref. 95. Copyright 2006 American Chemical Society.


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Fig. 5 (a) SEM and (b) TEM images of gold dendrite. Reprinted with permission from ref. 96. Copyright 2008 American Chemical Society.

hierarchical, three-fold symmetrical, single-crystalline gold dendrites by reaction between a zinc plate and a solution of HAuCl4 in the ionic liquid [BMIM][PF6] (Fig. 5).96 The growth process for formation of the dendritic gold nanostructures is proposed to be a self-organization process. First, the gold nuclei nanocrystals are formed on the zinc substrate by a direct surface reaction. Then, subsequent crystal growth preferentially occurs on the preformed gold crystals via a primary cell reaction, thus resulting in formation of gold dendrites. The authors found that the HAuCl4 concentration has considerable effect on the growth of the gold dendrites in ionic liquid. Using water rather than ionic liquid, Au–Zn alloy dendrites can be obtained, demonstrating that the ionic liquid medium could contribute greatly to formation of pure single-crystalline gold dendrites. 4.1.1.2 Carbon-based branched nanocomposites. Since their discovery in 1991, CNTs, a typical type of 1D material in the carbon family, have received a lot of attention because of their superior physicochemical, mechanical, and electronic properties.97,98 The large specific surface area, hollow structure, and extraordinary mechanical and unique electronic properties of CNTs make them attractive for potential applications in energy conversion, electrocatalysts, sensors, environmental remediation, and catalysts.99,100 In particular, the large electron-storage capacity and metallic conductivity of CNTs similar to metals suggest that they could perform as high-performance candidates for catalyst carriers or promoters.101–104 Through atomic

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layer deposition (ALD) of a uniform ZnO thin film on the outside surface of the multi-walled carbon nanotube (MWCNT) followed by hydrothermal growth of ZnO branches, Lian and Sawyer et al. synthesized a thorn-like ZnO–MWCNT hybrid, as displayed in Fig. 6a.105 The ZnO ALD is firstly formed on commercial surface-functionalized MWCNTs using alternating diethylzinc and H2O exposures. Then, ZnO branches are grown in an alkaline zinc chloride solution by a hydrothermal approach. As a result of the much higher carrier mobility of MWCNT than ZnO, less accumulation of electrons is achieved on the MWCNT side, which leads to a rapid rise in the photocurrent. Besides the high aspect ratio, the efficient carrier transport and collection efficiency of the hybrid paper are favorable for photodetector and photocatalyst applications (Fig. 6b and c). Carbon fiber (CF) as another typical 1D carbon nanomaterial, which is flexible, conductive, stable, and with a large surface area, is beneficial for constructing branched nanostructures toward solar energy conversion. Through a ‘‘dissolve and grow’’ method followed by a corrosion process in a strong acid solution, as illustrated in Fig. 7a, Wang’s group grew bunched TiO2 NRs on CFs from titanium to form branched architectures.106 The effect of concentration of HCl on the morphology was investigated. When the concentration of HCl is low, a large number of polycrystalline clusters, acting as nucleating centers for further growth of nanorods, form in the solution by homogeneous nucleation. Because there is no crystal plane trend for these clusters, nanorods can self-assemble into microspheres or tufted nanoflowers. When the concentration of HCl is moderate, the reaction proceeds more smoothly because of the ‘‘double buffer’’ effect of HCl. However, nothing is grown on the CFs at high concentration of HCl because the high acidity seriously restricts hydrolysis of Ti(III) and restrains both homogeneous and heterogeneous nucleations. They also showed that the etching and growth process is reversible and that the more reactive crystalline facet is more reactive in both growth and corrosion. Dense TiO2 NR rooting on the surface of conductive CFs could ensure the most effective path for charge transport, and intertwining the NR-coated CFs creates a 3D nanostructure with very large surface area. Therefore, the photocurrent over TiO2 NR-coated CFs is nearly doubled compared with simple TiO2 NR arrays, as shown in Fig. 7b. 4.1.1.3 Metal-based branched nanocomposites. Transition metals, such as platinum, silver, and iron, can improve charge

Fig. 6 (a) Schematic illustration showing the fabrication process of the flexible thorn-like ZnO–MWCNT hybrid paper, (b) schematic illustration of the UV photodetector fabricated from the ZnO–MWCNT hybrid paper, and (c) photoresponsivity spectra of the UV photodetector. Reprinted with permission from ref. 105. Copyright 2014 Royal Society of Chemistry.


Fig. 7 (a) Schematic representation of the growth of TiO2 NR arrays on carbon fibers CFs and (b) J–V curves of DSSCs based on TiO2 NR arrays and bunched TiO2 NR arrays on CFs. Reprinted with permission from ref. 106. Copyright 2012 American Chemical Society.

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transfer by trapping photoinduced charge carriers, and can enhance performance of the photochemical degradation process,107,108 because of the recombination of electron and hole prior to the superoxide activation process.109,110 On one hand, 1D metal nanostructures with excellent conductivity can increase the charge-carrier migration capability of the materials.111,112 On the other hand, transition metals with lowlying Fermi levels can also serve as an electron reservoir to decrease the likelihood of recombination of photoexcited electron–hole pairs from the semiconductor.113–117 Some noble metals such as Au and Ag, exhibit strong surface plasmon resonances (SPRs) under illumination of light because of strong coherent oscillation of free surface electrons in the NPs, resulting in strong absorption and scattering of incident light.118–120 Thus, 1D branched metal–semiconductor nanoheterostructures have drawn special attention for wide applications in fields related to solar-energy conversion. Lu and coworkers synthesized a branched Ag NW–ZnO NR hetero-assembly via a simple solution bottom-up strategy.121 First, Ag NWs were coated with ZnO seeds via a surface adsorption process for the growth of ZnO NRs. Then, the as-prepared Ag NW–ZnO NR assemblies were used as the template for morphology-preserved synthesis of ZnO BNR assemblies via secondary nucleation and growth procedures. The hierarchical structures of ZnO NR–grown Ag NWs have a similar growth process to ZnO NR arrays grown on planar substrates. In addition, the length and diameter of ZnO NRs can be tuned by adjusting the concentration of the nutrient solution. By means of a simple secondary growth method (Fig. 8), the inert surface of ZnO is switched to an active surface with increased surface area. The citrate ions are demonstrated to promote new site-specific heterogeneous nucleation and sequential secondary growth. The 1D structured Ag NWs and ZnO NRs with high carrier transport property can serve as spatially extended catalyzing centers to provide direct and fast electron–hole transfer, thus facilitating more effective

Fig. 8 A schematic illustration of the surface property switch of Ag NW-based ZnO nanorod arrays from inert (100) planes to active (001) planes after the second growth process. Reprinted with permission from ref. 121. Copyright 2012 Royal Society of Chemistry.

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separation of photoinduced electrons and holes and reducing the charge recombination. The branched structures endow the nanocomposites with a larger specific surface area and more accessible active sites for improved light harvesting as well as enhanced diffusion and adsorption capability. Finally, ZnO BNRs with highly polar (001) plane are more photoelectrically active than those with other non-polar planes. Thus, the synergistic effect of Ag–ZnO heterojunctions and the hierarchical fluffy worm-like morphologies are of benefit to solar energy conversion applications. Kuan et al. reported a branched hierarchical Zn–ZnO nanostructure obtained by direct thermal oxidation of metallic foil with single crystalline Zn micro-tip arrays (Fig. 9a).122 The influence of annealing temperature and time on the growth kinetics for the branched nanostructures illustrated that the length of the nanowhiskers and the density of hierarchical structures are a function of the annealing time and temperature, respectively (Fig. 9c and d). The thermal annealing temperature not only affects the oxidation rate but also influences the interfacial stress, surface and interface energies, and elastic properties of the oxide film. There are two main processes in hierarchical structure formation: formation of oxide layer and growth of branched nanostructures. Kuan et al. found that the growth mechanism for the ZnO branches is not only related to thermal oxidation of Zn micro-tips but also the mass transfer path of vapors. The formation mechanism for hierarchical Zn–ZnO structures can be simply described as a combination of the symmetric micro-tip arrays and the volume expansion caused by the formation of the oxide layer followed by solid-gas reaction. In addition, the branched morphology shows the best field-emission performance as shown in Fig. 9b. Although regular multidimensional patterned arrays have not yet been

Fig. 9 (a) TEM microscopic image of hierarchical ZnO structures, (b) J–E curves for the field emission behavior of the hierarchical ZnO structures, (c) a plot of the length of branches as a function of the reaction time at various temperatures, and (d) the relationship between density and annealing temperature. Reprinted with permission from ref. 122. Copyright 2009 American Chemical Society.


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obtained using a direct oxidation process, the 3D hierarchical arrays can be obtained using the anisotropically orientated Zn polyhedron under direct annealing processes, thus providing an alternative methodology for preparation and application of other nanostructures with symmetric arrays. 4.1.1.4 Semiconductor-based branched heterostructures. In recent years, 1D semiconductor nanostructures have been studied for solar energy conversion because of their distinctive properties and promise of superior solar energy conversion performance.26,27,42,87 Vertical 1D nanostructure arrays offer enhanced light absorption, reduced charge recombination, improved carrier collection, and increased surface area and reaction rate compared with planar bulk materials.123–125 However, 1D nanostructure arrays synthesized from single materials such as silicon are precluded for practical solar energy conversion application because of limited efficiency and chemical instability in the electrolyte solution.126,127 Having a 1D semiconductor heterostructure allows integration of properties from different materials.123 Specifically for solar energy conversion, 1D semiconductor heterostructures can improve the photocurrent and consequently the hydrogen production efficiency by enhanced light absorption and improved charge separation, while the formation of a heterostructure can simultaneously enhance chemical stability by use of a corrosion resistive material to interface with electrolytes.129 Kang and coworkers reported an architecture of ‘‘pine-tree’’ ZnO NRs on Si NWs hierarchical branched structure (HBS), sensitized with CdS–CdSe semiconductors as shown in Fig. 10.130 First, the intrinsic i-Si-NWs were grown randomly on an FTO glass via a metal-catalyzed VLS method and subsequently doped with PH3 to make n-Si-NWs for enhancing the electron conductivity. Then, the branched ZnO-NRs were grown directly on the Si-NWs, forming a pine-tree structure via a hydrothermal synthetic method without use of a metal catalyst. Finally, CdS QDs were grown

Fig. 10 (a) Fabrication scheme of Si–ZnO hierarchical structure, (b) SEM image of n-Si-NW–ZnO-NR, and (c) schematic Si–ZnO hierarchical structure for photoanode. Reprinted with permission from ref. 130. Copyright 2011 American Chemical Society.


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directly on ZnO-NR surface by successive ionic layer adsorption and reaction (SILAR), and CdSe QDs were deposited by CBD. Formation of p–n heterojunctions was found to result in efficient photon absorption and conductivity. Thus, the enhanced light-harvesting capability and photon localization arising from the large surface area, the direct charge extraction pathways throughout the device thickness, and the huge 3-D porous network, have potential for solar energy conversion applications. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO–CuO BNWs were grown on copper film substrates with various ZnO and CuO NW sizes and densities. This all-metal-oxide NW heterostructure with p–n heterojunction, reported by Wang et al., exhibited broadband photoresponse from UV to near infrared (IR) region, and higher photocathodic current (Fig. 11a and b) than ZnO-coated CuO (core–shell) NWs because of improved surface area and enhanced gas evolution.131 As shown in Fig. 11c, the approximate energy band diagram of ZnO–CuO heterojunction BNW in contact with the electrolyte at a reversed biasing potential of 0.45 V, the depletion region lies mostly in the p-CuO NW because of much higher doping concentration of n-ZnO NW. And, the radius of CuO NW core is generally smaller than the length of ZnO NW branch. These factors together cause large band bending to occur in the CuO NW core. By increasing the reversed biasing potential, the energy levels of ZnO move downward, leading to more band bending at the CuO–ZnO junction and reducing the barrier at the ZnO–electrolyte junction. The increased band bending and decreased barrier lead to enhanced light current. Other semiconductor 3D branched heterostructures such as InP–GaP,132 ZnS–CdS,133 ZnSe–CdSe,134 InAs–GaAs,135 and Si–Ge,136 using, for example, CVD, metal– organic chemical deposition, or molecular beam epitaxy methods are also reported, and various potential device applications have been demonstrated.128,136

Fig. 11 (a) Spectral incident photon-to-current efficiency (IPCE) and current density (b) of ZnO–CuO b-NW, and (c) approximate energy band diagram of the ZnO–CuO BNW in contact with the electrolyte at a reversed biasing potential of 0.45 V. The inset shows a STEM image indicating the relative size of the CuO NW radius to the ZnO NW length. Reprinted with permission from ref. 131. Copyright 2013 American Chemical Society.

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4.1.2 1D nanostructure arrays grown on 2D substrates forming carpet-like nanostructure arrays. Vertical orientation of 1D nanostructures on a 2D substrate to form carpet-like nanostructure arrays is a typical method for fabrication of 1D–2D spatial architectures. 1D nanostructure arrays grown on different 2D substrates including FTO coated glass, indium tin oxide (ITO) coated glass, metal substrates, graphene and 2D semiconductors have been reported and the physicochemical properties have been investigated. Establishment of integrated 1D nanostructure arrays renders numerous fast and directional electron transport accesses available to 2D carrier collectors. Also, an arrayed architecture can enhance light scattering and harvest sunlight efficiently. 4.1.2.1 1D nanostructure arrays grown on conductive glass substrates. Recently, growth of vertically aligned arrays of 1D nanostructures was reported on conductive glass substrates due to their innate transparency (B90% in visible region) and high electrical conductivity and potential for direct applications.137,138 By electrochemical deposition of CdTe on ZnO nanorod arrays in an electrolyte close to neutral pH, Li and coworkers fabricated vertically aligned ZnO–CdTe core–shell nanocable arrays on ITO coated glass.139 The CdTe shell thickness can be tuned from several tens to hundreds of nanometers by adjusting the total charge quantity applied during deposition. The nanocable array configuration, together with the type II band alignment between the CdTe shell and the ZnO core, led to excellent photovoltaic properties for solar energy applications of ZnO–CdTe with photocurrent density of 5.9 mA cm 2 under visible light illumination of 100 mW cm 2 with zero bias potential (vs. saturated calomel electrode), which is shown in Fig. 12. In such a configuration, the photogenerated electrons can be injected easily from the CdTe shell into the ZnO nanorod core, driven by the band alignment, and then transported to the ITO substrate along the single-crystal ZnO, which provides a direct path for electron transport. FTO coated glass is another typical conductive glass subńdez et al. strate for growth of 1D nanostructure arrays. Herna reported 1D ZnO@TiO2 core–shell nanostructures grown on FTO substrate.140 First, a vertical array of ZnO NWs was grown following a two-step synthetic approach, including deposition

Fig. 12 (a) Schematic of the operation of the semiconductor sensitized solar cell, (b) typical HRTEM image of ZnO–CdTe nanocable, showing the interface and crystalline structure of the nanocable and (c) current density versus potential curves for the ZnO–CdTe nanocable. Reprinted with permission from ref. 139. Copyright 2010 American Chemical Society.

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Fig. 13 Scheme of the prepared photoelectrodes, consisting of (a) ZnO NWs and (b) ZnO@TiO2 core–shell heterostructures and their band diagrams, which represent the charge carrier transport at the different watersplitting PEC cell interfaces. Reprinted with permission from ref. 140. Copyright 2014 American Chemical Society.

of seed layer and hydrothermal growth. Then, titania shell was deposited on the ZnO NWs using a non-acid sol–gel synthesis solution. A representative scheme of the two kinds of ZnO nanostructures, which consist of ZnO NWs and 1D ZnO@TiO2 core–shell nanostructures, is shown in Fig. 13, together with their relative simplified band diagrams. Growth of ZnO NWs on FTO (Fig. 13a) involves a common two-level electron transfer with a considerable number of CB electrons collected at the FTO coated glass substrate before recombination. The presence of the TiO2 shell in the core–shell nanostructure (Fig. 13b) improves the charge separation efficiency, because of the matched energy level position and the presence of a potential barrier at the semiconductor/electrolyte interface, which limits the charge recombination. Therefore, in ZnO@TiO2 core–shell nanostructures with optimized synthesis procedure and thermal treatments, efficient three-level electron transfer leads to enhanced solar conversion performance. The high electron mobility within the 1D ZnO nanocrystals, which is exploited for fast transport toward the FTO, is coupled with efficient separation of charge carriers between the TiO2 shell and the ZnO core.

4.1.2.2 1D nanostructure arrays grown on metal foil substrates. Metal foil substrates, such as Zn foil, Cu foil, Ni foil, and Ti foil, have merits for growing vertical 1D nanostructure arrays. First, metal foil endows formation of cells suitable for rooftop and building integrated applications.141 Second, metal foil substrates are economic as compared with conventional singlecrystal silicon substrates.142 Third, metal foils may enable the use of roll-to-roll-type processes, creating a new paradigm in manufacturing of crystalline silicon-based solar energy conversion.142 Fourth, the metal foil or film used as a precursor promotes growth of 1D nanostructure arrays and also can be used as an electrode and heat sink when the 1D nanostructure arrays are used in solar energy conversion. Finally, 1D nanostructure arrays can be self-grown from a metal substrate, and the strong bonding between 1D nanostructure arrays and the substrate ensures excellent electrical contact between them.143


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Fig. 14 Flowchart illustrating the LBL self-assembly of hierarchical T-NRNT and M/T-NRNT (M = Au, Ag, Pt) combined nanostructures. Reprinted with permission from ref. 144. Copyright 2012 Royal Society of Chemistry.

Xiao reported fabrication of TiO2 nanotube (TNT) arrays via a two-step anodization approach on titanium sheets (Fig. 14).144 After ultrasonically peeling off the disordered surface layers inherited from the first anodization leaving regular hexagonshaped imprints on the surface of the Ti foil, a second-step anodizing process was carried out. Then, uniform and ordered TNT arrays with high qualities were grown on the Ti foil. The physicochemical properties including optical properties, electrical properties, as well as performances for solar energy conversion of TNTs can be optimized by a tuned decoration of noble metal NPs via controlled layer-by-layer (LBL) self-assembly approach. By adjusting the surface charge properties of TNTs and metal NPs, tailor-made metal (Au, Ag, Pt) colloidal NPs are uniformly deposited to the framework of TNTs through the selfassembly monolayer of LBL buildup, which is afforded by substantial electrostatic attractive interaction between metal NPs and polyelectrolytes. It is believed that LBL self-assembly strategy could provide a far superior and efficient route to functionalization of 1D nanostructure arrays with metal NPs or other components. Ryan et al. presented growth of highly dense, crystalline Ge NWs on Cu substrates through a self-induced, solid seeding process, without use of discrete metal nanocrystal catalysts.145 It was found that the NW growth proceeded on Cu through spontaneous formation of catalytic Cu3Ge seeds within a Cu3Ge layer on exposure to Ge monomer. High-density growth of Ge NW mats directly onto a conductive Cu3Ge layer atop a Cu substrate results in excellent electrical properties over intermetallic Cu3Ge, which has been demonstrated by the efficient interconnect in Ge NW transistors. 4.1.2.3 1D nanostructure arrays grown on graphene substrate. Graphene as a 2D system is composed of hexagonal arranged carbon atoms with honeycomb lattice. Its excellent optical transparency, mechanical flexibility, thermal stability, and chemical inertness make it an ideal material for solar cells, transparent electrodes, light-emitting diodes, nanogenerators, and photodetectors.146–149 Owing to its linear energy-momentum dispersion relation, graphene has unique electronic structures of high electron mobility, relativistic massless Dirac particles,


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and excellent optical transmittance, while maintaining high mechanical strength and thermal conductivity similar to carbon nanotubes.146,150 Therefore, graphene possesses the required combination of long-range atomic periodicity, mechanical strength and flexibility, high electrical conductivity, and optical transparency to serve as an ideal substrate for growth of vertically aligned 1D nanostructures for solar energy conversion.151,152 Mohseni et al. grew high-density vertical self-organized core–shell heterostructured InAs–InGaAs NWs through a seedfree van der Waals epitaxy (vdW Epi) approach on graphene films by metal–organic chemical vapor deposition (MOCVD) (Fig. 15).151 A unique phase segregation phenomenon occurred, causing self-organization of coaxial heterostructures into InAs-core and InGaAs-shell segments. The self-assembly mechanism is attributed to the commensurate relationship between the InAs crystal structure and the 2D graphene lattice, and the lack of strain accommodation of graphene to InGaAs because of the weak vdW interaction. However, when grown on MoS2, or via the Au-assisted VLS mechanism on graphene, no InGaAs phase segregation was observed. This implies that the absence of atomic registry between substrate and Epi-layer forces growth of single compositional phase InGaAs NWs on 2D materials, which is the expected outcome of the vdW Epi growth mechanism. Although the physicochemical properties are hardly tested, such material combinations of compound III–V semiconductors and monolayer graphene films have potential in novel device applications including flexible electronics and optoelectronics. Recently, nanoarchitectured, 3D carbon nanostructures made by connecting 2D graphene and vertical 1D CNTs have attracted great interest, because their ultrahigh surface-tovolume ratio, which is greatly important in energy applications, and reduced agglomeration between nanomaterials can greatly enhance applicability in industrial fields. The 3D carbon hybrid materials have great advantages for solar energy conversion applications.153,154 Sridhar and coworkers grew vertical CNTs on graphene nanosheets to form unique 3D carbon nanostructures by a simple defect-engineered technique using an ionic liquid, a palladium catalyst, and microwave radiation (Fig. 16a and b).153 The mechanism of CNT growth on graphene sheets can be explained as follows: first, palladium NPs are anchored on a partial exfoliation of graphene platelets in ionic liquids

Fig. 15 Schematic model of InAs–InxGa1 xAs core–shell NWs grown on graphene and InxGa1 xAs NWs grown on MoS2. Reprinted with permission from ref. 151. Copyright 2013 American Chemical Society.

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Fig. 16 (a) Mechanism of one-pot microwave synthesis of three-dimensional carbon hybrid nanostructures, (b) SEM images of CNT forest extensively grown on graphene surfaces, (c) current–voltage responses and (d) specific capacitances of G-CNT–Pd, G-Pd, and RGO. Reprinted with permission from ref. 153. Copyright 2012 American Chemical Society.

with imidazolium by reducing palladium acetate. Then, under microwave irradiation, the graphene platelet is fully exfoliated to form graphene nanosheets with atomic defects, which act as nucleation and anchoring sites for imidazolium-shelled palladium NPs. Finally, with further microwave irradiation, the imidazolium component of the ionic liquid decomposes to produce carbonaceous gases that serve as carbon sources for the CNT growth. The vertically aligned CNTs between graphene sheets will not only improve the surface area of graphene materials but also act as spacers to facilitate a diffusion path for rapid transport of electrons and energy exchange in 3D nanostructures. Thus, this unique 3D architecture shows excellent electrochemical performance as displayed in Fig. 16c and d. The synthesis of 3D carbon nanostructures, whose functionality can be tailored by changing the catalyst with cobalt, nickel, and other metals, heralds a new era for practical applications. 4.1.2.4 1D nanostructure arrays grown on 2D semiconductor substrates. Growing 1D semiconductor nanostructure arrays on other 2D semiconductor substrates with matched band gap can further promote transport and separation of photogenerated charge carriers. The 1D–2D spatial system has been reported for various applications.151,155–158 For example, Li et al. reported high-quality vertical CdS nanowire arrays (NWAs) heteroepitaxially grown on a CdSe single-crystalline sheet (SCS) by a two-step thermal evaporation method governed by a typical VLS mechanism.159 In the first step, CdSe SCS were synthesized by thermal evaporation of CdSe powder and transported by argon using SiO2–Si as substrates for the collection of the sample. By depositing with a 10 nm thick Au film on the obtained CdSe SCS, tapered CdS vertical NWAs were grown via a similar thermal evaporation approach, forming 1D–2D spatial architectures (Fig. 17a and b). Introduction of CdSe as the growth substrate provides a perfect pathway to synthesize vertically

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tapered CdS NWs, and type-II heterostructures with a staggered alignment of CdS NWAs on CdSe SCSs, leading to enhanced field emission and enlarging the response spectrum with broad-band coverage from ultraviolet to red visible light and enhanced spatial charge separation by reducing electron–hole pair recombination, as shown in Fig. 17c and d. Furthermore, as a result of the larger number of NWAs contributing to the devices’ performance compared with single-NW based counterparts, the signal level and the stability of these devices are enhanced.

Fig. 17 (a) SEM image of CdS NWAs on CdSe, (b) a simulated structure image shows a CdS NW epitaxially grown on the CdSe SCS along the [0001] direction, (c) wavelength dependent photocurrent response of the CdS NWA–CdSe SCS heterostructure photodetector and (d) a schematic band structure of the CdS NWA–CdSe SCS heterostructures implies the enhancement of photoconductivity of these heterostructures. Reprinted with permission from ref. 159. Copyright 2014 Royal Society of Chemistry.


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4.1.3 1D nanostructure arrays grown on 3D substrates forming urchin-like architectures. Some recent efforts have focused on integration of 1D nanoscale building blocks into 3D ordered superstructures or complex functional architectures, which is a crucial step toward realization of functional nanosystems.160,161 These novel structures have a wide range of potential applications, such as electron field emitters, sensors, catalysis, biomarkers, microelectronics, and in energy storage.161–167 Huang and coworkers reported a core–shell urchin structure composed of high-density anatase TiO2 nanorod-built networks (shell part) that stand on a TiB2 microcrystal (core part).168 A three-step sequential ‘‘oriented attachment growth’’ model is proposed to explain the formation mechanism of a TiO2–TiB2 heterostructure with a sea-urchin shape, which is illustrated in Fig. 18. In the initial step, TiB2 with small particle size or the surface of TiB2 with large particle size were oxidized to a soluble titanium fluoride complex [TiF6 n(OH)n]2 , which led to a hydrous oxide precipitate by condensation reactions. In the second step, chelation between EDA and [TiF6 n(OH)n]2 provided the driving force for ‘‘oriented attachment’’ during the precipitation so that the surface energy is at a minimum through eliminating the surface; namely, the hydrous oxide precipitate attached to the surface of unreacted TiB2 particles in assembly. After completion of attachment, the precipitate further hydrolyzed and formed TiO2 after a long time (step 3). That is, the TiO2– TiB2 heterostructure with an urchin shape and high-crystal properties formed.

4.2

High dimension units grown on 1D individual substrate

4.2.1 2D nanosheets grown on 1D substrate. Growth of 2D nanosheets on 1D nanomaterials forming novel 1D/2D nanoheterostructures (NHSs) could fully integrate the merits of 1D nanostructures for efficient transport of electrons and optical excitation with those of 2D nanostructures for excellent properties owing to the confinement of electrons in their unique atomic layers, and mitigate the drawbacks of the single units, such as the low surface area of 1D NWs and the tendency of 2D

Fig. 18 Schematic illustration of a three-step sequential growth model for formation of the TiB2–TiO2 heterostructure. Reprinted with permission from ref. 168. Copyright 2012 American Chemical Society.


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nanosheets towards restacking.169,170 In addition, the interface between these two segments might produce new attributes that are different from the single components.171,172 Wang and coworkers demonstrated construction of 1D CdS NW/2D ZnIn2S4 helical and consecutive nanosheet NHSs attached along CdS NWs (Fig. 19a and b).173 Following the lattice and dangling bond matches, they affirmed that the most preferable bonding lateral face of the ZnIn2S4 nanosheet is the (010) one, which has Zn–S1–S2 and In–S1–S2 terminals; these correspond to two different 1D/2D interfaces, but have similar bonding modes. Besides the lattice mismatch, the bond mismatch allows the ZnIn2S4 nanosheets to rotate along the (120) axis. The greatly enhanced photoelectrochemical cell (PEC) performance of the NHSs may originate from the 1D/2D interface facilitating charge separation and transport, which is verified by experimental results and density functional theory (DFT) calculations. Compared with bulk ZnIn2S4, the atomically thick ZnIn2S4 nanosheet possesses a higher conduction band minimum (CBM) and a lower valence band maximum (VBM), inducing a smaller VBM offset between ZnIn2S4 and CdS, and thus facilitating injection of holes from the VBM of CdS as shown in Fig. 19c. 4.2.2 3D nanocubes grown on 1D substrate. 3D nanostructures grown on 1D substrate forming spatial architectures could inhibit bundling of accumulated single 1D nanostructures, making use of 1D nanostructures more effective. Ye’s group fabricated an Ag NW–Ag3PO4 cube necklace-like heterostructure by a hetero-growth process (Fig. 20a).174 Single-crystalline Ag NWs were firstly fabricated through a modified polyol process, and served as starting templates for subsequent selective growth and assembly of Ag3PO4 crystals. When the NWs were reacted

Fig. 19 (a) TEM image of the NW–nanosheet NHS, (b) the morphological evolution process, and (c) the band alignment of the nanoheterostructure. Reprinted with permission from ref. 173. Copyright 2014 Wiley.

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as well as artificial photosynthesis on CO2 reduction and water splitting.


5.1 One-dimension-based spatially ordered architectures for nonselective pollutant degradation

Fig. 20 (a) SEM images of the Ag NW–Ag3PO4 cube necklace-like heterostructure and (b) photocatalytic activities of Ag NW–Ag3PO4 cube necklacelike heterostructures, Ag NWs, and pure Ag3PO4 cubes for RhB degradation under visible-light irradiation (l 4 420 nm). Reprinted with permission from ref. 174. Copyright 2012 Royal Society of Chemistry.

with [Ag(NH3)2]+ complex and Na2HPO4 in aqueous solutions at room temperature, uniform and orderly Ag3PO4 submicrocubes were formed on the Ag NWs by a hetero-growth process. It was found that successful hetero-growth of Ag3PO4 cubes was independent of the number of Ag NWs, and the only requirement was their direct interface contacts. The epitaxial growth process of Ag3PO4 crystals on Ag NWs, as well as the compositions and structures of Ag–Ag3PO4 hetero-products, can be rationally tailored by simply adjusting the concentration of [Ag(NH3)2]+ complex. The low Fermi level of Ag means it serves as a good electron acceptor for facilitating quick electron transfer from both exterior and inner Ag3PO4 cubes, while the enriched electrons on the Ag NWs can be exported effectively by the novel necklace heterostructure. The efficient separation of photogenerated electron–hole pairs could lead to enhanced photocatalytic solar energy conversion performances as shown in Fig. 20b.

Nonselective photocatalysis has been investigated extensively owing to its great significance to environmental remediation, by which contaminants are mineralized to less toxic inorganic compounds, such as water, carbon dioxide, and salts.16,175,176 One-dimension-based spatially ordered architectures could be a class of novel photocatalysts in nonselective degradation of organic pollutants for environmental amelioration on account of their superior electron mobility, high adsorption capacity and large specific surface area.89,177–180 Zhang et al. reported 3D ZnO NWs–Si microrod (SiMRs) hybrid architectures constructed by combining bottom-up and top-down approaches in a controllable way for photodegradation of methyl red as shown in Fig. 21a.179 The 3D spatial architectures show improved light harvesting ability because of an increased optical path by multi-scattering (Fig. 21b). Thus, together with the ultrahigh surface areas which lead to an enlarged contact area with the sample solution, photocatalytic performances of 3D ZnO NWs–SiMR hybrid architectures with a medium ZnO NW length are evidently superior to those of ZnO NWs on flat Si substrates (Fig. 21c). High-quality, singlecrystalline ZnTe–ZnO core-branch NHSs were achieved via a facile and effective two-step synthesis process by Sun et al.180 It was demonstrated that higher temperature leads to denser and thinner ZnO NW growth, thus this could further affect the surface area of the samples. The branched ZnTe–ZnO NHS architecture exhibits enhanced photodegradation abilities with respect to pure ZnO NWs, which could be ascribed to the

5. Applications of one-dimensionbased spatially ordered architectures in photocatalytic solar energy conversion The fascinating optical and electrical properties associated with one-dimension-based spatially ordered architectures means they have potential for applications in photovoltaics and solar cells devices. Direct conversion of solar energy into chemical energy by photocatalysis is more flexible in some aspects. In addition to mimicking natural photosynthesis and providing an alternative approach to photovoltaic cells for production of carbon-neutral energy from sunlight, photocatalysis could also mitigate environmental issues and carbon dioxide emissions. Recent advances in scientific research have yielded exciting knowledge in the area of photocatalysis on one-dimensionbased spatially ordered architectures. However, no comprehensive summary has been made of their possible applications in photocatalytic solar energy conversion. In this section, we choose some typical examples and highlight the application of one-dimension-based spatially ordered architectures toward photocatalytic solar energy conversion including nonselective pollutant degradation and selective organic transformation

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Fig. 21 (a) Schematic diagram of the fabrication processes of 3D radial ZnO–SiMR arrays, (b) absorption spectra of ZnO NW@flat-Si, and 3D ZnO NWs–Si MRs with increasing length of the ZnO NWs from about 600 nm (sample S), and 1.7 mm (sample M) to 6–10 mm (sample L) and (c) photodegradation of methyl red with the assistance of different types of catalysts. Reprinted with permission from ref. 179. Copyright 2011 American Chemical Society.


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Fig. 22 (a) SEM image of branched ZnTe–ZnO nanoheterostructures, (b) a MR photodegradation rate of ZnO NWs and ZnTe–ZnO NHSs, compared with that without any catalyst under UV light irradiation and (c) schematic diagram showing band configuration and electron–hole separation at the interface of ZnTe–ZnO heterostructure under UV light irradiation. Reprinted with permission from ref. 180. Copyright 2011 Royal Society of Chemistry.

Fig. 23 (a) A schematic illustration of synthesis of branched hierarchical CdS–ZnO nanocomposites, (b) photocatalytic performance of CdS NWs, blank ZnO and branched hierarchical CdS–ZnO nanocomposites for selective oxidation of thioanisole, and (c) 4-nitroaniline reduction under artificial solar light irradiation. Reprinted with permission from ref. 78. Copyright 2014 Royal Society of Chemistry.

significantly increased effective surface area of these unique branched 3D architectures, while the band configuration at the interface of the ZnTe–ZnO heterostructure could also contribute to such improvement as displayed in Fig. 22. Furthermore, when the stem is prepared to be a metal or metallic NW, it can act as an electric cable for transferring photoelectrons from the semiconductor branches and help in facilitating the electron– hole separation.109

of thioanisole and anaerobic reduction of nitro compound 4-nitroaniline. As mirrored in Fig. 23b, the conversion for thioanisole and yield for methylsulfinylbenzene are about 60% and 52%, respectively, over the branched hierarchical CdS–ZnO nanocomposites, which are much higher than the values obtained over CdS NWs (conversion 36% and yield 33%) and blank ZnO (conversion 3.5% and yield 3%). In addition, Fig. 23c shows that hierarchical CdS–ZnO nanocomposites exhibit remarkably enhanced photoactivity (conversion of 60%) compared with CdS NWs (conversion of 30%) and blank ZnO (conversion of 2%) under artificial solar light irradiation for only 1 minute. These redox reaction results suggest that branched hierarchical CdS–ZnO nanocomposites with nanotree-like structure have potential as photocatalysts for fine-chemical synthesis. By planting CdS-sensitized 1D ZnO nanorod arrays on a 2D graphene sheet, we obtained ternary hierarchical nanostructures, in which graphene oxide as the precursor of graphene acts as a flexible substrate for formation of ZnO nanorod arrays (Fig. 24a).186 The fast electron transport of 1D ZnO nanorods, the well-known electronic conductivity of 2D graphene, the intense visible-light absorption of CdS, the unique hierarchical structure, and the matched energy levels of CdS, ZnO, and graphene efficiently boost the photogenerated charge carrier separation and transfer across the interfacial domain of hierarchical CdS-1D ZnO-2D graphene hybrids under visible light irradiation via a three-level electron transfer process (Fig. 24b). Therefore, the hierarchical CdS-1D ZnO-2D graphene hybrids can serve as an efficient visible-light-driven photocatalyst for selective organic transformations. We investigated the photocatalytic performance of these samples in anaerobic reduction of nitroaromatic compounds to corresponding amino compounds in water, a key transformation in synthesis of fine chemicals. As shown in Fig. 24c, reduced graphene oxide (RGO)–ZnO NRs–CdS nanocomposites exhibited enhanced photocatalytic performance

5.2 One-dimension-based spatially ordered architectures for selective organic transformations As organics are used extensively in diverse applications, selective organic transformation is of great industrial importance.181 Traditional organic synthesis always results in environmentally toxic/corrosive oxidants and produces large quantities of hazardous wastes.182 To overcome these disadvantages, the use of photocatalytic selective organic transformation has been demonstrated as an alternative with intrinsic merits including mild reaction conditions and the possibility of reducing generation of undesired byproducts.183–185 So far, typical examples for photocatalytic organic transformations have been reported including selective oxidation and reduction, isomerization reactions, C–H bond activations, and C–C and C–N bond-forming reactions.16 Our group prepared branched hierarchical CdS–ZnO nanocomposites for application toward photocatalytic fine-chemical synthesis.78 Growing ZnO nanorod arrays on individual CdS NWs can not only enhance light absorption but also effectively passivate surface defect states that act as charge traps on the CdS NWs, thus boosting the transfer of charge carriers in the hierarchical architecture nanocomposites (Fig. 23). Under irradiation of artificial solar light, these two light-absorbing materials provide a model ‘‘Z-scheme’’ for driving photocatalytic redox reactions, as evidenced from aerobic selective oxidation


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Fig. 24 (a) Schematic illustration of controllable synthesis of the hybrids of binary ZnO nanostars–CdS and ternary RGO–ZnO NRs–CdS, (b) schemes illustrating the transfer of charge carriers in ternary RGO–ZnO NRs–CdS nanohybrids under visible light irradiation and (c) photocatalytic activities for selective reduction of 4-nitroaniline to p-phenylenediamine over bare ZnO nanostars, ZnO nanostars–CdS, 5%RGO–ZnO NRs and RGO–ZnO NRs–CdS with different weight addition ratios of RGO under visible light irradiation (l 4 420 nm). Reprinted with permission from ref. 186. Copyright 2010 Wiley.

compared with ZnO nanostars, RGO–ZnO NRs, and ZnO nanostars– CdS. Furthermore, superior reusability of ternary hybrids was achieved by controlling the reaction parameters, that is using visible light irradiation and hole scavengers to prevent ZnO and CdS from photocorrosion. 5.3 One-dimension-based spatially ordered architectures for artificial photosynthesis With increasing concern over the global energy crisis and environmental contamination caused by the use of conventional fossil fuels, artificial photosynthesis with the aim of producing high-energy content chemical species from low-energy content chemical species using solar energy as the energy source, such as simultaneous reduction of CO2 and water, is a ‘‘Holy Grail’’ of modern science, as it could solve the problems connected with the intermittency and low density of solar energy.5,7,187–190 One-dimension-based spatially ordered architectures as efficient photocatalysts in artificial photosynthesis could help to address the issue of rising levels of greenhouse gas emissions and provide an alternative source of renewable energy. Some representative works on photocatalytic water splitting on one-dimension-based spatially ordered architectures have been reported.191–197 Of these, an integrated standalone system is fundamentally intriguing, to mimic the microscopic spatial control of natural photosynthesis. Using Si and TiO2 NWs as building blocks, a proof-of-concept solar-to-fuel conversion nanodevice was demonstrated for solar water splitting (Fig. 25) by Yang’s group.198 Using knowledge of the differences in catalytic and electrical transport properties of Si NW and TiO2, the nanoscale tree-like light-harvesting architecture could be of benefit in the water splitting performance. Under solar light irradiation, photogenerated electrons in Si NWs migrate to the surface where they reduce protons to generate hydrogen,

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and, meanwhile, photogenerated holes in TiO2 NWs oxidize water to produce oxygen, thus forming a ‘‘Z-scheme’’ mechanism. In terms of systems-level materials engineering, this provides a potential future research direction for newly discovered components, which might be able to replace existing ones to yield better performance. The photocatalytic CO2 reduction process can be sustained by harnessing solar energy, making it economically feasible and environmentally benign. However, it still suffers from very low conversion efficiencies, resulting from the fast electron–hole recombination rate in the photocatalysts.199 One-dimension-based spatially ordered architectures have great potential for photocatalytic CO2 reduction because of their unique properties.200–204 Wang et al. fabricated highly efficient Pt–TiO2 nanostructured

Fig. 25 A proof-of-concept fully integrated nanosystem for direct solar water splitting was demonstrated. (a) The tree-shaped heterostructure is designed on the basis of different material properties of Si and TiO2 photoelectrodes, (b) which leads to unassisted water splitting (c). The overall integrated structure has spatial separation of the photocathode and photoanode (d, e), analogous with natural photosynthesis. Reprinted with permission from ref. 198. Copyright 2013 American Chemical Society.


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films composed of a unique 1D structure of TiO2 single crystals coated with ultrafine Pt NPs (0.5–2 nm) for high CO2 photoreduction efficiency with selective formation of methane.199 They chose columnar morphology of TiO2 films for the high surface area, single crystallinity, and superior electron-transfer performance originating from highly oriented 1D structures. According to the photocatalytic performance of the samples as shown in Fig. 26a, the CH4 yield increased with irradiation time, reaching a peak value of 1361 mmol g-cat 1 h 1 at 5 h, with quantum yield at this condition calculated to be 2.41%. The slightly decreased CH4 production rates after 5 h of irradiation can be attributed to deterioration of photocatalytic activity because of diminishment of the adsorption power of the catalyst and saturation of adsorption sites on the TiO2 surface with intermediate products. The CO2 photoreduction mechanism of the Pt–TiO2 nanostructured films is shown schematically in Fig. 26c. The extremely high photoreduction efficiency of the films is believed to be a result of the synergistic effects of high surface area and minimized charge barriers by oriented single-phase crystallinity of the film and efficient electron–hole separation by the Pt NP. Similar enhancement of CO2 photoreduction efficiency is also achieved by Pt–TiO2 NT arrays, where the Pt NPs are deposited on the TiO2 NTs using a rapid microwave-assisted solvothermal approach. Although one-dimension-based spatially ordered architectures have potential for the photocatalytic solar energy conversion, geometric and structural effects are not the sole factors involved. Engineering a brilliant photocatalytic system with both suitable components and geometric structures may achieve improved efficiency in photocatalytic solar energy conversion. The photocatalytic processes require not only light absorption and charge separation in semiconductors that exhibit band-bending at a junction but also a minimum voltage output that is imposed by the chemical reaction involved.26,205 For example,

Fig. 26 Photoreduction analysis of Pt–TiO2 thin films. (a) Representative CH4 and CO yields on a Pt–TiO2 thin film as a function of irradiation time, (b) CO and CH4 yields of commercially available TiO2 powders (P25), pristine TiO2 columnar films (TiO2 film), and Pt–TiO2 films with different Pt deposition times and (c) schematic diagram of CO2 photoreduction mechanism by using Pt–TiO2 nanostructured films. Reprinted with permission from ref. 199. Copyright 2012 American Chemical Society.


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at standard conditions, splitting water into hydrogen and oxygen requires at least 1.23 V. This voltage restriction limits the choice of materials. If only one light-absorber is applied, a large band gap semiconductor is required. To achieve high efficiency, it is more desirable to use two light-absorbers with smaller band gaps to provide the necessary voltage for the electrochemical reaction.206 However, system engineering is not a simple additive operation between components and geometric structures. For instance, metal–semiconductor heterostructures where gold is grown on one side of CdSe nanorods offering intrinsic asymmetry, show different physical and chemical properties compared with two-sided growth nanostructures and common Au–CdSe nanocomposites.207–209 It was found that one-sided Au–tipped CdSe nanorods exhibit significant enhancement towards photocatalytic multiple-electron reduction of methylene blue molecules even at room temperature.210 Thus, for optimized photocatalytic performance, careful structural and material design is needed, along with fundamental insight into the interplay among the various components.

6. Summary and outlook Combining 1D and 2D/3D properties, including enhanced solar energy harvesting and high surface area, one-dimension-based spatially ordered architectures represent a class of attractive materials for photocatalytic solar energy conversion. In this review, we have classified and summarized one-dimensionbased spatially ordered architectures in terms of the dimension of composition for two categories: 1D nanostructure arrays grown on substrates with different dimension, and 2D or 3D units grown on 1D individual substrate. Some prominent and intriguing examples of using these spatially ordered architectures for solar energy conversion have been elaborated, which include nonselective pollutant degradations, selective organic transformations, CO2 photoreduction, and water splitting. Common techniques to integrate 1D nanostructures into highdimensional hierarchical structures range from one-step selfcatalyzed growth to multi-step seed-assisted growth. However, integration and optimization of various 1D materials into functional solar energy conversion devices by seamless control of diverse materials under continuous synthesis conditions remains a challenge. To date, there is no simple, high-efficiency method for synthesis of one-dimension-based spatially ordered architectures with precise structure control, high quality, high density, high aspect ratio, large-area uniformity, and perfect vertical alignment. Therefore, development of advanced nanomanufacturing processes such as large-scale self-assembly, advanced reactor design, and roll-to-roll processes for 1D nanostructures growth and assembly is critical for commercial adaptation of these structure-based technologies. The overall photocatalytic performance of one-dimensionbased spatially ordered architectures is attributed to a collective, harmonious integration of the individual components, interface composition, and material structure and morphology at the nanoscale. Systems-level rational engineering of functional devices, which is analogous to biological systems, is needed

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to optimize performance. On one hand, engineering of special interfaces in the systems with diverse individual components forming efficient interactions such as p–n junctions, heterojunctions and Z-scheme systems would be an effective strategy for enhancing the separation and transfer of photogenerated charge carriers and thus improving the overall photocatalytic efficiency of one-dimensionbased spatially ordered architectures. On the other hand, creating ‘‘compositionally complex’’ multi-component nanostructure systems will enable more sophisticated solar conversion device designs. The rational integration of various building blocks including 0D quantum dots (QDs), 1D nanostructures, 2D sheets, and 3D particles to built spatially ‘‘nano-house’’ systems could facilitate the microscopic charge carrier transfer pathway. Optimized system-level engineering of one-dimension-based spatially ordered architectures provides a versatile platform for study of photocatalytic mechanisms and some fundamental information such as the transfer of energy, force, charge, and/or mass across interfaces in the systems. So far, numerous one-dimension-based spatially ordered architectures have been grown on macro conductive substrates, such as metal foils and conductive glass substrates. These macro devices retain the advantages of decoupling the directions of light absorption and providing an ideal channel for effective directional carrier transport, leading to enhanced quantum efficiency and photoconversion efficiency. However, the reduced contact between the macro systems and the reactants limits their further applications in photocatalytic solar energy conversion. 1D individual nanostructures could have efficient contact with the reactants while being used in a suspended state in water; however, the disordered diffusion of the suspended state would result in excessive aggregation and thus in a dramatic decrease of the active surface area. Growing 1D nanostructure arrays on suitable substrates such as 1D nanofibers, 2D graphene, and 3D microspheres to form mesostructures might find a balance for optimizing the photocatalytic systems. 1D nanostructure arrays grown on substrates at the micron level forming meso-structures could provide high quantum efficiency and facilitate easy secondary processing. We believe that one-dimension-based spatially ordered architectures are important and promising materials for solar energy conversion, providing high surface area, improved light trapping and tunable pathways for efficient conversion and collection of photocarriers. Rational design of these architectures from a systemlevel angle could help impart the unique and transformative properties of 1D nanostructures into the composite system, thereby offering promising scope and widespread practical implementation for the unique composite material in the tantalizing field of photocatalytic solar energy conversion.

Acronyms 0D 1D 2D 3D ALD BNR

Zero-dimensional One-dimensional Two-dimensional Three-dimensional Atomic layer deposition Branched nanorod

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BNW CB CBD CBM CF CNT CVD DFT DPN EBID EBL ED EUV FTO HBS IBL IPCE IR ITO LBL MBE MOCVD MOVPE MRs MWCNT NB NHSs NR NT NW NWAs NIL NP OL PEC PECVD RGO SCS SILAR SLS SPL SPR STM TNT UV VB VBM vdW Epi VLS XRL

Branched nanowire Conduction band Chemical bath deposition Conduction band minimum Carbon fiber Carbon nanotube Chemical vapor deposition Density functional theory Dip-pen nanolithography Electron beam induced deposition Electron beam lithography Electrodeposition Extreme ultraviolet F-doped tin oxide Hierarchical branched structure Ion beam lithography Incident photon-to-current conversion efficiencies Infrared Indium tin oxide Layer-by-layer Molecular beam epitaxy Metal–organic chemical vapor deposition Metal organic vapor phase epitaxy Microrods Multi-walled carbon nanotube Nanobelt Nanoheterostructures Nanorod Nanotube Nanowire Nanowire arrays Nanoimprint lithography Nanoparticle Optical lithography Photoelectrochemical cell Plasma enhanced chemical vapor deposition Reduced graphene oxide Single-crystalline sheet Successive ionic layer adsorption and reaction Solution–liquid–solid Scanning probe lithography Surface plasmon resonance Scanning tunneling microscopy TiO2 nanotube Ultraviolet Valence band Valence band maximum van der Waals epitaxy Vapor–liquid–solid X-ray lithography

Acknowledgements The support by Key Project of National Natural Science Foundation of China (U1463204), the National Natural Science


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Foundation of China (NSFC) (20903023, 20903022, 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Returned High-Level Overseas Chinese Scholars of Fujian province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged. Dr Colmenares is grateful for the support from the Institute of Physical Chemistry of PAS.

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