In situ studies of solvothermal synthesis of energy materials.

CHEMSUSCHEM MINIREVIEWS DOI: 10.1002/cssc.201301042

In Situ Studies of Solvothermal Synthesis of Energy Materials Kirsten M. Ø. Jensen, Christoffer Tyrsted, Martin Bremholm, and Bo B. Iversen*[a] Solvothermal and hydrothermal synthesis, that is, synthesis taking place in a solvent at elevated temperature and pressure, is a powerful technique for the production of advanced energy materials as it is versatile, cheap, and environmentally friendly. However, the fundamental reaction mechanisms dictating particle formation and growth under solvothermal conditions are not well understood. In order to produce tailor-made materials with specific properties for advanced energy technologies, it is essential to obtain an improved understanding of these processes and, in this context, in situ studies are an important tool as they provide real time information on the reactions taking

place. Here, we present a review of the use of powder diffraction and total scattering methods for in situ studies of synthesis taking place under solvothermal and hydrothermal conditions. The experimental setups used for in situ X-ray and neutron studies are presented, and methods of data analysis are described. Special attention is given to the methods used to extract structural information from the data, for example, Rietveld refinement, whole powder pattern modelling and pair distribution function analysis. Examples of in situ studies are presented to illustrate the types of chemical insight that can be obtained.

1. Introduction Energy conversion and storage technologies such as solar cells, thermoelectric modules, Li batteries, hydrogen storage and fuel cells play a large role in the transition to a society based on renewable energy. However, to compete with fossil fuel technologies, improvements of the energy conversion and storage systems are needed. For example, the energy densities of batteries must be enhanced and the batteries need to become cheaper and safer; the energy conversion rate of solar cells must be improved; and fuels cells must become more cost efficient. The key to advancing all of these technologies is improved material performance, and extensive research is now directed at the development of new and enhanced energy materials.[1] The properties of functional materials are directly linked to the material characteristics, for example, crystal structure, structural defects, crystallite and particle size, micro- and nanostructure, and crystallinity. A major goal in modern materials chemistry is thus to develop advanced synthesis methods that allow precise control of the material characteristics, while ensuring that the synthesis is still cheap, green, and capable of quickly producing materials on a large scale. The solvothermal synthesis method, in which materials are produced in a solvent at elevated temperatures and pressures, is in this context very promising.[2] A solvothermal process is generally defined as a chemical reaction taking place in a solvent at temperatures above the solvent boiling point and at pressures above 1 bar.[2c] Any type of solvent can be used, but water is most [a] Dr. K. M. Ø. Jensen, Dr. C. Tyrsted, Dr. M. Bremholm, Prof. Dr. B. B. Iversen Center for Materials Crystallography, Department of Chemistry and iNANO Aarhus University Langelandsgade 140, 8000 Aarhus C (Denmark) E-mail: [email protected] Homepage: www.cmc.chem.au.dk

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commonly employed, and in this case, the synthesis is termed “hydrothermal”.[2c] Compared to solid state synthesis, which is often performed at elevated temperatures (generally above 700 8C) for several hours or days, the solvothermal method is fast, energy efficient, environmentally benign, inexpensive, and very versatile.[2a] The reactants used in most solvothermal syntheses of inorganic materials are simple metal salts, and unlike more advanced methods, for nanoparticle synthesis, the use of toxic or expensive organic reactants and solvents is not necessary.[3] Moreover, by changing very simple synthesis parameters such as temperature, pressure or reactant concentration, the characteristics of the particles can easily be altered. The method can be used for batch synthesis, but continuous flow hydrothermal synthesis is now also widely applied for many different functional energy materials.[3] The solvothermal technique thus shows great promise for the production of energy materials.[2a] However, the mechanisms controlling the fundamental particle formation processes taking place during solvothermal synthesis are not well understood, and extensive trial-and-error experiments are most often needed in order to design the synthesis of a certain material. To take materials science to the next level and produce energy materials with tailor-made properties for improved technologies, a deeper knowledge of these processes is crucial. For this reason, in situ studies of solvothermal processes, that is, studies of the chemical reactions in real time, have become important in the understanding of material formation. In situ investigations of material synthesis may be done through a plethora of experimental techniques, which can generally be categorized as spectroscopy [IR,[4] Raman,[5] NMR, and X-ray absorption spectroscopy (EXAFS)[6]] or scattering [small-angle X-ray scattering (SAXS),[7] wide-angle X-ray scattering (WAXS), ChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS Bo Brummerstedt Iversen received his Ph.D. in Chemistry from Aarhus University in 1995. Following a post doc period at University of California at Santa Barbara he became Assistant Professor at the Department of Chemistry, Aarhus University. In 2000, he was promoted to Associate Professor and in 2004 he became Chair of Inorganic Chemistry. He is director of the Center for Materials Crystallography, which is a Center of Excellence funded by the Danish National Research Foundation. Martin Bremholm received his Ph.D. in Chemistry from Aarhus University in 2009. Following a post doc position at Princeton University he became Assistant Professor at the Department of Chemistry, Aarhus University. In 2012 he received a Young Investigator Grant from the Villum Foundation. He is part of the Center for Materials Crystallography, a Center of Excellence funded by the Danish National Research Foundation. In addition to in situ studies of hydrothermal syntheses he also specializes in synthesis and characterization of solids at high pressure and high temperature. Kirsten Marie Ørnsbjerg Jensen received her Ph.D. in Chemistry from the Center for Materials Crystallography, Aarhus University in 2013. Her Ph.D. project focused on the hydrothermal synthesis of battery materials, applying in situ X-ray investigations for studies of synthesis processes. She is currently a Postdoctoral Research Fellow at Applied Physics and Applied Mathematics, Columbia University in New York, funded by an individual postdoc grant from the Villum foundation. Her research concerns the use of total scattering and Pair Distribution Function analysis for studies of nanomaterial structure. Christoffer Tyrsted received his Ph.D. in Nanoscience from Center for Material Crystallography and iNANO, Aarhus University in 2013. His Ph.D. research concerned the use of in situ X-ray studies of solvothermal reactions. Following his Ph.D., he was hired by Haldor Topsøe A/S in Copenhagen, one of the world’s leading catalysis companies. There, he works on structural characterization of catalyst materials using X-ray scattering techniques.


www.chemsuschem.org powder X-ray diffraction (PXRD),[8] and total scattering[9]]. Different methods may offer different insights and it is often useful to combine techniques to obtain a more comprehensive description of the processes occurring during material synthesis.[10] In this review, we focus on recent in situ studies using powder X-ray and neutron diffraction, which has been employed for a large range of materials since the development of the first hydrothermal reactors suitable for diffraction in the 1990s. Industrially important microporous materials have been studied extensively,[11] and more recently, functional energy materials have received much attention, for example, photocatalytic metal oxides (e.g., TiO2, SnO2),[7, 9b, 12] battery materials (LiFePO4, LiCoO2),[13] thermoelectrics (Bi2Te3, ZnO),[14] catalysts and catalyst support materials (CeO2, ZrO2, and Al2O3)[10a, 15] as well as a range of metal–organic frameworks used, for example, in gas storage.[8a, 16] The studies have revealed new knowledge on some of the fundamental processes that occur during the hydrothermal synthesis; this can help in designing new synthesis methods for a wide range of materials. Here, we discuss new experimental approaches for in situ studies of solvothermal reactions, as well as methods of powder diffraction data analysis which can yield novel information on particle and crystallite formation and growth. Special focus is given to recent progress in in situ total scattering experiments, and the article is thus intended to complement previous reviews in the field.[17] The paper is structured in the same manner as an in situ study would be performed: First, we describe the common approaches used for obtaining the in situ experimental data. This is followed by a description of the different ways of analyzing the experimental data. Finally, examples of results obtained from in situ solvothermal synthesis experiments with regards to energy materials are given. We focus on the methods more than on the results from specific material systems, and the reader is referred to the relevant publications for further details.

2. Experimental Set-Ups Over the past three decades, a multitude of experimental techniques have been employed for in situ powder diffraction investigations of nanomaterial formation. Different approaches to the studies have evolved and are often catered to specific experimental requirements or investigations of particular scientific questions. Here, the general techniques are presented along with the different reactors applied in central studies. 2.1. In situ energy dispersive X-ray diffraction In energy dispersive X-ray diffraction (ED-XRD) experiments, the sample is illuminated by a white X-ray beam consisting of a wide range of energies in the range from 10–150 keV.[18] The intensity of the scattered X-rays is then measured as a function of photon energy at a fixed diffraction angle. The fixed angle of the outgoing beam allows for great freedom in designing an in situ cell as only a narrow window for the outgoing beam is needed. In addition, the high flux of the incident white X-ray beam is advantageous for in situ setups as it ensures effective ChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS reactor penetration. This results in high sample scattering intensities, allowing for exposure times on the second scale or below..[17b] By post-sample collimation of the scattered beam, spatial mapping of the reactor cell can be obtained.[17c] The use of collimation therefore makes it possible to focus the beam on only a certain volume of the reactor, thus eliminating X-ray intensity scattered from the surrounding environment. The great possibility with ED-XRD is to make close representations of large volume ( 10 cm3) laboratory autoclave vessels, mimicking the conditions used for ex situ autoclave synthesis. This has the inherent advantage of making results obtained from in situ and ex situ studies readily comparable. In situ autoclave cells are commonly designed as an inert reaction vessel encased in a sturdier metal container. The inert vessel prohibits chemical reactions with the metal casing and usually determines the upper limit for the synthesis temperature. Often, a Teflon vessel is applied which can withstand temperatures up to around 250 8C. It is also common to apply magnetic agitation for easier processing and investigations of very dense material suspensions. The metal casing, often manufactured from steel, provides a way for containing elevated internal pressures of above 100 bar. To allow for the penetration of X-rays through the reactor, the metal casing thickness is reduced at two opposing points and/or replaced by small inserts of a lighter metal, which shows lower X-ray absorption such as beryllium.[19] The first true in situ X-ray diffraction experiment on hydrothermal material formation was performed as an autoclave experiment using ED-XRD by Munn et al.[11a] in the early 1990s. Energy dispersive diffraction on autoclave setups still accounts for a significant part of in situ solvothermal studies undertaken by various research groups, mainly focused on the beamlines of I12, Diamond Light Source (Oxfordshire, United Kingdom)[20] and F3, DORISIII (Hamburg, Germany, now closed).[21] More recently devised setups include the Oxford–Diamond in situ cell described by Moorhouse et al., depicted in Figure 1 A. The versatility of the setup allows for easy interchange of different types of reaction vessels for both solvothermal and solid state

www.chemsuschem.org reactions. Heating is applied through focused infrared radiation, permitting considerable higher heating rates compared to resistive heating that relies on effective thermal contact. Information about the reactions occurring at different positions in the reaction vessel can be obtained by changing the beam position in the rector. A recent example of this can be found in the in situ autoclave setup described by Grundwaldt et al.,[19a] seen in Figure 1 B. Originally designed for X-ray absorption spectroscopy, it allows one to simultaneously investigate the upper mother-liquid phase and lower solid/liquid suspension. The same principle may be used for spatial variations of the crystallization through ED-XRD.[22] The use of energy dispersive diffraction has some inherent implications on the following data analysis. The peak widths are affected by the energy resolution of the detector ( 2 %) and the dispersion in the fixed angle 2q angle (D2q) determined by post-sample collimation.[18] This can make it difficult or impossible to distinguish closely spaced reflections in the obtained diffraction pattern. Furthermore, to extract detailed structural information from the data, the energy dependence of scattering power and absorption needs to be taken into account as well as the shape of the incident X-ray spectrum and the detector response. This highly complicates structural analysis such as Rietveld refinement, which is thus rarely done for ED-XRD data.[23] Nonetheless, as described in section 4.1, EDXRD is an effective way for understanding crystallization pathways and kinetics. 2.2. In situ angular dispersive powder X-ray diffraction In angular dispersive diffraction, the X-ray wavelength is held fixed while the signal is measured as a function of diffraction angle. This puts more spatial restraints on the design of the experimental setup compared to energy dispersive experiments as a wider opening angle for the outgoing beam is required. Furthermore, the flux of the X-ray beam may be limited compared to energy-dispersive diffraction as the beam is monochromatized before being exposed to the sample. On the

Figure 1. a) The Oxford-Diamond in situ cell (ODISC), A) schematic and B) photograph.[20] b) A schematic representation of the apparatus developed for in situ X-ray absorption spectroscopy studies with two pathways to monitor the solid/liquid interface of heterogeneous catalysts and the liquid phase or solid–liquid reactions.[19a]


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CHEMSUSCHEM MINIREVIEWS other hand, angular dispersive diffraction allows detailed structural information to be extracted through, for example, Rietveld refinement (or even pair distribution function (PDF) analysis, described below), which is not possible from ED-XRD. A common approach for in situ studies using angular dispersive diffraction experiments is to use a thin capillary as the solvothermal reactor. The capillary material is chosen such that it can withstand pressure, temperature, and chemicals used in the solvothermal process, while still being relatively transparent to X-rays. Examples are quartz,[11p] single-crystal sapphire,[24] or even thin steel tubes.[11n, 25].The small sample volumes allows for rapid heating which may be provided by, for example, a jet of hot air,[26] resistive heating,[27] or microwave heating,[16b, 28] and others,[11n, p, 24, 25] designed to allow for a large scattering window. The early developments in capillary reactors for hydrothermal in situ studies were achieved in the 1990s by Norby et al.,[11c, 29] based on the work by Clausen and Topsøe et al. on in situ studies of catalysts.[30] The capillary setup first used by Norby et al. at the X7B beamline, Brookhaven National Laboratory is depicted in Figure 2 A.[31] The precursor for the hydrothermal synthesis is injected into the capillary and subsequently mounted on a standard goniometer.[11n] Pressure is applied by N2 gas and, simultaneously with initiation of heating by a hot air blower, X-ray exposures are commenced. Since then, several other groups have applied the same approach.[15a, 17a, 24, 32] Studies using setups of this kind can usually be done up to 250 8C, depending on the reactor material.[33] Recent in situ powder diffraction and total scattering studies of near- and supercritical synthesis done in our group have also used a capillary reactor (Figure 2 b), described in detail by Becker et al.[26] Here, a thin tube of, for example, sapphire or fused silica is used, depending on the experiment. This is sealed using Swagelok fittings and graphite ferrules and mounted directly on a standard diffractometer goniometer. Pressure is applied by water, and heating is done by a jet of hot air, coming from below the capillary. The capillaries and ferrules can withstand more than 300 bar and 500 8C. Due to the preheating of the air-flow, the set-temperature is quickly reached in the capillary. The high heating rate means that a steady reaction conditions in the capillary is reached rapidly and this is important for analysis of the growth kinetics. Furthermore, the heating profiles are comparable to those observed in, for example, a hydrothermal flow reactor.

Figure 2. a) First published in situ capillary setup by Norby et al.[31] Capillary mounted on goniometer. A: Capillary, measuring 0.5–1 mm. B: Goniometer head. C: Swagelok T-piece. D: Pressure tube. b) In situ capillary setup capable of reaching supercritical conditions for H2O and other solvents by Becker et al.[26]


www.chemsuschem.org The use of capillary reactors has some synthesis drawbacks compared to autoclave studies. The conditions used in a capillary reactor to obtain a certain product can usually not be directly transferred to those needed for synthesis on a larger scale. Furthermore, as stirring is not feasible in the small capillary volume, the X-ray beam only probes a limited portion of the sample which might not represent the average reaction process. However, due to the higher data resolution and possibility of detailed data analysis such as Rietveld, whole powder pattern modeling (WPPM), or PDF analysis, capillary reactor studies are ideal for detailed structural and nanostructural studies.[18] With the use of high energy X-rays (> 50 keV), reactors closer in design to laboratory conditions can be applied even for monochromatic X-ray radiation. This approach combines the synthesis merits known from ED-XRD setups with the possibility for detailed data analysis. Exhibiting large potential, this approach is expected to grow in interest and implementation. A few attempts have already been made including an in situ study on TiO2 sol–gel synthesis using a large volume (30 mL) batch reactor under relatively mild supercritical CO2 conditions (100 8C, 100 bar), as described by Jensen et al.[34] More recently, high-energy angular dispersive diffraction has been used to study synthesis in a supercritical continuous reactor. The increased focus on continuous supercritical material production has created a need for in situ studies of this sort of reaction.[2f, 35] However, it has proved challenging to study flow synthesis in real time at near- and supercritical conditions, because of the gradual deposition of solids on the inner walls of the reactors, which, in severe cases, leads to clogging of the reactors. The lack of steady state conditions makes data reduction problematic. To overcome these challenges, we recently developed a “pulsed” supercritical synthesis method. In contrast to flow synthesis, the pulsed reactor makes it possible to rapidly inject a well-defined volume of a chemical solution into a heated reactor zone in which it stays for a predetermined residence time. Subsequently, the product is rapidly ejected from the heated zone into a cooled zone thereby quenching the reaction.[36]Using high energy synchrotron X-rays (> 60 keV), the pulsed reactor (Figure 3) has been used for in situ investigations of TiO2,[12c] AlOOH,[37] and Fe3O4 synthesis.[38] The reactors used were steel tubes, measuring ~ 3 mm in outer diameter. The strong diffraction signal from the polycrystalline steel reactor were masked physically by two semi-circular 10 mm thick steel rings mounted in front of the detector. The pulsed synthesis method allows direct transfer of synthesis parameters from in situ experiments to subsequent laboratory scale production, but the robust reactor design also allows data measurement well beyond supercritical water conditions (P = 350 bar, T = 550 8C). The angular diffractive in situ experiments described in this review have been done at powder diffraction beamlines at synchrotrons around the world. Examples of beamlines used for in situ studies of solvothermal synthesis at the large facilities include, but are not limited to, 1-ID-C[10a, 37, 38] at the APS, Argonne National Laboratory; X7B;[11o, 33, 39] and X14A[32b] at NSLS, Brookhaven National Laboratory; ID15,[12c] BM01 A[10b, 40] and ChemSusChem 0000, 00, 1 – 19

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www.chemsuschem.org for X-ray in situ total scattering experiments are similar to the capillary reactors used for angular-dispersive powder diffraction setups.[26, 27b] In situ X-ray total scattering of solvothermal reactions have been done at high-energy X-ray beamlines, for example 11-ID-B, APS, USA;[9b] ID11, ESRF, France and P02.1, PETRAIII, Germany. 2.5. In situ neutron diffraction

Figure 3. In situ supercritical pulsed flow synthesis reactor. High-energy Xrays are directed at the reaction chamber and the resulting diffraction is recorded on the detector. Two steel rings are mounted directly on the detector to attenuate the diffraction from the steel reactor itself.[12c]

BM8[41] at the ESRF, Grenoble, France, and I711, MAX-lab, Lund, Sweden.[13a, 15b, 42] While the detector used for angular diffraction experiments can be point detectors or line detectors, large 2D area detectors with fast readout times are usually preferred.[43] These allow the signal from full Debye–Scherrer rings to be recorded, which increases the data statistics in the integrated patterns. High energy X-rays (12–80 keV) are usually advantageous, depending on the experiment and reactor material. This allows not only efficient penetration of the reactor and limited X-ray absorption in the sample, but also the concentration of a large q-range on the detector area, allowing further parameters to be extracted from the data.[18] 2.3. In situ laboratory X-ray diffraction Although most in situ X-ray diffraction studies are done using synchrotron radiation, laboratory studies may also give valuable results on slower evolving chemical reactions. For these experiments, capillary reactors that can be mounted directly on the diffractometer goniometer are most suitable. Heating can be done by means of an airflow hitting the capillary. Depending on the systems investigated, as well as the setup, it may be possible to follow transformations on a minute or hour scale. This approach has, amongst others, been applied by Vistad et al. in a study of zeolite crystallization.[44] 2.4. In situ X-ray total scattering At first glance, an X-ray total scattering experiment is very similar to a standard angular X-ray powder diffraction experiment. However, to interpret the diffuse scattering signal in the data and thus obtain information about local structural order, it is necessary to obtain data to high values of momentum transfer Q (< 16 1, where Q = 4p sinq/l). As a rule of thumb, the maximum Q-value affects the resolution of data analysis in real space on the order of Dr p/Qmax.[45] For this reason, high energy X-rays (40-100 keV) are required for total scattering experiments. For time-resolved studies, the rapid acquisition PDF (RA-PDF) method is usually applied.[43b] Here, a large 2D detector combined with a small distance between sample and detector ensures fast data collection as well as good counting statistics at high Q-values. The reactor setups currently used 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Compared to X-rays, neutrons interact weakly with matter and have a low absorption cross-section. This property makes them especially suited for studying materials in special environments such as autoclaves because bulkier setups and thicker reactor walls can be used. For this reason, the very first in situ study of a hydrothermal synthesis, done by Polak et al. in 1990[46] used neutrons. Here, an aluminum autoclave-like cell was used with monochromatic neutron radiation at D1B at ILL, France, in a study of zeolite crystallization. With neutrons, two different experimental approaches can be used: Monochromatic or time-of-flight (ToF) neutron diffraction and both methods allow structural information to be extracted from Rietveld analysis. Compared to X-ray experiments, several different considerations have to be done when working with neutrons due to the differences in scattering characteristics from various isotopes of the elements. For example, H gives a large incoherent scattering signal, and performing hydrothermal experiments in H2O is therefore problematic. Neutron in situ studies of hydrothermal reactions are thus done in D2O to minimize the background signal.[47] The general design of reactors used for in situ neutron experiments is related to the autoclave cells used for ED-XRD and high energy angular X-ray diffraction experiments. However, a very thin reactor wall in the beam position is not necessary due to the low absorption cross section of neutrons. Furthermore, when designing reactors for neutron experiments, one can take advantage of the negative scattering power of some elements. This is exploited in the ISIS/Oxford solvothermal cell[48] which is constructed from a Ti–Zr alloy with a composition which gives an average neutron scattering length of zero. The only coherent signal seen in the diffraction patterns thus arises from the sample itself. This cell, which is similar in volume to a conventional laboratory cell, has been used with great success for studies of, for example, BaTiO3,[47] and zeolites,[11l] using both ToF diffraction (at POLARIS, ISIS, UK) and monochromatic radiation (D20, ILL, France). A further development of the neutron batch hydrothermal cell was done for flow-through experiments at the Australian Nuclear Science and Technology Organization (ANSTO), Australia.[49] Here, a large volume (> 200 cm3) reactor was constructed to operate in a flow-through configuration, so that the hydrothermal fluid can be recycled continuously through the cell. This allows for continuous changes of the hydrothermal environment during the synthesis, to mimic the conditions during natural mineral formation. Recently, a new reactor (the CAU-Oxford-ISIS cell)[8b, 50] for neutron studies of supercritical synthesis was developed (Figure 4). This was applied to study material synthesis under ChemSusChem 0000, 00, 1 – 19

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be possible to combine more than two techniques simultaneously as exemplified by Nikitenko et al. in a SAXS/WAXS/EXAFS setup,[53] (Figure 5 A) and Kongmark et al., using XRD/EXAFS/ Raman spectroscopy.[10b] This gives great possibilities for complementary information that cannot be obtained in single experiments. However, the choice for combining several techniques simultaneously should be made with care as compromises on individual techniques are inevitable. The insight gained from simultaneous experiments should thereFigure 4. Schematic diagram of the CAU-Oxford-ISIS cell mounted in an instrument sample tank.[50] Components fore outweigh the resulting loss situated below the base plate are held under vacuum during data collection. of data quality compared to dedicated experiments. Sometimes, it may thus be preferable to combine the insight gained from complementary dedicated batch supercritical conditions at 450 8C and 380 bar.[8b] The experiments performed at intended equal conditions.[54] This large volume (45 cm3) autoclave cell has been used at the POLARIS ToF instrument at the neutron facility ISIS, UK. The cell is way, data quality is retained while perfect correlation might be constructed from Inconel steel, which can withstand the very sacrificed. Novel beamline developments and more advanced harsh environments and high pressure during supercritical synsetups with, for example, multiple detectors for SAXS/WAXS or thesis, unlike the Zr–Ti alloy used for synthesis under more SAXS/PDF experiments may allow further possibilities for commoderate subcritical conditions. Fast heating in the supercritibined experiments without sacrificing data quality.[17d] An excal cell is done from a cylindrical metal element surrounding ample of a simultaneous SAXS/WAXS experiment using individthe cell, with windows allowing the neutron beam to enter ual detectors for the SAXS and WAXS data is exemplified in and exit the cell. The use of a ToF instrument allowed the cell Figure 5 B in a study of TiO2 synthesis.[34] to be constructed such that only a small window for the outgoing beam, hitting the backscattering bank, was required, and in this way heating to supercritical conditions could be done within only 10 min. The extreme temperatures and pressures available in the cell made it possible to study the synthesis of species which form only under supercritical conditions.[8b] 2.6. Combination of experimental techniques in simultaneous experiments The understanding of material formation may often be improved by combining different complimentary in situ techniques in which different length scales in the sample can be probed. Ideally, these should be combined in a single simultaneous experiment directly correlating insight gained from all techniques. This includes simultaneous small-angle scattering/ powder diffraction which, amongst other things, makes it possible to distinguish between amorphous and crystalline nanostructure.[10a, 51] Another wellknown combination is powder diffraction/absorption spectroscopy, in which the average lattice structure and elemental specific local order may be probed simultaneously.[52] Under special circumstances, it may 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 5. Examples of setups allowing for simultaneous complementary techniques. a) Schematic of an SAXS/WAXS/EXAFS multi-technique setup with an enlarged “inside” image of the hydrothermal reactor cell. Angular dispersive SAXS and WAXS data were collected over a maximum theoretical k range of 0.3 < q < 2 nm1 and 2q range of 11– 458, respectively. XAFS data were collected from 9.465 to 10.30 keV.[126] b) Experimental setup for real-time in situ SAXS/WAXS studies of supercritical reactions.[34]

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CHEMSUSCHEM MINIREVIEWS 3. Data Analysis Methods 3.1. Powder diffraction Depending on both data quality and the experimental method, various levels of information can be extracted from powder diffraction data. The following sections describe different approaches to the data analysis, going from detailed structural analysis with Rietveld and WPPM methods to model-free fits to the data.

3.1.1. Rietveld refinement of the crystal structure Rietveld refinement[55] allows extraction of both the crystal structure and nanostructure of a crystalline sample. Several Rietveld software packages exist, for example, FullProf Suite,[56] GSAS,[57] JANA2006[58] and MAUD,[59] making the method highly accessible, also for the large amounts of data often obtained in time-resolved experiments. In general, a Rietveld refinement requires two inputs: An initial structural model, from which a theoretical diffraction pattern can be calculated, and an experimental pattern from which the structural parameters are to be extracted.[60] The structural refinement is then done as a least-square minimization[60] of the difference between the two by varying the refinable parameters in the model. Although time-resolved studies are always a compromise between data quality and time resolution, studies of LiFePO4 synthesis have shown it to be possible to refine both unit cell parameters and defect structures (Figure 6) on a second scale, as described in Section 4.[13b, 32b]

Figure 6. Rietveld refinement of in situ powder diffraction data obtained from LiFePO4 synthesis. The data were collected with 5 second exposure time.[13b]

Le Bail refinements are similar to the Rietveld approach except that only the Bragg positions and peak broadening is included in the model, whereas the Bragg intensities are fitted without any structural constraints.[61] This approach is useful for angular dispersive data of lower quality, or where the intensities cannot be trusted. Le Bail refinements thus allow extraction of unit cell parameters but not unit cell contents, that is, atomic positions, occupancies, or thermal parameters.


www.chemsuschem.org 3.1.2. Analysis of Bragg peak profiles by Rietveld or Le Bail refinement The peak broadening of Bragg reflections contain information on both crystallite size and microstrain which may be readily analyzed through Rietveld refinement. For angular dispersive data, the size of the coherently diffracting crystalline domains (often interpreted as crystallite or particle size) is related to the Bragg peak broadening through the Scherrer formula:[62] Kl bð2qÞ ¼ hDi cosðqÞ, where b is the Bragg peak integral breadth, l is the monochromatic X-ray or neutron wavelength, q is the mean scattering angle for the reflection, and < D > is the volume-averaged size of the coherently diffracting domains. K is a constant which value depends on the shape of the particles; however it can be usually be fixed at 1.[63] Anisotropic crystallites will give rise to < hkl > dependent broadening,[64] and the morphology of the coherently diffracting domains are thus expressed in the data. A rewritten version of the Scherrer equation may also be used for size determination of very small particles from ED-XRD data with larger uncertainties due to the low resolution.[65] Apart from size broadening, microstrain in the crystal structure also contributes to the Bragg peak profile. This can be quantified according to: b (2 q / he2i = tan (q), in which b is the integral breadth, q is the mean scattering angle for the reflection, and e is the crystal lattice strain given as the difference in d-spacing, compared to a relaxed lattice, Dd/d.[18] By analysis of the dependence of broadening on scattering angle, information about crystallite size and strain can thus be extracted and analyzed. This is usually done by Rietveld or Le Bail refinement, in which peak functions such as Gaussian, Lorentzian, or Voigt/Pseudo-Voigt functions are fitted to the data and the angular dependence of the peak width is refined with suitable parameters after corrections for the instrumental broadening.[60] However, care should be taken when interpreting size/strain results obtained from simple line broadening analysis. Often, the reality is not as simple as expressed in general size and strain formulas, as crystallite size distributions and various kinds of strain can give very different angular broadening dependencies.[66] Nevertheless, valuable information about trends in size and strain can be obtained. 1

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3.1.2. Whole powder pattern modeling of reflection profiles While standard Rietveld refinement of peak profiles relies on peak fitting with analytical functions, it is possible to obtain a more physical modeling of size and strain effects through whole powder pattern modeling (WPPM)[66, 67] using software such as PM2K.[66] Here, peak profiles are described directly in terms of physical models of the microstructure and lattice defects present in the sample. The profiles are thus modeled by convolutions of real physical expression describing for example, instrument broadening, lattice distortions, and crystallite size, but also further complex crystal imperfections such as crystal twinning or anti-phase boundaries. Much more detailed information about the sample can therefore be obtained compared to simple peak fitting analysis. This includes estimations ChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS of size-distributions of coherent domains with varying morphology[68] and investigation of strain contribution effects such as line dislocations.[69]

3.1.3. Structure-free analysis When it is not possible to rely on a structural model in the analysis, as for ED-XRD data, insight may be gained from simply analyzing the evolution of Bragg reflection intensities and positions. The peaks can be fitted with individual Gaussian, Lorentzian, or Voigt/Pseudo-Voigt functions to follow the evolution of the unit cell and extent of crystallization, as described in Section 4.1. To directly compare Bragg intensities from different time periods, it is important to normalize the intensities to account for fluctuation in incoming flux and the number of scattering crystallites in the beam. This may be achieved by normalizing it against the intensity of a fluorescence peak measured simultaneously with diffraction on a different detector.[21a] From angular dispersive data, particle size estimates can furthermore be obtained without a proper Rietveld model by simple fits to selected Bragg reflections. However, as this does not include a determination of the 2q dependence of the broadening, strain and size cannot be readily distinguished.

3.2. Total scattering analysis When using conventional crystallographic methods, only the Bragg peaks are used for extracting structural information. This reflects only the long-range order of the sample. Information about the short-range (local) order remains in the diffuse scattering located underneath and between Bragg positions, which in a Rietveld refinement is treated as a “background” along with, for example, incoherent effects. In the total scattering (TS) approach both Bragg intensities and the diffuse scattering is included in the data analysis, thus providing information about both local and global order.[70] The starting point for analysis of TS data is obtaining the total scattering structure function, S(Q). This function is acquired by determining the sample coherent scattering intensity, Icoh(Q), and subsequently normalizing the data by the average scattering power of the compound: SðQÞ ¼

1 ½Icoh ðQÞ þ hf i2 hf 2 i Nhf i2

Note that in the total scattering approach, data over a wide Q-range (Qmax > 16 1) is required, as described in Section 2. S(Q) contains both the diffuse and Bragg scattering and can be treated in Q-space (reciprocal space) with appropriate models for both the local and global order. However, most often, the total scattering is treated in r-space, that is, real-space. This is done by taking the sine Fourier transform over the measured Q-range to obtain the reduced pair distribution function, G(r), which is effectively a weighted histogram of the interatomic distances in the structure: 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org Gð r Þ ¼

2 p

Z

qmax

Q½SðQÞ 1 sinðQr ÞdQ qmin

Several software packages have been developed to obtain G(r) from total scattering data: for example, PDFgetX3[71] and GudrunX[72] for X-rays, and PDFgetN[73] and GudrunN for neutrons.[72] After this initial processing, G(r) can then be treated with different methods as exemplified below.

3.2.1. Model-free data analysis Since G(r) is a function in real space; the data analysis is intuitive compared to Q-space analysis. The peak positions represent interatomic distances, while the peak intensities contain information about the atomic species and the coordination number of a specific site. The width and shape of the PDF peak are determined by thermal motion and disorder in the structure.[70c] As the total scattering signal is used to obtain the PDF, this applies for both crystalline and non-crystalline species, and much information can be extracted by simply considering single peaks in the PDF. By fitting for example, Gaussian functions to specific peaks in the time-resolved data,[9a, 74] changes in bond length and coordination can be followed even without knowing the full structure, which may be difficult to model in the case of highly disordered systems. This approach has been employed in in situ studies of for example, catalysts[9a, 75] and battery cycling,[76] and is also applicable for studies of crystallization. The method is illustrated in Figure 7 A, where Gaussian fits to single PDF peaks were used to investigate the structure of a Pt-catalyst under oxidative conditions.[9a]

3.2.2. Real-space Rietveld refinement When the structure of the sample is known, the G(r) is often treated using real-space Rietveld[70c] analysis which conceptually corresponds to a conventional Q-space Rietveld refinement. A structural model is used to calculate G(r) which is then refined to fit an experimentally obtained PDF through the least squares method. The program suite PDFfit2[77] and PDFgui[77] is widely used for this. PDFfit2 uses a crystallographic approach, where the atomic positions are described in a unit cell, and is thus mostly applicable for crystalline materials without significant disorder, or nanocrystalline materials for which a structural model can easily be constructed. The size (and morphology) of nanoparticles is in the real-space Rietveld approach determined by applying an r-dependent particle envelope function.[78] An example of a real-space Rietveld fit to data collected during the synthesis of ceria nanoparticles is plotted in Figure 7 B. Small discrepancies are seen in the difference curve from the low-r region. These most probably arise due to surface structure, which is not included in the structural modeling. ChemSusChem 0000, 00, 1 – 19

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www.chemsuschem.org of amorphous precursors etc. For example, RMC modeling was used to study the local structure of amorphous zirconia in a solid state reaction (Figure 7 C).[80] RMC modeling may be performed in a variety of software suites including RMCprofile,[81] and RMC_POT.[82] 3.2.4. Nanoparticle modeling When working with very small nanoparticles, effects such as surface relaxation, core-shell correlations, and extended structural or surface defects dominate the scattering pattern and the corresponding PDFs.[83] To model this and extract the full nanoparticle structure, the atomic arrangements in the entire particle must be built up, atom by atom, and the effect compared to the scattering signal. This can be done either in rspace by calculating the PDF for the particle ensemble, or in qspace by applying the Debye function.[84] Both approaches can be used in the programs DISCUS[85] and DIFFEV[86] where the refinements can be done using an evolutionary algorithm.[86] This method has been applied for several nanoparticle systems,[87] for example, gold nanoparticles with organic ligands attached.[83] While this technique is yet to be used for in situ studies, it shows great promise for studies of the disordered nanoparticles forming in a hydrothermal synthesis.

4. Chemical Insight from In Situ Experiments In this section, focus will be on the type of results available through in situ powder diffraction or total scattering experiments, and how they can be quantified. For details on specific studies, readers are referred to the relevant publications. 4.1. Crystallization and phase transformation rates

Figure 7. a) Gaussian function fitting of the differential PDF obtained for oxidized Pt surface layers.[9a] b) Real space Rietveld refinement of the PDF obtained for nanocrystalline ceria. The blue line shows the calculated r-dependent particle envelope function for a coherent domain diameter of ~ 4 nm.[112] c) Reverse Monte Carlo modelling of the local range PDF for amorphous and crystalline zirconia.[80]

3.2.3. Reverse Monte Carlo analysis For extensive disordered or completely amorphous materials, real-space Rietveld methods are not always applicable. For further studies of these structures, Reverse Monte Carlo (RMC) methods may be more useful.[73–75] Here, a “large-box” approach is applied, describing individual atomic positions of a large collection of atoms. The configuration of the system is modified in successive steps according to user-provided constraints[79] until it agrees with experimental data. Although this approach is yet to be used for in situ studies of hydrothermal and solvothermal reactions, there is great potential for studies 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In situ powder diffraction is a powerful tool for studies of crystallization rates and mechanisms from solutions or crystalline precursors. By following the evolution of crystallization with time, varying for example, temperature, reactant concentration or pressure, the effect on induction times before initial crystallization of a certain phase[21a, 88] as well as crystallization rates[89] can be investigated. Furthermore, whereas ex situ studies often require lengthy parameter space studies to determine optimum conditions for obtaining a certain phase and avoid impurities, these investigations can easily be done during an in situ study in which the effect of certain parameters can be followed in real time.[33, 39, 90] Figure 8 A shows angular dispersive data obtained during CeO2 crystallization. It can be seen how CeO2 rapidly crystallizes directly from the solution. The final intensity of the CeO2 Bragg peaks is reached few minutes after the reaction was initiated. The properties of CeO2 in for example, ion conduction and catalysis are highly dependent on the crystallinity of the phase, and understanding the crystallization kinetics is thus important to optimize the synthesis parameters.[91] Figure 8 B shows ED-XRD data, illustrating the crystallization of a MOF-material, namely copper phosphonatoethanesulfonate. Here, it can be seen how the reaction takes place through a closely related crystalline intermediate phase, ChemSusChem 0000, 00, 1 – 19

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Figure 8. a) Time-resolved in situ angular dispersive diffraction data on the solvothermal synthesis of Ce0.5Zr0.5O2.[15b] b) Time-resolved in situ energy dispersive diffraction data on the solvothermal synthesis of copper phosphonatoethanesulfonates.[92]

as will be discussed further below.[92] The reaction from one phase to the next takes place rapidly, and the final crystallization happens quickly. Again, the in situ study reveals important information about the reaction mechanisms which can help deduce the optimum synthesis pathway for a phase pure, crystalline materials. Further studies of the crystallization kinetics of MOF-materials can facilitate the synthesis of new compounds to be used in gas storage or catalysis.[93] Quantitatively, the rate of crystallization of a certain phase can be determined by following the extent of crystallization as function of time. This may be done through the evolution of the scale factor refined in Rietveld analysis,[32b] or by simply following the increase in normalized integrated intensity of specific Bragg peaks.[8a] The latter approach is usually applied in ED-XRD data analysis. The integrated intensity can subsequently be used to describe the extent of reaction or crystallization as a(t) = Inorm(t) / Inorm(tinf), where tinf describes the time where the reaction is assumed complete.[21a] The resulting crystallization curves may be analyzed with different kinetic models. This includes Johnson–Mehl–Avrami kinetics, a model which was originally developed for solid-state reactions, but is now also widely used to describe solvothermal material transformations. In this approach, the extent of reaction is described via the Avrami–Erofev expression, aðtÞ ¼ 1 expð½kðt t0 Þm Þ, where k is a rate constant, t0 is an induction time and m is the Avrami exponent. Using a Sharp–Hancock plot, that is, ln½ lnð1 aÞ ¼ m lnðt t0 Þ þ m lnðkÞ, one may readily obtain the Avrami exponent, m, from the slope of the curve and the rate constant, k, may be found from intersection with the yaxis. The Avrami exponent can be used to distinguish between 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemsuschem.org different reaction mechanisms. A diffusion-controlled mechanism is indicated by values between 0.54–0.62 a zero order; first-order phase-boundary-controlled mechanism is assigned to values of 1.00–1.24, while values of 2.00–3.00 indicate a mechanism controlled by nucleation and growth. If the crystallization experiment is studied at varying temperatures T, the rate constant k may, in addition, be used to determine the crystallization activation energy Ea through the Arrhenius equation k (T) = A exp[Ea/(RT)], where A is a non-descriptive constant and R is the gas constant. The Avrami–Erofev approach has been used in numerous in situ studies of hydrothermal reactions,[22a, 28, 42, 54b, 94] with a recent example being investigations of crystallization of Bi2WO6 nanoplatelets,[54a] a process that is photocatalytic.[95] The properties of the compound are highly dependent on the size, morphology, and microstructure of the crystalline phase, and understanding the crystallization mechanisms is thus crucial. Here, Avrami–Erofev analysis was used to conclude that since the Avrami-coefficient m in all cases was close to 0.5, the reaction mechanism is diffusion- limited, and the mechanism is temperature independent. As the Sharp–Hancock plots, shown in Figure 9 A, yielded linear functions for reaction extent between 0.10 and 0.90, it was concluded that the mechanism does not change during the crystallization. When the pH value in the synthesis reactor was raised, the Avrami–Erofev coefficient increased, indicating a change of the reaction mechanism. This was accompanied by a change in sample microstructure, showing how important the crystallization kinetics is for the characteristics and properties of the final phase. For other syntheses, of for example, cobalt-containing frameworks, a change in the slope of the Sharp–Hancock-plot indicated a change in the reaction mechanism as the crystallization proceeds.[42, 94c, d, 96] Because the Avrami–Erofev expression was originally developed for solid state synthesis, it has some limitations for the studies of hydrothermal reactions. For example, it does not separate the nucleation and growth processes, which are described by a single set of parameters in the simple Avrami–Erofev formulation, and it is not always clear if the Avrami coefficient actually has physical meaning. In a study of zeolite crystallization from amorphous silica, Gualtiere therefore suggested a related model, in which the crystallization curves were modeled as:[11m] aðt Þ ¼

1 ta 1 exp kg t n 1 þ exp b

Here, a is the extent of crystallization and t is time a and b are constants related to nucleation, while kg is the rate constant of crystal growth. The parameter n expresses the dimension of crystal growth, and the value can thus help understand anisotropic crystal growth.[88b] This approach and expanded models have been used to determine activation energies for both nucleation and growth of MOFs and related materials.[21a, 88b, 97] By comparing activation energies for the separate nucleation and growth processes, the rate limiting step can be determined. Furthermore, from the constants obtained from Gualtieri analysis, the probability of nucleation can be calChemSusChem 0000, 00, 1 – 19

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www.chemsuschem.org ðtaÞ2

culated as PN = e b2 . Figure 9 B shows a plot of the extent of crystallization along with the nucleation probabilities for various temperatures obtained in the synthesis of the important MOF structure Mn-MIL-100,[98] which possesses outstanding thermal and chemical stability with permanent porosity, and is a promising catalyst and hydrogen carrier. By increasing the temperature, faster nucleation is seen with early maximum, while at later times, the crystallization process is dominated by crystal growth.[16b] In many systems, a crystalline phase will be present in the synthesis before heating is applied.[22b, 32c, 42, 94b, e, 99] The transformation mechanism to the final crystal structure can thus be studied, and the crystallization curves can be treated with similar models as presented above. An example of this is the formation of KTiOPO4 from TiO2 and KH2PO4 under supercritical conditions.[8b] KTiOPO4 is one of the best nonlinear optical materials for high-power lasers.[100] Due to the low solubility of the constituents, the hydrothermal synthesis is unusually difficult and require extreme, supercritical conditions.[101] Figure 9 C shows the extent of reaction, that is, the normalized refined scale factors as a function of time, along with the temperature in the reaction cell at a given time. Interestingly, the amount of TiO2 decreases before any KTiOPO4 has formed, indicating a dissolution–recrystallization process. As another example of phase transformations, the intercalation of Li into MnOOH to give LiMnO2 was studied by ED-XRD and neutron diffraction.[94b] The family of lithium manganese oxides are interesting cathode materials for Li-ion batteries,[102] and obtaining detailed knowledge of the various synthesis pathway is therefore highly interesting. Here, it was shown that the crystallization curve for LiMnO2 crossed with the disappearance of MnOOH at a of 0.5, indicating a direct transformation and no amorphous or crystalline intermediates.

4.2. Identification of intermediate phases

Figure 9. a) Sharp–Hancock plots for the kinetic data recorded at different temperatures over a range of 0.2 < a < 0.85 in the synthesis of Bi2WO6.[54a] b) The reaction progress (symbols) and the probability of nucleation PN (solid line) determined for the different temperatures.[16b] c) Time-resolved changes in integrated Bragg peak intensities for the TiO2 (101) and KTiOPO4 (420) reflections obtained during hydrothermal reaction at 450 8C. Lines are drawn to guide the eye.[8b]


The use of in situ techniques also allow the observation of intermediate phases, which often are difficult to isolate during an ex situ synthesis.[48, 92, 94d, e, 103] Identification of intermediate structures may help deduce reaction mechanisms, and thus get a deeper understanding of the crystallization of new phases. For example, Millange et al.[8a] showed that the metal organic framework MIL-53 crystallizes through a previously unknown intermediate phase. Quenching of the synthesis allowed the phase to be isolated for further structural characterization, and ex situ high resolution powder diffraction revealed that the intermediate phase was closely related to another MOF, MOF-235. A similar approach was applied by Feyand et al.[92] in a study of the synthesis of copper phosphonatoethanesulfonates, illustrated in Figure 8 B. Here, an unknown intermediate phase was observed by in situ ED-XRD, and isolated by quenching. Ex situ high resolution powder X-ray diffraction data subsequently allowed structural solution of the phase, and the changes in crystal structure could be understood in detail. Recently, in situ powder diffraction data from solid state processes have been used for structure solution.[104] The same ChemSusChem 0000, 00, 1 – 19

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approach can most likely be applied to studies of solvothermal synthesis in the near future. 4.3. Crystal structure evolution By Rietveld or PDF analysis of time-resolved X-ray or neutron data, the evolution of the crystal structure during the hydrothermal process can be followed. In X-ray diffraction, structural information can be extracted from angular dispersive data, although some information about unit cell and particle size can also be obtained from ED-XRD.[22b] Rietveld analysis can be employed using both ToF and monochromatic neutron data. An example of studies of structural evolution is the synthesis of LiFePO4.[13b] LiFePO4 is one of the most studied Li-ion battery cathodes and is now widely used in commercial applications.[105] However, there is still a need to develop new, cheap, and energy efficient synthesis methods of LiFePO4 to reduce the price of the final battery.[106] Here, the solvo- and hydrothermal method is promising, and in 2001, the first hydrothermal synthesis of LiFePO4 was described.[107] However, the electrochemical properties of the synthesized material were disappointing. This was later shown to be due to an anti-site defect, where Fe partly occupies the Li-site in the crystal structure.[108] The defects thus blocks the Li-ion diffusion pathways in the structure, which highly affects the capacity of the battery.[109] Using in situ powder diffraction, it was possible to characterize the defect formation in real time. From Rietveld analysis of second-scale time-resolved data, both the evolution of unit cell and defects in the olivine structure could be followed. As seen in Figure 10 A, large anisotropic changes in the unit cell parameters were observed during the synthesis. This was related to the structural disorder in the initially formed structure, and a large amount of Fe was observed at the Li site (Figure 10 B). The defect is initially present in the structure regardless of the synthesis conditions, but, with increasing synthesis time, the structure becomes ordered. The ordering occurs faster at elevated temperatures. The in situ study thus allowed optimum synthesis conditions to be identified, and gave a deeper understanding of the defect formation. Further ex situ studies resulted in a more detailed structural refinement, showing that the defects occur due to differences in the rate by which Fe and Li is incorporated into the crystal structure, and is really due to an Fe excess rather than an anti-site defect.[110] During phase transformations from one crystalline phase to another, strain and structural changes are often seen.[12b] This was quantified in a study of Li2TiO3 nanoparticle formation from a suspension of TiO2 nanoparticles in LiOH solutions. Lithium titanates are used in a number of applications, for example, Li-ion batteries, and understanding their chemistry and nanostructure is thus important.[111] Here, Avrami–Erofev analysis of the reaction mechanism was complemented by structural studies which showed that during the formation of the cubic Li2TiO3 phase, the unit cell parameters of tetragonal rutile TiO2 changed anisotropically towards the value of Li2TiO3. This observation could be explained by lithium ions being inserted into the anatase structure, but not transforming to Li2TiO3. Supported by analysis of the size of the reactant and 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 10. a) Evolution of unit cell dimensions for LiFePO4 during the hydrothermal synthesis at 170 8C. b) Fe occupancy on the Li site as function of time for temperatures varying from 170 8C to 380 8C.[13b] b) Unit cell volume dependence on crystallite size for ceria nanoparticles.[112]

product nanoparticles, it could thus be concluded that the mechanism for the phase transformation was topotactic.[42] For many nanoparticle systems, the unit cell parameter is dependent on the size of the crystalline particles.[13a, 15a, c, 38, 112] This is often related to the large ratio of surface-to-volume, as the surface structure is different from that of the bulk phase. With a combined size and structure analysis of time resolved data, it is possible to relate the two quantities. This is illustrated for CeO2 nanoparticles in Figure 10 C. As described above, CeO2 is used in a number of technological applications, for example, ion conduction and catalysis.[91] Here, the properties are highly dependent on the crystal structure and surface/bulk defects, and understanding the structure/size relation is important. A large expansion of the average lattice volume is seen for small particles, regardless of the synthesis conditions. The lattice behavior is thus intrinsic to the system. 4.4. Nanostructure Nanostructure refers to the presence of finite coherent crystalline domains. This includes small crystallite sizes, the presence of stacking faults or other deviations from the bulk crystalline state. The nanostructure is expressed in the broadening of the Bragg peaks, at described in Section 3. Working with nanocrystalline materials, one can readily obtain an estimate of the volume-averaged coherent domain sizes by analyzing the Bragg peak broadening of angular-dispersive diffraction data or decay of the PDF amplitude with ChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS distance r. It is therefore possible to, for example, follow the growth of average crystallite sizes from solution at various conditions.[7, 9b, 10a, 12b, 14, 15, 22b, 37, 90, 99, 112, 113] This is illustrated for TiO2[12c] in Figure 11 A, in which it is seen how the crystallite size is highly dependent on time and temperature. Extensive studies of TiO2 properties have shown the effect of crystallite size on both photocatalytic and electrochemical applications,[114] and being able to precisely control the size of the synthesized nanoparticles is of large importance.[115] The in situ study, performed using the lab-scale pulsed hydrothermal reactor described above, makes it possible to directly map the relation between temperature, reaction time, and nanosize, thus allowing tailor-made nanoparticles to be produced.[7] Conversely, the dissolution or amorphization of crystalline domains can also be followed. This effect was observed during the synthesis of LiCoO2,[13a] which is used as the cathode in most commercial Li-ion batteries.[116] The formation of LiCoO2 from CoOOH in LiOH was followed with PXRD, and the size analysis indicated that the size of the CoOOH particles decreased, that is, dissolved during the formation of LiCoO2. The study helped deduce the reaction mechanism for the material synthesis.[13a] In addition to average sizes, it is also possible to estimate and follow the change in the size-distribution of coherent domains by means of WPPM analysis to get a more detailed description of the evolution of crystallite sizes. The result of such

Figure 11. a) Growth curves for anatase TiO2 nanocrystals at varying temperatures.[12c] b) Size-distribution curves for growth of CeO2 nanocrystals.[112] c) LSW modelling of growth curves for SnO2.[9b, 112]


www.chemsuschem.org an analysis is shown in Figure 11 B for CeO2 nanoparticle synthesis. The plot shows that, as the synthesis proceeds, the average crystallite size increases, and the crystallite size distribution broadens. As the properties of nanomaterials are highly size dependent, characterizing the size distribution is very important to fully understand the material properties.[112] For anisotropic crystallites, the evolution of particle morphology and parameter dependence can furthermore be obtained by considering the hkl-dependence of the Bragg peak broadening.[7, 13a, 14, 15b, 42] This was done in the case of LiCoO2 where the data analysis showed that the nanocrystals forming during hydrothermal synthesis were disk-shaped, with the shortest dimension being along the crystallographic c-direction. The crystallite morphology affects the electrochemical properties as the Li-diffusion takes place in the ab-plane, and understanding how this can be altered is thus important for optimization of the particle properties. The aspect-ratio of the disk-shaped particles varied with synthesis conditions, and increasing the temperature elongated the crystals along c, thus making diffusion into the particles easier.[13a] To understand the particle growth processes further, the growth curves can be analyzed with kinetic models. The theory of solid species growing from a liquid phase has been investigated in detail for crystallization from melts. For solid species present at a solid/liquid interface, the chemical potential increases with decreasing particle size. This leads to redissolution of the smallest, newly formed particles, creating a concentration gradient in the solution. Uniformity of the concentration is re-established by material diffusion towards the larger particles, thus leading to growth of the larger particles at the expense of the smaller particles. The mechanism is known as Ostwald ripening,[117] and the process is often termed diffusion-limited growth. The full mathematical treatment of the process was done in the 1960s by Lifshitz and Slyozov,[118] followed by work by Wagner[119] and is known as LSW theory. It was shown that the diffusion-limited growth process leads to time-growth dependence of the form r3r 30 = Kt. Here, t is the time, r the particle radius, r0 the particle radius at time t = 0, and K is a constant containing the interfacial energy, the diffusion constant of the system, the average concentration of the species, and the temperature among other parameters. If the diffusion of material to the growing particles is faster than the actual reaction of the material with the particle surface, the growth is reaction-limited. In this case, it can be shown that the time dependency of the size is r2 = KRt. Even though this theory is developed for crystallization from melts, the same models are often used for other studies of crystallization in a liquid phase, for example, hydrothermal synthesis.[15b, 120, 121] Analysis of growth curves obtained from in situ Xray diffraction experiments with LSW and extended related models can therefore give indications of which step in the particle growth determines the growth rate. This was done for SnO2 nanoparticle crystallization,[9b] where it was shown that no matter the temperature, the growth was limited by dissolution. A LSW fit with the expression Dðt Þ DO ¼ k ðt t0 Þ1=x is shown in Figure 11 C, with a value of x close to 3. In the case of LiCoO2, growth analysis indicated a change in mechanism ChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS from diffusion limited to surface limited with increasing temperature.[13a] Although LSW analysis can give an indication of the growth mechanisms, the theory fails to supply any details about the further mechanism, and classical modelling is often insufficient in describing experimental observations. Much care should thus be taken when interpreting results. The LSW theory is very approximate, and for example, the chemical reactions and anisotropy of particles forming from molecular complexes is not considered. Following LSW theory, the coarsening phenomenon has been studied both theoretically and experimentally in several systems, and more complex models for particle growth have been developed by several different groups.[120, 122] Other models for particle growth, for example, oriented attachment[123] has now been reported, and, with the development of novel characterization methods, further advanced models regarding nucleation and growth of nanoparticles are likely to appear in the coming years. Information about strain in the structure also lies in the broadening of Bragg peaks. There are multiple ways of describing crystal strain, and the physical origin will often depend on the specific material type. However, for angular-dispersive diffraction, microstrain has a distinct contribution to peak profiles and, similarly to nanocrystalline sizes, it is therefore possible to estimate the change with material evolution during synthesis. By using the WPPM approach, further details on the origin of strain can be obtained.[66] For example, CeO2 nanocrystals are known to exhibit line dislocations[68a, 124] and, using in situ powder diffraction, it is possible to understand the decline in dislocation density (Figure 12 A) as nanocrystals grow from solution and mature over time under solvothermal conditions. A similar investigation can be made on the evolution of twin fault probabilities in ZnO nanocrystals as they grow from a precursor gel (Figure 12 B). ZnO, which is a wide gap semiconductor, is used for a multitude of applications, for example, in varistors, photovoltaics, catalysis and in several other technologies.[125] As the electronic properties of the compound depend directly on the structure of the material, knowing the effect of various synthesis parameters is crucial. 4.5. Precursor structures and formation mechanisms The earliest points of material formation occurring during solvothermal synthesis often involve either an amorphous gel or a solution containing ionic species or complexes. Due to the limited extent of structural order, it is not possible to investigate these structural motifs using conventional powder diffraction; this method can thus not assist in understanding the initial nucleation and crystallization processes. The first few coordination shells around specific elements can be obtained through X-ray absorption spectroscopy. Combined with powder diffraction studies, this has classically been a vital tool for investigating precursor complexes in situ.[103b] Study of the synthesis of microporous, Zn-substituted aluminophosphates showed that Zn2 + remains in a tetrahedral environment upon the ordering and crystallization of the initial amorphous gel structure.[126] Similarly, in situ EXAFS was used 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 12. a) Dislocations for ceria nanoparticles.[112] b) Twin fault probability for zinc oxide nanoparticles.

to investigate the change in local structure during the synthesis of Bi2MoO6 from an amorphous precipitate.[10b] Bismuth based mixed oxides have drawn significant attention due to their use in catalysis, as well as ion conduction in fuel cells.[127] Here, EXAFS/PXRD was used to obtain a deeper understanding of the reaction mechanism. The EXAFS data revealed that Mo transformed from non-centrosymmetric tetrahedral coordinated [MoO4] species to octahedral coordinated [MoO6] species during the formation of Bi2MoO6.(Figure 13 A) There has been a rapid growth in investigations of the local structure through the application of in situ total scattering. The advantage of total scattering and PDF analysis is that both the local- and long-range order is probed, and the whole crystallization process, from precursor complexes to nanoclusters to crystalline particles, can be studied using one single technique. An example of this is the investigation of cerium dimers present prior to the nucleation of CeO2 nanoparticles under solvothermal conditions.[112] Figure 14 A shows the PDF obtained from the precursor as well as the fitted model, whereas the refined dimer structure is seen in Figure 14 B. The time-resolved PDF analysis showed how this dimeric precursor structure condensed to crystalline CeO2 with time and temperature. In the case of SnO2 synthesis, the in situ total scattering analysis showed that the precursor, an aqueous solution of SnCl4·5 H2O, contained distinct precursor species with the majority of Sn bound in octahedrally coordinated aquachlorotin(IV) complexes, as illustrated in Figure 14 C + D. However, a second distinct complex, hexa-aquatin(IV) coexisted with the aquachlorotin(IV)-complex, and accounted for the PDF peak at 2.0 not described in the structural model. By analysis of the formation of SnO2, the PDF study showed that the crystalline particles formed uniquely from the hexa-aquatin complexes, and the formation mechanisms could be elucidated.[9b] The in situ investigations of local structures is expected to grow in intensity as the means for measuring and analyzing total scattering data is improved further.[9, 10, 22b, 75, 76, 89, 112, 128] ChemSusChem 0000, 00, 1 – 19

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Figure 13. In situ simultaneous PXD/XAS/Raman experiment on the hydrothermal synthesis of g-Bi2MoO6.[10b] a) In situ time-resolved Mo K-edge XANES measurements showing the transition of Mo from the precursor environment to crystalline Bi2n + 4MonO6(n + 1). b) Proposed transformation route deduced from insight from combined experiments.

Figure 14. Local structure investigations of precursor complexes present prior to particle nucleation. a) In situ total X-ray scattering PDF obtained for a 1 m cerium(IV) precursor solution. b) Molecular structure used to model experimental data shown in (a), (Ce: green, O: red, N: blue). c) In situ total Xray scattering PDF obtained for a 2 m tin(IV) chloride precursor solution. d) mer-aquachlorotin complex used for modelling of experimental data shown in (c).[9b, 112]

5. Summary and Outlook With the on-going development of powder diffraction synchrotron beamlines and neutron instruments around the world, the use of in situ experiments for studies of chemical reactions as they take place have grown rapidly. In the case of solvothermal syntheses of energy materials, studies of many different classes of materials are now leading to a better understanding of the fundamental processes taking place in the solvothermal reactions. By means of powder diffraction experiments, crystallization of materials can be followed on a second time-scale. As the effect of various synthesis parameters can be followed in real time, the extensive trial-and-error studies often needed in the laboratory for identifying conditions for obtaining a given material may be reduced substantially. The use of powder dif 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

fraction furthermore allows for identification of crystalline intermediate phases, and through structural analysis of these, further knowledge of the reaction mechanisms can be obtained by considering the possible rearrangements of structural units. With the development of software for structure solution using powder diffraction data, there is great promise for identification and analysis of new compounds, even from in situ data. The kinetics of the crystallization can be understood by applying models such as Avrami–Erofev expression or the more recent Gualtiere equation. The parameters obtained here can give indications of the reaction mechanisms taking place during crystallization. Further data analysis through Rietveld refinement allows detailed structural information to be extracted, even from second-scale time-resolved data. Thus, the evolution of unit cell and defect structures can be understood in real time. Analysis of the nanostructure through Rietveld or WPPM methods furthermore allows the evolution of particle size, morphology, and strain to be extracted. Apart from allowing easy identification of synthesis parameters for obtaining nanoparticles of a given size or morphology, the detailed structural and nanostructural characterization give further insight into the phase formation and growth mechanisms. The use of standard powder diffraction thus gives a wealth of information about the crystalline particles forming during the synthesis. However, to truly understand the crystallization processes, it is necessary to probe the atomic structure before crystalline particles have formed. This can be done through in situ EXAFS, which, combined with PXRD experiments, gives a broad insight into the formation process. However, the total scattering approach has recently shown great promise for in situ studies of hydrothermal reactions. Here, both the shortand long-range order is probed in one single technique, meaning that the whole formation process can be probed in detail through one experiment. This opens up a new realm of in situ studies in which the fundamental chemistry occurring in the material formation can be investigated because the precursor complexes and the nanoclusters forming before the presence of crystalline materials can be identified. With the continuous development of beamlines as well as software for total scattering data analysis, the PDF approach is expected to grow rapidChemSusChem 0000, 00, 1 – 19

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CHEMSUSCHEM MINIREVIEWS ly in the coming years. The new understanding obtained from experimental total scattering, coupled with theoretical calculations of energies of, for example, cluster structures may lead to new understanding of the particle formation and growth kinetic models. So far, studies have shown that the initial processes often involve large precursor clusters, and, to fully understand the particle formation, both the spatial restraints related to the assembly of these clusters as well as the reaction energies in the condensation have to be considered. The classical models for particle formation and growth, in which particles are assumed to grow isotropically to a spherical particle from single atoms thus cannot describe the complicated processes. Neutron studies of solvothermal reactions have so far been done for slow reactions investigated with minute- or hourscale time resolution. However, with the construction of new high flux neutron facilities such as the Spallation Neutron Source at Oak Ridge National Laboratory, USA, and the planned European Spallation Source in Lund, Sweden, new possibilities for faster neutron experiments are appearing. Both powder diffraction and total scattering experiments on the second scale will most likely become possible, meaning that neutron experiments can provide information on light elements and elements with similar atomic mass. Many new possibilities for in situ experiments and data analysis have thus arisen in recent years. The combined results of these developments brings hope that we may be one step closer to truly understanding the formation of some of the highest impact functional materials of today and the future.

Acknowledgements Espen Drath Bøjesen and Mogens Christensen are thanked for helping providing illustrative material and for general feedback. Professor Richard I. Walton, Department of Chemistry, University of Warwick is thanked for fruitful discussions on in situ experimental techniques. We gratefully acknowledge funding from the Danish National Research Foundation (DNRF93). Keywords: energy materials · hydrothermal · in situ · powder diffraction · solvothermal [1] a) M. S. Whittingham, MRS Bull. 2008, 33, 411 – 419; b) A. S. Aric, P. Bruce, B. Scrosati, J. M. Tarascon, W. Van Schalkwijk, Nat. Mater. 2005, 4, 366 – 377; c) C. G. Granqvist, Sol. Energy Mater. Sol. Cells 2007, 91, 1529 – 1598; d) C. J. Vineis, A. Shakouri, A. Majumdar, M. G. Kanatzidis, Adv. Mater. 2010, 22, 3970 – 3980; e) M. Ni, M. K. H. Leung, D. Y. C. Leung, K. Sumathy, Renewable Sustainable Energy Rev. 2007, 11, 401 – 425; f) L. Schlapbach, A. Zuttel, Nature 2001, 414, 353 – 358; g) A. B. Stambouli, E. Traversa, Renewable Sustainable Energy Rev. 2002, 6, 433 – 455; h) E. Fabbri, D. Pergolesi, E. Traversa, Chem. Soc. Rev. 2010, 39, 4355 – 4369. [2] a) R. I. Walton, Chem. Soc. Rev. 2002, 31, 230 – 238; b) K. Byrappa, T. Adschiri, Prog. Cryst. Growth Charact. Mater. 2007, 53, 117 – 166; c) M. Yoshimura, K. Byrappa, J. Mater. Sci. 2008, 43, 2085 – 2103; d) K. Namratha, K. Byrappa, Prog. Cryst. Growth Charact. Mater. 2012, 58, 14 – 42; e) T. Adschiri, Y. Hakuta, K. Arai, Ind. Eng. Chem. Res. 2000, 39, 4901 – 4907; f) C. Aymonier, A. Loppinet-Serani, H. Revern, Y. Garrabos, F. Cansell, J. Supercrit. Fluids 2006, 38, 242 – 251; g) D. R. Modeshia, R. I. Walton, Chem. Soc. Rev. 2010, 39, 4303 – 4325.


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Received: September 30, 2013 Revised: October 20, 2013 Published online on && &&, 0000

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MINIREVIEWS In the thick of it: In situ X-ray and neutron studies of solvothermal and hydrothermal reactions can yield new information on the synthesis of energy material and map the structure–synthesis relationship. Various approaches to in situ powder diffraction and total scattering are reviewed. This review discusses experimental methods as well as strategies for data analysis and highlights the chemical insights that can be obtained from in situ experiments.


K. M. Ø. Jensen, C. Tyrsted, M. Bremholm, B. B. Iversen* && – && In Situ Studies of Solvothermal Synthesis of Energy Materials

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Solvothermal synthesis of functional magnetic nanoparticles for biocatalyst.

Synthesis of Antimony Nanotubes via Facile Template-Free Solvothermal Reactions.

Large-scale solvothermal synthesis of fluorescent carbon nanoparticles.

Microwave solvothermal synthesis and characterization of manganese-doped ZnO nanoparticles.

WO₃ Nanocomposites.

Theory of solar energy materials.

Real-time powder diffraction studies of energy materials under non-equilibrium conditions.

Calorimetric Studies and Structural Aspects of Ionic Liquids in Designing Sorption Materials for Thermal Energy Storage.

Environmental efficiency of energy, materials, and emissions.

Solvothermal synthesis of orthorhombic Sb2WO6 hierarchical structures and their visible-light-driven photocatalytic activity.

Microwave-assisted solvothermal synthesis and optical properties of tagged MIL-140A metal-organic frameworks.

Facile and low temperature route to synthesis of CuS nanostructure in mesoporous material by solvothermal method.

Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields.

Solvothermal synthesis of CdIn2S4 photocatalyst for selective photosynthesis of organic aromatic compounds under visible light.

Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis.

A facile one-step solvothermal synthesis of bismuth phosphate-graphene nanocomposites with enhanced photocatalytic activity.

One-Pot Solvothermal Synthesis of Highly Emissive, Sodium-Codoped, LaF₃ and BaLaF₅ Core-Shell Upconverting Nanocrystals.

Synthesis of hierarchically structured ZnO nanomaterials via a supercritical assisted solvothermal process.

Solvothermal synthesis of gallium-indium-zinc-oxide nanoparticles for electrolyte-gated transistors.

Solvothermal synthesis and controlled self-assembly of monodisperse titanium-based perovskite colloidal nanocrystals.

Er3+ nanocrystals.

Microwave-assisted solvothermal synthesis of spinel AV2O4 (M = Mg, Mn, Fe, and Co).

Climate and energy challenges for materials science.

Ionic liquids for energy, materials, and medicine.