Mechanisms for Tungsten Oxide Nanoparticle Formation in

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Mechanisms for Tungsten Oxide Nanoparticle Formation in Solvothermal Synthesis: From Polyoxometalates to Crystalline Materials Mikkel Juelsholt, Troels Lindahl Christiansen, and Kirsten Marie Ørnsbjerg Jensen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12395 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry

Mechanisms for Tungsten Oxide Nanoparticle Formation in Solvothermal Synthesis: From Polyoxometalates to Crystalline Materials Mikkel Juelsholt, Troels Lindahl Christiansen and Kirsten M. Ø. Jensen* Department of Chemistry and Nanoscience Center, University of Copenhagen *[email protected] Abstract Understanding nucleation mechanisms of the solid state on an atomic scale is crucial in order to develop new synthesis methods for tailored materials. Here, we use in situ x-ray total scattering to follow the structural rearrangements that take place in the formation of tungsten oxide, all the way from the ionic precursor solution to the final crystalline nanoparticles. The reaction was performed in water and oleylamine to study the influence of solvent, and in both cases the clusters present in the precursor solution adopt the well-known -Keggin polyoxometalate structure. However, despite the similarity between precursor cluster and the final crystallographic phase, the reaction route is highly dependent on the solvent, shedding new light on nucleation mechanisms and their influence of defects in the final oxide structure. In water, the precursor cluster partly rearranges to the Tungstate Y cluster before crystallization of tungsten bronze nanoparticles with a large degree of [WO6] disorder along the c direction of the unit cell. In oleylamine, the reaction goes through several steps including an amorphous phase and an intermediate crystalline pyrochlore phase before forming small, ordered tungsten bronze nanoparticles. The solvent does thus not only affect the crystallite size, but also the atomic structure of the nanoparticles, which we link to the observed reaction mechanism.

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Introduction Solvothermal synthesis has proven to be a versatile and efficient method for producing a wide variety of functional materials.1-4 Complex materials with advanced functionalities can be prepared using simple methods, and material characteristics such as crystallite size and shape can often be controlled through tuning of the synthesis parameters. However, to a large extent, the development of synthesis strategies of new materials still rely on trial-and-error experiments and large parameter studies.5-6 The demand for increased performance and new material applications require a mechanistic approach, where synthesis methods for advanced materials with tailored structures are developed based on the atomistic, chemical mechanisms responsible for material formation. In classical nucleation theory, only thermodynamic considerations of the bulk energy of the solid and the interfacial energy between the solid and liquid phases are taken into account. However, with the development of non-classical nucleation theory, it is now clear that the interactions between clusters, aggregates and molecules present before and during crystallization play an important role for the formed material.5, 7-8 In this study, we set out to investigate the effect of solvent on the chemical pathway and the final product in the solvothermal synthesis of tungsten oxide nanoparticles. Tungsten oxide shows extremely rich structural chemistry, forming solid state bulk structures with stoichiometries between WO2 and WO3, which holds potential for applications in e.g. gas sensors, electrochromics, supercapacitors, and catalysis.9-10 WO2 is known in two forms, an orthorhombic phase and a monoclinic distorted rutile structure.11-12 The phase diagram of bulk WO3 is considerably more complex, and consist of 6 perovskite-like phases, where the room temperature structure is the monoclinic γ-WO3.10 A hexagonal WO3 structure also exist13 which is able to host a number of ions in the form of MxWO3 crystal structures,14-15 known as tungsten bronzes, illustrated in Figure 1A. In addition to its large structural complexity in the solid state,


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tungsten oxido clusters in solution also show rich structural chemistry where large polyoxometalates (POMs) are formed.16 The POMs are a family of well-defined polyanions whose structures are sensitive to the chemical environment, such as pH, solvent, temperature, etc.16-18 Examples of POMs ion structures are shown in Figure 1C-F.

Figure 1: The six structures used for data modelling in this study. In all structures, W is shown in blue, O in red, and N in white. H is omitted for clarity. (A) The hexagonal ammonium tungsten bronze, (NH4)0.25WO3. (B) The pyrochlore structure WO30.5H2O. (C) The metatungstate cluster, W12O40 with the -Keggin structure. (D) The paratungstate cluster, W12O42. (E) The Tungstate Y cluster, W10O32. (F) The tungstate X cluster with the -Keggin structure.


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In the synthesis of tungsten oxides, POMs can act as precursors for material formation, and their well-defined structure make them ideal model systems for studying the relation between the structure of the solution and the final crystalline phase in solvothermal synthesis.19-22 Here, we apply in situ x-ray total scattering and powder diffraction to study the solvothermal formation of tungsten bronze nanoparticles from POM ions in solution. X-ray Total Scattering (TS) with Pair Distribution Function (PDF) analysis has proven an excellent technique for studying the formation of materials, as it allows following atomic structure all the way from ionic clusters in solution over amorphous intermediates to the final crystalline phase.5, 7, 23-30 By combining in situ XRD and in situ PDF we can study both the crystalline phases and cluster species formed during solvothermal reactions. Saha et al has previously studied the formation of hexagonal tungsten oxide nanoparticles showing that the reaction rate determine the crystal structure of the formed nanoparticles.20 Here, we investigate the effect of the solvent, using water and oleylamine and do a detailed investigation of the POM structure and rearrangements during synthesis. Water is widely used as solvent for synthesis of metal oxides, including tungsten oxides.31 More recently. oleylamine has become a common, cheap and effective solvent for size-controlled nanoparticle synthesis, where it acts as both solvent and capping agent, limiting the particle growth.32 The large differences between the solvents in terms of e.g. polarity and chemical reactivity are likely to influence the mechanisms involved in the nanoparticle formation. In both solvents, nanoparticles with the hexagonal ammonium tungsten bronze structure form. Despite obtaining the same crystalline phase, we observe large differences in the chemical pathway leading to the final product. In both solvents, we initially observe how ammonium metatungstate hydrate (AMT) dissolves into -Keggin-structured clusters in solution. In water, these clusters slowly rearrange to Tungstate Y, followed by crystallization into tungsten bronze nanoparticles. The particles


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formed in water contain a large degree of [WO6] disorder along the crystallographic c axis. In oleylamine, the metatungstate clusters transform into an amorphous phase with local structure resembling the paratungstate cluster. Moments before crystallization, this phase breaks up, again forming isolated clusters related to the -Keggin structure which crystallizes into small pyrochlore WO30.5H2O nanoparticles. Upon continued heating, the pyrochlore nanoparticles transform into the hexagonal tungsten bronze structure. Surprisingly, the nanoparticles formed in oleylamine do not contain clear disorder along c, showing the effect of reaction mechanisms on the defect chemistry of the final nanoparticles.

Experimental methods The tungsten oxide syntheses were performed using a 0.05 M ammonium metatungstate hydrate (99.99 %, Sigma-Aldrich) solution. In water, the AMT completely dissolved, and the precursor appeared as a clear solution. In oleylamine (70%, analytical grade, Sigma-Aldrich, containing >98% primary amines) the AMT and oleylamine form a homogenous white slurry upon rigorous stirring. The solutions were used for synthesis in our custom-made setup for in situ studies, which is similar in design to that described by Becker, et al.

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The precursor is injected into the reactor, which

consists of a thin fused silica tube measuring 0.7 mm in inner diameter and 0.09 mm in wall thickness, ensuring a high transmission of x-rays. The tube is pressurized with the appropriate solvent using a HPLC pump and heated using a jet of hot air coming from below the sample. At the same time as the experiment is initiated by turning on the heating, sequential x-ray exposures are started. For all experiments, the tube was pressurized to 100 bar, while the synthesis temperature (as measured in the capillary) was varied between 250 oC and 300 oC.


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All in situ experiments were performed at beamline P02.1, PETRA III, DESY, Germany using a wavelength of  = 0.2072 Å. The total scattering data were obtained applying the RA-PDF setup, where the large 2D detector was placed close to the sample.34 The 2D data were integrated in Fit2D35-36 and subsequently Fourier transformed to the PDF in PDFgetX337 using the Q-range up to 16 Å-1. Background correction was done by subtracting the scattering signal obtained from an identical experiment performed without AMT. Analysis of the non-crystalline structures was performed using Diffpy-CMI,38 while PDF analysis of the crystalline structures was done using PDFgui.39 Rietveld refinements were done in Fullprof Suite,40 with further details on the modelling given in the supplementary information.

Results and discussion We start by considering in situ XRD data of the formation of tungsten oxide in water to follow the formation of the crystalline phases in the reaction. Figure 2 shows the time resolved data plotted as a contour plot. In water (Figure 2A), the XRD data show that crystalline particles form after ca. 6 minutes at 300oC as seen from the appearance of clear Bragg peaks. In oleylamine (Figure 2B), weak Bragg peaks are seen already after 2 minutes, although these are much harder to distinguish due to peak broadening and the intense background scattering from oleylamine.


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Figure 2: In situ time-resolved XRD data from the solvothermal reaction of AMT in water, (A), and in oleylamine (B) at 300 oC

The XRD data were treated through Rietveld refinement. A representative fit to the data obtained from the synthesis in water is seen in Figure 3A. As shown in Figure S1-S3, the final, crystalline phase obtained from synthesis in water is best described by the hexagonal tungsten bronze structure, (NH4)0.25WO3, whose structure is illustrated in Figure 1A.41 Hexagonal tungsten oxide phases are formed of six-membered rings of [WO6]-octahedra stacked upon each other in the a/b plane, forming columns along c throughout the structure. The tungsten bronze phases have been reported in a large number of space groups and structures with the only significant difference being the intercalated cation hosted in the hexagonal rings. From the current synthesis, NH4+ is


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expected to be intercalated from the AMT precursor salt, although we cannot exclude the presence of other molecules in the channels (e.g. water) from the Rietveld refinements alone.

Figure 3: (A) Rietveld refinement of the XRD pattern obtained from the nanoparticles formed in water after 13 min at 300 oC with the hexagonal tungsten bronze structure shown in Figure 1A. (B) The XRD data obtained from the particles formed in oleylamine. The insert shows the same data, but with the oleylamine signal subtracted. The visible peaks can be indexed to the pyrochlore and bronze structures seen in Figure 1B and Figure 1A, respectively.

Using the same structural model, we performed sequential Rietveld refinements on the in situ XRD data obtained from water, allowing to extract time-resolved structural parameters from the tungsten oxide phase, as described in detail in Table S4 and Figure S4. The growth curve obtained from the


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refinements is illustrated in Figure 4A, where the refined crystallite size is plotted against time for the experiment performed at 300oC. In the first frame with the presence of significant Bragg peaks allowing for Rietveld refinement, the crystallite size refines to ca. 20 nm, growing to around 70 nm during the synthesis process. The crystallite growth follows the evolution of the refined scale factor (Figure 4A, right) indicating that the crystallite growth happens from more material nucleating to the solid phase. As the particles grow, the lattice parameter increases along c while staying almost constant along a, as seen in Figure 4B. In order to fit peak intensities, the DebyeWaller factors for the W sites were refined anisotropically, and the resulting Debye-Waller factor along c is almost double the value obtained along a/b (Figure 4C), indicating significant stacking disorder along c in the layered structure.


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Figure 4: (A) Evolution of the crystallite size and the normalized scale factor obtained from Rietveld refinement of data obtained from synthesis in water at 300oC. (B) The relative expansion of the unit cell. (C) The anisotropic Debye-Waller factor along a (B11) and c (B33).

The broad Bragg peaks from the small tungsten oxide nanoparticles formed in oleylamine are almost completely hidden below the oleylamine scattering signal, as seen in Figure 2B and Figure 3B. By subtracting the background signal, two crystalline phases can be identified, as illustrated in the insert in Figure 3B. The two phases are identified as the hexagonal tungsten bronze (as also


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synthesized from water) and pyrochlore WO30.5H2O.42 The broad Bragg peaks from both phases show that the crystallites are significantly smaller than those synthesized in water. However, the large background and the broad peaks make it challenging to extract quantitative information from Rietveld refinement, and we must therefore apply other methods for further structural characterization.

X-ray diffraction with Rietveld refinement only provides information on the crystalline structures present in a sample. In order to follow the structural rearrangements taking place in solution before crystallization, we turn to in situ PDF. Figure 5 shows the time resolved PDFs obtained from the synthesis in water (Figure 5A-C) and oleylamine (Figure 5D-F), while the corresponding Q-space data are shown in Figure S6. We consider first the time resolved PDF obtained from the synthesis in water. Before crystallization, a structure with well-defined peaks to ca. 7 Å is seen, which persist until sudden crystallization after ca. 10 minutes. Note that this is 3 minutes later than in the corresponding XRD experiment, however this difference simply comes from slightly different heating rates in the experiments. To describe the structure of the precursor in solution, we first look to the atomic arrangement in the crystalline precursor, ammonium metatungstate, AMT. The structure of AMT has never been solved from single crystal diffraction, but according to spectroscopic studies it should adopt the structure of the -Keggin POM ion in water.43-44 Figure 6B shows the PDF obtained from the beginning of the reaction modelled using an isolated tungsten based

-Keggin

W12O40

cluster,

obtained

from

the

structure

of

crystalline

Cs5(Cr3O(OOCH)6(H2O)3)(CoW12O40)(H2O)2.45 The isolated cluster is shown in 6A. The Keggin structures gives a good description of the clusters, and as we show in Figure S5, PDF is generally well suited to distinguish between POM clusters. The two intense peaks at 3.28 Å and


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3.72 Å correspond to W-W distances between edge-sharing and corner-sharing [WO6] octahedra present in the Keggin structure, as illustrated in Figure 6A.

Figure 5 (A) Time-resolved PDFs from the synthesis in water. (B) A zoom of (A) showing the local structure just before crystallization. (C) Single PDFs after 2 s, 4 and 13.8 min from the synthesis in water. (D) Time-resolved PDFs from the synthesis in oleylamine. (E) A zoom of (D) showing the local structure just before crystallization. (F) Single PDFs obtained from the synthesis in oleylamine, after 6 s, 4 min and 13.8 min.


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Figure 6: (A) The -Keggin cluster with the edge- (green) and the corner-sharing (red) W-W distances indicated. (B) Refinement of the -Keggin cluster to the PDF obtained after 1 second in water. The arrows indicate W-W distances corresponding to the edge- (green) and the cornersharing (red) [WO6] octahedra. (C) A fit of the Tungstate Y and the -Keggin to the data obtained after 4 min. (D) Refined phase fractions of the -Keggin and Tungstate Y clusters present in solution before crystallization.


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Upon heating, the PDF peak at 3.3 Å, corresponding to the W-W distance between edge-sharing [WO6] octahedra, becomes significantly more intense than the W-W peak arising from cornersharing [WO6] octahedra as seen in Figure 5B-C. This indicates structural rearrangements of the clusters in the solution. It has previously been reported that the metatungstate ion ([H2W12O40]6-) in water is in equilibrium with other tungstate POM ions, including Tungstate X and Tungstate Y clusters.16, 43, 46-47 Tungstate X adopts the -Keggin structure (Figure 1F) and Tungstate Y (Figure 1E) is closely related to the Lindquist cluster.16, 43-44 Both contain more edge-sharing octahedra than the metatungstate ion. By fitting to the PDF, we see that only a combination of the -Keggin and Tungstate Y clusters can describe the data in all frames. A representative fit with -Keggin and Tungstate Y are seen in Figure 6C while fits using the -Keggin and Tungstate X are plotted in Figure S7. It should be noted that the Keggin cluster has 3 additional isomers, called ,  and Keggin. They are less stable and have not been observed in water but do contain increasingly more edge-sharing octahedra16, 43, 47-51. We cannot exclude that more of these isomers or other clusters are present in solution, but the simplest model that describes the data is combination of metatungstate and Tungstate Y. Figure 6C show results from fits including both the -Keggin and Tungstate Y cluster. Initially, no Tungstate Y is observed, and after heating, up to a third of clusters in solution take the Tungstate Y structure.

The XRD experiment described above revealed that the resulting phase from the experiment is the hexagonal tungsten bronze. However, the PDFs clearly reveal a very high degree of local structural disorder. In Figure 7 we compare the experimental PDF obtained in the end of the experiment with a calculated PDF of the bulk bronze structure. We also show a fit of the bronze structure to the PDF, both in the local range (1-15 Å) and in to the long-range order (15-60 Å). While the structure


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fits well to the long-range order, the local range is rather poorly described. If comparing the calculated and experimental PDFs in Figure 7A, we see that the first W-O peak at 1.8 Å is highly asymmetric, and that the second W-O peak at 4.2 Å, which is a sharp peak in the calculated PDF, is seen as a wide peak in the experimental PDF, indicating a large distribution of W-O distances in the crystal structure. The modelling of the PDF in Figure 7B confirms the oxygen disorder, but also reveal a misfit with the bulk crystal structure at higher r at approximately 5, 6 and 12 Å, corresponding to W-W distances in the c direction in the unit cell. What should be a single peak at ca. 5 Å according to the bulk model is in the experimental PDF split in two, while the 2 peaks just below and above 6 Å are merged to a single peak. The experimental PDF also show that the intensity in the peaks around 12 Å has shifted. As the Rietveld refinements indicated disorder along c, which we described in the model through anisotropic Debye-Waller factors, we refined the W B-factor anisotropic in the PDF refinements. The results from this is shown in Figure S8 and Table S5 in the SI. As in the Q-space Rietveld refinement, the W B33 parameters refined to much larger values than those in the a/b plane, confirming the structural disorder in the c direction. From this, we attempted to allow breaking of the space group symmetry and refine the zW fractional coordinates in the unit cell, adding 6 new parameters to the model. This lowered the Rwp value from 30.1% to 13.9% and gave a much better fit describing the local disorder. The effect on the structure model is seen in Figure 7E along with the final fit in Figure 7D. 4 out of 6 W atoms in the unit cell are moved from their crystallographic position along c by ca. 0.4 Å, breaking the 6fold rotational axis, while the last 2 is only moved by ca. 0.07 Å. We interpret this result as a large degree of [WO6] disorder in the layers, perpendicular to c. Rather than sitting in ordered layers in the a/b plane, a distribution of tungsten positions is seen along c while still keeping the local corner-sharing [WO6] coordination.


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Results from the sequential PDF modelling is shown in the supplementary information in Table S6-S7 and Figure S9-S10. The displacement of the W atoms along c is present throughout the entire reaction, suggesting that the disorder appears as the nanoparticles form.

Figure 7: A) A comparison between the nanoparticles formed in water after 30 min and the theoretical PDF of the bronze structure, which reveal great local oxygen disorder. (B) A realspace Rietveld refinement of the bronze structure to the data in (A). (C) Same fit as in (C) but over the full r-range, showing how we can describe the long-range structure, but not the local structure due to W- and O disorder. (D) Real-space Rietveld refinement from 3.3 Å to 15 Å where the W in the bronze structure has been allowed to move along c. (E) The structure obtained from the fit in (D). The local disorder is caused by the displacement of 4 W atoms along c. (F) The structure in (E) seen from the top.


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The contour plot of the PDF obtained from the particles formed in oleylamine at 300oC is seen in Figure 5D-F. Due to fast kinetics, an additional experiment was performed at 250oC, with the contour plot shown in Figure 8A-B. The corresponding Q-space data are shown in Figure S11. As described further in the SI, the scattering pattern from the oleylamine appears to change upon addition of AMT which makes background subtraction challenging. This affects the oleylamine data and all PDFs obtained from oleylamine contain a large peak at 1.5 Å and a minor peak at 2.5 Å corresponding to the first and second C-C distance in the organic oleylamine. However, the PDFs still allow analyzing the structures present in the solution. Similar to the AMT solution in water, AMT forms an -Keggin-structured cluster in oleylamine, with a fit to the initial structure seen in red in Figure 8C, bottom. From a macroscopic point of view, the AMT and oleylamine form a white slurry, but looking closer through PDF, the AMT is dissolved in the oleylamine as there are no peaks beyond 9 Å.


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Figure 8: (A) Time-resolved PDFs from the formation of nanoparticles in oleylamine at 250 degrees. (B) Data from the first 7 minutes, showing the cluster transformations and crystallization. The line highlights the W-W distance at 6 Å common the -Keggin, pyrochlore and bronze structures, but not present in the paratungstate cluster. (C) Examples of refinements of the paratungstate and -Keggin to the PDF obtained after 2 s and 4.1 min. After 2 s, only the -Keggin is present, while after 4.1 min only the amorphous paratungstate compound. The insert shows the long-range structure formed by the paratungstate clusters also after 4.1 min.


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The PDFs show a completely different cluster behavior in oleylamine compared to water. Here, the metatungstate clusters transform into a structure which is locally similar to paratungstate, another common POM. This transformation is apparent in the data through the appearance of the third W-W peak at 5.75 Å (Figure 8B), which is much shorter than the corresponding W-W distance of 6 Å in the -Keggin structure. Simultaneously, broad peaks appear in the PDF up to approximately 40 Å (Figure 8C), which is characteristic of the presence of an amorphous compound. In the last 20 seconds before crystallization, the broad, long-range features disappear, and the structure becomes again almost identical to the -Keggin, as illustrated in Figure 9A where we compare the first PDF from the precursor with a PDF obtained just before crystallization. The comparison of the two PDFs reveal that the only significant difference is the decrease in intensity of the peaks above 5 Å, which indicates that the final, non-crystalline phase is similar in structure to a -Keggin cluster that is breaking up. This structure, which can be considered a pre-nucleation cluster, then forms a crystalline phase. The cluster transformations from metatungstate to paratungstate and back at 250oC are shown in Figure 9B, where results from two-phase cluster refinements are illustrated. Representative cluster structure fits and results from the transformation at 300oC are shown in Figure S13-S14. The first crystalline phase forming in oleylamine is the pyrochlore structure, whose structure is illustrated in Figure 1B. As seen from the refined weight fractions in Figure 9C, the hexagonal tungsten oxide bronze structure subsequently appears as the nanoparticles undergo a phase change as they grow. Details on the refinements are given in Table S8-S11 and Figure S15. The bronze phase formed from oleylamine does not contain the local distortion observed in the nanoparticles formed in water, as the W-W peak at 5 Å is not split in 2, and the W-W peak at 12


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Å has the expected intensity, as illustrated in Figure S16 with an expansion of the local region of the PDF plotted in Figure 9D. However, Figure S16 also shows how oxygen disorder is still present in the nanoparticles formed in oleylamine. Firstly, the peak corresponding to the second W-O distance in the structure is very broad and is hidden beneath the first W-W peaks, and secondly, the first W-O peak is highly asymmetric. The asymmetry is worsened by the C-C peak present due to the challenges in background subtraction as discussed above.


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Figure 9: (A) Comparison between the first frame and the frame just before crystallization of the reaction in oleylamine at 250 degrees. (B) The relative amount of metatungstate and paratungstate through the reaction at 250 degrees in oleylamine. (C) Weight fraction of the two structures as the reaction proceeds. (D) Real-space Rietveld refinement of the bronze and


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pyrochlore structures to the PDF obtained from the nanoparticles formed in oleylamine after 34 min. An expansion of the fit to the local structure is seen in Figure S16.

The presence of the short lived Keggin-like cluster just before crystallization in the oleylamine synthesis provides new hints to the reaction mechanism for the formation of the hexagonal bronze structure. As indicated in Figure 8B, the paratungstate structure contains a PDF peak at 5.75 Å which is not present in the subsequently formed crystalline phases. In order to match the distances in the crystalline phases, the WO6-octahedra of the paratungstate cluster must therefore undergo significant restructuring. The -Keggin-like intermediate is likely formed in order to match the local structure of the crystalline phases before crystallization compared to the different structural environment of the paratungstate cluster. The presence of the -Keggin-like cluster formed just prior to crystallization is resembling the behavior and function of the pre-nucleation clusters reported and discussed over the last decade. 52-54 In the formation of the bronze phase, the edge-sharing WO6-octahedra in the pre-nucleation cluster must furthermore change their configuration to corner-sharing. The intermediate pyrochlore phase is likely formed in order to compensate for a mismatch between the octahedra arrangements during this process. As in the POM clusters, the octahedra in the pyrochlore are tilted with respect their neighboring octahedra compared to the bronze structure, where layers of wellaligned [WO6] octahedra build up the structure. The many steps present in the formation of the bronze structure in oleylamine may be responsible for the lack of disorder in the layers along c. The slower nucleation through several intermediates enable further possibilities for structural rearrangements, while in water, the burst nucleation leaves much less time for structural ordering.


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The highly different reaction mechanism is linked to the solvent properties. The restructuring of POMs in solution is known to be strongly dependent on the polarity of the solvent, which thus influences the formation of tungstate Y and paratungstate in the water and oleylamine synthesis, respectively.43 The solvent-dependent nucleation mechanisms illustrate the importance of understanding the solvent/POM interactions in solvothermal synthesis. The influence of solvent on resulting crystallite size has over the past years been studied in detail, however the effect on atomic structure is much less clear. By here relating the structural rearrangements in the POMs before crystallization to the defects in the crystal structure, we are getting one step closer to be able to fully control atomic structure in materials through synthesis design.

The reaction mechanisms appear unaffected by temperature, which only affects the kinetics of the reaction and the resulting crystallite sizes. The crystallization rate is significantly higher at 300oC compared to 250oC. The first nanoparticles detected at 300 oC already contain 75% of the bronze phase as seen in Figure S15, and only small changes in the phase fractions are observed as the reaction continues. At 250oC the pyrochlore phase forms first and then rapidly transforms into the bronze phase. As the phase fraction of the bronze structure approaches 60 %, the transformation rate is reduced significantly. The synthesis temperature also has a large impact on the crystallite size. The growth curves of the pyrochlore and bronze phases from the two experiments performed in oleylamine are shown in Figure S15. At the highest temperature of 300 oC, the bronze crystallite size is refined to 3 nm at the beginning of the reaction and grows to ca. 6 nm in the end of the experiment. At 250 oC, the initial crystallite size of the bronze nanoparticles is refined to 6 nm and ends at approximately 15 nm, i.e. somewhat larger than those synthesized at higher temperatures. In all cases, the pyrochlore


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crystallites are smaller than the co-existing bronze particles and appear to decrease in size as the reaction proceeds. As tungsten oxide nanoparticles often are anisotropic forming either platelets or rods, the particles, especially those formed at 300oC, are likely larger along at least one direction.10

Conclusion In situ PXRD and PDF studies have shown the large effect of solvent on the formation mechanism of tungsten oxide nanoparticles from ammonium metatungstate hydrate solutions. The solvent, here water and oleylamine, does not only affect the crystallite size, but also the reaction pathway, as well as the final crystal structure of the nanoparticles. The reaction pathways are summarized in Figure 10. In water, the crystalline metatungstate precursor salts dissolves and forms a solution of -Keggin structured clusters, which upon heating enter an equilibrium with the Tungstate Y cluster. From this solution, a hexagonal tungsten bronze, (NH4)0.25WO3, is formed and grows into nanoparticles of ca. 70 nm. The particles contain a high degree of local disorder, as [WO6] displacement is seen along the c direction of the unit cell. In oleylamine, metatungstate also dissolves to form -Keggin-structured clusters, but this is transformed into an amorphous phase with local structure similar to the paratungstate cluster. Moments before crystallization the amorphous phase breaks up and an -Keggin-like structure is again observed which crystalizes into a cubic pyrochlore structure WO30.5H2O. The nanoparticles then undergo a phase change into the hexagonal tungsten oxide bronze structure as seen in the synthesis in water, however, showing no clear structural disorder on the tungsten sites. Thus, the use of oleylamine compared to water as solvent not only affects the crystallite size, but also the atomic structure of the nanoparticles. The lack of stacking disorder is likely linked to the reaction pathway, where the


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prenucleation clusters formed in oleylamine and the formation of the intermediate pyrochlore phase allows structural relaxation. The study underpins the need for in-depth knowledge of reaction mechanisms in solid state chemistry for development of tailored nanomaterials.

Figure 10: The formation pathway for the solvothermal synthesis of hexagonal tungsten bronze nanoparticles on water and oleylamine. The equilibrium between the cluster prior to crystallization determines the pathway and the defects in the structure.


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Supplementary information Descriptions of all models, representative fits and results from modelling of time resolved data.

Acknowledgements We are grateful to the Villum Foundation for financial support through a Villum Young Investigator grant. K.M. Ø. J furthermore thanks the Carlsberg Foundation for financial support, and M.J. acknowledges the Siemens Foundation for support for his thesis project. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at beamline P02.1 at PETRAIII and we would like to thank Martin Etter for assistance in using the beamline.

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Author Biography for Young Investigator VSI Kirsten M. Ø. Jensen obtained her Ph.D in 2013 from Aarhus University under supervision of Prof. Bo B. Iversen in Center for Material Crystallography. She then spent two years as Postdoctoral Fellow at Department of Applied Physics and Applied Mathematics at Columbia University in New York working with Prof. Simon J. L. Billinge. In 2015 she became assistant professor at Department of Chemistry, University of Copenhagen, where she leads the Nanostructure Group, focusing on using X-ray and neutron scattering techniques to study the atomic structure of nanomaterials.


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Mechanisms for Tungsten Oxide Nanoparticle Formation in

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