Size Induced Structural Changes in Molybdenum Oxide Nanoparticles

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Size Induced Structural Changes in Molybdenum Oxide Nanoparticles Troels Lindahl Christiansen, Espen D. Bøjesen, Mikkel Juelsholt, Joanne Etheridge, and Kirsten M. Ø. Jensen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b01367 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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Size Induced Structural Changes in Molybdenum Oxide Nanoparticles Troels Lindahl Christiansen1, Espen D. Bøjesen2,3, Mikkel Juelsholt1, Joanne Etheridge2,3 and Kirsten M.Ø. Jensen1* 1: Department of Chemistry and Nanoscience Center, University of Copenhagen, 2100 Copenhagen Ø, Denmark 2: Monash Centre for Electron Microscopy, Monash University, Victoria 3800, Australia 3. Department of Material Science and Engineering, Monash University, Victoria 3800, Australia Correspondence email: [email protected]

Abstract Nanosizing of metal oxide particles is a common strategy for improving materials properties, however, small particles often take structures different from the bulk material. MoO2 nanoparticles show a structure that is distinct from the bulk distorted rutile structure, and which has not yet been determined. Here, we present a model for nanostructured MoO2 obtained through detailed atomic pair distribution function analysis combined with highresolution electron microscopy. Defects occur in the arrangement of [MoO6] octahedra, in both large (40 – 100 nm) nanoparticles, where the overall distorted rutile structure is preserved, and in small nanoparticles (< 5 nm), where a new nanostructure is formed. The study provides a piece in the puzzle of understanding the structure/properties relationship of molybdenum oxides, and further our understanding of the origin of structural changes taking place upon nanosizing in oxide materials.

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KEYWORDS: nanostructures, nanoparticles, structural characterization, total scattering, pair distribution function analysis, high resolution electron microscopy, molybdenum oxide

Nanomaterials have over the last decades garnered an increasing amount of interest as chemists have learned to exploit their properties via control of size, morphology, and composition with atomic scale precision through the development of tailored synthesis methods.1-4 The atomic structure of nanomaterials is often assumed to be simple cut-outs of the structure of the corresponding bulk materials, where only small structural changes, e.g. surface relaxation and lattice expansion or contraction separate them from the bulk. It has recently been observed that this can be far from the truth - nanoscaling can lead to significant changes to the atomic structure.5, 6 However, in spite of the enormous amount of nanoparticle syntheses reported in the literature, only a few examples can be found where size-dependent structural motifs are examined and thoroughly characterized.7-9 The lack of knowledge on structural changes taking place when nanosizing can be attributed to the difficulty in characterizing the atomic structure of nanostructures. The small particle sizes challenge current conventional crystallographic techniques for determining atomic structure due to the broad and diffuse appearance of their diffraction pattern.10 This ‘nanostructure problem’ can be overcome by applying Total Scattering (TS) with Pair Distribution Function (PDF) analysis8, 11, 12 in combination with high resolution electron microscopy. PDF analysis enables structural investigations of e.g. amorphous, disordered and nanostructured materials by taking into account the full scattering signal from the sample rather than just considering Bragg scattering.13 Ab initio structural solution from PDF has been shown to be feasible for structures with high symmetry,11 and in ultrasmall, well-defined metallic particles, the existence of known structural arrangements can help to deduce particle structures.5, 8, 9 However, for more disordered materials, accurate structure solution remains elusive. Nevertheless, the intuitive nature of the PDF allows identification of structural motifs, building on knowledge from e.g. conventional crystallographic studies of bulk materials.1416

Here, nanoparticles of MoO2 in sizes ranging from >5 nm to 50-100 nm are examined to obtain an

understanding of the effect nanosizing has on MoO2. Using PDF analysis combined with high resolution


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scanning transmission electron microscopy (HR-STEM), we are able to characterize structural motifs induced by the small size and give a full description of the nanostructure. Studying the nanostructure of MoO2 is particularly interesting, as in recent years several reports on nanosized MoO217-21 have been published concerning e.g. their potential as Li-ion battery anodes. General improvements in materials performance have been observed upon decreasing the crystallite sizes of MoO2.17,19 However, while the size dependent properties of molybdenum oxides have been thoroughly studied, the atomic structure of MoO2 nanoparticles has never been characterized and modelled properly, and thus the intricate relationship between structure, size and properties is not well understood.

Figure 1: A) Structure of distorted rutile MoO2. Octahedra are arranged in chains by edge-sharing along a, and the chains share corners in the bc-plane. B) The alternating Mo-Mo distances along a in the crystal structure give rise to a distortion of the rutile structure. C) Synchrotron PXRD data of all samples synthesized with different oleylamine/ethanol ratios ranging from 0% to 80% oleylamine. D) Rietveld refinements for the crystalline and (E) nanostructured sample. Large changes in size and structure with increasing oleylamine content in the solvent is observed, and Rietveld refinement demonstrate that the nanostructed MoO2 is fundamentally different from crystalline MoO2.


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Crystalline molybdenum oxides have a rich and well-studied structural chemistry and a multitude of structures exist, all with [MoO6] octahedra as the fundamental building block. Bulk MoO2 crystallizes in a distorted rutile structure.22 As illustrated in Figure 1A-B, [MoO6]-octahedra in MoO2 are arranged in corner-sharing sheets in the bc-plane, and form chains of edge-sharing octahedra along a.22 The distortion away from the conventional rutile structure is present in the chains along a, where the Mo-Mo distance alternates between approximately 2.5 Å and 3.1 Å, the short distances being associated with a pseudo metallic bonding (Figure 1B).23, 24 As a consequence of the distortion, MoO2 is described in the monoclinic P21/c space group as opposed to the tetragonal P42/mnm space group of the un-distorted rutile structure. Building on knowledge of common structural motifs and trends in the molybdenum oxide and rutile systems, we are able to construct structural models for molybdenum oxide particles of sizes ranging from >5 nm to 50-100 nm. We show that the structural transformation of the nano-sized MoO2 is caused by a high concentration of local defects resembling structural motifs observed in other rutile-like systems, which greatly alters the atomic arrangement on both a global and local scale compared to bulk MoO2.

Results and discussion Size and structure control in ethanol/oleylamine synthesis Molybdenum oxide samples were synthesized in a range of different ethanol/oleylamine mixtures. The PXRD patterns obtained from the six samples are shown in Figure 1C. The amount of oleylamine in the solvent clearly has a significant influence on the synthesis product. On the basis of their diffraction patterns, the samples can be divided into three categories. First, the sample synthesized in 80% oleylamine shows no significant Bragg peaks, and can be categorized as amorphous molybdenum oxide. Weak peaks indicate that an unidentified crystalline impurity co-exists with the amorphous structure in the sample. In contrast, samples synthesized in 0% and 20% oleylamine display clear, sharp Bragg peaks from distorted rutile MoO2 with a broadening corresponding to crystallite sizes of around 40 nm, as determined by Rietveld refinement assuming spherical particles. The Rietveld refinement (Figure 1D and Table S1) also shows that the average structure is well described by the expected distorted rutile MoO2 structure, and this pair of samples with well-fitting Rietveld profiles will be referred to as crystalline MoO2.


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In contrast to the crystalline samples, the samples prepared with 40%, 50% and 60% oleylamine show very different diffraction patterns, albeit with features similar to a mixture of the two previously described groups. Strong Bragg peak broadening is present, indicating a smaller crystallite size compared to the crystalline MoO2 samples. Furthermore, the first peak in the diffraction pattern at Q = 1.69 Å- 1 is shifted to much lower scattering angles compared to the crystalline samples and it is significantly broader and more asymmetric in shape than the other peaks. A simple Scherrer analysis of the diffraction peak at Q = 1.69 Å-1 gives a Scherrer size of ca. 3 nm, however, this number is associated with very large uncertainty. This group of samples will be referred to as nanostructured MoO2. When comparing the position of the Bragg peaks from the small and large MoO2 particle synthesized, we observe many similarities. A simple Rietveld refinement of the pattern was therefore attempted based on the disordered rutile structure. However, as seen in Figure 1E and Table S2, the bulk model is not able to correctly describe the peak positions, and it is not possible to refine a model using the standard MoO2 structure. This is a clear indication of the existence of a different structure for the nanostructured MoO2 particles as the changes in the PXRD cannot be explained by common size broadening effects. Interestingly, the appearance of the nanostructured MoO2 diffraction patterns seen here is not unique to the specific samples of our study. Similar diffraction patterns are presented in several other publications17-21 where nanosized MoO2 has been synthesized in a range of different ways. The structural transformation upon nanosizing of MoO2 thus appears to be a general, ubiquitous effect induced by the size of the particles, rather than an effect of the synthesis method applied. The changes in the diffraction patterns are easily overlooked without performing Rietveld refinement on high quality diffraction data.25 Consequently, the structural alteration in nanosized MoO2 either goes unnoticed, and the structure of the phase is believed to be identical to bulk MoO2, or the phase of nanosized MoO2 is erroneously identified as combinations of other molybdenum oxides. It has been proposed that nanosized MoO2 has the so-called hexagonal MoO2 structure (JCPDS no. 50-0739), however there is no referenced paper on the structure in the ICDD database entry, and to the best of our knowledge, X-ray diffraction data, or other data containing quantitative structural information, agreeing with the reported space group and unit cell have not been published.


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Figure 2: Representative ADF-STEM images of the three different types of samples at different magnifications. A, D and G show the amorphous type samples; B, E and H the nanostructured samples and C, F, and I the crystalline samples. Images C, F, H and I are collected with samples aligned along the [001] zone axis.

Figure 2 shows a collection of STEM images of the three different types of samples at various magnifications. A cursory inspection of these images corroborates the general trends discussed above. The amorphous sample (Figure 3A, D, and G) consists of aggregates with dimensions on the order of 50-2000 nm and shows no indication of long-range crystalline order. Energy dispersive x-ray (EDX) STEM and additional low magnification images can be found in Figure S1D and E. EDX maps confirm that the amorphous particles all are molybdenum oxides, but also contain nitrogen, likely originating from the oleylamine used in the synthesis. Wavelike changes in image contrast with a period of about 2 nm can be observed in Figure 2D and G. This distance is on the order of the length of one oleylamine molecule.26 The nanostructured sample (Figure 2B, E, and H) similarly consist of large aggregates (50-200 nm), however, in stark contrast to the amorphous sample, these consists of assemblies of numerous crystalline particles oriented in a seemingly random fashion as clearly observed in Figure 2H. The crystallite sizes observed are on the order of 2-5 nm with an average of around 3.4 nm (Figure S1A-C), in good agreement with the size estimated from PXRD. Finally, the crystalline sample at first sight also consists of large (50-250 nm) particles or assemblies of particles. The aggregates are composed


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of 50-150 nm wide plates, each plate 3-10 nm thick, stacked on top of each, but slightly rotated about the aaxis (Figure S2 & 3). Some of the crystalline particles are surrounded by amorphous material, most likely residual degraded oleylamine. Overall, the initial PXRD and STEM work shows that simple tuning of the solvent composition in the synthesis leads to significant variation in both atomic structure and particle size of the samples. Both techniques support the categorization of the samples as either amorphous, nanostructured or crystalline.

Figure 3: A) PDFs calculated from X-ray total scattering data for all samples synthesized with different oleylamine/ethanol ratios ranging from 0% to 80% oleylamine; intensities are scaled to the Mo – O peak at ca. 2 Å. The 1 – 30 Å range of the PDF reveals a decrease in correlation length with increasing oleylamine concentrations. B) The local range (1 – 10 Å) shows similarity in structural units which shifts from corner-sharing towards edge-sharing motifs with increasing oleylamine concentration. C) Intensity ratio between the 3.8 Å peak and 2.5 Å peak, a measure for the corner-/edge-sharing octahedra ratio

PDF modelling In order to further characterize the atomic structure of the nanostructured samples, we look towards X-ray Total Scattering and PDF analysis. The PDFs obtained from the samples (Figure 3A) show that the trend in extent of structural coherency agrees with the size analysis from the PXRD data and initial conclusions from STEM imaging. The extent of the structural correlations observed are shortest for the amorphous sample, followed by the nanostructured and the crystalline samples. The commercial sample shows no clear dampening of the PDF peak intensities with r within the 1-30 Å r-range as the crystallite size is on the order of


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micrometers. Using the extent of structural correlations as a rough estimate for the crystallite size, a size of around 2.5 nm is obtained for the nanostructured sample, which is significant less than the 3.4 nm observed in the STEM images. The fact that the PDF peaks are damped at smaller r values than the actual, average crystallite size is an indication of significant structural disorder. The PDF peaks may be directly interpreted as interatomic distances in the samples and thus offer an intuitive way of analyzing the structure of the MoO2 particles. Focusing at first on the positions of the most intense peaks, correlations at roughly the same distances seem to be present in all samples regardless of size, thus the structures of both nanostructured and amorphous molybdenum oxide contain many of the same basic structural units. Furthermore, in the nanosized samples, peaks originating from atomic correlations beyond the local structure (>5 Å) are broader and less well defined than in the crystalline samples, again indicating a highly disordered structure. We also note that no clear C – C or C - N peaks (at ca. 1.5 Å) from residual oleylamine are visible in the PDF. Oleylamine residue was detected in TEM (Figure 2, Figure S1D). However, this residual is insignificant for the PDF analysis due to the lower scattering power of C and N compared to Mo. Figure 3B, showing only the low r region, reveals some clear differences in structure between the different samples. The peaks at 2.55 Å and 3.10 Å correspond to the distance between Mo4+ in the distorted edge-sharing octahedra and the peak at 3.8 Å to the distance between corner-sharing octahedra. The development in the ratio of edge to corner-sharing octahedra may be followed by plotting the intensity ratio between the 2.5 Å peak (from edge-sharing octahedra) and 3.8 Å peak (from corner-sharing octahedra) (Figure 3C). The ratio has its lowest value for the commercial sample, and slowly increases for the subsequent more disordered samples, until reaching a maximum for the amorphous samples. It thus appears that this ratio is dependent on particle size; i.e. the smallest particles show the largest fraction of edge-sharing between octahedra compared to corner-sharing. The observed change in the relative number of octahedra that share edges and corners is tied to the disordered nature of the small particles and will be key to understanding the atomic arrangement. In Figure 4, we show fits of the distorted rutile MoO2 structure to the PDFs of the commercial bulk MoO2 sample (A) as well as the crystalline (B) and nanostructured (C) sample (details given in tables S3-5).


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As expected, the commercial sample is well described by the distorted rutile structure in both the local and global range of the PDF. The long-range correlation peaks of the crystalline sample (Figure 4B) are also in agreement with the reported distorted rutile structure. However, differences between model and data are seen in the local structure, specifically observed in the peaks from nearest neighboring Mo-Mo correlations. Analysis of the PDFs thus indicate that the crystalline samples contain structural defects likely to be associated with the ratio between edge- and corner-sharing octahedra, in agreement with the observed difference in edge/corner sharing ratio between the crystalline and commercial samples observed in Figure 3C. The defects are averaged out over longer distances, and do not affect the long-range average structure significantly. The local distortion must therefore be somewhat random in nature to allow the crystallographic long-range structure to remain unchanged, as evidenced by the good fit of the distorted rutile model to the long-range region of the PDF as well as the Rietveld fit to the PXRD data.


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Figure 4: Fits to the PDFs of A) the commercial, B) crystalline and C) nanostructured samples using the distorted rutile MoO2 model. The commercial sample shows an excellent fit, while the crystalline sample shows discrepancies in the local structure. The nanostructured sample shows large discrepancies across the entire range of the PDF.

For the nanostructured sample (Figure 4C), the misfit is significant across the entire r range, i.e. both the shortand long-range order is poorly described by the distorted rutile model, consistent with the poor Rietveld refinement seen in Figure 1E. All major peaks are present in both the experimental and calculated PDF, but the peak intensities differ substantially from each other. Building on the hypothesis of structural defects altering the ratio of edge- to corner-sharing octahedra in the rutile structure, the data indicate that a larger


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concentration of these defects must be present in the nanostructured samples than in the crystalline ones, again in agreement with the calculated ratio in Figure 3C. The crystalline samples only show discrepancies from the MoO2 structure in the low r PDF region, and any realistic model should leave the high r-region unperturbed by the inclusion of more edge-sharing octahedra. To test this, we constructed a simple two-phase model containing a distorted rutile phase describing the overall structure and size of the particle, and an additional phase derived from the K2Mo8O16 hollandite structure27 (Figure S4) with dimensions confined to model the local structure, i.e. the defects, solely. The fit with the local phase is shown in Figure 5 (additional information in Table S4 and Table S6). Similar to the distorted rutile structure, hollandite consists of chains of edge-sharing octahedra. However, in contrast to rutile, the connection of these chains alternates between edge- and corner-sharing as shown in inserts in Figure 5 (and S5). Prior to refinement, all K and O atoms were removed from the hollandite structure, thus ensuring that the hollandite phase was only used to model the additional Mo–Mo distances observed. Furthermore, a spherical dampening function,28 usually interpreted as a particle size,29 was applied to limit the contribution from the hollandite phase to be only in the local r-range with no influence on the long-range structure. It is evident that by including the local phase, a significant improvement of the fit is achieved, thus further attesting to the association between the connection of [MoO6] octahedra and the observed defects.


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Figure 5: Fit using a two-phase model based on the distorted rutile MoO2 and a modified hollandite K2Mo8O16 to the PDF obtained from the crystalline sample.

HR-STEM images shown in Figure 6 corroborate the notion that a simple rutile model is not sufficient to describe the crystalline sample satisfactorily. Placing an overlay of the rutile structure oriented along [100] on top of the images reveals that in some regions of the sample, the observed image contrast is inconsistent with a simple distorted rutile structure. Intensity is seen in-between the metal sites in the model (Figure 6 D-F), indicating the presence of molybdenum atoms in sites not occupied in the distorted rutile structure. Considering the STEM images in Figure 6, the ability of the proposed two-phase model to fit the PDF should not be understood as the presence of two distinct separate phases. Instead it should be interpreted as consisting of a distorted rutile structure with a rather large, but spatially un-correlated distribution of local point defects that are akin in nature to features in the hollandite structure, i.e. areas with occasional edge-sharing between octahedra in the b-c plane of the structure.


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Figure 6: HR-STEM images of the crystalline sample at two different magnifications. A and D are bright field images (detector collection angle, ≈∼10.5 mrad, B and E are medium angle annular bright field images (≈10.5∼18.5mrad) and C and F are annular dark field images (≈ 35.5∼200 mrad The overlaid structural drawing is the distorted rutile structure, all images and the structural drawing are oriented down the [100] zone axis. The images are purposely collected with a slightly defocused probe, since this region of the sample had residual amorphous surfactant adsorbed on it and the defects are more readily observed and interpretable using these conditions (see SI 18-20).30

In the nanostructured sample, both the local and long-range order is affected by the defects, and the same twophase treatment is not able to describe the observed PDF. A special crystallographic trait of molybdenum oxides and other early-transition metal oxides is the ability to form homologous series by ordered and reoccurring shear planes known as ‘Magnéli series’. For example, a Magnéli series is known to form in the stable ReO3-structured polymorph of MoO3. The parent structure consists entirely of corner-sharing octahedra31 and the reduction of Mo leads to formation of Mo8O23 , which is accommodated by a shift from corner-sharing octahedra to edge-sharing octahedra, resulting in two neighboring Mo atoms sharing two


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oxygens instead of one.24 The defects form ordered and reoccurring crystallographic shear planes32 and by adopting different spacings between the planes, a whole range of structures can be formed with a general formula MonO3n-1. The concept of the homologous series is visualized by MoO3 and Mo8O23 in Figure S6. However, the ReO3-based molybdenum oxide Magnéli series cannot explain the defect structures we observe, as the structures of our samples are close to the rutile structure. However, several different Magnéli series based on the same crystallographic shear principle exist. They have a range of different parent structures, including the rutile and distorted rutile structures of TiO2 and VO2,33-35 where formation of shear plane defects are known to occur on reduction.36, 37 To the best of our knowledge, rutile-based shear planes have never been observed for bulk molybdenum oxides. Nevertheless, a reduction in oxidation state of Mo related to the formation of shear planes is consistent with the observed shift towards longer Mo–Mo distances with decreasing particle size observed in the PDFs of the nanostructured samples, mainly visible for the cornersharing peak (3.8 Å) in Figure 3B. This observation supports the possibility of extended defects akin to a Magnéli series as an explanation for the altered nanostructure.

Figure 7: Illustrations of rutile-like structures; A) Magnéli type disorder in titanium oxide and B) De Wolff type disorder in manganese oxide. C) Structural models proposed to describe the nanostructured MoO2 samples. The distorted rutile MoO2 was modified to a model with two interwoven rutile lattices (D), which in turn was advanced to Magnéli–like (E) and De Wolff-like models (F).


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The Magnéli phases in rutile structures can be described as alternating ‘blocks’ of rutile extending infinitely in 2 dimensions, but only for a limited number of octahedra in the last dimension along the edge-sharing chains.38 The structure of various members of the TinO2n-1 Magnéli series are shown in Figure S7 and in Figure 7A (Ti4O7). In the Magnéli structures, the blocks of rutile share the same oxygen lattice, which largely corresponds to a regular rutile oxygen lattice. However, the metal atom positions in one block correspond to interstitials in an adjacent block, i.e. the metal atoms are shifted (0, ½, 0) compared to a standard rutile unit cell.39 An overlap region is present between these rutile blocks, and in the overlapping layer the structure resembles the corundum structure type where the octahedra share both edges and faces.34 The general formula for the members in the titanium oxide Magnéli series is TinO2n-1, where the stoichiometry determines the size of the alternating rutile blocks, i.e. 4 units of the characteristic rutile octahedra chain are repeated in Ti4O7 and 8 in Ti8O15 (Figure S7).38, 40 Calculated PDFs of the members of the TiO2 rutile Magnéli series with Mo replacing Ti are compared to the PDFs of nanostructured samples in Figure S8 and show clear similarities. The structures providing the best description of the nanostructured data are those based on Ti4O7, Ti5O9 and Ti6O11. However, when applying the Ti-based structures directly in our fit, issues arise. The modified structures based on Ti do not feature the alternating long and short edge-sharing metal-metal distance inherent to the distorted MoO2 bulk structure, resulting in large discrepancies for the nearest neighbor distances. However, the long-range order is surprisingly well described by these models considering that no refinement of the structural parameters was performed, and the atomic arrangement in the nanostructured samples are very likely related to the rutile Magnéli models, as discussed further below. A related type of disorder is De Wolff defects, commonly observed in another rutile-like system; γMnO2.41 De Wolff showed that poorly crystalline γ-MnO2 can be understood on the basis of a disordered intergrowth between two well-known MnO2 polymorphs; pyrolusite and ramsdellite.42 An example of such an intergrown structure is shown in Figure 7B. Pyrolusite is a rutile-type structure with single chains of edgesharing octahedra connected by corner-sharing. Ramsdellite features two adjacent chains connected by edgesharing, which in turn are connected by corner-sharing.43 Hollandite is structurally related to ramsdellite, and


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thus the intergrowth description of this disorder is very similar to the model applied to fit the crystalline PDFs presented in Figure 5. Considering only the very local, atomic scale order, De Wolff-type and Magnéli-type disorder are very similar. Fundamentally, both types of disorder can locally be described as point defects in the empty octahedral sites in rutile-like structures. Thus, a highly simplified model can be made that unifies both descriptions by adding additional Mo atoms shifted by (0, ½, 0) compared to the original Mo position in the distorted rutile unit cell, thus creating two super-positioned or interwoven rutile structures within the same oxygen lattice, as illustrated in Figure 7C-D. The new Mo position in the “interwoven rutile” system corresponds to the introduction of an edge-sharing octahedron in the b-c plane of the original lattice, making the structure akin to the parent NiAs structure, albeit a distorted version hereof. By refining the atomic occupancy of the new Mo site, a satisfactory correction of the corner-sharing to edge-sharing ratio can be achieved using very few parameters as is apparent from Figure 8B, where a fit is shown with the refined fractional occupancy of the secondary Mo being approximately 0.33. The refined values can be found in Table S7. A significant improvement in the fit to the PDF is observed when this model is implemented compared to the fit using the normal rutile MoO2 structure (Figure 8A). Only a few extra parameters are added and the model is kept very simple. The Annular Dark Field (ADF) STEM image shown in Figure 8C corroborates the suggested model. The overlaid rutile structure clearly does not suffice to describe the observed image contrast and again, added intensity is observed in the interstitial site, as will be discussed in detail below.


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Figure 8: PDF fits of different structural models for the nanostructured sample. The fit is improved from the distorted rutile model (A) by including a second rutile lattice (B). The ADF image (C) supports the need to include additional Mo atoms in the unoccupied sites of the rutile structure.

Despite the improvement of the fit, the suggested simple superposition model does not provide a good description of the data above 10 Å. This is especially evident when considering the two small peaks at ca. 15.7 Å and 17.5 Å. To rectify this and improve the fit above 10 Å, structural models based on Magnéli-type disorder and De Wolff-type disorder were built. The models were created by building supercells from the interwoven rutile model and removing Mo atoms until representative models were obtained. The Magnéli-type molybdenum oxide structure is nearly identical to the Ti4O7 structure yet including the alternating edge-sharing Mo-Mo distance that is characteristic for MoO2. The fits and structures are shown in Figure 9, while refined parameters are found in Table SI8-SI9.


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Figure 9: Fits to the PDF obtained from the nanostructured sample using extended defect structure models based on (A) Magnéli-type and (B) De Wolff disorder.

Both the Magnéli and De Wolff structures provide very good descriptions of the data, with the Magnéli model fitting slightly better. However, it is important to note that neither of the models should be seen as a unique structural solution to the structure of the nanoparticles. The particles are highly disordered, and most likely contain a range of different defect types and concentrations, and possibly both Magnéli and De Wollf-like disorder. Instead, we interpret the results as a clear indication that the nanostructured samples contain extended defects that are similar in nature to those present in the other rutile systems, but not in bulk molybdenum oxide. In contrast to related rutile materials, i.e. the Magnéli series for vanadium and titanium oxides, the defects are highly disordered, and not discernable with conventional crystallographic means.


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Note that the refinements shown in Figure 9 were done using P1 symmetry. Structural solutions of rutile family Magnéli phases in vanadium and titanium oxides have yielded solutions in low symmetry space groups with triclinic cells,34, 38-40, 44, 45 and therefore low symmetry solutions are to be expected. In order to further relate the structures of the nanostructured and crystalline samples, we studied the reaction taking place when treating both types of samples by simple post-synthesis sintering. The results are shown and described further in Figure S9. While the crystalline sample is stable upon heating, the nanostructured sample undergoes a gradual amorphization, and critically does not transform to the ordered distorted rutile, as was initially expected. This experiment also clearly shows that the nanostructure structure has implications on the physical properties of the material.

Multislice modelling of STEM data To investigate the proposed models further and extract more detailed information from the collected HRSTEM images, a broad range of multislice simulations of HR-STEM images were prepared. Figure 10 shows simulated ADF images based on some of the proposed models together with different areas selected from one STEM image of the nanostructured sample. It is evident that no single one-phase model is able to encapsulate the distribution of observed defects. It is also very clear that the structure is not simply the bulk MoO2 distorted rutile structure, as the additional intensity between the Mo sites is not observed in simulations for any sample thickness in the range 2-22nm (Figure S11). The figure supports the previous findings that the ‘interwoven’ model with all interstitial sites occupied (akin to a distorted NiAs structure) and the refined model with 0.33 occupancy on the interstitial site despite their simplicity match the observed contrast very well. Indications of features that agree well with a Magnéli or partially occupied Magnéli-like structure are observed. Images simulated using the DeWolff type structure fail to reproduce many of the main features of the experimental images. However, by allowing for additional occasional edge-sharing [MoO6] octahedra in the De Wolff structure one would assume that a relatively good match between experiment and simulation may be obtained. As a whole, the simulations and experimental data support the need for additional edge-sharing [MoO6] octahedra, as proposed from the PDF analysis, to explain the observed ADF image contrast.


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Figure 10: Experimentally obtained and simulated ADF images based on the different proposed models for the nanostructured samples. The simulations (assuming 3.4 nm thickness) are compared qualitatively to different areas found in an ADF image of the nanostructured sample (see Figure S10).

The crystalline sample did not show the same clear indications of additional edge-sharing octahedra in the structure based on ADF images alone (Figure 2I and 7D-F). However, by imaging using a range of different STEM collection angles and comparing the data to results from multislice simulations (Figure S11-S23) it was possible to clearly identify the presence of defects. Representative images and selected simulation results shown in Figure S23 reveal that for the crystalline sample some regions are pure distorted rutile while others contain some degree of defects. In certain areas an interwoven rutile model with 10% occupancy of the interstitial sites describes the observed images well while in others, the Magnéli model gives a good description. No quantitative comparison between the simulated images and the experimental ones was made due to the complex nano/microstructure of the particles. Even over small areas significant differences in thickness, surface structure, buckling, and amorphous adsorbents exists and makes proper quantitative comparisons practically impossible.


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Conclusions Applying PDF analysis and high-resolution STEM, we have shown the presence of a high concentration of defects in nanostructured molybdenum oxide (3-5 nm) which dramatically changes the atomic arrangement throughout the material. The defects can be related to extended defects often observed in bulk structures of early transition metal oxides, but unlike bulk molybdenum oxide, the defect structures occur in the rutile structure rather than the ReO3 structure and appear completely disordered. Applying known defect structures in rutile systems, i.e. Magnéli and De Wollf structures, we have constructed full models which agree both with PDF and HR-STEM data. Similar defect structures to those observed in small nanostructured particles are also seen in larger, crystalline nanoparticles of molybdenum oxide (50-150 nm). However, the lower defect concentration in the larger nanoparticles means that the average structure is not affected, and simple PXRD data could successfully be treated by conventional Rietveld refinement. Only when including the diffuse scattering in the analysis through PDF and performing a detailed analysis of electron microscopy data do the defects show up. This study showcases the strength of the intuitive nature of the PDF, which is best exemplified by how missing structural motifs are readily identified from peak position and intensity. This allows applying simple, small box models, while still allowing us to extract significant structural information. Combining the excellent statistical sampling of the x-ray PDF method with a technique with the spatial sensitivity of HR-STEM allows for an in-depth understanding of the nature of nanostructure. Similar defects have been linked to improve electrical conductivity in TiOx samples,46 and thus the structure model provides a possible explanation for the improved performance of nanostructured MoO2 as anode material for Li-ion batteries.17-19 The study presents a direct link between structural features studied as defects in bulk materials, to structural rearrangements occurring at the nanoscale. The work furthermore provides an example of how defects are an integral part of nanostructure, and at the very least suggest that known structural defects from related structure types should be considered when characterizing structures on the nanoscale.


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Experimental methods

Synthesis of molybdenum oxide particles Molybdenum oxide particles were synthesized via a simple one-step solvothermal synthesis.47 0.3 mmol (0.3708 g) Ammonium heptamolybdate, (NH4)6Mo7O24, (>99% Bioultra, Sigma Aldrich) was dissolved in an oleylamine (70% technical grade, Sigma Aldrich) and ethanol (96%) mixture with a total volume of 10 mL and heated to 250 °C in a 20 mL stainless steel autoclave for 24 h. The oleylamine/ethanol volumetric ratio was varied from 80% oleylamine to 0% oleylamine. The synthesis product was centrifuged and washed three times using ethanol before drying in air. Additionally, commercial MoO2 (99%, Sigma Aldrich) was characterized for comparison with the synthesized samples.

X-ray scattering experiments and analysis X-ray total scattering data were collected at the 11-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory, Argonne, USA and at beamline P02.1, PETRAIII, DESY, Hamburg, Germany.48 All data were collected at room temperature using a wavelength of 0.2112 Å (APS) and 0.2072 Å (PETRAIII) applying the RA-PDF setup.49 Powder X-ray diffraction (PXRD) data were collected at a detector distance of 950 mm. An in situ X-ray total scattering experiment on the sintering of molybdenum oxide particles was performed at DESY. A 1 mm kapton capillary was filled with the sample heated to 300 °C using a hot air blower while collecting time resolved X-ray total scattering patterns. All scattering patterns were integrated using pyFAI in Dioptas.50 The total scattering data were Fourier transformed to obtain the PDF using PDFgetX3,51 and modelled using DiffPy-CMI.52 The DiffPy scripts used in the modelling are available online.53 The following parameters were used in PDFgetX3: Qmin = 0.5 Å-1, Qmax = 22 Å-1, Qmaxinst= 25 Å-1 and rpoly = 0.9. Rietveld refinements of the PXRD data were performed using Fullprof.54 The crystallite size for the crystalline sample was obtained from Rietveld refinement (Q-range 1-7 Å-1), taking into account instrumental broadening. Details on all models and refinements can be found in the SI (Table S1 and S2 for PXRD and Table S3 to Table S9 for PDF).


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Electron microscopy Samples for electron microscopy investigations were prepared by crushing the washed powders in absolute ethanol (Merck, EMSURE® ACS, ISO, Reag. Ph Eur) using an agate mortar and pestle. The suspension was sonicated for 3 minutes and drop-cast onto holey carbon grids (300 mesh Cu, EM Resolutions). Samples were plasma cleaned for 20 s in an H2/O2 plasma (Gatan Solarus 950), a non-plasma cleaned sample was also inspected and no significant damage caused from plasma cleaning was evident. HR-STEM imaging was conducted on a double-aberration-corrected 80-300 keV Titan3 FEG-TEM (Field Emission Gun Transmission Electron Microscope) operating at 300 keV using a range of detectors for signal collection of signal scattered to various collection angles (β), Annular Dark Field (ADF) ( ≈ 35.5 ∼ 200mrad), Medium Angle Annular Bright Field (MAABF) (≈10.5∼18.5mrad) and Bright Field (BF) (≈0∼10.5mrad) signals. The probe was generated using a probe forming aperture semiangle of 15 mrad and aberrations were corrected to beyond 20 mrad, to give an essentially aberration free probe. To minimize the influence of any sample drift and scanning noise, stacks of 30 STEM images along two orthogonal scanning directions were collected using very short dwell times ( s per pixel). From these 60 images, a combined image was obtained using the program and processes developed by Savitzky et al.55 Multislice calculations of STEM images based on different possible structural models were performed. The calculations were performed using Prismatic, 56, 57 with additional details given in the SI.

Acknowledgements We are grateful to the Villum Foundation for financial support through a Villum Young Investigator grant (VKR00015416). KMØJ acknowledges funding from the Danish Research Council under the Sapere Aude Research Talent Program. EDB acknowledges financial support through a research grant (VKR023371) from the Villum Foundation. This research used equipment funded by Australian Research Council grant (ARC Funding (LE0454166). JE acknowledges funding from the Australian Research Council Discovery Project DP150104483. The authors acknowledge use of the facilities and the assistance of A/Prof. Matthew Weyland at the Monash Centre for Electron Microscopy.


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We furthermore thank DANSCATT (supported by the Danish Agency for Science and Higher Education) for support. 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 P02.1 and we would like to thank Martin Etter for assistance in using the beamline. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We thank Pavol Juhas (Brookhaven National Laboratory, USA) for help using Diffpy-CMI for structural analysis.

Supporting Information Available: Description of all Rietveld refinements with resulting parameters; description of all PDF refinements with resulting parameters; TEM images and particle size analysis. Structural models of molybdenum oxides and related titanium oxide rutile structures; multislice simulations, and a discussion of an experiment studying the thermal stability of nanostructured molybdenum oxides. This material is available free of charge via the Internet at http://pubs.acs.org.

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TOC figure:


Size Induced Structural Changes in Molybdenum Oxide Nanoparticles

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