Tungsten-niobium oxide bronzes: a bulk and surface structural study

[EN] Materials from the WO3-Nb2O5 system, presenting bronze-type crystal structures, display outstanding functional properties for several applications as thermoelectric materials, lithium-ion battery electrodes, or catalysts. In this work, a series of W-Nb-O oxide bronzes have been synthesized by the hydrothermal method (with Nb/(W + Nb) ratios in the range of 0-1). A combination of bulk and surface characterisation techniques has been applied to get further insights into: (i) the effect of thermal treatments on as-prepared materials and (ii) the surface chemical nature of W-Nb-O oxide bronzes. Thermal treatments promote the following structural changes: (i) loss of emerging long-range order and (ii) the elimination of NH4+ and H2O species from the structural channels of the as-synthesized materials. It has been observed that W-Nb-O bronzes with Nb at% of ca. 50% are able to retain a long-range order after heat-treatments, which is attributed to the presence of a Cs-0.5[W2.5Nb2.5O14]-type structure. Increasing amounts of Nb 5T in the materials (i) promote a phase transition to pseudocrystalline phases ordered along the c-axis; (ii) stabilize surface W s. species (elucidated by XPS); and (iii) increase the proportion of surface Lewis acid sites (as determined by the FTIR of adsorbed CO). Results suggest that pseudocrystalline oxides (with a Nb at% >= 50%) are closely related to NbO2 pentagonal bipyramid-containing structures. The stabilisation of Lewis acid sites on these pseudocrystalline materials leads to a higher yield of heavy compounds, at the expense of acrolein formation, in the gas-phase dehydration of glycerol.The authors would like to acknowledge the Ministerio de Ciencia, Innovacion y Universidades in Spain for the financial support (RTI2018-099668-B-C21 and SEV-2016-0683 projects), and the Electron Microscopy Service at Universitat Politecnica de Valencia for providing facilities and technical support. D. D. also thanks Severo Ochoa Excellence Program for his fellowship (SVP-2014-068669).Delgado-Muñoz, D.; Concepción Heydorn, P.; Trunschke, A.; López Nieto, JM. (2020). Tungsten-niobium oxide bronzes: a bulk and surface structural study. Dalton Transactions. 49(38):13282-13293. https://doi.org/10.1039/d0dt02058cS13282132934938D. J. M. Bevan and P.Hagenmuller , Non-Stoichiometric Compounds , Pergamon , 1973Quan, H., Gao, Y., & Wang, W. (2020). 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Mechanisms for Tungsten Oxide Nanoparticle Formation in Solvothermal Synthesis: From Polyoxometalates to Crystalline Materials

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 clusters in 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 adopted the well-known α-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 thus affects not only the crystallite size but also the atomic structure of the nanoparticles, which we link to the observed reaction mechanism

Linking structure to function at the solid electrolyte interphase: Insights from NMR spectroscopy

The performance of Li metal batteries is tightly coupled to the composition and properties of the solid electrolyte interphase (SEI). Even though the role of the SEI in battery function is well understood (e.g., it must be electronically insulating and ionically conductive, it must enable uniform Li+ flux to the electrode to prevent dendrite growth, it must accommodate the large volume changes of Li electrodeposition), the challenges associated with probing this delicate composite layer have hindered the development of Li metal batteries for practical applications. In this review, we detail how nuclear magnetic resonance (NMR) spectroscopy can help bridge this gap in characterization due to its unique ability to describe local structure in conjunction with ion dynamics while connecting these properties to electrochemical behavior. By leveraging NMR, we can gain molecular-level insight to aid in the design of Li surfaces that enable reactive anodes for next generation, high energy density batteries

Formation and growth mechanism for niobium oxide nanoparticles: atomistic insight from in situ X-ray total scattering

Understanding the mechanisms for nanoparticle nucleation and growth is crucial for the development of tailormade nanomaterials. Here, we use X-ray total scattering and Pair Distribution Function analysis to follow the formation and growth of niobium oxide nanoparticles. We study the solvothermal synthesis from niobium chloride in benzyl alcohol, and through investigations of the influence of reaction temperature, a formation pathway can be suggested. Upon dissolution of niobium chloride in benzyl alcohol, octahedral [NbCl$_{6−x}$O$_x$] complexes form through exchange of chloride ligands. Heating of the solution results in polymerization, where larger clusters built from multiple edge-sharing [NbCl$_{6−x}$O$_x$] octahedra assemble. This leads to the formation of a nucleation cluster with the ReO$_3$ type structure, which grows to form nanoparticles of the Wadsley–Roth type H-Nb$_2$O$_5$ structure, which in the bulk phase usually only forms at high temperature. Upon further growth, structural defects appear, and the presence of shear-planes in the structure appears highly dependent on nanoparticle size

Explaining an anomalous pressure dependence of shear modulus in germanate glasses based on Reverse Monte Carlo modelling

Unlike traditional silicate glasses, germanate glasses often feature non-monotonic variations in material properties (e.g., elastic moduli and glass transition temperature) with varying chemical composition, temperature, and pressure. However, the underlying atomic-scale structural origins remain poorly understood. This is because, in most oxide glasses, the structural changes are quantified through solid-state NMR spectroscopy, but unfortunately the only NMR active germanium isotope (73Ge) has very unfavorable NMR properties. Here, we circumvent this problem by using high-energy X-ray and neutron total scattering coupled with ab initio molecular dynamics simulations as input for Reverse Monte Carlo modeling. In detail, we study the structure and properties of two sodium germanate glasses (10Na2O-90GeO2 and 20Na2O-80GeO2) subjected to permanent densification through hot compression up to 2 GPa at the glass transition temperature. While density as well as Young's and bulk modulus increase with pressure as expected, shear modulus first increases and then decreases slightly at higher pressures. The refined atomistic structure models suggest that the glasses feature a distribution of 4, 5, and 6 coordinated Ge with a majority of 4 and 5 coordinated species. Only minor changes in the Ge–O coordination occur upon hot compression, but a notable transformation of edge- to corner-sharing Ge-polyhedra is found. This anomalous polyhedral packing causes a lower number of angular constraints upon higher pressure treatment, explaining the non-monotonic trend of shear modulus with pressure. We also find that the rings become smaller and less circular upon compression, contributing to the volumetric compaction. These findings may aid the future design of germanate glasses with tailored properties and the general understanding of structure-property relations in oxide glasses.</p

Towards a mechanistic understanding of the sol–gel syntheses of ternary carbides

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Sol Gel-Based Synthesis of the Phosphorus-Containing MAX Phase V2PC

More than 150 MAX phases are known to date. Their chemical diversity is the result of mixing-and-matching early-to-mid transition metals (M), main group elements (A), and carbon and/or nitrogen (X). The vast majority of the respective carbides and (carbo)nitrides contain group 13 and 14 as the A element, such as Al, Ga, and Si. V$_2$PC is among the least studied members of this family of materials; as a matter of fact, it is only mentioned in two pieces of original literature. The solid-state synthesis is extremely vaguely described and working with elemental phosphorus poses additional synthetic challenges. Here, we confirm these experimental difficulties and present an alternative sol gel-based approach to prepare almost single-phase V$_2$PC. The versatility of the sol gel chemistry is further demonstrated by variation of the gel-building agent moving beyond citric acid as the carbon source. DFT calculations support the experimentally obtained structural parameters and show V$_2$PC is a metal

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Characterisation of material structure with Pair Distribution Function (PDF) analysis typically involves refining a structure model against an experimental dataset. However, finding or constructing a suitable atomic model for PDF modelling can be an extremely labour-intensive task, requiring carefully browsing through large numbers of possible models. We present POMFinder, a machine learning (ML) classifier that rapidly screens a database of structures, here polyoxometalate (POM) clusters, to identify candidate structures for PDF data modelling. The approach is demonstrated to identify suitable POMs on experimental data, including in situ data collected with fast acquisition time. This automated approach shows significant potential for identifying suitable structure models for structure refinements to extract quantitative, structural parameters in materials chemistry research. The code is open source and user-friendly, making it accessible to those without prior ML knowledge. We also demonstrate that POMFinder offers a promising modelling framework for combined modelling of multiple scattering techniques compared to conventional refinement methods