There's no place like real-space:elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

The development of new functional materials builds on an understanding of the intricate relationship between material structure and properties, and structural characterization is a crucial part of materials chemistry. However, elucidating the atomic structure of nanomaterials remains a challenge using conventional diffraction techniques due to the lack of long-range atomic order. Over the past decade, Pair Distribution Function (PDF) analysis of X-ray or neutron total scattering data has become a mature and well-established method capable of giving insight into the atomic structure in nanomaterials. Here, we review the use of PDF analysis and modelling in characterization of a range of different nanomaterials that exhibit unique atomic structure compared to the corresponding bulk materials. A brief introduction to PDF analysis and modelling is given, followed by examples of how essential structural information can be extracted from PDFs using both model-free and advanced modelling methods. We put an emphasis on how the intuitive nature of the PDF can be used for understanding important structural motifs, and on the diversity of applications of PDF analysis to nanostructure problems

Exploration of Phase Compositions, Crystal Structures, and Electrochemical Properties of Na<i>_x</i>Fe<i>_y</i>Mn_1–<i>y</i>O₂ Sodium Ion Battery Materials

Layered manganese oxide materials are widely used in sodium and lithium ion batteries, but significant discrepancies are encountered in the literature with respect to their electrochemical properties. This could be due to difficulties in establishing the exact phase compositions and crystal structures (typically P2, P3, O2, and O3, their distorted analogues, e.g., P’2, hydrated - PH2, or carbonated - PC2, phases) of a given synthesis product, especially when only crude crystallographic indexing is used without structural analysis. Here we report a benchmark high-resolution synchrotron powder diffraction investigation of a broad composition range of the layered NaxFey­Mn1–y­O2 cathode materials (x = 0.5, 0.7, and 1.0 and y = 0.3, 0.5, and 0.7) with respect to phase composition, crystal structure, and electrochemical properties. On the basis of multiphase Rietveld refinements, it is shown that crystal structure can be controlled to a certain degree for different x and y. Most synthesis products contain a complex phase mixture, but in a few cases, almost phase pure P2 and O3 type materials can be produced. The P2 phase is observed to be air sensitive, whereas the O3 and P3 structures are not. Clear trends linking electrochemical performance to x and y are observed, where higher x and y result in worse performance. On the other hand, no clear trend is observed linking the type of layered crystal structure to electrochemical performance. Overall, the electrochemical performance of the NaxFey­Mn1–y­O2 samples seems to be mostly dependent on the initial oxidation state and the transition metal ratio

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

Size Induced Structural Changes in Molybdenum Oxide Nanoparticles

Nanosizing of metal oxide particles is a common strategy for improving materials properties; however, small particles often take structures different from the bulk material. MoO$_2$ 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 MoO$_2$ obtained through detailed atomic pair distribution function analysis combined with high-resolution electron microscopy. Defects occur in the arrangement of [MoO$_6$] 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

Influence of precursor structure on the formation of tungsten oxide polymorphs

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3, is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO30*5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggest that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest

Size-induced amorphous structure in tungsten oxide nanoparticles

The properties of functional materials are intrinsically linked to their atomic structure. When going to the nanoscale, size-induced structural changes in atomic structure often occur, however these are rarely well-understood. Here, we systematically investigate the atomic structure of tungsten oxide nanoparticles as a function of the nanoparticle size and observe drastic changes when the particles are smaller than 5 nm, where the particles are amorphous. The tungsten oxide nanoparticles are synthesized by thermal decomposition of ammonium metatungstate hydrate in oleylamine and by varying the ammonium metatungstate hydrate concentration, the nanoparticle size, shape and structure can be controlled. At low concentrations, nanoparticles with a diameter of 2-4 nm form and adopt an amorphous structure that locally resembles the structure of polyoxometalate clusters. When the concentration is increased the nanoparticles become elongated and form nanocrystalline rods up to 50 nm in length. The study thus reveals a size-dependent amorphous structure when going to the nanoscale and provides further knowledge on how metal oxide crystal structures change at extreme length scales

Size-induced amorphous structure in tungsten oxide nanoparticles

The properties of functional materials are intrinsically linked to their atomic structure. When going to the nanoscale, size-induced structural changes in atomic structure often occur, however these are rarely well-understood. Here, we systematically investigate the atomic structure of tungsten oxide nanoparticles as a function of the nanoparticle size and observe drastic changes when the particles are smaller than 5 nm, where the particles are amorphous. The tungsten oxide nanoparticles are synthesized by thermal decomposition of ammonium metatungstate hydrate in oleylamine and by varying the ammonium metatungstate hydrate concentration, the nanoparticle size, shape and structure can be controlled. At low concentrations, nanoparticles with a diameter of 2-4 nm form and adopt an amorphous structure that locally resembles the structure of polyoxometalate clusters. When the concentration is increased the nanoparticles become elongated and form nanocrystalline rods up to 50 nm in length. The study thus reveals a size-dependent amorphous structure when going to the nanoscale and provides further knowledge on how metal oxide crystal structures changes at extreme length scales