Niobium oxide (Nb 2 O 5 ) is an interesting active material for technologies ranging from catalysis and sensors to energy storage and electrochromic devices owing to its unique optical, electronic, and electrochemical properties. These properties vary between different phases and morphologies in the Nb 2 O 5 system, but systematic studies that correlate properties to phase and morphology are limited by current synthetic methods, which require postsynthetic high temperature treatments and suffer from a lack of direct and precise control over morphology, crystal structure, and stoichiometry. Here, we report a heat-up colloidal synthesis method that produces orthorhombic Nb 2 O 5 nanorods 1 nm in width by 31 nm in length that preferentially grow along the [001] direction. The synthesis is based on aminolysis of niobium oleate in octadecene, and nanorods are formed through three distinct steps: aminolysis-driven formation of niobium oxo clusters, condensation into amorphous Nb 2 O 5 seeds below the reaction temperature (240 °C, under atmospheric pressure), and crystallization and growth of Nb 2 O 5 nanorods. We investigated the electrochromic behavior of nanorod thin films upon Li + intercalation and observed predominantly near-infrared coloration, fast switching kinetics, and durability for at least 500 charge−discharge cycles.

Understanding growth mechanisms and the role of surface functionalization is of key importance to control shape and morphology of nanoparticles and their properties. Here, we describe the growth mechanism and the effect of hydrothermal synthesis parameters (pH, time and precursor/functionalization agent ratio) during in situ functionalization of anatase TiO2 nanoparticles with 3-aminopropyltriethoxysilane. Elongated crystallographically oriented TiO2 nanoparticles were formed by oriented attachment mechanism in addition to spherical nanoparticles. The growth mechanism is determined by a combination of ex situ techniques such as high-resolution transmission electron microscopy combined with in situ synchrotron X-ray diffraction and density functional theory calculations. Oriented attachment induced by the functionalization agent is shown to be the origin of the elongation of the nanoparticles, as only spherical nanoparticles were formed in the absence of surface functionalization. Finally, it was shown that the amount and the size of the elongated nanoparticles can be tuned by adjusting pH.

Hydrothermal synthesis is a well-established method to produce complex oxides, and is a potential interesting approach to synthesize stoichiometric lead-free piezoelectric K 0.5 Na 0.5 NbO 3. Due to challenges in obtaining the desired stoichiometry of this material, more knowledge is needed on how the end members, KNbO 3 and NaNbO 3 , are nucleating and growing. Here we report on the formation mechanisms and growth during hydrothermal synthesis of KNbO 3 and NaNbO 3 by in situ synchrotron powder X-ray diffraction. We show that tetragonal KNbO 3 crystallites form from

In situ monitoring of the formation of crystalline phases during conventional hydrothermal synthesis is experimentally challenging. Here, we report an in situ time-resolved synchrotron X-ray diffraction study during hydrothermal synthesis of NaNbO 3 using a high-pressure custom-made capillary cell penetrable to X-rays. The high time resolution (0.1 s) revealed a sequence of transient intermediate phases, including several unknown phases, before the final perovskite NaNbO 3 was formed. These new findings highlight the complexity of the hydrothermal synthesis of NaNbO 3 and demonstrate the potential for obtaining in-depth knowledge of the reactions taking place by time-resolved in situ X-ray diffraction.

In situ techniques are powerful for providing insight into the determining factors when preparing KxNa1−xNbO3 nanoparticles with a designed composition, structure and size.

Characterization of material structure with X-ray or neutron scattering using e.g. Pair Distribution Function (PDF) analysis most often rely on refining a structure model against an experimental dataset. However, identifying a suitable model is often a bottleneck. Recently, automated approaches have made it possible to test thousands of models for each dataset, but these methods are computationally expensive and analysing the output, i.e. extracting structural information from the resulting fits in a meaningful way, is challenging. Our Machine Learning based Motif Extractor (ML-MotEx) trains an ML algorithm on thousands of fits, and uses SHAP (SHapley Additive exPlanation) values to identify which model features are important for the fit quality. We use the method for 4 different chemical systems, including disordered nanomaterials and clusters. ML-MotEx opens for a type of modelling where each feature in a model is assigned an importance value for the fit quality based on explainable ML.

Sodium niobate (NaNbO 3 ) attracts attention for its great potential in a variety of applications, for instance, due to its unique optical properties. Still, optimization of its synthetic procedures is hard due to the lack of understanding of the formation mechanism under hydrothermal conditions. Through in situ X-ray diffraction, hydrothermal synthesis of NaNbO 3 was observed in real time, enabling the investigation of the reaction kinetics and mechanisms with respect to temperature and NaOH concentration and the resulting effect on the product crystallite size and structure. Several intermediate phases were observed, and the relationship between them, depending on temperature, time, and NaOH concentration, was established. The reaction mechanism involved a gradual change of the local structure of the solid Nb 2 O 5 precursor upon suspending it in NaOH solutions. Heating gave a full transformation of the precursor to HNa 7 Nb 6 O 19 ·15H 2 O, which destabilized before new polyoxoniobates appeared, whose structure depended on the NaOH concentration. Following these polyoxoniobates, Na 2 Nb 2 O 6 ·H 2 O formed, which dehydrated at temperatures ≥285 °C, before converting to the final phase, NaNbO 3 . The total reaction rate increased with decreasing NaOH concentration and increasing temperature. Two distinctly different growth regimes for NaNbO 3 were observed, depending on the observed phase evolution, for temperatures below and above ≈285 °C. Below this temperature, the growth of NaNbO 3 was independent of the reaction temperature and the NaOH concentration, while for temperatures ≥285 °C, the temperature-dependent crystallite size showed the characteristics of a typical dissolution–precipitation mechanism.

Ferroelectric materials are crucial for today's technological society and nanostructured ferroelectric materials are important for the downscaling of devices. Controlled and reproducible synthesis of these materials are, therefore, of immense importance. Hydrothermal synthesis is a well-established synthesis route, with a large parameter space for optimization, but a better understanding of nucleation and growth mechanisms is needed for full utilization and control. Here we use in situ X-ray diffraction to follow the nucleation and growth of BaTiO 3 formed by hydrothermal synthesis using two different titanium precursors, an amorphous titania precipitate slurry and a Ti-citric acid complex solution. Sequential Rietveld refinement was used to extract the time dependency of lattice parameters, crystallite size, strain, and atomic displacement parameters. Phase pure BaTiO 3 nanoparticles, 10-15 nm in size, were successfully synthesized at different temperatures (100, 125, and 150 • C) from both precursors after reaction times, ranging from a few seconds to several hours. The two precursors resulted in phase pure BaTiO 3 with similar final crystallite size. Finally, two different growth mechanisms were revealed, where the effect of surfactants present during hydrothermal synthesis is discussed as one of the key parameters.