Size-Controlled High-Temperature Synthesis of Crystalline Niobium and Tantalum Oxide Nanoparticles: Exploring Structural Variations at Nanoscale

Niobium and tantalum oxides are highly stable materials under harsh oxidizing conditions with diverse applications in catalysis, energy storage, and optoelectronics. Niobium oxides exhibit a rich variety of polymorphs and crystal structures (e.g., T-, M-, H-Nb₂O₅, NbO₂, and NbO), whose electronic properties depend strongly on their crystalline structure. Realizing these crystal structures typically requires thermal syntheses at high temperatures and different gas atmospheres, which causes particle sintering and growth, compromising nanoscale features and limiting functionality. Here, we present a synthesis route for niobium and tantalum oxide nanoparticles below 10 nm, showing unprecedented control over size and stoichiometry at temperatures up to 1100 °C. The synthesis utilizes a reverse microemulsion for the controlled synthesis of the nanoparticles, followed by silica shell encapsulation, which successfully preserves particle size while enabling access to different polymorphs. Structural characterization by HAADF-STEM and XRD confirms particle size preservation and the formation of various crystal structures through different heat treatments. For Nb₂O₅ polymorphs, the optical bandgap can be tuned by the crystal structure excluding size effects due to the uniform particle size. This approach yields highly crystalline nanoparticles with defined structures, providing model materials for a range of applications and potentially extendable to other material systems.