Morphological and structural evolution of gas-phase synthesized vanadium nanoparticle films induced by thermal treatment

Temperature-driven genesis of morphology and structure of vanadium nanoparticle films fabricated by magnetron-based gas aggregation sources has been investigated. Three temperature regions were identified. In the first one which covers the range from room temperature to approximately 200 °C, the increasing temperature results in the gradual oxidation of nanoparticle films and a slight increase of the individual nanoparticles that form the coating. Despite this, the nanoparticle films in this temperature range preserve their highly porous architecture. Above 200 °C, the vanadium nanoparticles undergo rapid oxidation, which is accompanied by an abrupt change in their mass, crystallinity, morphology, and optical properties (absorbance, photoluminescence). According to XRD, an orthorhombic V2O5 phase becomes the only detectable crystalline phase in the films in this temperature range. Furthermore, the individual nanoparticles start to coalesce, rapidly forming rod-like structures. The size of such formed structures, as well as the size of crystallites, rapidly increases with the temperature. This lowers the specific surface area of the coatings and causes a shift in the optical band gap from 2.68 eV to 2.48 eV. Finally, the subsequent heating of the nanoparticle films above 650 °C causes the complete collapse of the coatings due to their melting.