Annealing-induced oxidation state transition, crystal formation, optical properties, and photocatalytic activity of vanadium oxide nanoparticles

Vanadium has various oxidation states and multiple crystalline phases that make it interesting for various applications. The oxidation state transition and crystal formation of vanadium oxide (VOx) were affected by growth conditions and annealing temperatures. In this study, VOx nanopowders were prepared by hydrothermal method, and annealing-induced characterizations of VOx were analyzed. The morphologies, structures, composition, and optical properties of VOx were characterized by SEM, XRD, EDX, FTIR, and UV–Vis spectroscopy. The results demonstrated that the annealing temperature significantly affected the transition of oxide states from the VOOH and VOx clusters to V2O5 nanoparticles and the crystal size from amorphous to 38.96 nm which led to an increase in the optical band gap from 2.28, 2.26 to 2.39 and 2.38 eV as increasing calcination temperature and enhanced photocatalytic activity under sunlight irradiation. The energy dispersive X-ray (EDX) spectra reveal that the percentage molar mass between vanadium and oxygen changes due to the oxidation state transition and the formation of oxygen vacancies in V2O5. The relation between nanoparticle size, oxidation state, and crystal size was clarified by comparing EDX and XRD spectra.