Temperature effects on VO2 thin films deposited by RF sputtering for the degradation by photocatalysis of methylene blue and naproxen

The temperature effect on VO2 thin films synthesized by RF sputtering on their morphological, optical, electrical properties and their activity in the photocatalytic degradation of methylene blue and naproxen was studied. Characterization results presented microcrystallinity for VO2 films treated at 50 and 100 degrees C. Nevertheless, the untreated films and films treated at 200 degrees C revealed characteristic peaks of monoclinic and tetragonal phases. SEM micrographs with elemental mapping of VO2 films showed granular morphology and a good oxygen dispersion along the film surface, possibly due to a restructuring on the film occasioned by particle coalescence and vanadium oxide island conformation. The electronic transmittance spectra showed the d-d transition characteristic for the square-pyramidal stereochemistry of vanadium (IV) ion, where the optical band interval was high for films treated at 50 degrees C. Raman spectroscopy results presented an increment in the V = O/V-O ratio as a function of temperature, probably related to superficial vanadium species formation. X-ray spectroscopy results showed the Onon-lattice/O-lattice ratio values higher for films treated at 50 degrees C than the other films, related to an oxide character. The V 2p fit results presented V4+, V5+ regions and satellites for VO2 films thermal treated at 50 degrees C. The electrical resistivity on the VO2 films decreased as a function of temperature. Finally, the VO2 films thermal treated at 50 degrees C had higher photocatalytic activity in the degradation of methylene blue and naproxen compared to the other VO2 films, possibly associated with high electron mobility between the surface and the bulk, where the oxygen vacancies act as recombination sites for the e(-)/h(+) pairs during photocatalytic degradation.