Reduction of V_2O_5 thin films deposited by aqueous sol-gel method to VO_2(B) and investigation of its photocatalytic activity

摘要

A way of preparation of VO_2(B) thin films by reduction of V_2O_5 films synthesized from an aqueous sol-gel system has been developed and photocatalytic properties of the obtained films were studied. The reduction was performed by annealing of the V_2O_5 film in vacuum as well as in H_2/Ar atmosphere, which was followed by temperature dependent XRD. It has been shown that the reduction is influenced by the layered-structure of the vanadium oxides. It is a two-step process, where the mixed-valence vanadium oxide V_4O_9 is first formed before reaching the VO_2(B) phase. The film microstructure was characterized by SEM and AFM and the valence states of vanadium in VO_2(B) films were evaluated by XPS. The VO_2(B) polymorph shows an energy band-gap around 2.8 eV and it exhibits photocatalytic properties. It was measured by following the degradation of rhodamine B under UVA as well as metalhalogenide lamp irradiation, which has similar spectral distribution as natural sunlight. The VO_2(B) films show distinct photoactivities under both lamps, although they were found to be more active under the UVA irradiation. The film annealed under reducing hydrogen atmosphere, which exhibits higher granularity and surface roughness, shows higher photoactivity than the vacuum-annealed film.