Investigation of post-annealing treatment effect on film properties of sputter-deposited BiVO4 nanoporous photocatalyst

Nanoporous BiVO4 thin films were deposited on fused silica substrate using reactive magnetron sputtering. The effect of annealing temperature on the microstructure, morphology and optical properties was evaluated. The samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), ultraviolet-visible spectroscopy (UV-Vis) and X-ray photoelectron spectroscopy (XPS). XPS demonstrated the Bi+3 and V+5 oxidation states, as well as the adsorbed and lattice oxygen on the film surface. The as-deposited films proved to be amorphous by the XRD results, while the pure monoclinic scheelite BiVO4 crystal structure was obtained after a post-annealing treatment at 300 and 450 ℃. FESEM images displayed a uniform surface with no grain boundaries for the as-deposited film, whereas nanopores with an average diameter of 20˗40 nm were observable in the film annealed at 450 ℃ as opposed to the film annealed at 300 ℃ with a dense and cracked surface. The association of nanoporosity with the efficiency of visible-light absorption was demonstrated by the UV-Vis spectrophotometry results with the narrowest bandgap (2.5 eV) concerning the film annealed at 450 ℃. The photocatalytic experiment under visible light showed an 80 % photodegradation of Rhodamine-B solution after 7 h, and the recycling experiment proved the stability of the thin films after three cycles. These results show the great potential of BiVO4 thin films deposited by reactive magnetron sputtering in photocatalytic wastewater treatment applications.