Annealing effect on structural, morphological, and optical properties of reactive sputtered WO₃ films for mediated heterogeneous photocatalyst

The reactive facing-target sputtering method was used to deposit WO₃ thin films from a metal tungsten disk in an Ar+O₂ mixture gas atmosphere at different sputtering pressures. X-ray diffraction, scanning electron microscopy (SEM), UV-visible spectrophotometer, and Raman studies were performed to study structural, surface morphology, and optical properties of the as-deposited and annealed samples. Annealing treatments were done in oxygen atmospheres. All the as-deposited films were amorphous in structure. The films annealed at 400 °C and deposited at 200 W show preferential orientation. SEM images show nanorodlike growth for the films deposited at a sputtering pressure of 0.15 Pa and annealed at 400 °C. The optical absorption edge of the as-deposited films prepared at the sputtering pressures of 0.8–0.15 Pa varied between 340 and 380 nm and shifted up to 480 nm when the samples were annealed at 400 °C. From Raman spectra, we observed the O–W⁶⁺–O bonds, and the W⁶⁺O stretching mode of terminal atoms on the surface of WO₃ microcrystalline grains. The crystal structures of the annealed films were of monoclinic phase. The suitability of the films for the WO₃/TiO₂ heterogeneous photo- - catalyst are analyzed and discussed.