Defect concentration regulation in nanoflower-like WO3 film and its influence on photocatalytic activity

Traditional anodic oxidation followed by annealing was performed to prepare a nanoflower-like WO 3 film. The XRD results show that triclinic–monoclinic mixed-phased WO 3 formed. The TEM images confirmed the heterojunction formation of triclinic–monoclinic WO 3 . A further reduction treatment of the as-annealed specimens in a mixed solution of EG and NH 4 F was carried out at a low temperature of 333 K for 24–72 h. The XPS and EPR analysis revealed that the surface and bulk defects formed, and the defect concentration increased with the increase in reduction time. The defects’ introduction decreased the bandgap values and shifted the bandgap structure towards a more positive position. Although the defect introduction did not significantly improve photogenerated electrons and holes’ separation efficiency, it lowered the charge transfer resistance and enhanced the interfacial transfer efficiency. The visible-light-driven photocatalytic activity of the as-reduced WO 3 film increased to varying degrees. The as-reduced sample for 48 h exhibited the highest photocatalytic activity, which was 1.7 times that of the as-annealed specimen. A volcanic-like curve relationship between the photocatalytic activity, defect concentration, and reduction time established. A moderate defect concentration is beneficial to improve the photocatalytic performance of the WO 3 film. Anodic oxidation followed by a solution reduction is an efficient and economical way to prepare metallic oxide films with a large specific surface area.