Visible-Light Photocatalytic Activities Of Narrow-gap InVO₄ Coupled With TiO₂ Thin Films

InVO₄ material was found to be a visible light responding photocatalyst because of its narrow-gap energy band. The InVO₄ sol was synthesized by low-temperature reflux method using indium trichloride (InCl₃), ammonium metavanadate (NH₄VO₃) and potassium hydrate (KOH) as raw materials, which reveals the synthetic temperature of InVO₄ is effectively reduced. InVO₄ doped RS sol had been prepared through blending InVO₄ sol into RS sol under continual magnetic stirring. Novel InVO₄-TiO₂ thin films and functionally gradient thin films on glass slides were synthesized via a sol-gel dipping method from the as-synthesized composite sol. The influences of processing parameters on the phase of InVO₄ sol, the shape of the crystal grain, the stability of the sols and the photocatalytic performance of InVO₄-TiO₂ composite thin films were investigated in detail.The results of samples reveal that the pH value of the reaction system, reflux time and different processing method play important roles to the target phase. It is shown that the well-crystallized InVO₄ particles were obtained when the pH value was 8. With the increase of the reflux time, the dispersity and the size of the grains in the sol were obviously improved. According to the graded distribution of chemical environment on the crystal grain surfaces, the forming process of the InVO₄ sol under low-temperature reflux conditions is investigated in this paper based on crystallography. The whole process can be explained that in the graded chemical environment, the VO₃^- monomers and In³⁺ ions incorporate into the In(OH)₃ particle and replace OH^- to form orthogonal InVO₄ crystal.In this study, the visible-light activity of InVO₄-TiO₂ composite thin films and functionally gradient thin films are tested. The photocatalytic results by degradation of methyl orange indicate that the InVO₄-TiO₂ composite thin film with 10 layers shows excellent photocatalytic activities under visible-light irradiation; the films with 3.0wt% InVO₄ content without heat treatment exhibits the highest visible light photocatalytic activity, the degradation rate of methyl orange solution reaches 33.8% after irradiated for 12 h. It is due to the heterojunction formed by the two semiconductors. The outer-shell electrons of InVO₄ can absorb the energy of light and the electrons in the InVO₄ valence band can be excited into conduction band by visible light, and the excited electrons inject into the conduction of the TiO₂, which makes photo-generated electrons and holes separation more efficient and enhances the visible light photocatalytic activity of the thin film.