In the purpose of preparing highly efficient film photocatalyst and exploring rule of semiconductor photocatalytic and photoelectrocatalytic degradation of organic pollutants, (Ni-Mo)/TiO₂ composite thin films were prepared by composite electroplating at a constant current. The depositing conditions of (Ni-Mo)/TiO₂ film were as follows: suspension quantity of TiO₂ in solution was 6.0 g/L; pH value in solution was 10.5; electrodepositing time and current density were 15 min and 95 mA/cm², respectively. The surface morphology, phase structure, and spectral characteristics of the thin film were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectrum and ultraviolet-visible diffuse reflectance spectroscopy (DRS), respectively. Its photoelectrocatalytic properties were evaluated with Rhodamine B as a model compound under the conditions of negative bias and visible light. Using electrochemical technique and adding active species scavenger to the solution, the mechanism of photoelectrocatalytic degradation for the film were explored. The results indicate that the (Ni-Mo)/TiO₂ film consists of crystalline grains of TiO₂ in the size range of 50～100 nm and the nano-crystalline of Ni-Mo in solid solution. The (Ni-Mo)/TiO₂ film is photoelectrocatalytically more active than TiO₂/ITO (indium tin oxide) film. Reacting 60 min under the negative bias of -0.4 V and visible light irradiation, the photoelectrocatalytic degradation rate of (Ni-Mo)/TiO₂ film is 1.56 times as much as that of porous TiO₂(Degussa P25)/ITO film. The improvement in photoelectrocatalytic degradation activity for the composite film could be mainly attributed to the catalysis of Ni-Mo in the composite film for the reaction of excited electrons with oxygen. By adjusting the applied bias, the existing forms of the dye and active oxygen species at the electrode/electrolyte surface and their interactions can be controlled. It is the efficient method to study the visible light catalytic degradation reaction mechanism. Both the hydroxyl radical and the dye cationic radical have a decisive role on the photoelectrocatalytic degradation of dyes under the conditions of negative bias and visible light.

Key words: (Ni-Mo)-TiO₂ film, photoelectrocatalysis, negative bias, Rhodamine B, mechanism