Direction-regulated electric field implanted in multilayer Mo–TiO2 films and its contribution to photocatalytic property

• Propose a novel approach to implant internal electric fields in multilayer films.
• Optimized film structure with an ultrathin, heavy doped bottom layer.
• Investigate effects of various layer-assemble modes on internal carrier transfer.
• Confirm the effectiveness of electric field as carrier separation accelerator.

Multilayer Mo-doped TiO2 thin films were prepared by In-situ RF magnetron co-sputtering. Surface morphology, crystallite parameters, valence states and absorption band were investigated with atomic force microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible spectroscopy. AC impedance spectroscopy and photocatalytic capability of different layer-assemble modes were examined on an electrochemical workstation under visible light. The result indicates the electric fields resulted from Fermi level drops could remarkably accelerate the separation of photogenerated carriers. The double-layer film, which was prepared by covering the uniformly Mo doped layer with an undoped surface layer, has the smallest impedance. The strongest catalytic capability is demonstrated by the three-layer film, which has an undoped surface layer, a uniformly doped middle layer and an ultrathin, heavy doped bottom layer. Although different layer modes have little influence on absorption edge, our observations suggest that by manipulating doping content in each layer, we can implant upward electric field arrays, which has considerable potential to enhance the photocatalytic property, into multilayer Mo-doped TiO2 films.