Understanding the Mechanism of Photocatalysis Enhancements in the Graphene-like Semiconductor Sheet/TiO2 Composites

The electronic properties of monolayer transition-metal dichalcogenide MX2 (M = Mo and W; X = S and Se) interfaced TiO2(110) composites were investigated by hybrid density functional theory. In the MX2/TiO2(110) composites, MX2 serves as an efficient photosensitizer, and the electron hole pair can, therefore, be easily generated by visible-fight irradiation and be effectively separated by the electron injection from MX2 to TiO2. This mechanism is quite different from the one of the foreign elements doped TiO2, in which the electron is directly excited from the midgap impurity states into the CB of TiO2, leading to an optical absorption edge extending to the visible-light region. Moreover, we reveal that the prerequisite of designing the highly efficient semiconductor TiO2 photocatalytic composites is to select the proper semiconductor, which holds the band gap of similar to 2.0 eV and generates a built-in potential of 0.3-0.5 eV in the composite, as a photosensitizer, which can be also considered as a fundamental criteria to screen the suitable semiconductor and further to design the TiO2-based heterojunction composites for improving visible-light photocatalysis.