A new understanding of the photocatalytic mechanism of the direct Z-scheme g-C3N4/TiO2heterostructure

Constructing a TiO₂ based heterostructure is a very effective strategy for enhancing photocatalytic performance. The details of the electronic structure, interfacial interaction, and photogenerated carrier separation are important for explaining the photocatalytic properties of a heterostructure. Herein, the density of states, charge distribution, and the band offset of the monolayer g-C₃N₄/TiO₂ heterojunction are systematically investigated through the hybrid DFT method. Results indicated that the valence band offset and the conduction band offset of the g-C₃N₄/TiO₂ heterostructure were 0.40 and 0.18 eV, respectively. Interfacial interaction made the TiO₂ surface with negative charge, whereas the g-C₃N₄ surface with positive charge, which led to the formation of a built-in electric field at the interface. Under illumination, the built-in electric field accelerates the transfer of photoexcited electrons in the CB of TiO₂ into the VB of g-C₃N₄, thus resulting in the photoexcited electrons and holes naturally accumulating in the CB of g-C₃N₄ and the VB of TiO₂, respectively. The photoexcited electrons and holes gathering in different surface regions efficiently prolonged the lifetime of photogenerated carriers. Meanwhile, electrons in the CB of g-C₃N₄ and holes in the VB of TiO₂ had a stronger redox ability. Therefore, g-C₃N₄/TiO₂ is a direct Z-scheme photocatalyst, and the Z-scheme heterostructure mechanism can well explain the improved photocatalytic activity of the g-C₃N₄/TiO₂ heterostructure.