Synergistic Effect of Doping and Compositing on Photocatalytic Efficiency: A Case Study of La₂Ti₂O₇

Charge generation and separation are two key issues in developing a high-efficiency semiconductor for the visible-light-driven photocatalysis. Here, we use the layered perovskite-type wide-gap semiconductor La₂Ti₂O₇ (LTO) as a model to systematically explore the synergistic effect of doping (with sulfur or nitrogen) and heterojunction (with graphitic C₃N₄) on improving visible light absorption and photoexcited charge separation by means of density functional theory calculations. It is found that the anion (N or S) doping into the LTO(010) surface can not only shift the optical absorption edge to the visible region, but also creates some partially occupied or unoccupied states in the band gap that would facilitate the formation of recombination centers. For the purpose of promoting electron-hole separation, the (N or S-doped) LTO(010) surfaces were hybridized with the monolayer g-C₃N₄. Interestingly, we found that the (S-doped) LTO/g-C₃N₄ heterostructure forms a type-II heterojunction, with the valence band maximum residing in the (S-doped) LTO and the conduction band minimum in g-C₃N₄, respectively. This band alignment feature facilitates efficient electron-hole separation. Moreover, we found that the S-doped LTO/g-C₃N₄ composite has a short interfacial distance (about 2.1 Å), implying that the interfacial interaction of this composite might be a chemical bond rather than a weak van der Walls interaction. The chemical bonding can enhance charge separation. Our theoretical findings provide design principles for optimizing the photocatalytic performance of the wide-gap photocatalysts and demonstrate that the S-doped LTO/g-C₃N₄ composite would be a potential candidate for the photocatalysis of water splitting.