Proton migration pathways and hydrogen evolution mechanism in g-C3N4/TiO2-B and Li–F co-doped heterostructures: a theoretical study

Abstract

Solar water splitting has received a lot of attention due to its high efficiency and clean energy production potential. Herein, based on the band-alignment principle, the g-C₃N₄/TiO₂-B(001) heterostructure is strategically designed and then a Li–F co-doping approach is developed and implemented, leading to significant enhancement in the photocatalytic hydrogen evolution efficiency of the heterostructure systems. The decomposition of water molecule on the surface of the heterostructures, the migration and diffusion of proton across the interface, and the hydrogen evolution performance are systematically studied and comprehensively analyzed. The results demonstrate that the heterojunction surface exhibits a relatively low energy barrier for water decomposition, facilitating both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Proton transfer preferentially occurs from the TiO₂-B(001) surface to the g-C₃N₄ surface through the interface. The presence of polar covalent bonds establishes a substantial energy barrier for proton migration from the TiO₂-B(001) surface to the interface, representing a rate-determining factor in the hydrogen evolution process. The formation of hydrogen bond (O–H⋯N) significantly reduces the migration energy barrier for proton crossing the interface to the g-C₃N₄ surface. Notably, in the Li–F co-doped heterostructure system, while proton diffusion is observed at the interface, this interfacial proton diffusion does not constitute a determining factor for the overall HER performance. Furthermore, Li–F co-doping enhances interfacial polarization, leading to a substantial reduction in the migration energy barrier for protons moving from the interface to the g-C₃N₄ surface. Hydrogen adsorption free-energy analysis shows that the heterojunction surface exhibits optimal proton adsorption and desorption characteristics. The synergistic combination of low water decomposition energy barrier, reduced proton migration energy barriers and exceptional HER performance endows both the g-C₃N₄/TiO₂-B(001) heterostructure and the Li–F co-doped g-C₃N₄/TiO₂-B(001) heterojunction with remarkable potential as efficient HER photocatalysts.