Fast Carrier Recombination, Nanoconfinement, and Defects Boost Solar-Driven Hydrogen Evolution Reactions at Z-Scheme Heterojunctions

The vertical heterostructure between phase-different two-dimensional (2D) transition metal dichalcogenides (TMDs) and metal monochalcogenides exhibits a promising photocatalytic performance. We combine several computational methods to explore catalytic processes for solar-driven hydrogen evolution reactions (HERs) on thermodynamically stable MoX₂/SnS (X = S, Se, and Te). Except for MoTe₂/SnS, these heterostructures have prominent staggered band alignment, significantly reducing the band gap and enhancing optical response to visible light irradiation. The time-domain Kohn–Sham (TD-KS) theory and nonadiabatic molecular dynamics (NAMD) depict fast interlayer electron–hole recombination between band edge states, revealing prominent Z-scheme heterojunction formation. The band-to-band relaxation eventually leads to long carrier lifetimes and high redox potential for photogenerated electrons in the SnS layer, enhancing HER catalytic performance. Moreover, the inner layer of SnS that noncovalently interacts with MoS₂ emerges as a superior photocatalytic surface compared to the traditionally investigated outer one. Nanoconfinement influences the hydrogen bond formation between the reactant H atom and X of MoX₂ (X = S and Se), boosting the catalytic process at the interlayer space. The defect engineering, especially the facile formation of Sn vacancy sites within the nanoconfined SnS layer emerge as the most thermodynamically favourable sites for enhancing the HER activity in the SnS layer. These vacancies introduce a unique local electronic environment that promotes the adsorption and activation of reaction intermediates. The significant dynamic fluctuations of the transient S–H bond at these sites further depict the optimal binding of the reactant, ensuring the best catalytic activity of Sn vacancies in the inner layer. The in silico study provides a detailed atomistic understanding of the hydrogen evolution process on metal chalcogenide heterostructures, suggesting their strategic design principles to boost photocatalytic activities.