Unlocking ultrahigh solar-to-hydrogen efficiency in 2D GaN via S-scheme heterojunctions with HfS_xSe_2-x

Two-dimensional (2D) gallium nitride (GaN) is a promising photocatalyst owing to its robust stability and strong ultraviolet absorption. However, its practical application for overall water splitting is severely hindered by a low solar-to-hydrogen (STH) efficiency of ∼0.28%, mainly due to the insufficient driving force from its valence band maximum being too close to the water oxidation potential and rapid charge recombination. Herein, we overcome these limitations by designing 2D/2D S-scheme heterojunctions between GaN and transition metal dichalcogenides HfS_xSe_2-x (x = 0, 1, 2). Our comprehensive density functional theory calculations demonstrate that these van der Waals heterostructures are thermodynamically, dynamically, and mechanically stable. The interfacial contact induces a built-in electric field that drives the recombination of useless carriers while preserving the most redox-active electrons and holes, thereby optimizing the effective band edges for water splitting. Remarkably, the corrected STH efficiencies of 2D GaN/HfS₂, GaN/HfSSe and GaN/HfSe₂ heterojunctions reach 50.64%, 49.51%, and 45.28% respectively, over 2 orders of magnitude higher than that of pristine 2D GaN. Furthermore, 2D GaN/HfS₂ exhibits pH-universal photocatalytic activity, while the Janus-based counterparts perform optimally under alkaline conditions. This work establishes a highly efficient and versatile platform for 2D GaN-based photocatalysts.