Toward enhanced photocatalysis: High-mobility two-dimensional polar heterojunctions for water splitting

Abstract

The efficiency of overall water splitting is often hampered by the bandgap constraint of photocatalysts, restricting solar energy harvesting. To address this issue, two-dimensional (2D) polar materials offer a distinct advantage: their intrinsic internal electric field induces energy level bending, effectively extending the light absorption spectrum. Using first-principles calculations, we systematically evaluate the photocatalytic performance of 25 MXYN4/MXYN4 (M = Mo, W; X, Y = Si, Ge) van der Waals heterojunctions (vdWHs) as a model system for polar heterostructures. These vdWHs exhibit superior electronic properties, such as suitable band alignments, high carrier mobility, and strong light-harvesting capability. Meanwhile, a pronounced intrinsic electric field is generated, driven by considerable interfacial charge transfer between the constituent MXYN4 monolayers. This synergistic effect enhances the spatial separation of charge carriers and suppresses their recombination, thereby facilitating efficient photocatalysis. Among the 25 polar heterojunctions, 20 are spontaneously active for the hydrogen evolution reaction (HER) and 13 for the oxygen evolution reaction (OER) under illumination. Remarkably, the polar vdWHs show markedly superior HER activity under acidic conditions and significantly boosted OER performance in alkaline environments. These findings underscore the potential of 2D polar vdWHs for high-performance photocatalysis.