High solar-to-hydrogen efficiency in a direct Z-scheme GaAs/GeH van der Waals heterostructure for photocatalytic water splitting.

The development of efficient photocatalysts for solar-driven water splitting remains a critical challenge due to limited light absorption, rapid charge recombination, and insufficient redox capability. In this work, we propose a GaAs/GeH van der Waals (vdW) heterostructure and systematically investigate its structural, electronic, mechanical, optical, and photocatalytic properties using first-principles calculations. The results demonstrate that the heterostructure is energetically, mechanically, and dynamically stable and exhibits a direct band gap with a type-II band alignment, enabling effective spatial separation of photogenerated charge carriers. The GaAs/GeH heterostructure shows a reduced band gap and significantly enhanced optical absorption in the visible region. In addition, improved electron mobility is observed, which is beneficial for charge transport. Importantly, the band-edge positions are well aligned with the redox potentials of water, allowing both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) to proceed. The system follows a direct Z-scheme charge transfer mechanism driven by an intrinsic built-in electric field, which preserves strong redox ability while suppressing carrier recombination. As a result, a theoretical solar-to-hydrogen (STH) conversion efficiency of 24.41% is achieved, demonstrating competitive photocatalytic performance compared with previously reported 2D heterostructures.These findings highlight the strong potential of the GaAs/GeH heterostructure for efficient solar-driven water splitting and provide valuable insights for the rational design of advanced photocatalytic materials.