A direct Z-scheme GeS/GeSe van der Waals heterojunction as a promising photocatalyst with high optical absorption, solar-to-hydrogen efficiency and catalytic activity for overall water splitting: First-principles prediction

Seeking an efficient photocatalyst is the key to solve the problem of energy shortage and environmental pollution. We construct a GeS/GeSe heterojunction and explore its structural stability, electronic property, charge transfer, optical property and solar-to-hydrogen energy conversion efficiency by first-principles calculations. The results show that the GeS/GeSe heterojunction has high thermal and kinetic stability and is a direct Z-scheme photocatalyst with a smaller indirect band gap (1.73 eV) than two monolayers GeS (3.28 eV) and GeSe (3.00 eV), which can not only avoid the backward reaction and light-shielding effect caused by the carrier transfer mediators in traditional and all-solid-state Z-scheme photocatalysts, but also increase the light-harvesting ability, suppress recombination of the photoexcited charge carriers, promote surface catalytic reaction by preserving the strong redox capacity of photoexcited charge carriers with respect to the traditional type-II and p-n heterojunction photocatalyst. The charge transfer analysis shows that the electron accumulates on the GeS side and the hole accumulates on the GeSe side, forming a built-in electric field from GeSe to GeS, which can effectively restrain the recombination of photogenerated carriers. Besides, its band edge positions support overall water splitting reactions in acid and alkaline environments. The optical absorption of the heterojunction is higher than that of its monolayer materials and the solar-to-hydrogen energy conversion efficiency reaches up to 31.21%, which effectively improves the absorption and utilization rate of solar energy. We have good reasons to believe that the GeS/GeSe heterojunction provides an effective strategy for the research and development of efficient photocatalysts.