Two-dimensional germanium monochalcogenides for photocatalytic water splitting with high carrier mobility

Highly efficient utilization of solar energy to split water into hydrogen and oxygen is regarding as a promising strategy to deal with the future energy crisis and environmental problems. To explore highly efficient and low-cost photocatalysts is highly desired. Herein, phosphorene-like germanium monochalcogenides (GeS and GeSe monolayers) are proposed here as efficient photocatalysts for water splitting. After confirming their stabilities, we observe that GeS exhibits an indirect band gap of 2.29 eV while GeSe reveals a direct band gap of 1.59 eV by HSE hybrid functional. Remarkably, high and directionally anisotropic carrier mobilities (2430.50 cm2 V−1 s−1 for GeS and 4032.64 cm2 V−1 s−1 for GeSe) are quantitatively investigated by using deformation potential theory. In addition, GeS and GeSe monolayers exhibit a good separation of electrons and holes, which effectively reduces the photocatalytic activity with high efficiency. Upon the application of strain, the band structure can be modulated from semiconductor to metal and a direct-indirect bandgap transition is observed. Most intriguingly, the band gaps and band edge alignments at certain pH value can be effectively tuned to meet the requirement of the redox potential in water splitting. Finally, the adsorption and decomposition of water molecules on the surface of 2D GeS and the subsequent formation of hydrogen were explored, which unravels the mechanism of photocatalytic hydrogen production on 2D GeS. Our findings will be valuable for facilitating the exploration and application of GeS and GeSe for photocatalytic water splitting.