Two-dimensional blue-phosphorene-phase germanium monochalcogenide photocatalysts for water splitting: From ultraviolet to visible absorption

Hydrogen generation from photocatalytic splitting of water utilizing solar energy is the most ideal way to alleviate the energy crisis at present. Using first-principles calculations, two-dimensional blue-phosphorene-phase GeX (X = S, Se) monolayers are systematically evaluated as photocatalysts for water splitting. Results show that the band edge positions of GeX locate outside the reduction potential of hydrogen and oxidation potential of water, and the values of bandgap fall into the ultraviolet range, suggesting that GeX would be served as an ultraviolet-driven photocatalyst. Interestingly, with the applying of tensile strain, the band edge positions are still suitable whereas the band gaps are significantly lowered to the visible range, and meanwhile, the intrinsic dipoles and concomitant electrostatic potential differences between Ge- and X-sides are greatly enhanced. In particular, at the 10% strain, the electrostatic potential difference reaches around 1.23 eV, which breaks the requirement of bandgap restriction for the conventional photocatalysts, thereby broadening the spectral range of light absorption from ultraviolet to visible. Using GeSe as an example, further calculations indicate that strain can enhance the optical absorption in the visible range and decrease the exciton binding energy. At the same time, the effective masses of carriers are not strongly affected by the strain. The increased internal electric field would act as an auxiliary booster to break up an exciton pair and inhibit the probability of electron-hole recombination. Moreover, strain can dramatically lower the free energy of two elementary steps of hydrogen evolution reaction. All these evidences confirm that GeX monolayers with a certain strain can serve as promising photocatalysts for water splitting under the visible range. (C) 2019 Elsevier Inc. All rights reserved.