Monolayer Group IV–VI Monochalcogenides: Low-Dimensional Materials for Photocatalytic Water Splitting

Harvesting solar energy for artificial photosynthesis is an emerging area in alternative energy research. In the present article, we have investigated the photocatalytic properties of single-layer group IV–VI monochalcogenides, MXs (M = Ge, Si, Sn and X = S, Se) based on first-principles electronic structure calculations. Our dispersion corrected DFT calculations show that these materials have moderate cohesive energies (<120 meV/atom), which are indicative of favorable isolation of MX monolayers by mechanical, sonicated, or liquid-phase exfoliation. The calculated band gaps using hybrid density functional method (HSE06) reveal that all of the MXs show larger band gaps than the minimum energy required for the water splitting reaction (1.23 eV). Considering band edge alignments, all the MXs other than SiS have an acceptable alignment of conduction band minima but not the valence band maxima. We have evaluated the overpotentials for both oxygen and hydrogen evolution reactions. Interestingly, considering contribution from overpotentials, we have tuned the band alignments by varying the pH of the medium. At a basic pH, GeS and SiSe exhibit excellent photocatalytic properties whereas for SiS, an acidic pH is required. Additionally, the optical absorption spectrum shows excellent absorption in the visible region indicating efficient harvesting of solar radiation. They are substantially stable even in aqueous environment indicating their robust stability at ambient electrochemical conditions.