First-Principles Calculations on Semiconducting ε-GeS and ε-SnS Monolayer Nanosheets with Photocatalytic Activity for Sunlight-Driven Water Splitting

The design of two-dimensional (2D) ultrathin nanosheets with favorable electronic and optic properties that can satisfy the essential requirements of photocatalytic water splitting remains a challenge. Anisotropic Janus 2D materials exhibit fascinating potential due to their excellent photocatalytic performance, yet the scarcity of such materials limits their wide application. Herein, an allotrope of GeS and SnS monolayers with a ε-phosphorene-like atomic arrangement (ε-GeS and ε-SnS monolayers) is predicted to be a promising electrocatalyst for photocatalytic water splitting by first-principles calculations, and the results attest to its good stabilities. Both Janus monolayers prove to be semiconducting with direct band gaps of 2.63 eV for ε-GeS and 2.41 eV for ε-SnS, straddling the redox potentials of water. HSE calculations reveal that both GeS and SnS allotropes display notable anisotropy along with high hole mobility (∼103 cm2 V–1 s–1). Moreover, both monolayers present strong optical absorption in the visible-light excitation range (105 cm–1), thus ensuring efficient harvesting for solar energy. Particularly, spontaneous hydrogen (HER) and oxygen evolution (OER) half reactions can occur simultaneously, driven only by sunlight-induced carriers, which still work even under strain. More interestingly, after transition-metal atom (from Sc to Zn) decoration, Co/ε-GeS and Sc/ε-SnS are confirmed as the most efficient single-atom HER catalysts, capable of driving HER with low overpotentials of 0.02 and −0.05 V, respectively, even outperforming the commercial Pt catalyst (−0.09 V). Meanwhile, for OER, low-overpotential Ni/ε-SnS (0.42 V) proves to be better than the generally used IrO2 catalyst (0.55 V).