Self-defined monolayers, bilayers and trilayers of two-dimensional Janus MoSSe and Ga2SSe van der Waals homojunctions as potential photocatalysts for overall water splitting

Recently, two-dimensional Janus transition metal dichalcogenides have received significant attention in the field of photocatalytic water splitting due to their unique structural and electronic properties. The successful synthesis of Janus MoSSe, a highly representative material, has further accelerated research in this area. In this paper, the electronic structures and optical properties of self-defined MoSSe and Ga2SSe monolayer, bilayer and trilayer Janus homojunctions are systematically investigated using first-principles calculations. The results show that both [MoSSe]n and [Ga2SSe]n (with n = 1, 2 or 3) exhibit a type-II energy band arrangement, which is favourable for the effective spatial separation of electron-hole pairs. According to their band edge positions, [MoSSe]1 and [Ga2SSe]n are predicted to enable water splitting reaction under strongly acidic conditions, while the [Ga2SSe]1 may also operate under mildly alkaline conditions. Furthermore, analyses of their optical absorption coefficients and corrected solar-to-hydrogen (STH) efficiencies show that they all have strong light absorption and conversion capabilities. Specifically, [MoSSe]1 demonstrates strong absorption in the visible and ultraviolet regions, achieving a corrected STH efficiency of 20.39 %, while [Ga2SSe]n displays strong absorption primarily in the UV region, with corrected STH efficiency increasing from 6.49 % to 17.02 % as n increases. Additionally, under bias voltage, both [MoSSe]1 and [Ga2SSe]1 can enable spontaneous water splitting reactions. These findings suggest that [MoSSe]1 and [Ga2SSe]n are promising materials for efficient photocatalytic water splitting applications.