Janus structures with a built-in electric field have shown many exotic properties, especially for new mechanisms in water splitting and out-of-plane piezoelectricity. In this work, Janus 1T- and 2H-$M\text{SH}$ monolayers are designed, guided by the synthesized Janus 2H-MoSH monolayer. Among them, semiconducting Janus 1T- and 2H-$M\text{SH}$ ($M$ = Sc, Y, V, Nb, Ta) monolayers are first determined by first-principles and high-throughput calculations. Intriguingly, they emerge as promising photocatalysts with suitable band edge alignments for water splitting, owing to their intrinsic built-in electric field. In particular, 2H-TaSH shows strong light-harvesting ability in the infrared and visible regions, adequate overpotentials, high solar-to-hydrogen energy conversion efficiency and autonomous hydrogen and oxygen evolution. Furthermore, Janus 1T- and 2H-ScSH exhibit particularly large piezoelectric coefficients of $d_{31}$ = 3.93 and 3.10 pm/V, respectively, which are attributed to their exceptionally low Young's modulus combined with significant charge asymmetry between the two surfaces. Our work significantly expands the family of Janus transition metal sulfhydrates while simultaneously identifying promising candidate materials for both photocatalytic water splitting and piezoelectric applications.

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• Electrical properties
• Excitons
• Photocatalysis
• Structural properties
• 2-dimensional systems
• Piezoelectrics
• Semiconductors
• Density functional theory
• Many-body techniques