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In Janus photocatalysts, intrinsic dipole (P) plays an important role in the separation of photogenerated carriers and photocatalytic activity for water splitting. However, effectively switching the direction of the P vector between different surfaces is still an open issue. Based on first-principles calculation, we find that the P vector relies on charge distribution, while the distribution of the conduction band maximum-distributed reduction reaction surfaces (Sc) and valence band minimum-distributed oxidation surfaces (Sv) depends on the orbital hybridization in GaOI-based Janus. The direction of the P vector can be switched between Sc and Sv by substituting a metal or halogen atomic layer. The P vector in the direction from Sc to Sv releases bandgap limitation, broadens the light absorption range, and improves high carriers' mobility and solar-to-hydrogen efficiency (STH) efficiency, while the P vector pointing from Sv to Sc hinders the photocatalytic activity of GaOI-based Janus. Based on the investigation of STH efficiency and redox reactions, GaInOI is found to be a potential photocatalyst for overall water splitting. The stability of the Janus is studied by ab initio molecular dynamics and phonon diffusion spectrum. Current work proposes that switching the direction of the P vector between Sv and Sc is a potential route to regulate the band structure, optical absorption, and photocatalytic activity in GaOI-based Janus. [ABSTRACT FROM AUTHOR]

• Photocatalyst for highly-efficient overall water splitting on Janus group-III chalcogenide. • Band edges cross the water-splitting reaction's redox potential. • Efficiently driving HER and OER, separating photogenerated electron-hole pair. • Overpotential of HER can reach the level achieved by photogenerated electrons and holes. • Intrinsic electric field size of Janus heterostructure can be changed. The intrinsic electric field is crucial to the charge transfer and separation of photogenerated carriers in Janus photocatalysts. However, the issue of how to effectively regulate the intrinsic electric field to improve photocatalytic activity remains unclear. We show that the Janus group-III chalcogenide heterostructure (AlS/InAlS) is a promising photocatalyst for highly-efficient overall water splitting using first-principles calculations. By replacing different layers of metal elements in the group-III chalcogenide, intrinsic electric field size of Janus heterostructure can be changed, promoting photogenerated electron-hole pair separation, and thereby improving photocatalytic performance. Its band edges cross the water-splitting reaction's redox potential. The high optical absorption coefficient (4.4 × 105 cm−1), high solar power conversion efficiency (14.31 %) indicate that AlS/InAlS heterostructure can efficiently utilise solar power. In addition, the overpotential of HER in the AlS/InAlS heterostructure can reach the level achieved by photogenerated electrons and holes. [ABSTRACT FROM AUTHOR]