A GaSe/BiSCl van der Waals heterostructure, which exhibits an efficient Z-scheme photocatalytic mechanism, is systematically investigated using first-principles calculations. The results reveal that the heterostructure possesses a strong interfacial built-in electric field ( \(4.2 \times {10}^{9} \text{V }{\text{m}}^{-1}\) ), which facilitates the spatial separation of electrons and holes into the conduction band minimum (CBM = −3.18 eV) of GaSe and the valence band maximum (VBM = −7.49 eV) of \(\text{BiSCl}\) , respectively, suggesting pronounced reduction–oxidation (REDOX) capabilities. The band edges straddle water REDOX potentials over a wide pH range, enabling spontaneous overall water splitting. The theoretical solar-to-hydrogen (STH) conversion efficiency reaches 32.2%, substantially surpassing that of the monolayer counterparts (GaSe monolayers cannot perform oxygen evolution; BiSCl monolayer STH efficiency is only 0.3%). Remarkably, the heterostructure exhibits ultrahigh electron mobility ( \(4758.5 {cm}^{2}{V}^{-1}{s}^{-1}\) ) and a pronounced disparity in carrier mobility, effectively suppressing charge recombination. Both strain and electric field can effectively modulate the band structure of the material. Under a compressive strain of −6%, the bandgap decreases to 0.46 eV, whereas under a reverse electric field of 0.5 V/Å, it is reduced to 0.08 eV. Moreover, the −6% compressive strain shifts the optical absorption peak to the green light region. These results indicate promising prospects for using this material in highly efficient photocatalytic systems.
