Direct Z-scheme ZnI₂/GeTe van der Waals heterostructure for photocatalytic overall water splitting

The rising global energy crisis and environmental degradation highlight the urgent demand for efficient photocatalytic materials for solar-driven hydrogen production through water splitting. Two-dimensional (2D) heterostructures are attractive because interfacial coupling can combine efficient charge management with strong redox capability. In this study, first-principles calculations are used to investigate the structural, electronic, optical, and photocatalytic properties of a ZnI₂/GeTe van der Waals heterostructure. The optimized heterostructure exhibits indirect band gaps of 1.23 eV at the PBE level and 1.96 eV at the HSE06 level, while its equilibrium projected electronic structure has a staggered type-II alignment. Importantly, the charge-transfer mechanism is not assigned from this static band alignment alone. The combined charge-density difference, Bader charge transfer (∼0.037|e| from GeTe to ZnI₂), work-function/electrostatic-potential analysis, and band-decomposed charge densities support a direct Z-scheme-like photoinduced pathway in which the comparatively weaker ZnI₂-derived conduction electrons recombine with GeTe-derived valence holes at the interface, while strongly reducing electrons are retained in GeTe-derived conduction states for HER and strongly oxidizing holes remain in ZnI₂-derived valence states for OER. The CHE analysis gives an OER overpotential of approximately 0.70 V at pH = 0 and H-adsorption free energies of 0.360-0.453 eV for the examined HER configurations. The calculated direct Z-scheme solar-to-hydrogen efficiency is 1.18%. These results identify ZnI₂/GeTe as a promising theoretical platform for photocatalytic overall water splitting while providing a conservative interpretation of the charge-transfer mechanism and solar-energy conversion performance.