Abstract
Lead-free halide perovskite, kesterite, and delafossite semiconductors were integrated into a multilayer ternary heterostructure (Cs(2)SnCl(6)/Cu(2)ZnSnS(4)/CuFeO(2)) to enable direct solar-driven hydrogen production from sewage water. X-ray photoelectron spectroscopy confirms the expected elemental composition and oxidation states, while X-ray diffraction verifies the successful incorporation of all three layers with well-defined crystallinity. Optical measurements reveal a systematic narrowing of the effective band gap, decreasing from 1.73 eV for CuFeO(2) to 1.50 eV for the Cu(2)ZnSnS(4)/CuFeO(2) bilayer and further to 1.12 eV for the complete Cs(2)SnCl(6)/Cu(2)ZnSnS(4)/CuFeO(2) stack. The multilayered architecture enabled effective charge separation and transport, delivering a photocurrent density of -24.0 mA cm-2, approximately 77 times higher than the dark current density. The incident photon-to-current efficiency reaches 77%. These results demonstrate strong photoresponsivity and confirm the suitability of the multilayer heterojunction for efficient solar-driven hydrogen production. The extracted thermodynamic parameters (ΔH* = 3.452 kJ mol(-1) and ΔS* = 9.644 J mol(-1) K(-1)) indicate a low activation barrier for interfacial charge transfer, suggesting that the system effectively couples photonic and thermal contributions to enhance hydrogen-evolution kinetics. Collectively, these findings establish the all-lead-free Cs(2)SnCl(6)/Cu(2)ZnSnS(4)/CuFeO(2) heterostructure as a highly efficient photoelectrode for solar-to-hydrogen conversion in complex wastewater environments. Demonstrating hydrogen evolution directly from sewage water further highlights the dual functionality of this architecture for simultaneous wastewater valorization and sustainable fuel production.