All-lead-free Cs ₂ SnCl ₆ /Cu ₂ ZnSnS ₄ /CuFeO ₂ cascade band-aligned multilayer heterostructures for solar-driven hydrogen production from wastewater

Lead-free halide perovskite, kesterite, and delafossite semiconductors were integrated into a multilayer ternary heterostructure (Cs₂SnCl₆/Cu₂ZnSnS₄/CuFeO₂) 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 CuFeO₂ to 1.50 eV for the Cu₂ZnSnS₄/CuFeO₂ bilayer and further to 1.12 eV for the complete Cs₂SnCl₆/Cu₂ZnSnS₄/CuFeO₂ 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 (Δ<i>H</i>* = 3.452 kJ mol^-1 and Δ<i>S</i>* = 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 Cs₂SnCl₆/Cu₂ZnSnS₄/CuFeO₂ 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.