KCa ₂ Nb ₃ O ₁₀ -Carbon Doped g-C ₃ N ₄ 2D-2D Heterostructure for Gas-Phase Solar-Light Driven CO ₂ Photoreduction Using Water Vapor as a Reducing Agent

Dion–Jacobson layered perovskite KCa2Nb3O10 offers favorable redox potentials and structural robustness but remains limited in solar-driven CO2 conversion due to its wide band gap and inefficient charge utilization. Here, we report a carbon-doped g-C3N4/KCa2Nb3O10 2D–2D heterojunction that enables controlled modulation of interfacial charge-transfer pathways. Carbon incorporation into g-C3N4 enhances visible-light absorption and tailors the band alignment without altering the bulk crystal framework. In situ light–dark EPR analysis reveals a transformation from type-II transfer in CN/KCNO-NS to a direct Z-scheme in CN-C/KCNO-NS, supported by complementary electrochemical and spectroscopic analyses demonstrating improved interfacial charge separation and reduced recombination. Under continuous gas-phase CO2/H2O vapor flow without sacrificial agents or noble metals, the optimized heterojunction exhibits enhanced activity, generating H2 as the dominant product together with CH4 and CO. The dominance of H2 formation is consistent with rapid interfacial electron delivery and the kinetic advantage of HER under gas-phase CO2/H2O conditions. This study demonstrates electronic-structure-driven 2D–2D heterojunction engineering as an effective approach for regulating charge flow in solar-driven gas-phase CO2 conversion systems.