Twin S-Scheme g-C3N4/CuFe2O4/ZnIn2S4 Heterojunction with a Self-Supporting Three-Phase System for Photocatalytic CO2 Reduction: Mechanism Insight and DFT Calculations

The use of photocatalytic solar energy to drive CO2 reduction is beneficial for addressing fossil fuel shortages and environmental pollution issues. We synthesized a twin S-scheme g-C3N4/CuFe2O4/ZnIn2S4 heterojunction, which was used to construct a self-supporting three-phase system for photocatalytic CO2 reduction. Two built-in electric fields in this heterojunction induced effective migration of photogenerated carriers, resulting in a wide light response range and strong oxidation ability. This twin S-scheme photocatalytic system without a sacrificial agent had high CH4 selectivity (96.8%) and surprise production rate of CH4 (267.4 μmol g–1 h–1), and still maintained an excellent cycle rate (249–267.4 μmol g–1 h–1) during five cycles. In addition, g-C3N4/CuFe2O4/ZnIn2S4 heterojunction possessed both hydrophilicity and hydrophobicity, which achieved an efficient transformation of CO2 into CH4 by controlling interface wettability. g-C3N4 as a hydrophobic layer promoted CO2 mass transfer to achieve the enrichment of CO2 on the heterojunction surface; ZnIn2S4 as a hydrophilic layer could well adsorb H2O, which was further oxidized by the photogenerated holes into many protons (H+). Finally, DFT calculations found that Fe–N bonds located between g-C3N4 and CuFe2O4 played a crucial role during the photocatalytic CO2 reduction. They served as a bridge for electron transfer to induce the bending adsorption of CO2, which enhanced the adsorption of *CO and stabilization of *H.