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 CO 2 reduction is beneficial for addressing fossil fuel shortages and environmental pollution issues. We synthesized a twin S-scheme g-C 3 N 4 /CuFe 2 O 4 /ZnIn 2 S 4 heterojunction, which was used to construct a self-supporting three-phase system for photocatalytic CO 2 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 CH 4 selectivity (96.8%) and surprise production rate of CH 4 (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-C 3 N 4 /CuFe 2 O 4 /ZnIn 2 S 4 heterojunction possessed both hydrophilicity and hydrophobicity, which achieved an efficient transformation of CO 2 into CH 4 by controlling interface wettability. g-C 3 N 4 as a hydrophobic layer promoted CO 2 mass transfer to achieve the enrichment of CO 2 on the heterojunction surface; ZnIn 2 S 4 as a hydrophilic layer could well adsorb H 2 O, which was further oxidized by the photogenerated holes into many protons (H + ). Finally, DFT calculations found that Fe–N bonds located between g-C 3 N 4 and CuFe 2 O 4 played a crucial role during the photocatalytic CO 2 reduction. They served as a bridge for electron transfer to induce the bending adsorption of CO 2, which enhanced the adsorption of *CO and stabilization of *H.