Photocatalytic CO2 Reduction Using a Pristine Cu2ZnSnS4 Film Electrode under Visible Light Irradiation

The present study comprehensively investigated the metal sulfide Cu2ZnSnS4 (CZTS) as a visible-light-active photocatalyst for selective CO2 reduction on the basis of theoretical calculation and experimental evaluation of a highly pure CZTS film electrode. For our first-principles calculation, the Heyd–Scuseria–Ernzerhof functional and a slab model with a sufficiently thick vacuum layer for the crystal phase were used to determine the accurate band gap value and to discuss the proper positions of its conduction and valence bands, respectively. Consequently, CZTS could be expected to have sufficient high conduction band level to drive CO2 reduction under visible light irradiation. We fabricated a highly pure CZTS thin film via spin-coating, and elemental analysis indicated that the film consisted of almost stoichiometric chemical composition of CZTS. Our pristine CZTS film electrode functioned as a p-type photocathode in aqueous media bubbled with CO2, and it could generate carbon monoxide (CO) in addition to hydrogen (H2) production under visible light irradiation with the application of bias at −0.8 V (vs Ag/AgCl) even without cocatalyst modification condition. According to our isotope trace experiment using 13CO2, the origin of the produced CO was proved to be bubbled CO2. Furthermore, we confirmed oxygen evolution in the anodic side, indicating that pristine CZTS could produce CO from CO2 using water molecules as electron donors.