Photocatalytic CO2 reduction using water as an electron donor by a powdered Z-scheme system consisting of metal sulfide and an RGO-TiO2 composite

CuGaS2, (AgInS2)(x)-(ZnS)(2-2x), Ag2ZnGeS4, Ni- or Pb-doped ZnS, (ZnS)(0.9)-(CuCl)(0.1), and ZnGa0.5In1.5S4 showed activities for CO2 reduction to form CO and/or HCOOH in an aqueous solution containing K2SO3 and Na2S as electron donors under visible light irradiation. Among them, CuGaS2 and Ni-doped ZnS photocatalysts showed relatively high activities for CO and HCOOH formation, respectively. CuGaS2 was applied in a powdered Z-scheme system combining with reduced graphene oxide (RGO)-incorporated TiO2 as an O-2-evolving photocatalyst. The powdered Z-scheme system produced CO from CO2 in addition to H-2 and O-2 due to water splitting. Oxygen evolution with an almost stoichiometric amount indicates that water was consumed as an electron donor in the Z-schematic CO2 reduction. Thus, we successfully demonstrated CO2 reduction of artificial photosynthesis using a simple Z-scheme system in which two kinds of photocatalyst powders (CuGaS2 and an RGO-TiO2 composite) were only dispersed in water under 1 atm of CO2.