Photocatalytic CO 2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa) 0.5 ZnS 2 in the Presence of Basic Additives.

We demonstrated photocatalytic CO 2 reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa) 0.5 ZnS 2 prepared by a flux method as a CO 2 -reducing photocatalyst. The Z-scheme system employing the bare (CuGa) 0.5 ZnS 2 photocatalyst and RGO-(CoO x /BiVO 4 ) as an O 2 -evolving photocatalyst produced CO of a CO 2 reduction product accompanied by H 2 and O 2 in a simple suspension system without any additives under visible light irradiation and 1 atm of CO 2 . When a basic salt (i.e., NaHCO 3 , NaOH, etc.) was added into the reactant solution (H 2 O + CO 2 ), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO 2 reduction using SO 3 2- as an electron donor over the bare (CuGa) 0.5 ZnS 2 photocatalyst. The selectivity for the CO formation of the Z-schematic CO 2 reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa) 0.5 ZnS 2 metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O 2 evolution, indicating that water was an electron donor for the CO 2 reduction. Moreover, the present Z-scheme system also showed activity for solar CO 2 reduction using water as an electron donor. The bare (CuGa) 0.5 ZnS 2 powder loaded on an FTO glass was also used as a photocathode for CO 2 reduction under visible light irradiation. CO and H 2 were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.