Photocatalytic CO2 Reduction Using Water as an Electron Donor under Visible Light Irradiation by Z-Scheme and Photoelectrochemical Systems over (CuGa)0.5ZnS2 in the Presence of Basic Additives

We demonstrated photocatalytic CO₂ reduction using water as an electron donor under visible light irradiation by a Z-scheme photocatalyst and a photoelectrochemical cell using bare (CuGa)_0.5ZnS₂ prepared by a flux method as a CO₂-reducing photocatalyst. The Z-scheme system employing the bare (CuGa)_0.5ZnS₂ photocatalyst and RGO-(CoO_x/BiVO₄) as an O₂-evolving photocatalyst produced CO of a CO₂ reduction product accompanied by H₂ and O₂ in a simple suspension system without any additives under visible light irradiation and 1 atm of CO₂. When a basic salt (i.e., NaHCO₃, NaOH, etc.) was added into the reactant solution (H₂O + CO₂), the CO formation rate and the CO selectivity increased. The same effect of the basic salt was observed for sacrificial CO₂ reduction using SO₃^2- as an electron donor over the bare (CuGa)_0.5ZnS₂ photocatalyst. The selectivity for the CO formation of the Z-schematic CO₂ reduction reached 10-20% in the presence of the basic salt even in an aqueous solution and without loading any cocatalysts on the (CuGa)_0.5ZnS₂ metal sulfide photocatalyst. It is notable that CO was obtained accompanied by reasonable O₂ evolution, indicating that water was an electron donor for the CO₂ reduction. Moreover, the present Z-scheme system also showed activity for solar CO₂ reduction using water as an electron donor. The bare (CuGa)_0.5ZnS₂ powder loaded on an FTO glass was also used as a photocathode for CO₂ reduction under visible light irradiation. CO and H₂ were obtained on the photocathode with 20% and 80% Faradaic efficiencies at 0.1 V vs RHE, respectively.