Photocatalytic Hydrogen Evolution from Water Using Copper Gallium Sulfide under Visible-Light Irradiation

Copper gallium sulfide with a chalcopyrite-type structure (CuGa3S5) and a band gap of ca. 2.4 eV was prepared by a solid-state reaction. The as-prepared CuGa3S5 exhibited photocatalytic hydrogen evolution activity in an aqueous solution containing Na2S and Na2SO3 as sacrificial electron donors under visible-light irradiation (2 > 420 nm), even without a cocatalyst, such as platinum (Pt). The photocatalytic activity, however, was improved by the deposition of noble metal cocatalysts. Among the noble metals tested, rhodium (Rh) was found to be the most effective cocatalyst to improve the H-2 evolution activity, which was 2.5 times that achieved without a cocatalyst. Dispersion of base sulfides, including nickel sulfide (NiS) and iron sulfide (FeS), into the reactant solution containing CuGa3S5 also increased the activity. The activity under optimized conditions (5.0 wt % NiS/CuGa3S5) was 3 times that of a similarly optimized Rh/CuGa3S5. The results of electrochemical measurements and photocatalytic reactions suggested that NiS suspended in the reactant solution accepts electrons from the excited state of CuGa3S5 to reduce H+ into H-2 when NiS and CuGa3S5 particles collide with each other in the reactant solution.