Photocatalytic CO2 reduction by a Z-scheme mechanism in an aqueous suspension of particulate (CuGa)0.3Zn1.4S2, BiVO4 and a Co complex operating dual-functionally as an electron mediator and as a cocatalyst

A visible-light-driven Z-scheme photocatalytic CO 2 reduction reaction (CO 2 RR) to produce CO was demonstrated using an aqueous particulate dispersion containing two bare semiconductors, (CuGa) 0.3 Zn 1.4 S 2 for CO 2 RR and BiVO 4 for water oxidation. The semiconductors were mixed with a water-soluble cobalt tris(dimethylbipyridine) complex. The CO selectivity was 98% (against H 2 ), and the rate of CO generation was 1–2 orders of magnitude higher than those of previously-reported aqueous suspension photocatalytic systems. O 2 was continuously evolved, and isotope tracer analyses confirmed that CO 2 was the carbon source for CO. Experimental studies and calculations suggest that the Co complex acts dual-functionally in synergy with (CuGa) 0.3 Zn 1.4 S 2 and BiVO 4 : it behaves as an efficient ionic electron mediator, and also acts as a new active CO 2 RR cocatalyst after a structural change by accepting photoexcited electrons from (CuGa) 0.3 Zn 1.4 S 2 . This simple method, operating in a self-optimizing manner in solution, has great potential to help achieve sustainable, highly active artificial photosynthetic systems. • Co complexes were designed as redox electron mediators for visible-light photocatalytic liquid-phase Z-scheme CO 2 reduction. • Activity of aqueous suspension of two semiconductor particulates was highly dependent on redox potential of the Co complex. • Use of Co-tris(dimethyl-bipyridine) complex resulted in the Z-schematic CO 2 -to-CO conversion selectivity of 98%. • The CO generation rate was improved by one to two orders of magnitude over conventional aqueous suspension Z-scheme systems. • The Co complex acted dual-functionally as an electron mediator and as a cocatalyst for CO 2 reduction in the aqueous solution.