Selective Loading of a Cobalt Species Cocatalyst on an Oxygen-Evolution Photocatalyst in a Silver-Inserted Z-Scheme System Consisting of Bismuth Vanadium Oxide and Zinc Rhodium Oxide for Enhanced Overall Water Splitting and Carbon Dioxide Reduction

We selectively loaded various amounts of a cobalt species (CoOx) cocatalyst for oxygen (O2) evolution on bismuth vanadium oxide (Bi4V2O11 (BVO)) as an O2-evolution photocatalyst in a solid-state photocatalyst, a silver (Ag)-inserted BVO and zinc rhodium oxide (ZnRh2O4 (ZRO)) photocatalyst (BVO/Ag/ZRO (BAZ)), to form CoOx/BAZ. The amount of loaded CoOx was controlled by changing the photodeposition time to obtain CoOx/BAZ with up to 0.039 wt % Co vs BAZ. All the prepared CoOx/BAZ photocatalysts achieved overall water splitting irradiated with red light at a 700 nm wavelength, enhancing the hydrogen (H2) and O2 evolutions from water at a ratio of 2:1 compared with bare BAZ. Moreover, the apparent quantum efficiency (AQE) increased up to 0.11% under 700 nm-wavelength light irradiation. CoOx was demonstrated to function as a cocatalyst enhancing the overall water-splitting activity. It was confirmed that the CoOx/BAZ photocatalyst reduced carbon dioxide (CO2) to methane (CH4) and carbon monoxide (CO) and oxidize water to O2 using water as an electron donor and proton source under 700 nm-wavelength light. In addition, copper (Cu) was selectively photodeposited as a H2-evolution cocatalyst on ZRO, a H2-evolution photocatalyst, in CoOx/BAZ to form CoOx/BAZ/Cu. CoOx/BAZ/Cu further enhanced the water-splitting activity with an AQE of 0.22% under 700 nm-wavelength light irradiation. Moreover, CoOx/BAZ/Cu reduced CO2 and oxidized water to produce CH4 and O2, without producing CO, under 700 nm-wavelength light irradiation. Cu was confirmed to function as a cocatalyst for enhancing the overall water-splitting reaction and CO2 reduction to selectively generate CH4.