Bifunctional noble-metal-free cocatalyst coating enabling better coupling of photocatalytic CO2 reduction and H2O oxidation on direct Z-scheme heterojunction

Selective loading of spatially separated redox cocatalysts on direct Z-scheme heterojunctions holds great promise for advancing the efficiency of artificial photosynthesis, which however is limited to the photodeposition of noble metal cocatalysts and the fabrication of hollow double-shelled semiconductor heterojunctions. Moreover, the co-exposure of discrete cocatalyst and semiconductor increases the product diversity when both the exposed sites of which participate in CO2 photoreduction. Herein, we present a facile and versatile protocol to overcome these limitations via surface coating of Z-scheme heterojunctions with bifunctional noble-metal-free cocatalysts. With Cu2O/Fe2O3 (CF) as a model heterojunction and layered Ni(OH)2 as a model cocatalyst, it is found that Ni(OH)2 lying on the surfaces of Cu2O and Fe2O3 separately co-catalyzes the CO2 reduction and H2O oxidation. Thorough experimental and theoretical investigation reveals that the Ni(OH)2 outer layer: (i) mitigates the charge recombination in CF and balances their transfer and consumption; (ii) reduces the rate-determining barriers for CO2-to-CO and H2O-to-O2 conversion, (iii) suppresses the side proton reduction occurring on CF, and (iv) protects the CF from component detachment. As expected, the redox reactions stoichiometrically proceed, and significantly enhanced photocatalytic activity, selectivity, and stability in CO generation are achieved by the stacked Cu2O/Fe2O3@Ni(OH)2 in contrast to CF. This study demonstrates the significance of the synergy between bifunctional cocatalysts and Z-scheme heterojunctions for improving the efficacy of overall redox reactions, opening a fresh avenue for the rational design of artificial photosynthetic systems.