Activity, selectivity, and stability of earth-abundant CuO/Cu2O/Cu0-based photocatalysts toward CO2 reduction

Cu-based photocatalysts are widely used for the photocatalytic reduction of CO2 owing to their non-toxicity, earth abundance, extended light absorption, suppressed charge recombination, and good catalytic performance. In addition to low cost, abundant availability, and the ease of synthesis, they exhibit unique features, such as broad optical absorption, reaction intermediates stabilization, and C–C coupling ability, which lead to the formation of C2 + products. The photocatalytic activity of Cu-based photocatalysts is essentially linked to the optical absorption and interfacial charge transfer at the junction of Cu and the semiconductor substrate. However, the poor resistance of Cu to oxidation seriously perturbs the effective utilization of its unique features in practical applications. To date, various approaches, such as the use of metal/non-metal co-catalysts, Z-scheme heterostructures, and hole scavengers, have been proposed to improve its photocatalytic performance and maintain its stability in prolonged reactions. In addition to these approaches, as single metal atom catalysts, atomically dispersed Cu photocatalysts have gained immense attention because they can regain the desired oxidation state, and hence exhibit good stability. The designation of suitable oxidation states of Cu for various CO2 reduction reaction steps is challenging because of the rapid change in its oxidation states under ambient/irradiated environments. However, in situ spectroscopic analyses have nominated Cu2+ for CO2 adsorption, Cu1+ for the photoreduction reaction, and Cu0 for effective charge separation. In this review, the recent advancements in the photocatalytic activity, selectivity, and stability of Cu-based photocatalysts are discussed systematically. Certain concepts and mechanisms related to the photocatalytic performance of Cu-based catalysts have also been discussed. Finally, the future research directions are discussed based on the available relevant literature.