Cocatalysts in Semiconductor‐based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities

Abstract Ever‐increasing fossil‐fuel combustion along with massive CO 2 emissions has aroused a global energy crisis and climate change. Photocatalytic CO 2 reduction represents a promising strategy for clean, cost‐effective, and environmentally friendly conversion of CO 2 into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half‐reaction of CO 2 conversion with an oxidative half reaction, e.g., H 2 O oxidation, to create a carbon‐neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO 2 conversion: (i) solar‐light absorption, (ii) charge separation/migration, and (iii) catalytic CO 2 reduction and H 2 O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO 2 reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO 2 ‐reduction cocatalysts for semiconductor‐based photocatalytic CO 2 conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.