S-scheme heterojunction photocatalysts for CO2 conversion: Design, characterization and categories

Photocatalytic CO2 conversion into valuable hydrocarbon fuels via solar light is a promising strategy to simultaneously address energy shortage and environmental pollution. However, the photocatalytic CO2 reduction performance is too poor to be practically utilized due to the rapid recombination of photogenerated charge carriers. Constructing step-scheme (S-scheme) heterojunction photocatalysts can facilitate the charge separation and maximize the redox ability, thus remarkably enhancing the photocatalytic CO2 reduction activity. This review summarizes the progress of S-scheme heterojunction photocatalysts applied in the photocatalytic CO2 reduction reactions. Firstly, we introduce the fundamental design principles and characterization methods. The direct and indirect techniques to confirm the S-scheme charge transfer mechanism are disclosed. Secondly, we divide S-scheme composite photocatalyst into the following categories depending on their compositions: g-C3N4-based, metal-chalcogenide-based, TiO2-based, bismuth-based, other metal oxide-based and other semiconductor-based S-scheme photocatalysts. The synergistic effect of the S-scheme charge transfer pathway as well as the unique intrinsic properties of semiconductor materials on the photocatalytic CO2 reduction performance is discussed in detail. Finally, concluding perspectives on the challenges and opportunities for the further exploration of highly efficient S-scheme photocatalysts in photocatalytic CO2 conversion are presented.