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.