Tandem Photocatalysis on Highly Dispersed Copper Sites in Bismuth Nanocrystals for Thermodynamic-Optimized Photocatalytic CO ₂ -to-CH ₄ Conversion

Photocatalytic CO2 reduction to CH4 represents a promising strategy for CO2 utilization and the production of valuable chemical feedstocks. However, this strategy faces significant challenges due to charge recombination and the generation of non-selective reactive intermediates, which lead to parasitic side reactions that compromise both reaction selectivity and yield. In this study, we developed a tandem photocatalytic system featuring highly dispersed copper (Cu) sites within bismuth (Bi) nanocrystals (a CuBi bimetallic catalyst) to optimize the thermodynamic pathway of CO2-to-CH4 conversion on a titanium dioxide (TiO2) surface. This bimetallic catalyst simultaneously enabled directional electron transfer and enhanced photogenerated carrier separation efficiency in the TiO2. The optimized CuBi–TiO2 photocatalyst demonstrated a CH4 evolution rate of 84.1 μmol g−1 h−1 under sacrificial agent-free conditions, representing a nearly sevenfold improvement over pristine TiO2, while maintaining a selectivity of 97.3%. Mechanistic studies revealed that the CuBi bimetallic architecture exhibited enhanced electrical conductivity and an appropriate work function, facilitating the formation of an ohmic contact at the CuBi/TiO2 interface. This interface effectively directional transfer of photogenerated electrons from TiO2 to active CuBi sites. Density functional theory calculations further indicated that the highly dispersed Cu species within Bi nanocrystals modified the adsorption geometry of CO2 and significantly increased the binding energy of the *CO. The tandem catalytic cooperation between Bi and Cu sites renders the selective conversion of CO2 to CH4 thermodynamically favorable. This study underscores the potential of CuBi bimetallic catalysts to address the selectivity−activity trade-off in CO2 reduction, providing a promising pathway toward efficient and scalable CO2-to-CH4 conversion technologies.