3. PHOTOELECTROCATALYTIC CO2 REDUCTION
Photoelectrochemical (PEC) CO2 reduction possesses the advantages of both photocatalytic and electrocatalytic CO2 reduction and has attracted a great deal of interest.[82-84]
PEC CO2RR proceeds through the following steps:[83]
(1) Generation of electron-hole pairs.
(2) Charge separation/transport by an external bias.
(3) Surface redox reactions:
A large amount of electron-hole recombination occurs at each stage, releasing energy as light or heat. Therefore, the separation and transfer of photogenerated carriers in semiconductors are key factors for effectively improving the solar conversion efficiency.
Enormous efforts have been devoted to the design and engineering of photocathodes, such as doping,[85-86]nanostructuring,[87-89] and co-catalyst loading[84]. Among these, coupling a co-catalyst with a light-absorbing semiconductor is one of the most effective approaches for improving PEC performance. Normally, bare semiconductor surfaces are inert and can barely activate CO2molecules, leading to poor PEC CO2RR performance. To solve these problems, various types of co-catalysts have been loaded on the photoelectrode to reduce the activation energy for CO2 reduction and suppress surface charge recombination, thus accelerating the surface reaction kinetics.