3.1 Noble metal
Noble metals, including Au, Ag, Pt, Pd, Ru, and Rh, are known to be the most active co-catalyst for PEC CO2 reduction owing to their high catalytic activity and selectivity. Song et al. demonstrated a nanoporous mesh-type Au thin-film co-catalyst loaded onto a Si photocathode via mild electrochemical oxidation and reduction of the Au thin film (Figure 4(A)).[90] As shown in Figure 4(B), the current density and onset potentials of all RA-Au thin films were significantly improved compared with those of the untreated Au thin-film. The optimized co-catalyst showed outstanding performance toward PEC reaction for the reduction of CO2 to CO with an FE of up to 91 % at the CO2/CO equilibrium potential of −0.11 V vs. RHE in an aqueous solution under 1 sun illumination (Figure 4(C)). To design an efficient semiconductor/co-catalyst interface, Jang et al. prepared a ZnTe/ZnO photocathode with Au nanoparticles deposited using an e-beam evaporator.[91] By loading Au NPs onto a photocathode, a Schottky junction was formed at the interface between the Au NPs and ZnTe, resulting in the improved separation of photogenerated carriers and electron transfer into the electrolyte. As a result, Au-coupled ZnTe/ZnO-NW photocathode delivered outstanding PEC performance with a photocurrent density of −16.0 mA cm−2 and incident photon-to-current conversion efficiency of 97 % compared with those of a bare electrode (−7.9 mA cm−2, 68 %). Recently, Wang et al. developed a novel strategy for producing highly efficient PEC photocathodes by coupling plasmonic Au NPs and n+p-Si through a TiO2interlayer.[92] The Au/TiO2/n+ p- Si photocathode produced 86 % FECO with a partial current density of −5.52 mA cm−2 at −0.8 V vs. RHE (Figure 4(D) and (E)). DFT calculations indicated that the synergistic effect of layering Au and TiO2 facilitated *COOH formation and *CO desorption, thereby promoting the conversion of CO2 to CO (Figure 4(F)). An innovative method of controlled chemical etching on Si wafers using etching solutions containing Ag+ ions was reported to synthesize a Si surface uniformly deposited with an Ag particulate film.[93] The PEC performance of the method was excellent with a large photocurrent density of ~10 mA cm−2 under 0.5 sun, outstanding FE of ~90 % at 0.5 V vs. RHE for CO, and excellent stability of 8 h.