Isolated Cu Sites in CdS Hollow Nanocubes with Doping-Location-Dependent Performance for Photocatalytic CO₂ Reduction

Doping engineering has enabled the construction of homogeneous and abundant atomic-level catalytic sites for photocatalytic CO2 reduction with improved selectivity for the target product. However, little is known about the effect of the spatial position of the heteroatoms on the photocatalytic activity of the semiconductors toward CO2 reduction. Herein, uniform Cu doping into the bulk phase of hollow CdS cubes (HCC) and Cu doping onto the surface of HCC, denoted as Cu/HCC and HCC@Cu, respectively, are prepared by tuning the introduction order of Cu sources. Experimental analysis shows that the introduction of Cu by both methods can promote the separation and migration of photoinduced charge carriers in CdS. Notably, Cu doping onto the surface of CdS in HCC@Cu leads to much better proton reduction to H-2 production performance but lower CO2 reduction efficiency as compared to bare CdS. In sharp contrast, Cu doping into the bulk phase of CdS enhances the CO2-to-CO conversion while mitigating H-2 evolution. This should be ascribed to the smaller overpotential of Cu/HCC in the CO2 saturated system than that in the Ar system. In addition, doping Cu atoms into the bulk phase of CdS shifts the d band center of Cu/HCC upward to near the Fermi energy level, which promotes the adsorption and activation of CO2 on CdS. These results indicate that the photoelectrons with a prolonged lifetime in Cu/HCC preferably reduce CO2 molecules rather than protons. The density functional theory (DFT) calculation results show that the introduction of Cu heteroatoms can promote the desorption of CO*, and the adaptable sulfur vacancies (Vs) produced by in situ doping techniques can stimulate the formation of CO* intermediates, resulting in the high performance of photocatalytic CO2 reduction to CO. This work reveals the effect of different heteroatom doping locations on the catalytic activity and will provide a reference for the design of efficient photocatalysts with atomic-level fine structure.