Accelerating Electron‐Transfer and Tuning Product Selectivity Through Surficial Vacancy Engineering on CZTS/CdS for Photoelectrochemical CO2 Reduction

Abstract Copper‐based chalcogenides have been considered as potential photocathode materials for photoelectrochemical (PEC) CO 2 reduction due to their excellent photovoltaic performance and favorable conduction band alignment with the CO 2 reduction potential. However, they suffer from low PEC efficiency due to the sluggish charge transfer kinetics and poor selectivity, resulting from random CO 2 reduction reaction pathways. Herein, a facile heat treatment (HT) of a Cu 2 ZnSnS 4 (CZTS)/CdS photocathode is demonstrated to enable significant improvement in the photocurrent density (−0.75 mA cm −2 at −0.6 V vs RHE), tripling that of pristine CZTS, as a result of the enhanced charge transfer and promoted band alignment originating from the elemental inter‐diffusion at the CZTS/CdS interface. In addition, rationally regulated CO 2 reduction selectivity toward CO or alcohols can be obtained by tailoring the surficial sulfur vacancies by HT in different atmospheres (air and nitrogen). Sulfur vacancies replenished by O‐doping is shown to favor CO adsorption and the CC coupling pathway, and thereby produce methanol and ethanol, whilst the CdS surface with more S vacancies promotes CO desorption capability with higher selectivity toward CO. The strategy in this work rationalizes the interface charge transfer optimization and surface vacancy engineering simultaneously, providing a new insight into PEC CO 2 reduction photocathode design.