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) CO₂ reduction due to their excellent photovoltaic performance and favorable conduction band alignment with the CO₂ reduction potential. However, they suffer from low PEC efficiency due to the sluggish charge transfer kinetics and poor selectivity, resulting from random CO₂ reduction reaction pathways. Herein, a facile heat treatment (HT) of a Cu₂ ZnSnS₄ (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 CO₂ 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 CO₂ reduction photocathode design.