Bismuth-based photocatalysts enhanced by sulfur/oxygen vacancies for the selective reduction of carbon dioxide into methane

Reducing carbon dioxide (CO2) into methane (CH4) through photocatalysis technology can effectively decrease carbon emissions and ease the energy crisis. However, the low photocatalytic efficiency and high electron-hole recombination rates still hinder the large-scale utilization of photocatalysis. Vacancy engineering can reduce electron-hole pair complexation, however, the effects of oxygen (O) and sulfur (S) vacancies on the CO2 reduction properties of bismuth (Bi)-based photocatalysts are unclear. This study prepared different Bi-based photocatalysts, including alpha-Bi2O3, alpha-Bi2O3-X, Bi2S3, and Bi2S3-X. The introduction of vacancies was found to greatly increase the specific surface areas, effectively reduce electron-hole recombination, decrease charge transfer resistance, and enhance electron transfer, leading to enhanced CH4 production. Comparatively, Bi2S3-X has substantially demonstrated a superior efficiency in the photocatalytic conversion of CO2 to CH4 compared to Bi2S3, while alpha-Bi2O3-X exhibited slightly higher than alpha-Bi2O3. There are more S vacancies in Bi2S3-X (x=0.581) than O vacancies in alpha-Bi2O3-X (x=0.329), leading to its superior photocatalytic activity, with CO and CH4 production rates of 3.706 mu mol center dot g- 1 center dot h- 1 and 7.697 mu mol center dot g- 1 center dot h- 1, respectively. Moreover, the CH4 selectivity of Bi2S3X reaches 67.50 %, which is 1.67 times that of alpha-Bi2O3-X. Therefore, Bi2S3-X holds great promise as a photocatalyst for CO2 reduction. This work proves that in Bi-based photocatalyst, S vacancy is more effective than O vacancy in enhancing the selective reduction of CO2 to CH4, offering a pathway for achieving selective CO2 reduction to CH4.