Effect of oxygen vacancies on the photocatalytic CO2 reduction performance of Bi2WO6: DFT and experimental studies

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In order to improve the photocatalytic activity of Bi2WO6 (BWO) under solar light irradiation, the introduction of oxygen vacancies has attracted wide attention owing to its simple synthesis method. However, the intrinsic link between oxygen vacancies and photocatalytic CO2 reduction performance is still elusive. In this work, density functional theory (DFT) calculations combining with experimental measurements verify that oxygen vacancies are inclined to generate in [Bi2O2]2+ layers on BWO (010) surface, which results in narrowed band gap and improved electron-hole pair separation efficiency arising from the formation of Bi 6p impurity level. To the best of our knowledge, the reduction product CO is obtained via the reaction of CO2 with •H, and the intermediate species include •COOH, •CO, •OH, and H2O. Owing to BWO (010) surface chemical state variations, such as charge accumulation and depletion upon the introduction of oxygen vacancies, CO2 and intermediate species change remarkably in adsorption configuration, and so do the reaction energy barriers with or without oxygen vacancies. Apparently, the rate-limiting step for the overall reaction is the transition from •COOH to •CO and •OH, and the corresponding activation barrier value is reduced from 2.83 eV to 1.91 eV, because C–O bond breaks more easily after the introduction of oxygen vacancies. This work provides a new enlightenment and insight into the role of oxygen vacancies in the process of reducing CO2, and paves ways to design defective systems with higher photocatalytic activity.