Exceptionally Low-Coordinated Bismuth-Oxygen Vacancy Defect Clusters for Generating Black In₂O₃ Photocatalysts with Superb CO₂ Reduction Performance

Indium oxide (In₂O₃) is a widely used catalyst for CO₂ reduction, yet its inherent properties, such as a wide band gap and low-active surface, necessitate a modification to achieve broad-wavelength absorption and enhanced surface activity. However, simultaneously achieving these goals through a single material modulation approach remains challenging. Here, we present a simple yet innovative strategy to develop a black catalyst, Bi_xIn_2-xO_3-y, comprising notably low-coordinated bismuth on oxygen-defect-laden In₂O₃. This approach induces local structural and charge carrier changes, resulting in remarkably high visible light absorption and preeminent surface activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms the spontaneous dissociation of CO₂ species into CO even in the dark on the Bi_xIn _2-xO_3-y surface, underscoring the catalyst’s enhanced activity. Compared to pristine In₂O₃, Bi_xIn_2-xO_3-y exhibits approximately 24 times greater CO production. Characterization techniques, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) analyses, along with density functional theory (DFT) calculations, reveal that oxygen vacancies in the reduced sample decrease both the average coordination number of bismuth and its effective oxidation state. Our findings indicate that the unusually low-coordinated bismuth dopant preferably promotes the formation of oxygen vacancies close to bismuth (Bi-V_ö) rather than near indium, which induces local structural and charge carrier changes. These Bi-V_ö clusters enhance light harvesting, charge separation, and CO₂ adsorption/activation/reduction. Importantly, our approach demonstrates promise for a wide range of applications, addressing key challenges in catalyst modification for CO₂ reduction and offering opportunities for further advancement in this field.