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, BixIn2–xO3–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 BixIn 2–xO3–y surface, underscoring the catalyst's enhanced activity. Compared to pristine In2O3, BixIn2–xO3–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-Vö) rather than near indium, which induces local structural and charge carrier changes. These Bi-Vö 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.