Exceptionally Low-Coordinated\nBismuth–Oxygen\nVacancy Defect Clusters for Generating Black In<sub>2</sub>O<sub>3</sub> Photocatalysts with Superb CO<sub>2</sub> Reduction Performance

Indium oxide (In<sub>2</sub>O<sub>3</sub>) is a widely\nused catalyst\nfor CO<sub>2</sub> reduction, yet its inherent properties, such as\na wide band gap and low-active surface, necessitate a modification\nto achieve broad-wavelength absorption and enhanced surface activity.\nHowever, simultaneously achieving these goals through a single material\nmodulation approach remains challenging. Here, we present a simple\nyet innovative strategy to develop a black catalyst, Bi<sub><i>x</i></sub>In<sub>2–<i>x</i></sub>O<sub>3–<i>y</i></sub>, comprising notably low-coordinated bismuth on oxygen-defect-laden\nIn<sub>2</sub>O<sub>3</sub>. This approach induces local structural\nand charge carrier changes, resulting in remarkably high visible light\nabsorption and preeminent surface activity. In situ diffuse reflectance\ninfrared Fourier transform spectroscopy (DRIFTS) confirms the spontaneous\ndissociation of CO<sub>2</sub> species into CO even in the dark on\nthe Bi<sub><i>x</i></sub>In <sub>2–<i>x</i></sub>O<sub>3–<i>y</i></sub> surface, underscoring\nthe catalyst’s enhanced activity. Compared to pristine In<sub>2</sub>O<sub>3</sub>, Bi<sub><i>x</i></sub>In<sub>2–x</sub>O<sub>3–<i>y</i></sub> exhibits approximately 24\ntimes greater CO production. Characterization techniques, including\nextended X-ray absorption fine structure (EXAFS) and X-ray absorption\nnear-edge structure (XANES) analyses, along with density functional\ntheory (DFT) calculations, reveal that oxygen vacancies in the reduced\nsample decrease both the average coordination number of bismuth and\nits effective oxidation state. Our findings indicate that the unusually\nlow-coordinated bismuth dopant preferably promotes the formation of\noxygen vacancies close to bismuth (Bi-<i>V</i><sub>ö</sub>) rather than near indium, which induces local structural and charge\ncarrier changes. These Bi-<i>V</i><sub>ö</sub> clusters\nenhance light harvesting, charge separation, and CO<sub>2</sub> adsorption/activation/reduction.\nImportantly, our approach demonstrates promise for a wide range of\napplications, addressing key challenges in catalyst modification for\nCO<sub>2</sub> reduction and offering opportunities for further advancement\nin this field.