Unraveling the Intermediate Species of Co₃O₄ Hollow Spheres for CO₂ Photoreduction by In Situ X‑ray Absorption Spectroscopy

Nanostructured hollow materials have emerged as a promising class of materials for energy conversion and storage. In this work, we report a Co₃O₄ hollow sphere nanostructure that can serve as a CO₂ reduction catalyst to form CO with high selectivity upon visible-light illumination in the presence of a [Ru­(bpy)₃]²⁺ molecular photosensitizer. Using in situ X-ray absorption spectroscopy, we not only showed that the Co center in the Co₃O₄ hollow sphere is the active site for CO₂ reduction but also identified a key intermediate species, that is, a reduced Co center, due to electron transfer from the [Ru­(bpy)₃]²⁺ photosensitizer when the system can steadily generate CO.