Introducing O vacancies into the lattice of a semiconductor photocatalyst can alter its intrinsic electronic properties and band gap, thus enhancing the visible light absorption, promoting the separation/transfer of photogenerated charge carriers, and resultantly elevating the photocatalytic activity of oxide semiconductors. Moreover, O vacancies can help adsorb and activate CO 2 on photocatalyst surfaces, which, however, are prone to being filled by O atoms during the photoreduction reaction. In this work, Cu was introduced to increase the O vacancy concentration in CeO 2−x and promote the photocatalytic activity of CeO 2−x . The sample Cu/CeO 2−x -0.1 showed the highest photocatalytic activity with a CO yield of 8.25 μmol g −1 under 5 h irradiation, which is ∼26 times that on CeO 2−x . According to the analysis of Raman and X-ray photoelectron spectroscopy (XPS) spectra, it has been evidenced that Cu introduction benefits the chemical stabilization of O vacancies in CeO 2−x during photocatalytic CO 2 reduction, which is responsible for the improved and sustained photocatalytic activity.

The emerging metal single‐atom catalyst has aroused extensive attention in multiple fields, such as clean energy, environmental protection, and biomedicine. Unfortunately, though it has been shown to be highly active, the origins of the activity of the single‐atom sites remain unrevealed to date owing to the lack of deep insight on electronic level. Now, partially oxidized Ni single‐atom sites were constructed in polymeric carbon nitride (CN), which elevates the photocatalytic performance by over 30‐fold. The 3d orbital of the partially oxidized Ni single‐atom sites is filled with unpaired d‐electrons, which are ready to be excited under irradiation. Such an electron configuration results in elevated light response, conductivity, charge separation, and mobility of the photocatalyst concurrently, thus largely augmenting the photocatalytic performance.