As inspired by natural photosynthesis, semiconductor-based photocatalytic water splitting into hydrogen and oxygen has attracted extensive attention, which provides a promising and clean method to convert solar energy into chemical fuel, for alleviating human's excessive reliance on fossil fuels. [1][2][3] For the water splitting process, the half-reaction of water oxidation to oxygen has been well recognized as the rate-limiting step for its four-electron oxidation process and high activation energy barrier (≈700 mV) for O-O bond formation, [4,5] which thus remains as the main challenge to achieve highly efficient overall water splitting. Although enormous efforts have been dedicated to develop photocatalysts for improved water oxidation performances in the past decades, by far only a few photocatalysts (e.g., WO 3 , BiVO 4 , and Ag 3 PO 4 , etc.) have been proven to be efficient for lightdriven oxygen evolution from water. [5][6][7][8] Worse still, several drawbacks always limit their practical application in oxygen-generating photocatalytic systems, including poor photostability, low charge carrier mobility, and high costs of raw materials. Since the pioneering work reported in 2009 by Wang et al., [9] metal-free graphitic carbon nitride (g-C 3 N 4 ) with a 2D conjugated structure has been intensively investigated for photocatalytic water redox reactions, owing to its earth-abundance, environmental friendliness, chemical stability, and suitable thermodynamical potentials for both hydrogen and oxygen evolution reactions. [10][11][12] In the past ten years, various strategies, including thermodynamic (such as doping with heteroatoms, [13][14][15][16] engineering defects with nitrogen vacancies or carbon vacancies, [17,18] etc.) and kinetic (such as compositing with other semiconductors, [7,[19][20][21][22] morphology design, [23][24][25][26][27] and loading cocatalysts, [28][29][30] etc.) modifications, have emerged to enhance the photocatalytic hydrogen evolution activity of g-C 3 N 4 and a high quantum yield of 34.4% at 400 nm has been achieved. [31] However, only very few results have been reported on g-C 3 N 4 based photocatalysts for water oxidation, and most of them were focused on surface modification by loading cocatalysts, such as cobalt (hydr)oxide and selenide, [32][33][34] etc., onto g-C 3 N 4 . Unfortunately, the photocatalytic oxygen evolution Electronic structure greatly determines the band structures and the charge carrier transport properties of semiconducting photocatalysts and consequently their photocatalytic activities. Here, by simply calcining the mixture of graphitic carbon nitride (g-C 3 N 4 ) and sodium borohydride in an inert atmosphere, boron dopants and nitrogen defects are simultaneously introduced into g-C 3 N 4 . The resultant boron-doped and nitrogen-deficient g-C 3 N 4 exhibits excellent activity for photocatalytic oxygen evolution, with highest oxygen evolution rate reaching 561.2 µmol h −1 g −1 , much higher than previously reported g-C 3 N 4 . It is well evidenced that with c...

Photocatalysis suffers from rapid recombination of photoexcited charge carriers and limited active sites for the catalytic reactions, resulting in unsatisfactory performances that are far from practical application. Single metal atom catalysts not only increase the number of active sites due to the maximum atom‐utilization efficiency, but also enhance the intrinsic activity of each active site because of their unique geometric and electronic features. Furthermore, single metal atom catalysts provide a platform for photocatalysis research on the atomic level. This review discusses the unique characteristics, including geometric and electronic properties, of single metal atom catalysts, summarizes their synthesis strategies, and reviews their most recent development in photocatalysis. Finally, the challenges of single metal catalysts for both fundamental research and practical application are highlighted.

Exploring highly efficient non-noble-metal bifunctional catalysts for water electrolysis is vital for the commercial production of hydrogen.