hydrogen combustion only produces water. Therefore, hydrogen as a promising energy for the future development can help to meet the future energy needs and solve environmental problems. [4,5] Semiconductor-based photocatalytic water splitting utilizing renewable solar energy for hydrogen production is considered to be a feasible technology with great promise. [6][7][8][9] According to the research of titanium dioxide (TiO 2 ) single crystal in photoelectrochemistry, Fujishima and Honda [10] pioneered the field of semiconductor photocatalysis. However, the wide band gap of metal oxide such as ZnO, [11][12][13][14] SnO 2 , [15][16][17][18] Nb 2 O 5 , [19][20][21] etc., makes them only respond to near-ultraviolet or ultraviolet light, resulting in low efficiency of solar energy. In fact, visible light accounts for 43% of sunlight, while ultraviolet ray accounts for merely 4%. Therefore, developing visible light-driven photocatalysts attracts widespread attention for purpose of making better use of solar energy. [22][23][24] Graphite carbon nitride (g-C 3 N 4 ) is an attractive nonmetallic conjugated polymer semiconductor, which has been extensively applied in photocatalytic water splitting because of its attractive electronic structure, excellent thermal and chemical stability, unique 2D layered structure, suitable band gap, low toxicity, etc. [25][26][27] However, the actual application of the original g-C 3 N 4 still suffers from the limitations of underutilization of visible light, lacking of catalytic active sites, hydrophobic surface, low carrier mobility, and rapid recombination of photoinduced carriers. [28,29] For addressing these challenges, various strategies have been adopted so as to improve the photocatalytic performance of the original g-C 3 N 4 , such as nanostructure design, [30][31][32] doping of metallic and nonmetallic elements, [33][34][35][36] construction of heterojunctions, [37][38][39][40] copolymerization, [41][42][43] defect engineering, and dye sensitization. [44,45] Among them, the controllable synthesis of nanoscale morphology is one of the resultful means. Because of its characteristics including appropriate porosity, massive surface groups for anchoring, large specific surface area, slim thickness and as well as high aspect ratio, semiconductor nanosheets describe a brilliant future. [46] Therefore, 2D g-C 3 N 4 nanosheets may significantly improve the photocatalytic activity. [47,48] Besides, integration with other semiconductors to build heterojunctions is considered to be a reliable means. [49][50][51] Ultrathin 2D/2D heterostructures usually Designation of high-efficiency water splitting photocatalyst is still a challenge in converting solar energy into chemical fuels. Heterojunction can inhibit recombination of carriers which is considered to be a reliable strategy to improve photocatalytic performance on water splitting. In this work, a "face-to-face" 2D tight heterostructure is constructed by growing ZnIn 2 S 4 nanosheets on g-C 3 N 4 nanosheets. Due to the ultrathin 2D structure...