improve the hydrogen evolution reaction (HER) rate. [9][10][11] However, even with this approach, the quantum efficiency (QE) and, subsequently, the solar energy conversion efficiency for a single component material are still relatively low due to the fast recombination of photogenerated electron-hole pairs. [12][13][14][15] Reducing the dimensions of the photocatalyst can improve the photocatalytic activity due to the shortened diffusion length of photogenerated carriers. [16][17][18][19] In this context, graphitic carbon nitride (g-C 3 N 4 ) with atomic thickness has been recently investigated as a promising material for photocatalysis, since it can efficiently separate the photoexcited carriers, which then migrate to the surface with decreased possibility of recombination. [20][21][22] Nevertheless, the synthesis of ultrathin 2D g-C 3 N 4 nanosheets (monolayer or bilayer) with high crystallinity and uniform thickness on a large-scale remains a challenge. To further improve the quantum efficiency of HER, a second semiconductor with band positions complementary to that of g-C 3 N 4 can be introduced, creating an artificial Z-scheme junction, [23,24] able to suppress the recombination of electron-hole pairs and also enhance the light absorption. [25][26][27] By choosing an auxiliary semiconductor with deep valence band (VB), the Z-scheme structure formed with g-C 3 N 4 can also potentially drive the overall water splitting.Here, we, for the first, time developed a catalytic synthesis approach by employing a small amount of metal oxide (e.g., α-Fe 2 O 3 ) as a catalyst to produce ultrathin 2D g-C 3 N 4 nanosheets (one to two layers) on a large scale with a yield of 10 wt%. Meanwhile, the all-solid-state Z-scheme structure forms composed two photocatalysts (n-type α-Fe 2 O 3 nanosheet and n-type 2D g-C 3 N 4 ) in direct and tight contact, which mitigates the competing shuttle-mediator redox reactions and has a simple composition that is more attractive to the research community and industry. [28,29] For hydrogen evolution, the α-Fe 2 O 3 nanosheet/2D g-C 3 N 4 Z-scheme system exhibits a significantly enhanced quantum efficiency up to 44.35% (λ = 420 nm), which is the highest quantum efficiency so far reported for g-C 3 N 4based photocatalysts (see Table S1, Supporting Information). In addition, the quantum efficiency is also superior to most of the semiconductor photocatalysts containing metal oxides and Photocatalysis is the most promising method for achieving artificial photosynthesis, but a bottleneck is encountered in finding materials that could efficiently promote the water splitting reaction. The nontoxicity, low cost, and versatility of photocatalysts make them especially attractive for this application. This study demonstrates that small amounts of α-Fe 2 O 3 nanosheets can actively promote exfoliation of g-C 3 N 4 , producing 2D hybrid that exhibits tight interfaces and an all-solid-state Z-scheme junction. These nanostructured hybrids present a high H 2 evolution rate >3 × 10 4 µmol g -1 h -1 and externa...
