Metal-Oxide-Mediated Subtractive Manufacturing of Two-Dimensional Carbon Nitride for High-Efficiency and High-Yield Photocatalytic H₂ Evolution

g-C₃N₄ is a promising visible-light-driven photocatalyst for H₂ evolution reaction; however, the achievement of the high photocatalytic performance is primarily limited by the low separation efficiency of the photogenerated charge carriers and partly restricted by the slow kinetics of charge transfer. 2D g-C₃N₄ can significantly improve the charge generation, transfer, and separation efficiencies. The 2D g-C₃N₄-based Z-scheme heterostructure can further enhance the charge-carrier separation and simultaneously increase the redox ability, thereby further boosting the photocatalytic performance. Here we report a transition-metal-oxide (TMO)-mediated subtractive manufacturing process toward the large-scale synthesis of 2D g-C₃N₄ and the simultaneous formation of a 2D/2D TMO/g-C₃N₄ Z-scheme heterojunction. The TMOs serve as catalysts to facilitate the hydrolysis reaction of the bulk g-C₃N₄ in the presence of moist air, forming 2D g-C₃N₄. The resulting 2D/2D TMO/g-C₃N₄ catalysts, in particular, 2D/2D Co₃O₄/g-C₃N₄, exhibit high-efficiency and high-yield photocatalytic H₂ evolution due to the suppression of electron–hole pair recombination and enhanced redox ability. The 2D/2D Co₃O₄/g-C₃N₄ photocatalyzes the H₂ evolution with a rate of ∼370 μmol h^–1 within λ > 400 nm. The external quantum efficiency of 2D/2D Co₃O₄/g-C₃N₄ at λ = 405 nm reaches 53.6%, which is among the highest values for g-C₃N₄-based catalysts.