Synthesis of g‐C3N4 Nanosheets by Using a Highly Condensed Lamellar Crystalline Melamine–Cyanuric Acid Supramolecular Complex for Enhanced Solar Hydrogen Generation

A highly condensed lamellar melamine-cyanuric acid supramolecular (MCS) complex was synthesized in an autoclave at high pressure as a precursor for preparing g-C₃ N₄ nanosheets. Given the distinctive properties of the prepared MCS complex, an efficient g-C₃ N₄ nanosheet photocatalyst can be obtained by heat treatment of this MCS complex under Ar followed by calcination in air at 400 °C. The resulting nanosheets with in-plane nanoholes showed an extremely high specific surface area (≈270 m² g^-1 ) and significantly enhanced light absorption in the visible region. This phenomenon is observed for the first time in carbon nitride nanosheets. The enhanced light absorption results from the sizeable conjugated system of tri-striazine units in the carbon nitride framework, coupled with the structural defects arising from the presence of oxygen-containing groups induced during the synthesis. Consequently, the obtained carbon nitride nanosheets exhibited excellent performance for hydrogen generation under sunlight and especially under visible light. Its quantum efficiency (QE) of 20.9 % at 420 nm is one of the highest reported values for carbon nitride materials. A QE of 3.5 % could be observed even at 590 nm. The integrated QE of this material in the visible region (420-600 nm) is approximately 1 %. To the best of our knowledge this is the highest value compared to all other the carbon nitride nanosheet materials reported previously.