Synthesis of g-C3N4 Nanosheets Using Highly Condensed Lamellar Crystalline Melamine-Cyanuric Acid Supramolecular Complex for Enhanced Solar Hydrogen Generation.

Highly condensed lamellar melamine-cyanuric acid supramolecular (MCS) complex was synthesized in autoclave at high pressure as a precursor for synthesizing g-C3N4 nanosheets. Due to the distinctive properties of the prepared MCS complex, the efficient g-C3N4 nanosheets photocatalyst can be obtained by heating treatment of this MCS complex under argon followed by calcination in air at 400 ºC. The resulting nanosheets with in-plan 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 for carbon nitride nanosheets. The light enhancement results from the sizeable conjugated system of tri-s-triazine units in the carbon nitride framework coupling with structural defects by the presence of oxygen-containing groups being induced during the synthesis. Consequently, the obtained carbon nitride nanosheets exhibited excellent performance for hydrogen generation under full sunlight and especially, under visible light. Its quantum efficiency (QE) is 20.9% at 420 nm, one of the highest values for carbon nitride materials; even at 590 nm, the QE is 3.5%. To the best of our knowledge, the integrated QE of this material in the visible region (420 to 600 nm) is approximately 1% - the highest one compared to all other the carbon nitride nanosheet materials reported previously.