Extending the π-Conjugation of g-C3N4 by Incorporating Aromatic Carbon for Photocatalytic H2 Evolution from Aqueous Solution

This study details the synthesis of high-activity g-C3N4 catalysts for H-2 generation from a triethanolamine aqueous solution under visible light. We anneal a mixture of urea and NH4Cl to obtain g-C3N4 nanosheets, which are subsequently solvated with ethanol molecules and annealed to form aromatic carbon-doped g-C3N4. The results of analyses conducted using X-ray photoelectron, Fourier-transform infrared, and carbon-13 nuclear magnetic resonance spectroscopies demonstrated that annealing the ethanol molecules leads to the grafting of aromatic heterocycles on the g-C3N4 nanosheets and substitution of nitrogen with carbon. The grafted aromatic heterocycles and doped carbon atoms extend the pi-conjugation system in g-C3N4 to reduce the band gap and facilitate the separation of photogenerated charges. The carbon-incorporating also preserve the crystallinity of g-C3N4 during high-temperature annealing, which facilitates the suppression of the recombination of photogenerated charges at defect sites. The developed aromatic carbon-doped g-C3N4 effectively catalyzes H-2 generation from the aqueous solution, achieving apparent quantum yields of 14% and 2.2% under 420 and 550 nm monochromatic irradiation, respectively, whereas urea-derived g-C3N4 reached only 3.4% and 0.1%. The proposed strategy of extending the pi-conjugation system is promising for promoting the activity of carbon-nitride photocatalysts.