This study details the synthesis of high-activity g-C3N4 catalysts for H2 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 π-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 H2 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 π-conjugation system is promising for promoting the activity of carbon-nitride photocatalysts.

Developing a structural design of g-CN to enhance absorption in the solar spectrum, [11] facilitate separation of photogenerated charges, [12] and expose the active sites for charge transfer [13] is a major challenge for improving the applicability of g-CN.The structure of g-CN comprises 2D layers that are constituted by H-bondingconnected melon strains (Scheme 1). [14] The layers are linked through van der Waals attraction to form a 3D framework. Scheme 1 elucidates that the melon strain is produced through polymerization of 1,3,4,6,7,9,9b-Heptaazaphenalene-2,5,8-triamine (melem) molecules along with NH 3 removal; its structure comprises heptazine units connected by NH bridges. This g-CN framework results in an electronic structure exhibiting a bandgap straddling the redox potentials for H 2 and O 2 evolutions from water splitting and a gap width of ≈2.7 eV, [4] which enables g-CN to capture a considerable amount of solar irradiation.Previous studies have reported that crystalline graphite-like CN (i.e., g-C 3 N 4 ) exhibited negligible activity in photocatalysis, whereas g-CN, which is partially condensed and contains amino groups, was active in H 2 evolution under illumination. [15] Lotsch and co-workers reported that low-molecular-weight melon, a type of melem oligomer, exhibited a higher efficiency than melon in H 2 evolution under illumination at λ ≈ 420 nm. This was because of the increase in kinetics even though the energy gap of the melem oligomer was larger than that of melon. [16] Theoretical calculations have indicated that the lowest unoccupied molecular orbital of melem oligomers is contributed by hybrid systems comprising edged carbon and nitrogen atoms, central nitrogen atoms, and bridging amino groups, and by localized terminal primary amines. [16,17] Experimental studies on dendrimers and metal-organic frameworks have revealed that photodeposition of metallic platinum in H 2 PtCl 6 solution preferentially occurred at the amino groups. [18] These findings imply that the amino groups of melem oligomers acted as the electron-donating sites for photocatalytic H 2 evolution and their robust interaction with the deposited metallic platinum facilitated the transfer of excited electrons from the amino sites to the solution phase for H 2 evolution. [19] A framework comprising melem oligomers for providing amino sites can enable the production of g-CN materials with high photocatalytic activity. [13,20] However, melem oligomers are wide-bandgap This paper presents a structural design of graphitic carbon nitride (CN) for use as an active photocatalyst in H 2 evolution reactions. The active photocatalyst is synthesized by subjecting a CN sample to consecutive annealing and ammonia treatments. The CN sample is produced through urea condensation. The annealing treatment destroys the polymeric CN to form a defective polymeric layer framework incorporating 1,3,4,6,7,9,9b-Heptaazaphenalene-2,5,8-triamine (melem) oligomers. The annealed CN exhibits a wide absorption range in visible light because the distorted ...