Graphitic carbon nitride (g-CN) has emerged as a promising metal-free photocatalyst, while the catalytic mechanism for the photoinduced redox processes is still under investigation. Interestingly, this heptazine-based polymer optically behaves as a "quasi-monomer". In this work, we explore upstream from melem (the heptazine monomer) to the triazine-based melamine and melam and present several lines of theoretical/experimental evidence where the catalytic activity of g-CN originates from the electronic structure evolution of the C−N heterocyclic cores. Periodic density functional theory calculations reveal the strikingly different electronic structures of melem from its triazine-based counterparts. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy also provide consistent results in the structural and chemical bonding variations of these three relevant compounds. Both melam and melem were found to show stable photocatalytic activities, while the photocatalytic activity of melem is about 5.4 times higher than that of melam during the degradation of dyes under UV−visible light irradiation. In contrast to melamine and melam, the frontier electronic orbitals of the heptazine unit in melem are uniformly distributed and well complementary to each other, which further determine the terminal amines as primary reduction sites. These appealing electronic features in both the heterocyclic skeleton and the terminated functional groups can be inherited by the polymeric but quasi-monomeric g-CN, leading to its pronounced photocatalytic activity.

Polymeric graphitic carbon nitride (g-CN) has emerged as a promising metal-free photocatalyst; however, the polymerization process is still poorly understood, and the synthesized g-CN shows a structural complexity, with photocatalytic activities far from being optimized. Herein we present new insight into its polymerization reaction kinetics and develop a quasi-sealed condensation route to properly regulate the distribution of the degree of polymerization (DP) in the synthesized g-CN. The correlation throughout the condensation process, the structure−property relationship, and the photocatalytic performance of g-CN have been discussed in detail. The synthesized g-CN shows a narrower and uniform DP distribution, possesses improved crystallinity, and features a nanoporous texture with fruitful amine groups and better water dispersibility, which promotes the fast chargecarrier transport under aqueous conditions and give rise to substantially enhanced photocatalytic activity. Compared with the conventional counterpart, its visible-light activity is 4.88 times higher for hydrogen production, 7.81 times higher for the degradation of rhodamine B, and 2.47 times higher for the degradation of 4-chlorophenol. We further report that its solar-driven photocatalytic activity is superior to that of the representative Degussa TiO 2 P25 catalyst for scale-up RhB degradation, thus highlighting the great prospects of g-CN-based photocatalysts toward practical applications.