Fig. 1. (Color online) The schematic structure of triazine (a) and tri-s-triazine (heptazine) (b) in g-C₃N₄. Reprinted from Ref. [10].

Fig. 2. (Color online) The process of overall solar water splitting over a semiconductor photocatalyst.

Fig. 3. (Color online) (a) The UV–vis absorption spectra, (b) converted Kubelka–Munk vs. light energy plots and (c) XPS valence band spectra of CN and CNQs. (d) The schematic band structures of CN and CNQ 680. Reprinted from Ref. [22].

Fig. 4. (Color online) Schematic illustration of synthesis methods of DTLP-CN via thermal polymerization of melamine, urea, and KOH. Reprinted from Ref. [25].

Fig. 5. (Color online) (a) Schematic structure of the O-doped g-C₃N₄-based photocatalyst. (b) Band structure diagrams of g-C₃N₄ and O-doped g-C₃N₄. (c) Schematic of the fabrication of BDCNN originated from CNN and (d) the charge-transfer process in BDCNN-based heterojunction upon light irradiation. Reprinted from Refs. [36, 40].

Fig. 6. (Color online) (a) UV–vis diffuse reflectance spectra, (b) the band gap from (αhv)^1/2 vs. photon energy, (c) valance band XPS spectra, and (d) schematic illustration of the band gap structure of pristine and doped g-C₃N₄ samples. Reprinted from Ref. [45].

Fig. 7. (Color online) (a) UV–visible diffuse reflectance spectrum (DRS) and (b) HOMO and LUMO positions of CN, CN-LiNa, CN-NaK, and CN-LiK. (c) UV–vis DRS and (b) bandgap structures for CN, crystalline CN, CCN and crystalline CCN. Reprinted from Refs. [52, 54].

Fig. 8. (Color online) Schematic illustrations of basic structural units of polymeric carbon nitride with different C and N stoichiometric ratios: (a) triazine-based graphitic carbon nitride, (b) heptazine-based graphitic carbon nitride, (c, d) polymeric C₃N₅, (e) C₃N₆, (f) C₃N₇, and (g) C₃N₃. Reprinted from Ref. [56].

Fig. 9. (Color online) (a) Synthesis scheme of C₃N₅. (b) UV–Vis DRS for C₃N₅ compared with bulk g-C₃N₄. (c) Steady-state PL spectra of melem, g-C₃N₄ and C₃N₅. Reprinted from Ref. [61].