O-doped g-C₃N₄ was synthesized for the first time by a facile H₂O₂ hydrothermal approach. The O-doping in the g-C₃N₄ lattice could induce intrinsic electronic and band structure modulation, resulting in its absorbance edge up to 498 nm and enhanced visible-light photoactivity, consequently.

Nitrogen-deficient graphitic carbon nitride (g-C₃N_4−x) was synthesized by a hydrothermal treatment using ammonium thiosulfate as an oxidant. The as-prepared photocatalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption–desorption, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), electron paramagnetic resonance (EPR), UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and photoluminescence (PL) spectroscopy. The visible-light-driven photocurrent measurement was performed by several on–off cycles of intermittent irradiation. The photocatalytic activity of catalysts was evaluated by splitting water under visible-light irradiation (λ > 420 nm). Results demonstrated that the photoactivity of g-C₃N_4−x was enhanced greatly by the deficiency of the terminal amino species on the catalysts. The average H₂ evolution rate on g-C₃N_4−x was 31.6 μmol h⁻¹, which was ca. 3 times higher than that on pristine g-C₃N₄. It was revealed that the unique nitrogen-deficient structure of g-C₃N_4−x played an important role in broadened visible-light absorption and efficient electron–hole separation, mainly accounting for the improved photocatalytic activity.

Graphitic carbon nitride (g-C₃N₄) hybridized with a small number of multi-walled carbon nanotubes (CNT) was synthesized using cyanamide as precursor. The optimal CNT content is found to be ∼0.2 wt% in the composite, which displays a 2.4-fold enhancement in photocatalytic water splitting over pure g-C₃N₄. Characterizations by a series of joint techniques including Raman spectra, UV/vis diffuse reflectance spectra, steady and time-resolved fluorescence emission spectra, and photocurrent responses were carried out, aiming to reveal the determinative factor for the improved visible-light response. Our results indicate that the increased photoactivity originates from the enhanced charge-transfer effect due to the intimate interactions between g-C₃N₄ and conjugated CNT. The presence of CNT in the hybrids is beneficial for improving electron–hole separation on the excited g-C₃N₄ by prolonging the lifetimes of charge carriers and improving the population distribution of short-lived and long-lived charge carriers.

Deprotonation of g-C ₃ N ₄ with Na ions successfully inhibits the unfavorable H ₂ O ₂ intermediate in a photocatalytic process, and significantly facilitates nonsacrificial water splitting under visible light.

Metal-free 2D/2D heterostructured photocatalysts of black phosphorus/covalent triazine-based frameworks are constructed for water splitting and pollutant degradation.