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Graphitic carbon Nitride (g-C3N4) is one of the most utilized graphitic materials in hydrogen (H2) production via photocatalytic water splitting. Thus, a detailed critical overview, updated with the most recent works, has been performed on the synthesis methods, modification techniques, characterization, and mechanisms of g-C3N4 and g-C3N4-based composite materials, with the aim of clarifying the optimum course towards highly efficient hydrogen-producing photocatalysts based on this promising material. First, the synthesis methods for different morphologies of pure g-C3N4 (bulk, nanosheets, nanotubes and nanodots) are critically analyzed in detail for every step and parameter involved, with special mention regarding the modification methods of g-C3N4 (doping and composite formation). Next, the most common results of g-C3N4 characterization, regarding structural, morphological, optical, and electrical properties, are presented and analyzed. Then, a detailed critical survey of the mechanisms, using g-C3N4 and g-C3N4-based composites during photocatalytic activity, is performed with a focus on their effect on their hydrogen production capabilities via water splitting. This review aims to provide a clear image of all aspects regarding the use of g-C3N4 for photocatalysis, as well as a comprehensive guide for research targeted towards this promising graphitic material.

Ternary TiO2/g-C3N4@Ag nanocomposites with controlled composition were synthesized and used as highly effective bifunctional photocatalysts. TiO2 nanoparticles were initially prepared via a sol-gel method and afterwards combined with optimum amounts of graphitic carbon nitride (g-C3N4) by a hydrothermal procedure to finally produce TiO2/g-C3N4 heterostructures. TiO2/g-C3N4 was impregnated with a silver nitrate aqueous solution containing sodium citrate as a reducing agent resulting in elemental silver nanoparticle (Ag0 NP) integration, as confirmed by x-ray photoelectron spectroscopy (XPS). Following crystallographic, electron-microscopy and spectroscopic characterization, the prepared nanocomposites presented high photocatalytic activity for both oxidation and reduction reactions. The photodegradation of model organic pollutants: methylene blue (MB) and rhodamine B (RhB) dyes and oxytetracycline (OTC) pharmaceutical, together with decomposition of 4-nitrophenol (4-NP) and Cr(VI), was investigated and the reaction constants were determined. An optimum component ratio was determined, which leads to the effective elimination of pollutants under UV light, with degradation percentages up to 95% for MB and 89% for RhB and 90% removal efficiency of the OTC pharmaceutical. The degradation of both dyes was also confirmed under artificial solar light. Furthermore, the ternary TiO2/g-C3N4@Ag NCs were highly effective for the photocatalytic reduction of 4-NP to aminophenol (>95%) and Cr(VI) to Cr(III) (80%), under UV illumination.

In the current study, CoFe2O4 and TiO2 nanoparticles were primarily made using the sol-gel method, and subsequently, the hybrid magnetic composites of TiO2 loaded with CoFe2O4 (5–15 percent w/w) were made using a hydrothermal procedure. X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were all used to thoroughly characterize the materials. Additionally, the zero-charge point (ZCP) determination, the examination of the pore structure by nitrogen adsorption, and an evaluation of magnetic properties were performed. Six organic dye pollutants were selected to evaluate the performance of the synthesized nanocomposites toward photocatalytic degradation, including methylene blue (MB), methyl orange (MO), crystal violet (CV), acridine orange (AO), rhodamine B (RhB), and rhodamine 6G (R-6G). Photodegradation of tetracycline (TL), a model pharmaceutical pollutant, was also studied under UV and visible light. The composites exhibited a high degradation performance in all cases without using any oxidants. The photocatalytic degradation of tetracycline revealed that the CoFe2O4/TiO2 (5% w/w) composite exhibited a higher photocatalytic activity than either pure TiO2 or CoFe2O4, and thus attained 75.31% and 50.4% degradation efficiency under UV and visible light, respectively. Trapping experiments were conducted to investigate the photodegradation mechanism, which revealed that holes and super oxide radicals were the most active species in the photodegradation process. Finally, due to the inherent magnetic attributes of the composites, their easy removal from the treated solution via a simple magnet became possible.