**5. Carbon nitride g-C3N4**

Graphitic carbon nitride is a very versatile material discovered in 1843 by Berzelius and Liebig [6]. The interesting thing about is that several techniques have been reported for the synthesis of C3N4 with a variety of precursors such as the thermal decomposition of melamine. Moreover, it was reported that the synthesis of carbon nitrides can be proceeded through a condensation method with cyanurchloride and calcium cyanamide. In another synthesis procedure, high pressure and high temperature were applied to create carbon nitride from 2-amino-4, 6-dichlorotriazine. In another study, it was reported that the synthesis of carbon nitride can be performed by using cyanurchloride and sodium amide by heating at 200°C in benzene. Recently, carbon nitride has attracted the researchers in photocatalysis area because its small bandgap makes it utilize light in visible light areas of the solar spectrum. Hence, the choice of the precursor and the synthesis conditions are extremely important factors to achieve the demanded structure.

One of the famous composites was g-C3N4 with TiO2. Chang et al. [7] reported a sol-gel technique to create a series of TiO2/g-C3N4 composites. The composite showed an excellent liquid phase photocatalytic decolorization of rhodamine B (RhB) dyed solution. C3N4-TiO2 composites exhibited 2.4 to 7.0 times higher than solo TiO2 or N-TiO2. Gu et al. [8] reported the synthesis of anatase TiO2 nano-sheets composite with (g-C3N4). The synthesis procedure was the solvent evaporation method. The composite exhibited superior photocatalytic degradation activity of several organic

compounds under the illumination of UV and visible than the parent TiO2 and C3N4. Zhou et al. [9] reported the synthesis of g-C3N4/TiO2 by pyrolysis process of urea and titanium hydroxide. The formed material was evaluated in the gas phase photoreduction of carbon dioxide and water vapor to form CO and CH4. In this paper, the authors confirmed the formation of nitrogen-doped TiO2 together with g-C3N4 as a separate phase. The photocatalytic behavior of the composite was much higher than the commercial P25. Wang et al. [10] reported the heating of carbon nitride precursor together with TiO2; however, the formed composite was adjacent to particles C3N4 and TiO2. Although the formed composite was two separate phases, however, the photocatalytic activity was much higher than the parent TiO2 in H2 evaluation reaction. Another trial has been reported for the solid state reaction of C3N4 precursor and TiO2 by Boonprakob et al. [11]. The composite of g-C3N4/TiO2 was prepared under Ar flow, and the formed sheets were tested in the degradation of methylene blue under visible light. The composite exhibited also higher photocatalytic activity than the parent TiO2 and C3N4. However, again, the structure formed was adjacent to two separate phases of the mesoporous C3N4 and the crystalline TiO2. Core-shell structure was not feasible. A third trial was to perform the solid state reaction between the carbon nitride precursor and the pre-synthetized TiO2. Although the authors claimed the formation of a thin layer of carbon nitride around the titania particles, but the HR-TEM images they presented did not show such structure, in addition to, and based on our primary experiments, the ratio of C3N4 precursor/TiO2, which they presented, cannot lead to a core-shell structure.
