A review on TiO2/g-C3N4

2.1. Mesoporous heterojunction
The mesoporous photocatalysts possess intrinsic interconnected pore networks along with textural mesopores which facilitate the transport of guest species to the framework binding sites. Further, macro/mesoporous structures enhance the specific surface area of the material. High specific surface area and large pore volume allows to adsorb and transfer reactant molecules effectively through the interconnected porous structure. As a result, an enhanced photocatalytic performance is exhibited [119,120]. Formation of mesopores can be evident from type IV of N₂ adsorption- desorption isotherm [121]. Further, the existence of large macropores and mesopores can be substantiated if the isotherms exhibit strong absorption at a high P/P0 ratio (∼1.0) [122]. The low P/P0 range (0.5–0.8) is an indicative of the existence of interconnected pore structure [123]. Hao et al. synthesized macro/mesoporous TGCN heterojunction photocatalysts by a template free calcination method [124]. XRD and FTIR studies confirmed the formation of heterojunction whereas nitrogen adsorption-desorption isotherms, the pore size distributions curves and SEM results revealed the existence of mesoporous structure. XRD results shown in Fig. 2(i) revealed that peak positions of anatase TiO₂ in the TGCN composites were not shifted appreciably which is an indicative of deposition of g-C₃N₄ on the anatase surface [102]. FTIR spectra in the Fig. 2(ii) indicated the presence of prime characteristic peaks for g-C₃N₄ and TiO₂ in the heterostructure whereas shifting of Ti O Ti stretching frequency (presented in inset) from 490 cm⁻¹ to 483 cm⁻¹ confirmed the formation of a firm heterojunction between g-C₃N₄ and TiO₂ through a close interfacial contact [125]. Fig. 2(iii) shows that N₂ adsorption- desorption isotherm for TGCN composites were of type IV, which indicated the presence of mesopores in the composite. This was also evidenced from the SEM images presented in Fig. 2(iv). The low P/P0 values also confirmed the presence of interconnected pores. The mesoporous TGCN composite samples exhibited high surface area, which increased with the increase of g-C₃N₄ due to its low density and small particle size [117]. However, presence of excess g-C₃N₄ content leads to self-aggregation [121]. This results in decrease of surface area and pore volume of the mesoporous TGCN heterojunction photocatalysts. Therefore, the composite with 28.3 % g-C₃N₄ possessed highest surface area of 70.2 m² g⁻¹ and exhibited maximum photoactivity towards degradation of Rh B under visible light irradiation as presented in Fig. 2(v) [124]. Chen et al. compared photocatalytic activity of TGCN mesoporous heterostructure prepared by the growth of g-C₃N₄ on mesoporous TiO₂ spheres at 150 °C with that of physically mixed g-C₃N₄/TiO₂ in terms of photocurrent and EIS studies. Fig. 2(vi) shows that intensity of photocurrent for TGCN heterostructure is greater than that of physically mixed g-C₃N₄/TiO₂. Further, the authors have also mentioned that the photocurrent was immediately dropped for the latter after removal of light implying very fast rate of recombination of photoinduced charge carriers. This might be attributed to the fact that there is no existence of chemical bond between g-C₃N₄ and TiO₂ in physically mixed g-C₃N₄/TiO₂ sample. The gap thus formed between the two semiconductors prohibits the transfer of photogenerated electrons from CB of g-C₃N₄ to that of TiO₂. In contrast, the decay curve with long tail for TGCN composite is an indicative of significant electron-hole pair separation. Furthermore, the greater arc radius obtained from EIS study [Fig. 2(vii)] for the composite represents least charge transfer resistance. These results suggested that fusion of g-C₃N₄ into mesoporous TiO₂ at high temperature forms a well interconnected heterostructure that provides a free path for charge migration at the interface. These charges get accumulated at the CB of TiO₂ causing change in internal electric field that facilitates the charge transfer process and hence, the recombination of electron – hole pairs are considerably suppressed. In addition, light harvesting efficiency is enhanced due to the incorporation of nitrogen in the composite during high temperature synthesis [118]. Besides this, operative charge separation prevails at the interface of TiO₂ and g-C₃N₄ due to the delocalisation of electrons that obtained from local crystalinity and extensive π conjugation of g-C₃N₄ resulted during its formation through thermal condensation [126,127]. Therefore, TGCN mesoporous heterojunction exhibited substantially high photoactivity as compared to its physically mixed counterpart towards degradation of RhB and phenol.

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Fig. 2. (i) XRD results of (a) anatase TiO₂, (b) – (e) TGCN composites formed with different weight of melamine, (f) g-C₃N₄, (ii) FTIR spectra of anatase TiO₂ (CNT0), TGCN composite (CNT3) and g-C₃N₄ (iii) N₂ adsorption- desorption isotherm of anatase TiO₂ (CNT0), TGCN mesoporous heterojunction with different weight of melamine (CNT1, CNT2, CNT3, CNT4) and g-C₃N₄ (iv) SEM image of TGCN mesoporous heterojunction (v) Comparison of RhB degradation efficiency among anatase TiO₂ (CNT0), TGCN mesoporous heterojunction with different weight of melamine (CNT1, CNT2, CNT3, CNT4) and g-C₃N₄ [Reproduced from Ref. No. [124]], (vi) higher intensity of photocurrent and (vii) greater arc radius obtained from EIS study for TGCN heterostructure than that of physically mixed g-C₃N₄/TiO₂ [Reproduced from Ref. No. [118]].