A review on TiO2/g-C3N4

2.5. Face –to- face (2D-2D) Heterojunction
The interfacial contact area of 2D/2D heterojunction is extremely higher than that of the 0D/2D heterojunction (point-to-face contact) and 1D/2D heterojunction (line-to-face contact) [70]. Due to this, formation of 2D/2D heterojunction provides an enlarged area of intimate contact between the two components at the interface. This facilitates electron-hole separation by augmenting transfer of charge carriers, leading to an enriched photocataytic performance. Moreover, this heterostructure enhances the stability of the catalyst by prohibiting it from photocorrosion and agglomeration [88,[176], [177], [178]]. Zhong et al. compared the photocatalytic performance of 2D/2D nanocomposites of TiO₂ and g-C₃N₄ obtained from three different approaches namely co-calcination, charge induced aggregation and in-situ solvothermal treatment. The synergistic effect between g-C₃N₄ and TiO₂ due to formation of intimate 2D/2D interface enhanced light harvest efficiency both in visible and UV region, leading to remarkable photocatalytic performance for H₂ evolution. However, the sample prepared by charge induced aggregation method exhibited poor performance in spite of its high surface area as compared to that is prepared by co–calcination method. It is because former method involves formation of heterojunction through electrostatic interaction which can not hinder recombination of charge carriers as efficiently as Ti O N covalent bonds that is formed by co–calcination method at the interface of TiO₂ and g-C₃N₄ in the composite. The composite prepared by in-situ solvothermal treatment possesses close face to face contact through Ti O N covalent bonds and largest surface area of 263 m² g⁻¹ [179]. The formation of Ti O N covalent bonds at the interface is responsible for fast transfer and separation of charge carrier whereas enlarged surface area provides enough space for reactant adsorption, mass transfer acceleration and deposition of co-catalyst (Pt) that acts as a sink for accumulated electrons to prohibit excess accumulation and recombination of charge carriers [[180], [181], [182]]. These cause a remarkable photocatalytic activity, as a result of which the composite photocatalyst exhibited high H₂ evolution rate of 587 μ mol. g⁻¹ h⁻¹ [179]. The covalent heterojunction formation through Ti O N linkages was predicted by DFT calculations and was confirmed by electron energy loss spectra (EELS) and XPS studies. EELS [Fig. 7(a)] show that Ti and N edges are very prominent at the interface, demonstrating the establishment of well contacted heterojunction between O-g-C₃N₄ and TiO₂ [88]. Further, Fig. 7(b) shows EDS mapping in which the peaks corresponding to O K edges for both interfaces 1 and 2 are found to be broad due to the amplified extent of disorder in oxygen coordination environment at interfaces. This might be resulted from the formation of covalent bond between TiO₂ NS and O-g-C₃N₄ NS formed during the solvothermal treatment [183,184]. Shifting of the N O peak in XPS to higher binding energy (BE) by 0.4 eV as shown in Fig. 7(c), indicated the formation of N O Ti covalent bond in the O-g-C₃N₄/TiO₂ composite [185]. Gu group have also reported that the photo-generated carriers migrate efficiently from the surface of mildly oxidised g-C₃N₄ (O-g-C₃N₄) to that of TiO₂ through N O Ti covalent heterojunction as shown in the Fig. 7(d) and hence the recombination of charge carriers are comprehensibly prohibited [88]. In addition, the band bending at the interface of such heterojunction minimises the local band gap energy and widens the absorption range [186,187]. As a result, the composite exhibited superior visible light induced photocatalytic activity towards H₂ production. Photocatalytic activity also depends on the textural properties like surface area, pore size and pore volume. O-g-C₃N₄/TiO₂ composite with 1:1 wt ratio of O-g-C₃N₄ and TiO₂ possessed largest surface area (S_BET) of 263 m² g⁻¹, total pore volume (V_tot) of 0.57 cm³ g⁻¹ and average pore size of 14.7 nm. This composite photocatalyst performed maximum activity. Further, increasing the amount of O-g-C₃N₄, the textural properties were reduced significantly as a result of which poor photoactivity was observed [88]. Wu and his co-workers have also demonstrated that the ultrathin face-to-face (2D/2D) interfacial contact between g-C₃N₄ and anatase TiO₂ can improve the flat band potential, lower the band gap energy and modify the band locations. As an outcome, the photon generated electrons travel a very small distance from g-C₃N₄ to TiO₂ nanosheets within small time interval with higher charge mobility through the hetero-interface due to which the recombination of electron hole pairs in TGCN heterojunction is significantly reduced. Therefore, 2D/2D TGCN heterojunction exhibited significantly greater performance towards phtodegradation of MO, MB and RhB. About 98 % of MO was degraded with a rate of 0.189 min⁻¹ within 15 min under UV–vis. light. The hybrid photocatalyst also have shown an extraordinary photocatalytic H₂ evolution rate of 18,200 μ mol. g⁻¹ h⁻¹ [100].

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Fig. 7. (a) EELS spectra demonstrates well contacted heterojunction between O-g-C₃N₄ and TiO₂ (b) EDS mapping showing covalent bonding between TiO₂ NS and O-g-C₃N₄ NS (c) XPS results represent the formation of N-O-Ti covalent bond in the O-g-C₃N₄/TiO₂ composite (d) Efficient migration of photo-generated charged species from O-g-C₃N₄ to TiO₂ through N-O-Ti covalent heterojunction. [Reproduced from Ref. No. [88]].