A review on TiO2/g-C3N4

2.2. Core-shell heterojunction
Core–shell nanocomposite photocatalysts have recently achieved considerable attention owing to their inimitable properties and encouraging potential for energy conversion [128]. Among different kinds of core–shell architectures, semiconductor core@semiconductor shell composites provide better structural stability along with improved photo catalytic activity by tuning adsorption efficiency of reactants, amending the light transmission capability, and accelerating the alienation of photo-spawned species [129,130]. A good number of studies have been carried out on photocatalytic activity of TGCN core-shell heterostructure [92,108,109,131]. Wang and his co-workers explained the formation of core-shell quantum heterojunction of TGCN with the help of TEM, FTIR and XPS studies. TEM images presented in Fig. 3(a) revealed that TiO₂ is covered with 6–8 nm thickness of g-C₃N₄ quantum shell. The appearance of a weaker peak at 520 cm⁻¹ in FTIR spectra of TGCN core-shell heterostructure [Fig. 3(b)] corresponding to Ti O Ti stretching vibration also confirmed that TiO₂ was completely covered by g-C₃N₄ quantum shell. The strong peaks appearing in the region of 1200–1650 cm^-1 due to C N stretching mode of heterocycle rings are shifted to red end endorsing the robust contact between TiO₂ and g-C₃N₄ quantum shell of TGCN. As shown in Fig. 3(c), the shifting of Ti 2p peak in XPS spectra towards blue end indicated an intense interface interaction between g-C₃N₄ and TiO₂. On account of this interaction, the core-shell heterojunction possessed a greater ability for the separation of electron-hole pairs [132]. In addition to this, the presence of g-C₃N₄ quantum shell imparts intense quantum confinement effect as revealed from the blue-shift of much lower fluorescence emission peak of the core-shell structure shown in Fig. 3(d) [[132], [133], [134]]. The intense quantum confinement effect caused by g-C₃N₄ quantum shell broadens the band gap [135,136]. The specific surface area of core-shell TGCN structure was found to be 51.87 m² g⁻¹, which was higher than that of as-prepared g-C₃N₄ (33.12 m² g⁻¹). The high specific surface area of TGCN may countenance the adsorption of umpteen pollutants over it and facilitate their diffusion into its matrix. Moreover, the peaks at 3045 cm⁻¹ in Fig. 3(b) corresponds to the presence of abundant surface adsorbed OH groups that significantly adsorb the hydrophilic pollutants (tetracycline molecules) [132]. Thus, the synergistic effect of intense interface, quantum confinement effect and large specific surface area enabled the core-shell heterostructure to exhibit enhanced photoactivity. The effect of thickness of g-C₃N₄ shell in the TGCN core-shell heterostructure on photocatalytic efficiency was investigated by Wang et al [137]. On increasing the thickness of g-C₃N₄ layer up to 1 nm, photocatalytic activity increases whereas reverse trend is observed by further increasing the thickness up to 3 nm. It may be attributed to the presence of enough layer of g-C₃N₄ that prohibits the transfer of photoexcited electrons from g-C₃N₄ to TiO₂. The composite containing TiO₂ core with g-C₃N₄ shell layer of 1.0 nm thickness displays maximum visible light driven photocatalytic performance which is nearly 7.2 times more than that obtained by bulk g-C₃N₄.

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Fig. 3. (a) TEM image of TGCN core-shell heterojunction, (b) FTIR spectra of TGCN core-shell heterojunction (TCN), g-C₃N₄, physical mixing of TiO₂ and g-C₃N₄ (TCN mix) and TiO₂ (c) XPS of TCN and TiO₂ (d) Fluorescence emission peak of TiO₂, TCN mix, TCN and g-C₃N₄ [Reproduced from Ref. No. [132]].
Besides the effective separation of charge carriers, the hollow core shell structure has two more advantages for demonstrating substantial photoactivity. Firstly, the inside and outside walls of the hollow structure provides double specific surface area which brings about abundant reaction sites to facilitate photocatalytic reaction [[138], [139], [140], [141]]. Secondly, the multiple reflections caused due to the hollow structure, enrich the visible light absorption [141]. Li group have synthesised Ti³⁺ self-doped B (black)- TGCN hollow core-shell nano-heterojunction in which the surface of the hollow TiO₂ nanosphere was covered by g-C₃N₄ shell. Brunauer–Emmett–Teller (BET) specific surface area and average pore size determined by Barrett–Joyner–Halenda (BJH) equation was found to be 96.80 m² g⁻¹ and 10.24 nm respectively for the hollow core-shell structure. These textural properties are very much advantageous for meliorating the photoactivity. Therefore, the obtained core-shell structure boosted photocatalytic H₂ production to ∼808.97 μmol.g⁻¹ h⁻¹ because of its large specific surface area, adequate pore size distribution in addition to superior visible light utilisation and restricted recombination of electron-hole pairs [142]. The enhanced H₂ production may also be due to the presence of oxygen vacancy (Ov) and Ti³⁺ in black TiO₂ [143,144]. The unique sublevel state formed by Ov at the bottom of CB of TiO₂ could improve the absorption of visible light and fasten the separation of charge carriers [[145], [146], [147]]. The presence of Ti³⁺ induces hydrogen atoms to get diffused into TiO₂ at the interface to obtain self- hydrogenated shell, which could be able to overcome the activation barrier of the H₂, bring the proton of water to photo-excited stage and make hydrogen to get diffused into the subsurface easily for boosting H₂ production [[148], [149], [150]]. The more negative CB of the B-TiO₂ would also be advantageous to heighten photocatalytic H₂ evolution.
Core-shell TGCN system also exhibited improved photoelectrochemical activity. Jing et al. fabricated core-shell TGCN composite, which demonstrated efficient visible light responsive photoelectrocatalytic water splitting as evident from photocurrent and linear sweep voltammetry (LSV) measurements. The amperometric current–time curves in Fig. 4(a) have shown that prepared TGCN heterojunction imparted 2.5 times more photocurrent density compared to pristine TiO₂ during the irradiation time. It might be attributed to the prominent charge separation and migration resulting in minimal recombination rate owing to the formation of a robust TGCN interface. LSV study [Fig. 4(b)] revealed that TGCN electrode produces much higher photocurrent than that of TiO₂ electrode with rise in potential. This further evidenced a magnificent migration of charge carriers due to successful formation of heterojunction between TiO₂ and g-C₃N₄. The efficiency of charge transfer (η_transfer) for TGCN core-shell heterojunction with respect to potential is quite higher (88 %) than that obtained for TiO₂ electrode (77 %) as presented in Fig. 4(c). The result again indicated a significant charge separation and transfer through the well contacted heterojunction that led to availability of enough holes for photoelectrochemical oxidation of water. Authors have suggested a plausible mechanism [Fig. 4(d)] according to which photon induced electrons are transferred from CB of g-C₃N₄ to that of TiO₂ through the interface and the holes move to the VB of g-C₃N₄ from that of TiO₂ for an improved photoelectrocatalytic water splitting at TGCN core-shell electrode [151].

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Fig. 4. shows (a) Amperometric current–time curves, (b) linear sweep voltammetry curves measured with and without light irradiation in 0.1 M Na₂SO₄ solutions with and without 0.01 M H₂O₂ solution as electrolytes, (c) charge transfer efficiency of TiO₂ and TGCN core-shell (TiO₂@g-C₃N₄) electrode, (d) plausible mechanism for the enhanced PEC water oxidation at TGCN core-shell electrode [Reproduced from Ref. No. [151]].