A review on TiO2/g-C3N4

2. Strategies for synthesis and construction of TGCN heterojunction
Various synthetic approaches such as co-calcination [84,85], hydrothermal treatment [86,87], solvothermal [88] and microwave assisted synthesis [89] have been investigated to fabricate TGCN nanocomposites. Wang et al. reported that TGCN composites synthesised by calcination process, exhibited enriched light absorption in visible and UV region due to the synergistic effect between TiO₂ and g- C₃N₄ [90]. Li and his co-workers observed that hybridization of g-C₃N₄ with brookite TiO₂ through calcination route, exhibited superior activity in production of hydrogen by splitting water under irradiation of visible light in comparison with that of TiO₂ or g-C₃N₄ [91]. Hydrothermally synthesized TGCN heterojunctions were found to be beneficial in interfacial charge transfer as the layered structure of g-C₃N₄ provided heterogeneous nucleation template for anatase TiO₂ nanoparticles with the exposure of 001 facet [87]. Jiang et al. reported an enhanced photocatalytic activity by g-C₃N₄ hybridized hierarchical yolk–shell TiO₂ spheres fabricated by solvothermal method. The superior photoactivity may be attributed to the improved charge separation due to the formation of TGCN heterojunction [92]. A hydrothermal–sonication assisted technique was applied to combine g-C₃N₄ with Ti³⁺-TiO₂ photocatalyst with co-exposed {0 0 1} and {10 1} facets. The synergistic effect between g-C₃N₄ and Ti³⁺-TiO₂ bestowed high photoactivity [93]. Wang et al. used an in situ microwave-assisted strategy to build heterojunction between g-C₃N₄ and N-TiO₂ due to which an increased photocatalytic performance was observed [89].
Heterojunction construction by combining two semiconductors having matched energy bands is responsible for the synergetic effect of the enriched light garnering property and enlarged charge separation ability, which are advantageous to achieve significant photocatalytic (PC) activity [84,94]. Mott-Schottky equation is used to determine the flat band potentials and the type of two semiconductors, which are considered crucial for designing the nano-heterojunction. It is depicted in Eq. (1) [95,96].(1)1/C² = ± 2/ε ε₀ AA²eN_D (V−V_fb – k_BT/e)Where, C, V, V_fb and A are interfacial capacitance, applied potential, flat band potential and area respectively. T, N_D, and k_B are absolute temperature, donor density and Boltzmann’s constant respectively. e is the charge on electron, ε is the dielectric constant for semiconductor photocatalyst and ε₀ is the permittivity of the free space. Flat band potential of the semiconductor can be obtained from the intercept of the straight line obtained by the plot of 1/C² vs V. The positive value for the slope of this straight line predicts that the semiconductor is of n-type where as its negative values denotes it as p-type [[97], [98], [99]]. Gu et al. demonstrated that both TiO₂ and g-C₃N₄ are of n-type as positive slopes are obtained for these two semiconductors in the Mott-Schottky plot. The flat band potential for TGCN heterojunction obtained from Mott-Schottky plot was found to be more negative than that of anatase TiO₂ because of the low conductivity of g-C₃N₄. However, TGCN hybrid exhibited increased electron donor density with enriched reduction ability [100].
Appropriate alignment of band structures between two semiconductors resulted in accumulation/depletion of space charge at the interface, which enhances the separation efficiency of photo-induced charge carriers [101]. Further, adequate band edge potentials determine the effective separation of photo-induced charge carriers [102]. The band edge potentials of a semiconducting material can be determined by the empirical Eqs. (2) and (3) [103].(2)E_VB = χ − E₀ + 0.5 Eg(3)E_CB = E_VB – EgWhere, χ is the electronegativity. Its value for g-C₃N₄ is 4.73 eV and that for TiO₂ is 5.81 eV [104]. E_VB and E_CB are VB and CB edge potentials respectively. Eg is the energy of the band gap, Ee is the energy associated with free electrons on the hydrogen scale (about 4.5 eV vs. NHE). The band gap energies can be determined by Kubelka-Munk equation [104,105].(4)(ahν)ⁿ = A(hν−Eg)Where ν, Eg, A and a are frequency of light, band gap energy, absorption coefficient and a constant respectively.
The CBM of g-C₃N₄ is more negative than that of TiO₂ whereas VBM of the later is more positive. The combination of these two semiconductors usually forms type II heterojunction as shown in Scheme 1(a). Therefore, visible light induced electrons at CB of g-C₃N₄ are conveniently transferred to that of TiO₂ through the interface [106]. Moreover, the flow of electrons from g-C₃N₄ to TiO₂ is facilitated by the differences in their CBM positions [91,107]. Additionally, formation of this heterojunction provides enough interfacial area for charge transfer due to the high barrier of conduction bending for TGCN [108]. This accelerates a comprehensive transfer of electrons with suppressed recombination of photoinduced charge carriers leading to an enhanced phtocatalytic activity [100,108]. Ma et al. reported that the e⁻ at CB of TiO₂ with relatively less anodic potential can reduce the surface adsorbed O₂ (E^o _O2/_O2^.- = −0.33 V vs NHE at pH 7) to release O₂ ⁻, which decomposed Rhodamine B (RhB) to CO₂ and H₂O molecules. Similarly, the RhB molecules were also degraded by holes (h⁺) at VB of g-C₃N₄ [109]. However, the electrons at the CB of TiO₂ are weakly reducing than that of g-C₃N₄ as the CB level of later is less anodic and the holes at VB of g-C₃N₄ have weak oxidation ability to oxidise adsorbed water molecules (or surface hydroxyl groups) to produce OH· species because VB level of g-C₃N₄ is less positive than the potential of H₂O/HO  (2.29 V vs NHE at pH 7) [110]. As a result, desired photoactivity cannot be observed for type II TGCN heterojunction.

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Scheme 1. (a) Construction of Type II (b) Z-scheme TGCN heterojunction.
In order to overcome these difficulties, Z-scheme TGCN heterojunction was proposed [[111], [112], [113], [114], [115], [116]]. It was presented in Scheme 1(b). According to it, the photo-induced electrons and holes remain at CB and VB of TiO₂ respectively upon irradiation of UV light. Under irradiation of visible light, the CB of g-C₃N₄ contains photo-electrons while its holes remained at its VB. The electrons at CB of TiO₂ recombine with the photo-generated holes present at VB of g-C₃N₄. This leads to an efficient separation of charge carriers. As a result, the electrons available at CB are trapped by molecular oxygen near the surface of g-C₃N₄ to obtain reactive superoxide radicals (O₂ ⁻) which directly oxidize the organic pollutants. Since, the VBM potential of TiO₂ is more positive than that of  OH/H₂O couple, the holes at VB of TiO₂ undergo reaction with surface hydroxyl groups or adsorbed water molecules at TiO₂ surface to generate highly reactive hydroxyl radicals species ( OH) which decompose the pollutants in to harmless products [117].
Notwithstanding the promoted PC activity exhibited by TGCN heterojunction, it has been a great challenge to fabricate a heterostructure consisting of TiO₂ and g-C₃N₄ in which both are composited uniformly and connected well. It is because reduced interfacial connection can act as barrier at the interface for movement of charge carriers leading to restricted separation of photo-induced charged species [118]. For achieving intimate interfacial contact, the construction of various morphology based TGCN heterojunctions is recently being emphasised. These are presented in Scheme 2.

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Scheme 2. Morphology based TGCN heterojunctions and their photocatalytic applications for H₂ production and pollutant degradation.