A review on TiO2/g-C3N4

Name of heterojunction	Light source	Performance (μ mol g^−1 h^−1)	Times greater than g-C3N4	Times greater than TiO2
Reference
TGCN hollow core-shell	simulated sunlight	7.9 μmol h^−1	0.37	0.36	[108]
TGCN hollow core-shell	Visible light	808.97μmol g^−1 h^−1	65	18	[142]
Nano-hollowsphere g- C3N4@TiO2	300 W Xe lamp >420nm	466.43μmol g^−1 h^−1	–	–	[197]
TGCN yolk–shell structure	3W, UV-LED(420 nm)	5.6 μmol h^−1	31.1	–	[92]
0D-2D	300 W Xe lamp (λ > 420 nm)	513 μmol g^−1 h^−1	10.8	–	[199]
0D-2D	500 W Xe lamp	55.979 μmol h^−1	5	5	[200]
0D-2D	450 W high-pressure mercury lamp at 436nm	22.4 μmol h^−1	2	–	[201]
0D-2D	300 W xenon lamp(λ ≥ 420 nm)	68.76 μmolh^–1	6.8	–	[202]
1D-2D TiO2 nanobelts/g-C3N4 nanosheets	simulated solar light (AM 1.5)	555.8μmol g^−1 h^−1	5	–	[147]
1D-2D, g-C3N4 nanosheets/TiO2 nanofibers	visible light(λ > 400 nm)	251.7μmol g^−1 h^−1	1.6	167.8	[198]
2D-2D O-g-C3N4/TiO2	visible-light irradiation (λ > 400 nm).	587.1μmol g^−1 h^−1	3.2	–	[88]
2D-2D g-C3N4/TiO2	300 W xenon arc lamp	18200 μmol g^−1 h^−1	3.5	–	[100]

3.2. Environmental remediation
Organic compounds, dyes, antibiotics, oxides of nitrogen (NO_x) and CO₂ are the major contaminants in the environment [188,192,[203], [204], [205], [206], [207]].This section comprises of two parts. In the first part, photocatalytic CO₂ conversion by TGCN heterojunction was discussed whereas second part consists of photodegradation of pollutants.
3.2.1. Photocatalytic conversion of CO₂
Rapid consumption of fossil fuels is elevating the atmospheric level of CO_2, which adversely raises the temperature of earth surface. As a green approach, photocatalytic reduction of CO₂ can not only minimise the CO₂ concentration but also mitigate the global energy demand by producing energy fuels such as CH₄, CH₃OH etc. [207]. TGCN heterojunction can conveniently convert CO₂ into various solar fuels as the CBM potential of both g-C₃N₄ and TiO₂ is more negative than the standard redox potentials of redox couples as presented in Eqs. (9), (10), (11) [208].(9)CO₂ (g) + 8 H⁺ + 8 e⁻→ CH₄ (g) + 2H₂O, E^o = −0.24 V vs. NHE at pH 7(10)CO₂ (g) + 6 H⁺ + 6 e⁻→ CH₃OH (aq) + H₂O, E^o = −0.38 V vs. NHE at pH 7(11)CO₂ (g) + 4 H⁺ + 4 e⁻→ HCHO (aq) + H₂O, E^o = −0.48 V vs. NHE at pH 7
Adekoya et al. reported that visible light driven photoconversion of CO₂ over Cu-modified TGCN composite yielded 2574 mmol g⁻¹ of CH₃OH [209]. Au modified core-shell TGCN heterojunction designed by Wang et al. exhibited excellent photoreduction of CO₂ under visible light irradiation and 37.4 μmol g⁻¹ h⁻¹ of CH₄ was produced [210].
3.2.2. Pollutant degradation
The reactive species like O₂ ⁻,  OH, h⁺_VB and e-_CB under suitable thermodynamic conditions degrade the pollutants in to innocuous products [[211], [212], [213], [214], [215], [216], [217], [218], [219]]. TGCN heterojunction acts as a promising photocatalyst in complete removal of these pollutants through photoredox processes. Mechanism and kinetics of photocatalytic degradation of such pollutants over TGCN heterojunction are described in this section.
3.2.2.1. Mechanism of photodegradation
The main oxidative species that play pivotal role in photo-degradation process are h⁺,  OH, and  O²⁻ [220]. In order to elucidate the photodegradation mechanism carried by TGCN heterojunction, radical trapping experiments were conducted. The scavenging agents used for photoexcited holes (h⁺), superoxide radicals ( O²⁻), and hydroxyl radicals ( OH) are disodium ethylenediaminetetraacetate (Na₂-EDTA), p-benzoquinone (p-BQ) and tert-butyl alcohol (TBA) respectively [221,222]. As shown in Fig. 8(a), the photo degradation of RhB was significantly reduced by adding p-BQ and Na₂-EDTA. This recommended that RhB underwent decomposition mainly due to the presence of  O²⁻ and h⁺ as the reactive species. On the other hand,  OH had shown a relatively negligible role in photodegradation of RhB as degradation process was rarely affected by the addition of TBA [124].

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Fig. 8. (a) Active species trapping experiments of TGCN (CNT3) heterojunction (Reproduced from [124]), (b) UV–vis spectrum changes of RhB with increasing irradiation time (Reproduced from Ref. No. [154]), Electron spin resonance spectroscopy of TGCN core-shell photocatalyst without and with visible light irradiation(λ Vis ≥ 420 nm) (c) Superoxide radical test in methanol solvents with DMPO (50 mM) as radical trapper and (d) Hydroxyl radical test in aqueous solvents with DMPO (50 mM) as radical trapper (Reproduced from [137]), DMPO spin-trapping EPR spectra of M400 and M0 + g-C₃N₄ (e) methanol dispersion for DMPO  O₂ and (f) aqueous dispersion for DMPO  OH (Reproduced from Ref. No. [228]), (g) The samples of PL intensity@425 nm in 2 × 10⁻³ M NaOH solution in presence of 5 × 10⁻⁴ M terephthalic acid (Reproduced from Ref. No. [113]).
Yang et al. investigated the major reactive species in the photocatalytic dilapidation of RhB through the changes observed in UV–vis spectrum with the advances in degradation reaction. Fig. 8(b) shows that the maximum absorbance peak experiences a blue shift from 553 nm to 502 nm with the proceeding of reaction due to formation of copious N-demethylated intermediates [154]. Thus, N-demethylation is the key process in RhB photo-degradation rather than hydroxylation [223,224]. In contrast, the degradation reaction could have been proceeded with the decomposition of the conjugated aromatic structure of RhB without appearance of new peak if  OH was the actual reactive species [225]. This validated that the chief oxidative species was not  OH.
Wang and his co-workers carried out Electron Spin Resonance spectroscopy (ESR) studies to identify the prime oxidative species in phenol degradation process. The ESR signal of TGCN core-shell structure under visible light irradiation as shown in Fig. 8(c), exhibits obvious superoxide ( O₂) crack peak suggesting that  O₂ is likely to be one of the main active species. No signal in the dark and also under visible light irradiation as seen in Fig. 8(d) indicated that hydroxyl radicals ( OH) are probably not the active oxidative species [137]. After carrying out trapping experiments, Lu group reported that  O²⁻ and h⁺ act as the primary active species in the photocatalytic degradation of tetracycline [132]. This has also been thermodynamically predicted [226]. Since, the CB levels of both TiO₂ and g-C₃N₄ were more negative than the reduction potential of E°_O2/·O2-, the electrons on the CB of either g-C₃N₄ or TiO₂ could be taken up by surface adsorbed O₂ to release  O^2– [227]. Considering type-II heterojunction, h⁺ remaining on the VB level of g-C₃N₄ could not react with OH⁻ or H₂O to generate  OH radicals as the potential of  OH/H₂O and  OH/OH⁻ are extremely positive than VB level of g-C₃N₄ [110].
The stronger signal of  O²⁻ with respect to that of  OH (Fig. 8(e) and (f)) in DMPO spin-trapping electron paramagnetic resonance spectra (EPR) measurements under visible light irradiation predicted that  O²⁻ was the main active species and  OH played a minor role in NO_x oxidation [228]. The presence of  OH might be attributed to the reaction of  O^{2 –} with H₂O [229]. Over all,  O^2– and h⁺ are considered as the main reactive species in TiO₂/g-C₃N₄ heterojunction system for degradation of pollutants. However,  OH species may take part in degradation process of contaminants to small extent, when these are released by the reaction of h⁺ present on the VB of TiO₂ if any with OH⁻ or H₂O [226]. The photocatalytic decomposition of pollutants may then be represented by the Eqs. (12), (13), (14), (15), (16).(12)g- C₃N₄ /TiO₂ + hv → g- C₃N₄ /TiO₂ (e⁻ + h⁺)(13)e- + O₂ →  O²⁻(14)h⁺ + H₂O →  OH(15)H₂O +  O^{2 –}→ 2  OH(16) OH/  O²⁻/ h⁺ + Pollutant → degradation product
Li et al. constructed TGCN Z- scheme heterojunction and investigated the major active species in photooxidation of propylene by fluorescence method. Terephthalic acid was used as probe molecule to capture hydroxyl radicals. It could react only with HO  radicals to form 2-hydroxyterephthalic acid along with emission of fluorescence at about 425 nm, intensity of which determines the concentration of dissociative hydroxyl radical. Sharp increase in intensity with increase in irradiation time [Fig. 8(g)] indicated formation of large amount of HO· radicals by TGCN Z- scheme heterojunction under visible light interaction [113]. Thus, the plausible mechanism of photcatalytic degradation of pollutants over TGCN heterostructure may schematically be presented in two different ways as shown in Scheme 3. Morphology based TGCN heterostructure has a tremendous impact on photocatalytic degradation irrespective of reactive species. Degradation performance of different morphology based heterojunction photocatalytic systems are depicted in Table 2.

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Scheme 3. Plausible mechanism for photocatalytic pollutant degradation under visible light irradiation over morphology based TGCN.
Table 2. Comparison of TGCN with g-C₃N₄ and TiO₂ for photocatalytic pollutant degradation under visible light irradiation.