A review on TiO2/g-C3N4

3. Photocatalytic applications
Basic principles of a semiconductor photocatalytic system involves following three steps. First of all, absorption of photons having energy (hv) equal to or greater than that of band gap (Eg) resulted in the excitation of the electrons from the valence band (VB) of a semiconductor to the conduction band (CB). In the second step, the photo-induced charged species (holes and electrons) are transferred to the catalyst surface after being separated effectively. Third step involves various redox reactions of these photogenerated charge carriers for different photocatalytic applications. Besides this, there is possibility of the recombination of electon/hole pairs in the bulk and on the surface of the semiconductor [188]. Being a robust photocatalyst, TGCN system has extensively been studied in two major areas of photocatalytic applications such as energy generation (H₂ energy) and environmental remediation as presented in Scheme 2 and are discussed in following sections.
3.1. H₂ energy generation
The energy content of hydrogen varies from 120 to 142 MJ kg⁻¹, which is higher than that of hydrocarbon fuels and hence it is expected that H₂ will be major source of energy generation by 2080 [189]. Different technologies like gasification of coal and other hydrocarbons, steam methane reforming, hydrogen production powered by wind, water electrolysis, biological conversion, splitting of water over semiconductor photocatalyst etc. have been applied currently for production of H₂ [190,191]. Photocatalytic H₂ production technique utilises prolific solar light and abundantly available water resources. Further, H₂ gas is regarded as an emission free energy source since water or water vapour is produced as exhaust after its use as fuel. It is also estimated that the cost of hydrogen production through photocatalytic water splitting would be approximately US$1.60–10.40 kg⁻¹ which projects the hydrogen price of $2.00–4.00 kg⁻¹ by 2020 [192]. Therefore, photocatalytic H₂ generation over semiconductor photocatalysts through splitting of water has attracted worldwide attention as an environmentally benign, economically viable, renewable and versatile approach.
In the process of photocatalytic water splitting, the photogenerated electrons present at the semiconductor surface undergoes H₂ evolution reaction (HER) and O₂ evolution reaction (OER) as per Eqs. (6) and (7) respectively [193].(6)4H⁺ + 4e⁻ → 2H₂, E^o = -0.41 V vs. NHE at pH 7.0(7)H₂O + 2h⁺ → O₂ + 2H⁺ + 4e⁻, E^o = 0.82 V vs NHE at pH 7.0
So, the photocatalytic water splitting resulted in production of H₂ and O₂ as presented in Eq. (8) [193].(8)2H₂O → 2H₂ + O₂, ΔG^o = +237.2 kJ mol⁻¹
The thermodynamic perspective for Eq. (6) to occur is that the conduction band minimum of the semiconductor photocatalyst must be more negative than the reduction potential of H⁺/H₂ (E^o = −0.41 V vs. NHE at pH 7.0). Similarly, valence band maximum must be more positive than 0.82 V vs. NHE at pH 7.0 in order for feasibility of Eq. (7) [[194], [195], [196]]. Since the CBM of both g-C₃N₄ and TiO₂ is anodic than that of reduction potential of H⁺/H₂, the production of H₂ is thermodynamically feasible for different morphology based TGCN photocatalytic systems [100,142,[197], [198], [199], [200], [201], [202]]. Therefore, such heterojunction exhibited significant performance towards H₂ production under visible light irradiation with respect to g-C₃N₄ and TiO₂ as shown in the Table 1.
Table 1. Comparison of TGCN with g-C₃N₄ and TiO₂ for photocatalytic H₂ production under visible light irradiation.