A review on TiO2/g-C3N4

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Keywords
Photocatalysis
TiO₂/g-C₃N₄ morphology based heterojunctions
H₂ generation
Pollutant degradation
Stability
1. Introduction
Global energy demand is expected to be increased twice by 2050 as compared to the present energy stock because of rapid population growth vis-a vis vast advancement in industrialization. Petroleum, coal and natural gas are mostly used to meet the world’s energy mandate in recent times. Owing to their rapid consumption, these fossil fuels are not only at the verge of depletion but also are accountable for amassed environmental pollution [[1], [2], [3]]. Rampart industrial activities have also significantly contributed to the adversely rising environmental issues due to release of enormous amount of hazardous organic pollutants, toxic heavy metals, CO₂ and oxides of nitrogen (NO_x) into the ecosystem. These have placed an imperative challenge before the scientific community across the globe to mitigate energy crisis by exploring renewable energy sources and resolve environmental problems through the development of sustainable techniques. Semiconductor based photocatalysis has been deliberated as a sustainable technology to address these two major challenges through production of H₂ energy and degradation of pollutants by directly harnessing inexhaustible solar energy along with copiously available water resources [[4], [5], [6], [7], [8], [9], [10], [11]].
After the landmark invention of H₂ production through photoelectrochemical water splitting over TiO₂ electrode by Fujishima and Honda in 1972 [12], several semiconductor photocatalysts such as ZnO, BiPO₄, CdS, WO₃, g-C₃N₄, Ag₃PO₄, BiVO₄ etc. have been widely studied for various photocatalytic applications [[13], [14], [15], [16]]. Among these, TiO₂ has extensively been investigated because of its non-toxicity, low cost, high oxidizing power, exceptional photochemical stability, great chemical passiveness and environment friendly property [[17], [18], [19], [20], [21]]. In addition, excellent anti-fogging and self-cleaning abilities indicate excellent hydrophilic properties of photoexcited TiO₂ surfaces [22]. Besides this, it is thermodynamically considered as an efficient photocatalyst because its more negative conduction band (CB) potential (−0.5 V vs NHE at pH 7.0) permits the photo induced electrons to exhibit higher reduction ability as well as the spawned holes at its more positive valence band (VB) potential (+2.7 V vs NHE at pH 7.0) exhibit higher oxidation tendency [23,24].
TiO₂ appears in three crystalline forms namely brookite, anatase and rutile. In these crystalline phases, VB is formed by the overlapping of 2p orbitals of oxygen whereas the 3d orbitals of Ti⁴⁺ are responsible for the formation of lower part of CB. The first two forms are extensively applied in photocatalysis while brookite form gains restricted research attention. Anatase TiO₂ demonstrates greater photoactivity than rutile TiO₂. This is attributed to the different dominant facets for the two different structures. The dominant facets for the former are {101} and {001} whereas {110}, {100}, and {101} are the dominant faces for the later [25]. The availability of abundant under-bonded Ti atoms and hefty Ti O Ti bond angles at {001} facet makes anatase TiO₂ a superior photocatalyst [26,27]. Furthermore, Ti⁴⁺ ions are surrounded by six O²⁻ ions to form TiO₆ octahedron in these three polymorphs of TiO₂ as shown in Fig. 1 [28]. In rutile TiO₂, slightly orthorhombic structure is possessed by the octahedron, each of which is connected to 8 such octahedrons whereas the octahedron in anatase TiO₂ belongs to distorted orthorhombic structure and each octahedron is attached to 10 octahedrons surrounded by it [29]. These structural differences between anatase and rutile forms made them to possess the dissimilar mass densities as well as electronic band arrangements that resulted in higher photoactivity of anatase TiO₂ [30,31]. However, the recombination rate for photoinduced charge carriers in anatase TiO₂ is much higher than their rate of separation [32]. Matsuzaki et al. reported that the recombination time was found to be ∼100 ps and the diffusion length is greater than ∼24 nm [33]. This largely lessens the quantum efficiency of TiO₂ photoactivity [[34], [35], [36], [37]]. Moreover, TiO₂ possesses large band gap of 3.2 eV and 3.0 eV for anatase and rutile phase respectively. This causes electronic excitation from VB by absorption of UV radiation corresponding to the wavelength below 388 nm, which is less than 7 % of solar light [[38], [39], [40]]. Therefore, the photocatalytic activity of TiO₂ has not yet been achieved as per expectation mainly because of poor solar energy conversion efficiency, high recombination rate for electron – hole pairs, low rate of separation and migration of photoelectrons. It is highly desirable to vanquish these precincts by augmenting the life period of charge carriers and visible light utilization efficiency of TiO₂ for its practical applicability [[41], [42], [43], [44], [45]]. Several strategies such as surface modification with metals, doping of metals and non-metal elements, coupling with visible light active semiconductors etc. have been addressed to tune its photocatalytic properties [[46], [47], [48]]. Among these strategies, combining it with another semiconductor of narrow band gap energy to form heterojunction is proved to be extremely beneficial [[49], [50], [51], [52]] as the separation of charge carriers would significantly be enhanced by the transfer of photo induced electrons from the CB with more negative potential to that of less negative potential along with excitation of holes from more positive VB to less positive VB through the interface of heterojunction [53]. Additionally, the visible light active semiconductor would harvest abundantly available solar radiation. The heterojunction would also provide large surface area that facilitates the reactants to get adsorbed on the surface of the catalyst and the reaction is accelerated in presence of enough reaction sites. All these factors lead to an enriched visible light responsive photocatalytic activity.

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Fig. 1. Crystalline phases of TiO₂ (a) Anatase, (b) Rutile, (c) Brookite (Reproduced from Ref. No. [28]) and Structures of g-C₃N₄ (d) s-triazine (e) tri-s-triazine (heptazine) [Reproduced from Ref. No. [70]].
Graphitic carbon nitride (g-C₃N₄) is contemplated as a visible light responsive and robust photocatalyst among all organic semiconducting materials since the pioneering work by Wang and co-workers’ in 2009 when they produced hydrogen gas through splitting of water under visible light irradiation [54]. This metal free polymeric semiconductor has garnered a lot of interest in solar light driven photocatalysis owing to its myriads of advantages like non toxicity, inexpensiveness, abundant availablability, thermal stablity (up to 600 °C in air), good chemical inertness (in organic solvents, acids and bases), facile fabrication, appealing electronic band arrangement and moderate band gap of 2.7–2.8 eV corresponding to wave length 450–460 nm of solar spectrum [[55], [56], [57], [58], [59], [60], [61], [62], [63], [64]]. It’s more negative CB potential (−1.3 V vs NHE at pH 7.0) and positive VB potential (1.4 V vs NHE at pH 7.0) allow thermodynamically to carry out redox reactions for various photocatalytic reactions under solar irradiation [[65], [66], [67]].
Among the different allotropes of carbon nitride (i.e a-C₃N₄, b-C₃N₄, graphitic-C₃N₄, cubic-C₃N₄ and pseudo-cubic-C₃N₄), g-C₃N₄ is the most stable phase at ambient conditions [54,68]. It is fabricated by the thermal polymerisation of nitrogen and carbon rich precursors like melamine, cyanamide, dicyandiamide, urea, thiourea and ammonium thiocyanate [69]. It consists of s-triazine [Fig. 1(d)] or tri-s-triazine (heptazine) units [Fig. 1(e)] as the building blocks, which are interconnected with each other through tertiary amines to form honeycomb like two dimensional (2D) sheets [70]. The 2D sheets of g-C₃N₄ are held with one another by weak van der Waals force and the atoms present in each layer are bonded by covalent bonds. According to density functional theory (DFT) calculations, valence band maximum (VBM) of g-C₃N₄ consists of N 2p state and the conduction band minimum (CBM) is formed by the hybridization of N 2p and C 2p states. As a result, the photogenerated holes become available at N 2p state where as photoelectrons are present in N 2p and C 2p states. The hybridization of the N 2p and C 2p states in CB leads to high rate of charge carrier recombination which reduces the quantum efficiency of g-C₃N₄ in photocatalytic processes [[71], [72], [73]]. Besides this, the photocatalytic efficiency of g-C₃N₄ is also hindered due to its low specific surface area (10–15 m² g⁻¹), availability of less number of active sites, slow surface reaction kinetics, modest oxidising ability, grain boundary effects, restricted charge mobility and inefficient absorbance of solar spectrum [[74], [75], [76], [77]]. The difficulty in complete separation from water [78] and the possibility of secondary pollution caused due to dissolution [79] are also the shortcomings that confine g-C₃N₄ from photocatalytic applications. Several strategies like foreign elements doping, co-catalysts incorporation, construction of heterojunction etc. have been developed to overcome its limitations [80,81]. Among these, heterojunction construction is proved to be advantageous as the separation of photoexcited electrons is expedited through the interface due to its delocalized conjugated π structure [76].
A suitable photocatalytic system should be constructed for effective solar energy conversion to generate H₂ energy and degrade pollutants proficiently. Construction of heterojunction with another semiconductor having apposite band edge potential is a promising approach for achieving improved photoactivity [82]. The matched energy band of g-C₃N₄ and TiO₂ has been fruitful to couple them for constructing the heterojunction, which is considered as an alternative to obtain maximised photocatalytic performance [83]. Since the energy difference between the conduction and valence bands of these two semiconductors is sufficiently high, the irreversible spatial charge separation is facilitated across the heterojunction. This suppresses the recombination of photo-originated holes and electrons to a large extent as a result of which charge carriers are efficiently separated. The narrow band gap of g-C₃N₄ helps to harness light from visible region and acts as a sensitizer for TiO₂. The combination of UV activity of TiO₂ and visible light response of g-C₃N₄ extends the light absorption range of the coupled photocatalyst to harness sufficient solar radiation for photocatalytic reactions, making the hetero system suitable for practical applications. Moreover, TiO₂-g-C₃N₄ (TGCN) heterojunction provides higher surface reaction activity which is also beneficial for the photocatalytic reactions. Owing to these advantages, a good number of research papers on designing of heterostructure and photocatalytic applications highlighting water splitting, environmental purification and stability of TGCN system have been published in recent years. In this review, we discussed various synthetic routes for construction and designing strategies for TGCN heterojunctions. Different morphology based TGCN heterostructures such as mesoporous, core-shell, point-to-face (0D/2D), line-to-face (1D/2D) and face to face (2D/2D) architectures were systematically overviewed. The enhanced photocatalytic activity of morphology based TGCN heterojunctions with respect to H₂ generation and environmental applications in terms of NO oxidation, CO₂ conversion, Cr (VI) reduction, dye degradation, organics removal and antibiotics decontamination are depicted thoroughly. Kinetics of photodegradation of pollutants is also presented. The plausible mechanism for photodegradation of pollutants was demonstrated with the deliberation on radical trapping experiments, electron spin resonance spectroscopy (ESR) and electron paramagnetic resonance spectroscopy (EPR). Photocatalytic efficiency of different morphology based heterojunctions was compared with that of pristine TiO₂ and g-C₃N₄. Photoelectrochemical oxidation of water over TGCN electrode was briefly highlighted. An overview on the stability and reproducibility of this photocatalytic system has been bestowed in details. The present review concludes with advantages, limitations and emerging challenges of TGCN heterojunction photocatlytic system. The future perspectives of this system for the large scale production of H₂ energy and degradation of pollutants were manifested.