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Review

Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal

by
Zhanyong Gu
1,*,
Mengdie Jin
1,
Xin Wang
1,
Ruotong Zhi
1,
Zhenghao Hou
1,
Jing Yang
1,
Hongfang Hao
1,
Shaoyan Zhang
1,
Xionglei Wang
2,
Erpeng Zhou
1,* and
Shu Yin
3,4,*
1
Shijiazhuang Key Laboratory of Low-Carbon Energy Materials, College of Chemical Engineering, Shijiazhuang University, Shijiazhuang 050035, China
2
School of Material Science and Engineering, Hebei University of Engineering, Handan 056038, China
3
Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan
4
Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(1), 192; https://doi.org/10.3390/catal13010192
Submission received: 14 December 2022 / Revised: 8 January 2023 / Accepted: 11 January 2023 / Published: 13 January 2023
(This article belongs to the Special Issue Advances in Heterojunction Photocatalysts)
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Abstract

:
Nitrogen oxides (NOx) pollutants can cause a series of environmental issues, such as acid rain, ground-level ozone pollution, photochemical smog and global warming. Photocatalysis is supposed to be a promising technology to solve NOx pollution. Graphitic carbon nitride (g-C3N4) as a metal-free photocatalyst has attracted much attention since 2009. However, the pristine g-C3N4 suffers from poor response to visible light, rapid charge carrier recombination, small specific surface areas and few active sites, which results in deficient solar light efficiency and unsatisfactory photocatalytic performance. In this review, we summarize and highlight the recent advances in g-C3N4-based photocatalysts for photocatalytic NOx removal. Firstly, we attempt to elucidate the mechanism of the photocatalytic NOx removal process and introduce the metal-free g-C3N4 photocatalyst. Then, different kinds of modification strategies to enhance the photocatalytic NOx removal performance of g-C3N4-based photocatalysts are summarized and discussed in detail. Finally, we propose the significant challenges and future research topics on g-C3N4-based photocatalysts for photocatalytic NOx removal, which should be further investigated and resolved in this interesting research field.

1. Introduction

With the fast development of the economy and modern industrialization, environmental pollution and the energy crisis have become the two major challenges in the world [1,2,3,4,5,6,7]. Air pollution is one of the serious environmental problems [8,9]. There are a variety of air pollutants including sulfur oxides (SOx), nitrogen oxides (NOx), carbon monoxide (CO) and so on [10,11]. Air pollution would bring about a number of environmental issues: it can damage and corrode buildings and equipment; it is harmful to human beings, animals and vegetables; and it can lead to ecological deterioration of the environment. NOx is especially serious among air pollutants [12,13,14,15,16,17,18]. To be more specific, as shown in Figure 1, NOx pollutants result in ground-level ozone pollution, acid rain, photochemical smog, global warming and so on. Moreover, it could cause damage to human health and increase the risk of diseases such as emphysema, bronchitis and respiratory disease. If human beings want to realize sustainable development and have a bright future, we must solve air pollution as soon as possible.
An enormous amount of research work has been carried out to deal with NOx pollution [19,20,21,22]. For example, traditional adsorption, filtration and selective catalytic reduction technologies have been extensively used to solve NOx pollution [23,24,25]. However, these technologies have low efficiency to remove the low-concentration NOx pollutants and also have disposal and regeneration issues [26]. Recently, photocatalysis as a green technology has attracted a substantial amount of attention [8,27]. Compared with traditional physical or chemical methods, photocatalytic NOx removal technology is more efficient and environment-friendly with a semiconductor as the photocatalyst and solar energy as the driving force. Photocatalytic technology can convert the low concentration of NOx in the atmosphere into non-toxic products without disposal and regeneration issues. The greatest challenge in the photocatalytic process is to develop low-cost and highly efficient photocatalysts, which can make full use of solar energy to remediate environmental problems. Unveiling the photocatalytic mechanism is the premise of developing highly effective photocatalysts. Since Fujishima and Honda reported photoelectrochemical water splitting in 1972 [28], great progress has been made to unravel the photocatalysis mechanism [29,30,31]. Typically, as shown in Scheme 1, the photocatalytic process includes three consecutive steps [32,33,34]: (1) light harvesting using a semiconductor photocatalyst for charge carrier excitation; (2) photogenerated electron-hole pairs separation and migration and (3) surface reduction and oxidation reactions. It should be pointed out that a large number of photogenerated electrons and holes recombine during the charge carrier separation and migration process, i.e., volume recombination and surface recombination. Only the effective charge carriers, in other words, the remaining charge carriers, can attend the surface reduction and oxidation reactions [35,36,37]. Therefore, if we attempt to improve the photocatalytic performance of the photocatalysts, we can enhance the light-harvesting capability, promote the charge carrier separation efficiency and facilitate the surface reduction and oxidation reactions.
As one of the typical photocatalytic processes, the reaction mechanism of photocatalytic NOx removal has been carefully discussed. When the photocatalyst is irradiated by light with equal or greater energy than the bandgap energy of the photocatalyst, the electrons (e−) can be excited from the valence band to the conduction band, resulting in positive holes (h+) in the valance band [8,38]. It is well-understood that oxygen molecules are the second most abundant gases, consisting of 21% of the atmosphere. The oxygen molecules absorbed on the surface would be reduced, which brought about the superoxide anion radicals (·O2−) [39]. At the same time, holes (h+) in the valance band can oxide the water molecules and hydroxide ion (OH−) to obtain hydroxyl radicals (·OH) [10,39]. It should be noted that these radicals play significant roles during the photocatalytic NOx removal process. To be more specific, the NO absorbed on the surface of a photocatalyst would be oxidized by superoxide radicals (·O2−) and/or hydroxyl radicals (·OH) and/or positive holes (h+) to become the final products of the nitrate ion (NO3−), through the formation of intermediates related to NO2 and HNO2. These series of reactions are illustrated as follows [10,24,39,40,41,42]:
Photocatalyst + hv → e− + h+
O2 + e− → ·O2−
h+ + OH− → ·OH
NOx + ·O2− → NO3−
NO + ·OH → HNO2
HNO2 + ·OH → NO2 + H2O
NO2 + ·OH → NO3−
It is universally acknowledged that the key point of photocatalysis is to develop highly effective and stable photocatalysts. Since the pioneering work of Fujishima and Honda in 1972 [28], a great number of photocatalysts have developed to be used in energy conversion and environmental remediation [1,21,43,44,45,46,47,48,49,50]. The reported photocatalysts include metal oxides such as Fe2O3 [51,52], WO3 [53], TiO2 [54] and ZnO [55]; metal sulfides such as ZnS [56], CdS [57] and CuS [58]; bismuth-based photocatalysts such as BiOX (X = I, Br, Cl) [59] and Bi2MO6 (M = Mo, W) [60]; metal-organic frameworks (MOFs) [61]; covalent organic frameworks (COFs) [62] and many more. Among these semiconductor photocatalysts, TiO2 is considered the most classical photocatalyst owing to its many advantages such as non-toxicity, low cost, long-term stability and so on [63,64]. However, the wide bandgap of TiO2 (3.0–3.2 eV) constrains the photocatalytic response to only ultraviolet (UV) light [65]. As we know, the full solar spectrum consists of a near-infrared region (52%), a visible-light region (43%) and a UV region (5%) [66]. Therefore, metal and non-metal doping are performed to reduce the bandgap to make the best use of solar energy [67,68].
Recently, graphitic carbon nitride (g-C3N4), as a metal-free semiconductor photocatalyst, has attracted much attention since Wang et al. reported it could be used for H2 production in 2009 [31]. The g-C3N4 exhibited outstanding optical, electrical and structural properties, such as visible-light response due to the suitable bandgap (2.7 eV), nanoscale thickness and high surface-to-volume ratio owing to the two-dimensional (2D) structure, facile synthesis with a cheap precursor, such as melamine, dicyandiamide, urea thiourea and cyanamide (Figure 2a,b), high thermal and chemical stability and nontoxic nature [32,38]. What is more, the electronic structures of the 2D materials could be modulated by controlling the thickness or doping strategy. Thanks to its excellent advantages, g-C3N4 has been used in different fields including energy issues and environmental remediation. To be more specific, it has been reported in supercapacitors [69], electrocatalysis [70], photo-electro catalytic reactions [71], N2 fixation [72], pollutant degradation [73], CO2 reduction [74], water splitting [30], organic catalysis [75] and sensing [76].
The long history of g-C3N4 could trace back to 1834 when Berzelius and Liebig prepared it by igniting mercuric thiocyanate [77,78]. Since then, g-C3N4 has become a hot research area. g-C3N4 is normally considered as three basic structures: triazine-based g-C3N4, heptazine-based g-C3N4 and triazine and heptazine mixed g-C3N4 [32,38]. Figure 3a,b shows the typical structures of triazine-based g-C3N4 and heptazine-based g-C3N4, respectively. However, it should be noted that heptazine-based g-C3N4 is the most stable phase at ambient conditions according to the first-principles density functional theory (DFT) calculations carried out by Kroke et al. [79]. Therefore, more and more researchers and scientists tend to recognize heptazine as the building block for the formation of g-C3N4.

2. Modification Strategies of Pristine g-C3N4

g-C3N4, as the significant metal-free semiconductor photocatalyst, holds great potential in the application of the photocatalytic NOx removal process due to its plentiful extraordinary advantages, such as visible light response properties, mild bandgap, low cost, facile preparation and high thermal stability. However, pristine g-C3N4 prepared using the traditional high-temperature solid reaction suffers from low specific surface areas and low crystallinity owing to kinetic hindrance, which results in small specific surface areas, few reactive sites, limited light-harvesting capacity, rapid recombination of photogenerated charge carriers and unsatisfactory photocatalytic NOx removal performance. In order to improve the photocatalytic performance of pristine g-C3N4, a variety of modification strategies have been developed including metal doping, non-metal doping, defect engineering, crystallinity optimization, morphology controlling and heterojunction construction.

2.1. Morphology Controlling

Morphology controlling is considered a promising strategy to improve the photocatalytic performance of bulk g-C3N4 [80]. Since bulk g-C3N4 is synthesized using the high-temperature solid reaction, it suffers from low specific surface areas and few active sites, which is detrimental to photocatalytic performance. Moreover, bulk g-C3N4 exhibits a longer charge carrier migration distance, and thus the photogenerated electron and hole pairs achieve rapid charge recombination. In addition, bulk g-C3N4 is unfavorable for molecular mass transport, surface redox reactions and light harvesting in comparison with porous g-C3N4 [32,81]. In order to enlarge the specific surface areas, increase the active sites, promote the charge carrier separation efficiency and facilitate the molecular mass transport, much progress has been made such as the exfoliation of bulk g-C3N4 into nanosheets, template strategy including hard-template and soft-template and supramolecular preorganization method.

2.1.1. Nanosheets Structure

Inspired by the preparation of graphene nanosheets [82,83], scientific researchers attempt to exfoliate bulk g-C3N4 into nanosheets. Compared with bulk g-C3N4, g-C3N4 nanosheets exhibit a great deal of distinct benefits owing to morphology changes. It not only enlarges the specific surface areas and increases the active sites but also shortens the charge carrier’s transport distance, improves the solubility and modifies the electronic structures owing to the famous quantum confine effect. More specifically, the photogenerated electrons and holes coming from the g-C3N4 nanosheets can easily migrate to the surface of the photocatalysts to attend the surface reactions through the shortened paths. This phenomenon is instrumental in facilitating the charge carrier separation efficiency to improve photocatalytic performance. In addition, the enlarged bandgap of g-C3N4 nanosheets leads to enhanced oxidation potential energy and reduced potential energy, which is useful for surface reactions.
Generally, the g-C3N4 nanosheet structures could be achieved using two different strategies, i.e., liquid exfoliation of bulk g-C3N4 and thermal exfoliation of bulk g-C3N4. Various solvents with suitable surface energy, such as water, methanol, ethanol, N-methyl-pyrrolidone (NMP), 1-isopropanol (IPA), acetone and their mixtures, have been used to overcome the weak van der Waals forces between the two adjacent layers of bulk g-C3N4 using facile sonification. For example, Xie et al. reported a green liquid exfoliation strategy to obtain ultrathin nanosheets using cheap and environmentally friendly water as the solvent (Figure 4) [84]. The thickness of the exfoliated nanosheet is about 2.5 nm in height (around seven layers) with the size distribution ranging from 70 nm to 160 nm. In addition, Zhu et al. reported a concentrated H2SO4 (98%) assisted liquid exfoliation strategy to fabricate a single atomic layer of g-C3N4 ultrathin nanosheets [85]. The intercalation of concentrated H2SO4 (98%) into the interplanar spacing of bulk g-C3N4 resulted in the graphene-like single-layer g-C3N4 structure with a small thickness of 0.4 nm and a large size of micrometers.
Compared with the liquid exfoliation strategy, thermal exfoliation is more facile and more environmentally friendly because it does not involve toxic solutions such as aqueous ammonia, hydrochloric acid and concentrated H2SO4 (98%). The thermal exfoliation approach is fast, low-cost and low-pollution. However, the largest drawback of the thermal exfoliation strategy is the low yield due to thermal oxidation and thermal etching. For instance, Niu et al. obtained g-C3N4 nanosheets with a thickness of 2 nm (about six to seven layers) using the thermal exfoliation strategy [86]. The synthesized g-C3N4 nanosheets exhibited enhanced photocatalytic H2 evolution under simulated solar light irradiation. The excellent photocatalytic H2 production of the obtained g-C3N4 nanosheets was ascribed to the large specific surface area, low sheet thickness, enlarged band gap, increased electron-transport ability and prolonged lifetime of the charge carriers. In addition, Gu et al. reported that bulk g-C3N4 could be exfoliated into nanosheets to increase the specific surface areas and active sites using facile post-thermal treatment [87]. At the same time, the electronic structure of bulk g-C3N4 was optimized during the calcination process. The valence band of g-C3N4 nanosheets was increased owing to the quantum confinement effect and nitrogen vacancy, which led to the higher thermodynamic driving force during the photocatalytic NOx removal process. The g-C3N4 nanosheets showed about 3.0 times higher photocatalytic NOx removal performance than pristine g-C3N4 owing to the enlarged specific surface areas and optimized electronic structure. In addition, the impact of calcination temperature, calcination time and sample amount on the photocatalytic NOx removal performance and the yield of g-C3N4 nanosheets were systematically studied. This research work provides new insight into the thermal exfoliation approach for enhancing the photocatalytic NOx removal performance.

2.1.2. Porous Structure

The template strategy is an effective approach to fabricating porous nanostructured g-C3N4, which can increase the specific surface areas and active sites of bulk g-C3N4 [88,89]. Moreover, the high porosity of nanostructured g-C3N4 is beneficial for mass and gas transport. In addition, the voluminous void space in nanostructured g-C3N4 can enhance the light absorption efficiency owing to the light trapping effect. These plentiful advantages can bring about outstanding photocatalytic performance in comparison with the bulk counterpart [89]. In general, the template method is based on the use of inorganic or organic nanostructures as a template, i.e., a hard template and a soft template [90].
A hard template is a controllable and precious strategy to prepare nanostructured g-C3N4. The hard template method, in other words, solid material nano-casting, is performed using a physical structure agent to control the porous nanostructured g-C3N4. Up to now, a large number of hard templates have been studied. For example, Zhang et al. used HCl-treated SBA-15 silica as a hard template to prepare ordered mesoporous g-C3N4 [91]. The obtained mesoporous g-C3N4 displayed significantly enlarged specific surface area and pore volume, which were 517 m2 g−1 and 0.49 cm3 g−1, respectively. Similarly, Sun et al. reported hollow nanospheres of g-C3N4 when using a silica-based hard template [92]. The hollow nanospheres of g-C3N4 displayed excellent photocatalytic H2 evolution performance due to the hollow sphere structure. However, the biggest drawback of the silica-based hard template involves toxic reagents, such as ammonium hydrogen difluoride (NH4HF2), when we remove the silica-based hard template. Recently, Zhang et al. demonstrated that low-cost calcium carbonate (CaCO3) is a promising environmentally friendly hard template [93]. After the CaCO3 is removed using hydrochloric acid treatment, porous g-C3N4 was successfully prepared.
Since hard templates involve hazardous fluoride-containing reagents, a tremendous amount of work has been completed on the soft template [94]. The key point of a soft template is the molecular self-assembly process, which can chemically tailor the porosity and morphology of pristine g-C3N4. Various templates, such as non-ionic surfactants and amphiphilic block polymers, could be chosen as soft templates. In addition, ionic liquids have demonstrated an effective soft template [32,38]. The soft templates provide a facile and more environmentally friendly strategy to prepare nanostructured g-C3N4 [95]. Very recently, supramolecular preorganization has become an interesting topic to prepare nanostructured g-C3N4. Figure 5 shows the formation of the self-assembled structures used to prepare nanostructured g-C3N4 [75]. To some extent, the supramolecular preorganization strategy is similar to the soft template strategy. However, this strategy is based on the supramolecular interactions of g-C3N4 monomers, including hydrogen bonds, the π–π bond and so on. For instance, Zhang et al. reported a solvent-assisted strategy to prepare porous g-C3N4 with enhanced visible-light photocatalytic NO removal performance (Figure 6a) [96]. g-C3N4 prepared with the addition of water and ethanol exhibited significantly improved visible-light photocatalytic performance, with NO removal percentages of 37.2% and 48.3%, respectively (Figure 6b,c). The enhanced photocatalytic NOx removal performance was ascribed to the unique microstructure and prolonged lifetime of the charge carriers.

2.2. Band Structure Engineering

The band structure of photocatalysts plays a crucial role in the photocatalytic process. The optimized band structure can absorb more solar energy to generate more electron-hole pairs; improve the charge carrier separation efficiency to obtain more effective electrons and holes for the surface reactions; and optimize the reaction sites and promote the adsorption of intermediates to improve the surface reactions. Up to now, tremendous efforts have been devoted to modulating the electronic structure of g-C3N4. The strategies of band structure engineering can be roughly divided into two categories: metal element doping and non-metal element doping [38].

2.2.1. Metal Element Doping

A series of metal cations have been used to modulate the band structure of pristine g-C3N4. There are two kinds of metal element doping related to g-C3N4, which are cave doping and interlayer doping. The metal cations can be introduced into the triangular pores of g-C3N4 between the heptazine structures [33]. The strong coordination interaction between the metal cations and g-C3N4 matrix and negatively charged nitrogen atoms can realize cave doping [38]. According to previous literature, the transition metal elements including Fe, Mn, Co, Ni and Zn have been demonstrated to be effective at optimizing the electronic structure [32,38]. For example, Wang et al. showed that the band gap could be reduced to enhance the visible-light harvesting capability using Fe and Zn doping into g-C3N4 [97]. Ding et al. also demonstrated that Fe, Mn, Co and Ni could be incorporated into the g-C3N4 framework to extend the visible-light absorption range and improve the separation efficiency of the photogenerated electrons and holes, which resulted in enhanced photocatalytic performance [98].
In addition, according to the first principle DFT calculation, Pan et al. predicted that the incorporation of Pt and Pd into the g-C3N4 framework could promote the charge carrier transport rate to improve the charge carrier separation efficiency and reduce the band gap to improve the light absorption, which played positive effects in improving the photocatalytic activity [99]. Recently, Dong et al. found that K atoms brought about interlayer doping instead of caving doping in the g-C3N4 matrix. Pristine g-C3N4 displayed a limited photocatalytic NO removal rate of 16%. The K-doped g-C3N4 exhibited approximately 2.3 times higher photocatalytic NO removal performance than pristine g-C3N4. The outstanding photocatalytic performance of K-doped g-C3N4 was ascribed to the benefits of K intercalation including bridging the layers, charge redistribution, facilitating the charge carrier separation and tuning band structure (Figure 7) [100]. In addition, Zhu et al. revealed that K doping could decrease the VB level of g-C3N4, leading to the promoted separation and transportation of photo-induced electrons and holes under visible light irradiation [101].

2.2.2. Non-Metal Element Doping

Compared with metal doping, the strategy of non-metal doping may be more popular because it not only tunes the electronic structure but also retains the metal-free property. So far, many non-metal elements such as S, P, B, O, C and I have been demonstrated to be effective for band-gap engineering using chemical substitution. As shown in Figure 8 [38], C atom self-doping can substitute the bridging N atoms while O, S and I atoms tend to replace the N atoms in the aromatic heptazine rings. Thanks to non-metal doping, the delocalization of the Π-conjugated electrons is enhanced to improve the conductivity, mobility and separation of the charge carriers, which is beneficial for improving the photocatalytic performance. As for the P and B atoms, they are inclined to substitute the C atoms. For instance, Wang et al. successfully synthesized B-doped g-C3N4 hollow tubes for improved photocatalytic NOx removal performance [102]. The B-doped g-C3N4 hollow tubes were fabricated by calcining the assembly supramolecular precursors, which were obtained using the self-conversion of melamine with the aid of boric acid (Figure 9a). The B-doped g-C3N4 hollow tubes displayed the best photocatalytic NOx removal performance (30.4%), which was 1.5 and 1.3 times higher than pristine g-C3N4 (20.8%) and g-C3N4 hollow tubes (22.9%), respectively (Figure 9b,c). The excellent photocatalytic NOx removal performance of B-doped g-C3N4 hollow tubes was attributed to the extended light-harvesting range and enhanced charge carrier efficiency (Figure 9d).

2.3. Defect Engineering

At the same time, defect engineering is also an effective strategy to improve the photocatalytic performance of pristine g-C3N4. The defect engineering strategy is premature to modify the electronic structures of pristine TiO2, which may be due to the fact that TiO2 is the most classical and fully-studied photocatalyst [103,104,105,106]. For example, the band structures and optical properties of pristine TiO2 could be tuned by oxygen vacancies [107,108]. The oxygen vacancies-mediated TiO2 can extend the visible-light range, enhance the charge carrier separation efficiency and improve the molecules to be adsorbed on the surface of the photocatalysts, which would result in excellent photocatalytic performance.
Inspired by the oxygen vacancies-mediated TiO2, a defect engineering strategy is used to improve the photocatalytic performance of pristine g-C3N4. For example, Wang et al. reported the nitrogen vacancies-mediated g-C3N4 microtubes synthesized using a simple and green hydrothermal process (Figure 10a) [109]. The nitrogen vacancies-mediated g-C3N4 microtubes displayed significantly enhanced NO removal performance due to the enlarged specific surface areas and the curial roles of nitrogen vacancies. As shown in Figure 10b–g, the nitrogen vacancies-mediated g-C3N4 was beneficial for NO and O2 adsorption, which contributed to attending the surface reactions. The enhanced surface reactions and increased active sites resulted in improved photocatalytic NO removal performance in comparison with pristine g-C3N4 under visible-light irradiation. Li et al. successfully synthesized carbon vacancies-modified g-C3N4 nanotubes by calcining the hydrolyzed melamine–urea mixture [110]. The EPR spectra confirmed the formation of carbon vacancies in g-C3N4 nanotubes because the EPR signal of carbon vacancies-modified g-C3N4 decreased significantly due to the fewer unpaired electrons.
Gu et al. reported that the carbon vacancies and hydroxyls co-modified g-C3N4 were successfully prepared using a post-hydrothermal treatment [111]. Pristine g-C3N4 was first prepared using the thermally induced polymerization of melamine. Then a green hydrothermal treatment was employed to introduce the carbon vacancies and hydroxyls (Figure 11a). During the hydrothermal process, the water could induce the pristine g-C3N4 to partially hydrolyze, which introduced the carbon vacancies and hydroxyls into the pristine g-C3N4 simultaneously (Figure 11b). The obtained carbon vacancies and hydroxyls co-modified g-C3N4 showed 2.2 times higher photocatalytic NO removal activities than pristine g-C3N4. With the aid of DFT calculations and experimental calculations, Gu et al. revealed that carbon vacancies and hydroxyls played significant roles in enhancing the photocatalytic NO removal performance due to a synergistic effect. The carbon vacancies narrowed the band gap to extend the light-harvesting range and the hydroxyls could form the covalent bond acting as electron transport channels to facilitate the charge carrier separation efficiency (Figure 11c).

2.4. Crystallinity Optimization

Recently, the crystallinity optimization strategy has attracted much attention for improving the photocatalytic activity of pristine g-C3N4 [78]. It is well-understood that kinetic hindrance is the major issue in the traditional high-temperature solid-state synthesis of pristine g-C3N4, which results in semi-crystalline or amorphous structures and limited photocatalytic performance. Since kinetic hindrance is a great problem in traditional high-temperature solid-state reactions, a novel liquid reaction synthesis technology was developed to solve this problem. Bojdys et al. first reported that triazine-based crystalline g-C3N4 was successfully synthesized with the ionothermal method using the eutectic mixture of LiCl/KCl as a high-temperature solvent [112].
Up to the present, a great deal of research work has been carried out to prepare crystalline g-C3N4 for enhancing photocatalytic performance. For example, Wang et al. prepared heptazine-based crystalline g-C3N4 with the molten salt method using preheated melamine as precursors. The melamine was first heated at 500 °C in a muffle furnace and the preheated melamine was mixed with KCl and LiCl [113]. Then, the mixtures were calcined again in a muffle furnace to prepare heptazine-based crystalline g-C3N4. Detailed experimental characterization and theoretical simulation showed that heptazine-based crystalline g-C3N4 displayed higher photocatalytic performance than triazine-based crystalline g-C3N4 owing to the enhanced light-harvesting property and increased mobility of photogenerated charge carriers. In addition, Wang et al. studied the crystallization process of g-C3N4 using different precursors of the melem-based oligomer and melon-based polymer with a molten salts method (Figure 12a) [114]. The melem-based oligomer and melon-based polymer represented different polymerization degrees of g-C3N4, which were calcined at 450 °C and 550 °C in a muffle furnace, respectively. Xiang et al. demonstrated that the crystallinity of crystalline g-C3N4 synthesized using the molten salts method could be further improved by hydrochloric acid treatment (Figure 12b) [115].
It is important to point out that these reported molten salt methods were carried out under an inert gas atmosphere in a muffle furnace, which limited the large-scale production of crystalline g-C3N4. To solve this drawback, Gu et al. developed a modified molten salt method under ambient pressure using dicyanamide (DCDA) as the initial precursor (Figure 13a) [116]. The molten salts played two roles in the post-calcination process. One was improving the crystallinity of pristine g-C3N4 acting as the high-temperature solution, the other was protecting pristine g-C3N4 from contact with air since the pristine g-C3N4 was immersed in the solution. The crystalline g-C3N4 exhibited 3.0 times higher photocatalytic NO removal activity than pristine g-C3N4 under visible-light irradiation, with high stability under the cycling test (Figure 13b,c). The detailed experimental characterization and DFT calculation demonstrated that the optimized crystallinity played important roles in improving the photocatalytic NO removal activity of crystalline g-C3N4. The optimized crystallinity could decrease the band gap to extend the light-harvesting range, increase the conductivity to promote the photogenerated charge carrier separation efficiency and reduce the adsorption energy of NO and O2 molecules to activate the surface reactions, which led to the significantly enhanced photocatalytic NO removal performance (Figure 13d–k).

2.5. Heterojunction Construction

Charge carrier transport and separation is decisive in the photocatalytic process. A large number of photogenerated electrons and holes suffered from volume recombination and surface recombination, which result in unsatisfactory photocatalytic performance [117,118,119,120]. Constructing a g-C3N4-based heterojunction is an effective strategy to improve photocatalytic performance. The spatial separation of photogenerated electron-hole pairs can be achieved with efficient charge transfer across the interface between the two semiconductors. At the same time, the g-C3N4-based heterojunction can display the advantages of the counterpart. In other words, the g-C3N4- based heterojunction has both benefits of the two components. Up to the present, several types of g-C3N4-based heterojunction have attracted much attention including the traditional type-II heterojunction, all-solid-state Z-scheme heterojunction, step-scheme (S-scheme) heterojunction and g-C3N4/carbon heterojunction.
The traditional type-II heterojunction is facile constructed, and much progress has been made in this field. For example, Koci et al. reported that a series of TiO2/g-C3N4 heterojunction photocatalysts were easily prepared using mechanical mixing of TiO2 and g-C3N4 in a water suspension followed by calcination in a muffle furnace [121]. The TiO2/g-C3N4 heterojunction with the optimal weight ratio of TiO2 and g-C3N4 has shifted absorption edge energy towards longer wavelengths and decreased the recombination rate of charge carriers compared to pure g-C3N4.
Even though the traditional type-II heterojunction can improve the charge carrier separation efficiency to improve the photocatalytic performance, it sacrifices the oxidation potential energy and reduction potential energy. To overcome this drawback, the all-solid-state Z-scheme was developed inspired by the photosynthesis of plants [70]. The photosynthesis of plants consists of two isolated reactions of water oxidation and CO2 reduction, which are linked together through redox mediators. Thanks to the unique structure, it keeps the strong redox ability, improves the charge carrier separation efficiency and results in enhanced photocatalytic performance. For instance, Zhang et al. reported an all-solid-state Z-scheme g-C3N4/Au/ZnIn2S4 heterojunction photocatalyst for enhanced photocatalytic NO removal performance [122]. The noble Au nanoparticles played an important role in the charge carrier transfer process, which acted as an electron acceptor and conductive channel for enhancing the charge carrier separation efficiency. Additionally, the all-solid-state Z-scheme heterojunction exhibited oxidation potential energy and reduction potential energy during the photocatalytic NO removal process. Therefore, the optimized Z-scheme g-C3N4/Au/ZnIn2S4 heterojunction photocatalyst showed photocatalytic NO removal efficiency of up to 59.7%.
Recently, direct Z-scheme heterojunction has become a research hotspot [37,123]. The direct Z-scheme heterojunction is totally different from the all-solid-state Z-scheme because there is no intermediate, either Au or Ag nanoparticles, used in the direct Z-scheme heterojunction [25,122,124]. To describe the photocatalytic mechanism of the direct Z-scheme heterojunction clearly and vividly, the Yu group first nominated the direct Z-scheme heterojunction for a step-scheme (S-scheme) heterojunction [125,126]. Yu et al., for the first time, reported the g-C3N4-TiO2 direct Z-scheme heterojunction using the facile calcination method. Moreover, they found that the g-C3N4-TiO2 direct Z-scheme heterojunction was largely dependent on the content of g-C3N4 [54]. To be more specific, if the surface of TiO2 was partially covered by the g-C3N4, the g-C3N4-TiO2 direct Z-scheme heterojunction would be obtained; if the content of g-C3N4 was too much, the traditional type-II heterojunction would be obtained. Lu et al. reported an α-Fe2O3/g-C3N4 direct Z-scheme heterojunction prepared using an impregnation–hydrothermal method (Figure 14a) [52]. The unique direct Z-scheme heterojunction brought about wide visible-light absorption and facilitated charge carrier separation efficiency (Figure 14b) [52]. The α-Fe2O3/g-C3N4 direct Z-scheme heterojunction displayed approximately 1.78 times higher photocatalytic NO removal performance than pristine g-C3N4. As for the S-scheme heterojunction, Yu et al., for the first time, designed and constructed the S-scheme heterojunction of WO3/g-C3N4 using an electrostatic self-assembly strategy [126]. After that, a great number of S-scheme heterojunctions have been reported for improving photocatalytic performance. For example, Zhang et al. reported a Sb2WO6/g-C3N4 S-scheme heterojunction prepared using an ultrasound-assisted strategy for improved visible-light photocatalytic NO removal performance. In addition, Cao et al. successfully reported a 2D/0D g-C3N4/SnO2 S-scheme heterojunction using a hydrothermal and annealing strategy toward visible-light-driven NO degradation (Figure 15a) [127]. The S-scheme charge transfer mechanism was revealed using the Density-Functional Theory (DFT) calculation, trapping experiments and EPR spectra. Because of the unique structural features, the g-C3N4/SnO2 S-scheme photocatalysts displayed a photocatalytic NO removal percentage of 40% irradiated under visible light (Figure 15b).
Carbon materials including graphene have brought about widespread attention in the field of photocatalysis owing to their outstanding physical and chemical properties including excellent electron conductivity, good light harvesting properties, large specific surface areas, low cost and high stability [128,129,130]. Constructing a g-C3N4/carbon heterojunction can take advantage of carbon materials to improve the photocatalytic performance of pristine g-C3N4. Firstly, coupling carbon materials with the g-C3N4 can significantly extend the light-harvesting range to near-infrared. Secondly, carbon materials can facilitate the photogenerated charge carrier separation efficiency since it acts as conductive channels for electron transfer. Lastly, the large specific surface areas of carbon materials can provide plentiful of supporting sites for g-C3N4. For example, Gu et al. reported an alkali-assisted hydrothermal method to prepare g-C3N4/reduced graphene oxide (g-C3N4/rGO) nanocomposites (Figure 16a) [131]. During the hydrothermal process, the NaOH could improve the reduction of GO to increase the conductivity of rGO and etch the pristine g-C3N4 into nanosheets to enlarge the specific surface areas. The g-C3N4/rGO nanocomposites displayed 2.7 times higher photocatalytic NOx removal performance than pristine g-C3N4. The distinctly enhanced photocatalytic performance of g-C3N4/rGO nanocomposites is ascribed to the improved light-harvesting property, increased specific surface areas and active sites and facilitated charge carrier separation efficiency.

3. Conclusions and Prospects

In conclusion, photocatalysis is an environmentally friendly and low-cost technology to solve NOx pollution. As a typical 2D metal-free semiconductor, the g-C3N4 photocatalyst has drawn great attention owing to its outstanding physical and chemical properties including visible light response, adjustable band structure, low cost, facile synthesis, high stability and so on. Therefore, the g-C3N4 photocatalyst possesses great potential for application in the photocatalytic NOx removal process. However, pristine g-C3N4 synthesized using the traditional high-temperature solid reaction suffers from small specific surface areas and low crystallinity, which results in few reactive sites, limited light-harvesting capacity, rapid recombination of photogenerated charge carriers and unsatisfactory photocatalytic NOx removal performance. In this review, we briefly summarize the recent advances in g-C3N4-based photocatalysts for the NOx removal process. Various modification strategies are discussed including morphology controlling, band structure engineering, crystallinity optimization, defect engineering and heterojunction construction (Figure 17). The different modification strategies play different roles in improving the photocatalytic NOx removal performance of pristine g-C3N4. Specifically, morphology controlling can not only enlarge the specific surface areas and increase the actives, but also shorten the charge carrier migration distance and thus suppress the photogenerated electron and hole pairs recombination. Band structure engineering can be achieved using metal and non-metal doping, which can reduce the bandgap and extend the light-harvesting range. As for defect engineering, it can modify the electronic structures and improve the molecules to be adsorbed on the surface of pristine g-C3N4. The crystallinity optimization strategy can increase the crystallinity of pristine g-C3N4 and decrease its band gap. In addition, it increases the conductivity to improve the photogenerated charge carrier separation efficiency. When it comes to the heterojunction construction, it mainly promotes charge carrier separation efficiency through efficient charge transfer across the interface between the two semiconductors. Up to now, great progress has been made to enhance the photocatalytic NOx removal performance of g-C3N4. However, from our perspective, there are still some significant challenges for us to solve.
First of all, although a variety of modification strategies were developed to enhance the photocatalytic NOx removal performance of g-C3N4, many of them are carried out under harsh conditions even involving toxic reagents. For example, dangerous hydrofluoric acid is used to remove the silica-based hard templates for preparing different nanostructured g-C3N4. Therefore, in the near future, the environmentally friendly and low-cost synthesis process which is suitable for large-scale production needs to be exploited for fabricating hollow or porous nanostructured g-C3N4.
Secondly, collaborative strategies should be adopted to modify pristine g-C3N4 for improving the photocatalytic NOx removal performance. The majority of research works involve the single modification strategy, i.e., the element doping strategy, morphology control strategy, crystallinity optimization, defect engineering and heterojunction construction. The single modification strategy can enhance the photocatalytic activity of pristine g-C3N4 remarkably, but there is room for improvement. We can employ collaborative strategies to improve the photocatalytic NOx removal performance of pristine g-C3N4. For instance, we can optimize the crystallinity and introduce nitrogen simultaneously to enhance the photocatalytic activity of pristine g-C3N4. Collaborative strategies extend the visible light response range, promote charge carrier separation efficiency and activate the reactants molecules, resulting in excellent photocatalytic performance.
Thirdly, g-C3N4-based photocatalysts with full-spectrum-activated photocatalytic activities from UV to near-infrared still remains a great challenge. It is well-understood that pristine g-C3N4 cannot harvest near-infrared (NIR) light, which accounts for 50% of the full solar spectrum. In order to make full use of solar energy, it is urgent to develop full-spectrum responsive g-C3N4-based photocatalysts for photocatalytic NOx removal. Heterojunction construction with narrow optical materials, the combination of up-conversion materials and plasmonic materials and element doping collaboration with crystallinity optimization can be used to extend the light absorption of g-C3N4 to NIR light region.
Finally, the photocatalytic NOx removal process is very complicated, and the related catalytic mechanism is not thoroughly studied or understood. Therefore, advanced characterization techniques and theoretical simulations are recommended for revealing the photocatalytic NOx removal mechanism. Steady-state photoluminescence spectra and time-resolved photoluminescence spectra show the photogenerated charge carrier separation efficiency. The in situ infrared absorption spectroscopic analysis helps us to understand the mechanism of ROS generation and conversion. At the same time, the theoretical calculation should be widely used to analyze the effects of metal or non-metal doping on the band structure and the effects of carbon or nitrogen vacancies on the reactant molecule activation. Only when we clarify the photocatalytic NOx removal mechanism comprehensively can we synthesize highly efficient g-C3N4-based photocatalysts for the photocatalytic NOx removal process.

Author Contributions

Conceptualization, S.Y., E.Z. and Z.G.; Review and funding acquisition, Z.G. and X.W. (Xionglei Wang); Writing original draft preparation, Z.G., M.J., X.W. (Xin Wang), R.Z., Z.H., J.Y. and H.H.; writing review and editing, Z.G., S.Z. and X.W. (Xionglei Wang); supervision, S.Y., E.Z. and Z.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Hebei Natural Science Foundation (E2022106008 and B2022402024) and Shijiazhuang University (22BS009). This work was also partly supported by the JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Mixed anion” (No. 16H06439) Grant-in-Aid for Scientific Research (20H00297).

Acknowledgments

The authors acknowledge Ruotong Yang for her kind help.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The environmental issues caused by NOx pollutants.
Figure 1. The environmental issues caused by NOx pollutants.
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Scheme 1. The basic principle of photocatalysis over semiconductor photocatalyst.
Scheme 1. The basic principle of photocatalysis over semiconductor photocatalyst.
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Figure 2. (a) Synthesis of g−C3N4 using thermal polymerization with different precursors [32]. Copyright 2016, American Chemical Society. (b) reaction pathway of g−C3N4 using cyanamide as a precursor [38]. Copyright 2017, Elsevier.
Figure 2. (a) Synthesis of g−C3N4 using thermal polymerization with different precursors [32]. Copyright 2016, American Chemical Society. (b) reaction pathway of g−C3N4 using cyanamide as a precursor [38]. Copyright 2017, Elsevier.
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Figure 3. (a)Triazine and (b) tris−s−triazine (heptazine) structures of g−C3N4.
Figure 3. (a)Triazine and (b) tris−s−triazine (heptazine) structures of g−C3N4.
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Figure 4. (a–c) Schematic illustration for the liquid-exfoliation process of bulk g−C3N4 [84]. Copyright 2012, American Chemical Society.
Figure 4. (a–c) Schematic illustration for the liquid-exfoliation process of bulk g−C3N4 [84]. Copyright 2012, American Chemical Society.
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Figure 5. Schematic illustration for the self−assembled method to prepare few-layer porous structures of g−C3N4 [75]. Copyright 2019, American Chemical Society.
Figure 5. Schematic illustration for the self−assembled method to prepare few-layer porous structures of g−C3N4 [75]. Copyright 2019, American Chemical Society.
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Figure 6. (a) Schematic illustration for preparation of porous g−C3N4 with the addition of water and ethanol. Photocatalytic performance (b) and Arrhenius rate constants (c) of g−C3N4, g−C3N4−W and g−C3N4−E for the removal of NO irradiated under visible light [96]. Copyright 2017, Elsevier.
Figure 6. (a) Schematic illustration for preparation of porous g−C3N4 with the addition of water and ethanol. Photocatalytic performance (b) and Arrhenius rate constants (c) of g−C3N4, g−C3N4−W and g−C3N4−E for the removal of NO irradiated under visible light [96]. Copyright 2017, Elsevier.
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Figure 7. The proposed mechanism of K intercalation to improve photocatalytic performance of g−C3N4 [100]. Copyright 2016, American Chemical Society.
Figure 7. The proposed mechanism of K intercalation to improve photocatalytic performance of g−C3N4 [100]. Copyright 2016, American Chemical Society.
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Figure 8. Schematic illustration of the non−metal doping of the g−C3N4 framework [38]. Copyright 2017, Elsevier.
Figure 8. Schematic illustration of the non−metal doping of the g−C3N4 framework [38]. Copyright 2017, Elsevier.
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Figure 9. (a) Schematic illustration for the preparation of B−doped g−C3N4 hollow tubes. (b) Photocatalytic performance of bulk g−C3N4, g−C3N4 tubes and B−doped g−C3N4 tubes for the removal of NO irradiated under visible light. (c) Stability test of B−doped g−C3N4 tubes. (d) The proposed mechanism of B−doped g−C3N4 tubes to improve the photocatalytic performance of g−C3N4 [102]. Copyright 2018, Elsevier.
Figure 9. (a) Schematic illustration for the preparation of B−doped g−C3N4 hollow tubes. (b) Photocatalytic performance of bulk g−C3N4, g−C3N4 tubes and B−doped g−C3N4 tubes for the removal of NO irradiated under visible light. (c) Stability test of B−doped g−C3N4 tubes. (d) The proposed mechanism of B−doped g−C3N4 tubes to improve the photocatalytic performance of g−C3N4 [102]. Copyright 2018, Elsevier.
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Figure 10. (a) Schematic illustration for the preparation of nitrogen vacancies−mediated g−C3N4 microtubes. (b–g) Schematic illustration for the nitrogen vacancies in g−C3N4 photocatalysts for the enhanced adsorption behavior [109]. Copyright 2019. American Chemical Society.
Figure 10. (a) Schematic illustration for the preparation of nitrogen vacancies−mediated g−C3N4 microtubes. (b–g) Schematic illustration for the nitrogen vacancies in g−C3N4 photocatalysts for the enhanced adsorption behavior [109]. Copyright 2019. American Chemical Society.
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Figure 11. (a) Schematic illustration for the preparation of carbon vacancies and hydroxyls co−modified g−C3N4. (b) Schematic illustration for the g−C3N4 and carbon vacancies and hydroxyls co−modified g−C3N4. (c) The proposed mechanism of carbon vacancies and hydroxyls co−modified g−C3N4 for improved photocatalytic performance of g−C3N4 [111]. Copyright 2020, Elsevier.
Figure 11. (a) Schematic illustration for the preparation of carbon vacancies and hydroxyls co−modified g−C3N4. (b) Schematic illustration for the g−C3N4 and carbon vacancies and hydroxyls co−modified g−C3N4. (c) The proposed mechanism of carbon vacancies and hydroxyls co−modified g−C3N4 for improved photocatalytic performance of g−C3N4 [111]. Copyright 2020, Elsevier.
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Figure 12. (a) Schematic illustration for the crystallization process of g−C3N4 [114]. Copyright 2020, Elsevier. (b) Schematic illustration for improving the crystallinity of g−C3N4 using hydrochloric acid treatment [115]. Copyright 2020, Elsevier.
Figure 12. (a) Schematic illustration for the crystallization process of g−C3N4 [114]. Copyright 2020, Elsevier. (b) Schematic illustration for improving the crystallinity of g−C3N4 using hydrochloric acid treatment [115]. Copyright 2020, Elsevier.
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Figure 13. (a) Schematic illustration for the preparation of crystalline g−C3N4 using a modified molten salt method under ambient pressure. (b) Photocatalytic performance of pristine g−C3N4, crystalline g−C3N4 and P25 for the removal of NO irradiated under visible light. (c) Stability test of crystalline g−C3N4. (d–g) Schematic illustration for crystallinity in g−C3N4 photocatalysts for the enhanced light-harvesting properties. (h–k) Schematic illustration for crystallinity in g−C3N4 photocatalysts for the enhanced adsorption behavior [116]. Copyright 2021, Elsevier.
Figure 13. (a) Schematic illustration for the preparation of crystalline g−C3N4 using a modified molten salt method under ambient pressure. (b) Photocatalytic performance of pristine g−C3N4, crystalline g−C3N4 and P25 for the removal of NO irradiated under visible light. (c) Stability test of crystalline g−C3N4. (d–g) Schematic illustration for crystallinity in g−C3N4 photocatalysts for the enhanced light-harvesting properties. (h–k) Schematic illustration for crystallinity in g−C3N4 photocatalysts for the enhanced adsorption behavior [116]. Copyright 2021, Elsevier.
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Figure 14. (a) Schematic illustration for the preparation of α−Fe2O3/g−C3N4 direct Z−scheme heterojunction prepared using an impregnation−hydrothermal method. (b) The proposed mechanism of direct Z−scheme heterojunction to improve the photocatalytic performance of α−Fe2O3/g−C3N4 [52]. Copyright 2021, Elsevier.
Figure 14. (a) Schematic illustration for the preparation of α−Fe2O3/g−C3N4 direct Z−scheme heterojunction prepared using an impregnation−hydrothermal method. (b) The proposed mechanism of direct Z−scheme heterojunction to improve the photocatalytic performance of α−Fe2O3/g−C3N4 [52]. Copyright 2021, Elsevier.
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Figure 15. (a) Schematic illustration for the preparation of g−C3N4/SnO2 S−scheme heterojunction using a hydrothermal and annealing strategy. (b) The proposed mechanism of S−scheme to improve the photocatalytic performance of g−C3N4/SnO2 [127]. Copyright 2021, Elsevier.
Figure 15. (a) Schematic illustration for the preparation of g−C3N4/SnO2 S−scheme heterojunction using a hydrothermal and annealing strategy. (b) The proposed mechanism of S−scheme to improve the photocatalytic performance of g−C3N4/SnO2 [127]. Copyright 2021, Elsevier.
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Figure 16. (a) Schematic illustration for the preparation of g−C3N4/rGO nanocomposites using an alkali−assisted hydrothermal method. (b) Photocatalytic performance of pristine g−C3N4 and g−C3N4 treated using an alkali-assisted process and g−C3N4/rGO for the removal of NO irradiated under visible light. (c) Stability test of g−C3N4/rGO [122]. Copyright 2020, Elsevier.
Figure 16. (a) Schematic illustration for the preparation of g−C3N4/rGO nanocomposites using an alkali−assisted hydrothermal method. (b) Photocatalytic performance of pristine g−C3N4 and g−C3N4 treated using an alkali-assisted process and g−C3N4/rGO for the removal of NO irradiated under visible light. (c) Stability test of g−C3N4/rGO [122]. Copyright 2020, Elsevier.
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Figure 17. The different modification strategies to improve the photocatalytic NOx removal performance of g−C3N4.
Figure 17. The different modification strategies to improve the photocatalytic NOx removal performance of g−C3N4.
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Gu, Z.; Jin, M.; Wang, X.; Zhi, R.; Hou, Z.; Yang, J.; Hao, H.; Zhang, S.; Wang, X.; Zhou, E.; et al. Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal. Catalysts 2023, 13, 192. https://doi.org/10.3390/catal13010192

AMA Style

Gu Z, Jin M, Wang X, Zhi R, Hou Z, Yang J, Hao H, Zhang S, Wang X, Zhou E, et al. Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal. Catalysts. 2023; 13(1):192. https://doi.org/10.3390/catal13010192

Chicago/Turabian Style

Gu, Zhanyong, Mengdie Jin, Xin Wang, Ruotong Zhi, Zhenghao Hou, Jing Yang, Hongfang Hao, Shaoyan Zhang, Xionglei Wang, Erpeng Zhou, and et al. 2023. "Recent Advances in g-C3N4-Based Photocatalysts for NOx Removal" Catalysts 13, no. 1: 192. https://doi.org/10.3390/catal13010192

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