Emerging heterostructured C₃N₄ photocatalysts for photocatalytic environmental pollutant elimination and sterilization

Abstract

Photocatalysis is deemed a highly prominent technology to solve environmental problems such as pollution, CO₂ emission and bacterial contamination. As an important photocatalyst, g-C3N4 has attracted a great amount of attention in environmental remediation owing to its good stability, excellent light response, low cost and environmentally friendly properties. However, the pristine g-C3N4 photocatalyst generally suffers from serious photoinduced charge carrier recombination, poor surface active sites, and insufficient visible light harvesting, thereby leading to unsatisfactory photocatalytic performance. Heterostructured C₃N₄ photocatalysts have recently become a research focus in environmental fields thanks to their fast photoexcited electron–hole pair dissociation, broadened visible light response range, and sufficient photoredox capability. Herein, we critically review the up-to-date developments of heterostructured C₃N₄ photocatalysts in organic pollutant elimination, heavy metal ion reduction, CO₂ conversion and bacterial inactivation. Meanwhile, the strategies for constructing efficient C₃N₄ based heterostructures with enhanced environmental photocatalytic capability are thoroughly described, which should help readers to quickly acquire in-depth knowledge and to inspire new concepts in heterostructure engineering. Finally, the challenges and opportunities in fabricating heterostructured C₃N₄ photocatalysts for large-scale and commercial applications are discussed to give a clear study direction in this field.

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1. Introduction

Since the pioneering work regarding solar-light-driven water decomposition was reported in 1972,¹ the transformation of solar energy into chemical energy has attracted tremendous attention in environmental remediation and sustainable energy evolution.^2–6 Furthermore, the photocatalysis method has been considered as a feasible means for harvesting light energy in chemical bonds through different photoexcited reactions, like organic pollutant oxidation, CO₂ reduction, H₂O splitting, H₂O₂ preparation and valued organics synthesis.^7–15 Over recent decades, a considerable number of semiconductors such as metal oxides, metal sulfides, and nitride materials have been exploited as photocatalysts for various photocatalytic reactions.^16–20 Among them, graphitic carbon nitride (g-C₃N₄) possessing outstanding light-response ability has shown great promise because of its facile synthesis, tunable band gap structure, good chemical stability, and environmentally friendly properties.^21–25 Since the pioneering research regarding photoinduced water splitting over g-C₃N₄ was reported in 2009,²⁶ lots of studies have been conducted to broaden the application of C₃N₄ based photocatalysts in different photocatalytic reactions such as toxic organic pollutant elimination, heavy metal ion reduction, CO₂ conversion and bacterial inactivation.^27–30 However, pristine g-C₃N₄ generally suffers from limited visible light harvesting, sluggish photoexcited electron–hole pair separation and insufficient photoredox potentials, thereby leading to poor photocatalytic performance. Up to now, a variety of modification methods such as ion doping, defect introduction, heterostructure construction, surface modification, and noble metal nanoparticle decoration have been developed for improving photocatalytic reaction rates.^31–35 In particular, the combination of g-C₃N₄ with other semiconductors to obtain heterostructures has proven the most effective. Usually, engineering heterojunctions can effectively increase the visible light response range and promote photoinduced charge dissociation of photocatalysts.^36–38 In addition, the oxidation and reduction potentials of the heterostructured photocatalysts with Z-scheme configurations are strengthened, thus improving the photoredox ability and broadening the photocatalytic applications in environmental remediation fields.^39–43 More significantly, in comparison to other common photocatalysts like TiO₂ and ZnO, bulk C₃N₄ is much easier to exfoliate into two-dimensional (2D) nanoplates or nanosheets via facilely breaking the van der Waals forces and hydrogen bonds between the layered C₃N₄ units, which are flexible and able to be coupled with other semiconductor photocatalysts. These features make C₃N₄ more favorable to be used in establishing heterojunction photocatalysts compared with other semiconductor substrates.^44,45

As is well-known, environmental issues such as water pollution, air deterioration and microbial contamination have posed great threats to human beings. Although a variety of methods including adsorption, chlorination, and ultraviolet (UV) irradiation have been adopted for environmental remediation, there are still some drawbacks such as complicated operation and huge energy consumption. Therefore, it is urgent and meaningful to explore a facile, inexpensive, and environmentally friendly strategy to resolve environmental problems. Photocatalytic techniques have been verified to possess significant capability to eliminate organic/inorganic pollutants, purify air and kill bacteria in water via a green and sustainable process.⁴⁶ In the visible light driven organic pollutant degradation reactions, the photoexcited electrons on the conduction band (CB) of the semiconductor can reduce O₂ to form oxidative ˙O₂⁻ species, while the photoinduced holes on the valence band (VB) of the semiconductor can combine with OH⁻ to produce oxidative ˙OH radicals for further organic substance decomposition or to directly oxidize organic molecules. As for heavy metal ion pollutants like Cr⁶⁺, converting Cr⁶⁺ into the less toxic Cr³⁺ can be realized via a reduction process by the photoexcited electrons on the CB of the semiconductors. In addition to pollutant elimination, the reactive radicals like holes, ˙O₂⁻, and ˙OH can also exert important functions in microbial inactivation. Therefore, designing semiconductor photocatalysts with fast photoinduced charge separation, favorable generation of reactive species and sufficient photoredox potentials is highly anticipated for efficient photocatalytic environmental applications.^47–52

As important visible light response photocatalysts, heterostructured C₃N₄ photocatalysts with reliable activity in environmental remediation and protection including organic pollutant degradation, heavy metal ion reduction, CO₂ conversion, sewage purification and microbial inactivation have been attracting more and more attention.^53–56 Some reviews regarding the applications of heterostructured C₃N₄ photocatalysts in the photocatalytic field have been published.^53,56 Nonetheless, a critical review that thoroughly discloses the relationship between the construction of heterostructured C₃N₄ materials and improved performance in environmental applications is still imperatively required, and fundamentally crucial for further exploration of advanced heterostructured photocatalysts and expanding their application. In this review, the categories and photocatalytic environmental applications, including organic pollutant degradation, heavy metal ion reduction, CO₂ conversion, and microbial inactivation, of heterostructured C₃N₄ photocatalysts are thoroughly outlined and discussed (Fig. 1). Meanwhile, the relationships between the fabrication of efficient C₃N₄ based heterostructures and improved performance in photocatalytic environmental applications are correlated in depth. Finally, the challenges and perspectives of heterostructured C₃N₄ photocatalysts in real applications are discussed to offer a clear study direction in heterostructured photocatalyst design and application.

Fig. 1 Applications of heterostructured C₃N₄ photocatalysts.

2. Categories of heterostructured C₃N₄ photocatalysts

Until now, various semiconductors including metal oxides, metal chalcogenides and Bi based materials have been coupled with g-C₃N₄ to construct heterojunctions with efficient photocatalytic performance for different applications. According to the mechanism of photoinduced carrier separation and transfer, four main kinds of g-C₃N₄ based heterostructured photocatalysts in the environmental photocatalysis field are outlined and discussed in this review as follows: type II heterojunction, Z-scheme heterojunction, p–n heterojunction, and cascade electronic band heterojunction (Fig. 2).^57–62

Fig. 2 Common types of g-C₃N₄ based heterojunction for photocatalytic environmental applications: (a) type II heterojunction, (b) Z-scheme heterojunction, (c) p–n heterojunction and (d) cascade electronic band heterojunction.

2.1. g-C₃N₄-based conventional type II heterojunction systems

With regard to the pure C₃N₄ photocatalyst, the photoinduced electrons on the conduction band (CB) are inclined to come back to the valence band (VB), and the unfavorable recombination of photoexcited electrons and holes occurs, thus leading to the unsatisfactory photocatalytic performance. When a semiconductor with an appropriate electronic bandgap is decorated on the C₃N₄ photocatalyst, the facile separation of photoexcited charge pairs can be achieved via the interface between the two semiconductors for enhanced photocatalytic activity. As presented in Fig. 2a, the type II g-C₃N₄ based heterojunction photocatalyst gives rise to convenient photo-generated charge dissociation owing to the staggered bandgap configuration. Fig. 2a (left) depicts a heterojunction that consists of C₃N₄ and a semiconductor possessing a more positive VB level. Under light irradiation, the photoelectrons can be shifted from the CB of g-C₃N₄ to the CB of another semiconductor whereas the photoinduced holes are moved from the VB of another semiconductor to the VB of g-C₃N₄. The above charge transfer mechanism is frequently observed in lots of type II g-C₃N₄ based heterojunction photocatalysts such as TiO₂/g-C₃N₄, WO₃/g-C₃N₄, and ZnO/g-C₃N₄.^57–59 In addition to the aforementioned staggered bandgap structure, another interlaced bandgap configuration was also revealed in type II g-C₃N₄ based heterojunction photocatalysts (Fig. 2a, right). In this case, a semiconductor has a more negative CB position compared with that of g-C₃N₄, and the photoexcited electrons remain on the CB of g-C₃N₄ while the photoinduced holes remain on the VB of the other semiconductor after photogenerated charge dissociation and migration. In comparison with the previous type II heterojunction (Fig. 2a, left), the latter type II heterojunction is less reported because of the sufficiently positive VB level of g-C₃N₄ (about +1.9 eV vs. normal hydrogen electrode (NHE)). Although sufficient carrier separation can be reached in type II heterojunction systems, electronic repulsion phenomena may occur and thus limit the photocatalytic performance. Meanwhile, the energy utilization efficiency needs to be further improved. These shortcomings should be addressed by scientists in future research.

2.2. g-C₃N₄-based Z-scheme heterojunction systems

Although fast photocharge separation and transfer can be achieved in type II g-C₃N₄ based heterojunction photocatalysts, the insufficient photo-reduction and photo-oxidation energy levels after the two semiconductors couple limit the applications of g-C₃N₄ based heterojunction photocatalysts. Achieving efficient photoinduced carrier separation while simultaneously retaining sufficient photoredox performance is therefore significantly meaningful for fabricating g-C₃N₄ based heterojunction photocatalysts. In this context, the Z-scheme g-C₃N₄ heterojunction systems have been revealed by scientists. Actually, the most optimal means for charge diffusion and migration was found in the photosynthesis procedure of green plants.⁶⁰ Under sunlight illumination, the charges shift in the plant rootstock via a different route from the type II heterojunction with the assistance of electron mediators. This category of charge migration is the Z-scheme, which offers fast photoexcited hole–electron pair dissociation and simultaneous competent photoredox abilities. Inspired by the natural photosynthesis process, engineering Z-scheme g-C₃N₄ based heterojunctions has been considered a highly feasible strategy for achieving efficient photocatalytic activity. Fig. 2c (left) displays the typical Z-scheme charge migration route between two semiconductors. It is clear that the photoelectrons from the CB of semiconductor I are shifted to the VB of g-C₃N₄. Consequently, the photoexcited hole–electron pairs from semiconductor I and g-C₃N₄ can be effectively dissociated. More importantly, the electrons on the CB of g-C₃N₄ and holes on the VB of semiconductor I can endow highly sufficient reduction and oxidation abilities, respectively. In order to further improve the photocatalytic ability, a Z-scheme heterostructure with an electron mediator between the two semiconductors has been exploited. As presented in Fig. 2b (right), an electron mediator located at the interface between semiconductor I and g-C₃N₄ can promote charge migration for further improved photocatalytic performance. In general, noble nanoparticles with good electron conductivity such as Pd and Pt are frequently adopted as electron mediators. In addition to the fantastic Z-scheme charge migration route, the formation of an internal built-in electric field between the two semiconductors is another advantage for promoting carrier dissociation. In some cases, the Z-scheme heterojunction is also denoted as an S-scheme heterojunction.^40,41 Despite the different terms, the charge transfer and separation mechanisms for the Z-scheme and S-scheme heterojunctions are almost the same. Recently, g-C₃N₄ based Z-scheme heterostructures have been classified into three kinds by researchers, the redox mediator solution-phase Z-scheme, solid-state Z-scheme, and direct Z-scheme, which can certainly offer important knowledge for designing heterostructures.^44,45

2.3. g-C₃N₄-based p–n heterojunctions

Apart from the above-mentioned type II and Z-scheme heterostructure photocatalysts, building an inner electric field via engineering a p–n heterojunction between two semiconductors has also been investigated as a feasible means for efficient hole–electron pair separation.⁶¹ Commonly, g-C₃N₄ displays an n-type semiconductor character because of the existence of the electron donors of –NH and NH₂ on its surface. As shown in Fig. 2c (left), the Fermi level (E_F,_n) of n-type g-C₃N₄ is situated near its CB, whereas the Fermi level (E_F,p) of a p-type semiconductor is close to its VB. When g-C₃N₄ and a p-type semiconductor are combined together, the electrons from g-C₃N₄ can move to the p-type semiconductor via the interface because of the offset of Fermi levels. As a result, the interface with g-C₃N₄ becomes positive while the edge of the p-type semiconductor becomes negative. Therefore, an inner electric field is formed between g-C₃N₄ and the p-type semiconductor. Under light illumination, the photoelectrons transfer from the CB of the p-type semiconductor to the CB of g-C₃N₄ whereas the photoinduced holes migrate from the VB of g-C₃N₄ to the VB of the p-type semiconductor due to the driving force from the formed inner electric field, thus achieving fast photogenerated carrier separation and shift, and favorable for increased photocatalytic activity (Fig. 2c, right). Just like the g-C₃N₄ based p–n heterojunction, g-C₃N₄ can also form an n–n type heterojunction such as N,S-TiO₂/g-C₃N₄²⁴ with increased photocatalytic performance, which can be produced via combining two kinds of p-type semiconductors. After the combination, the n–n type heterojunction presents an inner electric field similar to that of the g-C₃N₄ based p–n heterojunction for promoted carrier separation. Furthermore, with p–n/n–n heterojunctions, the loading of a co-catalyst has been verified as an efficient way to improve photocatalytic ability.^23,24 Ti₃C₂ serving as co-catalyst combined with different photocatalysts has been reported. Specifically, the low Fermi level of Ti₃C₂ can promote the generation of a Schottky junction with the semiconductors to function as an electron trapper and facilitate charge dissociation. For example, a Ti₃C₂/N,S-doped TiO₂/g-C₃N₄ composite was prepared with Ti₃C₂ acting as co-catalyst to improve charge separation because of the generation of an effective charge transfer pathway involving an n–n heterojunction and Schottky junction.²⁴

2.4. g-C₃N₄-based cascade electronic band heterojunctions

By coupling a semiconductor with g-C₃N₄, various heterostructures such as type II and Z-scheme types have been obtained with improved photocatalytic ability. Nevertheless, the shortcoming that the photoinduced charges could still undergo recombination during the migration process to the surface active sites for photoredox reactions is rarely addressed. The photocatalytic ability of heterostructured photocatalysts has still not been fully explored.⁶² On account of this phenomenon, further combining a proper photocatalyst (semiconductor 2) with rich surface active sites with the heterostructured photocatalyst g-C₃N₄/semiconductor 1 to fabricate a ternary g-C₃N₄/semiconductor 1/semiconductor 2 system has been recently developed. As shown in Fig. 2d, when the typical ternary g-C₃N₄ based heterostructure photocatalyst is excited under light, all three components can generate holes and electrons, and the photoelectrons can shift from g-C₃N₄ to semiconductor 1 and semiconductor 2 via a cascade transport route whereas the holes move along an opposite direction. This g-C₃N₄ based cascade electronic band heterojunction gives rise to the quick dissociation of photoinduced carriers and therefore improves the photocatalytic performance.

Overall, a variety of heterojunctions have been designed for enhanced photocatalytic performance, which can clearly suggest directions for design principles of heterostructured C₃N₄ photocatalysts for efficient environmental applications. Firstly, a heterostructure with facilitated carrier dissociation ability should be the prerequisite. Secondly, for environmental pollutant elimination applications such as organics decomposition and Cr(VI) reduction, sufficient photoredox potentials, which can provide enough driving force to promote pollutant molecule elimination, are of significant importance and should be paid more attention. Lastly, for the preparation strategy of heterostructured C₃N₄ photocatalysts, the wet-chemical route may lead to a uniform morphology and avoid the undesirable agglomeration, thus ensuring more rich surface reaction sites.

3. Photocatalytic pollutant removal by g-C₃N₄ based heterostructures

So far, a considerable number of pollutants including dyes, pesticides, heavy metal ions, volatile organic compounds (VOCs), and bacteria have been emitted into air, water, and soil, leading to huge threats to human beings and the environment.^63–65 In order to eliminate these hazardous substances, a variety of techniques and devices, such as adsorption, filtration, UV light irradiation and biological decomposition, have been developed and desirable achievements have been realized.⁶⁵ Nevertheless, these techniques still have some intrinsic shortcomings including being time-consuming and involving complicated operation, the formation of toxic byproducts, and huge energy waste. From a long-term perspective with the strategic goal of carbon peaking and carbon neutralization, exploring a facile, green, sustainable and energy-saving strategy for removing environmental pollutants is becoming a mainstream focus of scientific research. Accordingly, the environmental photocatalysis technique has been attracting much attention owing to its obvious merits in comparison to the traditional techniques^66–70 (Fig. 3). In this section, various environmental photocatalysis applications of g-C₃N₄ based heterostructures including dye decomposition, VOC elimination, heavy metal ion reduction, and bacterial inactivation are thoroughly outlined and discussed. Meanwhile, the related mechanisms for enhanced environmental photocatalysis are elaborated.

Fig. 3 Schematic mechanism of environmental photocatalysis technique for pollutant treatment.

3.1. Dye pollutant degradation

Among the various photocatalytic pollutant removal methods, photocatalytic dye (e.g., Rhodamine B (RhB), methylene blue (MB), and methyl orange (MO)) degradation is an effective technique and has been inspiring a great number of studies.^71,72 In general, dye pollutant degradation over g-C₃N₄ based heterostructure photocatalysts comprises the following four major steps to totally mineralize organic molecules. Firstly, organic molecules transfer from the pollutant solution to the surface of the photocatalyst. Secondly, electron–hole pairs can be formed upon sunlight irradiation. Thirdly, photo-oxidation decomposition of organic molecules occurs on the surface of the photocatalyst with the participation of photoinduced oxidative species such as holes, hydroxyl radicals and super oxygen. Finally, the decomposition products like CO₂ and H₂O are released to the pollutant solution.^73–75 Taking advantage of their excellent optoelectronic characteristics and optimized energy level configuration, facile separation and shift of charges, enhanced surface photooxidation rates, and favorable generation of reactive species are achieved on g-C₃N₄ based heterostructure photocatalysts for organic pollutant degradation and mineralization.^76–82

Typically, Li et al. prepared MoS₂ nanosheets attached to g-C₃N₄ (MoS₂/C₃N₄) as a heterostructure photocatalyst through a simple ultrasonic chemical technique for efficient photocatalytic dye decomposition.⁷⁶ After MoS₂ loading, rapid dissociation of photoinduced carriers, broadened visible light absorption and increased photocurrent density were achieved on the MoS₂/C₃N₄ heterostructure. On account of the above merits, the optimal MoS₂/C₃N₄ heterostructure with 0.05 wt% MoS₂ loading exhibits the largest photocatalytic RhB degradation rate constant of 0.301 min⁻¹, 3.6 times that of pure C₃N₄ (Fig. 4a and b). The type II charge transfer pathway in MoS₂/C₃N₄ for photocatalytic RhB degradation was proposed. Specifically, MoS₂ acting as an electron trapper can prolong the lifetime of electron charges, and the accumulated holes on the VB of C₃N₄ directly oxidize RhB molecules (Fig. 4c). Similarly, a type II 3D g-C₃N₄/TiO₂ heterojunction photocatalyst was synthesized for efficient MB degradation.⁷⁷ The MB degradation rate over 3D g-C₃N₄/TiO₂ is about 4 times that of bulk g-C₃N₄. In addition to the benefit of the heterojunction in promoting electron–hole pair separation (Fig. 4d), the 3D structure can also elevate the adsorption enrichment ability and endow convenient mass and charge migration channels (Fig. 4e), therefore improving the MB degradation capability. Hao et al. synthesized a macro/mesoporous g-C₃N₄/TiO₂ heterojunction photocatalyst (Fig. 4f and g) via a template free route.⁷⁸ Owing to the advantages of morphology, porous structure and type II heterostructure, the macro/mesoporous g-C₃N₄/TiO₂ photocatalyst achieved an RhB degradation rate as high as 0.0478 min⁻¹, which is about 7.2 and 3.1 fold that of the pure TiO₂ and g-C₃N₄ photocatalysts, respectively (Fig. 4h).

Fig. 4 (a) Time dependent photocatalytic degradation of RhB over the as-obtained photocatalysts and (b) the corresponding degradation rates. (c) Schematic illustration of photoinduced charge transfer over the MoS₂/C₃N₄ heterostructure. Reproduced with permission.⁷⁶ Copyright 2014, American Chemical Society. (d) Photocatalytic mechanism for organic pollutant elimination over 3D g-C₃N₄/TiO₂. (e) SEM image of 3D g-C₃N₄/TiO₂. Reproduced with permission.⁷⁷ Copyright 2019, Elsevier. (f and g) SEM pictures of the macro/mesoporous g-C₃N₄/TiO₂. (h) Photocatalytic RhB degradation rates of the as-obtained samples. Reproduced with permission.⁷⁸ Copyright 2016, Elsevier.

Compared with the type II heterostructure, the Z-scheme heterostructure possesses higher capability for dye removal. In addition to facilitating photoinduced charge separation, the Z-scheme configuration enables sufficient photoredox ability, enabling dye oxidation and mineralization. For example, a Z-scheme 2D/2D g-C₃N₄/MnO₂ heterostructured photocatalyst was prepared through in situ loading MnO₂ nanosheets onto g-C₃N₄ nanosheets via a wet chemical strategy (Fig. 5a).⁷⁹ As expected, the g-C₃N₄/MnO₂ heterostructured photocatalyst displayed increased photocatalytic RhB degradation ability compared with the pure g-C₃N₄ or MnO₂ sample. The charge transfer mechanism for photocatalytic RhB degradation is depicted in Fig. 5b, wherein the photoexcited electrons on the CB of MnO₂ are coupled with the holes on the VB of g-C₃N₄, giving rise to efficient charge carrier dissociation and visible light harvesting. Moreover, more negative CB and positive VB potentials are obtained after the Z-scheme charge separation and migration, thus delivering enough photoredox ability. Specifically, the holes on the VB of MnO₂ can oxidize H₂O to form oxidative ˙OH radicals and the photoinduced electrons on the CB of g-C₃N₄ produce ˙O₂⁻ radicals. Both the reactive radicals are responsible for RhB molecule decomposition.

Fig. 5 (a) Schematic process for the preparation of a 2D/2D g-C₃N₄/MnO₂ heterostructure and (b) photoexcited charge transfer pathway over the g-C₃N₄/MnO₂ heterostructure. Reproduced with permission.⁷⁹ Copyright 2018, American Chemical Society. (c) Schematic illustration of the synthesis of a ternary g-C₃N₄/Al₂O₃/ZnO photocatalyst. EPR characterization of (d) DMPO-˙O₂⁻ and (e) DMPO-˙OH adducts over the as-synthesized photocatalysts. (f) Schematic cascade electron transfer route for MO degradation. Reproduced with permission.⁸¹ Copyright 2017, Elsevier.

In addition to binary C₃N₄ based composites with type II or Z-scheme heterostructures, the ternary C₃N₄ based heterostructure with a cascade electron energy structure has also aroused great interest owing to its unique and rapid electron shift capability. For instance, a ternary H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ composite was prepared by a facile impregnation and ion exchange strategy for enhanced photodecomposition of methyl orange (MO).⁸⁰ Firstly, a binary H₂SrTa₂O₇/g-C₃N₄ hybrid with an optimal g-C₃N₄ loading mass ratio of 60 wt% was synthesized. Then, Ag₃PO₄ is decorated on H₂SrTa₂O₇/g-C₃N₄ to create the ternary H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ photocatalyst with further improved photoactivity. An increased interface interaction between H₂SrTa₂O₇, g-C₃N₄ and Ag₃PO₄ is realized, which is favorable for separation of photoexcited charge carriers. Accordingly, the ternary H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ composite exhibits about 45, 44 and 38 fold higher rates of photocatalytic MO removal than those of the H₂SrTa₂O₇, g-C₃N₄ and H₂SrTa₂O₇/g-C₃N₄ photocatalysts, respectively. Electron spin resonance measurements indicated that photoinduced holes and superoxide radicals exert a significant function in decomposing MO molecules. Moreover, a cascade electron transfer route is followed on the ternary photocatalyst. Specifically, under light irradiation, the photoinduced electrons migrate from g-C₃N₄ to H₂SrTa₂O₇ and Ag₃PO₄, and combine with O₂ to generate oxidative superoxide radicals for phenol decomposition while the photogenerated holes on the VB of g-C₃N₄ directly take part in phenol degradation. This stepwise migration leads to the facile separation of photogenerated carriers and therefore elevates the photocatalytic activity.

Similarly, a ternary g-C₃N₄/Al₂O₃/ZnO photocatalyst was obtained through a hydrothermal and calcination route (Fig. 5c).⁸¹ The amorphous Al₂O₃ component with a disorganized atomic arrangement and defects can achieve a more favorable electron acceptance ability from g-C₃N₄ than that of other crystalline materials. EPR characterization disclosed that the ternary g-C₃N₄/Al₂O₃/ZnO photocatalyst exhibits greater ability in producing reactive oxygen species such as superoxide radicals and hydroxyl radicals than that of the binary g-C₃N₄/ZnO and g-C₃N₄/Al₂O₃ samples (Fig. 5d and e), which is favorable for photo-oxidation of dye molecules. Under visible light irradiation, the ternary g-C₃N₄/Al₂O₃/ZnO photocatalyst shows about a 1.7 times higher rate for methyl orange degradation than that of pure g-C₃N₄ due to the prominent cascade electron transfer route. As shown in Fig. 5f, the photoexcited electrons on the CB of g-C₃N₄ move to the CB of amorphous Al₂O₃ and then shift to the CB of ZnO via the cascade transport route, which effectively facilitates the hole–electron pair dissociation. Finally, the electrons on the CB of ZnO can combine and activate O₂ to produce ˙O₂⁻ and ˙OH for oxidizing and mineralizing MO molecules.

On the whole, via constructing a series of heterostructures including the type II heterostructure, Z-scheme and tandem type, fast charge separation and efficient photocatalytic dye decomposition can be realized. However, some aspects still should be clarified. For example, the TiO₂/g-C₃N₄ heterostructure system established via different methods usually displays various charge transfer pathways in dye elimination. The inherent mechanism leading to such a variation for the same system should be revealed by advanced characterization techniques such as in situ XPS.

3.2. VOC elimination

Volatile organic compounds (VOCs) are highly detrimental pollutants in the atmosphere and lead to decreased outdoor and indoor air quality.^83–85 Hence, effectively eliminating VOCs has aroused a great deal of concern in pollutant treatment. A variety of strategies such as adsorption, combustion and biological degradation have been adopted for eliminating VOCs. With the fast development of economy and human society, more facile, green and low-cost methods for VOC removal haves attracted great attention.⁸⁶ Accordingly, photocatalytic oxidation as a clean and sustainable technique has been widely studied for the oxidation decomposition of VOCs. The significant step for photocatalytic oxidation is to form photoinduced hole–electron pairs and dissociate the pairs to achieve photooxidation reaction under light illumination. Specifically, the photoinduced holes with strong oxidative ability can directly decompose VOC molecules. Meanwhile, other reactive oxygen species like ˙O₂⁻ and ˙OH can be also generated during the photocatalytic process, which are also useful for VOC molecule elimination.^87,88

Typically, a g-C₃N₄@Bi₂WO₆ core–shell structured heterostructure was fabricated by in situ C₃N₄ precursor encapsulation and reassembly.⁸⁹ As illustrated in Fig. 6a, the precursor molecules were polymerized to generate a thin g-C₃N₄ layer on the surface of Bi₂WO₆ nanosheets. The obtained g-C₃N₄@Bi₂WO₆ photocatalyst with a g-C₃N₄ layer thickness of about 1 nm (Fig. 6b and c) displayed the highest visible light driven photocatalytic phenol degradation rate, being about 6 and 2 times that of bulk g-C₃N₄ and pure Bi₂WO₆ nanosheets, respectively. Electron spin resonance spectroscopy indicated that the superoxide radicals and hydroxyl radicals are more likely to be generated on g-C₃N₄@Bi₂WO₆ compared with pure Bi₂WO₆ and bulk g-C₃N₄, which demonstrates that the formation of a core–shell structured heterostructure can enable a high oxidation ability. Accordingly, a type II heterojunction for photoexcited charge transfer on g-C₃N₄@Bi₂WO₆ for photocatalytic phenol degradation is depicted in Fig. 6d. Firstly, hole–electron pairs are formed on the CB and VB of the g-C₃N₄@Bi₂WO₆ photocatalyst under visible light illumination. Then, the photoinduced electrons can move easily from the g-C₃N₄ shell into the Bi₂WO₆ core because the CB of g-C₃N₄ is higher than that of Bi₂WO₆, while the photogenerated holes can shift from Bi₂WO₆ into g-C₃N₄. O₂ molecules can combine with the electrons on the CB of Bi₂WO₆ to generate superoxide radicals and the holes on the VB of g-C₃N₄ display high oxidation ability, resulting in an improvement in phenol decomposition of the g-C₃N₄@Bi₂WO₆ photocatalyst.

Fig. 6 (a) Schematic diagram of the synthesis of the g-C₃N₄@Bi₂WO₆ core–shell structured heterostructure and (b and c) HR-TEM images of the g-C₃N₄@Bi₂WO₆ core–shell heterostructure. (d) Photoinduced charge transfer mechanism over the g-C₃N₄@Bi₂WO₆ heterostructure. Reproduced with permission.⁸⁹ Copyright 2018, Elsevier. (e) TEM picture of a ternary BiPO₄/TiO₂/gC₃N₄ composite. (f) The cascade electron transfer pathway of the BiPO₄/TiO₂/gC₃N₄ composite for phenol degradation. Reproduced with permission.⁹⁰ Copyright 2016, Elsevier.

A ternary BiPO₄/TiO₂/gC₃N₄ heterostructured composite was obtained via a facile impregnation route and clearly exhibited an increased photocatalytic phenol degradation rate.⁹⁰ TEM images suggested the intimate attachment of the three constituents of BiPO₄/TiO₂/g-C₃N₄, which is an important precondition for constructing a ternary heterostructure (Fig. 6e). In comparison to the binary TiO₂/BiPO₄ and TiO₂/g-C₃N₄ heterostructures, the ternary BiPO₄/TiO₂/g-C₃N₄ heterostructure photocatalyst exhibited higher photocatalytic activity due to the optimization of its photoinduced carrier dissociation and shift properties. The radical capturing experiments indicated that the ternary BiPO₄/TiO₂/g-C₃N₄ heterostructured composite is more able to generate superoxide radical species compared with ternary photocatalysts and pure TiO₂, which are directly responsible for decomposing phenol molecules. A schematic diagram of the proposed cascade electronic band gap in the BiPO₄/TiO₂/g-C₃N₄ heterostructure is depicted in Fig. 6f. BiPO₄ as well as TiO₂, and g-C₃N₄ can be excited by UV and visible light, respectively, to generate hole–electron pairs. Owing to the staggered band gap configuration, the photoelectrons shift from g-C₃N₄ to TiO₂ and BiPO₄, whereas the holes move along the opposite direction to participate in the photooxidation of phenol on the VB of g-C₃N₄.

Although an increased photocatalytic VOC elimination rate can be reached via different kinds of heterostructures, the total decomposition and mineralization of some specific VOC molecules is still complicated and tedious. Therefore, in-depth studies are necessary in this field. For example, the analysis of intermediates during the degradation of VOC molecules is inevitable. Moreover, the photocatalytic technique must guarantee the non-formation of hazardous substances during the decomposition process.

3.3. Heavy metal ion reduction

Hexavalent chromium ions (Cr(VI)) generally exist in wastewater from chromate preparation, electrolyzing, leather tanning, metallurgy, paper making and other industrial manufacturing. Cr(VI) is facilely absorbed by the human body, thus posing a great threat to human beings via the skin, digestion and mucosa.^91–93 The toxicity of Cr(VI) can be significantly decreased as Cr(VI) is reduced to Cr(III). Hence, it is greatly meaningful to remove Cr(VI) from wastewater.⁹⁴ Up to now, a considerable number of works concerning the photocatalytic reduction of Cr(VI) into Cr(III) have been conducted to facilitate its application in wastewater purification. The obtained Cr(III) product is easily eliminated as solid precipitation waste such as Cr(OH)₃.⁹⁵ For the photocatalytic reduction of Cr(VI), the photoinduced electrons from semiconductor photocatalysts play an important role. Therefore, satisfactory photogenerated hole–electron pair separation ability of a photocatalyst is a precondition for Cr(VI) reduction. Moreover, a more negative CB level for photocatalysts is also significant for promoting Cr(VI) reduction. On account of the above considerations, the construction of heterostructured photocatalysts is deemed an ideal solution.^96–101

Typically, g-C₃N₄/ZnO nanorods were synthesized via a simple hydrothermal route for enhanced photocatalytic reduction of Cr(VI).⁹⁶ SEM and TEM images suggested that the g-C₃N₄/ZnO nanorods with a diameter of about 50 nm are well dispersed (Fig. 7a–c). A clear boundary between ZnO and g-C₃N₄ can be found in the TEM image in Fig. 7d, and the plane spacing ascribed to the ZnO (002) plane is 0.26 nm, demonstrating that a heterojunction formed. A type II heterostructure was engineered between g-C₃N₄ and ZnO without changing the structure of the single components, which increased the visible light absorption region. As illustrated in Fig. 7e, g-C₃N₄ can be excited under visible light illumination and photoelectrons on the VB shift to the CB, leaving holes on the VB, while ZnO cannot be irradiated by visible light due to its wide band gap. Because the CB of g-C₃N₄ is more negative than that of ZnO, the photoinduced electrons of g-C₃N₄ facilely migrate to the CB of ZnO and participate in the photocatalytic reduction of Cr(VI). The type II electron transfer pathway improves the photoinduced carrier separation rate and enhances photocatalytic ability. Similarly, a Cu-ZrO₂@g-C₃N₄ heterostructure was prepared by loading Cu-doped ZrO₂ nanoparticles on the surface of g-C₃N₄via a hydrothermal route.⁹⁷ An enhanced Cr(VI) photoreduction was achieved due to its wider solar spectrum response range and facilitated charge carrier separation via a type II heterostructure (Fig. 7f and g).

Fig. 7 SEM (a and b) and TEM (c and d) images of the obtained g-C₃N₄/ZnO heterostructure nanorods and (e) mechanism of photoreduction of Cr(VI) over the g-C₃N₄/ZnO heterostructure. Reproduced with permission.⁹⁶ Copyright 2020, Elsevier. (f) UV-visible light absorption over the obtained photocatalysts. (g) Mechanism of photoreduction of the Cu-ZrO₂@g-C₃N₄ heterostructure. Reproduced with permission.⁹⁷ Copyright 2022, Elsevier.

Apart from engineering a heterostructure, the introduction of effective substrates with good electron mobility is also significantly important for enhanced photocatalytic activity. In this context, a ternary CoS₂/g-C₃N₄-rGO hybrid photocatalyst was fabricated via a facile one-pot solvothermal route.⁹⁸ The CoS₂/g-C₃N₄-rGO hybrid photocatalyst shows increased utilization of visible light and more facile electron–hole pair dissociation in the heterostructure junction. More importantly, a high specific surface area and abundant exposed reactive sites of CoS₂ decorated on rGO are achieved. Ultimately, the ternary CoS₂/g-C₃N₄-rGO hybrid photocatalyst achieves a 99.8% elimination rate for Cr(VI) within 2 hours under visible light illumination.

A B-doped g-C₃N₄ decorated BiVO₄ (BCN-BV) composite with a type II p–n heterojunction was synthesized via loading n-type BiVO₄ on the surface of p-type B-doped g-C₃N₄ for enhanced photoreduction of Cr(VI).⁹⁹ In comparison with pristine BiVO₄ and B-doped g-C₃N₄, CN-BV exhibits a higher photocatalytic reduction rate of Cr(VI) owing to the formation of a type II p–n heterojunction, which can effectively increase visible light utilization and restrict the recombination of hole–electron pairs. Moreover, the TEM image demonstrated the co-existence of monoclinic and tetragonal phase BiVO₄, therefore generating an inner heterojunction for the further improvement of photocatalytic ability (Fig. 8a–c). The schematic mechanism for Cr(VI) reduction over the CN-BV composite is depicted in Fig. 8(d). When BiVO₄ is combined with BCN, a type II p–n heterojunction is formed. In order to form an equilibrium in the Fermi level potential between the two semiconductors, the Fermi level of BiVO₄ is shifted downward whereas the Fermi level of BCN moves upward. As a result, the CB edge position of BiVO₄ is lower than that of BCN. Under visible light excitation, both BVO and BCN can be simultaneously irradiated to generate electron–hole pairs. Along with that, an inner electric field is also formed that enhances the movement of photoexcited charge carriers. Because of the co-existence of both monoclinic and tetragonal phased BiVO₄, the photoelectrons from the CB of the tetragonal phase can facilely be shifted to the CB of the monoclinic phase, promoting Cr(VI) reduction reaction (Fig. 8d). Accordingly, the type II p–n heterojunction between BiVO₄ and BCN not only facilitates the dissociation of the photoexcited hole–electron pairs but also promotes the Cr(VI) reduction process.

Fig. 8 XRD patterns (a), HR-TEM image (b) and SAED pattern (c) of the obtained heterostructure photocatalysts. (d) Mechanism of photoreduction of Cr(VI) over the BCN-BV heterostructure. Reproduced with permission.⁹⁹ Copyright 2019, American Chemical Society. (e) TEM and (f) HR-TEM images of the obtained Ag@Ag₃PO₄/g-C₃N₄/NiFe photocatalyst. (g) Band gap structure and (h) mechanism of photoreduction of Cr(VI) over the Ag@Ag₃PO₄/g-C₃N₄/NiFe heterostructure. Reproduced with permission.¹⁰⁰ Copyright 2018, American Chemical Society.

In addition to creating binary photocatalysts with type II and p–n heterojunctions for efficient Cr(VI) reduction, the quaternary semiconductor composite with a cascade electron migration pathway has also been explored. For example, a quaternary Ag@Ag₃PO₄/g-C₃N₄/NiFe layered double hydroxide (LDH) photocatalyst (Fig. 8e and f) was synthesized via a combination of self-assembly and in situ photoreduction.¹⁰⁰ To be specific, p-type Ag₃PO₄ was electrostatically anchored onto the self-assembled n-type g-C₃N₄/NiFe (CNLDH) LDH composite. Meanwhile, a small number of Ag⁺ ions were reduced to metallic Ag nanoparticles via the photoexcited electrons and -OH groups on the surface of LDH upon visible light illumination. Both Ag nanoparticles and Ag₃PO₄ decorated on the thin layers of g-C₃N₄/NiFe hybrid enable convenient electron and mass transportation. The surface plasma effect of Ag nanoparticles and oxygen vacancies in the NiFe LDH can effectively promote charge separation and is the major reason for the enhanced photocatalytic activity. The band gap alignments of g-C₃N₄, NiFe LDH, Ag₃PO₄, and Ag nanoparticles before and after combination are systematically depicted in Fig. 8g. Before the combination, the band alignments of the four components are messy, and unable to enable the separation and migration of photoinduced carriers. After the combination, a cascade electron migration pathway in the Ag@Ag₃PO₄/g-C₃N₄/NiFe LDH was observed, in which the photoelectrons can flow from Ag₃PO₄ to g-C₃N₄ and then to NiFe LDH because of the Fermi level alignment of the constituent photocatalyst, which is favorable for the shift of photo-charge for Cr(VI) reduction (Fig. 8h).

In this section, g-C₃N₄ based heterostructures for dye decomposition, VOC elimination, and heavy metal ion reduction are thoroughly outlined and discussed. Owing to the convenient photoinduced carrier separation and sufficient photoredox potential after the formation of a heterostructure, improved environmental photocatalytic performance is achieved. On the whole, photoinduced holes are responsible for dye and VOC molecule oxidation while the heavy metal ion reduction is majorly determined by the photoelectrons. The experimental details and corresponding performance of various g-C₃N₄ based heterostructure photocatalysts in organic pollutant elimination and Cr(VI) reduction are listed in Table 1.

Table 1 Experimental details of various g-C₃N₄ based heterostructure photocatalysts for pollutant treatment

Photocatalyst	Conditions	Reaction	Performance	Type of heterojunction	Ref.
MoS2/C3N4	Xenon lamp, 0.1 g l^−1	RhB degradation	0.301 min^−1	Type II	76
3D g-C3N4/TiO2	Xenon lamp, 0.5 g l^−1	MB degradation	0.777 h^−1	Type II	77
Macro/mesoporous g-C3N4/TiO2	Xenon lamp, 2 g l^−1	RhB degradation	0.0478 min^−1	Type II	78
g-C3N4/MnO2	Xenon lamp, 1 g l^−1	RhB degradation	91.3%, 60 min	Z-scheme	79
H2SrTa2O7/g-C3N4/Ag3PO4	λ ≥ 400 nm, 0.5 g l^−1	MO degradation	98%, 8 min	Cascade electronic band	80
g-C3N4/Al2O3/ZnO	Xenon lamp, 1 g l^−1	MB degradation	0.0243 min^−1	Cascade electronic band	81
CN/rGO@BPQDs	Xenon lamp, 1 g l^−1	RhB degradation	97%, 20 min	n–n type	82
g-C3N4@Bi2WO6	Xenon lamp	Phenol degradation	0.078 h^−1	Type II	89
BiPO4/TiO2/g-C3N4	Hg–Xe lamp, 1 g l^−1	Phenol degradation	7.2 mol min^−1	Cascade electronic band	90
g-C3N4/ZnO	Xenon lamp, 1 g l^−1	Cr(VI) Reduction	98%, 90 min	Type II	96
g-C3N4/Cu-doped ZrO2	AM 1.5G, 1 g l^−1	Cr(VI) Reduction	0.0094 h^−1	Type II	97
CoS2/gC3N4-rGO	Xenon lamp, 0.5 g l^−1	Cr(VI) Reduction	99.8%, 120 min	Type II	98
B-doped g-C3N4/BiVO4	Xenon lamp, 1 g l^−1	Cr(VI) Reduction	85%, 30 min	p–n type	99
Ag@Ag3PO4/g-C3N4/NiFe LDH	Sunlight, 1 g l^−1	Cr(VI) Reduction	97%, 120 min	Cascade electronic band	100
g-C3N4/UiO-66	Xenon lamp	Cr(VI) Reduction	0.1102 min^−1	Z-scheme	101

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4. CO₂ reduction

The greenhouse effect induced by CO₂ emission has aroused great concern during the past few decades, as it causes global warming and poses significant challenges to the development of society and economy.^102–104 With environmental deterioration and energy shortages, the utilization of fossil fuels has significantly increased, giving rise to a large amount of CO₂ emission to the atmosphere and resulting in an environmental crisis. Among the various strategies for alleviating the results of CO₂ emission, the adoption of abundant and clean sunlight energy for reducing CO₂ into value-added products and fuels is deemed an ideal protocol to simultaneously address both environmental deterioration and energy shortage problems^105–108 (Fig. 9). However, the intrinsic inertness and thermodynamic stability of CO₂ molecules with a C=O bond breaking energy of around 750 kJ mol⁻¹ lead to the requirement of large energy inputs for light driven CO₂ conversion.^109,110 Up to now, a variety of photocatalysts have been explored for photoreduction of CO₂. Nevertheless, several drawbacks such as unsatisfactory energy conversion efficiency, low selectivity, and inability to restrict the competing reaction of hydrogen formation are still present that limit the development of advanced CO₂ conversion.^111–115 Therefore, the design and fabrication of highly efficient photocatalysts is a core challenge for achieving elevated performance in the photoreduction of CO₂.

Fig. 9 Schematic diagram of photocatalytic CO₂ conversion.

Among a series of semiconductor photocatalysts, heterostructured C₃N₄ photocatalysts with appropriate electronic structures have been considered as a promising candidate for increased photoreduction of CO₂ owing to their long lifetime of photoexcited charge carriers via the spatial dissociation of holes and electrons within the interface region.^116–122 As an efficient visible light converter, the CdS semiconductor has an outstanding optical response and has been frequently adopted as a photo-harvester for various photoredox reactions.^123–125 In this regard, employing CdS to hybridize with a C₃N₄ photocatalyst to construct a type II CdS/C₃N₄ heterostructure with improved CO₂ conversion capability was achieved via a simple photoinduced deposition technique.¹¹⁶ The SEM image in Fig. 10a shows that the CdS nanoparticles are tightly decorated on the surface of C₃N₄. The HR-TEM picture of CdS/C₃N₄ suggests that the distinct lattice fringes correspond to the well-defined crystal structures of C₃N₄ and CdS, indicating the formation of a heterostructure (Fig. 10b). A proposed mechanism for reducing CO₂ over the CdS/C₃N₄ heterostructure is depicted in Fig. 10c. Hole–electron pairs are formed on the CdS and C₃N₄ semiconductors upon visible light illumination. Because CdS has a more negative CB level compared with that of BCN, the generation of an internal electric field occurs, which is located in the space charge position, therefore promoting the dissociation and migration of photoexcited hole–electron pairs. Afterwards, the photoelectrons can shift quickly from the CB of CdS to that of C₃N₄. The accumulated photoelectrons react with the adsorbed CO₂ molecules on the surface of the CdS/C₃N₄ heterostructure to generate CO. Meanwhile, the holes migrate from the VB of C₃N₄ to that of CdS and are finally quenched by the sacrificial agent TEOA. Thus, the redox cycle of the light driven CO₂ conversion is accomplished. Specifically, the optimal CdS/C₃N₄ heterostructure displays excellent performance in the photoreduction of CO₂ with a CO productivity of about 13 μmol h⁻¹, being 10 fold that of pristine C₃N₄ (Fig. 10d).

Fig. 10 SEM (a) and HR-TEM (b) images of a CdS/C₃N₄ heterostructure. (c) Mechanism of photoreduction of CO₂ over the CdS/C₃N₄ heterostructure. Reproduced with permission.¹¹⁶ Copyright 2018, American Chemical Society.  (d) Photocatalytic CO and H₂ evolution rate over the obtained samples. (e) Photocatalytic CO₂ reduction performance of different samples. (f) Mechanism of photoreduction of CO₂ over the In₂O₃@g-C₃N₄ heterostructure. Reproduced with permission.¹¹⁷ Copyright 2014, Elsevier. SEM (g) and HR-TEM (h) images of the obtained g-C₃N₄/NaNbO₃ nanowire photocatalyst, and the (i) corresponding photoinduced charge transfer for CO₂ reduction. Reproduced with permission.¹¹⁸ Copyright 2014, American Chemical Society.

Similarly, Cao et al. reported the in situ growth of In₂O₃ nanoparticles on the surface of g-C₃N₄ nanosheets.¹¹⁷ The resulting In₂O₃@g-C₃N₄ hybrid structures exhibited a considerable enhancement in photocatalytic CO₂ conversion into CH₄ (Fig. 10e), resulting from convenient charge separation and transfer via a type II heterostructure between the In₂O₃ and g-C₃N₄ components (Fig. 10f). Zou's team fabricated a g-C₃N₄/NaNbO₃ nanowire photocatalyst by loading polymeric g-C₃N₄ on NaNbO₃ nanowires.¹¹⁸ NaNbO₃ possesses a unique crystal structure comprising a framework of corner-shared [NbO₆] octahedral units, which is beneficial for enhancing charge movement in the crystal. Meanwhile, the NaNbO₃ nanowires generally offer higher surface-to-volume ratios and provide more efficient ballistic charge migration along the single nanowire than the diffusive shift in powdered photocatalysts (Fig. 10g and h). Under visible light illumination, the photoinduced charges shift along a type II route (Fig. 10i). On account of the above merits in structure, composition and charge transfer mechanism, the g-C₃N₄/NaNbO₃ heterojunction photocatalyst displays almost 8 times higher CO₂ reduction than that of single C₃N₄ upon visible light irradiation.

Although a large number of works combining C₃N₄ with both broadband (ZnO and TiO₂) and narrowband (In₂O₃, NaNbO₃, Ag₃PO₄, and SnS₂) semiconductors for photocatalytic CO₂ reduction were carried out,^116–122 achieving high CO₂ conversion efficiency and selectivity is still challenging. In the combination of two semiconductors, the Z-scheme electron transfer mechanism has been found to be the optimal strategy for photocatalytic reactions. In this case, the photoexcited electrons with strong reduction capability are situated in one semiconductor, while photoinduced holes with sufficient oxidizing potential are located in the other semiconductor, which can be subsequently used for corresponding surface reactions. Therefore, fast charge separation and efficient photocatalytic CO₂ conversion along with a high selectivity of products are reached. Unfortunately, up to now, most of the developed Z-scheme heterostructures for photoreduction of CO₂ have employed an additional sacrificial reagent, which is known as the indirect Z-scheme mechanism. Therefore, for large scale photocatalytic CO₂ conversion reaction with C₃N₄, developing a direct Z-scheme heterostructured photocatalyst with an appropriate electronic structure match is urgently required to realize efficient spatial dissociation of charge carriers and thus ideal performance as well as high product selectivity.

Typically, a series of Z-scheme g-C₃N₄/FeWO₄ heterostructure photocatalysts (Fig. 11a) was synthesized for enhanced CO₂ reduction performance with high selectivity into CO as solar fuel upon solar light illumination.¹¹⁹ Specifically, the Z-scheme heterostructure with an advanced charge transfer route exhibited an increased CO formation rate of 6 μmol g⁻¹ h⁻¹ at room temperature, which is around 6 and 15 times that of the pure C₃N₄ and FeWO₄ photocatalysts (Fig. 11b). In addition, the product selectivity for CO was 100% over other carbon products from CO₂ reduction. As depicted in Fig. 11c, FeWO₄ possesses a relatively negative CB position with respect to the VB level of C₃N₄, which is favorable for achieving the Z-scheme charge shift mechanism. Therefore, the photoelectrons on the CB of C₃N₄ having sufficient reducing ability are well preserved (which may otherwise recombine with the photoinduced holes from the VB), while the photoexcited holes on the VB of FeWO₄ with strong oxidizing capability remain. On account of the above illustration, more photoelectrons are able to take part in the photoreduction of CO₂. As a result, both CO production and selectivity are elevated in the C₃N₄/FeWO₄ heterostructure photocatalyst.

Fig. 11 (a) HR-TEM image of the g-C₃N₄/FeWO₄ heterostructure. (b) Time dependent photocatalytic CO productivity over the synthesized samples. (c) Mechanism of photoreduction of CO₂ and corresponding charge migration route in the g-C₃N₄/FeWO₄ heterostructure. Reproduced with permission.¹¹⁹ Copyright 2019, American Chemical Society. (d) CO₂ adsorption isotherms of the prepared g-C₃N₄, SnS₂ and g-C₃N₄/SnS₂ samples. (e) Schematic illustration of photoreduction of CO₂ and charge migration pathway of the g-C₃N₄/SnS₂ heterostructure. (f) In situ FTIR spectra of the g-C₃N₄/SnS₂ photocatalyst under different conditions. Reproduced with permission.¹²⁰ Copyright 2017, Elsevier. (g) UV-visible light absorption spectra of the fabricated photocatalysts. (h) Photocatalytic reduction of CO₂ over the fabricated photocatalysts and the (i) corresponding photoinduced charge transfer for CO₂ reduction over the ternary Ag/Ag₃PO₄/g-C₃N₄. Reproduced with permission.¹²² Copyright 2015, American Chemical Society.

Similarly, a direct Z-scheme g-C₃N₄/SnS₂ heterojunction was obtained by in situ decorating SnS₂ quantum dots on the surface of g-C₃N₄ by a convenient hydrothermal route.¹²⁰L-Cysteine as the sulfur precursor can also graft ammine species onto g-C₃N₄ during the hydrothermal treatment, which significantly promotes the CO₂ uptake of the photocatalyst (Fig. 11d). As illustrated in Fig. 11e, Z-scheme charge transfer occurs under light irradiation, with the photoelectrons on SnS₂ combining with the photoinduced holes on g-C₃N₄, which promotes the extraction and utilization of photoelectrons on g-C₃N₄. As expected, the g-C₃N₄/SnS₂ heterojunction shows superior photocatalytic CO₂ reduction in comparison to the individual g-C₃N₄ and SnS₂ samples, which is attributed to the direct Z-scheme charge transfer and enhanced CO₂ adsorption ability. In situ FTIR spectra suggested that HCOOH is formed as an intermediate during light driven CO₂ reduction (Fig. 11f), which can only be produced by g-C₃N₄ based on the energy level of the photoexcited electrons, further proving the existence of the Z-scheme heterostructure in the g-C₃N₄/SnS₂ system. A g-C₃N₄/ZnO composite microsphere was synthesized via an electrostatic self-assembly protocol.¹²¹ This method skillfully exploited the opposite surface charge of g-C₃N₄ and ZnO, acquiring a hierarchical structure with close contact between them. The composite achieves increased light absorption and convenient photoinduced charge transfer originating from the Z-scheme heterostructure. Therefore, an increased photocatalytic CO₂ reduction rate was attained. Specifically, g-C₃N₄/ZnO exhibits a CH₃OH productivity of 1.32 μmol h⁻¹ g⁻¹, which is around 2.1 and 4.1 times that of the pure ZnO and g-C₃N₄, respectively.

In addition to the binary Z-scheme heterojunction discussed above, ternary Z-scheme heterojunctions containing noble metal ions have also been developed for efficient photoreduction of CO₂. For example, a ternary Ag/Ag₃PO₄/g-C₃N₄ composite photocatalyst was fabricated using an in situ deposition route.¹²² As shown in Fig. 5, the absorption of Ag₃PO₄/g-C₃N₄ composites is progressively enhanced with increasing Ag₃PO₄ content, which favors the efficient utilization of solar light and elevates the photocatalytic activity (Fig. 11g). Moreover, it can be noted that the ternary composites also display light absorption in the range of 520–700 nm, which can be ascribed to the plasmonic effect of Ag nanoparticles. Accordingly, the ternary photocatalyst exhibits a prominent CO₂ reduction rate of 57.5 μmol h⁻¹, which is about 6.1 times that of pristine g-C₃N₄ (Fig. 11h). The mechanism for the photoreduction of CO₂ over the ternary Ag/Ag₃PO₄/g-C₃N₄ composite is depicted in Fig. 11i, wherein Ag nanoparticles can act as a charge transportation bridge to activate the Z-scheme Ag₃PO₄/g-C₃N₄ system. Because the CB edge of Ag₃PO₄ is more negative than the Fermi level of the Ag nanoparticles, the photoexcited electrons on the CB of Ag₃PO₄ thus shift to the Ag nanoparticles. Meanwhile, the photoinduced holes on the VB of g-C₃N₄ can move to metallic Ag and further combine with the photoelectrons. This type of charge shift pathway efficiently improves the association of hole–electron pairs and enables the electrons and holes to stay on the CB of g-C₃N₄ and VB of Ag₃PO₄, respectively, which is favorable for the photoredox capability. Owing to the negative CB position of g-C₃N₄, these photoelectrons possess strong reduction ability and can facilely reduce CO₂ into CO, CH₄, CH₃OH, and CH₃CH₂OH.

By effectively separating the photoinduced charges and maintaining appropriate photoreduction capability, the photoreduction of CO₂ into organics was achieved. However, the major product of CO₂ reduction is CO over most g-C₃N₄ based heterostructures. Thus, more focused heterostructure engineering should be carried out to facilitate the production of more valuable C1 and C2 products such as methanol, ethylene and ethanol. The experimental details and corresponding performance of various g-C₃N₄ based heterostructure photocatalysts in CO₂ reduction are listed in Table 2.

Table 2 Experimental details of various g-C₃N₄ based heterostructure photocatalysts for CO₂ reduction

Photocatalyst	Conditions	Reaction	Performance	Type of heterojunction	Ref.
CdS/BCN	Xe light, bpy, acetonitrile, TEOA, and CoCl2	CO2 reduction	CO productivity of 250 μmol h^−1 g^−1	Type II	116
In2O3/g-C3N4	Xe light, 0.5 g l^−1	CO2 reduction	CH4 productivity of 76.7 ppm	Type II	117
g-C3N4/NaNbO3	Xe light, photocatalyst (50 mg)	CO2 reduction	CH4 productivity of 6.4 μmol h^−1 g^−1	Type II	118
C3N4/FeWO4	Xe light, 2.5 g l^−1 0.5 M Na2SO3	CO2 reduction	CO productivity of 6 μmol g^−1 h^−1	Z-scheme	119
g-C3N4/SnS2	Xe light, 5 g l^−1	CO2 reduction	CH3OH productivity of 2.3 μmol g^−1	Z-scheme	120
g-C3N4/ZnO microspheres	Xe light, 10 g l^−1 0.5 M Na2SO3	CO2 reduction	CH3OH productivity of 1.32 μmol h^−1 g^−1	Z-scheme	121
Ag3PO4/g-C3N4	Xe light, 2.5 g l^−1	CO2 reduction	CO2 reduction rate of 57.5 μmol h^−1 gcat^−1	Z-scheme	122

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5. Photocatalytic bacteria inactivation over g-C₃N₄ based heterostructured photocatalysts

Water contamination by bacteria is becoming a tremendous threat to human health.^126–128 Although traditional techniques like chlorination and ozonation have proven to be efficacious in the disinfection of water, they suffer from some shortcomings, such as high costs, tedious operation and huge energy consumption.^129–132 Photocatalytic bacteria inactivation via a green, facile, and sustainable process has attracted much attention (Fig. 12). The design of highly efficient photocatalysts is actually the key factor for achieving efficient bacteria inactivation.^133–135 In fact, owing to their extraordinary photoactivity and stability, g-C₃N₄ based heterostructures have significant potential in disinfecting water and great progress has been achieved.

Fig. 12 Illustrated mechanisms of photocatalytic bacteria inactivation by common semiconductors. Reproduced with permission.¹³² Copyright 2018, Frontiers Media S.A.

For example, a series of g-C₃N₄/xAgBr composites was obtained by loading various amounts of AgBr nanoparticles on g-C₃N₄.¹³⁶ Compared with the pure g-C₃N₄ and AgBr samples, the disinfection capability of g-C₃N₄/xAgBr was evidently improved. As shown in Fig. 13a, g-C₃N₄/AgBr displays the highest rate of E. coli cell inactivation. The control experiment showed that the cell amount does not change in the absence of photocatalyst or in the dark, suggesting that photocatalyst and light are crucial for bacteria inactivation. With higher AgBr loading, the disinfection activities of g-C₃N₄-2AgBr and g-C₃N₄-4AgBr are attenuated because the agglomeration of AgBr nanoparticles on the surface of g-C₃N₄ restricts photoinduced carrier separation. In order to directly reveal the evolution of E. coli cells during the disinfection procedure, SEM images of E. coli cells were recorded at different inactivation times. As depicted in Fig. 13b, the E. coli cells show a smooth appearance before light irradiation. After visible light photocatalytic disinfection with g-C₃N₄-AgBr for 0.5 h, the cells become cataplastic (Fig. 13c), exhibiting destroyed cell wall and damaged cytoplasm. After 1 h of visible light illumination (Fig. 13d), the cells appeared to be totally broken down and effective disinfection was realized. Moreover, the scavenger trapping experiments indicated that photogenerated holes have the major role in the bacterial disinfection process (Fig. 13e). Moreover, the disinfection ability of g-C₃N₄-AgBr toward S. aureus was slower than that of Gram-negative E. coli., which was attributed to the distinct cell wall configuration of the two kinds of cells (Fig. 13f).

Fig. 13 (a) Time dependent photocatalytic disinfection efficiencies of E. coli over g-C₃N₄ and g-C₃N₄/xAgBr. SEM pictures of E. coli cells at various photocatalytic disinfection times of (b) 0 min, (c) 30 min, and (d) 60 min. (e) Disinfection performance of E. coli over g-C₃N₄-AgBr with the addition of different scavengers and (f) disinfection ability of S. aureus over g-C₃N₄ and g-C₃N₄-AgBr. Reproduced with permission.¹³⁶ Copyright 2017, Elsevier.

A variety of ZnO/g-C₃N₄ heterojunction photocatalysts with increased disinfection performance were synthesized by a thermal polycondensation route (Fig. 14a).¹³⁷ Under simulated sunlight illumination, the ZnO/g-C₃N₄ heterojunction photocatalyst with 10 wt% ZnO loading displays the optimum bactericidal efficiency in natural water. Specifically, a bactericidal rate of 97.4% is achieved after 60 min of photocatalytic reaction with the addition of 10% ZnO/g-C₃N₄, while a bactericidal rate of 47.3% was reached for pure g-C₃N₄ (Fig. 14b). The results indicate that the construction of the ZnO/g-C₃N₄ heterojunction leads to enhanced photocatalytic disinfection. More in-depth research demonstrated that ZnO/g-C₃N₄ heterojunction photocatalysts give rise to increased H₂O₂ production, which can be used for in situ bacterial inactivation in water. As shown in Fig. 14c, after the end of photocatalytic disinfection, the amount of colonies continually reduces. Meanwhile, the concentration of H₂O₂ gradually decreases. This phenomenon can be ascribed to the fact that there is still a certain amount of H₂O₂ in the reaction system after photocatalytic disinfection, which can kill and restrict the survival of bacteria. Hence, it was demonstrated that the disinfection mainly relies on the H₂O₂ in the solution. Moreover, reactive oxygen species including ˙O₂⁻ and ˙OH are easily generated on the ZnO/g-C₃N₄ photocatalyst due to the facile photoinduced hole–electron pair separation and shift via a Z-scheme heterostructure (Fig. 14d), and they are also responsible for the efficient photocatalytic bacterial inactivation.

Fig. 14 (a) Preparation process of a ZnO/g-C₃N₄ heterojunction. (b) Sterilization rates for bacteria over the obtained photocatalysts. (c) Residual sterilization ability after the photocatalytic process. (d) Pathway of H₂O₂ formation and photocatalytic sterilization over the ZnO/g-C₃N₄ heterojunction. Reproduced with permission.¹³⁷ Copyright 2021, Elsevier. (e) HRTEM image of g-C₃N₄/NiFe₂O₄. (f) Radical trapping experiments for photocatalytic inactivation of A. flavus over g-C₃N₄/NiFe₂O₄ under visible light illumination. (g) A. flavus colony on peanuts after the photocatalytic process in the presence of g-C₃N₄/NiFe₂O₄. Reproduced with permission.¹⁴⁰ Copyright 2021, Elsevier.

Aspergillus flavus can cause crop production reduction and generate mycotoxin, thus causing huge threats to human beings.^138,139 In this regard, a Z-scheme g-C₃N₄/NiFe₂O₄ (CN/NFO) heterostructure photocatalyst was prepared via a facile hydrothermal method for efficient photocatalytic Aspergillus flavus inactivation.¹⁴⁰ TEM characterization suggested that the ultrathin g-C₃N₄ nanosheets homogeneously and intimately combine with rhombic NiFe₂O₄ nanosheets to form a g-C₃N₄/NiFe₂O₄ heterostructure (Fig. 14e), which can provide more sufficient surface active sites for photocatalytic processes and promote photoexcited charge separation and transfer, which are beneficial to the photocatalytic ability. Among a series of CN/NFO composites with various amounts of g-C₃N₄, 0.2CN/NFO presented the highest capability for Aspergillus flavus inactivation with a bactericidal rate of over 90% under visible light illumination for 1.5 h. The efficient photocatalytic disinfection performance was attributed to the optimal photoexcited charge separation, excellent photoelectric properties and appropriate band structure. Radical trapping tests indicated that ˙O₂⁻ and ˙OH are major active species for killing Aspergillus flavus (Fig. 14f). In general, peanut is highly vulnerable to Aspergillus flavus contamination. Therefore, the photocatalytic disinfection ability of the 0.2CN/NFO photocatalyst was checked on contaminated peanuts. As shown in Fig. 14g, it is clear that the colony unit number of Aspergillus flavus on the peanuts progressively decreased with prolonged irradiation time.

Although the g-C₃N₄ based heterojunction photocatalysts have displayed certain promise in the inactivation of several bacteria, the precise optimization of the photocatalytic process is still needed for overall microbial inactivation. In particular, finding competent materials with a sufficient photoredox potential, low cost, and long-term durability are preconditions for the photocatalytic disinfection of microbes. In addition, the facile separation of photocatalyst powders from aqueous solution after the disinfection procedure should be addressed.

6. DFT calculations of heterostructured photocatalysts

Since there are many DFT calculations of the above heterostructured C₃N₄ photocatalysts, we thus added one section and briefly highlighted the important points and most representative cases of DFT calculations including Fermi level, work function and electron density difference for heterostructured C₃N₄ photocatalysts.

In order to investigate the heterojunction generated between two different semiconductor photocatalysts, their work functions have frequently been calculated. Generally, electrons will flow from a semiconductor with a smaller work function to another semiconductor with a larger work function via the interfacial heterojunction. Fig. 15a depicts a schematic configuration of a g-C₃N₄/MnO₂ hybrid for calculating the corresponding Fermi levels of MnO₂ and g-C₃N₄. The values of work function for g-C₃N₄ and MnO₂ were determined to be 4.5 and 6.8 eV, respectively (Fig. 15b), implying that the electrons will transfer from g-C₃N₄ to MnO₂via the heterojunction. As a result, g-C₃N₄ becomes a positive component whereas MnO₂ presents a negative feature close to the heterojunction interface. The above conclusion was further verified by the electron density difference diagram displayed in Fig. 15c. The yellow and cyan domains denote electron accumulation and depletion, respectively, demonstrating that electrons flee from g-C₃N₄ to MnO₂via the heterojunction. Similarly, electron migration from g-C₃N₄ to SnS₂via the interface of the g-C₃N₄/SnS₂ heterojunction was verified by the calculated work functions of g-C₃N₄ and SnS₂ (Fig. 15d and e).

Fig. 15 (a) Schematic configuration of the g-C₃N₄/MnO₂ hybrid. (b) The calculated work functions of g-C₃N₄ and MnO₂ materials, respectively. (c) Electron density difference diagram of the g-C₃N₄/MnO₂ hybrid. Reproduced with permission.⁷⁹ Copyright 2018, American Chemical Society. The calculated work functions for (d) g-C₃N₄ and (e) SnS₂. Reproduced with permission.¹²⁰ Copyright 2017, Elsevier.

7. Conclusions and outlook

g-C₃N₄ is a highly promising semiconductor photocatalyst as a substitute for the traditional TiO₂ owing to its low cost, good stability and abundance. Nevertheless, the unfavorable recombination of photoexcited charges and insufficient visible light utilization capability of pristine g-C₃N₄ seriously reduce its photocatalytic activity and prohibit its practical application. Among a variety of modification strategies, only the rational fabrication of heterojunctions with two or more semiconductor photocatalysts can combine the merits of multicomponents to facilitate photo-irradiated charge dissociation, improve visible light harvesting and retain the sufficient redox potential of hole–electron pairs simultaneously.

This comprehensive review summarizes the categories of g-C₃N₄ based heterojunction photocatalysts, including the type-II heterojunction, Z-scheme heterojunction, p–n heterojunction and cascade electronic band heterojunction. The applications of g-C₃N₄ based heterojunctions in environmental fields involving organic pollutant removal, Cr(VI) ion reduction, CO₂ conversion and bacterial disinfection are thoroughly outlined and discussed. Briefly, on account of the strong oxidation ability of the g-C₃N₄ based heterojunction, efficient organic pollutant removal and bacterial disinfection can be realized, whereas promoted Cr(VI) ion reduction and CO₂ conversion over the g-C₃N₄ based heterojunction result from the sufficient photoreduction potential due to the formation of the heterojunction. Moreover, the construction of a heterostructure also facilitates these environmental applications driven by visible light illumination, thus broadening their practical applications.

Certainly, the design and fabrication of g-C₃N₄ based heterostructure photocatalysts will still be further developed and expanded in the future. Up to now, great progress and achievements have been made in the utilization of g-C₃N₄ based heterojunction photocatalysts in environmental fields. Nevertheless, there are still some challenges and shortcomings in this promising field.

First of all, although g-C₃N₄ based heterojunction photocatalysts actually give rise to increased photocatalytic capability in environmental applications, it should be apparent that the photocatalytic rate is still far from commercial utilization standards and the large-scale production of uniform heterojunction structures is highly challenging. Secondly, the photoexcited charge transfer pathway in various types of g-C₃N₄ based heterojunctions and the corresponding photocatalytic mechanism should be further elucidated. Therefore, some advanced characterization methods, like in situ analysis technique, in situ XPS, and even theoretical calculations should be well explored and applied. Thirdly, the stability of heterostructured C₃N₄ photocatalysts for environmental applications should be addressed. The heterostructured C₃N₄ photocatalysts fabricated via calcination treatment generally show better stability and rigidity than those obtained from wet chemical methods such as hydrothermal reaction in long-term photocatalytic reactions. However, calcination treatment often leads to an aggregated morphology with a low surface area and poor surface active sites. Thus, how to balance good stability and uniform morphology with plentiful surface active sites should be deeply investigated.

Finally, the construction of heterojunction photocatalysts is a typical surface-based process. Therefore, the photoexcited charge transfer will be faster when a larger contact area is present at the heterojunction interface. However, bulk g-C₃N₄ adopted in lots of g-C₃N₄ based heterojunctions generally possesses a low surface area and insufficient active sites, limiting the improvement of photocatalytic activity and exploration of exact active sites. The nanostructured g-C₃N₄ photocatalysts like nanosheets, nanotubes and nanowires possessing a large surface area, fast charge transfer, and sufficient charge redox capability are thus considered to have great potential in g-C₃N₄ based heterojunctions, and this could be a hopeful protocol to drastically enhance the photocatalytic performance and precisely reveal the exact active sites.

All in all, we cordially hope that this review can offer deep knowledge to readers and spur on the development of some new concepts and thoughts in manufacturing highly efficient g-C₃N₄ based heterojunction photocatalysts for commercial applications.

Author contributions

Conceptualization, J. Z., L.-H. C. and B.-L. S.; investigation, Y. D.; writing – original draft, Y. D.; writing – review & editing, Y. D., C. W., L. P., S. M., M. Q., R. Z., M. L., Y. N., J. Z., L.-H. C. and B.-L. S.; supervision, J. Z., L.-H. C. and B.-L. S.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

L.-H. Chen acknowledges Hubei Provincial Department of Education for the “Chutian Scholar” program. This work is financially supported by National Natural Science Foundation of China (U1663225 and U20A20122), Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R52) of the Chinese Ministry of Education and Program of Introducing Talents of Discipline to Universities-Plan 111 (Grant No. B20002) from the Ministry of Science and Technology and the Ministry of Education of China. This research is also supported by the European Commission Interreg V France-Wallonie-Vlaanderen project “DepollutAir”.

References

1. F. Akira and H. Kenichi, Electrochemical Photolysis of Water at a Semiconductor Electrode, Nature, 1972, 238, 37–38.
2. Q. C. Wei, Y. Chen, Z. Wang, D. Z. Yu, W. H. Wang, J. Q. Li, L.-H. Chen, Y. Li and B.-L. Su, Light–Assisted Semi-Hydrogenation of 1, 3-Butadiene with Water, Angew. Chem., Int. Ed., 2022, 61, e202210573.
3. H. Zhao, C. F. Li, L. Y. Liu, B. Palma, Z. Y. Hu, S. Renneckar, S. Larter, Y. Li, M. G. Kibria, J. Hu and B.-L. Su, n-p Heterojunction of TiO₂-NiO core-shell structure for efficient hydrogen generation and lignin photoreforming, J. Colloid Interface Sci., 2021, 585, 694–704.
4. M. Yang, Y. X. Guo, Z. Liu, X. Y. Li, Q. Huang, X. Y. Yang, C.-F. Ye, Y. Li, J.-P. Liu, L.-H. Chen, B.-L. Su and Y. L. Wang, Engineering Rich Active Sites and Efficient Water Dissociation for Ni-Doped MoS₂/CoS₂ Hierarchical Structures toward Excellent Alkaline Hydrogen Evolution, Langmuir, 2023, 39, 236–248.
5. H. Zhao, J. Liu, C. F. Li, X. Zhang, Y. Li, Z. Y. Hu, B. Li, Z. Chen, J. Hu and B.-L. Su, Meso–microporous nanosheet–constructed 3DOM perovskites for remarkable photocatalytic hydrogen production, Adv. Funct. Mater., 2022, 32, 2112831.
6. S. Maitra, S. Halder, T. Maitra and S. Roy, Superior light absorbing CdS/vanadium sulphide nanowalls@TiO₂ nanorod ternary heterojunction photoanodes for solar water splitting, New J. Chem., 2021, 45, 7353–7367.
7. T. W. Wang, Z. W. Yin, Y. H. Guo, F. Y. Bai, J. Chen, W. Dong, J. Liu, Z.-Y. Hu, L. Chen, Y. Li and B.-L. Su, Highly Selective Photocatalytic Conversion of Glucose on Holo-Symmetrically Spherical Three-Dimensionally Ordered Macroporous Heterojunction Photonic Crystal, CCS Chem., 2022, 1–16.
8. C. Wang, B. Weng, Y. Liao, B. Liu, M. Keshavarz, Y. Ding, H. Huang, D. Verhaeghe, J. A. Steele, W. Feng, B.-L. Su, J. Hofkens and M. B. Roeffaers, Simultaneous photocatalytic H₂ generation and organic synthesis over crystalline-amorphous Pd nanocube decorated Cs₃Bi₂Br₉, Chem. Commun., 2022, 58, 10691–10694.
9. Y. Ding, S. Maitra, C. Wang, R. Zheng, T. Barakat, S. Roy, L.-H. Chen and B.-L. Su, Bi-functional Cu-TiO₂/CuO photocatalyst for large-scale synergistic treatment of waste sewage containing organics and heavy metal ions, Sci. China Mater., 2023, 66, 179–192.
10. L. Pei, Z. Ma, J. Zhong, W. Li, X. Wen, J. Guo, P. Liu, Z. Zhang, Q. Mao, J. Zhang, S. Yan and Z. Zou, Oxygen Vacancy–Rich Sr₂MgSi₂O₇:Eu²⁺, Dy³⁺ Long Afterglow Phosphor as a Round–the–Clock Catalyst for Selective Reduction of CO₂ to CO, Adv. Funct. Mater., 2022, 32, 2208565.
11. Y. Ding, C. Wang, R. Zheng, S. Maitra, G. Zhang, T. Barakat, S. Roy, B.-L. Su and L.-H. Chen, Three-dimensionally ordered macroporous materials for photo/electrocatalytic sustainable energy conversion, solar cell and energy storage, EnergyChem, 2022, 4, 100081.
12. S. Maitra, A. Sarkar, T. Maitra, S. Halder, K. Kargupta and S. Roy, Solvothermal phase change induced morphology transformation in CdS/CoFe₂O₄@Fe₂O₃ hierarchical nanosphere arrays as ternary heterojunction photoanodes for solar water splitting, New J. Chem., 2021, 45(28), 12721–12737.
13. X. Y. Li, S. J. Zhu, Y. L. Wang, T. Lian, X. Y. Yang, C. F. Ye, Y. Li, B.-L. Su and L.-H. Chen, Synergistic regulation of S-vacancy of MoS₂-based materials for highly efficient electrocatalytic hydrogen evolution, Front. Chem., 2022, 10, 578.
14. T. L. Madanu, S. R. Mouchet, O. Deparis, J. Liu, Y. Li and B.-L. Su, Tuning and transferring slow photons from TiO₂ photonic crystals to BiVO₄ nanoparticles for unprecedented visible light photocatalysis, J. Colloid Interface Sci., 2022, 634, 290–299.
15. C. Wang, B. Weng, M. Keshavarz, M. Q. Yang, H. Huang, Y. Ding, F. Lai, I. Aslam, H. Jin, G. Romolini, B.-L. Su, J. A. Steele, J. Hofkens and M. B. Roeffaers, Photothermal Suzuki Coupling Over a Metal Halide Perovskite/Pd Nanocube Composite Catalyst, ACS Appl. Mater. Interfaces, 2022, 14, 17185–17194.
16. W. Wang, H. Zhang, Y. Chen and H. Shi, Efficient degradation of tetracycline via coupling of photocatalysis and photo-fenton processes over a 2D/2D α-Fe₂O₃/g-C₃N₄ S-scheme heterojunction catalyst, Acta Phys. – Chim. Sin., 2022, 38, 2104030.
17. L. Zhou, Y. Li, Y. Zhang, L. Qiu and Y. Xing, A 0D/2D Bi₄V₂O₁₁/g-C₃N₄ S-scheme heterojunction with rapid interfacial charges migration for photocatalytic antibiotic degradation, Acta Phys. – Chim. Sin, 2022, 38, 2112027.
18. Z. Bao, M. Xing, Y. Zhou, J. Lv, D. Lei, Y. Zhang, Y. Zhang, J. Cai, J. g Wang, Z. Sun, W. Chen, X. Gan, X. Yang, Q. Han, M. Zhang, J. Dai and Y. Wu, Z-Scheme Flower-Like SnO₂/g-C₃N₄ Composite with Sn²⁺ Active Center for Enhanced Visible–Light Photocatalytic Activity, Adv. Sustainable Syst., 2021, 5, 2100087.
19. N. Li, J. Ma, Y. Zhang, L. Zhang and T. Jiao, Recent Developments in Functional Nanocomposite Photocatalysts for Wastewater Treatment: A Review, Adv. Sustainable Syst., 2022, 6, 2200106.
20. X. Gu, T. Chen, J. Lei, Y. Yang, X. Zheng, S. Zhang, Q. Zhu, X. Fu, S. Meng and S. Chen, Self-assembly synthesis of S-scheme g-C₃N₄/Bi₈(CrO₄)O₁₁ for photocatalytic degradation of norfloxacin and bisphenol A, Chin. J. Catal., 2022, 43, 2569–2580.
21. B. Zhang, X. Hu, E. Liu and J. Fan, Novel S-scheme 2D/2D BiOBr/g-C₃N₄ heterojunctions with enhanced photocatalytic activity, Chin. J. Catal., 2021, 42, 1519–1529.
22. F. A. Qaraah, S. A. Mahyoub, A. Hezam, A. Qaraah, Q. A. Drmosh and G. Xiu, Construction of 3D flowers-like O-doped g-C₃N₄-[N-doped Nb₂O₅/C] heterostructure with direct S-scheme charge transport and highly improved visible-light-driven photocatalytic efficiency, Chin. J. Catal., 2022, 43, 2637–2651.
23. S. Das, G. Swain, B. P. Mishra and K. Parida, Tailoring the fusion effect of phase-engineered 1T/2H-MoS₂ towards photocatalytic hydrogen evolution, New J. Chem., 2022, 46, 14922–14932.
24. L. Biswal, S. Nayak and K. Parida, Rationally designed Ti₃C₂/N, S-TiO₂/g-C₃N₄ ternary heterostructure with spatial charge separation for enhanced photocatalytic hydrogen evolution, J. Colloid Interface Sci., 2022, 621, 254–266.
25. Z. Liang, J. Bai, L. Hao, R. Shen, P. Zhang and X. Li, Photodeposition of NiS Cocatalysts on g-C₃N₄ with Edge Grafting of 4-(1H-Imidazol-2-yl) Benzoic Acid for Highly Elevated Photocatalytic H₂ Evolution, Adv. Sustainable Syst., 2023, 7, 2200143.
26. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater., 2009, 8, 76–80.
27. Y. Ding, S. Maitra, D. A. Esteban, S. Bals, H. Vrielinck, T. Barakat, S. Roy, G. V. Tendeloo, J. Liu, Y. Li, A. Vlad and B.-L. Su, Photochemical production of hydrogen peroxide by digging pro-superoxide radical carbon vacancies in porous carbon nitride, Cell Rep. Phys. Sci., 2022, 3, 100874.
28. M. Yang, R. Lian, X. Zhang, C. Wang, J. Cheng and X. Wang, Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction, Nat. Commun., 2022, 13, 1–9.
29. Y. Ding, S. Maitra, C. Wang, R. Zheng, M. Zhang, T. Barakat, S. Roy, J. Liu, Y. Li, T. Hasan and B.-L. Su, Hydrophilic bi-functional B-doped g-C₃N₄ hierarchical architecture for excellent photocatalytic H₂O₂ production and photoelectrochemical water splitting, J. Energy Chem., 2022, 70, 236–247.
30. T. Yuan, L. Sun, Z. Wu, R. Wang, X. Cai, W. Lin, M. Zheng and X. Wang, Mild and metal-free Birch-type hydrogenation of (hetero) arenes with boron carbonitride in water, Nat. Catal., 2022, 5, 1157–1168.
31. Y. Ding, S. Maitra, C. Wang, S. Halder, R. Zheng, T. Barakat, S. Roy, L.-H. Chen and B.-L. Su, Vacancy defect engineering in semiconductors for solar light–driven environmental remediation and sustainable energy production, Interdiscip. Mater., 2022, 1, 213–255.
32. S. Maitra, S. Pal, T. Maitra, S. Halder and S. Roy, Solvothermal Etching-Assisted Phase and Morphology Tailoring in Highly Porous CuFe₂O₄ Nanoflake Photocathodes for Solar Water Splitting, Energy Fuels, 2021, 35, 14087–14100.
33. Y. Ding, S. Maitra, S. Halder, C. Wang, R. Zheng, T. Barakat, S. Roy, L.-H. Chen and B.-L. Su, Emerging semiconductors and metal-organic-compounds-related photocatalysts for sustainable hydrogen peroxide production, Matter, 2022, 5, 2119–2167.
34. J. X. Yang, W. B. Yu, C. F. Li, W. D. Dong, L. Q. Jiang, N. Zhou, Z. Zhuang, J. Liu, Z. Hu, H. Zhao, Y. Li, L. Chen, J. Hu and B.-L. Su, PtO nanodots promoting Ti₃C₂ MXene in situ converted Ti₃C₂/TiO₂ composites for photocatalytic hydrogen production, Chem. Eng. J., 2021, 420, 129695.
35. H. Zhao, P. Liu, X. Wu, A. Wang, D. Zheng, S. Wang, Z. Chen, S. Larter, Y. Li, B.-L. Su, M. G. Kibria and J. Hu, Plasmon enhanced glucose photoreforming for arabinose and gas fuel co-production over 3DOM TiO₂-Au, Appl. Catal., B, 2021, 291, 120055.
36. H. S. Mohamed, M. Rabia, X. G. Zhou, X. S. Qin, G. Khabiri, M. Shaban, H. A. Younus, S. Taha, Z.-Y. Hu, J. Liu, Y. Li and B.-L. Su, Phase-junction Ag/TiO₂ nanocomposite as photocathode for H₂ generation, J. Mater. Sci. Technol., 2021, 83, 179–187.
37. L. Pei, Y. Yuan, W. Bai, T. Li, H. Zhu, Z. Ma, J. Zhong, S. Yan and Z. Zou, In situ-grown island-shaped hollow graphene on TaON with spatially separated active sites achieving enhanced visible-light CO₂ reduction, ACS Catal., 2020, 10, 15083–15091.
38. J. Hu, Y. Lu, X. L. Liu, C. Janiak, W. Geng, S. M. Wu, X.-F. Zhao, L.-Y. Wang, G. Tian, Y. Zhang, B.-L. Su and X. Y. Yang, Photoinduced terminal fluorine and Ti³⁺ in TiOF₂/TiO₂ heterostructure for enhanced charge transfer, CCS Chem., 2020, 2, 1573–1581.
39. Y. Ding, L. Huang, T. Barakat and B.-L. Su, A Novel 3DOM TiO₂ based multifunctional photocatalytic and catalytic platform for energy regeneration and pollutants degradation, Adv. Mater. Interfaces, 2021, 8, 2001879.
40. K. Das, S. Mansingh, R. Mohanty, D. Sahoo, N. Priyadarshini and K. Parida, 0D–2D Fe₂O₃/Boron-Doped g-C₃N₄ S-Scheme Exciton Engineering for Photocatalytic H₂O₂ Production and Photo-Fenton Recalcitrant Pollutant Detoxification: Kinetics, Influencing Factors, and Mechanism, J. Phys. Chem. C, 2023, 127, 22–40.
41. K. Das, S. Mansingh, D. Sahoo, R. Mohanty and K. Parida, Engineering an oxygen-vacancy-mediated step-scheme charge carrier dynamic coupling WO_3−X/ZnFe₂O₄ heterojunction for robust photo-Fenton-driven levofloxacin detoxification, New J. Chem., 2022, 46, 5785–5798.
42. Y. Q. Yan, Y. Chen, Z. Wang, L. H. Chen, H. L. Tang and B.-L. Su, Electrochemistry-assisted selective butadiene hydrogenation with water, Nat. Commun., 2023, 14, 2106.
43. H. Zhao, J. Liu, N. Zhong, S. Larter, Y. Li, M. G. Kibria, B.-L. Su, Z. Chen and J. Hu, Biomass Photoreforming for Hydrogen and Value-Added Chemicals Co-Production on Hierarchically Porous Photocatalysts, Adv. Energy Mater., 2023, 2300257.
44. B. Mishra and K. Parida, Orienting Z scheme charge transfer in graphitic carbon nitride-based systems for photocatalytic energy and environmental applications, J. Mater. Chem. A, 2021, 9, 10039–10080.
45. L. Biswal, L. Acharya, B. Mishra, S. Das, G. Swain and K. Parida, Interfacial Solid-State Mediator-Based Z-Scheme Heterojunction TiO₂@Ti₃C₂/MgIn₂S₄ Microflower for Efficient Photocatalytic Pharmaceutical Micropollutant Degradation and Hydrogen Generation: Stability, Kinetics, and Mechanistic Insights, ACS Appl. Energy Mater., 2023, 6, 2081–2096.
46. W. Yu, J. Liu, M. Yi, J. Yang, W. Dong, C. Wang, H. Zhao, H. Mohamed, Z. Wang, L. Chen, Y. Li and B-L. Su, Active faceted Cu₂O hollow nanospheres for unprecedented adsorption and visible-light degradation of pollutants, J. Colloid Interface Sci., 2020, 565, 207–217.
47. L. Acharya, S. Nayak, S. P. Pattnaik, R. Acharya and K. Parida, Resurrection of boron nitride in p-n type-II boron nitride/B-doped-g-C₃N₄ nanocomposite during solid-state Z-scheme charge transfer path for the degradation of tetracycline hydrochloride, J. Colloid Interface Sci., 2020, 566, 211–223.
48. L. Acharya, S. P. Pattnaik, A. Behera, R. Acharya and K. Parida, Exfoliated boron nitride (e-BN) tailored exfoliated graphitic carbon nitride (e-CN): an improved visible light mediated photocatalytic approach towards TCH degradation and H₂ evolution, Inorg. Chem., 2021, 60, 5021–5033.
49. J. Yang, Y. Peng, S. Li, J. Mu, Z. Huang, J. Ma, Z. Shi and Q. Jia, Metal nanocluster-based hybrid nanomaterials: Fabrication and application, Coord. Chem. Rev., 2022, 456, 214391.
50. Y. Wang, Y. X. Chen, T. Barakat, T. M. Wang, A. Krief, Y. J. Zeng, M. Laboureur, L. Fusaro, H. Liao and B.-L. Su, Synergistic effects of carbon doping and coating of TiO₂ with exceptional photocurrent enhancement for high performance H₂ production from water splitting, J. Energy Chem., 2021, 56, 141–151.
51. S. Das, G. Swain and K. Parida, One step towards the 1T/2H-MoS₂ mixed phase: a journey from synthesis to application, Mater. Chem. Front., 2021, 5, 2143–2172.
52. Y. Lu, Y. X. Liu, L. He, L. Y. Wang, X. L. Liu, J. W. Liu and B.-L. Su, Interfacial co-existence of oxygen and titanium vacancies in nanostructured TiO₂ for enhancement of carrier transport, Nanoscale, 2020, 12, 8364–8370.
53. A. Balakrishnan and M. Chinthala, Comprehensive review on advanced reusability of g-C₃N₄ based photocatalysts for the removal of organic pollutants, Chemosphere, 2022, 297, 134190.
54. L. Biswal, S. Nayak and K. Parida, Recent progress on strategies for the preparation of 2D/2D MXene/g-C₃N₄ nanocomposites for photocatalytic energy and environmental applications, Catal. Sci. Technol., 2021, 11, 1222–1248.
55. L. Acharya, B. P. Mishra, S. P. Pattnaik, R. Acharya and K. Parida, Incorporating nitrogen vacancies in exfoliated B-doped g-C₃N₄ towards improved photocatalytic ciprofloxacin degradation and hydrogen evolution, New J. Chem., 2022, 46, 3493–3503.
56. X. Liu, R. Ma, L. Zhuang, B. Hu, J. Chen, X. Liu and X. Wang, Recent developments of doped g-C₃N₄ photocatalysts for the degradation of organic pollutants, Crit. Rev. Environ. Sci. Technol., 2021, 51, 751–790.
57. H. Chen, Y. Xie, X. Sun, M. Lv, F. Wu, L. Zhang, L. Li and X. Xu, Efficient charge separation based on type-II g-C₃N₄/TiO₂-B nanowire/tube heterostructure photocatalysts, Dalton Trans., 2015, 44, 13030–13039.
58. X. Han, D. Xu, L. An, C. Hou, Y. Li, Q. Zhang and H. Wang, WO₃/g-C₃N₄ two-dimensional composites for visible-light driven photocatalytic hydrogen production, Int. J. Hydrogen Energy, 2018, 43, 4845–4855.
59. A. Bhosale, A. Gophane, J. Kadam, S. Sabale, K. Sonawane and K. Garadkar, Fabrication of visible-active ZnO-gC₃N₄ nanocomposites for photodegradation and cytotoxicity of methyl orange and antibacterial activity towards drug resistance pathogens, Opt. Mater., 2023, 136, 113392.
60. X. Zheng, G. Shen, C. Wang, Y. Li, D. Dunphy, T. Hasan, C. J. Brinker and B.-L. Su, Bio-inspired Murray materials for mass transfer and activity, Nat. Commun., 2017, 8, 1–9.
61. V. Soni, P. Singh, A. A. P. Khan, A. Singh, A. K. Nadda, C. M. Hussain and P. Raizada, Photocatalytic transition-metal-oxides-based p-n heterojunction materials: synthesis, sustainable energy and environmental applications, and perspectives, J. Nanostruct. Chem., 2023, 13, 129–166.
62. H. Zhao, M. Zalfani, C. F. Li, J. Liu, Z. Y. Hu, M. Mahdouani, R. Bourguig, Y. Li and B.-L. Su, Cascade electronic band structured zinc oxide/bismuth vanadate/three-dimensional ordered macroporous titanium dioxide ternary nanocomposites for enhanced visible light photocatalysis, J. Colloid Interface Sci., 2019, 539, 585–597.
63. A. Karmakar, E. Velasco and J. Li, Metal-organic frameworks as effective sensors and scavengers for toxic environmental pollutants, Natl. Sci. Rev., 2022, 9, nwac091.
64. Y. Yin, X. Kang and B. Han, Two-dimensional materials: synthesis and applications in the electro-reduction of carbon dioxide, Chem. Synth., 2022, 2, 19.
65. M.-H. Sun, S.-Z. Huang, L.-H. Chen, Y. Li, X.-Y. Yang, Z.-Y. Yuan and B.-L. Su, Applications of hierarchically structured porous materials from energy storage and conversion, catalysis, photocatalysis, adsorption, separation, and sensing to biomedicine, Chem. Soc. Rev., 2016, 45, 3479–3563.
66. J. Yu, W. Wang, B. Cheng and B. L. Su, Enhancement of photocatalytic activity of mesporous TiO₂ powders by hydrothermal surface fluorination treatment, J. Phys. Chem. C, 2009, 113, 6743–6750.
67. H. Wang, L. Wang, Q. Luo, J. Zhang, C. Wang, X. Ge and F.-S. Xiao, Two-dimensional manganese oxide on ceria for the catalytic partial oxidation of hydrocarbons, Chem. Synth., 2022, 2, 2.
68. Y. Li, D. Zhang, W. Qiao, H. Xiang, F. Besenbacher, Y. Li and R. Su, Nanostructured heterogeneous photocatalyst materials for green synthesis of valuable chemicals, Chem. Synth., 2022, 2, 9.
69. L. H. Chen, X. Y. Li, J. C. Rooke, Y. H. Zhang, X. Y. Yang, Y. Tang, F.-S. Xiao and B.-L. Su, Hierarchically structured zeolites: synthesis, mass transport properties and applications, J. Mater. Chem., 2012, 22, 17381–17403.
70. L. Wu, Y. Li, Z. Fu and B.-L. Su, Hierarchically structured porous materials: Synthesis strategies and applications in energy storage, Natl. Sci. Rev., 2020, 7, 1667–1701.
71. A. Ajmal, I. Majeed, R. N. Malik, H. Idriss and M. A. Nadeem, Principles and mechanisms of photocatalytic dye degradation on TiO₂ based photocatalysts: a comparative overview, RSC Adv., 2014, 4, 37003–37026.
72. M. Zalfani, B. Schueren, M. Mahdouani, R. Bourguiga, W. B. Yu, M. Wu, O. Deparis, Y. Li and B.-L. Su, ZnO quantum dots decorated 3DOM TiO₂ nanocomposites: Symbiose of quantum size effects and photonic structure for highly enhanced photocatalytic degradation of organic pollutants, Appl. Catal., B, 2016, 199, 187–198.
73. M. Rochkind, S. Pasternak and Y. Paz, Using dyes for evaluating photocatalytic properties: a critical review, Molecules, 2014, 20, 88–110.
74. S. Sakthivel, B. Neppolian, M. V. Shankar, B. Arabindoo, M. Palanichamy and V. Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO₂, Sol. Energy Mater. Sol. Cells, 2003, 77, 65–82.
75. M. Zalfani, B. V. Schueren, Z.-Y. Hu, J. C. Rooke, R. Bourguiga, M. Wu, Y. Li, G. V. Tendeloo and B.-L. Su, Novel 3DOM BiVO₄/TiO₂ nanocomposites for highly enhanced photocatalytic activity, J. Mater. Chem. A, 2015, 3, 21244–21256.
76. Q. Li, N. Zhang, Y. Yang, G. Wang and D. H. Ng, High efficiency photocatalysis for pollutant degradation with MoS₂/C₃N₄ heterostructures, Langmuir, 2014, 30, 8965–8972.
77. Y. Sheng, Z. Wei, H. Miao, W. Yao, H. Li and Y. Zhu, Enhanced organic pollutant photodegradation via adsorption/photocatalysis synergy using a 3D g-C₃N₄/TiO₂ free-separation photocatalyst, Chem. Eng. J., 2019, 370, 287–294.
78. R. Hao, G. Wang, H. Tang, L. Sun, C. Xu and D. Han, Template-free preparation of macro/mesoporous g-C₃N₄/TiO₂ heterojunction photocatalysts with enhanced visible light photocatalytic activity, Appl. Catal., B, 2016, 187, 47–58.
79. P. Xia, B. Zhu, B. Cheng, J. Yu and J. Xu, 2D/2D g-C₃N₄/MnO₂ nanocomposite as a direct Z-scheme photocatalyst for enhanced photocatalytic activity, ACS Sustainable Chem. Eng., 2018, 6, 965–973.
80. Y. Ren, W. Zhan, L. Tang, H. Zheng, H. Liu and K. Tang, Constructing a ternary H₂SrTa₂O₇/gC₃N₄/Ag₃PO₄ heterojunction based on cascade electron transfer with enhanced visible light photocatalytic activity, CrystEngComm, 2020, 22, 6485–6494.
81. S. J. Liu, F. T. Li, Y. L. Li, Y. J. Hao, X. J. Wang, B. Li and R. H. Liu, Fabrication of ternary g-C₃N₄/Al₂O₃/ZnO heterojunctions based on cascade electron transfer toward molecular oxygen activation, Appl. Catal., B, 2017, 212, 115–128.
82. J. Xiong, X. Li, J. Huang, X. Gao, Z. Chen, J. Liu and Y. Zhu, CN/rGO@ BPQDs high-low junctions with stretching spatial charge separation ability for photocatalytic degradation and H₂O₂ production, Appl. Catal., B, 2020, 266, 118602.
83. R. Khawaja, S. Sonar, T. Barakat, N. Heymans, B.-L. Su, A. Löfberg and S. Siffert, VOCs catalytic removal over hierarchical porous zeolite NaY supporting Pt or Pd nanoparticles, Catal. Today, 2022, 405, 212–220.
84. T. Barakat, J. C. Rooke, R. Cousin, J. F. Lamonier, J. M. Giraudon, B.-L. Su and S. Siffert, Investigation of the elimination of VOC mixtures over a Pd-loaded V-doped TiO₂ support, New J. Chem., 2014, 38, 2066–2074.
85. E. Haye, T. Barakat, L. Chavee, B.-L. Su, S. Lucas, L. Houssiau and A. Achour, Low-pressure plasma process for the dry synthesis of cactus-like Au-TiO₂ nanocatalysts for toluene degradation, App. Surf. Sci., 2022, 571, 151313.
86. J. C. Rooke, T. Barakat, J. Brunet, Y. Li, M. F. Finol, J. F. Lamonier and B.-L. Su, Hierarchically nanostructured porous group Vb metal oxides from alkoxide precursors and their role in the catalytic remediation of VOCs, Appl. Catal., B, 2015, 162, 300–309.
87. T. Barakat, V. Idakiev, R. Cousin, G. S. Shao, Z. Y. Yuan, T. Tabakova and S. Siffert, Total oxidation of toluene over noble metal based Ce, Fe and Ni doped titanium oxides, Appl. Catal., B, 2014, 146, 138–146.
88. J. C. Rooke, T. Barakat, M. F. Finol, P. Billemont, G. D. Weireld, Y. Li and B.-L. Su, Influence of hierarchically porous niobium doped TiO₂ supports in the total catalytic oxidation of model VOCs over noble metal nanoparticles, Appl. Catal., B, 2013, 142, 149–160.
89. Y. Wang, W. Jiang, W. Luo, X. Chen and Y. Zhu, Ultrathin nanosheets g-C₃N₄@ Bi₂WO₆ core-shell structure via low temperature reassembled strategy to promote photocatalytic activity, Appl. Catal., B, 2018, 237, 633–640.
90. S. Obregón, Y. Zhang and G. Colón, Cascade charge separation mechanism by ternary heterostructured BiPO₄/TiO₂/g-C₃N₄ photocatalyst, Appl. Catal., B, 2016, 184, 96–103.
91. S. S. Kerur, S. Bandekar, M. S. Hanagadakar, S. S. Nandi, G. M. Ratnamala and P. G. Hegde, Removal of hexavalent Chromium-Industry treated water and Wastewater: A review, Mater. Today: Proc., 2021, 42, 1112–1121.
92. S. Loyaux-Lawniczak, P. Lecomte and J. J. Ehrhardt, Behavior of hexavalent chromium in a polluted groundwater: redox processes and immobilization in soils, Environ. Sci. Technol., 2001, 35, 1350–1357.
93. X. Deng, Y. Chen, J. Wen, Y. Xu, J. Zhu and Z. Bian, Polyaniline-TiO₂ composite photocatalysts for light-driven hexavalent chromium ions reduction, Sci. Bull., 2020, 65, 105–112.
94. G. Ghosh and P. K. Bhattacharya, Hexavalent chromium ion removal through micellar enhanced ultrafiltration, Chem. Eng. J., 2006, 119, 45–53.
95. N. A. Azeez, S. S. Dash, S. N. Gummadi and V. S. Deepa, Nano-remediation of toxic heavy metal contamination: Hexavalent chromium [Cr(VI)], Chemosphere, 2021, 266, 129204.
96. Q. Zhong, H. Lan, M. Zhang, H. Zhu and M. Bu, Preparation of heterostructure g-C₃N₄/ZnO nanorods for high photocatalytic activity on different pollutants (MB, RhB, Cr(VI) and eosin), Ceram. Interfaces, 2020, 46, 12192–12199.
97. C. V. Reddy, R. Koutavarapu, J. Shim, B. Cheolho and K. R. Reddy, Novel g-C₃N₄/Cu-doped ZrO₂ hybrid heterostructures for efficient photocatalytic Cr(VI) photoreduction and electrochemical energy storage applications, Chemosphere, 2022, 295, 133851.
98. Y. Wang, S. Bao, Y. Liu, W. Yang, Y. Yu, M. Feng and K. Li, Efficient photocatalytic reduction of Cr(VI) in aqueous solution over CoS₂/g-C₃N₄-rGO nanocomposites under visible light, App. Surf. Sci., 2020, 510, 145495.
99. P. Babu, S. Mohanty, B. Naik and K. Parida, Serendipitous assembly of mixed phase BiVO₄ on B-doped g-C₃N₄: an appropriate p-n heterojunction for photocatalytic O₂ evolution and Cr(VI) reduction, Inorg. Chem., 2019, 58, 12480–12491.
100. S. Nayak and K. Parida, Dynamics of charge-transfer behavior in a plasmon-induced quasi-type-II p–n/n–n dual heterojunction in Ag@Ag₃PO₄/g-C₃N₄/NiFe LDH nanocomposites for photocatalytic Cr(VI) reduction and phenol oxidation, ACS Omega, 2018, 3, 7324–7343.
101. X. H. Yi, S. Q. Ma, X. D. Du, C. Zhao, H. Fu, P. Wang and C. C. Wang, The facile fabrication of 2D/3D Z-scheme g-C₃N₄/UiO-66 heterojunction with enhanced photocatalytic Cr(VI) reduction performance under white light, Chem. Eng. J., 2019, 375, 121944.
102. K. Gillingham and J. H. Stock, The cost of reducing greenhouse gas emissions, J. Econ. Perspect., 2018, 32, 53–72.
103. D. Liu, X. Guo and B. Xiao, What causes growth of global greenhouse gas emissions? Evidence from 40 countries, Sci. Total Environ., 2019, 661, 750–766.
104. I. Arto and E. Dietzenbacher, Drivers of the growth in global greenhouse gas emissions, Environ. Sci. Technol., 2014, 48, 5388–5394.
105. N. N. Vu, S. Kaliaguine and T. O. Do, Plasmonic Photocatalysts for Sunlight–Driven Reduction of CO₂: Details, Developments, and Perspectives, ChemSusChem, 2020, 13, 3967–3991.
106. N. N. Vu, S. Kaliaguine and T. O. Do, Critical aspects and recent advances in structural engineering of photocatalysts for sunlight–driven photocatalytic reduction of CO₂ into fuels, Adv. Funct. Mater., 2019, 29, 1901825.
107. D. Grammatico, H. N. Tran, Y. Li, S. Pugliese, L. Billon, B.-L. Su and M. Fontecave, Immobilization of a Molecular Re Complex on MOF–derived Hierarchical Porous Carbon for CO₂ Electroreduction in Water/Ionic Liquid Electrolyte, ChemSusChem, 2020, 13, 6418–6425.
108. E. B. Stechel and J. E. Miller, Re-energizing CO₂ to fuels with the sun: Issues of efficiency, scale, and economics, J. CO₂ Util., 2013, 1, 28–36.
109. D. Grammatico, A. J. Bagnall, L. Riccardi, M. Fontecave, B.-L. Su and L. Billon, Heterogenised molecular catalysts for sustainable electrochemical CO₂ reduction, Angew. Chem., Int. Ed., 2022, 61, e202206399.
110. S. Pugliese, N. T. Huan, J. Forte, D. Grammatico, S. Zanna, B.-L. Su, Y. Li and M. Fontecave, Functionalization of Carbon Nanotubes with Nickel Cyclam for the Electrochemical Reduction of CO₂, ChemSusChem, 2020, 13, 6449–6456.
111. X. Li, J. Yu, M. Jaroniec and X. Chen, Cocatalysts for selective photoreduction of CO₂ into solar fuels, Chem. Rev., 2019, 119, 3962–4179.
112. C. Aprile, F. Giacalone, P. Agrigento, L. F. Liotta, J. A. Martens, P. P. Pescarmona and M. Gruttadauria, Multilayered Supported Ionic Liquids as Catalysts for Chemical Fixation of Carbon Dioxide: A High–Throughput Study in Supercritical Conditions, ChemSusChem, 2011, 4, 1830–1837.
113. R. Li, W. Zhang and K. Zhou, Metal-organic–framework–based catalysts for photoreduction of CO₂, Adv. Mater., 2018, 30, 1705512.
114. C. Calabrese, F. Giacalone and C. Aprile, Hybrid catalysts for CO₂ conversion into cyclic carbonates, Catalysts, 2019, 9, 325.
115. P. Agrigento, S. M. Al-Amsyar, B. Sorée, M. Taherimehr, M. Gruttadauria, C. Aprile and P. P. Pescarmona, Synthesis and high-throughput testing of multilayered supported ionic liquid catalysts for the conversion of CO₂ and epoxides into cyclic carbonates, Catal. Sci. Technol., 2014, 4, 1598–1607.
116. M. Zhou, S. Wang, P. Yang, C. Huang and X. Wang, Boron carbon nitride semiconductors decorated with CdS nanoparticles for photocatalytic reduction of CO₂, ACS Catal., 2018, 8, 4928–4936.
117. S. W. Cao, X. F. Liu, Y. P. Yuan, Z. Y. Zhang, Y. S. Liao, J. Fang and C. Xue, Solar-to-fuels conversion over In₂O₃/g-C₃N₄ hybrid photocatalysts, Appl. Catal., B, 2014, 147, 940–946.
118. H. Shi, G. Chen, C. Zhang and Z. Zou, Polymeric g-C₃N₄ coupled with NaNbO₃ nanowires toward enhanced photocatalytic reduction of CO₂ into renewable fuel, ACS Catal., 2014, 4, 3637–3643.
119. R. Bhosale, S. Jain, C. P. Vinod, S. Kumar and S. Ogale, Direct Z-scheme g-C₃N₄/FeWO₄ nanocomposite for enhanced and selective photocatalytic CO₂ reduction under visible light, ACS Appl. Mater. Interfaces, 2019, 11, 6174–6183.
120. T. Di, B. Zhu, B. Cheng, J. Yu and J. Xu, A direct Z-scheme g-C₃N_4/SnS₂ photocatalyst with superior visible-light CO₂ reduction performance, J. Catal., 2017, 352, 532–541.
121. N. Nie, L. Zhang, J. Fu, B. Cheng and J. Yu, Self-assembled hierarchical direct Z-scheme g-C₃N₄/ZnO microspheres with enhanced photocatalytic CO₂ reduction performance, App. Surf. Sci., 2018, 441, 12–22.
122. Y. He, L. Zhang, B. Teng and M. Fan, New application of Z-scheme Ag₃PO₄/g-C₃N₄ composite in converting CO₂ to fuel, Environ. Sci. Technol., 2015, 49, 649–656.
123. L. Cheng, Q. Xiang, Y. Liao and H. Zhang, CdS-based photocatalysts, Energy Environ. Sci., 2018, 11, 1362–1391.
124. M. A. Correa-Duarte, M. Giersig and L. M. Liz-Marzán, Stabilization of CdS semiconductor nanoparticles against photodegradation by a silica coating procedure, Chem. Phys. Lett., 1998, 286, 497–501.
125. Q. Li, X. Li, S. Wageh, A.-A. Al-Ghamdi and J. Yu, CdS/graphene nanocomposite photocatalysts, Adv. Energy Mater., 2015, 5, 1500010.
126. Y. Ding, C. Wang, J. Zhong, Q. Mao, R. Zheng, Y. H. Ng, M.-H. Sun, S. Maitra, L.-H. Chen and B.-L. Su, Three-dimensionally ordered macroporous materials for pollutants abatement, environmental sensing and bacterial inactivation, Sci. China: Chem., 2023 DOI:10.1007/s11426-023-1574-1.
127. L. Wang, B.-B. Zhang, X.-Y. Yang and B.-L. Su, Alginate@ polydopamine@SiO₂ microcapsules with controlled porosity for whole-cell based enantioselective biosynthesis of (S)-1-phenylethanol, Colloids Surf., B, 2022, 214, 112454.
128. L. Wang, Y. Li, X. Y. Yang, B. B. Zhang, N. Ninane, H. J. Busscher, Z. Hu, C. Delneuville, N. Jiang, H. Xie, G. Tendeloo, T. Hasan and B.-L. Su, Single-cell yolk-shell nanoencapsulation for long-term viability with size-dependent permeability and molecular recognition, Natl. Sci. Rev., 2021, 8, nwaa097.
129. P. Xu, M. L. Janex, P. Savoye, A. Cockx and V. Lazarova, Wastewater disinfection by ozone: main parameters for process design, Water Res., 2022, 36, 1043–1055.
130. A. Zhang, H. Xie, N. Liu, B.-L. Chen, H. Ping, Z.-Y. Fu and B.-L. Su, Crystallization of calcium carbonate under the influences of casein and magnesium ions, RSC Adv., 2016, 6, 110362–110366.
131. A. Simpson and W. Mitch, Chlorine and ozone disinfection and disinfection byproducts in postharvest food processing facilities: A review, Crit. Rev. Environ. Sci. Technol., 2022, 52, 1825–1867.
132. C. Regmi, B. Joshi, S. K. Ray, G. Gyawali and R. P. Pandey, Understanding mechanism of photocatalytic microbial decontamination of environmental wastewater, Front. Chem., 2018, 6, 33.
133. O. K. Dalrymple, E. Stefanakos, M. A. Trotz and D. Y. Goswami, A review of the mechanisms and modeling of photocatalytic disinfection, Appl. Catal., B, 2010, 98, 27–38.
134. S. H. Xue, H. Xie, H. Ping, Q. C. Li, B.-L. Su and Z.-Y. Fu, Induced transformation of amorphous silica to cristobalite on bacterial surfaces, RSC Adv., 2015, 5, 71844–71848.
135. W. Wang, G. Huang, C. Y. Jimmy and P. K. Wong, Advances in photocatalytic disinfection of bacteria: development of photocatalysts and mechanisms, J. Environ. Sci., 2015, 34, 232–247.
136. J. Deng, J. Liang, M. Li and M. Tong, Enhanced visible-light-driven photocatalytic bacteria disinfection by g-C₃N₄-AgBr, Colloids Surf., B, 2017, 152, 49–57.
137. X. Geng, L. Wang, L. Zhang, H. Wang, Y. Peng and Z. Bian, H₂O₂ production and in situ sterilization over a ZnO/g-C₃N₄ heterojunction photocatalyst, Chem. Eng. J., 2021, 420, 129722.
138. H. Gourama and L. B. Bullerman, Aspergillus flavus and Aspergillus parasiticus: Aflatoxigenic fungi of concern in foods and feeds: A review, J. Food Prot., 1995, 58, 1395–1404.
139. B. B. Zhang, L. Wang, V. Charles, J. C. Rooke and B.-L. Su, Robust and biocompatible hybrid matrix with controllable permeability for microalgae encapsulation, ACS Appl. Mater. Interfaces, 2016, 8, 8939–8946.
140. D. Sun, J. Mao, L. Cheng, X. Yang, H. Li, L. Zhang, W. Zhang, Q. Zhang and P. Li, Magnetic g-C₃N₄/NiFe₂O₄ composite with enhanced activity on photocatalytic disinfection of Aspergillus flavus, Chem. Eng. J., 2021, 418, 129417.

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