Nanostructured materials based on g-C3N4 for enhanced photocatalytic activity and potentials application: A review

Muhammad Sohail; Usama Anwar; T.A. Taha; H.I.A. Qazi; Abdullah G. Al-Sehemi; Sami Ullah; Hamed Algarni; I.M. Ahmed; Mohammed A. Amin; Arkom Palamanit; Waseem Iqbal; Sarah Alharthi; W.I. Nawawi; Zeeshan Ajmal; Hamid Ali; Asif Hayat

doi:10.1016/j.arabjc.2022.104070

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Review article

09 2022

:15;

104070

doi:

10.1016/j.arabjc.2022.104070

Nanostructured materials based on g-C₃N₄ for enhanced photocatalytic activity and potentials application: A review

Muhammad Sohail^{^a}, Usama Anwar^{^b}, T.A. Taha^{^c,^d}, H.I.A. Qazi^{^e}, Abdullah G. Al-Sehemi^{^f,^g}, Sami Ullah^{^f,^g}, Hamed Algarni^{^f,^h}, I.M. Ahmed^ⁱ, Mohammed A. Amin^{^j}, Arkom Palamanit^{^k}, Waseem Iqbal^{^l}, Sarah Alharthi^{^j}, W.I. Nawawi^{^m}, Zeeshan Ajmal^ⁿ, Hamid Ali^{^o}, Asif Hayat^{^p,⁎}

a Yangtze Delta Region Institute (Huzhou), University of Electronic Science and Technology of China, Huzhou 313001, China

b Soochow Institute for Energy and Materials Innovations, College of Energy, Soochow University, Suzhou 215006, China

c Physics Department, College of Science, Jouf University, P.O. Box 2014, Sakaka, Saudi Arabia

d Physics and Engineering Mathematics Department, Faculty of Electronic Engineering, Menoufia University, Menouf 32952, Egypt

e College of Optoelectronic Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China

f Research Center for Advanced Materials Science (RCAMS), King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia

g Department of Chemistry, College of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia

h Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia

i Chemistry Department, College of Science, Jouf University, P.O. Box 2014, Sakaka, Saudi Arabia

j Department of Chemistry, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia

k Energy Technology Program, Department of Specialized Engineering, Faculty of Engineering, Prince of Songkla University, 15 Karnjanavanich Rd., Hat Yai, Songkhla 90110, Thailand

l Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Cosenza, Italy

m Faculty of Applied Sciences, Universiti Teknologi MARA, Cawangan Perlis, 02600 Arau Perlis, Malaysia

n School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, Xian 710072, China

o Multiscale Computational Materials Facility, Key Laboratory of Eco-Materials Advanced Technology, College of Materials Science and Engineering, Fuzhou University, Fuzhou 350100, China

p State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, China

⁎Corresponding author. asifncp11@yahoo.com (Asif Hayat)

Received: 2022-1-16, Accepted: 2022-6-21,

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Semiconductor-based photocatalytic technology is regarded as an efficient pathway for resolving the energy scarcity across the globe. In this regard, graphitic carbon nitride (g-C₃N₄)-based materials could be alternatively employed in photochemical applications such as photovoltaic energy generation via CO₂ photoreduction and water splitting, along with natural resource purification via organic/inorganic pollutant degradation. Indeed, this kind of assertion has been made by considering the intrinsic physicochemical properties of g-C₃N₄ nanomaterials, owing to their increased surface area, quantum yield, surface charge isolation, distribution, and ease of modification through material configuration or incorporation of preferred interfacial capabilities. This review article has been designed to provide the most up-to-date information regarding the further assessment of the important advancements in fabrication along with photochemical applications of various g-C₃N₄ nanomaterials, while specifically focusing on the scientific reason behind its success in each assessment. The discovery of interventions to alleviate such restrictions and boost photocatalytic performance has gained substantial interest. Following photo-excitation fundamentals, this work explains two distinct photoexcitation mechanisms, the carrier and charge transfer techniques, wherein the significant exciting state impact of g-C₃N₄ has still not been widely focused on in past studies. In this regards, we cautiously introduce the updated advances and associated functions of the alteration techniques, including morphological features, elemental dopants, deficiency engineering, and heterojunction implemented in photocatalytic performance, which are equated from the carrier and charge transport perceptions. The future perspectives in designing and properly tuning the highly active hierarchical or copolymer g-C₃N₄ nanoparticles in a photocatalytic system, which may improve the renewable energy cultivation and reduction efficiency are critically deciphered in detail and outlined thoroughly.

Keywords

g-C3N4

Photocatalytic water splitting

Photocatalytic CO2 reduction

Various dimensions

Physiochemical properties

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1 Introduction

1.1

1.1 Background

The utilization of clean and renewable energy is critical for fulfilling the growing global energy demands and resolving the fiscal energy problems caused by excessive consumption of fossil fuels (Child, 2018; Wang, 2018; Wei, 2018; Sohail, 2017; Sohail, 2018; Qadeer, 2020; Wageh, 2021; Wageh, 2021 ). With the development of science and technology, solar energy is an inexhaustible source of renewable energy on the earth, and has the extensive potential to overcome all of these limitations precincts (Faunce, et al., 2013; Mills et al., 1993; Qadeer, 2022; Hao, 2018; Wageh and Zhao, 2019; Swelm, 2018; Wageh, 2018; Wageh et al., 2016; Wageh, 2016 ) and its further use for the attended scientific consensus, since Honda and Fujishima used semiconductor based materials for water splitting in 1972 (Fujishima and Honda, 1972; Inoue, 1979). One such discovery, sparked an extensive research interest into the titanium dioxide (TiO₂) based photocatalysts, which are being widely used for the efficient photocatalytic CO₂ reduction, water splitting, pollutants degradation, sensors, photoelectronics, photobacterial agents, etc. under solar light irridiation (Yi, 2014; Gaya and Abdullah, 2008; Naciri, 2022; Naciri, 2020; Naciri, 2021; Huang, 2021; Zhang, 2016; Wageh, 2015; Wageh, 2014 ). Nonetheless, the efficient photocatalytic utilization of visible light (key constituent of photovoltaic irradiance as it produces the Earth texture) in order to accelerate the oxidation process, which is still a challenging task for scientists (Asahi, 2001; Kumar and Devi, 2011; Naciri, 2020; Qadeer, 2019 ), due to huge difficulty of identifying the compounds with narrow band gaps, an enhanced electronic structure, sufficient electron transfer mobility, and low level of replication, along with excellent consistency. Various materials have been used in multiple research fields (Ajmal, 2020; Ahmed, 2021; Hsini, 2021; Hsini, 2020; Hou, 2014; Hou, 2013; Wageh et al., 2013; Wageh et al., 2013; Wageh, 2013 ). Indeed, numerous outstanding studies on fabrication and modification of graphitic carbon nitride (g-C₃N₄)-based photocatalysts, as well as their uses in energy and ecological challenges, are already accessible. Only a few papers, however, have discussed on the varied features of g-C₃N₄-based photocatalyst rational design and its impact on the activity of the catalyst. As a result, it is relevant to provide a complete review of the current advancements in g-C₃N₄-based catalysts for photocatalysis and environmental remediation. We focus on the basics, diverse characteristics, design, and possible uses of g-C₃N₄ based photocatalysts in this review. We assume that this review will encourage the synthesis of state-of-the-art g-C₃N₄ based photocatalysts and designs that improve light utilization and photocatalytic performance, but will also assist with confronting the difficulties that g-C₃N₄-based photocatalysts face in manufacturing and storage of renewable and sustainable energy. The development of such unique photocatalysts will pave the way for their extensive use to complement the sustainable energy and the contaminant monitoring methods. The main postulates of our review are manifested in the fellowing graphical abstract as shown in Fig. 1.

Scheme exhibiting the applicability of g-C3N4-based nanomaterials in photocatalysis research and the accompanying methodologies for photocatalytic efficiency improvement.

1.2

1.2 Semiconductor photocatalysis

Heterogeneous photocatalysis accelerates the chemical reactions (oxidations and reductions) caused by the stimulation of a photocatalyst, that combines with metals, non-metals, and metal oxides to facilitate the optical absorptions with consecutive energy-conversion cultivator sources. Notably, the photocatalysis process is concerned with the considerable stimulation of reagents and derivatives by optical absorption and assessing the photocatalytic conversion. Additionally, the reaction process shows, that the photocatalyst assimilates the electrons rather than the sorbent (Rochkind, 2014; Pallavi, 2010; Ullah, 2016; Sohail, 2020; Wageh, 2007 ). The photoinduced charged particles basically stimulate the photoconversion of water and CO₂ into associated renewable compounds in a photocatalytic production known as “fossil energy” (Styring, 2012; Chen et al., 2015). Such chemical reactions are comparable to natural photosynthesis (in which chloroplasts assimilate natural light in plants to encourage carbohydrate and oxygen fabrication from carbon dioxide and water) and are thus referred as artificial photosynthesis (Fig. 2). Moreover, the photoinduced electrons could also promote the total combustion (calcification) of environmental pollutants in marine ecosystems, either via a direct or indirect pathway, thereby, resulting into the formation of potential oxidizing agents such as a hydroxyl source (Nosaka and Nosaka, 2017; Wang and Domen, 2019).

(a) Natural and (b) artificial photosynthesis processes. Reprinted with permission from Wang and Domen (2019).

1.3

1.3 Methodologies of photocatalysis

The photocatalysis process is initiated with the absorption of light from the ground state (valance band) toward the excited state (conduction band) (Step I) (Wageh, 2016). The subsequent clusters of electrons and holes could be thoroughly combined within or at the photocatalyst surface, with associated fuel released by luminescence or radiative activation of the crystalline structure (Step II). The charge recombination is critical, as it limits the photocatalytic performance for photoelectron absorption. Similarly, the photoinduced electrons/holes are deflected away from the material interface without charge recombination. Various photooxidation/reduction reactions with adsorbed species, including water, oxygen, and other organic or inorganic species (Steps III and IV) (Karamian and Sharifnia, 2016; White, 2015; Aleksandra, 2014 ), are presented in Fig. 3. These processes, which are essential for photocatalytic environmental remediation and solar fuel production, are ultimately limited by the reduction potential of photoexcited electrons in the conduction band (CB) and the oxidation potential of photogenerated holes in the valence band (VB). Therefore, the redox potential, band energies, and band gap of semiconductors (SC) play a significant role in determining the possibility and rate of charge transfer, which are considered to be the critical design parameters for selecting suitable photocatalysts (Sakata and Kawai, 1983; Zou, 2001; Qadeer, 2022 ). The semiconductor photocatalysis, whose fundamental configuration and surface-electronic structure differ between materials and applications, is an interfacial interaction between electrons and holes caused by band gap stimulation.

Principal photophysical processes for semiconductors (SC) under light irradiation. Reprinted with permission from Aleksandra (2014).

1.4

1.4 Photocatalytic materials

The use of titanium-based electrodes under UV irradiation for the overall water splitting (OWS) process has led to an extensive investigation for the evolution of H₂ energy (Park et al., 2006). TiO₂ powder was proven to be an effective catalyst for the photocatalytic decomposition of cyanide ions in water (Frank and Bard, 1977), and in the field of environmental purification. However, due to the wide band gap, recent research has focused on finding alternative semiconductors, which should provide better photocatalytic performance under solar (rather than UV) irradiation (Kubacka et al., 2016). According to the band gap of a semiconductor, a photocatalyst can be used in a variety of ways, including ZnO (Lee, 2016), Fe₂O₄ (Li, 2007), WO₃ (Wang, 2012), SrTiO3 (Kato and Kudo, 2001), NaTaO₃ (Yu et al., 2004), CdS (Jing and Guo, 2006), Ag₃PO4 (Wang, 2012), BiPO4 (Pan and Zhu, 2010), and g-C₃N₄ (Venkatathri and Kumar, 2017) (Fig. 4). Despite substantial literature on the subject, it is still difficult to use of photocatalysts in order to generate solar fuel or the decomposition of organic pollutants owing to poor light-harvesting or ineffective light energy conversion.

Band gap energy and band edge energies of different semiconductors. Reprinted with permission from Venkatathri and Kumar (2017).

1.5

1.5 Graphitic carbon nitride (g-C₃N₄)

The use of visible light photocatalysis provides the best possible way to capture the maximal solar energy owing to the dominance of three sections of the electromagnetic spectrum, UV (5 %), visible light (45 %), and IR (50 %), as manifested in (Fig. 5) respectively, (Wu, 2011). Several photocatalysts, such as TiO₂ (3.0–3.2 eV), have broad band gaps and are, therefore, only active under UV irradiation (385 nm) (Yi, 2014).

Spectral irradiance of solar light. Reprinted with permission from Wu (2011). .

There has been a rapid advancement in the number of scientific papers and patents (Kumar and Devi, 2011) dealing with photocatalytic OWS process, CO₂ reduction, and pollutants degradations (Hernández-Alonso, 2009; Sohail, 2021; Hayat, 2022 ). Fig. 6a shows that g-C₃N₄ is a promising metal-free polymeric semiconductor with a low band gap (Fig. 6b) (Wang et al., 2009), which is ideal for visible light absorption and is versatile for large-scale application. Unlike many other organic semiconductors, the g-C₃N₄ has a high thermal and chemical resistance to oxidation, even at temperatures of 500 °C, which makes it an ideal material for photovoltaic applications. The synthesis of g-C₃N₄ and its derivatives for diverse applications has been extensively documented (Vhna, 2021; Santosh, 2013; Qadeer, 2022 ). The photocatalytic applications in environmental remediation and solar fuel generation, which emphasize emerging synthetic strategies in order to improve the photoactivity of g-C₃N₄ nanostructures by controlling their size, morphology, light absorption, charge separation, and ultimately their surface reactions are covered in this review. In addition, future research directions are also highlighted in this review.

(a) g-C3N4 structure comprising melem units, and (b) UV–vis diffuse reflectance (DRS) spectrum and image (inset) of g-C3N4 (Wang et al., 2009).

1.5.1

1.5.1 g-C₃N₄ nanostructures: size and shape

There has been an increased interest in the application of nanoparticles, including electronics (Lewis and Nocera, 2006), catalysis (Rhind, 2009), biomedical (Faunce, 2013), sensing (Mills et al., 1993), and smart materials (Fujishima and Honda, 1972) throughout the science and engineering fields. The nanomaterials are distinct from their bulk precursors in some essential ways. It is due to their high surface-to-bulk atom ratio, as nanomaterials have a lower melting point and a higher solid–solid phase transition temperature. The electrical and optical properties of nanomaterials are greatly influenced by quantum confinement effects, which originate from the evolution of their band structure and the formation of atomistic-like behavior and several heterogeneous catalysts with strong size-dependencies due to quantum confinement (Inoue, 1979). For example, Gold (Fujishima et al., 2008; Ma, 2014) has a large surface area characterized by low coordination, and high energy sites exposed. Charge separation transfer from the photocatalyst surface to an adsorbed species may be accelerated, if these variables act synergistically (Gaya and Abdullah, 2008; Schneider, 2014; Habisreutinger et al., 2013; Qadeer, 2021 ). Mo et. al. investigated the effect of calcination factor onto the melamine-derived g-C₃N₄ crystalline lattice, morphological progression, and energy band construction (Mo et al., 2015). The XRD diffraction patterns revealed that g-C₃N₄ could be completely justified as the temperature was increased >500 °C (Fig. 7a). Significantly, the peaks observed of the material synthesized at 450 °C differed from another sample, indicating that the melem analogues were present. At 600 and 650 °C, the framework of g-C₃N₄ grew extremely loose, shorter, and plump, with the formation of numerous holes on the g-C₃N₄ interface (Fig. 7b, d). Conversely, Fig. 7e shows a significant red-shift in the sharp absorption edges stretching from 470 to 570 nm, as well as a color transition from yellow to deep oranges (Fig. 7e), thereby, signifying the higher light absorption for materials with rising temperatures (Mo et al., 2015). The red shift was caused by an increase in the π-plane conjugated and increased in the degree of crystallinity. According to the transitory current density responses (Fig. 7f), the g-C₃N₄ produced at 650 °C had the maximum current density with excellent repeatability and consistency, thereby, making it more suitable for photocatalytic performance.

(a) XRD patterns, TEM at (b) 550 °C, (c) 600 °C, and (d) 650 °C. (e) UV–vis DRS and (f) transitory current densities responses of g-C3N4 produced at various annealing temp. The color of the g-C3N4 photocatalysts produced at distinct temps is shown in the inset of (e). Reprinted with permission from Mo et al. (2015).

Despite, several efforts have been made in order to exfoliate the bulk g-C₃N₄ into sheets morphology, but yet there is still a challenging task for achieving a good yield, precise morphology, and ultrathin nanosheets, thereby, necessitating further research in this field (Ong et al., 2016). The g-C₃N₄ has transformed into nanosheets (three to two dimensional), which results in the transformation of various morphologies like nanorods/nanotubes (one dimensional) and quantum dots (zero-dimensional) aimed at industrial applications and energy harvesting (Ong et al., 2016). In this context, the g-C₃N₄ was obtained using a complicated, soft, and self-templating technique, and quantum dots were generated using a hydrothermal etching technique. From a salt-template assisted process, the honeycomb-structured g-C₃N₄ remained effectively organized (Ong et al., 2016; Naseri, 2017). Zhao et. al. proposed the current trends in merging 0D with 2D g-C₃N₄ to form 0D/2D composites with enhanced photocatalytic performance (Zhao, 2018). Wang et. al. (Wang, 2017) compared 2D/2D interface of g-C₃N₄ with other 2D nanomaterials. Unquestionably, heterojunctions such as type II, Schottky, and Z-schemes improve photo-driven reactions by reducing the charge diffusion paths, and decreasing photogenerated charge recombination rates, along with increasing the surface area of the photocatalyst (Wang, 2018).

1.6

1.6 Elemental doping

In additional to the structural regulation outlined previously, elemental doping (both non-metal and metals) is another efficient approach of alteration (Khan, 2020; Raziq, 2020; Ullah, 2020; Shaishta, 2020; Rehman, et al., 2019; Khan, 2020; Hayat, 2019; Hayat, 2020; Hayat, 2019; Majeed, 2021; Rahman and Hayat, 2019; Hayat and Li, 2019; Hayat, 2019; Khan, 2019; Hayat, 2019; Hayat, 2019; Khan, 2018 ). By inserting heteroatoms, the Eg values of the compound materials may be modified to enhance the photo response range (Fig. 8a), and the resistance between the layers could be reduced to speed up the separation of charge, thereby enhancing the photocatalytic performance of g-C₃N₄ (Hasija, 2019; Liu et al., 2021; Hayat, 2022; Kongto, 2022; Alenad, 2022; Hayat, 2021; Hayat, 2021; Rehman, 2021 ). Fig. 8b depicts the proportions of distinct elemental doping (Hasija, 2019; Li, 2022). Notably, metal and nonmetal doping have markedly differing impacts on the corresponding photoexcitation processes.

(a) Band configurations for pristine and modified g-C3N4 and (b) a description of the ratios of several modified g-C3N4 photocatalysts. Reprinted with permission from Hasija (2019)). Copyright © 2019, Elsevier.

1.6.1

1.6.1 Metal doping

Metal doping consists of precious metals/non-metals (such as alkali elements and transition elements) co-doping and single doping (Hayat, 2022; Pan, 2022; Taha et al., 2021; Sohail, 2021; Hayat, 2022 ). In particular, metals could be incorporated within g-C₃N₄ framework by calcining or hydrothermally method of their main precursors. The doping positions of the dopants are determined by the size of the heteroatoms and their length from or within the substrate plane. For metal doping, orbital hybridization could happen among the g-C₃N₄ doped orbital and molecular orbital, producing additional energy levels to reduce the E_g value and increase the optical absorption. Furthermore, additional defective phases could be incorporated into electronic frameworks, resulting in greater active sites and a stronger e^–/h⁺ separation and transport. Wang and coworker synthesized K-doped ultrathin porous g-C₃N₄ by hydrothermal crystallization for degradation of TC (Wang et al., 2018). X-ray photoelectron spectroscopy (XPS) measurements revealed that K has been effectively added into the g-C₃N₄ structure by creating K-N bonds (Fig. 9a). Furthermore, K-doped g-C₃N₄ demonstrated improved separation and transfer rates, that resulted in increased the photocatalytic activity. Moreover, porous design can supply additional active sites, that can reduce the charge recombination even more. As a result, K-doped g-C₃N₄ had the greatest degradation performance, which was roughly 2.88 times greater than pure g-C₃N₄. Yan et. al. studied the impact of Na, K, Ca and Mg doping on the morphology and photocatalytic activity of g-C₃N₄ (Yan et al., 2018). The metal doping discussed earlier caused by the modifications in the electronic structure of g-C₃N₄ (Fig. 9b), that were ascribed due to electrons supplied by metal to the nitrogen atom or those escaping from the surface of g-C₃N₄. Additionally, Na, Ca, Mg and K were introduced into g-C₃N₄ framework by creating cationic covalent bonds or by producing oxides with g-C₃N₄. This finally increased the degradation rate of TC. Metal doping impacts not just carrier mechanisms as well as the exciton behaviors of g-C₃N₄. For instance, Shu and coworkers inserted Na⁺ as impurities into mesoporous g-C₃N₄ nanosheets (referred as Na-MCN) by calcining a solution of sodium chloride and dicyandiamide and examined the carriers and excitons within the resulting material. The lower fluorescence intensity of the above material suggested a lower charge recombination rate. In addition, the strong photocurrent response and low impedance circle size of Na-MCN validated the enhanced exciton dissolution and carrier transfer performance. The aforementioned findings were ascribed to the production of cyano groups in the fundamental g-C₃N₄ structure by sodium ion doping, the insertion of sodium sites, and morphological alteration inside and among the unit layers.

(a) Illustration of modified K ions in the g-C3N4 layer. Reprinted with permission from Wang et al. (2018). ; (b) Band configuration of modified g-C3N4. Reprinted with permission from Zhang (2021). Copyright © 2021, Elsevier; (c) Proposed P loaded sites within the g-C3N4 skeleton. Reprinted with permission from Mao (2018). Copyright © 2019, Elsevier; (d) framework of pristine g-C3N4 and O/g-C3N4 after activation. Reprinted with permission from Yuejie (2019). Copyright © 2019, Elsevier.

1.6.2

1.6.2 Non-metal doping

Despite metal doping could significantly enhance the efficiency of g-C₃N₄ by regulating its band gap, constraints its photo-corrosion, elevated the charge recombination but expensive cost were observed. Non-metallic atoms could be integrated within the skeleton of g-C₃N₄ by using thermal polymerization or the hydrothermal technique for precursors mixture. Due to their higher electronegativity and ionization potential, non-metallic atoms can readily form covalent bonds by gaining electrons (Nor, 2019; Hayat, 2022; Hayat, 2022; Hayat et al., 2022; Hayat, 2022; Hayat, 2022; Hayat, 2021; Hayat, 2021; Hayat, 2021 ). In comparison to metallic doping, non-metal doping could avoid the above drawbacks and achieve the identical results as of metal doping, while maintaining the non-metallic properties of g-C₃N₄. Furthermore, contrary to metallic doping, non-metal doping is mostly accomplished through interatomic displacement (i.e., C, H and N) (Li, 2009; Hamid, 2020; Raziq, 2020; Khan, 2021 ). This interatomic replacement could have a considerable effect on carbon and nitrogen hybridization, influencing the electrical and optical performances of doped g-C₃N₄ (Jh et al., 2020; Hayat, 2022; Ghufran, 2021 ). Zhou and coworkers developed a phosphorus (P) doped g-C₃N₄ by thermal copolymerization for degradation of RhB dye (Zhou, 2015). P was effectively doped into g-C₃N₄ structure by replacing two distinct carbon sites to generate P–N or P––N bonds, resulting in more defects and greater active sites. The addition of P can facilitate the separation of charges even more, leading to the formation of a P⁺ center, that acts as a Lewis acid site. As a result, the Rhodamine B degradation efficiency of P-doped g-C₃N₄ enhanced 2.9-fold, when compared to pure g-C₃N₄. Despite this, Wu and coworkers discovered that P was exclusively doped into g-C₃N₄ skeleton via P-N bonds (Fig. 9c), and no P-N bond was identified (Mao, 2018). This meant that the exact doping location of P might alter based on the synthesis conditions. Recent studies have shown that inserting an oxygen (O) atom, which has merely an extra electron than nitrogen atom, enables an extremely effective substitution process. Zhang and coworkers created the double oxygen doped g-C₃N₄ (OPCN) materials through easy thermal copolymerization (Yuejie et al., 2019). They used DFT to determine the optimum oxygen replacement sites and discovered that the inserted oxygen atoms preferred to simultaneously substitute both the sp²-hybridized nitrogen atoms at the para-position of melem unit (Fig. 9d). In this situation, a delocalized arrangement developed on the OPCN surface, allowing carriers to migrate, while efficiently preventing electrons/holes recombination. Furthermore, under VL, the removal performance of the optimized OPCN for several organic contaminants was enhanced, and the photocatalytic performance for BP was roughly 9 times than pure g-C₃N₄ under solar light illumination.

1.7

1.7 Defect engineering

In contrast to the approaches described earlier, adding vacancies defects in g-C₃N₄ structure is an excellent approach for modifying the thermodynamic properties, kinetics, and mechanism of photocatalytic processes (Taha et al., 2021; You, 2017; Hayat, 2021; Khan, 2021; Ullah, 2021 ). Vacancy defects have following impacts on g-C₃N₄ photocatalytic efficiency. (1) capturing and storing electrons, leading to separation of charge; (2) Improvement of the ability to capture light and extension of the light response edge into the near-IR region through defect state; (3) Modification of the adsorption properties of the specific contaminant, hence enabling the targeted adsorption of various contaminants by g-C₃N₄; (4) The concentration, geometry, and position of vacancy defects all have an impact on the characteristics of g-C₃N₄. As a result, investigating vacancy flaws, particularly N and C vacancies, is crucial.

1.7.1

1.7.1 N-vacancy defects

N-vacancies may develop the electronic structural distortions, resulting in disrupted energy areas and aiding the exciton dissociation and transmission (Yw et al., 2020). Hydrogen bonds serve as strands in polymeric melon units having amino groups (NH/NH₂) thus, g-C₃N₄ having N vacancy might be generated by removing the amino groups by means of high calcination. Zhou and colleagues discovered that difference of energy at the disordered junction induced by a N vacancy is at least 0.35 eV, that is enough to split singlet excitons having Eb of 0.2 eV into free holes and electrons (Zhou, 2020). These available electrons are used to produce activate oxygen, resulting in a significant amount of OH ions, which gives superior photocatalytic performance of the modified g-C₃N₄. Wang and coworkers created an N vacancy containing g-C₃N₄ with increased crystalline phase by using oxalyl dihydrazide (ODH) as an atmospheric modifier, which they named ODH-CNX can be seen in Fig. 10a (Yw et al., 2020). DFT simulations revealed that lowest occupied molecular orbital (LUMO) of ODH-CNX tilted toward the C-C bond, when contrasted to the electronic distribution of pure g-C₃N₄, resulting in non-coplanar HOMO and LUMO, that were advantageous for carrier production (Fig. 10b). Furthermore, the insertion of N vacancy resulted in a 0.13 eV reduction in the Eg value. Furthermore, the experimental findings revealed that crystalline structure and N vacancies improved charge transfer, exciton dissociation and VL capture. Furthermore, N vacancy promotes charge separation and transfer. Cao et. al. synthesized g-C₃N₄ having rich N vacancies (NV-g-C₃N₄) by processing melamine in a nitrogen environment and used it for carbamazepine degradation. (Cao, 2019; Li, 2020). The findings indicated that the deterioration rate of NV-g-C₃N₄ enhanced towards 270 % in comparison with bulk g-C₃N₄, that was due to capturing of electrons by N vacancies, hence limiting charge recombination. Liang et. al., examined the relation between degradation efficiency and N-vacancies (Liang, 2019). The developed catalysts greatly improved photocatalytic efficiency, because negative shift occurs in CB region of g-C₃N₄ due to N vacancies and improved both photo response and separation of charges.

(a) Structures description of ODH-CNX with N defects, and (b) the best systemic electronics pattern of CN and ODH-CNX, and the predicted potential levels. Reprinted with permission from Yw et al. (2020) Copyright © 2020, Elsevier; (c) Pathway of g-C3N4 activation upon C vacancies alteration. Reprinted with permission from Liang et al. (2018).

1.7.2

1.7.2 C-vacancy defects

Multiple investigations have shown that introducing C-defects into g-C₃N₄ framework is a very attractive technique for optimizing carrier separation/transport (Dong, 2017). In general C vacancies could be produced by heating g-C₃N₄ in different environment such as ammonia (Liang, 2016; Ge, 2020), carbon dioxide (Ho, 2018), or argon atmosphere (Dong, 2016), etc. Liang and coworkers created C-vacancy modified g-C₃N₄ (VC–C₃N₄) using a simple two-step calcination procedure for increased photocatalytic performance (Liang, et al., 2018). C-vacancies can trap electrons, limit charges recombination, and transmit captured electrons to absorbed oxygen, boosting O₂ production (Fig. 10c). Wang et. al. demonstrated that photocatalytic efficiency of C-vacancy integrated g-C₃N₄ was 1.65 times greater than pure g-C₃N₄ (Wang, 2021). The researchers created oxygen doped g-C₃N₄ having C-vacancy (VC-OCN) and utilized it to degrade atrazine (ATN) and p-nitrophenol (PNP). The C-vacancies concentration might be altered by adjusting the formaldehyde concentration. Likewise, the enhanced photocatalytic effectiveness of VC-OCN was correlated to the C vacancies, that suppressed the charge recombination and encouragement of adsorption of oxygen molecule. In the end, both C and N vacancies cause the Eg value of g-C₃N₄ to drop, because of its impurity state (Li, 2020)^. The CB of g-C₃N₄ is connected with the 2p orbital of carbon, and the C-vacancy causes the CB to shift positive (Li, 2016), so the intermediate gaps occur below the CB. In contrary, the VB of g-C₃N₄ is connected to the 2p orbital of nitrogen, and the N vacancy produces a negative shift in the VB, allowing the intermediate gap to form above the VB. Insufficient vacancies have little influence on the electronic configuration, while an excess of vacancies may act as carrier recombination centers, thus lowering its efficiency.

1.8

1.8 Heterojunction building

g-C₃N₄ has a multilayer structure that can be combined with other semiconductors substances. For carrier excited photocatalysis, heterostructures formed by intimate interaction between different substances have the following advantages: First, photoinduced electron hole pair could be spatially separated due to the of two distinct semiconductors heterojunction (Kongto, 2022; Zhang, 2021; Hayat, 2021; Rehman, 2021; Uddin, 2021; Hayat, 2021; Arif, 2021; Hayat, 2021 ). Second the spectral response could be enhanced or the oxidation/reduction ability could be increased by combining semiconductors of various band gaps (Wang, 2021). For exciton-generated photocatalysis, the construction of a heterostructure has been demonstrated to aid in the regulation of exciton energy state and binding energy (Zhou, 2020). Furthermore, the heterojunction internal electric field can cause the reverse flow of electrons and holes, resulting in exciton dissociation. Based on how charge carrier move, heterojunction can mostly be put into three groups: traditional/conventional heterojunction (types I-III), P-N heterojunction, and Z-scheme heterojunction, as seen in Fig. 11 (Zhang, 2021).

Electron transport diagram using (a) three kinds of traditional heterostructures, (b) P–N heterostructure, and (c) three phases of Z-scheme heterostructure. Reprinted with permission from Zhang (2021). Copyright © 2021, Elsevier.

1.8.1

1.8.1 Conventional/traditional heterojunctions

1.8.1.1

1.8.1.1 Type-I and Type-III heterojunctions

In accordance with band position of the two semiconductors that make up the heterojunction. There are usually 3 kinds of traditional heterojunctions (Fig. 11a) (Zhang, 2021). On photoexcitation, the electrons in VB of these 3 kinds of heterojunctions could be stimulated into the associated CB, generating positive holes in the VB. For type-I, semiconductor A (S-A) exceeds the semiconductor B (S-B), meaning that S-A has a much more substantial redox ability than S–B. Consequently, the electrons in the CB of S-A and holes from the VB of S–B move to the CB and VB of S-B. This form of charge transport channel could lower the Eg of the composite, extending the light response range, which is helpful for photocatalytic efficiency. Consequently, both electrons and holes are saturated on S–B, preventing effective separation of photoinduced carriers. Consequently, the redox ability of composites deteriorates dramatically. In type-III, the two semiconductors are incapable of forming a heterostructure, because their energy band locations do not overlap and the photoinduced electrons and holes cannot transfer to one another.

1.8.1.2

1.8.1.2 Type-II heterojunction

Type-II heterojunctions are the extensively kind of traditional heterojunctions (Fig. 11a) (Li, 2020). Two kinds of Type-II heterojunction formed by g-C₃N₄ and semiconductors. (1) Whenever a semiconductor has a high CB value (serving as S-A), g-C₃N₄ function as S–B, and electrons on S-A transfer to the CB of g-C₃N₄, while holes gather on the VB of S-B. Ehsan et. al. developed a g-C₃N₄/ZnSe hybrid photocatalyst for high photocatalytic performance. Analysis demonstrated that type-II heterojunction developed between ZnSe and g-C₃N₄. Due to its excellent surface charge transport pathway, the composite exhibited 1.81 and 1.57 times greater efficiency than g-C₃N₄, respectively (Ehsan, 2020). (2) When the CB of semiconductors is low (functioning as S–B), g-C₃N₄ functions as S–A. In this situation, the electrons in g-C₃N₄ can move to the CB of S–B, and this is also very rare type. Zhu and coworkers synthesized g-C₃N₄/BiPO₄ heterostructure catalysts using varying concentrations of g-C₃N₄ for the degradation of TC using the ball milling technique. After 120 min of exposure to light, the optimum material had 97% degradation of the TC, and the greatly improved photocatalytic activity was closely linked to the efficient transport channel owing to the successful building of Type-II heterojunction. Hu and coworkers (Hu, 2019) obtained comparable findings when they developed carbon doped g-C₃N₄ (BCCN)/TiO₂ composites (BCCNT) for improved photocatalytic performance. The improved photocatalytic efficiency was strongly linked to the creation of a type-II heterojunction between BCCN and TiO₂. DFT simulations can be used to examine the viability of Type-II heterojunction fabrication. (Sun, 2021). Sun and coworkers used electrostatic self-assembly to create 0D SnO₂ nanoparticle composite with 2D g-C₃N₄ for Rhodamine B degradation. (Sun, 2021). The direction of charge transport was identified using DFT simulations and 3D charge density. A positive mean of electron density change was discovered on SnO₂ (1 1 0), whereas a negative mean of electron density variation was seen on g-C₃N₄ nanosheets (CNNSs), revealing that electrons concentrated on SnO₂ (1 1 0). As a result, it was verified that a type-II heterojunction can occur among SnO₂ (1 1 0) and CNNSs. The degrading performance of the material was 1.5 and 32.3 times greater than that of g-C₃N₄ and SnO₂.

1.8.1.3

1.8.1.3 P–N heterojunction

Even Type-II can help in the separation of e⁻––h⁺ pairs, by preventing the charge separation due to electrostatic repulsion. In general, the CB and VB of N-type semiconductor (S-N) are greater than a P-type semiconductor (S-P). The fermi level (EF) of semiconductors is in the middle of CB, but it is in the band gap for semiconductors. Furthermore, the EF of S–N is near to CB, whereas the EF of S-P is near to VB. Work function (Wf) is inversely proportional to the fermi level (EF). Fig. 11b depicts the charge transfer mechanism. Once S-N and S-P together, trend is towards equilibrium, because the EF values are different. Consequently, electrons moved from P-type to N-type, until their EF values achieve equilibrium. The surface of N-type and P-type semiconductor are loaded with positive and negative charges, therefore generating the internal electric field from S-N to S-P direction. The photoinduced holes and electrons are propelled in the reverse way by the internal electric field, boosting charge separation ability under solar light illumination. The creation of P-N heterojunction could be verified by analyzing the compound semiconductor M S curve, which has both positive and negative slopes (Li, 2020). For example, Babu et. al. created P-N heterojunction in boron doped g-C₃N₄ composites with BiVO₄ (BCN/BiVO₄) by deposition of N-type BiVO₄ on P-type 1 %wt boron doped g-C₃N₄ and used it to remove Cr(VI) metal (Babu, 2019). The highly efficient catalyst was 50% BiVO₄/B-doped g-C₃N₄, for enhanced photocatalytic performance. Mass spectrometry studies found that the optimum material had an inverted U-shaped curve, suggesting the existence of negative and positive slopes, revealing the existence of P-N junction. The enhanced photocatalytic capabilities of the resultant material might be attributed to two reasons: (1) P-N junction facilitates the improvement of VL absorption; (2) P–N heterojunction can help electrons/holes pair separation while also decreasing their recombination likelihood.

1.8.1.4

1.8.1.4 Z-scheme heterojunction

The Z-scheme heterojunction system, that could be separated into three generations for addressing the problem of photocatalyst redox capacity caused by P-N and Type-II was created (Fig. 11c). Bard in 1979 introduced the concept of Z- scheme heterojunction (first generation), in which the transfer of charges was accomplished via acceptor–donor electron, that was mostly used in the aqueous phases (Bard, 1979). To expand the Z-scheme system uses, Tada and coworkers in 2006 studied Z-scheme system having all Solid state phase (second generation) in which the noble metal replaced by the electron mediator (Tada, 2006). But, due to the costly nature of valuable metals and its great light absorption capability, it is not suitable for practical uses. Vast studies on mediator free photocatalysis started in 2009 to develop more cost-effective photocatalysts (QingáLu, 2009), and Yu and colleagues introduced the direct Z-scheme system in 2013 (Third generation). Yu and his team made Ag₃PO₄/AgBr/g-C₃N₄ with dual Z-scheme system for improved the photocatalytic performance of Z-scheme heterostructure (Yu, 2020). Ag₃PO₄/AgBr-20%/g-C₃N₄ was indeed the best photocatalysts, because it was able to build a dual Z-scheme junction. The photoexcited electrons present in AgBr moved to the VB of g-C₃N₄, whereas the photoexcited holes in AgBr recombined with the photoexcited electrons in the CB of Ag₃PO₄ under solar light. In comparison to the binary Z-scheme, this ternary Z-scheme increased the separation of charges and composite material performance. Furthermore, unlike in a normal heterojunction (Fig. 12a), the oxidation and reduction reactions of dual Z-scheme system happened separately in the CB of g-C₃N₄ and the VB of Ag₃PO₄ (Fig. 12b), showing that the Eg of the composite rose, hence enhancing photocatalytic efficiency.

Proposed pathway employing Ag3PO4/AgBr-20 % g-C3N4: Traditional heterostructure (a) and double Z-scheme heterostructure (b). Reprinted with permission from Yu (2020). Copyright © 2020, Elsevier.

1.8.1.5

1.8.1.5 Isotype heterojunction

Furthermore, g-C₃N₄ may also produce an isotype heterojunction by forming heterojunctions with other semiconductors. It is widely recognized that g-C₃N₄ could be synthesized through condensation of precursors (urea, thiourea and melamine). g-C₃N₄ having varied band gap configurations could be synthesized from several precursors. The isotype heterostructure g-C₃N₄/g-C₃N₄ like Type-II (Wang, 2017), Type-I (Dong, 2015), P-N type (Liu, 2016), N-N type (Zhou, 2017), could be prepared by various precursors. For instance, Xu and coworkers used thermal copolymerization to synthesized 3D g-C₃N₄ heterojunction blocks by assembling nanowires and nanosheets building blocks (Fig. 13a) (Xu, 2019). The hierarchical porous framework created by combining nanowires and nanosheets boosted the utilization of light dramatically. Concurrently, the widespread Type-II isotype heterojunctions created an internal electric field, that aided separation of carrier and exciton dissociation (Fig. 13b). Consequently, the photocatalytic efficiency of generated 3D g-C₃N₄ units rose by >10 folds.

Schematics of (a) the fabrication of 3D microstructural g-C3N4 with isoform heterostructures and (b) the isoform heterostructure fabricated at the interface region of g-C3N4 nanostructure and nanorods. Reprinted with permission from Xu (2019).

In particular, the isotype heterostructure could also be created by altering the crystalline nature of g-C₃N₄. Wang and coworkers synthesized the semi crystalline g-C₃N₄ by chloride assisted hydrothermal treatment and they got Type-II isotype heterojunction at disordered-ordered interface (Fig. 14a–b) (Wang, 2017). The rapid passage of electrons at crystalline units and the blockage of holes in networks having poor crystallinity enabled for efficient rise in exciton dissociation at the junction.

(a) HR-TEM photograph of semicrystalline g-C3N4, and (b) Type-II isoform heterostructure created at the orderly disorderly junction and modelled using DFT analysis. Reprinted with permission from Wang (2017).

2 2-Dimensional g-C₃N₄

2D-based materials have an extensive specific surface area, good crystallinity, numerous choices for host–guest interactions, maximum light absorption, and enhanced charge carrier separation due to their 3D analogues (Wang, 2012). Recently, g-C₃N₄ has become one of the most promising heterogeneous photocatalysts in the 2D nanomaterials field. To synthesized g-C₃N₄ nanosheets, the Ping and his colleagues proposed a simple approach called “etching” by annealing of bulk g-C₃N₄ in the air directly (Kato and Kudo, 2001). The bulk g-C₃N₄ can be reduced to the appropriate nanoscale thickness using this approach, which removes hydrogen-bonding interlayers through one oxidation, representing an easy, inexpensive and scalable synthesis. A blue shift in the intrinsic absorption edge of the UV–vis spectra is observed in resulting nanosheets compared to the bulk (Fig. 15) (Yu et al., 2004).

Thermal exfoliation is a low-cost and green method to prepare ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water. Reprinted with permission from Niu (2012).

Similarly, the blue shift in the fluorescence emission spectra at 20 nm further confirms the increase in the band gap of nanosheets (2.97 eV), compared to their bulk intermediates (2.77 eV). The band gap has been larger, because of quantum confinement (Niu, 2012). It was found that the pristine g-C₃N₄ nanosheets exhibited semiconducting qualities, implying that the electron transport takes place within the nanosheet plane by analyzing their IV curves for related electronic properties. The bulk g-C₃N₄ had low electrical conductivity, as evidenced by the absence of any current, when a bias range of −10 to 10 V was applied. Furthermore, the lifetime of charge carriers in the nanosheets using time-resolved fluorescence decay spectra exceeded from that of bulk g-C₃N₄. According to Xiadong and co-workers, a new liquid exfoliation strategy has been proposed as a low-cost and environmentally friendly method in order to fabricate the ultrathin nanosheets from bulk g-C₃N4 (Rochkind et al., 2015). Through the liquid exfoliation route, ultrathin nanosheets of g-C₃N₄ were designed, which may be due to their high polarity. The exfoliated g-C₃N₄ revealed free-standing nanosheets at 120 nm wide that were practically transparent and showed a well-defined Tyndall effect. The photoluminescence spectra (Pl) of these g-C₃N₄ nanosheets were highly stable, when exposed to an acidic or alkaline environment. However, it was dependent on PH of material. The g-C₃N₄ nanosheets exhibit a superior photo-absorption over the bulk counterpart, leading to an unusually high PL quantum yield of up to 19.6 %. Researchers found that liquid exfoliation of g-C₃N₄ led to nanosheets with better photocatalytic efficiency for the breakdown of organic contaminants than the bulk g-C₃N₄ (Fig. 16) (Barbero and Vione, 2016; Styring, 2012; Zhang, 2013 ).

Liquid exfoliation route as a low-cost and green method to prepare the ultrathin g-C3N4 nanosheets from bulk g-C3N4 in water. Reprinted with permission from Zhang (2013).

2.1

2.1 1-Dimensional g-C₃N₄

One-dimensional nanostructures have recently gained immense interest as photocatalysts, due to their distinctive morphology and photophysical properties (Listorti et al., 2009; Chen et al., 2015; Bai, 2013 ). The nanorods with variable aspect ratios were synthesized by refluxing g-C₃N₄ nanoplates in different solvents and variations of time (Nosaka and Nosaka, 2017). An exfoliation process and subsequent re-growth of individual nanosheets resulted into the 'rolling up' nanorods from g-C₃N₄ nanoplates (Bai, 2013). This study investigated the photocatalytic activity of the as-prepared nanorods for the degradation of methylene blue (MB) under visible light (>420 nm). These nanorods were shown to be 50–100 % more active and stimulated under visible and solar light as compared to g-C₃N₄ nanoplates. Zhihong and co-workers demonstrated that the reactive thermolysis of mechanically activated molecular precursors, such as C₃N₆H₆ and CN₃Cl₃, under heat treatment led to the large-scale production of well-aligned g-C₃N₄ nanorods (Kubacka et al., 2012). These PL emission and UV–vis absorption of nanorods demonstrate their unique optical characteristics. A template of monodispersed, chiral, mesostructured silica nanorods was easily generated via ammonia-catalyzed hydrolysis of tetraethyl orthosilicate with F127 cetyltrimethylammonium bromide (CTAB) surfactants in order to construct the uniform g-C₃N₄ nanorods. The hexagonal mesostructured porous silica nanorods allowed g-C₃N₄ condensation within the pores (Frank and Bard, 1977). The photocatalytic activity of the g-C₃N₄ nanorods was found to be significantly higher than that seen in the presence of triethanolamine and 1 % of Pt (Lee, 2016) as cocatalysts. Nanorods made from hydrous melamine nanofibers precipitated from an aqueous solution of melamine can also be synthesized by direct calcination (Li, 2007). The as-synthesized materials demonstrate high photocatalytic performance due to high interfacial area supplied by porosity. For hydrogen (H₂) evolution, a visible light photocatalytic activity in the presence of triethanolamine was enhanced by adding oxygen atoms to the g-C₃N₄ matrix, which resulted in a more effective separation of electron/hole pairs. Similarly, a simple wet-chemical approach was also reported for the synthesis of nanofiber-like g-C₃N₄ structures with an average diameter of few nm and a length of 100 nm (Wang, 2012)., The nanofibers of g-C₃N₄ showed a slight blue shift of 0.13 eV compared to that of bulk g-C₃N₄, due to better packing, electrical coupling, and quantum confinement effects. The g-C₃N₄ nanofibers had a substantially greater photocatalytic activity than bulk g-C₃N₄ and were more stable for Rhodamine B photodegradation. The g-C₃N₄ nanotubes can be synthesized by heating melamine in a compact arrangement that favors tubular forms (Fig. 17a, d) (Wang, 2014). Commercial, low-cost, and large-scale applications were possibly made due to lack of organic templates in this method. There is potential for blue light fluorescence in this newly synthesized material, as it exhibits strong fluorescence at approximately 460 nm. The visible-light photocatalytic activity of these g-C₃N₄ nanotubes was superior to that of bulk g-C₃N₄ or P25 TiO₂ as a reference (the latter is unsurprising since pure titania is a UV band gap material). Moreover, Muhammad and coworkers also used HNO₃ pre-treatment of melamine in order to fabricate tubular g-C₃N₄ (Kato and Kudo, 2001). The g-C₃N₄ nanotubes once again became capable of degrading both MB and methylene orange (MO) and were found to be more stable than bulk g-C₃N₄ under visible light. The superior activity of the nanotubes was attributed to their larger surface area (182 m²/g¹) and improved light absorption and charge separation/transfer. The g-C₃N₄ nanotubes can also be generated through rolling up nanosheets via an easy water-induced morphological transition, without the use of organic solvents and thus in support of green chemical principles (Jing and Guo, 2006).

(a–c) Synthetic strategy and corresponding (d) TEM image of g-C3N4 nanotubes. Reprinted with permission from Wang (2014).

Dicyandiamide (DCDA) and NaCl crystals have been used as structure-directing agents in order to generate the ribbon-like nanostructures of the g-C₃N₄. These ribbon-like g-C₃N₄ nanostructures have intriguing optical and electrical features, including a substantial blue shift in their absorption spectra corresponding to a rise in the band gap from 2.7 eV to 3.0 eV (Fig. 18) (Yuan, 2014). For that, a possible explanation is that the nitride pores are functionalized with cyano groups and Na⁺ ions. While bulk g-C₃N₄ emitted yellow-green light, ribbon-like g-C₃N₄ emitted blue light roughly at 400 nm under 365-nm excitation. Nevertheless, no research has been done on the potential photocatalytic properties of these ribbon-like g-C₃N₄ nanostructures.

Synthesis strategy (A) Ribbon-like g-C3N4 nanostructures (B) TEM image. Reprinted with permission from Yuan (2014).

2.2

2.2 0-Dimensional g-C₃N₄

0D materials like quantum dots are of tremendous interest (Wang, 2014). As, thermochemical etching has been used in order to fabricate the g-C₃N₄ quantum dots. In this multi-step process, the g-C₃N₄ is thermally exfoliated into 2D nanosheets, and then etched with concentrated H₂SO₄ and HNO₃ in order to form 1D nanoribbons. This process is adjustable. The functional groups, such as carboxylic acid, could be found at the margins and on the base of tri-s-triazine units, when some C–N linkages are oxidized. The preferential cleavage and the preferred orientation result in the formation of nanoribbons, that are approximately 10 nm in diameter and several hundred nanometers in length. The hydrothermal treatment converts nanoribbons into 0D quantum dots of 5–9 nm (Fig. 19) that are highly soluble in water and stable in solution for approximately eight months (Wang, 2014). For example, when 705–862 nm of light was irradiated onto a quantum dot, the emitted light was 350–600 nm in wavelength, thereby covering the whole visible spectrum. Anti-Stokes photoluminescence has been postulated as the mechanism for this up-conversion. The g-C₃N₄ quantum dots hold great promise as a photocatalytic component for long-wavelength solar energy harvesting, Since they can convert NIR to visible light. The quantum dots increased the photocatalytic H₂ generation by platinized bulk g-C₃N₄ and P25. Remark rate-enhancements (up to 52-fold) were observed for the latter under visible light irradiation in the presence of a methanol sacrificial hole scavenger. Guoping and coworkers fabricated quantum dots for two-photon fluorescence imaging of cellular nucleus (Herron, 2015). A multi-step synthesis was used comprising the acid treatment of bulk g-C₃N₄ in order to generate a porous material and ultrathin nanosheets, followed by the addition of ammonia, hydrothermal treatment, and ultrasonication of the porous g-C₃N₄ nanosheets to liberate g-C₃N₄ quantum dots.

(a) Synthesis of g-C3N4 nanosheets, and (b) TEM images of g-C3N4 quantum dots. Reprinted with permission from Wang (2014).

2.3

2.3 3-Dimensional g-C₃N₄

The 3D nanomaterials can be used in order to create highly ordered 3D nanoporous g-C₃N₄ microspheres that can be synthesized without the need of templates, to create unique and highly efficient photochemical systems. The nano-porous g-C₃N₄ micro-spheres were first made from cyanuric chloride and melamine in acetonitrile, then thermally processed at 550 °C under argon in order to turn them into hierarchical g-C₃N₄ microspheres (Fig. 20) (Gu, 2015). These highly ordered g-C₃N₄ microspheres displayed a redshift compared to their bulk counterparts, due to a higher condensation and filling among different layers inside the micro-spheres. Hydraulic charge transfer in hierarchical g-C₃N₄ was suppressed by calcination, and the photoluminescence emission intensity of the bulk and uncalcined microspheres, respectively. A narrower band gap (2.42 eV), reduced electrical resistance, and increased photo-response of these microspheres enables highly effective harvesting and charge carriers separation generated by visible light. Jinshui and co-workers also generated highy ordered g-C₃N₄ nano-spheres, constituted of nano-sheet assemblies, using high surface area of silica nanospheres as sacrificial templates (Ong, 2016).

(a) Synthetic strategy, (b) TEM image, and (c) room-temperature photoluminescence spectra of porous g-C3N4 microspheres. Reprinted with permission from Gu (2015). Copyright © 2015, Elsevier.

The silica template provides excellent cyanamide adsorption and a framework for producing the interconnected 2D g-C₃N₄ nanosheets during self-polymerization of the g-C₃N₄ polymer after heating. As a result, the silicon spheres demonstrate an outstanding thermal and mechanical durability, elevated temperature based synthesis of highly ordered g-C₃N₄ was possible, and g-C₃N₄ nanospheres might be easily detached via etching with an (NH₄HF₂) solution, while maintaining their spherical morphology. Fig. 21 shows how the highly ordered nano-spheres are fabricated from flat nano-sheets that radiate from center (sphere surface) and formerly interconnect by forming a meso-porous shell (Zhang, 2014). This structure may be advantageous in photocatalysis, because it promotes both separation of charges and transport of mass (Fig. 21). Such kind of as-synthesized nano-spheres exhibited broader band gap as compared to bulk g-C₃N₄ owing to the effect of their quantum size. Also they have higher potential of light garnering toward whole optical range, particularly between (430 and 590 nm). It might be caused by several reflections (absorption) of incident light within the highly ordered structure and due to the existence of a higher number of defective sites linked through the revelation of low-coordination positions over strident ends of the integral nanosheets at the ends of the highly ordered meso-porous structure.

(a) Synthetic strategy, and (b, c) Scanning electron microscopy and TEM images of hierarchical g-C3N4 microspheres. Reprinted with permission from Zhang (2014).

2.4

2.4 Hollow g-C₃N₄

The researchers have focused on designing and synthesizing hollow nanostructures having high intricacy by deploying their interior architecture, building units, chemical composition, and its geometry for improving the as-prepared materials performance in a wide variety of fields (Fu, 2018; Wang et al., 2012; Kizito, 2017 ). The hollow nanostructures is considered additional auspicious morphology for energy conversion and storage applications. The silica nanoparticles were used as templates in order to fabricate the hollow g-C₃N₄ nanospheres (Dong, 2014; Zhu, 2014; Sun, 2012; Kizito, 2017 ). Such hollow nanospheres are made of polymeric g-C₃N₄ and can withstand at 400 °C processing without the core–shell configuration (Fig. 22) (Sun, 2012). For example, the co-polymerization process could extend the π-system by anchoring aromatic motifs, which could red-shift the optical absorption by improving the separation of charge in shell deprived of d-blocking. This technique has turned the shell of hollow g-C₃N₄ nanospheres in order to boost the photocatalytic performance towards H₂ evolution in visible light. Young-Si et al. (Jariwala, 2013) demonstrate a direct facile chemical approach for assembling the triazine unit within the hollow g-C₃N₄ enabling the simultaneous textural tuning and photo-physical characteristics of g-C₃N₄.

Synthetic strategy for fabricating hollow g-C3N4 nanospheres. Reprinted with permission from Sun (2012).

2.5

2.5 Mesoporous g-C₃N₄

Mesporous photocatalysts have gotten a lot of research interest, because of their (relatively) high molecular mass transport in-pore, superior surface areas, higher quantum efficacy and, prospects of improved light collecting via inner dispersion of incident light. The photocatalytic efficiency for H₂ generation was demonstrated in a recently synthesized mesoporous network of g-C₃N₄ nano-sheets (Fig. 23) (Han, 2016).

(a) SEM images, (b) Sketch of photocatalytic H2 from water splitting, and (c) TEM image of atomically thin, mesoporous g-C3N4 nanosheets. Reprinted with permission from Han (2016).

2.6

2.6 g-C₃N₄ based quantum dots

In recent years, quantum dots (QD, s) sensitized semiconductors have gotten a lot of attention, because their active sites are more visible. They have a quantum confinement effect, and have a special effect at the interface (Liu et al., 2020). Research findings have shown that quantum dots sensitized could effectively change the width of the band structure and increase the light response rate of semiconductors (Liang, 2020). Moreover, small size of quantum dots may lower the diffusion time of photoexcited electrons/holes from bulk to the surface, that enhance and boost their separation/isolation ability on catalyst surfaces, and significantly increase the quantum efficiency of the whole system (Liang, 2020; Yang, 2019). Liang and coworker (Liang, 2020) synthesized the P g-C₃N₄ (P-CNT) added with CdS QDs to make a new highly efficient (CdS QDs/P-CNT) using an in-situ oil bath method. The synthesized materials demonstrate a superior performance of photocatalytic water reduction (H₂ evolution) and was more stable. It also demonstrated a significantly increased in the optical absorption of light (Fig. 24a), which was attributed to CdS broad photo absorption edge (560 nm) (Zhou, 2018), that significantly improved the photocatalytic efficiency. Wang and coworker (Wang, 2020) used ice-assisted ultrasonic approach to synthesized black phosphorus quantum dots (BPQDs) incorporated with tubular g-C₃N₄ (BPTCN) nanohybrid with a 1D tubular framework. BPQDs refelecting an average size of 3.32 nm were scattered on the tubular CN in this system (TCN). The addition of BPQDs has greatly increased the photon absorption of BPTCN (Fig. 24b). Alternatively, BPQDs (act as electron mediator) were successfully added on TCN surface to generate a unique 0D/1D structure (interfacial PeC bonds) that slowed charge recombination (Liu, 2019). The PeC bonds established a close association between BPQDs and TCN, thereby increasing cycle stability and electron transfer (Fig. 24c–d) (Wang, 2020). Carbon quantum dots (CQDs) could change the transfer of photoexcited electrons and holes by acting as electron acceptor. Moreover, CQDs/g-C₃N₄ band alignment can help the isolation of photogenerated electron-hole pairs (Liu, 2016; Qin and Zeng, 2017). Li and coworkers (Di, 2016) fabricated g-C₃N₄ nanosheets loaded with CQDs using a simple solvothermal process in ethanol and improved the photocatalytic HER. When g-C₃N₄ nanosheets (CNNS) stimulated in light, CQDs act as photosensitizer, causing more absorption of light in order to transfer electrons to the CB of CNNS. Liu and coworkers (Liu, 2019) also employed CQDs to adorn the g-C₃N₄ by a simple polymerized process without affecting its skeletal framework. CQDs expanded the absorbance spectra of g-C₃N₄ and acted as charge carriers for photogenerated electrons, leading to increased its photocatalytic performance. Qin (2020) created a seed-mediated hydrothermal g-C₃N₄ modified by NiS₂QDs as a co-catalyst. The quantum confinement effect and small size of synthesized materials may give additional active sites and help in the formation of an intimate interface contact (Zhang, 2018; Chen, 2017). Furthermore, existence of NiS₂QDs in a NiS₂QDs/g-C₃N₄ might increase the quick transfer of photogenerated electrons from g-C₃N₄ to NiS₂QDs, considerably enhancing the effective photoexcited electrons and holes separation (Lu, 2018). Lu and coworkers fabricated a 0D/2D heterostructure by modifying Ni₂PQDs as a co-catalyst over the g-C₃N₄ surface, that could significantly boost HER and PEC activity due to improved light absorption and better photogenerated separation of electrons and holes.

(a). The optical absorption of synthesized materials (Zhou, 2018), (b) the BPQDs has greatly increased the photon absorption of BPTCN. (c, d) the increasing cycle stability and electron transfer between BPQDs and TCN materials. Reprinted with permission from Wang (2020). Copyright © 2020, Elsevier.

Shakeelur and coworkers (Ar, s.r., 2020) created new type of catalyst composed of CdSeQDs loaded on thiol added g-C₃N₄ (TF-g-C₃N₄) and investigated for H₂ production. The fast rate of H₂ generation could be ascribed to the broad wavelength absorption and excellent reduction in recombination of electrons-holes pair of photocatalyst. Yang (2019) used a two-step approach for the synthesis of highly dispersed Co₃S₄QDs/g-C₃N₄ (Co₃S₄/CNNS) nanocomposites (water bathing and deposition). Co₃S₄QDs supplied additional active sites for HER, considerably widened the visible light range of CNNS, eased carrier separation and migration, and reduced the photoinduced electron migration distance, resulting in high HER performance. Electrons from the VB of CNNS migrated to CB, and the sacrificial agent such as triethanolamine generated the holes on VB. Electrons were then transported to Co₃S₄QDs. Co₃S₄QDs served as electron storage sites and hindered the recombination of photoexcited holes and electrons, resulting in a large quantity of active sites for HER. Following, the electrons had transferred from the CNNS to the Co₃S₄QDs reacted with the H₂O molecules to generate H₂, that was collected on the photocatalysts surfaces. QDs is a form of semiconductor material, have been identified as one of the promising architectures for suppressing the photoexcited electron-hole pair recombination (Fang, 2016). The strong quantum confinement provided by QDs is advantageous for shortening the path length of photogenerated charge transfer (Yang, 2019). On the other hand, faster the charge transport can boost the photocatalytic rates (Cao, 2013). While, QD can be used as a source of photoelectric response for the optical excitation (Meng, 2018). The considerably increased performance of g-C₃N₄ is due to the quantum dots acting as electron sources to capture electrons for improved separation and better light absorption.

3 Use of g-C₃N₄ in photocatalysis

g-C₃N₄ is an outstanding photocatalyst, which has diverse applications in different research fields (Su, 2012; Mandal and Ganguly, 2011; Anpo, 1987; Chen and Mao, 2007; Lu et al., 2009 ), including water splitting for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) (Daniel and Astruc, 2004; Watanabe, 2014; Claus, 2000 ), fuel cells (Bell, 2003), CO₂ reduction to value-added products (Hu et al., 2008; Alenad, 2022; Hayat, 2021 ), organic synthesis (Kumar, 2016) and environmental remediation (Mclaren, 2009). Herein, we will specifically discuss the application of g-C₃N₄ in photocatalysis.

3.1

3.1 Solar fuel generation

During photocatalysis, solar fuel generation from water and carbon dioxide (C0₂) is the best way to produce H_2, O₂, hydrocarbons, and syngas as chemical feedstocks and sustainable energy (Li and Liu, 2011). For that purpose, g-C₃N₄ is considered as the best metal-free and excellent photocatalyst in the visible range. In the recent times, a lot of semiconductor materials were fabricated, for the photocatalytic water splitting (WS) under solar illumination. Feasibly, g-C₃N₄ photocatalyst have great potential for water splitting, since its CB and VB locations are in the right places (Yang, 2017). In general, there isn't much informative about photocatalytic oxygen progression and WS. This is even though the OER is quite complex than the HER, that only involves two electrons transfer. Fig. 25 is a simplest diagram of the photocatalytic water splitting (WS) into H₂ and O₂ over g-C₃N4 (Tian, 2019). When g-C₃N₄ is exposed to illumination, the photoinduced electrons move from VB to CB, having left gaps in VB. Electron-hole pairs move individually to the activated and oxidation locations on the interface of g-C₃N₄ to turn water molecules into H₂ and O₂ in the mentioned expressions:

Depiction of photocatalytic WS employing pristine g-C3N4 for H2 and O2 energy sources under illumination. Reprinted with permission from Tian (2019).

Reduction reaction:

(1)

2 H^{+} + 2 e^{-} \to H_{2}

Oxidation reaction:

(2)

{2H}_{2} O + 4 H^{+} \to 4 H^{+} + O_{2}

Overall water splitting reaction:

(3)

{2H}_{2} O \to 2 H_{2} + O_{2}

The photochemical H₂ and O₂ production, which existed at the g-C₃N₄/water surface is strongly based on the thickness, shape, and deficiencies of g-C₃N₄. The huge particular surface areas, powerful elevated optical absorbance, maximum charge isolation performance and plentiful surface reacting spots are all helpful for the photocatalytic H₂ formation of the g-C₃N₄. Since g-C₃N₄ does have a comparable complex architecture with graphite, the surface area might potentially be enhanced up to 2500 m²/g¹ for ideal multilayer g-C₃N₄ (Sano, 2013). Although it is generally approximately upto 10 m²/g¹ for the pure g-C₃N₄ material owing to the compaction of polymer nanostructure. Here are numerous papers stated earlier emphasizing the advantages for photocatalytic H₂ and O₂ development by morphological manipulation of g-C₃N₄. The AQE of the 2D g-C₃N₄ nanostructure with a diameter of ∼1.2 nm, equivalent to ∼4 layer, that was synthesized utilizing urea as starting material by a sustainable steaming reformer technique, may obtain up to 11.3 % at 405 nm (Fig. 26) (Yang, 2017). A surface area of 210 m²/g¹ is achieved for such ∼1.2 nm g-C₃N₄ nanostructure, that is 4.38 times greater than that of conventionally synthesized g-C₃N₄ by urea calcination. The produced nanostructure additionally promotes the charge transport, participating in the increased effective H₂ generation function.

(a) Graphical depiction of the electronic framework. (b) Photocatalytic performance and durability of the PCN-U and PCN-U nanostructure. (c) Wavelength dependent of AQE on the H2 development in the PCN-U nanostructure. (d) Photocatalytic H2 production kinetics of the PCN and PCN nanostructure produced from various intermediates. Reprinted with permission from Yang (2017).

Basically, developing nanosized unique shaped nanomaterials may reduce the diffusing route of charged particles, enabling fast movement of charges over the interface of materials to contribute in the oxidation processes (Cui, 2018). The subsequent illustration is an effective spatially anisotropy charged isolation technique for acknowledging the benefit of morphological controlling on g-C₃N₄ (Liu, 2017). Using an NH₄Br based intermediate-mediated technique, the researchers generated the numerous phenomena (porous/microporous/ultraportable/horn-like) incorporated g-C₃N₄ tubes (Fig. 27a–d) (Liu, 2017). The g-C₃N₄ tubes possessed larger surface area and aided electron transport, carrier concentrations and interface charge transport performance, therefore exhibiting an exceptional photocatalytic performance for H₂ generation with an AQE of 14.3 percent at 420 nm (Fig. 27e, f). As seen in Fig. 27g, the selected photo-deposition findings reveal that the photoinduced electrons transfer to the external layer and holes might preferred to migrate onto the interior layer of the g-C₃N₄ tubes, offering a deep understanding into charges transport dynamics and increased photo performance.

(a, b) The enlarged TEM pictures of a singular g-C3N4 tube. (c) TEM pictures of CN-Br-3, (d) AFM imaging and diameter of g-C3N4 tubes shell. (e) the estimated rate ratios for HER of pristine g-C3N4 and CN-Br-X. (f) AQE and DRS spectra of CN-Br-3. (g) Graphical representation for the peculiar spatially charges isolation of g-C3N4 tubes in the photocatalytic reaction. Reprinted with permission from Liu (2017). Copyright © 2017, Elsevier.

Owing to rising technical emphasis on attaining “ecofriendly” power conservation and, continued endeavors was already made to the WS reaction for the research of a conceptual modeling on g-C₃N₄ catalysts. Besides novel nanomaterial g-C₃N₄, metal and non-metal loading, deficiencies variations construction, noble metal coating etc. for boosting the WS performance were also thoroughly published and evaluated in the preceding (Xiao, 2015). Although, it is also tough and prospective to generate completely employment of non-metallic g-C₃N₄ as an OWS photocatalysts. Acquiring a profound acknowledgement of the chemistry underlying the association among water molecules and g-C₃N₄, since the photocatalytic WS reaction is an essential part to alleviate the aforementioned problem. Yu et. al. (Tian, 2017) demonstrated that the lower dimensional charge transfer and electron mediated by interfacial hydroxylation might increase the photocatalytic H₂ evolution of g-C₃N₄. The plentiful interfacial hydroxyls (—OH) inserted on the —C≡N location, therefore enlarge the 2D coupling charged particle mechanism of g-C₃N₄ to 3D space to recognize multiple scales electron transport, and radicalize the adjacent N unit (C—N⚌C) to encourage the electron (H⁺) adsorbent and stimulation on them (Fig. 28). Incorporation of fundamental ammonium salts offers a pH-dependent hydroxylation-level mechanism and produces a ∼11-time boost in H₂ productivity. These finding might alter a potential route for producing extremely active HER composites basis on surface polarization of ligands.

(a) Interactive localized functions, (b) charged variation at upper aspect and side angle; (c) charged variation of protons deposited on CN, O-CN. Charging production is in blue and dissipation in yellow. Eads and Δq represents for the absorbance and carrying charges for protons. Reprinted with permission from Tian (2017).

Many single compounds based photocatalysts for HER have weak photocatalytic activity under visible light. Moreover, combining the g-C₃N₄ with hole scavenger and metal, resulting into higher photocatalytic activity. Shubin et. al. (Niu, 2012) used thermal exfoliation approach, in order to synthesize the g-C₃N₄ nanosheets with outstanding HER in a triethanolamine/water solution, compared to bulk g-C₃N₄ with a thickness of 2 nm HER rates of 3 and 5.5 under visible and UV–vis, respectively. The activity of one layer g-C₃N₄ nanosheets for HER is better than that of bulk g-C₃N₄, which is being synthesized by exfoliation method (Alivisatos, 1996) due to better charge transport and separation. The mesoporous g-C₃N₄ thin films synthesized by the solvothermal method have an outstanding photocatalytic activity for HER (Zhang, 2013) about 8510 mol/h¹ g¹ (with a quantum efficiency of 5%, 420 nm), greater than the 1560 mol/h¹ g¹, which was observed for 2D non-porous g-C₃N₄ nanoparticles or the 350 mol/h¹ g¹, observed for bulk g-C₃N₄. The platinum (Pt) nanoparticles loaded with g-C₃N₄ nanorods with TEOA and 1% wt Pt co-catalyst (Yang, 2013). These materials also outperform their mesoporous counterparts (Pan, 2012). Even though the reason for this difference is unknown, and Jones et. al. suggested lone pairs in nitrogen are the reason for excellent performance (Tian, 2014). The TEOA is mainly used as a hole scavenger for g-C₃N₄ and gives methanol as an extra performance (a 14-fold rate boost). When TEOA is used instead of methanol (as hole scavenger), the P25 gives excellent performance for HER, but less than g-C₃N₄. The g-C₃N₄ nanomaterials synthesized by the rolling up method show perfect HER (Weng, 2014). When used with TEOA (as sacrificial agent) and Pt as co-catalyst, the g-C₃N₄ nanosheets or QDs as a catalyst give three times better performance than pristine g-C₃N₄. The band structure g-C₃N₄ QDs could be altered in order to make them more sensitive for visible and near-infrared light, allowing for even greater photocatalytic HER. Other metal-free g-C₃N₄ nanosheets have photocatalytic HER of 1596 mmol/h¹ g¹ (quantum efficiency 3.56% at 420 nm), while P-doped g-C₃N₄ nanosheets have photocatalytic HER of 1596 mmol/h¹ g¹ (quantum efficiency of 3.56%). Similarly, the WS reaction was done with 3D g-C₃N₄ nanoporous material, TEOA, and 3% wt Pt as co-catalyst (Bai, 2013) that gives 2.5 times better HER than pristine g-C₃N₄ and remains stable after five cycles under visible light. The hollow g-C₃N₄ having a quantum efficiency of 7.5 %, has also been significant photoactivity for WS (Zhang, 2001). The inclusion of a MoS₂ co-catalyst considerably improved HER over hollow g-C₃N₄ spheres, forming a MOS₂/g-C₃N₄ hybrid (Fig. 29), thereby, increasing its light absorption and charge transfer ability (Zheng, 2016).

(a) Synthetic strategy and (b, c) TEM images and (d) Energy dispersive X-ray spectroscopy (EDX) elemental maps of MoS2@hollow g-C3N4. Reprinted with permission from Zheng (2016). Copyright © 2016, Elsevier.

In order to broaden the spectrum range of g-C₃N₄, other semiconductors and metal nanoparticles have been added, which further induce surface plasmon resonance. In such heterojunction materials, the photoexcited charge carriers are better segregated, reducing charge recombination and loss of energy for fluorescence (Li et al., 2012; Zeng, 2018; Ajmal, 2020 ). The metal doped g-C₃N₄ has several benefits, including high diffusion of molecules, better light absorption, and more active sites (Tahir, 2014; Wang, 2014; Tahir, 2013 ). For creating TiO₂/g-C₃N₄ composite, the melamine was used for hydrothermal calcination, followed by solid-state reaction (Zeng, 2014). When compared the pristine g-C₃N₄ (108 mol/h¹ g¹) with TiO₂/g-C₃N₄ heterostructures, the TiO₂/g-C₃N₄ had low band gap and high photoactivity (556 mol/h¹ g¹) for HER under visible-light irradiation (130 mol/h¹ g¹). The core/shell heterojunction nanocomposites were found to be more beneficial, because of the high facial contact between core and shell components (Yuan, 2014). The core/shell nanowires of CdS/g-C₃N₄ with different g-C₃N₄ content were synthesized using the solvothermal and chemisorption method (Fig. 30) (Zhang, 2013). The g-C₃N₄ has absorbed on CdS nanowires by giving an excellent HER rate of 4152 mol/h¹ g¹ for 2 wt% g-C₃N₄. A one-step self-assembly approach for the synthesis of the core/shell nanomaterial of carbon spheres coated with g-C₃N₄ has recently been devised. These composites exhibit better light absorption, mechanical stability, and charge transfer conductivity (Weiss, 2017), resulting into maximum HER rates of 129 mol/h¹, an 8-fold excellent over new g-C₃N₄ (16 mol/h¹). Other nanocomposites of g-C₃N₄ (Gu, 2015; Zhang, 2014; Zheng, 2016; Fattakhova-Rohlfing et al., 2014; Cheng, 2017; Nguyen et al., 2015; Sun, 2012; Zheng, 2015; Jun 2013; Hong, 2014; Han, 2016; Lang et al., 2014; Zhang et al., 2015; Ye, 2015; Zheng, 2012; Maeda, 2011; Ahmad, 2015; Xu, 2013; Bandyopadhyay, 2017; Ran, 2015; Di, 2010; Ajmal, 2018 ) were studied using a variety of materials and morphologies in order to get a deeper insight into the possible charge transfer pathways between g-C₃N₄ and other materials. Examples of different g-C₃N₄ heterojunctions include Z-scheme heterojunction (Ou, 2017; Wang, 2017), the Type II (Shen, 2017), p-n type (Zhang, 2013; Zhang, 2017; Ge, 2012; Xiang et al., 2011; Sun, 2017; Fang, 2017; Naseri, 2017; Lu, 2017; Xu, 2017; Zeng, 2017; Mao, 2017; Yu, 2017; Wen, 2017; Zhao, 2017; Barrio et al., 2018; Jin, 2017; Pramoda, 2017 ), g-C₃N₄ with metal (Ye, 2016; Xu, 2015) and g-C₃N₄ with carbon (Yuan, 2013). Controlling the structure of g-C₃N₄ heterojunctions for better light absorption is an interesting strategy in recent times.

(a, b) TEM and (c) HR TEM image of core–shell CdS/g-C3N4 heterojunction nanocomposite. Reprinted with permission from Zhang (2013).

3.2

3.2 Carbon dioxide (CO₂) reduction

The ongoing use of fossil fuels for chemical and energy production is causing severe problems (Tong, 2015; Martin, 2014). Among them, the photocatalytic CO₂ reduction to sustainable hydrocarbons and oxygenates might be a long-term solution. Because CO₂ reduction includes the multi-electron transfer, methanol, methane, formaldehyde, formic acid, and carbon monoxide formation kinetics are much slower than H₂ formation kinetics. The CO₂ reduction involves the molecule absorption onto the catalyst surface, where photoexcited electrons are transported across the semiconductor bandgap to form the anion radical. In an aqueous environment, the CO₂ reduction is caused by photoexcited holes transferred to hydrogen atoms from water, and then the proton is shifted to the CO₂ anion (Fig. 31) (Tian, 2019). The CO₂ reduction gives various products, which are shown below (relative to NHE at pH = 7):

(4)

C O_{2} + e^{-} \to C O_{2}^{\cdot -} + O_{2} E^{°} = - 1.90 e V

(5)

C O_{2} + H^{+} + 2 e^{-} \to H C O_{2}^{\cdot -} E^{°} = - 0.49 e V

(6)

{CO}_{2} {+ 2H}^{+} {+ 2e}^{-} \to {CO + H}_{2} O E^{°} = - 0.53 eV

(7)

{CO}_{2} {+ 4H}^{+} {+ 4e}^{-} \to {HCHO + H}_{2} O E^{°} = - 0.48 eV

(8)

{CO}_{2} {+ 6H}^{+} {+ 6e}^{-} \to {CH}_{3} {OH + H}_{2} O E^{°} = - 0.38 eV

(9)

{CO}_{2} {+ 8H}^{+} {+ 8e}^{-} \to {CH}_{4} {+ 2H}_{2} O E^{°} = - 0.24 eV

(10)

{CO}_{2} {+ 10H}^{+} {+ 10e}^{-} \to C_{2} H_{4} {+ 4H}_{2} O E^{°} = - 0.22 eV

(11)

{CO}_{2} {+ 12H}^{+} {+ 12e}^{-} \to C_{2} H_{5} {OH + 3H}_{2} O E^{°} = - 0.33 eV

Graphical depiction of photocatalytic conversion of CO2 to fossil energy under photoexcitation upon pure g-C3N4. Reprinted with permission from Tian (2019).

The essential features of photocatalyst, CO₂ absorption capacity, and strength are the main factors in the photocatalytic CO₂ reduction process (Gu, 2015). Electron and hole transfer kinetics to CO₂, charge separation, bandgap, and reductant are essential for photocatalytic CO₂ reduction process. g-C₃N₄ material has been studied for CO₂ reduction due to their negative CB, stability, and low bandgap (Li et al., 2015; Liang, 2015). The g-C₃N₄ activity is enhanced by doping of metal and nonmetal, heterojunction construction, and Z scheme hybrid (Han, 2015; Zhu et al., 2015; Huang, 2015; Wu, 2014; Han, 2015; Ajmal et al., 2021 ). Pengfei et. al. (Li, 2015) used ultrathin sheets of g-C₃N₄ for excellent CO₂ reduction, which gives tremendous charge carrier potential, greater surface area for absorption, and improved light absorption with methane and methanol productivities of 1.39 and 1.87 mol/h¹ g¹, respectively. According to Jianguo and colleagues (Wu, 2014), the decline in overpotential of CO₂ is due to the better charge separation of Pt nanoparticles on metal/semiconductor surface area. According to Qingqing et. al. (Wang, 2013) the Pd nanoparticles with double defect increase the reduction of CO₂ into CH₄ and carbon monoxide (CO) over g-C₃N₄ nanosheets. The 61.4 % conversion of CO₂ achieved and average CO production was 4.3 mol/g¹ h¹, with an average methane (CH₄) production rate of 0.45 mol/g¹ h¹, which showed reactive CO₂ and many active sites. The g-C₃N₄ nanotubes doped with oxygen synthesized by thermal oxidation and condensation of pristine g-C₃N₄, were demonstrated to have considerable photocatalytic CO₂ reduction capability under visible light (Zhang, 2012). The oxygen-doped g-C₃N₄ nanotubes have multi-walled tubes and have a diameter of 30 nm, and give methanol at the rate of 0.88 mol/g¹ h¹, which is five times greater than individual g-C₃N₄ nanotubes (0.17 mol/g¹ h¹). The ZnO/g-C₃N₄ heterojunction was synthesized by one-step calcination (Sui, 2013). The bulk g-C₃N₄ demonstrate 2.5-fold performance due to a direct Z-scheme mechanism indicating robust transportation of electrons from g-C₃N₄ towards ZnO surface. Zhong Xing et. al. showed that modified g-C₃N₄ with CeO₂ representing an excellent photocatalytic activity for CO₂ reduction to CH₄, with an average production rate of 4.79 mmol/g¹ h¹, which is 3 times greater than g-C₃N₄. Wang et. al. did a situ redox reaction between MnSO₄ and KMnO₄ and then synthesized a 2D/2D-MnO₂/g-C₃N₄ heterojunction photocatalyst for CO₂ reduction to CO (9.6 mmol/g¹), which gives excellent charge separation. The g-C₃N₄/SnS₂ was used for photocatalytic CO₂ reduction in which SnS₂ electrons interact with holes of g-C₃N₄ to give CH₃OH (2.3 mol/g¹) and CH₄ (2.3 mol/g¹), respectively. A unique Z scheme method was used in MoO₃/g-C₃N₄ heterojunction (Liu, 2015). TRyo et. al. used a different method in which they connected Ru(biby) complex into the structure of g-C₃N_4, which enhanced the performance of the reduction of CO₂ to formic acid, having an average quantum yield of 5.7% at 400 nm (Fig. 32) (Kuriki, 2015). The active photocatalysts for CO₂ reduction are generated, when polyoxometalate clusters are attached to g-C₃N4 (Zhang et al., 2015). After 10 h of visible light irradiation, the Co₄/g-C₃N₄ hybrid photocatalyst produced 107 mol/g¹ h¹ of CO and 94% selectivity for CO, significantly exceeding unpromoted g-C₃N₄.

The photocatalytic reduction of CO2 via Ru complexes structure and Ru complex/C3N4 hybrid photocatalyst. Reprinted with permission from Kuriki (2015)).

An intercorrelated super hybrid formed of g-C₃N₄ with AgBr ornamented onto N-doped graphene (produced by chemical synthesis) was also employed to show photo-catalytic CO₂ reduction to CH₃OH and C₂H₅OH (Fig. 33) (Li, 2015). Using Cu/TiO₂/g-C₃N4 (Cao, 2013), nanocomposites fabricated by impregnation and pyrolysis, the Oluwatobi et. al. demonstrated an increased rate of photo-reduction of CO₂ to CH₃OH and HCOOH under UV–vis radiation, with peak rates of CH₃OH at 2574 and HCOOH at 5069 mmol/g¹, respectively. The increased performance was attributed to the nanocomposite photoexcited electrons distribution and metal position. The Hailong et. al. demonstrated that AG/TiO₂/g-C₃N₄ heterojunction has excellent performance toward CO (28 mol/g¹) and CH₄ (19 mol/g¹) production from CO₂ reduction. However, depending upon lower Fermi level, the photoexcited electrons were transferred towards the TiO₂/g-C₃N₄ homojunction and then after TiO₂ to Ag nanoparticles, thereby, resulting into a significant reduction in electron-hole recombination onto the TiO₂ surface-modified Ag nanoparticles.

(a) Synthetic strategy, and (b, c) photocatalytic performance for CO2 reduction of intercorrelate super hybrid g-C3N4 nanocomposites under visible light and corresponding apparent quantum efficiencies. Reprinted with permission from Li (2015).

Yu et. al. (Fu, 2017) developed hierarchy permeable O incorporated g-C₃N₄ nanotubes (OCN-Tube) via serial heat oxidizing exfoliating and curling-condensation of pristine g-C₃N₄. The OCN-Tubes composed of interconnecting multi-walled nanotube with consistent sizes of 20–30 nm. Upon light irradiation, the photocatalytic methanol generation ratio of OCN-Tubes is 0.88 µmol/g¹ h¹, that is 5-fold greater that of pure g-C₃N₄ (0.17 µmol/g¹ h¹) (Fig. 34a). The improved photocatalytic performance of OCN-Tube is attributed to the synergism impact of hierarchy nanotube configuration and O coupled interaction. The hierarchy nanotubes design imbues OCN-Tube, increased particular surface area, increased light usage reliability, and enhanced mobility dynamics, owing to the numerous exposure energetic sides and various illumination dispersion pathways. Furthermore, the integration of O improves the electronic structure of g-C₃N₄, leading in a smaller absorption edge, stronger CO₂ affinities and an improved isolation of photoinduced electrons transportation. As demonstrated in Fig. 34b, whenever 13CO₂ was employed as the fuel, a significant pattern with the m/z value of 29 ascribed to 13CO was found (Ou, 2018)^. Consequently, it is established that CO is contributed from CO₂ in the photocatalytic reaction.

(a) Time trends of photocatalytic performance for CH3OH generation over pure g-C3N4 and O/CN Tubes (Fu, 2017). (b) GC–MS data of 13CO generated over 20 CPB-PCN from 13CO2 isotopes analysis. Reprinted with permission from Ou (2018).

Qin et. al. employed barbituric acid (BA) integrated g-C₃N₄ for the reduction of CO₂ to CO and H₂ as a biproduct (Qin, 2015). Despite alleviating the physical/chemical parameters of g-C₃N₄ with improved photostability, the molecular doping of urea and barbituric acid considerably simplified the charged transfer and isolation through the creation of interfacial chemical heterostructures. As demonstrated in Fig. 35a, the optimum CNU-BA_0.03 displayed a substantially improved photocatalytic CO generation, surpassing 15 times (31.1 μmol/h¹) more pristine g-C₃N₄ (2.1 μmol/h¹). Furthermore, high loading of BA resulted to the reduction in the CO generation ratio, that was ascribed to the degradation of g-C₃N₄ polymer structure due to BA monomer. It obviously illustrates the necessity of specific handling of the level of co-polymerization in boosting the photoreduction performance. Furthermore, the tendency of the materials such as CO and H₂ coincided effectively with the light absorbance of CNU-BA_0.03 predicated on the reaction spectra (Fig. 35b) and also the material held a good reliability with no declination in the final products (Fig. 35c). To extensively reveal the involvement of molecular doping process, various monomers comprising ABN, ATCN and DAMN were also applied to combine with urea raw material. Surprisingly, an elevated photocatalytic efficiency in all comonomer integrated g-C₃N₄ materials was detected contrasted with the simple g-C₃N₄ (Fig. 35d). This signifies those plentiful options of organic monomers may be merged with g-C₃N₄ using the co-polymerization approach to boost the reduction reaction of CO₂ source. To validate the formation of CO from the CO₂ photoreduction, a 13C tagged isotopic tracing assay utilizing 13CO₂ was demonstrated (Fig. 35e–f). Depending on the MS data, a fundamental signal pertaining to the 13CO (m/z value of 29) was depicted rather than 12CO (m/z value of 28). It contended the generation of CO exclusively resulted from the photocatalytic CO₂ reduction instead of the breakdown of carbon contaminants from the surface of photocatalysts.

(a) Production of CO and H2 from the photocatalytic CO2 reduction employing CNU-BAx with varying quantities of barbituric acid embedded g-C3N4. (b) Wavelength based production photocatalytic CO2 reduction. (c) Durability analysis of the generation of CO and H2 ove CNU-BA0.03 material. (d) Influence of several monomers integrated g-C3N4 on the synthesis of CO and H2 under illumination. (e–f) GC–MS findings of the produced CO from the reduction reaction of 13CO2 over the CNU-BA0.03 material. Reprinted with permission from Qin (2015). Copyright © 2015, Elsevier.

The integration of metals with g-C₃N₄ is also an intriguing technique for the creation of noble metal-based g-C₃N₄ heterostructure materials for efficient CO₂ reduction to sustainable compounds. Lately, a researcher team has showed an improved conversion of CO₂ to CH₄ using Pt integrated g-C₃N₄ composites, that has been synthesized via a chemical reduction technique in the solvent of ethylene glycol (Ong, 2014). This research enabled the photocatalytic reaction extra competitively possible and operationally practicable by employing a low-power of 15 W energy-saving daylight bulb as the illumination. As indicated in Fig. 36A–B, the photocatalytic CH₄ generation over the 2Pt/g-C₃N₄ with an ideal 2 wt% level of Pt can be obtained up to 13.02 μmol/g¹ after 10 h of solar illumination, that was 5.1 times larger than the pristine g-C₃N₄ (2.55 μmol/g¹). Therefore, the considerably enhanced photoactivity of Pt/g-C₃N₄ was attributed to the enhanced optical absorbance and remarkable surface electron transport from g-C₃N₄ to Pt owing to its reduced Fermi level to restrict the replication of electrons/hole pairs (Fig. 36C). Correspondingly to another prior report by Yu and team utilized Pt/g-C₃N₄ mixture photocatalysts for the reduction of CO₂ under visible illumination (Yu, 2014). With the ideal amount of Pt, CO₂ was decreased to potential hydrocarbon products, i.e., CH₄, CH₃OH and HCHO in the liquid–liquid interactions (Fig. 36D). Following the use of Pt, the alternating voltage was drastically lowered, thereby enabling the electron transport the photoreduction of CO₂. Table. 1 compares the performance of different g-C₃N₄ photocatalysts for photocatalytic CO₂ reduction and H₂ production.

(A) Photoreduction time function, (B) overall CH4 output over g-C3N4 and Pt/g-C3N4 catalysts for 10 h of operation. (C) Charge transport process for CO2 reduction using H2O to CH4 using Pt/g-C3N4 composite. Reprinted with permission from Ong (2014). (D) Output rate of hydrocarbons with regard to various integration of Pt (0–2.0 wt%) coated on the g-C3N4 nanostructure under visible light for 4 h of operation. Reprinted with permission from Ref. (Yu, 2014).

Table 1 Photocatalytic H₂ production and CO₂ reduction over g-C₃N₄.

Serial No.	Photocatalysts	Co-Catalysts (Loading)	Experimental details	H₂/CO₂ productivity/μmol·g⁻¹·h⁻¹	Enhancement Relative to Conventional g-C₃N₄	Refrences
1	g-C₃N₄ nanosheets	Pt (1 wt%)	10 vol% TEOA full sunlight and λ > 400 nm	1395	5.6	(Tong, 2015)
2	K-g-C₃N₄	Pt (0.5 wt%)	10 vol% TEOA 300 W Xe (λ > 400 nm	1028	14	(Wu, 2014)
3	AuPd/g-C₃N₄	Au and Pd	10 vol% TEOA 300 W Xe (λ ≥ 400 nm)	326	3.5 1.6	(Han, 2015)
4	Cu₂O@g-C₃N₄ core@shell		10 vol% TEOA 300 W Xe	202.28	5.7	(Liu, 2015)
5	CdS/g-C₃N₄ core/shell	Pt (0.6 wt%)	0.35 M Na₂S and 0.25 M Na₂SO₃ 300 W Xe (λ ≥ 420 nm)	4152		(Ou, 2017)
6	MoS₂/g-C₃N₄	N/A	10 vol% TEOA, 300 W Xe (λ > 400 nm	252	8	(Zhao, 2015)
7	CaIn₂S₄/g-C₃N₄	Pt (1 wt%)	0.5 M Na₂S and 0.5 M Na₂SO₃ 300 W Xe	102	3	(Jiang, 2015)
8	CdS nanorods/g-C₃N₄	NiS	10 vol% triethanolamine, 300 W Xe (λ ≥ 420 nm)	2563	1.6	(Yuan, 2015)
9	AgQCs/g-C₃N₄	Pt (1 wt%)	25 vol% methanol simulator AM 1.5 G	5.59	1.7	(Sridharan, 2015)
10	Thiourea and urea derived g-C₃N₄		300 W Xe/420 nm, 40 mg catalysts	Urea derived g-C₃N₄ CO: 0.56, CH₃CHO: 0.44, CH₄: 0.04 thiourea derived g-C₃N₄ CO: 0.36, CH₃CHO: 0.26, CH₄ = 0.025	N/A	(Wang, 2016)
11	Melamine & urea derived g-C₃N₄		300 W Xe (420 nm), 0.2 g and 1.0 M NaOH solution (100 mL)	Urea derived g-C₃N₄ CH₃OH: 6.28, C₂H₅OH: 4.51, O₂: 21.33 melamine derived g-C₃N₄ CH₃OH: TRACE, C₂H₅OH: 3.64, O2: 10.29	N/A	(Mao, 2013)
12	Sulfur-doped g-C₃N₄		300 W simulated solar Xe and 200 mL Pyrex reactor, 100 mg 1 wt% Pt co-catalyst, 0.12 g NaHCO₃ and 0.25 mL 4 M HCl solution	CH₃OH: 0.37	1.37	(Wang, 2015)
13	Pt-loaded g-C₃N₄		15 W energy-saving daylight bulb, flow rate of CO₂ fixed at 5 mL·min⁻¹	CH₄: 1.3	5.2	(Ong, 2015)
14	Ag₃PO₄/g-C₃N₄		500 W Xe/420 nm, stainless-steel reactor 132 mL, 10 mg in 4 mL H₂O, 0.4 MPa CO2 at 80 °C	CO: 44, CH₃OH: 9, CH₄: 0.2, C₂H₅OH: 0.1	CO: 11	(He, 2015)
15	BiOI/g-C₃N₄		300 W Xe (400 nm), 0.10 g catalyst, CO₂ bubbled through water	CO: 3.58, O₂: 1.96, H₂: 0.4, CH₄: 0.2	CO: 17.9	(Wang, 2016)
16	CeO₂/g-C₃N₄		300 W Xe, reactor volume 500 mL, 50 mg catalyst, CO₂ bubbled through water	2 wt% CO: 11.8 and CH₄: 9.08 3 wt% CO: 10.16 and CH₄: 13.88	CH4: 69.4	(Li, 2016)
17	g-C₃N₄/NaNbO₃		300 W Xe, reaction volume 230 mL, 50 mg catalyst, reactor purged with CO₂, then 2 mL H₂O injected	CH₄: 6.4	8	(Shi, 2014)

3.3

3.3 Environmental remediation

Polluted water is being discharged into the aquatic environment by several large-scale petrochemical, textile, and food sectors (Yuan, 2015). The use of organic dyes are often is being oftently observed in the printing, textile, as well as photography sectors. A significant amount of such colors is released into the effluent wastewater streams during the dying process. These colors are toxic to people and animals even at low concentrations (Jiang, 2015; Yuan, et al., 2014). As a result, modern oxidation methods are being developed in order to treat the contaminated drinking water and wastewater containing hazardous or non-biodegradable compounds (Sridharan, 2015; Li, 2015). The photocatalysis via semiconductors (Hou, 2013; Wu, 2014; Wen, 2015; Pany and Parida, 2015 ), is regarded as a low-cost, high-efficiency technique towards efficient removal of refractory organic/inorganic compounds from wastewater. The procedure of photocatalytic degrading of contaminants during light treatment on g-C₃N₄ is depicted in Fig. 37 (Tian, 2019). The principal active components in photocatalytic ecosystem cleaning are photoexcited holes, which are effective oxidants in and of themselves or combine with water in order to form hydroxyl radicals (^•OH), those are strong oxidants having oxidation potential about (2.8 eV). (NHE). Adsorbed pollutants on photocatalyst surfaces or in the solution can be attacked by reactively generated OH, causing them to mineralize as water and CO₂. The literature has broadly premeditated the organic pollutants photocatalytic oxidation in H₂O (Chen, 2015; Zhang, 2015; Kamat, 2007 ).

(12)

SC + h + h v \to SC \times {(e}_{CB}^{-} {+ h}_{VB}^{+})

(13)

{hVB}^{+} + H_{2} O \to^{\cdot} OH + H^{+}

(14)

O_{2} + e_{CB}^{-} \to O_{2}^{\cdot -}

(15)

O_{2}^{\cdot -} + H^{+} \to {HO}_{2}^{\cdot}

(16)

{HO}_{2}^{\cdot} + {HO}_{2}^{\cdot} \to H_{2} O_{2} + O_{2}

(17)

O_{2}^{\cdot} + {HO}_{2}^{\cdot} \to O_{2} + {HO}_{2}^{-}

(18)

{HO}_{2}^{-} + H^{+} \to H_{2} O_{2}

(19)

H_{2} O_{2} + hv \to 2^{\cdot} OH

(20)

H_{2} O_{2} + O_{2}^{\cdot} \to^{\cdot} OH + {OH}^{-} + O_{2}

(21)

H_{2} O_{2} + e_{CB}^{-} \to^{\cdot} OH + {OH}^{-}

(22)

Organic compund +^{\cdot} OH + e_{CB}^{-} \to Degradation products

(23)

Organic compund + SC (h^{+}) \to Degradation products

(24)

Organic compund + SC (e^{-}) \to Degradation products

Scheme of photocatalytic degrading of different contaminants under solar illumination employing pristine g-C3N4. Reprinted with permission from Tian (2019).

Mineralization causing oxidants have been related to a number of reactive species, including OH, HO₂, and H₂O₂, with ^•OH being the most probable candidate as illustrated in Eq. (22). The 'photo-Kolbe reaction,' in which photoexcited holes oxidize carboxylic acids, which directly produce the CO₂, has also been shown. The VB and CB must be sited, so hydroxyl radicals E (NHE) and superoxide radicals E (NHE) oxidation and reduction potentials for semiconductor photocatalysts are well located within the band gap of materials. However, the requirement for photocatalysts thermodynamically dictate that the hydroxyl radicals E (NHE) and superoxide radicals E (NHE) oxidation and reduction potentials are well located within the band gap. To put it another way, holes must have a sufficiently positive redox potential to make OH^• radicals, whereas photoexcited electrons must have a sufficiently negative redox potential to generate O₂^•. It has taken a long way to synthesize photocatalysts for water purification under the sunlight. The g-C₃N₄ materials physical properties are modified by doping some heteroatom and synthesizing heterojunction with the aim of increasing its surface area and porosity, thereby, making nanostructured based g-C₃N₄ as probable photo-catalysts for a wide variety of pollutants (Kumar, 2012; Yu et al., 2015). The ultrathin g-C₃N₄ nanosheets fabricated through methanol exfoliation from bulk g-C₃N₄ showed better photocatalytic activity towards methylene blue (MB) degradation (Kuriki, 2015). Under visible light, g-C₃N₄ nanotubes have a higher photoactivity towards MB degradation as compared to P25 or pristine g-C₃N4 (Walsh, 2016). Tahir and colleagues discovered that tubular g-C₃N₄ displayed improved stability and activity than bulk g-C₃N₄ towards photocatalytic MB degradation and methyl orange (MO) under visible light due to larger surface area of (182 m²/g¹), more excellent light absorption, and efficient separation/transfer of charges (Yu, 2014). Under visible light (>420 nm) and imitation sun light (>290 nm), one-dimensional g-C₃N₄ nanorods of varied ratios were studied for MB degradation (Gao, 2016). The g-C₃N₄ nanorods had 1.5–2.0 times greater photocatalytic activity and photocurrent responsiveness of g-C₃N₄ nanoplates. Aother effective but a facile chemical approach was employed in order to synthesize the nanofiber-like g-C₃N₄, which showed a probable photodegradation performance during Rhodamine B (RhB) degradation (Wang, 2015). One typical technique in order to drive the photo-generated charge carriers is the use of g-C₃N₄ doping to increase spectrum utilization and band alignment. In environmental pollution applications, metals like Fe and Cu (Fu, 2017; Mao, 2013; Shi, 2014 ), non-metal such as C, B, S or O (He, 2015; Li, 2015; Lin et al., 2014; Xia, 2017; Han, 2017 ), and Co doping (Wang, 2017; Di, 2017; Feng, 2017 ) all have been used. For the comparison purpose, with primeval g-C₃N₄ nanosheet, the O and S co-doped g-C₃N₄ were synthesized via melamine polymerization process followed by H₂O₂ activation subsequent to the functionalization of trithiocyanuric acid (Fig. 38a) which brought about the significant increased in the photo-catalytic degrading of RhB (Fig. 38b) up to 6-fold (You, 2017). It is due to the incremental increase in the total number of surface active sites, possibly leads to the enhanced separation and ultimate transfer of photo-generated electrons and holes, which in turn results into the high degree of delocalization in the HOMO and LUMO.

(a) Fabrication process, and (b) O and S co-doped g-C3N4 phtocaalytic performamce towards RhB degradation. Reprinted with permission from You (2017).

Similarly, a popular antibiotic photocatalytic degradation namely tetracycline chloride, whose uncontrolled release cause problem was examined (Li, 2017; Brini, 2021; Brini, 2021 ). The process containing the photodeposition of (7–15) nm of (Au and Pt) nanoparticles over g-C₃N₄ surface sites are good examples. The surface plasmon resonance of Au increases the optical adsorption range, but Pt sinks the photoexcited electrons. The multicomponent heterostructures like Ag₃PO₄/g-C₃N₄ for MO degradation (Huang, 2015; Niu, 2014) are viable for environmental pollution (Wang, 2016; Ong, 2015; Ni, 2016; Huang, 2015; Essekri, 2021 ). The coupling of g-C₃N₄ and metals provide adjustable heterojunctions with excellent charge transfer than typical nanocomposites (Wang, 2016; Li, 2016; Raziq, 2017; Maeda, 2014; Ong, 2016; Zhang, 2016; Wang, 2016 ), and multicomponent heterostructures like Ag₃PO₄/g-C₃N₄ systems for MO degradation (Ong, 2015; Zhou, 2014; Wu, 2017 ). The composite such as Ag₃PO₄/g-C₃N₄ core–shell converted MB at a rate of 97% in 30 min, compared to 79% Ag₃PO₄ and g-C₃N₄ components physical mixture and 69% untainted Ag₃PO₄. Actually, the g-C₃N₄ shell may make the composite more stable by preventing Ag₃PO₄ from dissolving. The core–shell g-C₃N₄@TiO₂ were seven times more photoactive than pristine g-C₃N₄/TiO₂, which was prepared via in situ sol–gel re-assembly pathway and then utilized to remove phenol during light source. Photo-degradation rate constant was fund to be enhanced from (0.0018–0.0386 h¹) by increasing the thickness of g-C₃N₄ shell from (0 to 1 nm). However the bigger shells impeded transport of charge towards external surface of photocatalyst, and ultimately decreasing the overall performance. A simple, environmentally friendly, and low-cost ultrasonic dispersion approach was used in order to efficiently construct a Z-scheme N-doped g-C₃N₄/ZnO core–shell nanomaterials (Fig. 39a, b). The thickness of the g-C₃N₄ shell can be adjusted with g-C₃N₄. A novel energy level was generated into the N-doped ZnO band gap by creating straight interaction between g-C₃N₄ shell and N-doped ZnO core by significantly reducing it. Therefore, these such kind of composite based core–shell demonstrated a considerably higher visible-light photocatalytic efficiency towards RhB deterioration than simple g-C₃N₄ and N-doped ZnO counterparts (Fig. 39a, c) (Kumar, 2014). Magnetically detachable g-C₃N₄/Fe₃O₄ hybrid composite were made via simple, repeatable, template-free synthesis (Fig. 39b, a) (Kumar, 2014). The mono-dispersed Fe₃O₄ nanoparticles with an (8 nm) diameter were consistently produced on g-C₃N₄ sheets, displaying better charge separation and photocatalytic performance for RhB degradation (Fig. 39b, c). After six recycles, the photocatalytic activity of these g-C₃N₄/Fe₃O₄ nanocomposites remained unchanged, permitting magnetic catalyst recovery (Fig. 39b, d) (Kumar, 2013). The Xiao et. al. demonstrated that during organic pollutants presnece, the g-C₃N₄ photocatalytic oxidation stability is due to the last intense of hydroxyl radicals in normal conditions, thereby, ensuing into the preferential decomposition of the pollutants over g-C₃N₄ (Maeda et al., 2013).

(a, b) As prepared N-ZnO-g-C3N4 core–shell nanoplates TEM pictures, along with (c) Z-scheme mechanism. Reprinted with permission from Kumar (2014). (B) (a) fabrication approach, (b) TEM photograph, and (c) g-C3N4–Fe3O4 nanocomposite based photodegradation mechanism along with (d) photocatalyst post-reactions magnetic separation. Reprinted with permission from Kumar (2013).

Similarly, Li (2018) devoted greater emphasis on addressing two interfaces of 2D layer g-C₃N₄, including the disorganized of electron transport and inefficient intercellular electrons propagation. Researchers created interior van der Waals heterojunction (IVDWHj) inside g-C₃N₄ using a straightforward synthetic method, greatly increasing the interfacial coulomb association and allowing the axially directed charged dissociation (Fig. 40a, b). The resulting g-C₃N₄ containing IVDWHs showed an improved protracted photocatalytic performance, with over 100 percent augmentation of NO effectiveness comparison with pristine g-C₃N₄ (Fig. 40c, d). NO purifying over CN premised materials is performed by NO oxidation to NO₂⁻ and NO₃⁻ by photoinduced superoxide radicals, including •O₂^– and •OH. The improved •O₂^– and •OH levels (Fig. 40e, f) clearly demonstrate the accelerated electrons activation and charge transport in O, K⁻ integrated CN.

Graphical depiction of the interior van der Waals heterojunction (IVDWHj) (a) “cake Framework” configuration of OCN-K-CN; (b) Band design of the OCN-K-CN IVDWH; (c, d) Photocatalytic performance contrast of CN and OCN-K-CN against NO purifying and (c) stability assess of OCNK-CN (d); (e, f) DMPO ESR peaks of CN and OCNKCN in dark/light region (λ ≥ 420 nm). Reprinted with permission from Li (2018).

3.4

3.4 g-C₃N₄-based 3D hydrogels for waste water treatment

Direct discharge of wastewater containing organic contaminants is particularly hazardous for environment and human health (Zubair, 2021; Liang, 2019). These organic contaminants can be removed by absorption using porous materials (Tan, 2015), but degradation of these organic contaminants using photocatalyst (Hao, 2018), could help to solve this problem (Huang, 2021). In particular, polymeric hydrogels having high specific surface area might be effective contaminant adsorbents. To enhance the waste water treatment performance, g-C₃N₄-based hydrogels has shown to be great approach for degradation of organic contaminants photcatalyically. It is because photocatalyst-based hydrogels have greater ultra violet and light response rate, producing active species to convert contaminants into CO₂ and water, but they have greater porous structure, large pore volume and surface area, enabling for effective contaminant adsorption. Self-assembled g-C₃N₄/graphene type and polymeric g-C₃N₄-based hydrogels are utilized for the photodegradation of hazardous organic and inorganic contaminants in wastewater. The 3D CNGA porous framework enhanced light absorption and boost the photoexcited electrons transfer from g-C₃N₄ to the surface of GO through interconnected pores (Tong, 2015; Hao, 2021). Furthermore, organic pollutants with high adsorption capacity (32 mgmg/1 for methyl orange (MO) dye) implies the availability of numerous active sites, so kinetically advantageous for photocatalytic reactions on surface. The degradation efficiency was significantly reduced after adding methanol and EDTA in a photocatalytic mechanism test, demonstrating that CO₂@ and COH are species involve in methyl orange degradation utilizing CNGA materials (Fig. 41a).

(a) Degradation of MO dye with CNGA in the presence of scavengers and inset showing the cyclic test of CNGA, (b) Highly selective dye adsorption toward MB, azure B, acriflavine, safranin O, RhB, eosin Y, and MO using h-CN. Inset of (b) shows photographs of dyes. Reprinted with permission from Tong (2015).

Wu and colleagues found that g-C₃N₄/GO based aerogel can degrade MO and MB (20 mgL@1) >90% in just half an hour under solar light and active CO₂ @ species were main element for MO degradation in oxygen environment (O₂ + e@!CO₂ @) (Tang, 2017). Zhang et. al. discovered self-assembled g-C₃N₄ hydrogel exhibited incredibly preferential dye adsorption due to its large surface area (190 m² g⁻¹) and induced by high negative or positive charge character (Zhang, 2016). Furthermore, the negative zeta potential of self-assembled hydrogels (h-CN), have strong electrostatic π-π interaction among self-assembled hydrogels and dye molecules having distinct charge states, determine the extraordinary capacity for photocatalytic reaction and selective adsorption of h-CN system (Fig. 41b). Shalom and team also demonstrated the adsorption of cationic dyes for MB using self-standing h-CN (Sun, 2017). The fact that the adsorbate and h-CN have strong chemical bonds suggests that dye adsorption on hydrogel is important for its removal as consistent with previous study. In addition, as a hybrid hydrogel composite comprising conductive material (PANI), synthetic polymer (PA) and biopolymer (Agar) were synthesized with g-C₃N₄ (Zhang, 2016; Jiang, 2016; Lamkhao and Randorn, 2017 ). Zhu and colleagues demonstrate a direct co-condensation process for the synthesis of g-C₃N₄/agar hydrogel with easy transfer of mass for adsorption and decontamination (Zhang, 2016). A PANI/h-CN having 3D framework demonstrates suitable HOMO and LUMO for good transmission of photoinduced electrons/holes pair, leading to increased the photocatalytic performance (Jiang, 2016). Zhu and colleagues discovered that a g-C₃N₄/SiO₂ hybrid hydrogel had adsorption capabilities of 3.5 and 48 mgg@1 for phenol and MB, respectively (Zhang, 2016). The zeta potential of the SiO₂ hydrogel got more negative as the g-C₃N₄ content increased, but the adsorption ability for organic contaminants don’t have any effect. Zhang and co-workers discover the idea that the zeta potentials and surface charge natures of g-C₃N₄/SiO₂ hydrogels alter their adsorption abilities, especially for MB and phenol adsorption (Zhang, 2016). Among all, the coupling of BiOBr or Cu₂O can result in more effective charge separation for g-C₃N₄-based hydrogels (Yu, 2017; Yan, 2017). The photocatalytic performance of these oxide loaded g-C₃N₄ based hydrogels depend on two factors. First one is of good surface area due to 3D porous structure, which allow for multiple surface-active sites, while the other is the p–n heterostructure interface among g-C₃N₄ and oxide materials, which allows for excellent separation of charges (Fig. 42). The performance of oxide loaded g-C₃N₄ hydrogel is increased by working together in both mechanisms.

Schemattic representation of heterostructure of the g-C3N4/GO hydrogel with (a) BiOBr. Reprinted with permission from Yu (2017). Copyright © 2017, Elsevier, and (b) Cu2O. Reprinted with permission from Yan (2017). Copyright © 2017, Elsevier.

3.5

3.5 Nitrogen photo-fixation

Nitrogen fixation is a revolutionary achievement by using g-C₃N₄ photocatalyst. Nitrogen could be transformed to ammonium, nitric acid, nitrous acid, and ammonia that are more valuable products than normal nitrogen. Fig. 43 depicts the nitrogen fixation procedure utilizing g-C₃N₄ as a photocatalyst (Ajiboye et al., 2020). In recent decades, photocatalytic N₂ fixation technique has been attracted more attention in energy and fuel production for its easy reaction conditions, low utilization of energy, cheap, and sustainable method that relies solely on limitless clean sunlight energy (Chen, 2018; Hao, 2020). Especially metal-free g-C₃N₄ photocatalysts, having nitrogen vacancies and matched heterostructure surfaces, have attracted the attention of N₂ fixation process (Li, 2018; Liang et al., 2017; Shi, 2018; Cao, 2018 ). Nitrogen vacancies are useful in chemical adsorption and activations of nitrogen, because the nitrogen atoms in dinitrogen have identical size and shape.

Nitrogen fixation using g-C3N4 mechanism. Reprinted with permission from Ajiboye et al. (2020). Copyright © 2020, Elsevier.

Dong and coworkers demonstrated integration of nitrogen vacancies within g-C₃N₄ framework that could photo reduced the nitrogen content and considerably improve isolation of charge carriers by capturing the photoexcited electrons and stimulating their transfer to the adsorbed nitrogen (Dong et al., 2015). Liu and coworkers demonstrated the fabrication of MXene derived TiO₂@C/g-C₃N₄ heterostructure by calcination of melamine and Mxene Ti₃C₂T_x (Chen, 2015). The fabrication method yields carbon nanosheets (2D-Planer) supported on TiO₂ with a high amount of Ti₃⁺ entities, that are compactly enfolded by in situ synthesized g-C₃N₄ nanosheets. Surprisingly, as-prepared catalyst showed good photocatalytic performance for reduction of nitrogen to NH₃ with an average rate of 250 mmol/g¹ h¹. Such high performance is owing to the presence of adequate surface defects, increased light capture, and remarkable charge separation and transfer ability of materials. For 2D graphene, Hu and coworkers created metal free 2D/2D nanocomposites of g-C₃N₄/rGO by connecting protonated g-C₃N₄ and rGO, in which reduced GO works as a conductive material (Hu, 2016). The as-prepared heterostructure produced five times more NH₃ than bulk g-C₃N₄, owing to appropriate interaction of rGO and protonated g-C₃N₄ across the g-C₃N₄/rGO heterostructure, that effectively suppressed the radiative charge carrier recombination and improved charge separation and transfer ability. Other than 2D/2D heterostructure, multi metal oxides with tunable band structures suit well with g-C₃N₄ semiconductor photocatalyst through intact heterostructure. Wang and coworkers effectively demonstrated the fabrication of a g-C₃N₄/MgAlFeO nanorod heterostructure for excellent performance for nitrogen reduction (Wang, 2017). Similarly, in Fig. 44a, a Z scheme heterostructure was formed, in which the photoinduced electrons from the CB of MgAlFeO moved to VB of g-C₃N₄, leading in better separation of charge and higher interfacial charge transfer, resulting in strong N₂ photofixation. Cao et. al. discovered a comparable Z-scheme heterostructure method for nitrogen photofixation, in which 3,4-dihydroxybenzaldehyde doped Ga₂O₃/g-C₃N₄ (Ga₂O₃-DBD/g-C₃N₄) was synthesized by introducing Ga₂O₃-DBD nanoparticles within the g-C₃N₄ structure through a simple post-grafting approach (Fig. 44b) (Cao, 2017). In the composites, the benzene ring act as a electron mediators that enhanced the separation of photogenerated charges by making well-matched interface among Ga₂O₃-DBD/g-C₃N₄ and making it easier to change nitrogen into ammonia with help of reducing ability of CO2 (Hao, 2021. 2021.), as seen in Fig. 44c. The preceding research effectively demonstrated that the metal free g-C₃N₄ photocatalyst with rational heterostructure fabrication can provide impressive performance towards ecofriendly nitrogen photofixation technology for global growth.

(a) g- C3N4/MgAlFeO possible mechanism, (b) The synthesis scheme and, (c) photocatalytic N2 reduction mechanism of Ga2O3-DBD/g-C3N4. Reprinted with permission from Cao (2017). Copyright © 2017, Elsevier.

3.6

3.6 Photoelectronic application

The addition of nitrogen into the carbon matrix brought an improvement in its electrical, structural, and mechanical properties, particularly its electronic properties, which in turn makes it a potential candidate to be used in rechargeable batteries (He, 2015; Li, 2017; Reli, 2016; Ohno, 2014; Hao, 2021 ), light-emitting devices (He, 2015; Zhao, 2017), fuel cells (Li, 2015; Bai, 2016; Li, 2017; Liu, 2017; Ye, 2016 ), solar cells, and other applications (Shi, 2015; Bai, 2016). Moreover, the g-C₃N₄ has been regarded as a most promising candidate in the overhead application domains. The sp² hybridization of nitrogen and carbon results in forming a π-conjugated electronic structure, which has an excellent photoelectronic characteristic (Richardson and Ternes, 2014).

3.6.1

3.6.1 Light-emitting devices

Due to its semiconductor features, the researchers are now concentrating onto the luminescence of g-C₃N₄ (Tyborski, 2013). In general, the optical band gap of g-C₃N₄ dominates the photoluminescence area, and it has been shown that the band gap of g-C₃N₄ is tunable by adjusting the processing temperature. The optical band gap and synthesized temperature are thought to be efficiently connected in the tectonic unit of g-C₃N₄, as shown in Fig. 45 (Tyborski, 2012).

Values of direct (squares) and indirect bandgaps (triangles) for carbon nitride powders processed at different temperatures. The solid black lines denote linear fits in the range between 490 and 610 ◦C, the dotted lines being extrapolations. Reprinted with permission from Tyborski (2012).

The photoluminescence functionality of g-C₃N₄ was explored in depth by Dong's group. As shown in Fig. 46 (Zhang, 2013), the resultant emission area of g-C₃N₄ ranging from 400 nm to 510 nm, due to constant modulation of the processing temperature. On the basis of two-dimensional conjugated polymeric system and the lone pair of g-C₃N₄, a promising energy level mechanism diagram has been proposed, thereby, concluding that the narrow band gap was primarily instigated through the advancement of the conjugated polymeric structure and electron delocalization.

(a) The normalized PL emission spectra of the g-C3N4 products were synthesized via melamine's thermal condensation at different temperatures for 2 h (ex = 280 nm). The inset shows the digital photos of g-C3N4 products with (bottom) and without (upper) 254 nm excitation; (b) The Commission International de L’Eclairage (CIE) chromaticity diagram of the carbon nitride products synthesized at different temperatures for 2 h. (1) 350 ◦C, (2) 400 ◦C, (3) 450 ◦C, (4) 500 ◦C, (5) 550 ◦C, (6) 600 ◦C, (7) 650 ◦C. Reprinted with permission from Zhang (2013).

Chen et. al. lately stated the ECL of g-C₃N₄ nanoflake films. The substantial ECL performance with (435 nm) blue light emission was estimated, under reductive-oxidative co-reactants environment (Jo, 2017). This is noteworthy that the ultrathin (g-C₃N₄) nanosheets might be fabricated via exfoliating bulk g-C₃N₄, and its PL quantum vintage could be high about 19.6% (Li et al., 2016). The optical characteristics and ECL activity of g-C₃N₄ demonstrate a superior performance in light-emitting devices. In light-emitting devices, the fluorescence powers of commercial nitrides via doped with transition ions or rare-earth ions are commonly employed. Although their luminescence qualities and quantum efficiency are good, metal toxicity, standard synthesis parameters, and their limit utilization (Tonda, 2014; Gao, 2015; Essekri, 2020 ). The Reyes et. al. further employed amorphous-CNx as a light-emitting material in order to develop a light maneuver, that exhibited a white–blue electro-luminescence at ambient temperature, which is presented in Fig. 47 (Yan et al., 2010; Ji, 2017). According to their investigations on the white light-emitting device, the C-N link of amorphous g-C₃N₄ emits green and blue light, whereas the C-H bond emits red light. However, it has been further proposed that g-C₃N₄ might be employed as a semiconductor layer in light-emitting devices, since it has a similar emission area to amorphous-CNx.

Emitting light device configuration (Ji, 2017).

3.6.2

3.6.2 Photocathodes

Semiconductor materials used in photoelectrochemical cells (PEC) for solar energy conversion into value-added chemicals are vital in developing sustainable energy to address current energy issues. The photoanodes for PEC water oxidation by employing n-type semiconductors i.e., WO₃, Fe₂O_3, ZnO, and TiO_2, are extensively investigated. Furthermore, few investigations have been conducted on the use of p-type semiconductors as photocathodes for photoelectrochemical cell reduction of water. The g-C₃N₄ is considered as a possible semiconductor for photocathodes, because of its high chemical durability and unusual electrical structure. The Zhang et. al. integrated active carbon sites to fabricate a long-lasting photocathode with three times more cathodic PEC performance as compared to pristine g-C₃N₄ (Panneri, 2017). Carbon inclusion is anticipated to significantly impact the electrical characteristcis of the g-C₃N₄ structure via the following mechanisms: (1) Charge separation efficiency is demonstrated by a decrease in photoemission intensity, which enhances charge mobility; (2) the π-conjugated framework is improved. Both of these elements help to increase the mobility of the charge. Furthermore, the nanostructure influences the free carrier diffusion, mass transfer, and active sites, impacting PEC efficiency. Fig. 48 depicting the differences in PEC activity amid pure and carbon-doped g-C₃N4 (Zhang, 2013). The PEC activity is projected to increase as electrical structures, change-carrier mobility, and surface area are all balanced.

Photoelectrochemical (PEC) properties of carbon nitrides in 0.1 M KCl aqueous solution. (a) The PEC cell configuration and possible charge transfer (the reference electrode was omitted for clarity). (b) Characteristic photocurrents at g-CN (black) and Alg-5-CN (red) photocathode under a chopped sunlight simulator at a scan rate of 10 mV/s. (c) The transient current response of g-CN (black) and Alg-5-CN (red) at −0.2 V vs Ag/AgCl (sat. KCl) under chopped monochromatic light at 440 nm. (d) Photo action spectra of g-CN (black), Alg-5-CN (red), and Gel-5-CN (blue) biased at −0.2 V vs Ag/AgCl (sat. KCl). Reprinted with permission from Zhang (2013).

3.6.3

3.6.3 Optical sensors

Optical sensor systems have attracted much research interest, because of their precise and reasonable detection limit and guaranteed biocompatibility and flexibility. Usually, the optical sensor is a molecule receptor with variable optical capabilities based on the specific visitors (Dai, 2013). Presently, fluorescent receptors are commonly used as optical sensors for detections by the interaction of fluorescent with adherent guests. The optical response is dominated by the transport of electrons from receptors to their binding guests. It is well known that g-C₃N₄ is an excellent catalyst to substantially absorb metal ions via chelation or redox reaction, since it has functions such as NH₂/NH/N over surface site. Despite many sensors with an optical receptor attaches to permeable materials, the g-C₃N₄ is the receptor itself and has excellent sensitivity. Lee et. al. investigated the possibility of using g-C₃N₄ as an optical sensor to check the presence of metal ions in an aqueous solution by examining its photoluminescence quenching effects (You, 2017; Dai, 2013). The findings in Fig. 49 reveal that g-C₃N₄ has good Cu²⁺ sensitivity. The Cu²⁺ entirely quenches photoluminescence thru no visible interfering from other metals, because photo-generated electrons are collected by Cu2+ (Lee, 2010). Furthermore, the Stern–Volmer equation may be used in order to quantify the degree of quenching: I0/I = 1 + KSV[Q], where I₀ and I indicate the luminescence intensity in the absence and presence of metal ions, respectively. The Stern–Volmer constant (You, 2017) is denoted by [Q], which is molar concentration of the metal ion. This comparation depicts a roughly linear relationship between the effect of Cu²⁺ concentration on g-C₃N₄ photoluminescence.

(a) PL spectra of both c-mpg-C3N4 (black bar) and bulk (gray bar) after 1 mm of metal ion solutions treatment, and (1 mm) of other metal ions intrusion with (1 mm) Cu2+ (white bar); (b) PL spectra of c-mpg-C3N4 to Cu2+ in 10–100 nm concentration range. Reprinted with permission from Lee (2010).

3.7

3.7 Bacterial disinfection

It is critical for human health to remove pathogenic bacteria from water. Although earlier disinfection processes such as chlorination and ozonation were highly successful for inactivation, as they frequently produced hazardous disinfection by-products having carcinogenic and mutagenic potential (Muellner, 2007; He, 2016; Malik, 2021 ). In 1985, the Matsunaga et. al. (Matsunaga, 1985), were the first to demonstrate photocatalytic sterilization of microbial cells in water. The first time Huang et. al. found the bactericidal effects of g-C₃N₄ on Escherichia coli (E.coli) K-12 in water, when irradiated with visible light (Huang et al., 2014). Mechanistic illustration showed that, the light-induced holes over g-C₃N₄ surface sites were shown to be a major source of the inactivation of bacteria. As a result, using a hole scavenger like OH had minimal influence on the inactivation efficiency (Huang et al., 2014). The use of single layer g-C₃N₄ was invetigated by Zhao et. al. in order to examined the photocatalytic disinfection of E. coli in visible-light (Zhao, 2014). The Fig. 50a displays the disinfection activity of several photocatalysts for E. coli, with single-layer g-C₃N₄ by providing the best results. As demonstrated in (Fig. 50b), adding isopropanol or Cr (VI) caused in killing around five log10 of E. coli cells in 4 h. Though, only about two log10 of E. coli cells were smashed, when KI was added. As a result of the findings above, the disinfection in visible light seems to be a hole oxidation process. The effects of disinfection on E Coli were investigated further using microstructure and morphology analysis earlier and afterward disinfection. Fig. 50c shows the E Coli cell wall, before photocatalytic disinfection, which is highly conserved. The cell wall of bacteria is destroyed after 12 h of irradiation, as seen in Fig. 50d, f.

Photocatalytic disinfection performace of (a) various g-C3N4 and (b) single-layer g-C3N4 with diverse scavengers. g-C3N4 NS and SL g-C3N4 indicates g-C3N4 nanosheets and, single-layer g-C3N4, correspondingly. The TEM picture of E. coli before and after photocatalytic disinfection: (c) Before reaction and after irradiation for (d) 4 h, (e) 8 h, and (f) 12 h. Reprinted with permission from Zhao (2014). Copyright © 2014, Elsevier.

Wang et. al. developed a metal-free heterostructure of cow wrapping Sulfur (−S8) on rGO sheets and g-C₃N₄ for bacteria disinfection during visible light. They examined the disinfection of E. coli K-12 cells via two diverse heterostructures (Fig. 51a) and rGO was found between -S8 and g-C₃N₄ in the first structure (CNRGOS8), whereas g-C₃N₄ was found between -S8 and rGO in the second structure (CNRGOS8). The aforesaid synthesized heterostructures (CNRGOS8 and RGOCNS8) exhibit different photocatalytic performance towards E. coli K-12 in anaerobic and aerobic conditions (Fig. 51b, c). As shown in Fig. 51a, b, the CNRGOS8 has a more excellent photocatalytic activity for bacterial inactivation in an aerobic environment than RGOCNS8, whereas the RGOCNS8 works better in anaerobic situations (Fig. 51C). The photocatalytic route in an anaerobic environment was more reductive than the oxidation process. Therefore, such discovery unfastens inventive paths for studying the use of metal-free catalysts to improve bacterial photoinactivation effectiveness (Wang, 2013).

(A) Graphic represnetation of mechanism of photocatalytic bacterial inactivation (a) CNRGOS8 and (b) RGOCNS8 in aerobic and (c) CNRGOS8 and (d) anaerobic conditions of RGOCNS8. Photocatalytic inactivation activity vs E. Coli K-12 during as-prepared samples presence (B) aerobic and (C) anaerobic conditions under visible-light irradiation. Reprinted with permission from Wang (2013).

4 Future prospective

Despite tremendous advances in past years, the major challenges persist in rationally designed extremely effective g-C₃N₄-based photocatalyst in terms of the number of their uses and the full knowledge of the fundamental tailoring process of g-C₃N₄-based photocatalysts. Hence, several unresolved concerns and areas exist, where more research is needed. So far, deep studies are urgently needed to fully explore the structural and electrical characteristics of g-C₃N₄ composites. Conversely, the diverse features of g-C₃N₄-based photocatalysts also need to be thoroughly investigated. Owing to the challenges in developing extremely efficient and sustainable g-C₃N₄-based photocatalysts with decreased band gaps, the creation of low band gap conjugated polymers may yield additional concepts for speeding organic semiconductor-based photocatalysis advances. The g-C₃N₄ nanosheets further require accurate surface defect regulation and simple scale preparation techniques. Advanced photo-electrocatalysts must be employed to explore the applications of a g-C₃N₄-based catalysts for the scientific community. In constructing practical g-C₃N₄-based photocatalysts, dimension controll, pore-size modifications, heterostructure building, cocatalyst and catalyst loading, and nanocarbon loading appear to be the most promising design strategies. More research into this method is required at this time. In these photocatalytic areas, developing Earth-abundant co-catalysts is still a challenging task. Nanocarbons will forever play vital roles in the construction of highly effective g-C₃N₄-based photocatalysts. In addition, the uses of g-C₃N₄-based photocatalyst are primarily concentrated on H₂ evolution and pollutant degradation. In contrast, during the photocatalytic CO₂ reduction process, these catalysts have grown in popularity over the past few years. The fundamental character of the surface of g-C₃N₄ dictates its promising future in CO₂ reduction fields. Moreover, study of OER from the water splitting half reaction, which involves both CO₂ reduction and overall water splitting, should receive more attention in the coming years. Furthermore, the process's precise mechanism, especially CO₂ reduction by g-C₃N₄-based catalyst, remains unclear and unexplored. Deep research in reaction pathways is mandatory for elucidating the fundamentals improvements of photocatalysis and designing effective g-C₃N₄-based photocatalysts in the future. Moreover, a few key concerns that are necessary for good photocatalytic activity, such as light absorption, charge separation and transfer, and band structure, must be thoroughly evaluated using both DFT and practical experiments to gain theoretical insights. Charge separation and transfer ability, substrate adsorption sites and molecular orbitals should all be comprehensively studied pertaining to experimental efforts. The collaborative efforts of researchers from many fields and countries should result in an exciting period for g-C₃N₄-based photocatalysts.

5 Conclusions

This review article mainly deals with recent progress in the improvement of photocatalytic performance of a wide variety of properly tuned highly active carbon based g-C₃N₄ photocatalysts in photocatalysis research field. However, due to electrical, optical, and mechanical properties, the g-C₃N₄ could be further tailored for solar energy use in environmental applications and other energy conversions. The g-C₃N₄ nanomaterials with their varying porosity and dimensionality and their incorporation in multi-functional nanocomposites, have brought about the significant enhancement in the solar light absorption, charge separation/transportation, quantum yields, and photocatalytic performance. At the same time, the main difficulty still being experienced is the lack of multidisciplinary actions, and the use of sacrificial reagents to improve the photoreactor design persists. The preliminary step in the photocatalytic reduction of CO₂ is the utmost selection of the best specific product—either energy (e.g., CO, CH₄, or formic acid) or chemicals (e.g., olefins or alkanes)—to improve the activity and the capacity as well. Although the quantitative comparisons are found to be difficult owing to the decoupling of adsorption from the reaction medium to the photocatalysis process as through photo-chemical stimulation of chromophores, the dearth of calibration in experimental protocols, and the design of the reactor during wastewater treatment using a g-C₃N₄-based photocatalyst appears to be remarkable.

Acknowledgments

We gratefully acknowledge the support of this research by funding from the Foundation of Yangtze Delta Region Institute (Huzhou) of UESTC, China, (Nos. U03210057). Authors acknowledge support and funding of King Khalid University through Research Center for Advanced Materials Science (RCAMS) under grant no: RCAMS/KKU/008/21.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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