Page | 5021 A review on g-C3N4 based photo-catalyst for clean environment Manas Ranjan Pradhan 1 , Snehashis Panda 1 , Binita Nanda 1, * 1 Department of Chemistry, Institute of Technical Education and Research, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India *corresponding author e-mail address: binitananda@soa.ac.in | Scopus ID 53980125400 ABSTRACT In recent years, our mother earth is facing drastic environmental and energy crisis. To resolve this crisis, the search for suitable strategy becomes a challenge to the sciencetific community. In this aspect, graphitic carbon nitride (g-C3N4) attracts the attention because of its super hardness, low density, chemical, thermal stability and biocompatibility. Among various analogues, g-C3N4 is considered as one of the most stable allotrope in the research hotspot drawn attention as metal-free and visible light responsive photocatalyst. Moreover, g- C3N4 acts as an n-type semiconductor and unique electrical, optical, structural and physiochemical properties. This makes g-C3N4 and g- C3N4 based materials, a new class of multifunctional nano platform. The polymeric structure of g-C3N4 enables the tuning of surface of g-C3N4 at a molecular level. However it has the lowest band structure among all the seven phases of CN. These properties enable the g- C3N4, an emerging and ideal candidate for clean energy production and environmental remediation. Last but not least, this review article emphasizes on the summary and some invigorative perspective on the challenges and future directions towards the development of sustainability without environmental detriment. Keywords: g-C3N4; photocatalysis; hydrogen energy generation. 1. INTRODUCTION In recent years, the global energy problem and environmental crisis are hindered the sustainability. This encouraged the scientists and researchers to develop some noble pathway for clean energy production as well as pollutants degradation strategies with the help of efficient photocatalytic materials [1]. Over the past decades semiconducting based visible light driven photocatalytic materials are the most suitable alternative that has attracted the attention of the researchers. Conversion of water into hydrogen is the best technique and regarded as a promising route with a higher efficiency [2, 3]. Many photocatalytic systems have been explored for efficient absorption, utilization, and conversion of solar photons in a sustainable and greener mode [4, 5]. A handful of semiconducting metals, metal oxides, and metal composites are extensively used as photocatalyst to harness solar energy (green technology) for cleaner environment. Among all, the story of semiconductor photocatalysis starts with TiO2by Fujishima and Honda, and termed as ‘golden’ photocatalyst for a long period. The large band-gap of TiO2 (anatase) is 3.2 eV restrict the broad spectrum of sun light. To handle these problem, scientist and researchers have taken the challenges and search a new semiconducting visible light responsive material which tackle the environmental remediation and solve the energy crisis. Recently, a simple efficient, highly stable and sustainable polymeric material, graphitic carbon nitride (g-C3N4),a n-type semiconductor, taken as a metal-free visible light responsive photocatalyst for H2 evolution and pollutant degradation. Berzelius was the first man to synthesize it and after that Liebig was named “MELON” [6]. But later on more simple and feasible method (thermal condensation) was implemented for synthesizing g-C3N4using nitrogen rich precursors like cyanamide, dicyandiamide, urea, thiourea and melamine as precursors [7]. It is a stable allotrope of carbon nitride as there is a strong covalent bond between carbon and nitrogen [8]. Another way it is a poly-conjugated semiconductor which contains earth’s most abundant elements C and N atoms with a molar ratio C/N is 0.75, suggesting g-C3N4 is low cost material with two-dimensional stacking of π conjugated planes analogous to graphite. It is iso-structural with graphite where van der Waals’ force holds the stacking layers (covalent C–N bonds) with each layer composed of tri-s-triazine units. It is bio- compatible, thermally and chemically stable in both acidic and alkaline environments. It has photo-electrochemical property in- oxidizability and waterproofing nature for which it is most wanted in photo-catalytic application and energy conservation [9-10]. Wang et al. synthesized bulk g-C3N4for the first time in 2009, using cyanamide were applied to photocatalytic H2 generation from water [11]. After that, a number of review articles have been focused on the synthetic strategies of g-C3N4. Basing on the heat and chemical resistance, g-C3N4 exhibits unique stability. As prepared g-C3N4is non-volatile till the temperature reaches up to 600 0 C, but completely decomposed at temperature 700 0 C, confirm from thermogravimetry analysis (TGA) [12-14]. In single layer of g-C3N4, mainly consist of triazine and tri-s-triazine, is considered as tectonic unit [15]. Different synthesis methods such as hydrothermal, physical vapour deposition, chemical vapour deposition, solid state methods, and thermal condensation are commonly adopted. Among all, thermal condensation is the most commonly adopted technique, where basically nitrogen rich species are used as precursors. The most commonly used precursors are melamine, cyanamide, dicynamide, urea and thiourea etc [16]. When melamine undergoes thermal reaction, self-condensation occurs along with deammonication process and produces intermediate products such as melam, melem, and melon respectively. Which again thermal condensation at 400-5000C, g- C3N4 is formed, which is shown in scheme-1. Despite the presence of many transition metal oxides like ZnO, TiO2 which are active within UV region, corresponding to Volume 10, Issue 2, 2020, 5021 - 5027 ISSN 2069-5837 Open Access Journal Received: 03.09.2019 / Revised: 11.01.2020 / Accepted: 14.01.2020 / Published on-line: 27.01.2020 Original Review Article Biointerface Research in Applied Chemistry www.BiointerfaceResearch.com https://doi.org/10.33263/BRIAC101.021027 https://www.scopus.com/authid/detail.uri?authorId=53980125400 http://orcid.org/0000-0002-7011-894X http://orcid.org/0000-0002-5267-765X http://orcid.org/0000-0003-1132-2753 https://doi.org/10.33263/BRIAC101.021027 Manas Ranjan Pradhan, Snehasis Panda, Binita Nanda Page | 5022 an optical wavelength 460 nm, makes g-C3N4active under visible light. Clearly, it is seen that the band gap of g-C3N4 is 2.7eV. Scheme-1. Systematic approach synthesis of polymeric g-C3N4. Due to thermodynamic losses and over potentials in the photocatalytic process the band gap lies between 2.1-3 eV, which enhances the hydrogen energy generation with enough endothermic driving force and light absorption in the visible region. Though g-C3N4has many applications towards clean energy and environment, but unfortunately its efficiency is low due to grain boundary effect, marginal absorption of visible light (< 460 nm), very low surface area (10 m 2 g -1 ), low quantum efficiency, high recombination rate of electron-hole pair less active sites for interfacial photon reactions, slow surface reaction kinetics, and low charge mobility interrupt electron delocalization [17]. Its photo-catalytic activity thus be enhanced by modifying its electronic structure by doping [18], templating [19], co-catalyzing [20], copolymerization [21], heterogeneous catalysis [22], and surface modification [23]. Amongstnumerous prospects, photocatalysis by establishing hetero-junction between twofold semiconductors, identified as Z-scheme photo-catalysis [24–25]. It has been fascinatedbecause of consumption ofa greatpercentage of the solar spectrum and reduces the rate of recombination by driving the redox process at different sites of the catalyst. Even photo-catalysis by doping is testified by Zhang et al. that phosphorus atoms is introduced into the framework of g-C3N4 by substituting C atoms [26-27]. As tri-s-triazine motif in g-C3N4 is considered to be electron acceptor, then P atom becomes electron rich and tri-s-tri-azine will be electron-deficient which is favourable for the effective separation of photo-generated e – and h + pairs and improve the photo-catalytic efficiency [28-29]. In this book chapter, we are mainly focus on the green technology for a clean environment by pollution abetment and energy conservation. Basing on green technology, photocatalysis is the safer, cleaner and greener method to reduce the environmental hazards and give an alternative path for hydrogen evolution to mitigate the energy crisis. Taking this into consideration, finding a suitable catalyst is a challenging task. In this way, g-C3N4 is a very good photocatalyst for hydrogen generation and pollutant degradation. This book chapter, also provide a comprehensive recent data generated by different authors regarding photocatalysis and energy evolution. 2. RESULTS Photocatalysis: A Promising route for energy and environment Photocatalysis The goal of 21 st century chemistry is to adopt energy efficient routes to avoid environmentally hazardous processes with replacement of harmful chemicals during production and uses. Photocatalysis, a conventional green technology, the advanced oxidation process (AOP) has the pivotal role in today’s clean environmental [30]. As definition denotes, photocatalysis is a ‘green’ technique which accelerates the photoelectron in the presence of promoter. In the photocatalytic reaction, light is absorbed by an adsorbed substrate. Therefore, in photo generated catalysis, semiconducting nano materials preferred as photocatalyst due to narrow band gap between the valance band and conduction band [31-32]. Mechanism Semiconducting materials having filled valence band (occupied by electrons) and an empty conduction band (unoccupied electronic states) are mostly sensitizes redox processes in presence of light. The two bands are separated by an energy gap particular to each semiconductor referred to as the band gap (Eg). In the photocatalytic process, when a semiconducting material is subjected to radiation exceeding its band gap, establishing a redox environment. This helps in generation of e – and h + pairs in the valance band and conduction band [33-34]. These e – and h + pairs enable redox reaction and form different oxidative radicals on surface of semiconductor and enhance the photocatalysis process through three mechanistic ways such as: proper excitation, bulk diffusion and surface charge transfer [35]. Scheme 2. Mechanism of photocatalysis reaction process. In a typical photocatalytic process, when the semiconductor photocatalyst is illuminated with light of sufficient energy (either UV/visible light source), the e – s of VB jumped to the CB, thus leaving an h + in VB is shown in scheme-1. The holes (h + ) produced in VB, are trapped by the surface adsorbed water molecule and generates OH • radical. At the same time photo generated electrons (e – ) in CB react with the dissolved oxygen forming superoxide radical (O2 –• ), which again reacts with photon (H + ) to produce hydroperoxyl radical (HO2 • ) followed by the formation of H2O2. The charge transfer and production of highly oxidative radicals during the redox process enhances the photocatalytic ability of a semiconductor. The mechanism of photocatalysis is explained below and the schematic representation is shown in scheme-2 Photocatalyst + hυ → e – (CB) + h + (VB) (1) OH – + h + (VB) → OH • (2) O2 + e – (CB) → O2 – • (3) O2 – •+ H + → HO2 • (4) 2 HO2 • → H2O2 + O2 (5) H2O2 + e – (CB) → OH • + OH – (6) A review on g-C3N4 based photo-catalyst for clean environment Page | 5023 In this regard, the electronic structure of g-C3N4 accountable the photocatalytic ability. The presence of two abundant element carbon and nitrogen in g-C3N4 and the lone pair electron of nitrogen plays an important role in electronic structure. The combination of π bonding electronic states and the lone pair of nitrogen stabilizes the electronic lone pair state [36]. The role of nitrogen content in optical properties of g-C3N4 was again confirmed by Abd El-Kader et al. [37]. To enhance the positive improvement in optical properties and photocatalytic performance, band gap is to be modified with negative ions in g-C3N4 by doping [38] as the position of VB and CB are responsible for oxidation and reduction levels. Generally, the HUMO –LUMO band gap of melem is 3.5 eV and it is reduced to 2.6 eV in case of melon and finally attain the band gap 2.1 eV with fully formation of condensed g-C3N4. The wave function investigated that the nitrogen Pz orbitals and carbon Pz orbitals drove VB and CB, where the photogenerated e – s and h + s are separated. This creates the oxidation and reduction cites in VB and CB respectively, which is responsible for splitting of water (H2 generation) and pollutant degradation independently in the nitrogen and carbon atom. Wei et al. synthesized visible light responsive photocatalyst mesoporous TiO2/g-C3N4 for phenol degradation. Amorphous TiO2 microsphere was coated with thin layer of g-C3N4 was clearly visible from SEM and TEM study. From HRTEM, it is clear that heterojunction formed due to the connection of the crystal lattice of TiO2 and g-C3N4 at the interface of TiO2/g-C3N4. The irregular mesoporosity of TiO2 and the composites (TiO2/g- C3N4) were confirmed from N2 adsorption and desorption study, which exhibited the type (IV) isotherm. UV-vis DRS shows that the light absorption of TiO2 was restricted to wavelength range at 390 nm. But, after the decoration of g-C3N4 on the surface of TiO2 extended the absorption of light towards visible region. The PL spectra of TiO2 was found to be in the range 350-420 nm due to band-band emission and surface oxygen vacancy effect. In the case of TiO2/g-C3N4 composite the peak intensity was negligible which confirms the reduction of electron-hole recombination rate by the efficient separation of electron-hole pairs. During the photocatalytic degradation of phenol, there is a charge transfer through TiO2/g-C3N4 heterojunction. The heterojunction was formed inside the anatase of TiO2 structure, the growth of g-C3N4 was restricted and only a thin layer of g-C3N4 was embedded on the surface of TiO2. During the addition of precursor (cyanamide) and the intermediate product (melem and s-triazine) possibly react with TiO2 and form TiO2-xNx after calcination (well confirmed through XPS). In the heterojunction both g-C3N4 and TiO2-xNx are irradiated under visible light, generates electron-hole pair. During this process, the transfer of electrons occurs from CB of g- C3N4 to CB of TiO2-xNx, whereas the holes present in the VB of TiO2-xNx move towards VB of g-C3N4. The electrons accumulated in the CB of g-C3N4 and TiO2-xNx react with atmospheric oxygen to produce superoxide radical (O2 -. ), which responsible for the degradation of phenol [39]. Sun et al. fabricated g-C3N4/ZnO photocatalyst with different ZnO content for decomposition of para nitrophenol and methyl orange. The thermal stability and weight of g-C3N4/ZnO deceases, when the temperature rises from 520 0 -690 0 C. This shows that at temperature 500 0 C, the composite is stable and above that combustion occurs and the weight of the g-C3N4 decreases and at the same time ZnO increases. This again confirms from PXRD which reveals that the intensity of peak of ZnO in the composite increases with increasing calcination time. The bonding between ZnO and g-C3N4 is probably due to condensation reaction between amino group of tri-s-triazine of g- C3N4 and surface OH group of ZnO. From HRTEM image it was revealed that there is an interface between g- C3N4 and hexagonal wurtzite phase of ZnO. UV-vis spectroscopy discloses that the absorption shift to the lower region to 470 nm corresponds to band gap of the composite is 2.8 eV. This arises due to the strong chemical bond between ZnO and g- C3N4 and enhances the photocatalytic activity towards methyl orange and paranitro phenol. Increasing Zn content in the composite, remarkably enhances the photocatalytic activity. The photocatalytic activity of the composite is found to be increasing with increase in ZnO content as degradation of MO is about 97% for 16 wt% of ZnO in g-C3N4 in 80 min. This shows that there is a strong synergism between ZnO and g-C3N4 in g- C3N4/ZnO photocatalyst. The increased photocatalytic property of g-C3N4/ZnO is due to lower –ve conduction band potential of g- C3N4 (-1.12eV) than that of ZnO (0.5 eV). Photo induced e – s on conduction band of g-C3N4 goes to that of ZnO through the interface and produces O2 .- from atmospheric oxygen. MO degradation charged the g-C3N4 surface and again restore to the ground state. The two semiconductor also rebuild the electric field and creates the electron-hole separation, which accumulates a large number of electron on the surface of ZnO and enhances the photocatalytic activity towards MO degradation [40]. He et al. fabricated Z-scheme type MoO3/g-C3N4 photocatalysts and evaluated for its photo degradation activities towards methyl orange (MO). From thermo gravimetric analysis shows that there is a weight loss in the composites as g-C3N4 volatilized within temperaure range of 600-750 0 C. Again vaporization of MoO3 in the composites (MoO3/g-C3N4) further loss the weight occurs within 750 to 800 0 C. The surface areas of MoO3/g-C3N4 composites are more as compared pure MoO3 and pure g-C3N4. The surface area of the composite (MoO3/g-C3N4) increases due to the modification of MoO3 grains on the surface of g-C3N4 is and again decreases due to agglomeration of a higher amount of MoO3 onto the surface of g-C3N4. UV-vis DRS spectra delineates the optical properties of MoO3/g-C3N4 fall in between those of MoO3 and g-C3N4. This is because of both MoO3 and g- C3N4 possess nearly equal band gap. For this reason both the semiconductors exhibit nearly the same photo absorbance. MoO3 shows less photocatalytic activity towards MO, after modification with g-C3N4 the activity increases. As the % of MoO3 increases the photocatalytic activity of MO increases (1.5 wt % is the highest) and again decreases because at high wt % of MoO3 (2 wt %) on g-C3N4 may block the visible light absorbance and decreases the degradation percentage. Furthermore MoO3/g-C3N4 obeys the Z-scheme mechanism as O2– • and h + the two active species during photocatalytic degradation of MO [41]. (II) Photocatalytic water spiltting: In recent year, production of clean energy is a big challenge. The conversion of solar energy to chemical energy is the most suitable greener method to mitigate the future energy crisis in cleaner way. This is because when hydrogen gas is burned as fuel, water vapour is released, which restricts global warming Manas Ranjan Pradhan, Snehasis Panda, Binita Nanda Page | 5024 and emission of other air pollutants. In this regard, photocatalytic water splitting, is the alternative way to conserve energy sources. Because in this process relies on sunlight and semiconductor and considered as solar-to-hydrogen conversion (STH). This can be calculated as: STH = Output energy as H2/ energy of incident solar light (7) Apparent quntum efficiency is the appropriate method to evaluate the photocalytic performance AQY=nR/I (8) Where, n is the number of electrons involved, R is the rate of production of H2 and I is the rate of incident radiation (photons). For photocatalytic hydrogen generation photochemical or photoelectrical cells are generally used but thermodynamically the process is difficult due to very high change in free energy of the process which is about +237.2kj/mole. For the generation of hydrogen basically three steps are used. First of all electron-hole pair is to be produced by absorption of light with energy equal or greater than band gap energy of photocatalyst. Secondly, the electron-hole pairs called charge carriers to migrate to the surface of photocatalyst or combine in the core. Last of all the photogenerated electron in CB reduces water to liberate hydrogen whereas the holes oxidise water molecules to liberate oxygen at VB [42]. The mechanism of the reaction is given below and explained through schematic diagram 3. 2H2O + 4 hυ → 2H2 + O2 (9) 4H + + 4e ‒ → 2H2 (reaction at CB) (10) H2O → 2h + → O2 (reaction at VB) (11) Qin eta al. developed a heterostructure photocatalyst by coupling Cu3P with g-C3N4 which are p and n type semiconductors respectively for effective charge separation there by improving photocatalytic hydrogen generation. This heterocatalyst shows 95 times more activity than bare g-C3N4 with AQE of 2.6% at 420 nm. As Cu3P has low cost and is earth abundant with a band gap of 1.5 eV that can improve the charge separation and enhance photocatalytic performance of g-C3N4. Hydrogen generation by g- C3N4 is faint but adding Cu3P increases the rate of hydrogen production which is about 95.7 times higher than bulk g-C3N4. It is to be noted that Cu3P is not an active photocatalyst but mixture of Cu3P and g-C3N4 increases slightly the generation of hydrogen but when a heterojunction is formed between Cu3P and g-C3N4 the production of hydrogen increases many more times. It is evident from SPV spectra. The SPV signal identifies the light response of wavelength range and how efficiently the charged pairs are separated in the photocatalyst. The spectroscopic and electrochemical analysis says that the heterojunction possesses efficient charge separation capacity which facilitates photochemical activity. When Cu3P is loaded over g-C3N4 an electron transfers from g-C3N4 to Cu3P takes place until the Fermi level becomes equal. When the heterojunction photocatalyst is exposed to visible light, the photogenerated electrons in CB of Cu3P diffuses into CB of g-C3N4 through p-n junction. Simultaneously the photogenerated holes in VB of g-C3N4 diffuses into VB of Cu3P. Now the electron in g-C3N4 reduces H + ion liberating hydrogen and holes oxidizes water to liberate oxygen henceforth justifying water splitting capability [43]. Scheme 3. Schematic representation of H2 gas generation. Wang and his team have synthesized g-C3N4 and Mn doped g-C3N4 nanoribbon for photocatalytic degradation of MB coupling with water splitting. The controlled synthesis of Mn doped g-C3N4 allows better results compared to bulk g-C3N4. HRTEM confirms the nanoribbon morphology of Mn doped g-C3N4 with high surface area 134 m 2 /g as compared to bulk g-C3N4 (58.7 m 2 /g). The successful doping of Mn onto the surface of g-C3N4 was observed by XPS and EDS mapping. From the band gap of both g- C3N4 and Mn doped g-C3N4 nanoribbon (2.67 and 2.56 eV respectively) confirms that both are visible light active. Mn dped g-C3N4 nanoribbon shows better H2 and O2 yield (380.39 and 74.78 µmol/gcat) respectively. Again coupling of photocatalytic performance and water splitting shows better results in the case of Mn doped g-C3N4 nanoribbon. This is because construction of nanoribbon and doping of Mn helps in separation of electron ad hole in the photocatalyst. Mainly doping of Mn facilitates the formation of OH • from H2O2 intermediate product. The coupling reaction solves the problem of H2O2 production and offers the possibility of energy utilization mode of waste water [44]. Different materials used in photocatalysis process are summarized in the Table 1. Table-1. Tabulations of different synthetic methods of g-C3N4 based composites and their applications. S.L. No Catalysts Methods of Synthesis Application References 1 Na-doped g-C3N4 Solid state method Photodegradation of 17α-ethynylestradiol 45 2 Cu (I) – g-C3N4 Wet impregnation-ultrasonication method Photodegradation of atrazine 46 3 g-C3N4 – WO3 Hydrothermal method H2 energy production 47 4 Se – doped g-C3N4 Ultrasonication method H2- production 48 5 g-C3N4/Silica gels Solvothermal method White light emitting devices 49 6 g-C3N4 Pyrolysis process Photo electrochemical sensing of methylene blue 50 7 MnO2/Ag/g-C3N4 Hydrothermal method Photoreduction of CO2 51 8 g-C3N4/Bi2WO6 Hydrothermal method CO2 photoreduction 52 9 g-C3N4-Ti3C2Tx Calcination process H2 evolution 53 10 CuO/g-C3N4 Sonication process 4-nitrophenol degradation and O2 evolution 54 11 g-C3N4-TiO2 Stirring followed by calcination Methylene blue degradation 55 12 NiO/g-C3N4 Hydrothermal Photocatalytic water splitting 56 13 WO3-g-C3N4 Hydrothermal Sulfamethoxazole degradation 57 A review on g-C3N4 based photo-catalyst for clean environment Page | 5025 S.L. No Catalysts Methods of Synthesis Application References 14 MoS2-QDs/g-C3N4 Wetness impregnation method H2 evolution 58 15 TiO2-g-C3N4 Hydrothermal method Organic pollutant degradation 59 16 WO3-g-C3N4 Calcination process Degradation of methylene blue and 4-chlorophenol 60 17 N-TiO2-g-C3N4 Impregnation method H2 evolution 61 18 g-C3N4-BiOBr Ultrasonic method Pollutant degradation 62 19 3C-SiC/g-C3N4 Pyrolysis method Degradation of methyl orange 63 20 PANI-g-C3N4 Oxidative polymerization process Methylene blue degradation 64 4. CONCLUSIONS In summary, this review article mainly highlights green technology for clean environment. In this regard, g-C3N4 based composites are proven to be a good photocatalyst for environmental remediation and energy recovery. To this end, the enhancement of photocatalytic performance of g-C3N4 is mainly due to three factors such as: 1) visible light responsive, ii) low rate of recombination of photoinduced charge carriers, and iii) high surface area. 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