Mini Review on the Structure and Properties (Photocatalysis), and Preparation Techniques of Graphitic Carbon Nitride Nano-Based Particle, and Its Applications
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Graphite carbon nitride (g-C3N4) is well known as one of the most promising materials for photocatalytic activities, such as CO2 reduction and water splitting, and environmental remediation through the removal of organic pollutants. On the other hand, carbon nitride also pose outstanding properties and extensive application forecasts in the aspect of field emission properties. In this mini review, the novel structure, synthesis and preparation techniques of full-bodied g-C3N4-based composite and films were revealed. This mini review discussed contemporary advancement in the structure, synthesis, and diverse methods used for preparing g-C3N4 nanostructured materials. The present study gives an account of full knowledge of the use of the exceptional structural and properties, and the preparation techniques of graphite carbon nitride (g-C3N4) and its applications.
KeywordsPhotocatalysis Graphite carbon nitride (g-C3N4) Carbon nitride nano-based particle
Graphite carbon nitride
The central energy source elated from the extraterrestrial space, solar energy capacities to surpass the almanac world’s energy request by a large border . Given the long forecast era of the Sun, solar energy is also considered the ultimate renewable source that can be harvested on the planet, Earth [2, 3]. The unending and discontinuous nature of this energy source, however, presents key challenges in relationships of harvesting, storage, and utilization . At the moment, there are a measure of technologies in place that may be used to face them. Solar energy can be flexibly gathered, transformed and kept in the form of heat, which can either distribute heat to residence or be further converted into electricity, as well as into other forms of energy . The most innovative investigated technologies concerning solar photon gaining may be on those by the photocatalysis, as described by Edmond Becquerel, 1839 .
Predominantly, wastewater is the major source of pollution, specifically, wastewater produced due to chemical industrialization, because this wastewater contains pronounced concentration of large organic fragments which are tremendously poisonous and carcinogenic in nature . Previously, the environmental remediation technology (which comprises of adsorption, biological oxidation, chemical oxidation, and incineration) has been used in the treatment of all types of organic and toxic wastewater and also has its effective application in solar energy utilization, environmental treatment, and biomedical and sensing applications. Fujishima and Honda revealed the exceptional knowledge about the photochemical splitting of water into hydrogen and oxygen in the presence of TiO2 in 1972; research interest has been focused in heterogeneous photocatalysis [3, 4, 5]. The speeding up of photoreaction in the existence of a catalyst is described as photocatalysis. Photocatalysis reaction is best known to be carried out in media such as gas phase, pure organic liquid phases, or aqueous solutions. Also, in most chemical degradation methods, photocatalytic degradation vis-à-vis photons and a catalyst is often identified as the best in controlling of organic wastewater, solar energy utilization, environmental treatment, and biomedical and sensing applications [3, 5]. Hence, the utmost technology used for the treatment of organic wastewater and related applications is attributed to the evolving solar light-driven photocatalysts .
Semiconductor photocatalysts can be used for the removal of ambient concentrations of organic and inorganic species from aqueous or gas phase systems in drinking water treatment, environmental tidying, and industrial and health applications. This is due to the massive ability of these semiconductors (g-C3N4, TiO2- and ZnO) to oxidize organic and inorganic substrates in air and water through redox processes for its effective application in solar energy utilization, wastewater, and environmental treatment, biomedical and sensing applications without any second pollution.
Polymeric graphitic carbon nitride (g-C3N4) has become the prime center for consideration in photocatalysis research . g-C3N4 is a visible-light-response element with band gap of 2.7 eV, and the energy location of CB and VB is at − 1.1 and 1.6 eV via normal hydrogen electrode respectively [Wang et al. 2009]. In addition, g-C3N4 has the ability to resist attacks from heat, strong acid, and strong alkaline solution . g-C3N4 has a unique ability to be simply prepared by thermally polycondensing the cheap N-rich precursors, such as dicyanamide, cyanamide, melamine, melamine cyanurate, and urea, and this is unlike the other metal-containing photocatalysts that require costly metal salts for preparation [6, 8]. Thermal condensation, solvothermal, chemical vapor deposition, microwave-assisted, polymerization, and hydrothermal synthesis are examples of preparative strategies (Table 2) which have been commendably applied in the preparation of carbon nitride for distinctive purposes and analysis in the area of photocatalysis and others .
Due to these outstanding properties of g-C3N4, the use of this promising g-C3N4 in water splitting, CO2 photo reduction, organic contaminants purification, catalytic organic synthesis, and fuel cells is more efficient and effective . The number of admirable researches and reviews on g-C3N4 structure and preparation in the last few years has increased tremendously . Authors mainly laid emphasis on the most contemporary advances on the structure, synthesis, and preparation techniques of g-C3N4 and carbon nitride (CNx) films vividly in this concise mini review. The unique structure and the novel synthesis and preparation techniques of g-C3N4, and CNX films are nicely presented, and the enlightened concepts on extending the preparation of g-C3N4 in this mini review are then emphasized. Also, the authors discussed the applications on g-C3N4, and the perspectives in future researches were also advocated.
Graphitic Carbon Nitride and Photocatalysis
Graphitic Carbon Nitride Nano-Based Particle
Materials with 1D nanostructures having distinct electronic, chemical, and optical properties could have their size and morphology adjusted. This ability of the 1D nanostructured materials has led to a novel advancement of diverse approaches to improve their photocatalytic activity . In addition, there is guidance of electron movement in the axial direction and lateral confinement of electrons by these 1D nanostructures. There has been advancement of 2D materials from graphene to metal oxide and metal chalcogenide nanosheets and then to 2D covalent organic frameworks (g-C3N4).
Structure of Graphitic Carbon Nitride Nano-Based Particle
g-C3N4 are a class of two-dimensional (2D) polymeric materials comprising entirely of covalently linked, sp2-hybridized carbon and nitrogen atoms. Carbon and nitrogen have the distinction of various valence states forming bonding; therefore, in g-C3N4, there are diverse of valence bond structures. Research works have initiated that some C3N4 defect structures and amorphous structures of g-C3N4 are still the metastable structures, but with the upturn of N vacancy, these two kinds of configuration of g-C3N4 material usually lessen in bulk modulus. The structural characteristics, composition of materials, and crystallinity of g-C3N4 can be characterized and evaluated by XRD, XPS, and Raman techniques. In 1830, Berzelius described the general formula (C3N3H) n and Liebig also devised the notation “melon,” and these predictions had then led to more research focused on carbon nitride oligomers and polymers [19, 20]. Furthermore, these crystal structures have been found and stated in experiments [21, 22, 23]. The α-C3N4 is earlier found by Yu and coworkers . A graphite-like loaded 2D structure of the graphite C3N4 is usually observed as nitrogen heteroatom- substituted graphite framework which mainly includes p-conjugated graphitic planes, and it is with sp2 hybridization of carbon and nitrogen atoms. Crystalline graphite is 3% less dense than the g-C3N4. Shifting the localization of electrons and then consolidating the bonds that is between the layers due to the nitrogen heteroatom substitution can help enlighten the interlayer distance of g-C3N4 .
Electronic Structure and Properties of g-C3N4
Currently, g-C3N4 is considered as a new-generation photocatalyst to recover the photocatalytic activity of traditional photocatalysts like TiO2, ZnO, and WO3. g-C3N4 is assumed to have a graphitic-like structure [26, 27, 28, 29, 30]. Thermal polycondensation method is generally used to prepare g-C3N4 and, hence, to investigate the electronic structure of g-C3N4.
The α-C3N4 is earlier found by Yu and coworkers . These scientists used the calculation procedure of quantum mechanics clusters model and developed α-C3N4 by optimization the electronic structure of g-C3N4 for photocatalysis and others. In the structure of alpha-C3N4, C and N atoms linked by sp3 key was to used design the tetrahedron structure of g-C3N4. Liu and Cohen anticipated the existence of beta-C3N4 by means of band concept of first principles and prepared beta-C3N4 based on β-Si3N4 electronic structure. Liu and Cohen then revealed that the structure of β-C3N4 was hexagonal encompassing 14 atoms for each unit cell.
The outstanding prediction anticipated by Liu and Cohen in 1989 that the b-polymorph C3N4 would have exceptional high hardness values in comparison with diamond has enthused scientific research to date . In 1993, C3N4 thin films via magnetron snorting of a graphite target on Si (100) and polycrystalline Zr substrates under a pure nitrogen ambience and consideration of the structure of C3N4 with analytical electron microscopy and Raman spectroscopy were synthesized by Chen and co-authors [27, 31]. Scientists, Teter and Hemley , foretold that alpha-C3N4, beta-C3N4, cubic-C3N4, pseudo cubic-C3N4, and graphite C3N4 show pronounced hardness approaching that of diamond in their experiment which they performed 3 years later as already described in accordance with first-principle calculations of the relative stability, structure, and physical properties of carbon nitride polymorphs.
Wang and coworkers [26, 32] applied ab initio evolutionary algorithm structure searches to calculate the precise structure of g-C3N4 prepared by thermal polycondensation and salt-melt synthesis methods for an enhanced visible-light-responsive photocatalysis. The most stable structure 1–3 were predicted for heptazine-based g-C3N4. The order of phase stability was 1 > 2 > 3. Contrary to other layered structures, distorted phases in heptazine-based g-C3N4 (see Fig. 3) were the most stable. This structure contributes the enhanced photocatalytic activity of the promise. In g-C3N4, lone pair electrons of nitrogen are mostly accountable for band structure and development of valence band.
Preparation of Graphitic Carbon Nitride Nano-Based Particle
Comparisons between hard templating and soft templating approaches used for g-C3N4 synthesis
1. Hard templating approach
i. The nano-casting technique using a hard template is the most widely reported and successfully applied method used for the introduction of mesoporosity in solid materials such as carbons, nitrides, polymers, and ceramics.
ii. Nano and casting differ in terms of the length scale involved. While casting is predominantly done on a macroscopic scale, nano-casting on the other hand is done on the nanoscale, and hence, the prefix “nano” is used while referring to the casting process as applied to the synthesis of materials with nano dimensions.
iii. This synthetic method involves the following three important steps: (a) synthesis of the ordered mesophase silica template; (b) infiltration of the template with the necessary precursors, including the conversion of precursors into a solid; and (c) removal of the template.
2. Soft templating approach
i. A soft templating approach has been extensively used for the synthesis of many mesoporous materials.
ii. Unlike the broadly reported hard template-based nano-casting procedures for the synthesis of graphene carbon nitride reports, on the use of soft templates for the synthesis of graphene are quite limited.
iii. The soft templating approach was appreciated by Antonietti and group for the preparation of carbon nitride through simple self-assembly between the organic structure directing agents and the CA.
Typical g-C3N4 preparation techniques
Melamine, cyanuric chloride
Fine nickel powder
Melamine, cyanuric chloride, urea
Crystalline, fine particles (Fig.7)
Chemical vapor deposition
Melamine, uric acid
Heptazine blocks, jaggy-like shape (Fig. 8), crystallinity, nanometric texture
Melamine, cyanuric chloride, urea
high (90 m2 g−1)
Comparisons of some selected Fabricating strategies of g-C3N4 synthesis
1. Supramolecular pre-assembly
a. Molecules adopt a well-defined arrangement into stable aggregates by non-covalent bonds under equilibrium conditions
b. Hydrogen bonding is highly essential in arranging the structure of supramolecular aggregates due to the directionality and specificity of this kind of inter-actions.
c. The precursor melamine can link with triazine derivatives, such as cyanuric acid, into supramolecular aggregates through hydrogen bonds
a. The use of melamine–cyanuric acid (CM) complex as starting materials was reported by Thomas and coworkers and Antonietti and coworkers. It was found that the CM morphologies depend on the used solvent for melamine–cyanuric acid molecular assembly, leading to various well-organized g-C3N4 with different morphologies
2. Molten salt strategy
a. Salt-melt synthesis usually acts as a solvent for high-temperature materials synthesis including many organic and inorganic reactions
a. Zou et al. successfully synthesized a carbon nitride intercalation compound by heating the melamine with a low melting point eutectic mixed salts under air and ambient pressure. Interestingly, g-C3N4 nanotubes were produced. The resultant g-C3N4 nanotubes are very stable and active for solar H2 production (Fig. 8b).
3. Ionic strategy
a. This strategy possesses high chemical and thermal stability, small vapor pressure, and the liquid nature at ambient.
b. Makes ionic liquid to be used as solvents in many fields
a. Reported the usage of 1-butyl-3 methylimidazolium tetrafluoroborate (BmimBF4) ambient ionic liquid as soft template and dicyandiamide (DCDA) as precursor to synthesize the boron- and fluorine-containing mesoporous-g-C3N4. Very interestingly, no micropores are present in obtained g-C3N4
Mesoporous structure, when mineralized and the specific surface area amplified, helps to fine-tune the physicochemical properties and then increases the photocatalytic performance of graphite carbon nitride (g-C3N4). Nano-casting/replication of mesoporous silica matrices is the first method used to prepare graphite carbon nitride (g-C3N4), these were famous for their cohort of the corresponding carbon nanostructures . Great efforts were then put in place to bring out more innovative schemes for g-C3N4 modification, which was enthused by the hard template method (Table 1). Liu and Cohen then discovered the (Table 1) soft template technique , and the other g-C3N4 modification schemes such as acidic solution impregnation, the ultrasonic dispersion method, and chemical functionalization  were also discovered. These methods as described above were good signs of the principle in modifying the surface chemical properties and the texture of g-C3N4, alone with its electronic potentials.
Thermal treatments, such as physical vapor deposition (PVD) , chemical vapor deposition (CVD) , solvothermal method , and solid-state reaction , are used for polymerizing plentiful nitrogen-rich and oxygen-free compound precursors comprising pre-bonded C–N core structures (triazine and heptazine derivatives), and these serve as the basic techniques for graphite carbon nitride (g-C3N4) synthesis. The commonly used precursors for the preparation of graphite carbon nitride (g-C3N4) through polymerization include cyanamide , dicyandiamide , melamine , urea , thiourea , guanidinium chloride , and guanidine thiocyanate . The use of accomplished elements directly is actually challenging in many areas; this is due to the weak dispersity and ordinary nature of the bulk g-C3N4. The use of ample micro/nanostructures and morphologies to prepare different kinds of g-C3N4 has been intensely researched by scientist over the few last years of photocatalysis studies. For example, ultrathin g-C3N4 nanosheets which were prepared by exfoliating bulk g-C3N4 materials [46, 47, 48] were negatively charged and could be well dispersed in water.
Techniques Used in Preparing Graphitic Carbon Nitride Nano-Based Particle
The study on the syntheses of carbon nitride (g-C3N4 and CNx) has enthused the curiosity of researchers from all over the world. g-C3N4 and films with precise photocatalytic properties have been synthesized [57, 58]. Thermal condensation, solvothermal, chemical vapor deposition, microwave-assisted, polymerization, and hydrothermal synthesis approached are methods (Table 2) which have been effectively used in the preparation of carbon nitride for different purposes and analysis in the area of photocatalysis and others .
Thermal and Solvothermal Treatment Methods
Niu and co. also reported the morphological changes when solvothermal technique was used . Loumagne and coworkers  testified the physicochemical possessions of SiC-based deposits, achieved via the thermal decomposition of CH3SiCl3 in hydrogen. Kelly and group  reported synthesis of TaC by using reactants tantalum (V) chloride and carbon mixed under an argon-filled glove box through the thermal process. Successively, thermal condensation method, which mostly consists of conjugated aromatic heptazine system with graphitic assembling characteristics, has been used several moments to prepare g-C3N4 . The use of solvothermal technique for g-C3N4 synthesis has great remunerations such as even and fine particle formation, little energy consumption, and higher economic feasibility as compared to the outdated thermal condensation method. Conversely, these methods are still time-consuming, demanding to a certain extent of a few hours to complete particle formation and crystallization.
Chemical Vapor Deposition
Investigation by Roberto and coworkers  suggested the use of chemical vapor deposition (CVD) for graphitic carbon nitride synthesis by the reaction between melamine and uric acid has high photocatalytic activity. It was found that the formed graphitic carbon nitride was with a structure based on heptazine blocks.
Roberto and coworkers then proposed that these carbon nitrides’ nature revealed a jaggy-like shape (Fig. 7), crystallinity, and a nanometric texture. Kelly et al.  has reported the synthesis of TaC by using reactants tantalum (V) chloride and carbon mixed under an argon-filled glove box via thermal technique and later transformed to TaC nanoparticles via chemical technique. CVD is one of the most useful methods to prepare monolayer graphene of high structural quality for use in different devices for catalytic activities . Wang and group [26, 32] obtained CN푥 films on Ni substrate by using HFCVD method firstly. Because the preparation of these films is more likely to produce C–H and N–H linkage under the CVD conditions, most of the CN푥 films are amorphous. From previous studies, CVD procedures are used to prepare carbon nitrides, the choice of substrate materials is very critical to be considered. Large area samples can be prepared by exposing a metal to different hydrocarbon precursors at high temperatures. There are different types of CVD methods available such as plasma-enhanced CVD, thermal CVD, and hot/cold wall CVD. CVD methods mainly consist of electron cyclotron resonance, hot filament-assisted, DC glow discharge, radiofrequency discharge, and microwave plasma chemical vapor deposition. Bias of auxiliary hot filament chemical vapor deposition (HFCVD) is one of the local tools used in the deposition of diamond films and others. The exact mechanism of the formation of graphene depends on the growth substrate but typically initiates with the growth of carbon atoms that nucleate on the metal after decomposition of the hydrocarbons, and the nuclei grow then into large domains . Recently, produced high-quality monolayer graphene by using resistive heating cold wall CVD was also 100 times faster than conventional CVD.
Sol–gel synthetic technique is a process through which a solid product or a nano-material is formed from a solution after the transformation of the gel intermediate. In this synthesis method, reactants are mixed at the molecular level allowing fast reactions and lead to more homogeneous products with higher surface area. Remarkably, this technique has been used to synthesize different types of nanoparticles including metal carbide, and nitride processes for photocatalysis . The synthesis of metal nitride using sol–gel processes can be traced back to the use of metal-organic compounds (synthesized from metal element and dialkylamine) .
Physical Vapor Deposition
It consists of magnetron sputtering, ion beam deposition (IBD), reaction sputtering, and pulsed laser deposition, and so forth. Reaction sputtering is the elementary method for preparation of composites. When this technique is used to prepare g-C3N4, the mass fraction of nitrogen is usually less than 40%. Conversely, to form 훽-C3N4, the system should consist of an adequate amount of nitrogen and stoichiometric ratio should reach 57%. Niu and his group  achieved the g-C3N4 on silicon substrate by using pulse laser evaporation C target, auxiliary deposition of atom nitrogen. Niu et al. studies found that the amount of N reached 40% in the films and then C, N atoms combined with nonpolar covalent bond. Successively, Sharma et al.  and Zhang et al.  also did some critical studies and then obtained CN푥 films by a similar method as discussed. Mihailescu and coworkers  also used ammonia instead of N2-manufactured hard CN푥 films with carbon nitrogen single bond, double bond, and triple bond and then found out that its optical band gap is 4.5 eV. From the recent study, what scientists frequently get are mixture films which comprise several crystal phases.
To consider the efficacy of prepared g-C3N4, photocatalytic hydrogen evolution using crystalline carbon nitrides (CNs) was proposed by Takanabe and his group . Takanabe et al. acquired carbon nitrides by supramolecular aggregation (Table 3) which was further monitored by (Table 3) ionic melt polycondensation (IMP) using melamine and 2, 4, 6-triaminopyrimidine as a dopant. There are other few methods similar to what Takanabe and his group used in their experiment, see Table 3.
Applications of Graphitic Carbon Nitride
There are several emerging applications of this graphitic carbon nitride and such applications include based sensing, biomedical applications, wastewater and environmental treatment, solar energy utilization and being used in device making.
Solar energy Utilization
Wastewater and Environmental Treatment
In summary, the unfeasible applications in wastewater and environmental pollution of most of the utmost well-versed photocatalysts is due to some of their demerit deterrents which includes, high cost, small scale, little photocatalytic activity, and thought-provoking recycle. Reasonably, in the area of environmental remediation, g-C3N4, TiO2-, and ZnO-based nano-material exhibit the most promising applications as result of their low cost, high photocatalytic activity, and no second pollution on the environment .
Biomedical and Sensing Applications
To increase the ability of g-C3N4 for sensing, biotherapy, and bioimaging usage, there is a need to alter the molecular structure, thereby enhancing the handling of the material in water. Due to the light photoluminenscence, highly recommended for biological related use, g-C3N4 nano-material is a very essential candidate for biomedical and sensing applications. The application of g-C3N4 for sensing, biotherapy, and bioimaging mainly considers its structure, synthesis, and preparative mechanisms. Zhang and coworkers  proposed that ultrathin g-C3N4 nanosheets could be used as biomarkers for the labeling of the cell’s membranes. g-C3N4 has also been suggested by Lin and co. to be a potential photosensitizers and pH-responsive drug nanocarriers for cancer imaging and therapy.
From the discussion, the future research of the g-C3N4 nano-based compound may focus on synthesizing innovative g-C3N4 nano-based particle which are responsive to morphology monitoring, evaluating the photocatalysis practicality and efficacy of traditional synthesis and preparative strategies of g-C3N4 nano-based compound, and then exploring the applications of diverse g-C3N4 nano-based particles in treating commercial wastewater, its effective application in solar energy utilization, environmental treatment, biomedical and sensing applications by fully assessing their photocatalytic ability, cost, energy consumption, and reusability.
In conclusion, this mini review climaxes the current advances on the structure and preparation techniques of full-bodied g-C3N4 nano-based material. Understandably, g-C3N4 has demonstrated to be one of the greatest favorable entrants suitable for scheming and assembling innovative composite photocatalysts. Thus, there is little uncertainty that the massive advancement of g-C3N4 nano-based particle will endure to develop in the near future. In view of that, more studies are also needed to making full use of the exceptional structural, synthesis, properties, and the preparation techniques of g-C3N4 nano-based particle.
Williams Kweku Darkwah was the recipient of a scholarship from the China Scholarship Council (CSC) for the duration of this work.
This work was financially supported by grants from the National Science Funds for Creative Research Groups of China (No. 51421006), the Natural Science Foundation of China (51679063), the Key Program of National Natural Science Foundation of China (No. 41430751), the National Key Plan for Research and Development of China (2016YFC0502203), Fundamental Research Funds (No. 2016B43814), and PAPD.
YA conceived the study and supervised the whole study. WKD drafted the manuscript including the design of the figures. Both authors read and approved the final manuscript.
WKD is a Master’s degree student (supervised by Prof. Yanhui Ao) at Environmental Engineering Department, College of Environment, Hohai University, Nanjing, China. He received his BSc degree in University of Cape Coast, Cape Coast, Ghana. His research interest mainly focusses on photocatalysis based water remediation technology using nano-materials.
YA is working as a Professor in Hohai University, Nanjing, China. He received his Doctorate degree in Southeast University, Nanjing, China. He has published more 130 academic papers and also has more than 7 patents. His research interests mainly focus on new photocatalysis-based water remediation technology using nano-materials, water resources protection, behavior of manufactured nano-materials in environment, and environmental friendly materials.
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