Saadat Majeed, Jianming Zhao, Ling Zhang, Saima Anjum, Zhongyuan Liu and Guobao Xu

Synthesis and electrochemical applications of nitrogen-doped carbon nanomaterials

De Gruyter | 2013

Abstract

Nitrogen doping is an effective way to tailor the properties of the shaped carbon materials, including the nanotubes, nanocups, nanofibers, as well as the nanorods, and render their potential use for various applications. The common bonding configurations obtained on the N insertion is the pyridinic N and pyrrolic N, which impart the characteristic properties to these carbon materials. This review will focus on the nitrogen-doped carbon materials, the doping effect on the electrochemistry of the doped nanomaterials, and the various synthetic methods to introduce N into the carbon network. The potential applications of the N-doped materials are also reviewed on the basis of the experimental and theoretical studies in electrochemistry.

1 Introduction

The carbon-based nanomaterials have been vigorously studied for the development of the many biological, electrical, and mechanical applications. Substantial research on their properties and potential applications has since ensued. Among them, the study of the shaped carbon nanomaterials (Figure 1) [1–6] such as the carbon nanotubes (CNTs), have stimulated enormous interest for constructing the sensors due to their unique physical and chemical properties, such as high surface-to-volume ratio, high conductivity, high strength, and chemical inertness. The CNTs are rolled up cylinders of graphene sheets. Many studies were carried out on these types of materials by several researchers for a long time. However, it is generally accepted that they were first observed as CNTs by Dr. Iijima in 1991 [7]. There are two distinct families of the CNTs [8], multiwalled CNTs (MWCNTs) and single-walled carbon nanotubes (SWCNTs). The SWCNT is a single, seamless, wrapped graphene sheet, which has the form of hexagonal aromatic ring patterns. It can be rolled up in many different ways, and the structure of the SWCNT can be classified into three basic types: armchair, zigzag, and chiral. The MWCNTs consist of coaxial SWCNT sheets and are materials with various textures. The CNTs have two common features [9]. One is the so-called herringbone texture, in which the graphene layers are at an angle with respect to the nanotube axis. The other is the so-called “bamboo” texture, in which the graphene sheets are oriented perpendicular to the nanotube axis. To take advantage of the attractive properties of the CNTs, the present review is a design to investigate the methods how the CNT has been modified by attaching onto the sidewalls, tips of the CNTs, and bulk modification. The properties of the MWCNTs and SWCNTs vary depending on the structure and the texture of the CNTs, but the major drawback in the utilization is concerned with the dispersions of the CNTs in the different solvents. Therefore, the CNTs have been doped and functionalized with many heteroatoms including B, Si, P, S, and N (Figure 2) [9–12]. The chemical reactivity of the graphene surface increases on the addition of the dopant and insertion of N that adds an extra electron to the surface. The type of the defect created by the heteroatom influences the kind of conduction generated ranging from the n-type transport (N substitution doping) to the p-type conduction (B substitution of boron in lattice) [21]. To solve the dispersion problem and to enhance the reactivity, the CNTs have been modified and functionalized for their application in electronics [22–24], clinical and biomedical [23], liquid-phase reactions [25], power sources filed [26–28] as nanoenergetic materials [29], and as gas sensors [30–32]. For the purpose of this review, we will also focus their representative/promising applications in the described fields covering from building materials to biological materials and from the electrical devices to the high nanoenergetic materials.

Figure 1 The common carbon reported shapes: (A) graphene sheet [1], (B) fullerenes [2], (C) carbon nanospheres [3], (D) nanocups [4], (E) bamboo-like CNTs [5], (F) nanorods [6]. (A) Reprinted (adapted) with permission from Ref. [1]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [2]. Copyright (2013) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [3]. Copyright (2004) American Chemical Society. (D) Reprinted (adapted) with permission from Ref. [4]. Copyright (2009) American Chemical Society. (E) Reprinted (adapted) with permission from Ref. [5]. Copyright (2011) American Chemical Society. (F) Reprinted (adapted) with permission from Ref. [6]. Copyright (2011) American Chemical Society.

Figure 1

The common carbon reported shapes: (A) graphene sheet [1], (B) fullerenes [2], (C) carbon nanospheres [3], (D) nanocups [4], (E) bamboo-like CNTs [5], (F) nanorods [6]. (A) Reprinted (adapted) with permission from Ref. [1]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [2]. Copyright (2013) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [3]. Copyright (2004) American Chemical Society. (D) Reprinted (adapted) with permission from Ref. [4]. Copyright (2009) American Chemical Society. (E) Reprinted (adapted) with permission from Ref. [5]. Copyright (2011) American Chemical Society. (F) Reprinted (adapted) with permission from Ref. [6]. Copyright (2011) American Chemical Society.

Figure 2 (A) The most common doping of the CNTs with heteroatom [9–12]. (B) Applications of the doped carbon nanomaterials [13–20].

Figure 2

(A) The most common doping of the CNTs with heteroatom [9–12]. (B) Applications of the doped carbon nanomaterials [13–20].

2 Synthesis of nitrogen-doped carbon nanotubes

The functionalized nanotubes are promising candidates for the reinforced and conductive plastics, sensor and photovoltaic materials, scanning probe microscopy tips, and much more. Many review articles have been dedicated to this topic as well [33–36]. Two broad synthetic methods have been proposed to make a variety of substituted nitrogen-doped CNTs (N-CNTs) (Figure 3), which includes (i) the nitrogen atom insertion into the CNTs during the reaction (in situ synthesis) [37–42], (ii) the post functionalization of the CNTs with nitrogen using the different precursors and compounds like organic moieties. The later method is reported in the literature but not investigated extensively [43–45]. Arc discharge [46, 47], laser ablation [48, 49], and the more recent plasma-etching [50, 51] technology are used for the synthesis of these nanomaterials. The major requirements for these techniques include the high-temperature conditions, limited variety of nitrogen/carbon precursors, rapid evaporation of the precursor targets along with the need of nitrogen or ammonia atmosphere, growth, and deposition of the N-CNT over the substrate materials. An alternative method, which can function at the lower temperatures in the presence [37, 52–54] and in the absence [50, 55] of an organometallic catalyst and numerous simple C/N sources [56], is recognized as the chemical vapor deposition (CVD) [31, 49]. The CVD method can give high yields of N-CNTs, typically 20–25 g per gram [45] of catalyst and posses nitrogen atoms embedded in the hexagonal carbon network at different ratios (10 atoms). For all the above synthesis methods, the nitrogen contents that range anywhere from the minimal incorporation of <1 atom % up to 20 atom % are reported [7, 34]. Czerw et al. named these highly oriented nanotubes/nanofibers of uniform diameter and length as “carpet-like structures” [57]. In the post functionalization of their work, N is added to an already synthesized CNT. Thus, nitrogen is deposited on the outer walls of an already made CNT, and regarding this method, the major drawback lies in its complexity owned by the multistep synthesis. This approach first leads to chemically oxidize the tips or structural defects of the CNTs and then couple them further with the other molecules via carboxylic, carbonyl, and/or hydroxyl groups located at the nanotube tips or its defects. For both methods, the covalent functionalization (through bonding to the π-conjugated skeleton of the CNT) is of the majority and is associated with the rehybridization of the sp2 bond. Interestingly, in this approach, the nitrogen is bonded to the carbon atoms in two fashions: (i) the pyridine-type N, in which each nitrogen atom is bonded to two carbon atoms; this type of doping creates cavities within the side wall of the tube, and (ii) substitution nitrogen, which corresponds to the nitrogen atoms bonded to the three carbon atoms (Figure 3). Nitrogen contains an additional electron when compared to carbon in network, so the CNTs with an N atom exhibit metallic properties [57, 58]. Moreover, the nitrogen groups increase the reactivity on the graphene wall when compared to the pure CNTs, which is basically inert. This enhanced surface reactivity manipulate the N-CNTs to be used as fast responsive sensors [59], efficient and intense field-emission sources [60], and as polystyrene (PS) and epoxy composites [32], protein [61] and nanoparticle [36] immobilizers, and so forth. The most common covalent functionalities involve the addition of the carbonyl and carboxyl groups via an aggressive treatment with a mixture of HNO3/H2SO4 or by plasma etching [62]. The latter technique can also be used to introduce the basic functionalities when applied in a nitrogen atmosphere. The carboxyl groups may then be acylated with thionyl chloride to make a basis for the various amine compounds [63] or to attach to the various proteins and DNA [64, 65]. The other commonly used chemical reactions to attach the organic groups include the cyclo additions (e.g., Bingel, Diels Alder), electrophilic and nucleophilic additions, ozonolysis, halogenations, or radical reactions (oxidative and reductive) [66–68]. The CNTs can also be synthesized by the arc evaporation of graphite [47, 49, 69] or by exploiting the pyrolysis of the hydrocarbons in the presence of the metal particles, particularly using the CVD method [42, 70, 71] or more attempting method of chemical modification [72, 73]. The noncovalent functionalization is also possible and mostly has been achieved by the adsorption or by the wrapping the CNTs in polymer polynuclear aromatic compounds, surfactants, or biomolecules via van der Waals forces and π-π interactions. The main advantage of the noncovalent functionalization of the CNTs, compared to the covalent one, is that with the former, the chemical functionalities can be introduced to the CNTs without affecting the structure and electronic network of the tubes. Owing to the vast number of the developed functionalization methods, only some highlights of the derivatization methods related to the CVD and chemical modification for the N-CNTs are discussed in detail here.

Figure 3 Representative N doping in the carbon nanomaterial structures.

Figure 3

Representative N doping in the carbon nanomaterial structures.

2.1 Chemical vapor deposition (CVD)

The most common technique for producing bulk amounts of the pure CNTs is based on the CVD route, which dwell in pyrolyzing the organic molecules (e.g., CH4, C6H6, C2H2, etc.) over a metal catalyst (e.g., Ni, Co, Fe) in an inert atmosphere [74, 75]. The catalytic CVD processes are simple and low cost; therefore, most of the synthesis and functionalization is carried out through the CVD method. The CVD synthesis involving the organic solvents like pyridine with Fe/Co in 2:1 volume ratio as the single C/N precursor and catalyst material produces highly pure aligned nitrogen-doped MWCNTs with a maximum of 9.2% nitrogen [76]. In another method, toluene with benzylamine is used to grow CNx nanotubes successfully [62]. The nanotube growth proceeded and emerged into two distinct morphologies of CNx nanotubes having the nitrogen groups attached on the tube surface, pyridine-type nitrogen, and substitutional nitrogen [71]. Sometimes, the CNTs from the ferrocene sources can also be obtained with all the four types of the N-functional groups, quaternary groups, and pyridine-N-oxide groups, including the former two, and are reported at the CNT surfaces by [77] and on heating the CNT under ammonia alone at low and high temperature conditions, respectively. Similarly, the pyrolysis of melamine under argon atmosphere yields bamboo-like CNTs [42] of a high pyridinic nature with the nitrogen content up to 10.4 at% at a low temperature of 800°C [29], while the face-centered cubic structures of the nitrogen-doped carbon possessing a high electrocatalytic activity have been obtained from the mixture of melamine formaldehyde resin/cobalt acetate at 700°C successfully [78]. The straight structure of the nitrogen-doped MWCNTs are reported from the nitrogen-rich metal phthalocyanin derivatives [79] and the mixture of C2H6/H2 and ammonia in the presence of alumina-supported iron catalyst at a temperature around 680°C in a CVD furnace [80]. The N-MWCNTs obtained by this method also possesses high N/C atomic ratios. The nitrogen content within the nitrogen-doped CNTs can be effectively tuned by employing the different amounts of nitrogen precursors [44, 81]. The growth rate of the nanotubes increases with the increase in precursor significantly, so the intensity ratio of the D to G bands in the Raman spectra of the nanotubes also increase. The inner structure of the nitrogen-doped nanotubes displayed a regular morphological transition from the straight and smooth walls (0 atom % nitrogen) to the cone-stacked shapes or bamboo-like structure (1.5%), then to the corrugated structures (3.1% and above) [82]. The melamine as a C/N initiator is used to synthesize the pure metal-free CNTs with a nitrogen-doping level as high as 20 atom %. The nitrogen atom in the reaction medium represents their unique property of self-assembly for the N-doping from the gaseous carbon without the assistance of metal [5]. The nitrogen-doped MWCNTs, synthesized by an aerosol-assistant catalytic CVD technique [54], manipulating the pyrolysis of xylene/pyridine or by ferrocene (catalyst)/N precursor imidazole [83], and/or by ferrocene with a modified precursor feeding technique using the CVD, produces a gradient of nitrogen concentration with varying nitrogen amounts. The CNTs on doping grows from a hollow cylinder to a bamboo-shaped structure. These structures contain a series of compartments whose lengths are gradually reduced, which is supposed to be related directly with the nitrogen concentration [84].

2.2 Chemical modification

The chemical modification approaches, as described earlier, usually includes the covalent and noncovalent approaches. The covalent modification of the nanotubes requires the formation and generation of the oxygen-containing functional groups, more specifically, the hydroxyl and carboxylic on the surfaces. The carboxylic acid groups are considered as one of the best choices because they can undergo a variety of reactions for further modification and are easily formed via the various oxidizing treatments, e.g., sonication in sulfuric and nitric acid, refluxing in nitric acid, ozonolysis, and air. The carboxyl-functionalized CNT can be further grafted (Figure 4) by the functional moieties with the terminal amine oxidization including the different mechanisms from the defect site chemistry oxidation reactions, esterification/amidation reactions to the oxidized CNTs [88, 89], nucleophilic additions/electrophilic additions [85, 90], mechanochemical functionalization [91, 92], cycloaddition reactions, ionic liquids (ILs) [93, 94], electrochemical modifications, diazotization [95], and radical additions [96]. The CNT functionalization via amidation and esterification of the nanotube-bound carboxylic acids [97], poly(benzyl ether) [62], and poly(amido-amine) (PAMAM) dendrons results in the dendrimer-MWCNTs hybrid materials of two enzymes, porcine pancreas lipase (PPL) and aminolipase (AL). In a similar way, lysine was covalently attached onto the MWCNT side walls, resulting in the nanomaterials that show a high dispersibility in deionized water (10 mg/ml) [66]. The other materials used, including the alkylamines [98], octaamino-substituted erbium bisphthalocyanine [99], and the metal-based [79] ferrocene and melamine [68, 82, 100], n-propylamine and n-butylamine [67], poly-amido amine [63], ferrocene/acetonitrile [84], cationic polyelectrolyte poly(diallyldimethylammonium)chloride [64], dendrimer [65, 101], increases the CNT’s activity/properties on doping. Hydrazine hydrate and diethyltriamine were doped to increase the water dispersion ability of the CNT [102]. The intermolecular conjugation of the proteins, and the uniform attachment of the proteins on the CNTs follow a two-step process including N-ethyl-N-(3-dimethylamino-propyl) carbodiimides hydrochloride (EDAC)-activated carboxylation and then through amidation forming an amide bond between the nitrogen-doped MWNTs and proteins by reacting it with the amine groups on the proteins of ferritin or bovine serum albumin (BSA) [89]. It is believed that in the case of the N-CNT, the carboxylation increases, and this causes the superior coverage of the metallo-proteins compared to the undoped MWCNT [103]. This approach provides an efficient method to attach the biomolecules to the CNTs.

Figure 4 Some schematic methods for the functionalization of the CNTs. (A) Dissolution and dichlorocarbene reaction of the SWCNTs [85]. (B) The electrochemical modification [86]. (C) 1,3 Dipolar cycloaddition to the CNTs using 3,4-dihydroxybenzaldehyde [87]. (A) Reprinted (adapted) with permission from Ref. [85]. Copyright (2003) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [86]. Copyright (2002) John Wiley and Sons. (C) Reprinted (adapted) with permission from Ref. [87]. Copyright (2008) American Chemical Society.

Figure 4

Some schematic methods for the functionalization of the CNTs. (A) Dissolution and dichlorocarbene reaction of the SWCNTs [85]. (B) The electrochemical modification [86]. (C) 1,3 Dipolar cycloaddition to the CNTs using 3,4-dihydroxybenzaldehyde [87]. (A) Reprinted (adapted) with permission from Ref. [85]. Copyright (2003) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [86]. Copyright (2002) John Wiley and Sons. (C) Reprinted (adapted) with permission from Ref. [87]. Copyright (2008) American Chemical Society.

2.3 Electrochemical modification

The successful doping and efficient tailoring methods for the CNTs can be carried out by controlling the redox properties of the dopant. The template synthesis of the various nitrogen-containing CNTs using the different nitrogen-containing polymers and the variation of the nitrogen content in the CNTs compared to the other electrodes like Pt showed a higher catalytic activity. This is attributed to the existence of the additional active sites on the surface of the N-containing CNT-supported electrodes, which favors the better dispersion of the Pt particles over the N-CNT and show an enhanced catalytic activity for methanol oxidation [104]. The doped CNT electrodes increase the output power of the thermoelectrochemical cells, which is associated with the fact that the doping state increases the electrochemically active surface area of the CNT electrodes proportionally. The charge transfer over these electrodes remained approximately equal to that of the pristine CNTs, and the accumulation of the potassium ions at the doped CNT electrodes was found to reduce the short-circuit current [105]. The doped nanotubes mixed with the glutaraldehyde-functionalized chitosan (GCS) show an improved biocompatibility and high conductivity for the enzyme immobilization, which is attributed to the increased kinetics from the N-CNTs [106]. A versatile approach to the electrochemical modification of the individual CNTs (small SWCNT bundles) is demonstrated by the attachment of the substituted phenyl groups fairly manipulating the two types of coupling reactions, working under oxidative (anodic) or reductive (cathodic) conditions. The anodic coupling to the SWCNTs was accomplished with the two aromatic amines, 4-aminobenzylamine and 4-aminobenzoic acid. The nitrogen-containing functional group created over the surface facilitates the further tailoring of the desired functional groups [86]. The porphyrin assembled on the nitrogen-doped MWNTs, a picket fence, through the Fe-N coordination providing the so-called noncovalent functionalization. This noncovalent modification with porphyrin makes the doped multiwalled nanotubes water insoluble but can be used for the highly efficient catalysis and biosensing [107]. The bamboo-shaped N-CNTs can also be synthesized from the Fe-containing SBA-15 molecular sieve as the catalyst with thermal decomposition. These nanotubes possess the different nitrogen contents presenting larger amounts of the defective sites in the structure. These nanotubes with higher nitrogen contents possess a lower graphitic ordering in the framework [81].

3 Effect of doping on the properties of carbon-based nanomaterials

The incorporation of the heteroatoms into a CNT modifies the tube characteristics [22]. Therefore, this phenomenon of substitution for the nitrogen atoms favors the formation of the pentagons and heptagons (Figure 3) and increases the reactivity of the neighboring carbon atoms resulting in a higher degree of disorder in the N-CNT relative to a “pure” CNT. The doping alters the electronic structures of the CNTs by charge transfer. This effect, therefore, influences the optoelectronic properties of the SWCNTs [108] because the band gap of the CNT is reduced to form the metallic CNTs with a high catalytic activity [109] (Figure 5). The pyridine-like N structures of the N-CNTs are found responsible for the metallic behavior and express the prominent features near the Fermi level. These electron-rich structures behave as the n-type nanotubes and are the good examples for the real molecular heterojunction devices [109]. The specific capacitance of the nitrogen-doped CNT electrode is also higher than that of the pure CNT electrode after cycling [112] because, as an electrode material for the electrochemical capacitors, the nitrogen functional groups contribute to the pseudo-Faradic capacitance [113]. The different types and sources of the N functional groups give different properties to the CNT. The structural characterization suggests the presence of more defects and rugged morphology on the N-CNTs synthesized from ferrocene compared to iron(II) phthalocyanin presenting a higher activity for the oxygen reduction reaction (ORR) catalysis based on the observed current behavior [114]. But the hydrophilic character of the CNTs seems to be dope-dependent, which increases with an increase in the N content. It is observed that the water dispersibility of the N-CNTs treated by hydrazine hydrate (having more N atoms) was better than that of the N-CNTs treated by diethylenetriamine (having less N atoms). Good hydrophilicity of the N-doping CNTs ameliorate the wettability of the CNTs for the electrolyte expanding its applications in the biomedical and clinical jurisdiction. The N-CNT provides a large surface area compared to the CNTs so the biomolecules’ immobilization on them follows the simple physical adsorption method [115], but it is also believed that the proteins can be denatured due to the uncontrolled interactions between the proteins and surfaces [116]. In view of it, it is found that the giant biomolecules, including the proteins, get attached at the surface of the doped tubes through the noncovalent interactions of the van der Waals forces, and during immobilization, no conformational changes seems to occur. The detailed conformational analysis determines that the N-doped MWCNT retain the correct confirmation of the metalloproteins irrespective of their size, charge, and folding motif compared to their counterpart, MWCNT [115], while increasing the nitrogen content, the surface adsorptibility becomes high, and the lower nitrogen content favors the electron transfer between dihydroxybenzene and the electrode surface [81]. In the in vivo and in vitro analysis, this property shows no toxicity but an enhanced and good hemocompatibility and cytocompatibility for the doped MWCNT [23]. The electrochemical data also indicated that CNx electrode with the lowest nitrogen content (1 atom %) showed the highest reversible capacity 270 mAh g-1 at the current density of 0.2 mA cm-2 and the highest value of exchange current density [54, 56]. The higher nitrogen content from both origins, i.e., the pyridine-like N and the graphite-like N, compared to the C atoms in the N-CNTs framework possesses higher electron densities at the surface of the CNx nanotubes. The extra electrons of the CNxNTs exhibit a negative active site on treating with acid, hence, increasing the adsorption properties of the tubes [117]. On the contrary, Tharamani et al. embrace the fact that the higher activity is not proportional to the total amount of nitrogen, and second, the quaternary nitrogen (another type of N insertion) also plays an important role for the enhanced activity in alkaline solution. The N-CNT obtained at the high temperature of 800°C with ammonia contains the highest amount of quaternary groups, but the lowest amount of nitrogen on the surface exhibits enhanced kinetic currents and electrocatalytic activity [77] than the undoped CNT. The nitrogen incorporation also strongly improved the stability and selectivity toward the C=C bond hydrogenation exhibiting much higher and significant improvement of the hydrogenation compared to those observed on the N-free CNT catalysts [25, 118].

Figure 5 (A) The effect of acid treatment on the structure of the functionalized nanotubes. (a) The CNTs/G and (b) N-CNTs/ G before the acid treatment and (c) CNTs/G and (d) N-CNTs/ G after the acid treatment [110]. (B) The TEM images and size distribution histograms (insets) of the Pt nanoparticles deposited on the CNTs (a) and CNx (b) [111]. (C) The gravimetric capacities and cycling performance of the as-grown N-CNTs and commercial multiwalled CNTs [27]. (A) Reprinted (adapted) with permission from Ref. [110]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [111]. Copyright (2011) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [27]. Copyright (2012) American Chemical Society.

Figure 5

(A) The effect of acid treatment on the structure of the functionalized nanotubes. (a) The CNTs/G and (b) N-CNTs/ G before the acid treatment and (c) CNTs/G and (d) N-CNTs/ G after the acid treatment [110]. (B) The TEM images and size distribution histograms (insets) of the Pt nanoparticles deposited on the CNTs (a) and CNx (b) [111]. (C) The gravimetric capacities and cycling performance of the as-grown N-CNTs and commercial multiwalled CNTs [27]. (A) Reprinted (adapted) with permission from Ref. [110]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [111]. Copyright (2011) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [27]. Copyright (2012) American Chemical Society.

4 Electrochemistry of doped carbon nanomaterials and their applications

Among all the electrochemical methods, cyclic voltammetry is the most widely used technique for acquiring quantitative information about the oxidation and reduction (redox) reactions. The cyclic voltammetry studies the redox properties of the chemicals and interfacial structure and rapidly provides considerable information on the thermodynamics of the redox processes, the kinetics of the heterogeneous electron transfer reactions, and on the coupled chemical reactions or adsorption processes. The high specific surface area and excellent structural, electronic, and mechanical properties make the CNTs a favorable ambassador for the electrochemical studies. However, producing a homogeneous dispersion of the CNT for electrode modification in the polar solvents is difficult due to its hydrophobic nature; therefore, the pretreatment or functionalization of the CNT is taken into account. The previous reports show that the electrodes modified with the pretreated CNT exhibit very potential applications (Figure 2) from their undoped progenitor.

4.1 Optoelectronics and as an electrode material

The picket-fence design of the porphyrin assembled on the nitrogen-doped MWCNTs display itself as a novel biofunctional material for biosensing and the photovoltaic devices [107]. The aligned N-CNTs synthesized from melamine with modulated morphology, controlled nitrogen concentration, and superior stability find their potential applications in developing the various nano-devices such as the fuel cells, fluidized-bed reactors, and nano-energetic functional components [82, 119]. The bilayer thin film-based N-CNTs [13] and the free-standing N-CNT arrays [14] are the promising candidates for constructing the flexible, low-cost electrode. These electrodes replace the fluorine-doped tin oxide/platinum and Pt-free substrate-independent counter electrodes for the dye-sensitized solar cells. The fast reaction rate on these N-CNT arrays and electron transfer rate from the substrate to the N-CNTs manifest much higher cathodic and anodic peak current densities than those of the Pt electrode, indicating a higher electrochemical activity for these tubes [14]. The “graphite-like” N dopant in the as-grown N-CNTs enhances their surface chemical activity and conductivity and presents the desirable performance for the electrochemical supercapacitors [67]. The electrochemical intercalation of the N-CNTs with lithium makes them a promising anodic source in the Li ion batteries [54]. The N-CNTs are successfully employed as the charge trap materials for the solutions [15] and used as the air-cathode electrocatalysts in order to improve the performance of the zinc-air batteries [16]. The kinetic-diffusion-controlled region indicates the first-order ORR kinetics for these materials with respect to oxygen. The four-electron reduction process makes it a promising and effective air-cathode catalyst. The electrochemical modification is also useful in transforming the metallic SWCNTs into the semiconducting tubes, which are occupying their remarkable position for the field-effect devices. The attachment of the ligand- or receptor-like functionalities offers the potential of producing nanowires. These moieties are compatible with the biological structural motifs for the applications such as contacting of the nerve cells on a solid support [86].

4.2 Sensors/biocatalyst

The doped CNTs are extremely attractive and important nanomaterials in bioanalytical applications due to their unique intrinsic properties. The electrical properties of the nitrogen-doped carbon nanomaterials are used for the fabrication of the electrical devices such as biosensors. The biosensors can be classified as the enzyme electrodes, immunosensors, DNA sensors, and microbial sensors. The glucose biosensors are often called the enzyme electrodes. The functionalized CNTs have been studied to properly immobilize GOx [120]. Shengyuan et al. [121], in their study, showed that nitrogen doping provides a simple, robust, and unique platform for biosensing and accelerate the electron transfer from the electrode surface to the immobilized GOx, leading to the direct electrochemistry of GOx. The direct electron transfer of the immobilized GOx led to the stable amperometric biosensing with a linear range from 0.02 to 1.02 mm and a detection limit of 0.01 mm. The biofunctional surface showed good biocompatibility, excellent electron-conductive network, and large surface-to-volume ratio. Similarly, the nanocomposites of the GCS mixed with the N-CNTs displayed an excellent enzymatic activity toward the reduction of oxygen or the oxidation of glucose [121]. The accelerated electron transfer occurs between the bioactive center embedded in the enzyme and the electrode, which is presented by the N-CNTs. This transfer is due to the significantly increased effective area for enzyme loading, and when compared with the conventional glucose fuel cells, this fuel cell yields much higher power output [106]. The nitrogen-doped MWCNTs are a good candidate material for the construction of the third-generation enzyme biosensors based on the direct electrochemistry of the immobilized enzymes.

The carbon-based nanomaterials are used in various forms for the development of the immunosensors. The N-CNTs are used as a platform to fabricate the cytosensor by decorating the N-CNT with thionine and gold nanoparticle exploiting layer-by-layer method. The cytosensor showed an excellent analytical performance for the enzymatic catalytic reaction of horseradish peroxide (HRP) following the two-step immunoreactions, toward the oxidation of thionine by H2O2 [61]. A simple strategy for using the Au/N-CNT nanocomposites as platform for the immobilization scaffold of the antibody for sensitive immunosensing of microcystin-LR is presented. This strategy allows the successful immobilization and enhances the catalytic efficiency of the antibodies by conjugations of the doped nanotubes and GNPs compared with the naked N-CNTs in this experiment [17]. Recently, Deng et al. have explored a novel double functionalization of a CNT delivery system containing polyelectrolyte and CdS quantum dots as the fluorescent labeling probes via electrostatic assembling [122]. With this novel functionalization, it has demonstrated an efficient and nearly twice stronger adsorption ability toward the dissolved O2 than the CNTs.

4.3 Electrocatalytic applications

Nitrogen is known to be able to create defects on carbon, which may then increase the edge plane exposure and, thus, enhance the catalytic activity [123, 124]. It plays an important role as an active site for the nanocarbon ORR catalysts and provides choices of pyrrolic, pyridinic, and graphitic form in the carbon network. Studies showed that the N-CNT surfaces, eliminates the need for the additional peroxides or electron-transfer mediators. The catalytic activity of the N-CNT toward the ORR and subsequent H2O2 disproportionation creates a sensitive electrochemical response to the enzymatically generated H2O2 [125]. The N-CNTs produced from the different N-sources using a single-step CVD process having much higher nitrogen content and more distortion properties express a better ORR catalytic performance in the acidic electrolytes [56]. The comparable analysis of the N-doped MWCNT and the undoped CNT showed a remarkable enhanced activity and efficient four-electron transfer for H2O2 in the former case with 87 times more sensitivity making it a good H2O2 sensor [126]. The doped CNTs can be used as platforms for the multiple derivatization by loading their surface with other therapeutic agents (treatment), fluorescent, magnetic, or radionuclide probes (tracking), active recognition moieties (targeting), and with nanoparticles. Substantial work has also been carried out in alkaline/acidic/neutral media anchoring metals and their oxides such as Pt [127], Pd [128, 129], Au [17, 64], Cu [130], Fe [131], Ga [132], ZnO [133], and SnO2 [70] over the N-CNT surfaces. The past studies showed a much higher electrocatalytic oxidation for the alcohols over Pt/N-CNT compared to the activity shown by Pt/C [5]. In conclusion, it is demonstrated that the stacked N-CNTs have a similar catalytic ability in the ORR as the Pt-CNTs and also could be used in the electrochemical detection of H2O2 and glucose successfully. Further, these doped nanotubes have shown the potential to replace the costly Pt-based catalyst in the fuel cells and sensors [134]. The incorporation of the electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on the adjacent carbon atoms [135]. It is reported that this effect, coupled with aligning the N-CNTs, provides a pathway consisting of the four-electron system for the ORR, so, in this case, the vertically aligned N-CNTs act as a metal-free electrode with a much better electrocatalytic activity, long-term operational stability, and tolerance to the crossover effect than platinum for oxygen reduction in the alkaline fuel cells. When the other composite materials like AU/PANI are loaded over the surface of the N-CNT, their catalytic behavior increases for the detection of H2O2 even at the low reduction potentials and oxidation potentials [136]. The catalytic reduction of the dissolved oxygen at the side wall of the SWCNT leads to signal-on sandwich immunoassay [137]. The cyclic voltammetry and EIS results indicate that the N-CNTs decorated with the SnO2 and CeO2 nanoparticles show even far better electrocatalytic activity for the NO electro-oxidation due to the much faster rate of charge transfer compared to the undoped CNT and conventional CNT-based composites, representing their potential application in electrochemistry for the NO sensor [117]. The N-CNT synthesized in the metal free environment from melamine exhibited an excellent catalytic activity for oxygen reduction in the fuel cells, comparable to the traditional platinum-based catalysts. The four- and two-electron combined pathway, in the case of the N-CNTs as a catalyst, exhibits an excellent selectivity in the ORR, with no visible response to methanol oxidation in competition to the Pt-C catalysts [5], and the two-electron pathway for the undoped MWCNTs could be use to generate the H2O2 sensor even at a low potential of +0.3 V [126].

4.4 Biomedical applications

The doped CNTs can be used in the biosensors, as a cytosensor in cancer cell detection and cell surface carbohydrates and p-glycoprotein [61], in the drug delivery systems, as anticancer agents, and in the imaging applications as most biomolecules can be immobilized on the doped CNTs. The immobilization method for the biomolecules governs the noncovalent approach via the electrostatic interaction, van der Waals force, or π-π stacking [138]. The carbon nanomaterials have better biocompatibility and mitigate cytotoxicity compared to the undoped pristine CNM (Figure 6) [8]. These nitrogen-doped materials are successfully employed for the simultaneous determination of ascorbic acid, dopamine, and uric acid in the biological samples [139].

Figure 6 The biocompatibility and toxicological studies of the pure MWCNTs and N-doped MWCNTs [8]. Reprinted (adapted) with permission from Ref. [8]. Copyright (2006) American Chemical Society.

Figure 6

The biocompatibility and toxicological studies of the pure MWCNTs and N-doped MWCNTs [8]. Reprinted (adapted) with permission from Ref. [8]. Copyright (2006) American Chemical Society.

The various low-molecular weight proteins can adsorb spontaneously on the walls of the doped CNTs. The great compatibility of the N-CNTs with the biomolecules suggested that they can be functionalized with the proteins and can adsorb more quantity of antibody and proteins [61, 86, 115] than the CNT due to the availability of the large number of the functionalities produced on the surface. The protein is adsorb individually, strongly, and noncovalently along the length of the CNTs [140, 141]. Ferritin, cytochrome, and polysaccharides, such as starch, have been reported to be successfully immobilized [141]. The immobilization is fairly endorsed by the electron transfer due to the nitrogen groups between the metalloproteins and the electrode surface [142]. Hence, the N-CNT provides a novel new class of protein transporters for the various in vitro and in vivo studies.

5 Doping and electrochemistry of other carbon-based nanomaterials

Imparting the CNTs, the other shaped carbon-based nanomaterials with basal nitrogen functionalities have also been synthesized and are accounted for their novels traits (Figure 1). All these carbon-based materials have been synthesized by the reported methods, which includes the CVD, the catalytic-reduction route, the sol-gel method, the solid template approach, and the solvothermal method. These materials share the common characteristics on doping with the electron-rich heteroatom like the high specific activity, large surface area, low density, high surface permeability, and good electronic properties [143, 144].

5.1 Graphene

Graphene is considered as an interesting, exciting, and eye-capturing two-dimensional (2-D) structure formed by the sp2 hybridized carbon [145–149]. The nitrogen-enriched graphene (NG) materials have been prepared by the various synthesis techniques and routes (Figure 7) [151, 152] in order to meet the desired requirements for the various electrochemical and biochemical applications [139, 153–156]. For example, the thermal annealing of graphene oxide in ammonia [157], a facile, catalyst-free thermal annealing approach for the large-scale synthesis of NG up to 10.1% nitrogen content using the low-cost industrial material melamine as the nitrogen source [158] and the solvothermal synthesis of N-doped graphene by the reaction of tetrachloromethane with lithium nitride under mild conditions [159]. The nitrogen-doped graphene of high specific surface area, controllable “N” content, and high percentages of pyridinic- and graphitic-N sites can be produced by following the route of the self-assembled molecule functionalization, ultra-rapid thermal expansion-exfoliation, and the covalent transformation of the mixture of graphene oxide and melamine as a nitrogen source [160]. Another simple route of the hydrothermal reaction of graphene oxide and urea produces the nitrogen-doped graphene nanosheets with the nitrogen level as high as 10.13 atom % [161]. The temperature besides the nitrogen sources has distinctive effect on the doping of the nitrogen in the graphene network (Figure 8) [157, 158]. Mayavan et al. generated the pyrrolic- and pyridinic-type nitrogen doping in graphene at 300°C and 500°C, respectively, by the thermal treatment of graphite oxide (GO) with glycine and silver nitrate [162]. The nitrogen-doped graphene obtained by this method contains 13.5% atomic percentage of nitrogen in the as-prepared sample [162]. The synthesis of the nitrogen-doped graphene has also been reported by exploiting the chemical vapor deposition technique using methane in the presence of ammonia as a metal-free electrocatalyst [163], and by using ammonia on the Cu film/Si single crystal substrate [154], and with the microwave plasma-assisted CVD using polydimethylsiloxane (PDMS) as a solid carbon source [152]. The arc discharge of the graphite electrodes in the presence of H2, He, and pyridine vapor have also been developed recently for the efficient synthesis of the nitrogen-doped graphene [164, 165]. Huli-cova et al. polymerized melamine in the interlayer space of a fluorinated mica template and subsequently carbonized the material at high temperatures ranging between 650°C and 1000°C for the synthesis of the nitrogen-doped graphene [166]. The ordered mesoporous silica SBA-15 was also used as a template for the metal-free nanocasting technology along with the nitrogen and carbon source, N,N′-bis(2,6-diisopro-pyphenyl)-3,4,9,10-perylenetetracarboxylic diimide (PDI) [166, 167]. It has been reported that the doping of graphene with the N heteroatoms could effectively alter the chemical, optical, and magnetic properties of the carbon materials [59, 167–170] and the changes of the band gap of graphene to introduce the new properties [171] for the electric devices like the fuel cells, for sodium [172] and lithium ion batteries [173–175], supercapacitors [151, 176], ORRs (Figure 9) [158, 177–180], dye-sensitized solar cells [181], and other energy conversion and storage devices [161].

Figure 7 (A) The growth of graphene from food, insects, or waste in a tube furnace [150]. (B) The nitrogen-doped graphene by plasma treatment [151]. (A) Reprinted (adapted) with permission from Ref. [150]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [151]. Copyright (2011) American Chemical Society.

Figure 7

(A) The growth of graphene from food, insects, or waste in a tube furnace [150]. (B) The nitrogen-doped graphene by plasma treatment [151]. (A) Reprinted (adapted) with permission from Ref. [150]. Copyright (2011) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [151]. Copyright (2011) American Chemical Society.

Figure 8 (A) The effect of temperature on the nitrogen doping in the graphene sheets by XPS [158]. (B) The Raman spectra of the pristine graphene (violet) and NGs with different nitrogen atomic percentages [159]. (A) Reprinted (adapted) with permission from Ref. [158]. Copyright (2009) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [159]. Copyright (2011) American Chemical Society.

Figure 8

(A) The effect of temperature on the nitrogen doping in the graphene sheets by XPS [158]. (B) The Raman spectra of the pristine graphene (violet) and NGs with different nitrogen atomic percentages [159]. (A) Reprinted (adapted) with permission from Ref. [158]. Copyright (2009) American Chemical Society. (B) Reprinted (adapted) with permission from Ref. [159]. Copyright (2011) American Chemical Society.

Figure 9 Ultracapacitors (UC) based on nitrogen doped graphene and their electrochemical testing. (A) The charging and discharging curves measured by galvanostatic characterization [151]. (B) The gravimetric capacitances of UCs based on various nitrogen doped graphene and pristine graphene measured at a series of current densities [151]. (C) (a) The ORR obtained at a bare GCE (a), graphene/GCE (b), and NG5/GCE (N% =7.1%) (c) [159]. (b) The CVs for ORR at NGs, synthesized with different mass ratio of GO and melamine (1:1, 1:2, 1:5, 1:10, 1:50) [159]. (A, B) Reprinted (adapted) with permission from Ref. [151]. Copyright (2011) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [159]. Copyright (2011) American Chemical Society.

Figure 9

Ultracapacitors (UC) based on nitrogen doped graphene and their electrochemical testing. (A) The charging and discharging curves measured by galvanostatic characterization [151]. (B) The gravimetric capacitances of UCs based on various nitrogen doped graphene and pristine graphene measured at a series of current densities [151]. (C) (a) The ORR obtained at a bare GCE (a), graphene/GCE (b), and NG5/GCE (N% =7.1%) (c) [159]. (b) The CVs for ORR at NGs, synthesized with different mass ratio of GO and melamine (1:1, 1:2, 1:5, 1:10, 1:50) [159]. (A, B) Reprinted (adapted) with permission from Ref. [151]. Copyright (2011) American Chemical Society. (C) Reprinted (adapted) with permission from Ref. [159]. Copyright (2011) American Chemical Society.

5.2 Carbon nanotube cups

The nano-containers, usually referred to as the CNT cups (Figure 1) in the nanotechnology world, with diameters of 12–40 nm, are synthesized by the CVD through the incorporation of the nitrogen atoms into the graphitic carbon structure [182]. These nanocups were also converted into the hollow structures, and these hollow nanocups (nanocapsules), possessing nitrogen functionalities, thus, serve as potential nano-containers. The nanocups lack covalent interaction but are physically stacked with each other so the nitrogen-doped nano-containers have been used to design the specific cell targeting and controlled release of the drugs in the target cells with high efficiencies due to their enzymatically biodegradable nature [183]. The stacked nitrogen-doped containers exhibit a comparable performance to that of the Pt catalyst. The oxygen reduction reaction, in comparison to the commercial Pt-decorated CNTs, exhibits a superior performance in effectively catalyzing O2 reduction for H2O2 and glucose detection [135, 184].

5.3 Nitrogen-doped carbon cylindrical nanostructures

The cylindrical nanostructures (carbon nanofibers, carbon nanorods) (Figure 1) have the graphene layers arranged as stacked cones, cups, or plates. The CNFs tend to be wider (>100 nm) than the CNTs (typically having internal diameters of <50 nm). These highly amorphous structures can be synthesized at lower temperatures than required for the CNT synthesis. These cylindrical nanostructures can be synthesized by the carbonization method using polyaniline as the carbon precursor and presents the selective and specific determination of dopamine in the presence of ascorbic acid [185], and by the CVD method using the nitrogen and carbon precursors consisting of ferrocene and either xylene or pyridine, respectively. The electrode fabricated from the N-doped CNF presents a significant catalytic activity toward oxygen reduction from the neutral to basic pH over the undoped carbon nanofibers. The ORR catalysis occurs over these nanostructured electrodes by the two-electron pathway [124]. These nanofibers are also used as the anode in the lithium ion batteries due to the enhanced reactivity and electric conductivity at the nitrogen-rich surfaces [186] and the binder-free cathode in the microbial fuel cells [187].

5.4 Nitrogen-doped carbon hollow spherical structures

A carbon sphere (CS) (Figure 1) generally refers to a spherical form of carbon that can be either semicrystalline or crystalline (graphitic) and can have a solid, hollow, or core shell morphology. The nitrogen-doped hollow spherical CSs (carbon nanocage or nanoparticles) have been extensively investigated in the recent years. These are the kind of nanocarbons with a graphitic shell and a hollow interior, show great potential applications in the widespread fields with the advantages of their low density, large surface area, good electrical conductivity, as well as performance stability [188–190].

These core/shell nanostructures of the carbon materials are adequately synthesized by several methods [191, 192], including the pyrolysis of the carbon nanofibers and the nitrogen-doped graphitic layer pyrolysis as a core and shell, respectively. The internal surfaces of these cages are freely available for the electrolytes and entrapping molecules [193]. The other synthetic methods reported are the quenching graphite rods in the aqueous NH4HCO3 at 1000°C [194] and the most popular templating process. The nanoporous graphitic nanocage, synthesized in the presence of the nitrogen template, display their efficient methanol oxidation property. The excellent characteristics depicted by these nanostructures are due to the generation of the nanopores on the shell of the nanocage, after templating, which act as the active sites. The whole molecular diffusion takes place by these nanopores and demonstrate an excellent ORR performance with high activities in the alkaline medium [195] and positively been used for the lithium ion batteries as the anode [196]. The nitrogen-doped hollow nanospheres, which are prepared via the hydrothermal method, also provide a fast electron and lithium ion transport [197]. The nitrogen-doped nanoparticles due to their special structure have inspired the interest as counter electrodes in the quantum dot-sensitized solar cells, revealing a low-charge transfer resistance and a high exchange current density between the electrolyte (polysulfide) and the electrode. The N doping to the hollow carbon nanoparticles accord the electrode excellent stability and tolerance against sulfur poisoning [198], over the Pt electrode. The hollow spheres have also been designed for the immobilization and biosensing of the proteins and laccase as the enzyme electrodes [199, 200], oxygen reduction reaction, and as storage devices [201, 202]. The small spheres are synthesized using 3-ethyl-1-vinylimidazolium tetrafluoroborate as the source of carbon and nitrogen [201], poly(o-phenylenediamine) [202], pyrrole [203], and by the polymerization of dopamine [204], using the different synthetic techniques.

5.5 Nitrogen-doped porous nanocarbon structures

The porous nanocarbon, also known as the sponge-like mesostructures [205], are lightweight in nature [206], with a larger surface area. These nitrogen-doped porous moieties are reported to be synthesized by several methods. The NH3/N2 mixture gas treatment at high temperature during the carbonization process on the resorcinol-formaldehyde cryogels for N-RFCC [207], pyrolyzing gelatin between 700°C and 900°C with a nano-CaCO3 template containing nitrogen in the material in the form of pyridinic, pyrrolic/pyridonic, and graphitic nitrogen [208], well-ordered nitrogen-containing mesoporous carbon via the nanocasting method using the gelatin biomolecules, and the SBA-15 as a template [209], and the highly N-enriched microporous carbon/CNT conjugates by carbonization [210], replication of the mesoporous silica thin films through electrodeposition for the synthesis of the mesoporous carbon thin films doped with nitrogen [211], and the carbon-based gels by melamine, formaldehyde doped with Co and Fe metals [212]. The mesoporous nitrogen-based carbon can also be achieved by using DNA as the carbon and nitrogen precursor in the presence of the hard-template (nano-CaCO3) [213], and the phenol-melamine-formaldehyde carbonization in the presence of the soft template like Pluronic F127 [214]. The N-doping endowed the structure of the mesoporous carbon as the hard carbon materials have a more disordered discharge and is restoring at recharge, and these structured materials are used in lithium storage [208] applications, in the fuel cells, a metal free catalyst [214], as supercapacitors [215–217] and have greater adsorption capacity and selectivity for CO2 [218], as a gas sensor. The mesoporous carbon thin films doped with nitrogen are used in the dye-sensitized solar cells. The N embodiments in the mesoporous carbon thin films provides a larger capacitance current compared to that of Pt [211]. The nitrogen-doped nanoporous carbon materials prepared by the sol-gel method are effectively used for the trans-esterification reactions due to the high amount of nitrogen over the surfaces that participate in the catalytic activities [219]. Despite the thin films and fibers, the other nanostructures prepared and modified are the monoliths. The available methods for the nitrogen-doped monolithic carbon aerogels include, the hydrothermal method using the protein (nitrogen source) and d-glucose (saccharide) as a surface-stabilizing agent [220], and the ammonia-assisted carbonization [221].

6 Concluding remarks

The doped-carbon nanomaterials seek a competitive position in the field of nanotechnology due to their synthesis, properties, and electrochemical applications. These nanomaterials, compared to the undoped counterpart, possess a remarkable range of morphologies and structures, shows much larger functional surface area, higher ratio of surface active groups to volume, and a more disordered behavior. Moreover, the article reveals the pronouncing effects from the physical parameters such as temperature, pressure, gas flow rates, type, and concentration of the nitrogen source on the type, size, and yields as well as the total nitrogen content of the N-CNTs. These N-CNTs express their extraordinary properties in the fuel cell and solar cell applications, oxidation and hydrogenation reactions, sensors, membranes, therapeutics and diagnostics studies, and as a catalyst. Regardless of the marvelous outcome in the synthesis and electrochemical properties of the nitrogen-doped carbon materials, many challenges still exist. For example, the methods to produce the doped nanomaterials in high purity with controlled morphology, the actual mechanism of the doped material growth, and the influence of the defects on the physical properties, are the key issues that are open to conceive and debate. Some other issues relating to the surface properties, electrical transport properties, and binding of the molecules are comparatively less investigated despite the reports of their better biocompatibility. The C-N microenvironment that holds the biomolecules via a weak binding force and causes them to leak from the surface during the experiments is likely to be digging in. The work on these attachments, thus, implies more horizons for the applications in nanotechnology, biomedical, and energy storage through the utilization of the hollow cavities of the carbon nanomaterials including the CNTs, carbon nanofibers, carbon nanocups, carbon nanospheres for drugs, or chemotherapeutic agents, followed by cross linkage, encapsulation, and noncovalent interactions. Graphene research is currently another area of intense scientific interest due to a wide variety of potential applications in the biomedical, optical, and electronic fields. There are still many avenues of human interests that seems to be untouched and needs more progress in carbon-based nanotechnology. These majorly include the grapheme-based sensors to diagnose diseases, development of grapheme-based batteries with shorter rechargeable time, but much faster electron transfer, enhancing efficiency of the grapheme-based fuel cell and ultracapacitors to overcome the energy crisis.

This project was kindly supported by the Natural Science Foundation of China (No. 21175126), Faculty Development Program of the Bahauddin Zakaryia University, Multan, Pakistan (100 Foreign Scholarships) (No.PF/Cont./2-50/Admin/5398), the Academy of Sciences for the Developing World (TWAS), and the Chinese Academy of Sciences (CAS).

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