Journal of Science: Advanced Materials and Devices (2017) 141e149 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Review Article Synthesis, characterization and prospective applications of nitrogen-doped graphene: A short review Roshni Yadav*, C.K Dixit Material Science Research Laboratory, Department of Physics, Dr Shakuntala Misra National Rehabilitation University, India a r t i c l e i n f o a b s t r a c t Article history: Received 18 March 2017 Received in revised form 18 May 2017 Accepted 19 May 2017 Available online 30 May 2017 Graphene is a high crystalline material possessing the high electronic qualities Doping of nitrogen in graphene is to tailor/control the electronic, chemical and structural properties of graphene by manipulating it through the means of doping such as its surface area and functional sites Different configurations i.e Pyridinic N, Pyrrolic N, Graphitic N are obtained while doping nitrogen into graphene This review paper focusses on various synthesis and characterization techniques for the analysis of structural configurations of the nitrogen-doped graphene and its potential applications in various fields such as nanoelectronics, energy storage and electrochemical biosensing © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Graphene Nitrogen doping Nanoelectronics Electrochemical biosensing Energy storage Introduction Graphene represents a major advancement in modern science and is one of the most promising materials for implementation in the next generation electronic devices Because of graphene's unique properties, devices based on mechanisms alternative to classical charge transport come into research that would allow the unprecedented speed of graphene based transistors [1], and thus graphene has emerged as one of the most prominent research fields The enchanting properties of graphene, such as high surface area (2630 m2/g) [2], high thermal conductivity (~5000 W/mK) [3], fast charged carrier mobility [3] and strong Young's modulus (~1 Tpa) [4], has been well documented Different morphologies have also been observed, including two-dimensional graphene nanosheets, one-dimensional graphene nanoribbons [5e7], and zero-dimensional graphene quantum dots (GQDs) [8,9] A promising resemble for tuning and controlling the electronic properties of graphene is doping with heteroatoms Thus, doping with nitrogen atoms allows graphene transformation into p- or n-type semiconductor respectively, accompanied by the opening of a bandgap [16] Chemical doping is one of the important factors in tailoring the properties of graphene, which has been proved * Corresponding author E-mail address: roshniyadav05@gmail.com (R Yadav) Peer review under responsibility of Vietnam National University, Hanoi effective in the doping of carbon nanotubes (CNTs) and has greatly broadened their applications [10e14] When a nitrogen atom is doped into graphene, three common bonding configurations within the carbon lattice, including quaternary N (or graphitic N), pyridinic N, and pyrrolic N are observed (Fig 1) Generally, Pyridinic N bonds with two C atoms at the edges or defects of graphene and contribute one p electron to the p system Pyrrolic N refers to N atoms that contribute two p electrons to the p system, although unnecessarily bond into the five-membered ring, as in pyrrole [16,17] Quaternary N is the N atoms that substitute for C atoms in a hexagonal ring In these nitrogen types, pyridinic-N and quaternary-N is sp2 hybridized and pyrrolic-N is sp3 hybridized Other than three common nitrogen types, N-oxides of pyridinic-N have been observed in both the N-graphene and N-CNT studies [18,19] In the present review, we have focussed on the synthesis and characterization techniques, effects of nitrogen doping in graphene and its potential applications A nitrogen atom contains one additional electron and when replacing a carbon atom in the graphene lattice, a novel electronic property can be predicted Commonly, incorporating nitrogen into a matrix of carbonbased materials in order to acquire the desirable semiconducting properties is a rapidly growing field in the carbon technology [20e26] Table shows a comparative study of the various synthesis methods, precursors used, as well as the Nitrogen content obtained and its potential applications in different fields http://dx.doi.org/10.1016/j.jsamd.2017.05.007 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 142 R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 Fig Three common bonding configurations of Nitrogen-doped Graphene [15] Synthesis methods of N-graphene Various methods are employed towards the synthesis of nitrogen-doped graphene Basically, two common methods involved for the synthesis are categorized as Direct Synthesis and Post treatment Direct synthesis method involves chemical vapor deposition (CVD), segregation method, solvothermal and arc discharge methodologies whereas, Post-treatment method involves thermal treatment, plasma treatment, and N2H4 treatment Further explanations for the synthesis of Nitrogen doped Graphene are explained as below 2.1 Direct synthesis 2.1.1 Chemical vapor deposition (CVD) Chemical vapor deposition is one of the recent modern synthesis techniques and is widely used in the synthesis of various carbon nanomaterials, such as Carbon nanosheets [35], Carbon nanofibre [36], Graphene [37], Boron-doped Graphene [38], nitrogen-doped graphene [39] Chemical vapor deposition has been successfully applied in the synthesis of nitrogen-doped graphene In this method, a metal catalyst is used as a substrate, a carbon source gas mixed with nitrogen containing gas is introduced at a high temperature and further the precursors segregate and recombine into N-graphene by means of precipitation on the surface of the catalyst [40,41] Other than gas mixture, liquid organic precursors are also used to obtain N-graphene Comparative study of various precursors shows that precise skeletal bonds of liquid precursors are important for the formation of N-graphene Acrylonitrile incorporating the CeC single bond, C]C double bond, and C^N triple bond cannot form N-graphene whereas, pyridine containing only the double bond forms N-graphene because the single bond is easily breakable, even at low temperature, leaving CeC and C]N bonds at the catalyst surface Then CN bond is preferentially eliminated from the surface forming volatile molecules at temperatures higher than 400 C thus, only C]C will be left to form undoped graphene above 500 C whereas, the skeletal bonds in pyridine have similar bond energies, leading to the formation of Ngraphene [31] In a CVD process, the nitrogen content can be restrained by varying the flow rate and the ratio between carbon source and nitrogen source It was also observed that the doping level subsequently reduced from 8.9 to 3.2 or 1.2 at % if the NH3/ CH4 ratio was lowered from 1:1 to 1:2 or 1:4 [23] 2.1.2 Arc-discharge method An arc-discharge method is another synthesis technique applied to obtain CNTs and doped CNTs by evaporating a carbon source, generally Graphite [42] at high temperature Panchakarla et al [27] have successfully obtained nitrogen-doped graphene (NG) by an arc ỵ discharge method in the presence of Hỵ pyridine (NG1) or H2 ammonia (NG2) and also carried out the transformation of nanodiamond in the presence of pyridine (NG3) The scale of graphene and N-graphene obtained by this method is generally below mm [27] 2.1.3 Solvothermal method The solvothermal method was first introduced in the gram-scale production of graphene [43] Generally, gram-scale production of N-graphene is obtained by introducing this method at ~300 C Liu et al [33] reported the preparation of N-GQD by a facile solvothermal method employing dimethylformamide as a solvent and a nitrogen source The two-photon absorption cross-section of NGQD passes 48,000 Goppert-Mayer units, which exceeds that of the organic dyes and is comparable to that of the high-performance semiconductor QDs, acquiring the highest value ever obtained for carbon-based nanomaterials (Fig 2) 2.2 Post-synthesis treatment 2.2.1 Thermal treatment Thermal treatment methodology employs high temperature to produce N-graphene Electrical annealing, which yields high Table Nitrogen-doped graphene synthesis methods and its applications S no Synthesis method Parameters of synthesis Nitrogen content (at %) Applications/References Arc discharge e [27] CVD Plasma treatment Thermal treatment 1.2e8.9 8.5 2.8 Field effect transistor [28] Electrochemical [29] Anodes for lithium ion batteries [30] Pyrolysis 7.86 Electrodes [31] Annealing the freeze-dried graphene oxide foams (GOFs) Facile solvothermal method ỵ Hỵ pyridine (NG1) or H2 ammonia (NG2), transformation of nano-diamond in the presence of pyridine (NG3) to obtain Ndoped Graphene Cu film on Si substrate as catalyst, CH4/NH3 Graphene to nitrogen plasma Exfoliated via a thermal treatment at 1050 C under nitrogen atmosphere with the product of GNS Graphene Oxide and a solid N precursor, urea Ammonia, fluorine-doped tin oxide (FTO) glass substrates Dimethylformamide as a solvent and nitrogen source Melamine diborate as precursor 7.6 Metal-free counter electrodes in highperformance dye-sensitized solar cells [32] Cellular and deep-tissue imaging [33] Co-polymerization ((B, N) co-doped Graphene formation) e B 13.47 N 9.17 Superior stable LieS half cell and GeeS full battery [34] R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 143 Fig Schematic illustration of the strategy for the N-GQD preparation through solvothermal method [33] temperature, has also been applied to obtain N-GNRs [22] The nitrogen content in the synthesis of N-graphene using this method is quite low Li et al [30] reported that the received graphite oxide was swiftly exfoliated via a thermal treatment at 1050 C under the nitrogen atmosphere, resulting in Graphene nanosheets Further annealing of the GNR is done in the presence of ammonia gas (NH3) resulting transformation into N-GNS 2.2.2 Plasma treatment Under nitrogen plasma atmosphere, carbon atoms will be partly replaced by nitrogen atoms and hence this method is employed for the synthesis of N-CNTs [44,45] The new approach has been directed towards the formation of N-graphene from graphene or graphene oxide by exposing it to the nitrogen plasma Nitrogen content can be controlled by the plasma strength and exposure time, which differs by 3e8.5 at % in various research work Shao et al [29] reported (Fig 3) the formation of nitrogen-doped graphene (N-graphene) by exposing graphene to nitrogen plasma 2.2.3 Hydrazine hydrate (N2H4) method Hydrazine hydrate method is also one of the most widely used methods for the synthesis of graphene from graphene oxide Latter, N-graphene has been achieved by reducing GO in a NH3 and N2H4 mixed solution [47] and thus it is one of the most successful methods employed for synthesis that shows that nitrogen content reaches up to at % when the reduction temperature is 80 C When the reaction temperature rises to 160 C or higher, N2H4 will be desorbed and the nitrogen content decreases to ~4 at % One of the amazing results reported is that the morphology of N-graphene also changes with the variation in temperature With the reduction of GO at low temperature, the relative flat N-graphene is generated ( 120 C), whereas the obvious aggregation in N-graphene will occur at higher temperature Characterization techniques Characterization plays a pivotal role for observing the surface morphology as well as the determination of the doping concentration and nature of dopant substitutions in the carbon lattice 3.1 X-ray photoelectron spectroscopy (XPS) technique XPS is the standard quantitative spectroscopy technique to study the nitrogen doping effect in graphene In the XPS spectrum of N-graphene, at about 400 and 284 eV the peaks appear that resemble the N1s and C1s, respectively The ratio of peak intensity between N1s and C1s is employed to determine the nitrogen content in N-graphene The N1s spectrum is used to determine the nitrogen configurations N1s spectrum usually can be deconvoluted to several individual peaks that are allocated to pyridinic-N (398.1e399.3eV), pyrrolic-N (399.8e401.2 eV), and quaternary-N Fig Plasma doping process for the preparation of N-doped Graphene The carbon atoms are replaced by the nitrogen atoms in the plasma process Inset represents the possible nitrogen configurations [46] 144 R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 Fig (a) XPS spectra of graphene and N-graphene, where N1 represents Pyridinic N, N2 represents Pyrrolic N, N3 represents Quaternary N and N4 represents the N-oxides of Pyridinic N [28], (b) XPS spectra of N s fitting for N-GNS anode [30] (401.1e402.7 eV) [28] (Fig 4(a)) The peak position of three nitrogen types varies in a relatively wide range in different experimental studies Li et al [30] reported that the obtained N-GNS contains 2.8 at% nitrogen As shown in Fig 4(b), the N1s signal splits into three peaks at 398.1, 399.9 and 401.3 eV, according to three types of doping nitrogen, i.e pyridine-like, pyrrole-like and graphitic nitrogen 3.2 Microscopic techniques Microscopic techniques are the important tools for the imagining of the doped graphene that includes Transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) techniques Rao et al [48] have reported that graphene contains to layers with an adjacent inter-layer spacing of 0.3e0.4 nm and electron diffraction pattern specifies that the graphene sheets are crystalline The crystallinity of graphene sheets varies with the number of layers [49] Wen et al [50] reported TEM images to examine the transformation of Graphene Oxide to nitrogen-doped graphene Atomic force microscopy (AFM) is also an important tool for the determination of the structure of graphene samples Scanning tunneling microscopy (STM) is another microscopic technique which provides useful access to the topography with atomic resolution Fig (a) SEM image of the Nitrogen doped Graphene acquired by the CVD process [23] (b) TEM image [23] (c) High resolution TEM image of Nitrogen doped Graphene [23] R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 Extremely resolved STM image can provide the topography of an individual doped nitrogen [51] Scanning tunneling microscopy can probe the charge density at the Fermi level After applying the bias voltage (V bias) between the tip and sample is positive, electrons tunnel from the tip into the specimen, as a result, lowest unoccupied states of the specimen are probed Applying a negative V bias electrons tunnel from the specimen into the tip, and the highest occupied states of the specimen are probed as reported by Herz and co-workers [52] Because of the sharp tip, STM images of atomic resolution can be viewed From Fig 5(a), it is clearly observable that the SEM image of the Ndoped graphene shows the substrate covered with the large area layers, continuous and crumpled membrane; further Fig 5(b and c) shows the TEM images of N-doped graphene indicating that the membrane has a morphology of large crumpled paper and its flexibility [23] 3.3 Raman spectroscopy Raman spectroscopy is a very useful method to characterize Ngraphene The paramount features in the spectrum of N-graphene are D, G, and 2D bands respectively Raman spectroscopy is commonly used to characterize the structure and electronic properties of carbon materials Single-layer graphene shows two intense Raman features, due to the formation of G-band at 1588 cmÀ1 and 2D-band The G-band is generally a doubly degenerated phonon mode of the sp2 carbon network and the 2D-band is the second-order Raman scattering process However, due to the defects, a weak D-band determined at 1348 cmÀ1 is seen Lin et al [31] synthesized the N-doped graphene using pyrolysis of graphene oxide and Urea, Raman microscopy offered a clear view of N-doping in the graphene lattice respectively From Fig 6(a,b), G peaks of the graphene oxide, graphene and nitrogen doped graphene appeared at 1600, 1588 and 1580 cmÀ1, the downshift of the G peak from GO to graphene can be considered as the conjugated structure during pyrolysis respectively and further downshift of the G peak in N-graphene can be related to the electron donating capability of N-heteroatoms ID/IG ratio in the Raman spectra was observed for the evaluation of the disorder in the graphene materials ID/IG ratio in graphene oxide was 1.10, which increased to 1.13 for graphene to 1.15 for nitrogen-doped graphene Zhang et al [53] observed that the ID/IG of graphene and N-graphene containing lower (NG1) and higher (NG2) nitrogen doping levels are 0.26, 0.8 and 2.1, corresponding to the crystallite sizes of 65, 21 and nm respectively 145 Applications 4.1 Supercapacitors Due to high power, long cycle effectiveness, energy density, and cost effectiveness electrochemical capacitors are widely applied in various fields such as mobile electronics, hybrid vehicles and power supply devices [54] Carbon based supercapacitors exhibit magnificent capacitive behaviour due to their high surface area, excellent electrical conductivity [55], mechanical flexibility [56] Graphene is widely used nowadays as the qualitative base material for supercapacitor due to its electron mobility around room temperature and high surface area Latter, diverse carbon nanostructures are explored as the electrode materials in capacitors for the advancement of the supercapacitors performance [57] Zhu et al [58] initiated a microwave-expanded graphite oxide (a-MEGO) by stimulating a graphene-like precursor with potassium hydroxide, compressed aMEGO exhibits a surface area as high as 3100 m2 gÀ1 and shows a peculiar performance as electrodes of supercapacitors Wen et al [50] reported an economic and apparent approach for fabrication of highly crumpled nitrogen-doped graphene nanosheets (C-NGNSs) consisting a pore volume as efficient as 3.42 cm3 gÀ1 The C-NGNSs manifest significant enhancement in terms of different performance parameters of supercapacitors (e.g., capacity, rate, cycling) due to the ample wrinkled structures, high pore volume, nitrogen doping, and enhanced electrical conductivity The supercapacitor behavior of the C-NGNSs explored by a symmetrical two-electrode system in organic electrolytes was shown by Wen et al [50] are discussed in Fig Fig shows the cyclic voltammograms (CVs) of the C-NGN900 supercapacitor with M [Bu4N]BF4 acetonitrile (CH3CN) solution as electrolyte at various scan rates CV curves at different scan rates display a typical rectangular shape ranging from À1.5 and 1.5 V representing pure electric double layer capacitive properties of CNGNSs in organic electrolytes Thus, the unique characteristic properties of the C-NGNs make them as a favourable electrode for supercapacitors with high capacity, high durability and stability Due to the scalable synthesis and outstanding properties, CNGNs can provide an important opportunity for both fundamental study and potential applications in industrial areas such as catalysis, adsorption, energy storage and conversion and many more 4.2 Lithium ion batteries Lithium ion batteries are another major advancement in the field of batteries because of its attractive properties such as high Fig (a) Raman spectra of graphene oxide (GO), Graphene, Nitrogen-doped Graphene (NG) [31] (b) Raman spectra of pristine graphene and N-graphene containing 0.6 (NG1) and 2.9 (NG2) at % doping level It shows the enlarged 2D band of NG2 [53] 146 R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 keeps increasing and reaching a maximum value of 684 mAh gÀ1 in 501st cycle and then becomes constant and thus N-GNS shows a superior cycle presentation as an anode for lithium ion batteries 4.3 Field-effect transistor Fig Cyclic voltammograms (CVs) of the C-NGN-900 supercapacitor with M [Bu4N] BF4 acetonitrile (CH3CN) solution as an electrolyte at various scan rates [50] electrical properties, efficient surface area, magnificent mechanical flexibility, high cycle life and high reversible capacity Despite the fact that graphene-based materials can acquire a high reversible capacity (1013e1054 mA h/g) at a low charge rate [59], it is still limited at eminent charge or discharge rate (!500 mA/g) [60] To overcome this, an N-graphene based device is recommended with an intent to obtain a high reversible capacity at the eminent charge or discharge rate Reddy et al [61] synthesized a N-doped graphene beneath the control of a CVD method, and the notable results were obtained as the reversible discharge capacity of N-graphene was approximately doubled relative to the pristine graphene due to elevated Li-ion intercalation on the introduction of nitrogen atoms Li et al [30] synthesized N-GNS at a large scale which exhibited an exceptional electrochemical performance of Liỵ intercalation or deintercalation as observed in the charge or discharge capacity of N-GNS with increasing with the charge or discharge cycles before the discharge capacity leading to a maximum value of 684 mAh gÀ1 Li et al (Fig 8) showed that the graphene nanosheets possessed a normal cycling performance, declining gradually in the specific capacity with the increased charge or discharge cycles The reversible discharge capacity in the graphene nanosheets is observed to be 269 mAh gÀ1 after the 100 cycles, whereas the 454 mAh gÀ1 higher discharge capacity after the second cycle is observed in the Nitrogen doped graphene sheets Surprisingly, the specific capacity of the N-GNS Fig Reversible charge/discharge capacity verse cycle number of (i) GNS and (ii) N-GNS [30] Nitrogen and boron doped graphene possess field emission properties alike carbon nanotubes [61,62] Doped graphene materials possess a very low turn-on fields The flow of electricity for semiconductors needs some kind of activation like heat or light absorption to get over the gap between the valence band and conduction band [63] If a semiconductor is activated by the external electric field to switch “on” and “off”, then it is referred to as FET Schwierz [64] reported that a large layer or bilayer graphene does not have a bandgap and hence constraining large scale graphene in a single dimension (GNRs) or providing an electric field perpendicularly on the bilayer graphene can induce the bandgap as shown in Fig Deifallah et al [65] revealed the C3N4 and C6N9H3 bandgaps of nitrogen doped graphene possessing a high N/C ratio reached approx eV, indicating that bandgap can be altered by the presence of an external field source or atoms As such, nitrogen doping can effectively change the electrical properties of graphene Kashid et al [66] reported the recorded field emission characteristics of nitrogen doped graphene employing in situ transmission electron microscopy, the turn-on voltage of N-doped graphene was revealed to be less than that of pristine graphene To obtain nA current, the turn-on voltage applied should be 230 V for pristine and 110 V for N-doped graphene because of the improved electrical conductivity of the nitrogen doped sample Nitrogen doping is an important aspect in modulating the electrical properties of the graphene Wei et al [23] prepared the bottom gated field effect transistors using both Nitrogen doped and pristine graphene, at the ambient conditions after measuring fifty devices astonishing and distinguished features of N-doped graphene were observed in comparison to the pristine graphene, as shown in Fig 10(a) Pristine graphene exhibits a good conductivity and a linear Ids e Vds behavior representing good ohmic contacts between the Au/Ti pads and the graphene Vg decreases as Igs is increased slowly and the neutrality point is reached at 15e20 V representing a P type behavior and as comparing pristine graphene with the nitrogen doped graphene, N-graphene exhibits distinguishingly relative lower conductivity and greater on/off ratio Graphene is a zero-gap semiconductor thus, band structure of graphene consists of two bands as Valence and Conduction band intersecting at two inequivalent points in the reciprocal space leading to good conductivity and a unique electric field effect of 1013 cmÀ3 high charge concentrations and because of its zero Fig Band structure at the k point of (i) large-area, Graphene, (ii) graphene nanoribbons, (iii) unbiased bilayer graphene, and (iv) bilayer graphene with an applied perpendicular field [64] R Yadav, C.K Dixit / Journal of Science: Advanced Materials and Devices (2017) 141e149 147 Fig 10 (a) Transfer characteristics of the pristine graphene (Vds at À0.5 V) and the N-doped Graphene (Vds at 0.5 and 1.0 V) [23] (b) & (c) Ids/Vds characteristics at various Vg for the pristine graphene and the N-doped Graphene FET device with presumed band structures [23] bandgap, pristine graphene possesses a low on/off ratio and shows a P-type behavior due to the absorption of oxygen or water in air but in the case of N-doped graphene foreign atoms and other topological defects acting as scattering centres are introduced in the graphene lattice, leading to the decrease in conductivity and thus nitrogen doping is one of the effective ways for modification of the electrical structure of graphene and suppressing the density of graphene states near the Fermi energy (Fm) level, resulting in the opening of gap between the valence and conduction band [23] 4.4 Electrochemical application An electrochemical device includes a fuel cell that generates electricity through the oxidation of a fuel at an anode electrode and the reduction of an O2 at the cathode electrode At the progress of the reaction, O]O bond in a typical oxygen reduction should be broken as to obtain remarkable current density and thus by lowering the activation energy the kinetics of Oxygen reduction reaction (ORR) must be increased Further by the introduction of nitrogen in carbon network elevates the ORR activity because of increasing electron density of states beside the Fermi level Nitrogen-doped graphene acts as a promising electrocatalyst for ORR The catalyst used in ORR is Platinium which is costly and also not available in high quantity in earth resources Qu et al [40] reported a four-electron pathway in alkaline solutions for nitrogen doped graphene, which represents a higher current density and good amperometric response for ORR relative to the commercial platinumecarbon catalyst High tolerance against carbon monoxide with an operation stability of more than 200,000 cycles is observed Conclusion Several research processes are directed towards the synthesis of N-graphene, it has become one of the prominent fields of research due to enhanced properties and prospective applications in different fields of science As a result, several synthesis methods are employed with the newly explored characterization techniques to discover new enhanced properties The chemical doping of nitrogen in graphene helps in controlling the properties of graphene which makes N-graphene a highly prominent material Despite its good quality, the large scale production of N-graphene is still a challenging task and thus new prominent synthesis methods are in demand However, the control of nitrogen content and specific position remains a problem in the case of N-graphene which is highly required 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