Synthesis, characterization and prospective applications of nitrogen-doped graphene: A short review

Fig.. electrical properties, ef ficient surface area, magnificent mechanical flexibility, high cycle life and high reversible capacity. To overcome this, an N-graphene based device is rec[r]

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Review Article

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

Article history: Received 18 March 2017 Received in revised form 18 May 2017

Accepted 19 May 2017 Available online 30 May 2017 Keywords:

Graphene Nitrogen doping Nanoelectronics

Electrochemical biosensing Energy storage

a b s t r a c t

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 manip-ulating it through the means of doping such as its surface area and functional sites Different configu-rations 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/)

1 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 prom-ising resemble for tuning and controlling the electronic properties of graphene is doping with heteroatoms Thus, doping with nitro-gen 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

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 thepsystem Pyrrolic N refers to N atoms that contribute two p electrons to thep system, although unnecessarily bond into theﬁve-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 sp2hybridized and pyrrolic-N is sp3 hybrid-ized 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 pre-dicted Commonly, incorporating nitrogen into a matrix of carbon-based 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 differentﬁelds

* Corresponding author

E-mail address:roshniyadav05@gmail.com(R Yadav)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.05.007

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2 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 in-volves thermal treatment, plasma treatment, and N2H4treatment 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 syn-thesis techniques and is widely used in the synsyn-thesis 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 sur-face 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 Acry-lonitrile incorporating the CeC single bond, C]C double bond, and C^N triple bond cannot form N-graphene whereas, pyridine con-taining 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 400C thus, only C]C will be left to form undoped graphene above 500C whereas, the skeletal bonds in pyridine have similar bond energies, leading to the formation of N-graphene [31] In a CVD process, the nitrogen content can be restrained by varying theﬂow 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/ CH4ratio 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 discharge method in the presence of H2ỵ pyridine (NG1) or H2ỵ ammonia (NG2) and also carried out the transformation of nano-diamond in the presence of pyridine (NG3) The scale of graphene and N-graphene obtained by this method is generally below 1mm

[27]

2.1.3 Solvothermal method

The solvothermal method wasﬁrst introduced in the gram-scale production of graphene[43] Generally, gram-scale production of N-graphene is obtained by introducing this method at ~300C Liu et al [33] reported the preparation of N-GQD by a facile sol-vothermal method employing dimethylformamide as a solvent and a nitrogen source The two-photon absorption cross-section of N-GQD 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 Fig Three common bonding conﬁgurations of Nitrogen-doped Graphene[15]

Table

Nitrogen-doped graphene synthesis methods and its applications

S no Synthesis method Parameters of synthesis Nitrogen

content (at %)

Applications/References

1 Arc discharge H2ỵpyridine (NG1) or H2ỵammonia (NG2),

transformation of nano-diamond in the presence of pyridine (NG3) to obtain N-doped Graphene

e [27]

2 CVD Cuﬁlm on Si substrate as catalyst, CH4/NH3 1.2e8.9 Field effect transistor[28]

3 Plasma treatment Graphene to nitrogen plasma 8.5 Electrochemical[29]

4 Thermal treatment Exfoliated via a thermal treatment at 1050C under nitrogen atmosphere with

the product of GNS

2.8 Anodes for lithium ion batteries[30]

5 Pyrolysis Graphene Oxide and a solid N precursor,

urea

7.86 Electrodes[31]

6 Annealing the freeze-dried graphene oxide foams (GOFs)

Ammonia,ﬂuorine-doped tin oxide (FTO) glass substrates

7.6 Metal-free counter electrodes in high-performance dye-sensitized solar cells[32]

7 Facile solvothermal method Dimethylformamide as a solvent and nitrogen source

e Cellular and deep-tissue imaging[33]

8 Co-polymerization ((B, N) co-doped Graphene formation)

Melamine diborate as precursor B N Superior stable LieS half cell and GeeS full battery[34]

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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 1050C 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 for-mation 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 NH3and 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 80C When the reaction temperature rises to 160C or higher, N2H4will 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 relativeﬂat N-graphene is generated (120 C), whereas the obvious aggregation in N-graphene will occur at higher temperature

3 Characterization techniques

Characterization plays a pivotal role for observing the surface morphology as well as the determination of the doping concen-tration 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 con-tent 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 Schematic illustration of the strategy for the N-GQD preparation through solvothermal method[33]

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(401.1e402.7 eV)[28](Fig 4(a)) The peak position of three nitro-gen 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 inFig 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 imag-ining 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) 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 sﬁtting for N-GNS anode[30]

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Extremely resolved STM image can provide the topography of an individual doped nitrogen [51] Scanning tunneling micro-scopy 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 N-doped graphene shows the substrate covered with the large area layers, continuous and crumpled membrane; furtherFig 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 N-graphene 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 cm1 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 cm1is 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 FromFig 6(a,b), G peaks of the graphene oxide, graphene and nitrogen doped gra-phene appeared at 1600, 1588 and 1580 cm1, 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/IGratio in the Raman spectra was observed for the evaluation of the disorder in the graphene mate-rials ID/IGratio 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

4 Applications 4.1 Supercapacitors

Due to high power, long cycle effectiveness, energy density, and cost effectiveness electrochemical capacitors are widely applied in variousﬁelds such as mobile electronics, hybrid vehicles and power supply devices[54] Carbon based supercapacitors exhibit magnif-icent capacitive behaviour due to their high surface area, excellent electrical conductivity[55], mechanicalﬂexibility[56] Graphene is widely used nowadays as the qualitative base material for super-capacitor 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 a-MEGO exhibits a surface area as high as 3100 m2g1and 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 cm3g1 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 7.Fig 7shows the cyclic voltammograms (CVs) of the C-NGN-900 supercapacitor with M [Bu4N]BF4acetonitrile (CH3CN) solu-tion as electrolyte at various scan rates CV curves at different scan rates display a typical rectangular shape ranging from1.5 and 1.5 V representing pure electric double layer capacitive properties of C-NGNSs 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, C-NGNs can provide an important opportunity for both funda-mental 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

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electrical properties, efficient surface area, magnificent mechan-icalflexibility, 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 recom-mended 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 Liỵintercalation 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 g1 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 g1after the 100 cycles, whereas the 454 mAh g1higher discharge capacity after the second cycle is observed in the Nitrogen doped gra-phene sheets Surprisingly, the speciﬁc capacity of the N-GNS

keeps increasing and reaching a maximum value of 684 mAh g1 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

Nitrogen and boron doped graphene possess field emission properties alike carbon nanotubes[61,62] Doped graphene mate-rials possess a very low turn-onfields 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 electricfield 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 gra-phene in a single dimension (GNRs) or providing an electricfield perpendicularly on the bilayer graphene can induce the bandgap as shown inFig Deifallah et al.[65]revealed the C3N4and 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 externalfield source or atoms As such, nitrogen doping can effectively change the electrical properties of graphene Kashid et al.[66]reported the recordedfield emission charac-teristics of nitrogen doped graphene employing in situ trans-mission 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 prop-erties 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 measuringfifty 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 Idse Vdsbehavior representing good ohmic contacts between the Au/Ti pads and the graphene Vgdecreases as Igsis 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 electricﬁeld effect of 1013 cm3 high charge concentrations and because of its zero Fig Cyclic voltammograms (CVs) of the C-NGN-900 supercapacitor with M [Bu4N]

BF4acetonitrile (CH3CN) solution as an electrolyte at various scan rates[50]

Fig Reversible charge/discharge capacity verse cycle number of (i) GNS and (ii) N-GNS[30]

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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 to-pological 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 O2at 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 so-lutions 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

5 Conclusion

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decrease the electron mobility extremely while opening an appropriate bandgap In the supercapacitor application, C-NGNs show a unique property as a promising electrode and offers an attractive opportunity for fundamental study and potential indus-trial applications such as catalysis and adsorption separation

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