Perspective pubs.acs.org/JPCL
Strategies on the Design of Nitrogen-Doped Graphene Haibo Wang,† Mingshi Xie,† Larissa Thia,‡,§ Adrian Fisher,⊥ and Xin Wang*,† †
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459, Singapore ‡ Interdisciplinary Graduate School, Nanyang Technological University, 50 Nanyang Avenue, Block S2 - B3a − 01, Singapore 639798, Singapore § Residues and Resource Reclamation Centre, Nanyang Environment and Water Research Institute, Nanyang Technological University, 1 Cleantech Loop, Singapore 637141, Singapore ⊥ Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge, CB2 3RA, United Kingdom ABSTRACT: Substitutional nitrogen doping in graphene has been a very powerful tool to tailor the pristine property of graphene and furthermore extend its application. While nitrogen-doped graphene (N-graphene) has shown many potential applications in catalysis, electronics, sensors and so on, there is still a lack of accurate control of substitutional nitrogen doping, and higher performance toward various applications is always needed. This Perspective summarizes the ongoing developments toward better control of nitrogen doping. Moreover, two recent strategies aiming to promote the activity of N-graphene are also discussed.
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deposition (CVD) and solvothermal approach. N-graphene is fabricated from the recombination of carbon and nitrogen atoms or contained fragments.8,16 The second method involves post-treatment of graphene and graphite oxide (GO). Using this method, N-graphene can be produced by annealing or reducing GO in the presence of nitrogen-contained precursors,17,18 exposing graphene or GO in N2 plasma.19,20 Rapid development of N-graphene in recent years indicates the huge interest in nitrogen doping control and its effect on improving performance. Several strategies have been proposed in these two aspects. The Perspective gives a brief review of recent strategies and discusses their roles in nitrogen doping of graphene. Recent Progress on Nitrogen Doping Control in N-Graphene. To get a better understanding of the doping effect in many
n the past decade, the unique properties of two-dimensional graphene, such as high surface area, good thermal conductivity, and strong Young’s modulus, have been revealed in a myriad of studies. These features render it promising for a wide range of applications, such as energy conversion and storage, sensors, and transistors.1−4 However, due to the zero band gap of pristine graphene and inert sp2 hybridized carbon atoms, its application is still limited. To tackle this, introducing new properties through substitutional doping has attracted continuous and intense interest.5−7 Recent studies report that various heteroatoms (nitrogen,8 boron,9 sulfur,10,11 phosphorus,12 etc.) can be doped into graphene carbon lattice to effectively tailor the properties of pristine graphene. Among various doping sources, nitrogen is the most widely available and thus most commonly used. Nitrogen doping can effectively alter the spin density and charge distribution of neighboring carbons;13 this in turn induces the activation of the carbon region on the surface of nitrogen-doped graphene (Ngraphene). As a result, N-graphene shows promising catalytic activity toward electrocatalytic reactions, such as the oxygen reduction reaction (ORR). Moreover, the electron-rich N dopant easily modulates the electrical property of graphene by controlled doping, making the N-graphene a potential candidate for sensors and electronics. Therefore, the development of N-graphene opens new opportunities for materials scientists to further extend applications of graphene.14,15 N-graphene is generally synthesized in two ways.6 First is the bottom-up method, such as the commonly used chemical vapor © 2013 American Chemical Society
To get a better understanding of the doping effect in many applications, the ability to control the doping level and type is a prerequisite.
Received: November 8, 2013 Accepted: December 11, 2013 Published: December 11, 2013 119
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Figure 1. Schematic representations. (a) Reaction between GO and iGO or pGO, followed by the annealing at 900 °C. Reprinted from ref 31. (b) Synthesis procedure of quantum dots 1−3. Conditions: (i) phenylboronic acid, Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, 80 °C; (ii) (a) n-BuLi, THF, −78 °C, (b) 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, −78 °C to room temperature (rt), (c) 1,3-dibromo-5-iodobenzene, Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, 85 °C; (iii) 9,11-diphenyl-10H-cyclop-enta[e]pyren-10-one, Ph2O, 240 °C; (iv) RuCl3, NaIO4, CH2Cl2/ CH3CN/H2O, rt; (v) benzene-1,2-diamine, EtOH/CHCl3, 65 °C; (vi) Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, 80 °C; (vii) FeCl3, CH2Cl2/ CH3NO2, rt; (viii) o-xylylenebis(triphenylphosphonium bromide), LiOH (aq, 5 M), CH2Cl2, rt. Reprinted from ref 32.
provides a new option to doping control. Compared with thermal annealing and CVD, the doping control is handled more easily as high temperatures are not required during the introduction of nitrogen. In earlier efforts, Long et al. showed that N-graphene could be synthesized in NH3/N2H4 mixed aqueous solution by the hydrothermal method.18 After reaction with oxygen-containing functionalities, nitrogen atoms from NH3 or N2H4 were incorporated into a graphene lattice through cyclization/rearrangement. Apart from this work, urea and dicyandiamide were also observed to yield N-graphene with a similar method.29,30 Unfortunately, nitrogen configurations in those studies still cannot be well-controlled during the doping process. In a recent study, N-graphene was synthesized through acid-catalyzed dehydration between GO and 4-fluoroaniline (iGO) or 1,2-diamino-4-fluorobenzene (pGO) (Figure 1a).31 X-ray photoelectron spectroscopy (XPS) revealed that imine-N was dominant in iGO, while pyrazine-N, imine-N, and amine-N existed in pGO. Thus, combining different reactants with GO introduces different nitrogen types. After annealing, quaternary N and pyridinic N were observed (Figure 1a). pGO applied as a FET device showed the obvious n-type behavior with the hole and electron mobility of 11.5 and 12.4 cm2V−1 s−1. In another work, three kinds of graphene quantum dots (GQDs) (samples 1, 2, and 3) were synthesized by a series of chemical reactions (Figure 1b).32 As expected, sample 3 without the nitrogen doping showed a much poorer ORR activity. However, sample 1 with lower nitrogen content showed the higher ORR activity compared with sample 2. This is in agreement with a previous study that an optimal nitrogen content is required for higher ORR activity.33 More importantly, because doping is controlled during the chemical reaction, the nitrogen type in both samples 1 and 2 can be regarded as the same. Thus, it indicates that the conjugation size is an important parameter in the activity toward ORR. The nitrogen doping by chemical reactions represents a promising route to better control the doping, which will benefit the further study of nitrogen doping for various applications. In addition to the nitrogen configuration, the position of nitrogen can also be controlled to the edge of the graphene
applications, the ability to control the doping level and type is a prerequisite. Adjusting the annealing conditions of N-graphene is the most frequent way to control the nitrogen bonding configurations based on their different thermal stability.21 Increasing the temperature decreases the content of nitrogen and the percentage of pyrrolic N while increases the percentage of pyridinic N and quaternary N. The varied nitrogen configuration and content provide a meaningful way to study the influence factors in many applications, such as ORR,22 dyesensitized solar cells (DSSCs),23 and field-effect transistors (FETs).24 For instance, through annealing GO with cyanamide at different temperatures, Hou et al. synthesized N-graphene with varied nitrogen content and configurations.23 Comparing the performance of different samples reveals that pyridinic and quaternary N rather than the nitrogen content in N-graphene are the key factors responsible for catalytic activity in DSSCs. CVD represents another powerful approach to control the nitrogen configuration. Single nitrogen bond type dominant graphene can be achieved despite the fact that it is influenced by many factors such as precursors and growth temperature.6 In previous study, pyridinic N-containing graphene can be obtained by low-pressure CVD using C2H4/H2 as the carbon precursor and NH3/He as the nitrogen precursor.25 In recent studies of N-graphene produced through atmospheric pressure chemical vapor deposition (APCVD), Xue et al. synthesized Ngraphene-containing quaternary N as the dominant configuration by using pyridine as the precursor at low temperature (300 °C) on a Cu surface.26 Lv et al. were also able to synthesize N-graphene-containing nitrogen mostly as quaternary N by using CH4 as the carbon source and NH3 as the nitrogen source,27 while in another study, pyrrolic N dominant graphene was reported after modifying the working parameter with the same precursors.28 CVD shows excellent ability to obtain N-graphene with specific nitrogen configurations. However, compared with simply annealing GO with nitrogen precursors, the CVD procedure is relatively more complex, and the production scale is also limited. Introducing nitrogen dopants via chemical reactions with graphene or graphene oxide at relatively low temperature 120
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Figure 2. Schematic representation of nitrogen-doped graphene nanoplatelets synthesized by ball milling. Reprinted with permission from ref 35. Copyright Nature Publishing Group.
nanoplatelets.34,35 An earlier study has already shown that nitrogen edge doping can alter the properties of a graphene nanoribbon (GNR) with negligible mobility loss.36 Thus, edge doping of graphene nanoplatelets may also be used to introduce active sites to the edges while maintaining the high quality of graphite on the rest of the sheet of graphene. This means that edge doping may improve the activity because of better conductivity. Baek and other co-workers achieved edge doping through exfoliating and functionalizing the graphene nanoplatelets with 4-aminobenzoic acid and then annealing at 900 °C.34 In another report, nitrogen atoms from nitrogen gas were anchored to the edge of graphene by ball milling (Figure 2).35 The nitrogen content detected by elemental analysis reached ∼14.8 wt %, in which pyrazole (pyrrolic-N) and pyridazine (pyridinic-N) were the dominant N components. N-graphene synthesized by either method showed the four-electron ORR pathway in KOH electrolyte. Moreover, N-graphene synthesized by ball milling was also tested in DSSCs; it exhibited a higher power conversion efficiency (9.34%) than Pt (8.85%).
counterpart heteroatom. In this way, B, N separately codoped carbon materials could be obtained.41,42 In the report of BNgraphene synthesized by this method,42 no N−B configurations were observed in the N1s and B1s peaks of the XPS spectra (Figure 3a,b). The BN-graphene exhibits an improved current
Co-doping nitrogen with other heteroatoms such as B, S, and P has been proposed as a strategy to further tune the properties of monodoped N-graphene. Strategies to Design Higher-Performance N-Graphene. Codoping nitrogen with other heteroatoms such as B, S, and P has been proposed as a strategy to further tune the properties of monodoped N-graphene. In the earlier studies, B,N-codoped graphene (BN-graphene) was obtained by annealing GO with ammonium and boric acid at 1000 °C37 or hydrothermally treating GO with NH3BF3.38 The former exhibited a fourelectron pathway in ORR, and the latter showed improved capacitance. In other studies, BN-graphene exhibited a possibly tunable “band gap”39 and enhanced H2O2 detection when used as a sensor.40 Although BN-graphene displays many possible applications, the formation of covalently bonded B−N during synthesis may be detrimental to some electrocatalytic applications, for example, ORR. Studies in both BN-graphene and BN-CNT have demonstrated the inertness of the B−N bonded structure in ORR.41,42 To avoid this limitation, a twostep method has been proposed to achieve separate doping of B and N. B- or N-doped carbon materials were first prepared, and then, they were annealed with precursors containing the
Figure 3. (a,b) The high-resolution spectra of (a) N1s and (b) B1s in BN-graphene. (c) The cyclic voltammograms of BN-graphene and hBN/graphene in O2-saturated 0.1 M KOH with a 0.1 V/s scan rate. (d) Rotating disk electrode (RDE) voltammograms of B-graphene, Ngraphene, h-BN/graphene, BN-graphene, and Pt/C in O2-saturated 0.1 M KOH. The rotation speed was 1500 rpm, and the scan rate was 10 mV/s. (e) The HO2 adsorption energies (Ead) on N-graphene, Bgraphene, and BN-graphene. N(p) and N(g) represent the pyridinic and quaternary N, respectively. (f) Ead on BN-graphene with various pyridinic and quaternary N models. Reprinted with permission from ref 42. Copyright John Wiley and Sons.
density and onset potential toward ORR compared to the graphene-containing hexagonal boron nitride (h-BN) structure or monodoped N or B (Figure 3c,d). Furthermore, DFT calculation reveals that B,N codoping induces a much higher adsorption energy toward HO2 than monodoping of N or B (Figure 3e). The synergistic effect observed originates from 121
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pyridinic N, which polarizes the 2p orbital of neighboring C atoms and then donates an extra electron to electron-acceptor B atoms. On the contrary, quaternary N suppresses the activity of B atoms rather than activating it (Figure 3f). N can also be codoped with S, P, and even Si atoms. S,Ncodoped graphene (SN-graphene) shows the enhanced synergistic effect in the ORR, resulting from spin and charge density redistribution induced by dual doping.43 Xu et al. reported that CVD-fabricated SN-graphene even exhibited the onset potential comparable to Pt/C.44 In the study of P,Ncodoped graphene (PN-graphene), a specific capacitance of 165 F/g and good capacitance retention ratio (>80%) were observed.45 Despite its low surface area (∼152 m2/g), abundant O groups and distortions were introduced during codoping with P heteroatoms, and this greatly improved the material’s capacitance. Si atoms were also used to increase the holedoping level in the p-type Si,N-codoped graphene.46 From the above discussion, nitrogen codoping has proven to be an effective way to promote the activity of N-graphene through a synergistic effect induced by N and other heteroatoms. Moreover, codoping presents new opportunities for Ngraphene by introducing other properties from codoped heteroatoms. Typically, nitrogen-codoped graphene is synthesized from GO. However, because GO possesses many defects and functional groups, reduction and doping cannot fully recover the structure of the carbon lattice. Therefore, the performance of nitrogen-codoped graphene from GO is still not so ideal in certain aspects. Another strategy to promote activity is by fabricating hybrid structures based on N-graphene. Hybrid structures based on carbon nanotubes (CNTs) and N-graphene are an interesting topic. Compared to N-graphene commonly synthesized from GO, CNTs fabricated by CVD possess fewer defects and distortions. The interconnection between CNTs and Ngraphene thus provides faster electron transport, which is always beneficial for electrocatalytic applications such as ORR.47 Moreover, the CNT’s interaction with N-graphene through covalent or noncovalent binding may also provide more reaction sites in a CNTs/N-graphene composite than highly aggregated N-graphene from GO. In a recent report, Li et al. obtained an Fe−N carbon nanotube/graphene complex (Fe−N−CNT/G) by first oxidizing CNTs with a modified Hummers’s method and then annealing the oxidized CNT/G in an NH3/Ar atmosphere (Figure 4a).48 Iron atoms originating from seed nanoparticles (NPs) tended to bond with the carboxylic acid groups at the edge of graphene pieces and then converted to Fe−N bonds after annealing. The final product exhibited high ORR activity in both alkaline and acidic electrolytes (Figure 4b,d). The Tafel slope of Fe−N−CNT/ G (∼68 mV) was close to that of Pt/C catalysts (∼70 mV) even in acidic electrolytes (Figure 4c). Interestingly, the study revealed that the intact inner tubes with high conductivity were indispensible for high ORR activity because they could facilitate charge transport. In another work, oxidized CNTs were believed to help prevent serious aggregation among GO sheets during the nitrogen doping process.49 The improved ORR activity was attributed to the increased accessible area. Hybrid CNT/N-graphene also has been synthesized from the in situ growth and graphitization of polyaniline by applying CNTs as the support and Co(NO3)2 as the catalysts.50 The hybrid catalysts exhibit superior ORR activity and better performance as Li−O2 battery cathodes, compared to Pt/C catalysts. It is found that the addition of Co species in the synthesis is
Figure 4. (a) Aberration-corrected TEM image of Fe−N−CNT/G. (b,d) Rotating ring-disk electrode (RRDE) polarization curves and peroxide yield of Fe−N−CNT/G and 20% Pt/C in (b) O2-saturated 0.1 M HClO4 and (d) O2-saturated 0.1 M KOH. (c) Kinetic current density versus potential derived from the mass-transport correction of the corresponding disk current density in (b). The rotation speed was1600 rpm, and the scan rate was 5 mV/s. Reprinted with permission from ref 48. Copyright Nature Publishing Group.
indispensable for achieving high activity due to their effects on the resulting catalyst morphology and structure. Notably, in the hybrid structure of CNTs/N-graphene, there is always the presence of metal impurities originating from the synthesis of CNTs. In order to exclude the metal influence, long treatment time is normally required to remove metal impurities as even residual amounts of metal may influence the performance of the hybrid significantly.48,51 A metal-based catalyst supported on N-graphene is another common type of hybrid structure. Studies on N-graphenesupported Fe3O452 and Co3O453 NPs demonstrate that the interaction between N dopants and nonprecious metal oxides can effectively promote its oxygen reduction ability. In other instances, the incorporation of p-type MoS2 nanoplatelets into n-type N-graphene greatly increases the solar hydrogen production compared with MoS2 and MoS2 on rGO.54 The nNanoscale p−n junction induced from the hybrid structure is believed to be the critical factor as it facilitates charge generation and migration while suppressing charge recombination. Another report showed that the integration between WS2 and N-graphene contributed significantly to the high specific capacity (905 mA h/g at 100 mA/g) in a lithium ion battery.55 Therefore, a proper design to introduce and tune the interaction between N-graphene and various metal-based catalysts allows the modification of the properties of the resulting hybrid material. Various polymer/GO hybrid materials have also been used to produce N-graphene. Annealing polymer/GO may provide more reaction sites by avoiding aggregation compared with simply annealing GO at high temperature. Although Wen et al.’s report has shown that polymerized C3N4 on GO could decompose into nitrogen-containing species, which further induces the formation of crumpled N-graphene at high 122
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temperature,56 in most cases, nitrogen-containing polymers (polypyrrole (PPy), polyaniline (PANI), polymerized melamine, etc.) are converted into nitrogen-contained carbon and retained on the surface of graphene. Recently, Lai et al. annealed PPy/GO and PANI/GO at high temperature;22 the carbonized PPy/rGO (C-PPy/rGO) and PANI/rGO (CPANI/rGO) showed very different ORR activity in alkaline electrolyte. Zhao et al. annealed a three-dimensional (3D) PPy/ GO hybrid material at 1050 °C;57 the as-synthesized 3D CPPy/rGO still exhibited a high capacitance (415 F/g) at the current density of 100 A/g. In these reports, it is suggested that nitrogen from polymer was doped into the graphene lattice based on the XPS spectra of the N1s peak. However, some other studies also reported that the C-PPy and C-PANI without GO showed similar N1s XPS spectra.58,59 Therefore, there is no consensus yet, and nitrogen doping of GO may not be the reason for the difference observed in ORR activity and capacitance. In a recent study, Lin et al. synthesized PANI/ GO in a similar way and annealed it at different temperatures.60 PANI/GO annealed at 1000 °C (NG/rGO-1000) exhibited the typical four-electron ORR pathway. It also demonstrated a better oxygen evolution reaction (OER) than Pt/C (Figure 5c,d). This was ascribed to the formation of N-graphene from
To sum up, this Perspective reviews several recent strategies to control nitrogen doping and improve performance. Although many advances have been made in this area, further efforts are still required in various aspects to further advance this area. For instance, without the presence of metal, although N-graphene based materials obtained from codoping and the hybrid strategy show better ORR activity than N-graphene synthesized under the same conditions, its performance is still lower than that of commercial Pt/C catalysts. As indicated in recent studies,32,64 the number of active catalytic sites and the effective utilization of π electrons are crucial to the catalysts in terms of ORR activity. The conductivity difference and even small difference in doping configuration will also influence the final ORR activity. The wide diversified results observed by various researchers may simply be due to the inaccurate control of all of these factors. Therefore, better control of all of these factors during synthesis is highly preferred in order to pinpoint the exact mechanism of N-graphene based on the material for electrocatalytic applications and beyond. Moreover, although doping control via chemical reaction has been proposed as an effective approach to obtain specific nitrogen configurations, the type of nitrogen configuration realized by this approach so far and the performance of the resulting catalysts are still limited. More efforts are needed on the development of new methods along this direction. Finally, although most current research studies on N-graphene focus on ORR and FET, there will be huge opportunities to apply N-graphene in other fields, as already demonstrated in some reports in the application of supercapacitors, DSSCs, and sensors. Particularly, recent studies, for example, N-graphene/MoS2 hybrid materials for increased hydrogen production54 or nitrogen-doped carbon nanofibers as catalysts for CO2 reduction because of their strong interaction,65 undoubtedly elucidate the big opportunity to explore N-graphene based composite materials for a variety of new applications.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Haibo Wang is a Ph.D student in the Wang group at Nanyang Technological University. His research interests include the electrochemical oxidation of alcohols and the oxygen reduction reaction on graphene-based materials.
Figure 5. (a) The schematic representation of the formation of NG/ rGO from GO/PANI. (b) The schematic representation of nitrogen bonding configurations. (c) RDE voltammograms of NG/rGO-1000 in O2-saturated 0.1 M KOH. (d) Linear sweep voltammograms of NG/rGO-1000, graphene, Pt/C, and glassy carbon electrode in 0.1 M KOH. The scan rate was 10 mV/s. Reprinted with permission from ref 60. Copyright Elsevier.
Mingshi Xie is a Ph.D student in the Wang group at Nanyang Technological University. He is working on the electroreduction of carbon dioxide. Larissa Thia is a Ph.D student in the Wang group at Nanyang Technological University. She is currently working on the selective electrochemical oxidation of glycerol for the cogeneration of electricity and valuable chemicals.
PANI on a rGO surface based on the Raman and FTIR spectra (Figure 5a,b). In another study, Kwon et al. directly observed that a thin-layer PPy film could be successfully converted into the nitrogen-doped few-layer graphene on the surface of Cu under 1 Torr of pressure at 70 °C.61 These studies may indicate that the rearrangement of the polymer on the rGO surface is a key factor for the improved performance. Moreover, the improved conductivity of rGO62 and the morphology of the carbonized polymer63 may also contribute to the performance of hybrid materials.
Adrian Fisher is a reader in the Department of Chemical Engineering and Biotechnology at the University of Cambridge. His current research focuses on the design, manufacture, and application of microscale reactor systems for the development of the next generation of (bio)chemical sensors using a range of interdisciplinary technologies, including microfabrication, fluid dynamics, numerical simulations, spectroscopic analysis, and electrochemical methodology. 123
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Xin Wang is an associate professor in the School of Chemical and Biomedical Engineering at Nanyang Technological University, Singapore. He has been working on electrochemical technology for energy harvesting for the past 15 years, with particular focus on the electrocatalysis and nanostructured electrocatalyst development for various important electrochemical reactions, such as electro-oxidation of organic fuels, oxygen reduction, and hydrogen production. See http://www.ntu.edu.sg/home/wangxin/WangXin.htm for more details.
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ACKNOWLEDGMENTS We acknowledge financial support from the Singapore National Research Foundation under its Campus for Research Excellence And Technological Enterprise (CREATE) programme and the academic research fund AcRF tier 1 (M4011020 RG8/12) Ministry of Education, Singapore.
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