DOI: 10.1002/chem.201405748
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Selective Nitrogen Functionalization of Graphene by BuchererType Reaction Chun Kiang Chua,[a] Zdeneˇk Sofer,[b] Jan Luxa,[b] and Martin Pumera*[a] Abstract: Nitrogen functionalization of graphene offers new hybrid materials with improved performance for important technological applications. Despite studies highlighting the dependence of the performance of nitrogen-functionalized graphene on the types of nitrogen functional groups that are present, precise synthetic control over their ratio is challenging. Herein, the synthesis of nitrogen-functionalized graphene rich in amino groups by a Bucherer-type reaction under hydrothermal conditions is reported. The efficiency of
Introduction Intrinsic modification of graphene has been an important focus of graphene research in recent years. Chemical functionalization and doping with metals or heteroatoms provides opportunities to tailor the electronic properties and modify the surface chemistry of graphene.[1, 2] Common metal dopants include precious and nonprecious metals, while heteroatom dopants comprise boron, nitrogen, sulfur, and phosphorus.[3–5] Enhanced electrical and thermal conductivity as well as increased density of free charge carriers of such modified graphene materials have improved their performances in applications such as bioelectrochemical sensing systems,[6] supercapacitors,[7] and lithium-ion batteries.[8] One of the earliest attempts to modify graphene was to introduce chemically bonded nitrogen species onto the sp2 carbon structure of graphene. In such nitrogenated graphenes, nitrogen atoms are chemically bonded in pyridinic N, amino N, pyrrolic N, quaternary N, and N oxide moieties. The numerous well-known methods to synthesize nitrogenated graphene include nitrogen plasma,[6] electrothermal reaction,[9] chemical vapor deposition,[10] solvothermal synthesis,[11] and post-thermal treatment with nitrogen-containing species.[12, 13] [a] Dr. C. K. Chua, Prof. M. Pumera Department of Chemistry & Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University 21, Nanyang Link, Singapore 637371 E-mail: [email protected] [b] Prof. Z. Sofer, J. Luxa Department of Inorganic Chemistry University of Chemistry and Technology Prague Technick 5, 166 28 Prague 6 (Czech Republic) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405748. Chem. Eur. J. 2015, 21, 8090 – 8095
the synthetic method under two hydrothermal conditions was examined for graphite oxide produced by Hummers and Hofmann oxidation routes. The morphological and structural properties of the amino-functionalized graphene were fully characterized. The use of a synthetic method with a well-known mechanism for derivatization of graphene will open new avenues for highly reproducible functionalization of graphene materials.
Despite the efficiency of these methods in producing nitrogenated graphene, the exact nature of the nitrogen moieties is often random and hard to control without precise instrumentation. Nitrogenated graphene usually shows exceptional performance in important catalytic reactions.[3, 4] Nitrogenated graphene materials that are rich in graphitic N moieties have been shown to have enhanced capacitive behavior and electrocatalytic effects towards the oxygen reduction reaction (ORR).[7, 14, 15] While the role of amino groups is taken into consideration in numerous ORR studies on nitrogenated graphene materials, Hou and co-workers systematically broke down the roles of the nitrogen moieties in the ORR to reveal that the graphitic N atoms and amino groups dictate the onset potential and electron-transfer number.[15] It is therefore imperative to develop novel functionalization methods capable of selectively introducing a specific type of nitrogen moiety to produce useful tailored nitrogenated graphene for technological applications. In this work, a selective functionalization method to produce amino-functionalized graphenes (NH2-Gs) based on a Bucherertype reaction under hydrothermal conditions (Scheme 1) is proposed. In accordance with the Bucherer-type reaction, it was anticipated that the resultant graphene materials would be functionalized with amino groups through exchange with hydroxyl groups. Hydrothermal reactions of graphite oxide (GO) obtained by the Hummers[16] and Hofmann[17] methods under two hydrothermal conditions were compared. The structural, morphological, and electrical properties of the NH2-Gs were fully characterized. To investigate possible applications of the NH2-Gs, their ORR performances and capacitive behaviors were analyzed. The reactivity in the Bucherer-type reaction was found to be dependent on the type of GO starting materials (i.e., Hummers or Hofmann). This subsequently affects and determines the electrical and electrochemical properties of the NH2-Gs.
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Figure 1. SEM images of NH2-Gs. Magnification: 30000 Õ .
Scheme 1. Synthesis of amino-functionalized graphene from GO by Bucherer-type reaction under hydrothermal conditions.
Results and Discussion To produce NH2-Gs, graphite was first oxidized to GO by Hummers (Hu) and Hofmann (Ho) methods. Subsequently, GO was subjected to a Bucherer-type reaction in a sodium hydrogensulfite/ammonia mixture without prior conversion to graphene oxide under two different hydrothermal conditions: 1) 150 8C for 5 h (denoted Hu-NA and Ho-NA) and 2) 240 8C for 24 h (denoted Hu-NB and Ho-NB). The obtained NH2-Gs were characterized by SEM, XRD, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and FTIR spectroscopy. The conductivities, ORR capabilities, and capacitive behaviors of the materials were also determined. Morphological and structural characterization The morphologies and topological details of the NH2-Gs were initially analyzed by SEM (Figure 1). In general, Hu-N materials showed a greater extent of exfoliation than Ho-N materials. Hu-N materials existed as layers of wrinkled graphene sheets analogous to those of chemically reduced graphene oxide. On the other hand, Ho-N materials retained the layered structure of GO with porous/spongy character (for 2000 Õ and 10 000 Õ images, see Figure SI1 of the Supporting Information). This coincided with the XRD analyses (Figure 2 A), which showed HuNA and Hu-NB had on average four and five graphene layers, respectively, whereas Ho-NA and Ho-NB were calculated to have eight and nine graphene layers, respectively. Such disparities between Hu-N and Ho-N materials could originate from the oxChem. Eur. J. 2015, 21, 8090 – 8095
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Figure 2. A) XRD patterns and B) Raman spectra of NH2-Gs.
idation techniques applied to obtain GO, since the Bucherertype reaction was performed directly on the GO without prior exfoliation.[18] The (002) diffraction peak of the NH2-Gs at 2 q = 24–268 corresponds to an interlayer spacing in the range of 0.36–0.38 nm. Additional structural information was obtained by Raman spectroscopy, which provided details of the density of defects and average crystalline size of the NH2-Gs (Figure 2 B). The density of defects, as represented by the intensity ratio ID/IG of D ( 1350 cm¢1, corresponding to sp3-hybridized carbon) and G ( 1560 cm¢1, corresponding to sp2-hybridized carbon) bands, was higher for Hu-N than for Ho-N materials. The former had ID/IG ratios of 1.2, and the latter 0.8 (Table SI1, Supporting Information). The low density of defects on Ho-N materials may be due to the low extent of exfoliation, as seen in the highly stacked graphene sheets. This results in an overall lower availability of edge sites or defective basal planes at which chemical reactivity is higher.[2, 19] Furthermore, 2D
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Full Paper ( 2690 cm¢1, corresponding to zone boundary phonons) and D’ ( 1610 cm¢1, corresponding to an intravalley, defect-induced, double-resonance process) bands were observed in the spectra. Moreover, the density of defects reflected the average crystallite size La of the NH2-Gs. If La is considered to be the average distance between two defect sites, a material with high defect density will have a small La value. Thus, Hu-N materials generally showed a smaller average La of approximately 15 nm, whereas that of Ho-N materials was 21 nm. Subsequent FTIR analyses showed very weak bands at approximately 3400 and 1230 cm¢1 corresponding to typical N¢H and C¢N stretching, respectively (Figure SI2, Supporting Information).[20–22] Carbon¢oxygen bands were less apparent for the NH2-Gs. To ascertain the success of the selective functionalization process, the surface chemical compositions of NH2-Gs were examined by XPS. Survey scans were performed to determine the atomic percentages of carbon, oxygen, and nitrogen (Figure 3 A). The survey spectra of NH2-Gs revealed an average oxygen content of 6.3–9.3 atom % (Table 1) and hence an average C/O ratio of approximately 10–14, which is comparable to those of chemically reduced graphene oxide materials.[23] Up to 2.7 atom % of nitrogen was found on Hu-N materials but not on Ho-N materials. This suggested that the reactivity of Hu-N is higher than that of Ho-N, which could be a result of the greater extent of exfoliation of Hummers-based GO resulting in the exposure of more defects and edge sites, which are more chemically active than the basal plane. Especially for
electron-transfer chemistry, the chemical reactivity of singlelayer graphene is higher than that of bilayer graphene.[24] Moreover, since the Bucherer-type reaction proceeded mostly at OH groups on the edges and defect sites of graphene, the nitrogen contents in the Hu-N materials were expected to be lower compared to previous works on nitrogenated graphene materials.[15, 20, 22] High-resolution C 1s and N 1s core-level spectra were recorded to provide greater insights into the nature of their chemical bonding. The high-resolution C 1s core-level spectra were deconvoluted into several components at approximately 284.5 (C=C), 285.5 (C¢C/C¢N), 286.5 (C¢O), 287.7 (C=O), 288.7 (O¢C=O), and 290.7 eV (p–p* shake-up). In general, the contents of oxygen-containing groups (C¢O, C=O, and O¢C=O) were higher in Hu-N than in Ho-N materials (Table S1, Supporting Information). Moreover, the exposure to higher reaction temperature and longer reaction time increased the amount of CO functional groups across all NH2-Gs. Deconvolution of the N 1s core-level spectra provided five components at 398.1 (pyridinic N), 399.4 (amino N), 400.3 (pyrrolic N), 401.2 (graphitic N), and 403.2 eV (N oxide).[12, 14] The Hu-N materials bear predominantly amino goups (up to 37.2 atom %). The amount of graphitic-N and pyrrolic-N species on the Hu-N materials were approximately 14.2 and 15.5 atom %, respectively, without much fluctuation even on increasing the temperature and reaction time of hydrothermal treatment. On the other hand, the amount of N oxide de-
Figure 3. XPS spectra of NH2-Gs. A) Survey scans. B) C 1s core-level scans. C) N 1s core-level scans.
Table 1. Elemental compositions from XPS survey spectra and percentages of nitrogenous moieties from N 1s core-level spectra of NH2-Gs. Samples
XPS survey [atom %] C O
N
N oxide
Graphitic N
Hu-NA Hu-NB Ho-NA Ho-NB
90.8 91.2 91.4 90.7
2.7 2.6 – –
15.0 7.6 – –
14.3 14.1 – –
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6.5 6.3 8.6 9.3
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N 1s core level [%] Pyrrolic N 15.9 15.3 – –
Amino N
Pyridinic N
36.9 37.2 – –
17.9 25.8 – –
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Full Paper creased by 7.4 %, while that of pyridinic N increased by 7.9 %. Given the mechanism of Bucherer-type reactions, selective amino functionalization of Hu-N most likely occurred along the edges or defect sites of the GO sheets, which are more chemically active.[14] Such disparities between the NH2-Gs were expected to bring about different electrochemical properties, especially in heterogeneous electron-transfer processes.[25]
in the Supporting Information). Further analysis of ORR performance was performed by linear sweep voltammetry (LSV) on the NH2-Gs (Figure 4 B). The obtained onset potentials, peak potentials, and peak currents for the NH2-Gs are listed in Table 2. Hu-NB gave the best performance, as shown by its low onset potential and high peak current. This is likely to be due to larger amounts of Mn impurities in Hu samples than in Ho samples (Hu-NA 510, Hu-NB 226, Ho-NA 4, Ho-NB 3 ppm by ICPMS).[26, 27, 28] While the NH2-Gs performed better than bare GC Electrical and electrochemical characterization electrode, Pt/C was still the best-performing material for ORR. In an attempt to expand the application of NH2-Gs, their caThe conductivities of NH2-Gs were probed with an interdigitatpacitive behaviors were analyzed in KOH (6 m) solution by ed electrode (Figure 4 A). The I–V curves were recorded, and means of a three-electrode cell system. CVs of NH2-Gs at since the materials showed ohmic behavior, the slope of the I– a scan rate of 100 mV s¢1 showed a capacitive background curV curves could be directly correlated to their conductivities. As rent in an N2-saturated solution (see Figure SI4A in the Supcan be seen in Table 2, despite the higher density of defects porting Information). The Hu-N materials evidently have more on Hu-N materials, the higher conductivities of Hu-N materials pronounced capacitive behaviors than the Ho-N materials. A in comparison to Ho-N materials suggested a possible ensimilar trend was observed in the almost symmetrical charge/ hancement of conductivity due to the presence of nitrogen discharge curves at different current densities. The charge/disfunctional groups on graphene.[13] charge profiles of the NH2-Gs at a current density of 0.1 A g¢1 are shown in Figure SI4B in the Supporting Information. The corresponding gravimetric capacitances of the materials were measured from the galvanostatic discharge curves, for which the responses at different current densities (0.1–5 A g¢1) are shown in Figure SI4C in the Supporting Information. At a current density of 0.1 A g¢1, the gravimetric caFigure 4. A) I–V curves of NH2-Gs. B) Linear-sweep voltammograms for ORR of NH2-Gs. Conditions: 0.1 m KOH solu¢1 pacitances measured for Hu-NA, tion at 100 mV s . Hu-NB, Ho-NA, and Ho-NB were 20, 11, 2, and 2 F g¢1. The overall Table 2. Conductances G and electrocatalytic properties based on CVs of low gravimetric capacitances could result from the low extent NH2-Gs. of exfoliation of the graphene sheets. Nevertheless, the aminofunctionalized Hu-N materials showed consistently higher Sample G Oxygen reduction reaction gravimetric capacitances than the Ho-N materials, which were Onset Peak Peak [mAV¢1] potential [V] potential [V] current [mA] not functionalized with nitrogen moieties. Hu-NA Hu-NB Ho-NA Ho-NB bare GC Pt/C
54 58 43 45 – –
¢0.28 ¢0.28 ¢0.30 ¢0.31 ¢0.32 ¢0.23
¢0.45 ¢0.41 ¢0.45 ¢0.47 ¢0.47 ¢0.37
8.6 9.6 7.8 6.9 6.4 11.8
Conclusion
To evaluate the electrocatalytic performance of NH2-Gs in the ORR, cyclic voltammetry (CV) measurements were performed on bare glassy carbon (GC) and GC electrodes coated with NH2-Gs and Pt/C (20 wt % Pt supported on activated carbon). Approximately 1 mg of each material was drop-cast onto a GC electrode and triplicate results were collected. The presence of a well-defined cathodic peak at approximately ¢0.35 V in KOH (0.1 m) solution and the subsequent absence of that peak in N2-purged KOH (0.1 m) solution suggested electrocatalytic behavior of NH2-Gs towards the ORR (see Figure SI3 Chem. Eur. J. 2015, 21, 8090 – 8095
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A selective synthesis of amino-functionalized graphene was achieved by a Bucherer-type reaction under hydrothermal conditions. The synthesis was performed on two types of GO, produced by Hummers and Hofmann oxidation routes, which showed contrasting extents of reaction as well as morphological and structural features. The Hummers GO was successfully functionalized and gave up to 2.7 atom % of nitrogen, whereas the Hofmann GO was unreactive. Amino groups were found in the highest concentration compared to the other nitrogencontaining moieties in amino-functionalized graphene materials. These findings will be valuable for future use of GO to produce selectively functionalized nitrogenated graphene materials.
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Procedures
Materials
Bucherer reaction: GO (0.4 g) was placed in Teflon-lined autoclave (27 mL autoclave volume), and concentrated ammonia (25 wt %; 8 mL) and sodium hydrogensulfite (8 mL) were added. Sodium hydrogensulfite was prepared by saturating a cooled solution of sodium hydroxide (20 wt %) with SO2. The autoclave was tightly closed and heated at 240 and 150 8C for 5 and 24 h, respectively. The reaction mixture was separated by suction filtration and washed with deionized water, carbon disulfide, and methanol.
Graphite oxide (GO) was prepared according to the Hummers or Hofmann method from high-purity microcrystalline graphite (2– 15 mm, 99.9995 %, Alfa Aesar, Germany). Sulfuric acid (98 %), nitric acid (68 %), potassium chlorate (> 99 %), potassium permanganate (> 99.5 %), sodium nitrate (> 99.5 %), hydrogen peroxide (30 %), hydrochloric acid (37 %), ammonia (> 25 %), methanol (99.9 %), and carbon disulfide (99.5 %) were obtained from Penta (Czech Republic). Sulfur dioxide (99.99 %) was obtained from SIAD (Czech Republic). Platinum (20 wt %) on activated carbon was obtained from Alfa Aesar (Singapore). A glassy carbon (GC) electrode with a diameter of 3 mm was obtained from Autolab, the Netherlands. Milli-Q water (resistivity: 18.2 MW cm) was used throughout the experiments.
Equipment XPS was performed with a Phoibos 100 spectrometer and a nonmonochromatic Mg X-ray radiation source (SPECS, Germany). Wide survey scans and high-resolution N 1s and C 1s spectra were collected. XPS measurements were taken on homogeneously covered carbon conductive tape. A JEOL-7600F semi-in-lens FE-SEM, operating in LEI and GB-H modes at 2 kV, was used to acquire the SEM images. The solid samples were transferred to a carbon tape held on an SEM holder for analyses. Attenuated total reflectance (ATR) FTIR measurements were carried out on a PerkinElmer Spectrum 100 system coupled with a universal ATR accessory. Diamond/ZnSe was used as the ATR crystal. The spectra were measured from 1000 to 3000 cm¢1. Raman spectroscopy was performed by using a confocal micro-Raman LabRam HR instrument from Horiba Scientific in backscattering geometry with a CCD detector, a 514.5 nm Ar laser, and a 100 Õ objective mounted on an Olympus optical microscope. The calibration was initially done with an internal silicon reference at 520 cm¢1 and gave a peak-position resolution of less than 1 cm¢1. The crystallite size La was calculated from the formula La = 2.4 Õ 10¢10 Õ l4laser Õ IG/ID. All samples were analyzed by powder XRD. Data were collected with a PANalytical X’Pert PRO diffractometer in Bragg–Brentano parafocusing geometry. CuKa radiation was used. Diffraction patterns were collected between 2 q = 5 and 808. The obtained data were refined by the Rietveld method. The Scherrer equation was used to calculate the number of layers. Trace metal compositions were determined by using an Agilent model 7700x ICP-MS, and microwave digestion with ultrapure HNO3 was performed on a Mars CEM system. Samples for ICP analysis were prepared by accurately weighing into a clean Easy Prep vessel followed by addition of 1.5 mL of ultrapure HNO3. The digestion procedure consisted of microwave treatment at 1600 W (100 %), ramping the temperature from 25 to 120 8C over 20 min ramping time, and maintaining the final temperature for 15 min. Final temperatures of 180 and 200 8C were also adopted to examine the influence of temperature and ramping time on elemental composition. The calculated results showed good agreement between the different methods and their replicates. All voltammetric experiments were performed on a mAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a PC and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). Chem. Eur. J. 2015, 21, 8090 – 8095
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GO synthesis: GO prepared by the Hofmann method was termed HO. Concentrated sulfuric acid (87.5 mL) and nitric acid (27 mL) were added to a reaction flask containing a magnetic stir bar. The mixture was then cooled at 0 8C, and graphite (5 g) was added. The mixture was vigorously stirred to avoid agglomeration and to obtain a homogeneous dispersion. While keeping the reaction flask at 0 8C, potassium chlorate (55 g) was slowly added to the mixture to avoid a sudden increase in temperature and the consequent formation of explosive chlorine dioxide gas. On complete dissolution of the potassium chlorate, the reaction flask was then loosely capped to allow the escape of the evolved gas and the mixture continuously vigorously stirred for 96 h at room temperature. On completion of the reaction, the mixture was poured into deionized water (3 L) and decanted. The graphite oxide was first redispersed in HCl (5 %) solution to remove sulfate ions and then repeatedly centrifuged and redispersed in deionized water until all chloride and sulfate ions were removed. The GO slurry was then dried in a vacuum oven at 50 8C for 48 h before use. The second GO, HU was synthesized in a similar way to the Hummers method. Graphite (5 g) and sodium nitrate (2.5 g) were stirred together with concentrated sulfuric acid (115 mL). The mixture was then cooled at 0 8C. Potassium permanganate (15 g) was then added with vigorous stirring over 2 h. During the following 4 h, the reaction mixture was allowed to reach room temperature before being heated to 35 8C for 30 min. The reaction mixture was then poured into a flask of deionized water (250 mL) and heated to 70 8C for 15 min. The mixture was then poured into deionized water (1 L). The unconsumed potassium permanganate and manganese dioxide were removed by addition of 3 % hydrogen peroxide. The reaction mixture was then allowed to settle before being decanted. The obtained GO was then purified by repeated centrifugation and redispersed in deionized water until all sulfate ions were removed. The GO slurry was then dried in a vacuum oven at 50 8C for 48 h before use. Electrochemical measurements: The electrochemical experiments were carried out in a 10 mL voltammetric cell at room temperature in a three-electrode configuration. A platinum electrode served as auxiliary electrode, an Ag/AgCl electrode as reference electrode, and a GC electrode as working electrode. Prior to measurements, the GC electrode was polished with 0.05 mm alumina on a polishing cloth. The CV and LSV measurements for ORR were performed in a solution of 0.1 m KOH at a scan rate of 100 mV s¢1. The working electrode was modified by applying 1 mL of DMF containing 1 mg mL¢1 of carbon material and allowed to dry. The linear sweep voltammograms for ORR were baseline-corrected and the onset potentials were determined as potential at 10 % of the peak wave current. The CV and galvanostatic charge/discharge curves for capacitance analyses were performed in 6 m KOH solution. The amount of active material was taken as 5 mg, since 1 mL of DMF containing 5 mg mL¢1 carbon material was drop-cast onto the GC working electrode and allowed to dry.
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Full Paper Conductivity measurements: I–V measurements were conducted with an interdigitated gold electrode platform (Au-IDE) by depositing 2 mL of the material suspension (prepared at 1 mg mL¢1 concentration in water) onto an electrode surface with 10 mm spacing. The electrode was then dried under a lamp for 20 min, leaving a randomly deposited material film on the interdigitated area bridging the two Au electrode bands. The I–V curves were obtained by linear sweep voltammetric measurements at 20 mV s¢1 scan rate. The displayed data correspond to the average from three measurements.
Acknowledgements M.P. acknowledges Tier 2 grant (MOE2013-T2-1-056; ARC 35/ 13) from Ministry of Education, Singapore. Z.S. and J.L. were supported by the Czech Science Foundation (GACR No. 1509001S) and by Specific University Research (MSMT No. 20/ 2015). We thank Dr. B. Khezri for ICP-MS measurements. Keywords: amination · electrochemistry · graphene hydrothermal synthesis · oxygen reduction reaction
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Received: October 21, 2014 Revised: February 22, 2015 Published online on March 26, 2015
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Selective Nitrogen Functionalization of Graphene by BuchererType Reaction Chun Kiang Chua,[a] Zdeneˇk Sofer,[b] Jan Luxa,[b] and Martin Pumera*[a] Abstract: Nitrogen functionalization of graphene offers new hybrid materials with improved performance for important technological applications. Despite studies highlighting the dependence of the performance of nitrogen-functionalized graphene on the types of nitrogen functional groups that are present, precise synthetic control over their ratio is challenging. Herein, the synthesis of nitrogen-functionalized graphene rich in amino groups by a Bucherer-type reaction under hydrothermal conditions is reported. The efficiency of
Introduction Intrinsic modification of graphene has been an important focus of graphene research in recent years. Chemical functionalization and doping with metals or heteroatoms provides opportunities to tailor the electronic properties and modify the surface chemistry of graphene.[1, 2] Common metal dopants include precious and nonprecious metals, while heteroatom dopants comprise boron, nitrogen, sulfur, and phosphorus.[3–5] Enhanced electrical and thermal conductivity as well as increased density of free charge carriers of such modified graphene materials have improved their performances in applications such as bioelectrochemical sensing systems,[6] supercapacitors,[7] and lithium-ion batteries.[8] One of the earliest attempts to modify graphene was to introduce chemically bonded nitrogen species onto the sp2 carbon structure of graphene. In such nitrogenated graphenes, nitrogen atoms are chemically bonded in pyridinic N, amino N, pyrrolic N, quaternary N, and N oxide moieties. The numerous well-known methods to synthesize nitrogenated graphene include nitrogen plasma,[6] electrothermal reaction,[9] chemical vapor deposition,[10] solvothermal synthesis,[11] and post-thermal treatment with nitrogen-containing species.[12, 13] [a] Dr. C. K. Chua, Prof. M. Pumera Department of Chemistry & Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University 21, Nanyang Link, Singapore 637371 E-mail: [email protected] [b] Prof. Z. Sofer, J. Luxa Department of Inorganic Chemistry University of Chemistry and Technology Prague Technick 5, 166 28 Prague 6 (Czech Republic) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405748. Chem. Eur. J. 2015, 21, 8090 – 8095
the synthetic method under two hydrothermal conditions was examined for graphite oxide produced by Hummers and Hofmann oxidation routes. The morphological and structural properties of the amino-functionalized graphene were fully characterized. The use of a synthetic method with a well-known mechanism for derivatization of graphene will open new avenues for highly reproducible functionalization of graphene materials.
Despite the efficiency of these methods in producing nitrogenated graphene, the exact nature of the nitrogen moieties is often random and hard to control without precise instrumentation. Nitrogenated graphene usually shows exceptional performance in important catalytic reactions.[3, 4] Nitrogenated graphene materials that are rich in graphitic N moieties have been shown to have enhanced capacitive behavior and electrocatalytic effects towards the oxygen reduction reaction (ORR).[7, 14, 15] While the role of amino groups is taken into consideration in numerous ORR studies on nitrogenated graphene materials, Hou and co-workers systematically broke down the roles of the nitrogen moieties in the ORR to reveal that the graphitic N atoms and amino groups dictate the onset potential and electron-transfer number.[15] It is therefore imperative to develop novel functionalization methods capable of selectively introducing a specific type of nitrogen moiety to produce useful tailored nitrogenated graphene for technological applications. In this work, a selective functionalization method to produce amino-functionalized graphenes (NH2-Gs) based on a Bucherertype reaction under hydrothermal conditions (Scheme 1) is proposed. In accordance with the Bucherer-type reaction, it was anticipated that the resultant graphene materials would be functionalized with amino groups through exchange with hydroxyl groups. Hydrothermal reactions of graphite oxide (GO) obtained by the Hummers[16] and Hofmann[17] methods under two hydrothermal conditions were compared. The structural, morphological, and electrical properties of the NH2-Gs were fully characterized. To investigate possible applications of the NH2-Gs, their ORR performances and capacitive behaviors were analyzed. The reactivity in the Bucherer-type reaction was found to be dependent on the type of GO starting materials (i.e., Hummers or Hofmann). This subsequently affects and determines the electrical and electrochemical properties of the NH2-Gs.
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Figure 1. SEM images of NH2-Gs. Magnification: 30000 Õ .
Scheme 1. Synthesis of amino-functionalized graphene from GO by Bucherer-type reaction under hydrothermal conditions.
Results and Discussion To produce NH2-Gs, graphite was first oxidized to GO by Hummers (Hu) and Hofmann (Ho) methods. Subsequently, GO was subjected to a Bucherer-type reaction in a sodium hydrogensulfite/ammonia mixture without prior conversion to graphene oxide under two different hydrothermal conditions: 1) 150 8C for 5 h (denoted Hu-NA and Ho-NA) and 2) 240 8C for 24 h (denoted Hu-NB and Ho-NB). The obtained NH2-Gs were characterized by SEM, XRD, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and FTIR spectroscopy. The conductivities, ORR capabilities, and capacitive behaviors of the materials were also determined. Morphological and structural characterization The morphologies and topological details of the NH2-Gs were initially analyzed by SEM (Figure 1). In general, Hu-N materials showed a greater extent of exfoliation than Ho-N materials. Hu-N materials existed as layers of wrinkled graphene sheets analogous to those of chemically reduced graphene oxide. On the other hand, Ho-N materials retained the layered structure of GO with porous/spongy character (for 2000 Õ and 10 000 Õ images, see Figure SI1 of the Supporting Information). This coincided with the XRD analyses (Figure 2 A), which showed HuNA and Hu-NB had on average four and five graphene layers, respectively, whereas Ho-NA and Ho-NB were calculated to have eight and nine graphene layers, respectively. Such disparities between Hu-N and Ho-N materials could originate from the oxChem. Eur. J. 2015, 21, 8090 – 8095
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Figure 2. A) XRD patterns and B) Raman spectra of NH2-Gs.
idation techniques applied to obtain GO, since the Bucherertype reaction was performed directly on the GO without prior exfoliation.[18] The (002) diffraction peak of the NH2-Gs at 2 q = 24–268 corresponds to an interlayer spacing in the range of 0.36–0.38 nm. Additional structural information was obtained by Raman spectroscopy, which provided details of the density of defects and average crystalline size of the NH2-Gs (Figure 2 B). The density of defects, as represented by the intensity ratio ID/IG of D ( 1350 cm¢1, corresponding to sp3-hybridized carbon) and G ( 1560 cm¢1, corresponding to sp2-hybridized carbon) bands, was higher for Hu-N than for Ho-N materials. The former had ID/IG ratios of 1.2, and the latter 0.8 (Table SI1, Supporting Information). The low density of defects on Ho-N materials may be due to the low extent of exfoliation, as seen in the highly stacked graphene sheets. This results in an overall lower availability of edge sites or defective basal planes at which chemical reactivity is higher.[2, 19] Furthermore, 2D
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Full Paper ( 2690 cm¢1, corresponding to zone boundary phonons) and D’ ( 1610 cm¢1, corresponding to an intravalley, defect-induced, double-resonance process) bands were observed in the spectra. Moreover, the density of defects reflected the average crystallite size La of the NH2-Gs. If La is considered to be the average distance between two defect sites, a material with high defect density will have a small La value. Thus, Hu-N materials generally showed a smaller average La of approximately 15 nm, whereas that of Ho-N materials was 21 nm. Subsequent FTIR analyses showed very weak bands at approximately 3400 and 1230 cm¢1 corresponding to typical N¢H and C¢N stretching, respectively (Figure SI2, Supporting Information).[20–22] Carbon¢oxygen bands were less apparent for the NH2-Gs. To ascertain the success of the selective functionalization process, the surface chemical compositions of NH2-Gs were examined by XPS. Survey scans were performed to determine the atomic percentages of carbon, oxygen, and nitrogen (Figure 3 A). The survey spectra of NH2-Gs revealed an average oxygen content of 6.3–9.3 atom % (Table 1) and hence an average C/O ratio of approximately 10–14, which is comparable to those of chemically reduced graphene oxide materials.[23] Up to 2.7 atom % of nitrogen was found on Hu-N materials but not on Ho-N materials. This suggested that the reactivity of Hu-N is higher than that of Ho-N, which could be a result of the greater extent of exfoliation of Hummers-based GO resulting in the exposure of more defects and edge sites, which are more chemically active than the basal plane. Especially for
electron-transfer chemistry, the chemical reactivity of singlelayer graphene is higher than that of bilayer graphene.[24] Moreover, since the Bucherer-type reaction proceeded mostly at OH groups on the edges and defect sites of graphene, the nitrogen contents in the Hu-N materials were expected to be lower compared to previous works on nitrogenated graphene materials.[15, 20, 22] High-resolution C 1s and N 1s core-level spectra were recorded to provide greater insights into the nature of their chemical bonding. The high-resolution C 1s core-level spectra were deconvoluted into several components at approximately 284.5 (C=C), 285.5 (C¢C/C¢N), 286.5 (C¢O), 287.7 (C=O), 288.7 (O¢C=O), and 290.7 eV (p–p* shake-up). In general, the contents of oxygen-containing groups (C¢O, C=O, and O¢C=O) were higher in Hu-N than in Ho-N materials (Table S1, Supporting Information). Moreover, the exposure to higher reaction temperature and longer reaction time increased the amount of CO functional groups across all NH2-Gs. Deconvolution of the N 1s core-level spectra provided five components at 398.1 (pyridinic N), 399.4 (amino N), 400.3 (pyrrolic N), 401.2 (graphitic N), and 403.2 eV (N oxide).[12, 14] The Hu-N materials bear predominantly amino goups (up to 37.2 atom %). The amount of graphitic-N and pyrrolic-N species on the Hu-N materials were approximately 14.2 and 15.5 atom %, respectively, without much fluctuation even on increasing the temperature and reaction time of hydrothermal treatment. On the other hand, the amount of N oxide de-
Figure 3. XPS spectra of NH2-Gs. A) Survey scans. B) C 1s core-level scans. C) N 1s core-level scans.
Table 1. Elemental compositions from XPS survey spectra and percentages of nitrogenous moieties from N 1s core-level spectra of NH2-Gs. Samples
XPS survey [atom %] C O
N
N oxide
Graphitic N
Hu-NA Hu-NB Ho-NA Ho-NB
90.8 91.2 91.4 90.7
2.7 2.6 – –
15.0 7.6 – –
14.3 14.1 – –
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6.5 6.3 8.6 9.3
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N 1s core level [%] Pyrrolic N 15.9 15.3 – –
Amino N
Pyridinic N
36.9 37.2 – –
17.9 25.8 – –
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Full Paper creased by 7.4 %, while that of pyridinic N increased by 7.9 %. Given the mechanism of Bucherer-type reactions, selective amino functionalization of Hu-N most likely occurred along the edges or defect sites of the GO sheets, which are more chemically active.[14] Such disparities between the NH2-Gs were expected to bring about different electrochemical properties, especially in heterogeneous electron-transfer processes.[25]
in the Supporting Information). Further analysis of ORR performance was performed by linear sweep voltammetry (LSV) on the NH2-Gs (Figure 4 B). The obtained onset potentials, peak potentials, and peak currents for the NH2-Gs are listed in Table 2. Hu-NB gave the best performance, as shown by its low onset potential and high peak current. This is likely to be due to larger amounts of Mn impurities in Hu samples than in Ho samples (Hu-NA 510, Hu-NB 226, Ho-NA 4, Ho-NB 3 ppm by ICPMS).[26, 27, 28] While the NH2-Gs performed better than bare GC Electrical and electrochemical characterization electrode, Pt/C was still the best-performing material for ORR. In an attempt to expand the application of NH2-Gs, their caThe conductivities of NH2-Gs were probed with an interdigitatpacitive behaviors were analyzed in KOH (6 m) solution by ed electrode (Figure 4 A). The I–V curves were recorded, and means of a three-electrode cell system. CVs of NH2-Gs at since the materials showed ohmic behavior, the slope of the I– a scan rate of 100 mV s¢1 showed a capacitive background curV curves could be directly correlated to their conductivities. As rent in an N2-saturated solution (see Figure SI4A in the Supcan be seen in Table 2, despite the higher density of defects porting Information). The Hu-N materials evidently have more on Hu-N materials, the higher conductivities of Hu-N materials pronounced capacitive behaviors than the Ho-N materials. A in comparison to Ho-N materials suggested a possible ensimilar trend was observed in the almost symmetrical charge/ hancement of conductivity due to the presence of nitrogen discharge curves at different current densities. The charge/disfunctional groups on graphene.[13] charge profiles of the NH2-Gs at a current density of 0.1 A g¢1 are shown in Figure SI4B in the Supporting Information. The corresponding gravimetric capacitances of the materials were measured from the galvanostatic discharge curves, for which the responses at different current densities (0.1–5 A g¢1) are shown in Figure SI4C in the Supporting Information. At a current density of 0.1 A g¢1, the gravimetric caFigure 4. A) I–V curves of NH2-Gs. B) Linear-sweep voltammograms for ORR of NH2-Gs. Conditions: 0.1 m KOH solu¢1 pacitances measured for Hu-NA, tion at 100 mV s . Hu-NB, Ho-NA, and Ho-NB were 20, 11, 2, and 2 F g¢1. The overall Table 2. Conductances G and electrocatalytic properties based on CVs of low gravimetric capacitances could result from the low extent NH2-Gs. of exfoliation of the graphene sheets. Nevertheless, the aminofunctionalized Hu-N materials showed consistently higher Sample G Oxygen reduction reaction gravimetric capacitances than the Ho-N materials, which were Onset Peak Peak [mAV¢1] potential [V] potential [V] current [mA] not functionalized with nitrogen moieties. Hu-NA Hu-NB Ho-NA Ho-NB bare GC Pt/C
54 58 43 45 – –
¢0.28 ¢0.28 ¢0.30 ¢0.31 ¢0.32 ¢0.23
¢0.45 ¢0.41 ¢0.45 ¢0.47 ¢0.47 ¢0.37
8.6 9.6 7.8 6.9 6.4 11.8
Conclusion
To evaluate the electrocatalytic performance of NH2-Gs in the ORR, cyclic voltammetry (CV) measurements were performed on bare glassy carbon (GC) and GC electrodes coated with NH2-Gs and Pt/C (20 wt % Pt supported on activated carbon). Approximately 1 mg of each material was drop-cast onto a GC electrode and triplicate results were collected. The presence of a well-defined cathodic peak at approximately ¢0.35 V in KOH (0.1 m) solution and the subsequent absence of that peak in N2-purged KOH (0.1 m) solution suggested electrocatalytic behavior of NH2-Gs towards the ORR (see Figure SI3 Chem. Eur. J. 2015, 21, 8090 – 8095
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A selective synthesis of amino-functionalized graphene was achieved by a Bucherer-type reaction under hydrothermal conditions. The synthesis was performed on two types of GO, produced by Hummers and Hofmann oxidation routes, which showed contrasting extents of reaction as well as morphological and structural features. The Hummers GO was successfully functionalized and gave up to 2.7 atom % of nitrogen, whereas the Hofmann GO was unreactive. Amino groups were found in the highest concentration compared to the other nitrogencontaining moieties in amino-functionalized graphene materials. These findings will be valuable for future use of GO to produce selectively functionalized nitrogenated graphene materials.
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Procedures
Materials
Bucherer reaction: GO (0.4 g) was placed in Teflon-lined autoclave (27 mL autoclave volume), and concentrated ammonia (25 wt %; 8 mL) and sodium hydrogensulfite (8 mL) were added. Sodium hydrogensulfite was prepared by saturating a cooled solution of sodium hydroxide (20 wt %) with SO2. The autoclave was tightly closed and heated at 240 and 150 8C for 5 and 24 h, respectively. The reaction mixture was separated by suction filtration and washed with deionized water, carbon disulfide, and methanol.
Graphite oxide (GO) was prepared according to the Hummers or Hofmann method from high-purity microcrystalline graphite (2– 15 mm, 99.9995 %, Alfa Aesar, Germany). Sulfuric acid (98 %), nitric acid (68 %), potassium chlorate (> 99 %), potassium permanganate (> 99.5 %), sodium nitrate (> 99.5 %), hydrogen peroxide (30 %), hydrochloric acid (37 %), ammonia (> 25 %), methanol (99.9 %), and carbon disulfide (99.5 %) were obtained from Penta (Czech Republic). Sulfur dioxide (99.99 %) was obtained from SIAD (Czech Republic). Platinum (20 wt %) on activated carbon was obtained from Alfa Aesar (Singapore). A glassy carbon (GC) electrode with a diameter of 3 mm was obtained from Autolab, the Netherlands. Milli-Q water (resistivity: 18.2 MW cm) was used throughout the experiments.
Equipment XPS was performed with a Phoibos 100 spectrometer and a nonmonochromatic Mg X-ray radiation source (SPECS, Germany). Wide survey scans and high-resolution N 1s and C 1s spectra were collected. XPS measurements were taken on homogeneously covered carbon conductive tape. A JEOL-7600F semi-in-lens FE-SEM, operating in LEI and GB-H modes at 2 kV, was used to acquire the SEM images. The solid samples were transferred to a carbon tape held on an SEM holder for analyses. Attenuated total reflectance (ATR) FTIR measurements were carried out on a PerkinElmer Spectrum 100 system coupled with a universal ATR accessory. Diamond/ZnSe was used as the ATR crystal. The spectra were measured from 1000 to 3000 cm¢1. Raman spectroscopy was performed by using a confocal micro-Raman LabRam HR instrument from Horiba Scientific in backscattering geometry with a CCD detector, a 514.5 nm Ar laser, and a 100 Õ objective mounted on an Olympus optical microscope. The calibration was initially done with an internal silicon reference at 520 cm¢1 and gave a peak-position resolution of less than 1 cm¢1. The crystallite size La was calculated from the formula La = 2.4 Õ 10¢10 Õ l4laser Õ IG/ID. All samples were analyzed by powder XRD. Data were collected with a PANalytical X’Pert PRO diffractometer in Bragg–Brentano parafocusing geometry. CuKa radiation was used. Diffraction patterns were collected between 2 q = 5 and 808. The obtained data were refined by the Rietveld method. The Scherrer equation was used to calculate the number of layers. Trace metal compositions were determined by using an Agilent model 7700x ICP-MS, and microwave digestion with ultrapure HNO3 was performed on a Mars CEM system. Samples for ICP analysis were prepared by accurately weighing into a clean Easy Prep vessel followed by addition of 1.5 mL of ultrapure HNO3. The digestion procedure consisted of microwave treatment at 1600 W (100 %), ramping the temperature from 25 to 120 8C over 20 min ramping time, and maintaining the final temperature for 15 min. Final temperatures of 180 and 200 8C were also adopted to examine the influence of temperature and ramping time on elemental composition. The calculated results showed good agreement between the different methods and their replicates. All voltammetric experiments were performed on a mAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a PC and controlled by General Purpose Electrochemical Systems Version 4.9 software (Eco Chemie). Chem. Eur. J. 2015, 21, 8090 – 8095
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GO synthesis: GO prepared by the Hofmann method was termed HO. Concentrated sulfuric acid (87.5 mL) and nitric acid (27 mL) were added to a reaction flask containing a magnetic stir bar. The mixture was then cooled at 0 8C, and graphite (5 g) was added. The mixture was vigorously stirred to avoid agglomeration and to obtain a homogeneous dispersion. While keeping the reaction flask at 0 8C, potassium chlorate (55 g) was slowly added to the mixture to avoid a sudden increase in temperature and the consequent formation of explosive chlorine dioxide gas. On complete dissolution of the potassium chlorate, the reaction flask was then loosely capped to allow the escape of the evolved gas and the mixture continuously vigorously stirred for 96 h at room temperature. On completion of the reaction, the mixture was poured into deionized water (3 L) and decanted. The graphite oxide was first redispersed in HCl (5 %) solution to remove sulfate ions and then repeatedly centrifuged and redispersed in deionized water until all chloride and sulfate ions were removed. The GO slurry was then dried in a vacuum oven at 50 8C for 48 h before use. The second GO, HU was synthesized in a similar way to the Hummers method. Graphite (5 g) and sodium nitrate (2.5 g) were stirred together with concentrated sulfuric acid (115 mL). The mixture was then cooled at 0 8C. Potassium permanganate (15 g) was then added with vigorous stirring over 2 h. During the following 4 h, the reaction mixture was allowed to reach room temperature before being heated to 35 8C for 30 min. The reaction mixture was then poured into a flask of deionized water (250 mL) and heated to 70 8C for 15 min. The mixture was then poured into deionized water (1 L). The unconsumed potassium permanganate and manganese dioxide were removed by addition of 3 % hydrogen peroxide. The reaction mixture was then allowed to settle before being decanted. The obtained GO was then purified by repeated centrifugation and redispersed in deionized water until all sulfate ions were removed. The GO slurry was then dried in a vacuum oven at 50 8C for 48 h before use. Electrochemical measurements: The electrochemical experiments were carried out in a 10 mL voltammetric cell at room temperature in a three-electrode configuration. A platinum electrode served as auxiliary electrode, an Ag/AgCl electrode as reference electrode, and a GC electrode as working electrode. Prior to measurements, the GC electrode was polished with 0.05 mm alumina on a polishing cloth. The CV and LSV measurements for ORR were performed in a solution of 0.1 m KOH at a scan rate of 100 mV s¢1. The working electrode was modified by applying 1 mL of DMF containing 1 mg mL¢1 of carbon material and allowed to dry. The linear sweep voltammograms for ORR were baseline-corrected and the onset potentials were determined as potential at 10 % of the peak wave current. The CV and galvanostatic charge/discharge curves for capacitance analyses were performed in 6 m KOH solution. The amount of active material was taken as 5 mg, since 1 mL of DMF containing 5 mg mL¢1 carbon material was drop-cast onto the GC working electrode and allowed to dry.
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Full Paper Conductivity measurements: I–V measurements were conducted with an interdigitated gold electrode platform (Au-IDE) by depositing 2 mL of the material suspension (prepared at 1 mg mL¢1 concentration in water) onto an electrode surface with 10 mm spacing. The electrode was then dried under a lamp for 20 min, leaving a randomly deposited material film on the interdigitated area bridging the two Au electrode bands. The I–V curves were obtained by linear sweep voltammetric measurements at 20 mV s¢1 scan rate. The displayed data correspond to the average from three measurements.
Acknowledgements M.P. acknowledges Tier 2 grant (MOE2013-T2-1-056; ARC 35/ 13) from Ministry of Education, Singapore. Z.S. and J.L. were supported by the Czech Science Foundation (GACR No. 1509001S) and by Specific University Research (MSMT No. 20/ 2015). We thank Dr. B. Khezri for ICP-MS measurements. Keywords: amination · electrochemistry · graphene hydrothermal synthesis · oxygen reduction reaction
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Received: October 21, 2014 Revised: February 22, 2015 Published online on March 26, 2015
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