Nitrogen-Doped Reduced Graphene Oxide Prepared by Simultaneous

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Nitrogen-Doped Reduced Graphene oxide Prepared by Simultaneous Thermal Reduction and Nitrogen-Doping of Graphene Oxide in Air and Its Application as Electrocatalyst Donghe Du, Pengcheng Li, and Jianyong Ouyang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07757 • Publication Date (Web): 18 Nov 2015 Downloaded from http://pubs.acs.org on November 20, 2015

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ACS Applied Materials & Interfaces

Nitrogen-Doped Reduced Graphene oxide Prepared by Simultaneous Thermal Reduction and Nitrogen-Doping of Graphene Oxide in Air and Its Application as Electrocatalyst Donghe Du, Pengcheng Li, and Jianyong Ouyang* Department of Materials Science & Engineering, National University of Singapore, Singapore 117576 KEYWORDS: graphene, reduced graphene oxide, thermal reduction, nitrogen doped, air

ABSTRACT: Graphene is considered as one of the most interesting materials due to its unique two-dimensional structure and properties. However, the commercialization and large scale production of graphene still face great challenges at the moment. Thermal reduction of graphene oxide can be an effective method to fabricate graphene in large scale, but the necessity of inert gas protection and high reaction temperature leads to high cost of production thus limited production capacity of graphene. In this paper, for the first time we report a facile, safe and scalable method to achieve simultaneous thermal reduction and nitrogen doping of graphene oxide (GO) in air at much lower reaction temperature, while upholding a high quality end product. The reduction and nitrogen doping of GO are evidenced by ultraviolet-visible absorption spectroscopy, X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The N-doped rGO fabricated via this method has a high C/O ratio of 15 and a nitrogen content of 11.87 at.%. The N-doped rGO is

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also investigated by applying it as the electrocatalyst for the oxygen reduction reaction. As a result, the catalytic activity has presented itself to be much higher than the undoped rGO.

1. Introduction Graphene has been one of the most popular research topics in the last decade due to its unique structure and properties, since it was reported for the first time from mechanical exfoliation of graphite in 2004.1–7 Graphene has potential applications in many areas, e.g. electronics, catalysis and energy storage, owing to its high charge carrier mobility and high specific surface area. The most commonly used commercial method to fabricate large quantity of graphene-based sheets is based on Hummers method.8 The graphite oxide can be readily exfoliated into single layer graphene oxide (GO) sheets, and GO will be converted to reduced graphene oxide (rGO) through reduction. The reduction methods of GO are mainly categorized into chemical reduction and thermal reduction. Typical chemical reduction of GO often requires strong reductants, such as hydrazine9, sodium borohydride10 and HI11. Such reductants are hazardous or toxic. Other chemical reduction methods, such as photocatalyst reduction12, electrochemical reduction13,14 and solvothermal reduction15, were also demonstrated for the GO reduction. Nevertheless, they are not applicable for industrial scale production. Thermal reduction is a highly effective method to reduce GO, and the degree of reduction depends on temperature. When temperature is below 500 °C, GO is lightly reduced with a C/O ratio less than 7. The C/O ratio can be higher than 13 if temperature is above 750 °C.16 To avoid oxidation at high temperature, vacuum or protective inert gas such as Ar or N2 is required.17,18 The main issue for thermal reduction is the high cost incurred from the application of high temperature (~900 °C) and requirement of large quantities of inert gas. In addition, there is a serious safety issue for the thermal reduction of GO powder in large scale.19,20 Even a few milligrams of GO powders can cause micro-explosion during thermal


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annealing. The GO’s mass-specific enthalpy of decomposition associated with its thermal reduction is comparable with that of known explosives such as trinitrotoluene (TNT), or hazardous industrial chemicals such as cumene hydroperoxide and benzoyl peroxide.21–23 Moreover, chemical doping of foreign atoms is another concern for graphene to be successfully applied in electronics and energy devices. For instance, nitrogen-doped rGO (NrGO) exhibits excellent electrocatalytic activity for oxygen reduction reaction (ORR).24–26 Although the exact mechanism remains unclear, both theoretical calculations and experimental results suggest that the doped nitrogen atoms, especially pyridinic and graphitic N, may provide more active sites for ORR.26,27 Several methods were reported to simultaneously reduce and dope GO with nitrogen. One popular rout reported by Dai’s group is to thermal anneal GO in the presence of NH3 gas at ~900 °C28, and this method has been widely employed to produce Ndoped graphene or N-doped carbon composite materials in energy applications.29,30 However, high temperature thermal reduction of GO in the presence of hazardous NH3 gas will expose great risks in large scale industrial production. Another research by Sheng et al. showed that thermal annealing of physically mixed GO and melamine powder can successfully synthesize Ndoped graphene at 800°C under argon atmosphere protection24 , but safety issue and the high cost incurred from the requirement of inert gas protection and high temperature remains unsolved. Other than thermal annealing, various methods have also been proposed. For instance, hydrothermal reduction of GO in the presence of nitrogen containing chemicals, e.g. ammonia and ammonium carbonate, were reported to prepare NrGO.31,32 Kumar et al. demonstrated a plasma-assisted microwave treatment, which achieved simultaneous reduction and N-doping of GO in a gas mixture of H2 and NH3.33 Zhang et al. utilized N2+ ion sputtering in an ultra-high vacuum chamber to implant N atoms into GO at room temperature, and gradually reduced GO


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after about 2 hours of sputtering.34 Nevertheless, the above-mentioned methods have similar problems as rGO fabrication, such as high cost, low feasibility for large scale production, usage of toxic gas and risk of possible explosion. Novel and facile methods are urgently needed in order to achieve large scale commercialization of graphene. In this work, for the first time we report a facile, safe and scalable method to achieve simultaneous thermal reduction and nitrogen doping of graphene oxide (GO) in air at much lower reaction temperature, while upholding a high quality end product. In this method, GO is mixed with melamine (MA), and the conversion and nitrogen doping are completed by heating the mixtures to 430 oC in air environment. The NrGO produced by this method has high catalytic activity towards ORR.

2. Experimental 2.1. Materials. Highly concentrated GO dispersion in water (Graphene Supermarket, 5 mg/mL), rGO powder (Graphene Supermarket, high surface area) and melamine (SigmaAldrich) were used as received. Melem was synthesized according to the method reported by Bann and Miller.35 In brief, NH4SCN (100.0 g) was placed in a porcelain evaporating dish and heated at ca. 300 °C on a hot plate until boiling ceased and all volatile products were expelled. Melon was prepared by further polycondensation of melem at 450 °C until the white melem powder turned yellow. The GO:MA mixtures were prepared by the following process. 150 mg of melamine was first dispersed in 10 mL deionized (DI) water and then added into 30 ml concentrated GO dispersion (melamine to GO mass ratio is 1:1). The mixture was ultra-sonicated and stirred till the melamine powder was evenly mixed in the dispersion. The mixture was then freeze dried to obtain well mixed GO:MA powder. 100 mg of GO:MA mixture powder was put into a crucible


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and undergone thermal annealing at 430 °C in air for three hours. The final product denoted as NrGO430 was collected for further characterizations and analysis. A control experiment was conducted, in which 100 mg of GO powder without mixing with other chemicals had undergone the same thermal treatment. 2.2. Characterizations. The thermal gravimetric analyse (TGA) tests were carried out with a DuPont 2950 TGA. The samples were heated up to 600 °C at a heating rate of 20 °C/min under a constant air or nitrogen flow of 100 cc/min. The transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained with a JOEL JEM 3010F transmission electron microscope equipped with a field emission gun. The ultraviolet-visible (UV-Vis) absorption spectra were measured with a Shimadzu UV-1800 Spectrophotometer. X-ray diffraction (XRD) patterns were acquired using a Bruker D8 Advance XRD Instrument with Cu Kα radiation (λ=0.154 nm). The Raman spectra were obtained with a Horiba Scientific LabRAM HR Evolution system with a 514 nm Ar+ laser as the excitation source. Fourier transform infrared spectroscopy (FTIR) was recorded with an Agilent 660 FTIR spectrometer. The FTIR samples were prepared by dispersing the materials in KBr pellets. X-ray photoelectron spectra (XPS) were collected with an Axis Ultra DLD X-ray photoelectron spectrometer equipped with an Al Kα X-ray source (1486.6 eV). The CasaXPS program (2.3.14 version) was used to substrate Shirley background, compositional analysis and deconvolution of the XPS peaks. 2.3. Electrochemical tests. The active materials were dispersed in water through sonication with concentration of 2 mg/mL. 5 µL of the sample suspension were dropped on a 3 mm glass carbon rotation disk electrode (RDE), and cover with 3 µL of 0.05% Nafion solution when sample was half-dried. Electrochemical measurements were made in a three-electrode cell using an Autolab 302N potentiostat/galvanostat. Ag/AgCl (3 M KCl), Pt electrode, and the prepared


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glass carbon RDE were used as the reference, counter, and working electrodes respectively. All electrochemical experiments were performed in a solution of 0.1 M KOH aqueous solution. All cyclic voltammetry (CV) experiments were performed at a scan rate of 50 mV s-1. The linear sweep voltammograms (LSV) of the modified glass carbon electrode were recorded in O2saturated 0.1 M KOH with a scan rate of 10 mV s-1 at various rotating speeds from 500 to 2500 rpm.

3. Results and discussion 3.1. Thermal reduction and N-doping of GO in air. Melamine and GO with a mass ratio 1:1 were homogeneously mixed in DI water by ultrasonication. The mixture was freeze dried to obtain GO:MA powder. GO:MA powder was then heated to 430 °C in air for 3 h. The lightbrown mixture turned into black. The weight of the product was about 40 wt.% of the original mixture. The product is denoted as NrGO430. The presence of MA in GO is important to obtain the product. When GO powder had undergone the same thermal process, the pristine GO sample completely burned out with no residue left. In order to understand the observations, TGA tests were carried out on several samples (Figure 1). The presence of MA in GO remarkably changes the thermal process of GO. With the absence of MA, pure GO samples exhibit thermal behaviours depending on the GO loading. When the GO mass was 5 mg, a sharp weight loss to 0 wt.% occurred at 196 °C (Figure 1a). This drastic weight loss can be attributed to a micro explosion that was also observed by other labs at a large GO mass.19,20 GO retards heat and mass transfer, thus causing the rise in the local temperature and built up of the internal pressure.20 This is consistent with our observation of the complete burnt off of the pristine GO in the crucible. When a GO sample of 0.4 mg was investigated, the sample exhibited two weight losses at onset


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temperature 208 °C and 493 °C, respectively (Figure 1e). The residue at 550 °C was less than 10 wt.%. The thermal behaviour becomes quite different with the presence of MA (Figure 1d). The micro-explosion did not happen even at a large sample mass (3.48 mg). The TGA curve indicates weight losses at two stages for GO:MA in air. The initial weight loss at onset temperature 173 °C is originated from the decomposition of GO, but the onset temperature is 35°C lower than pure GO (0.4 mg). The other weight loss of about 25 wt.% takes place at 280 °C. By comparing with the TGA curve of melamine, the second weight loss can be ascribed to the sublimation and deamination of melamine in a non-closed system36, though the onset temperature is 25°C lower than the pure melamine. There is no remarkable weight loss from 300 to 600 °C. The decomposition of GO at 493 °C was not observed for GO:MA. The weight loss at 430 °C is around 60 wt.%, which is consistent with the thermal heating of GO:MA in a crucible. It is also interesting to note that this weight loss of GO:MA mixture is indifferent to the atmosphere. The TGA result of GO:MA in nitrogen is almost the same as in air. In addition, both TGA products of GO:MA mixture under nitrogen and air atmosphere were examined by XPS (Figure S1). The XPS spectra suggest almost the same compositions for the products in air or in nitrogen. This suggests that melamine can effectively prevent the oxidation of GO in air. The product obtained by heating GO:MA at 430 oC for 3 h has good thermal stability. There is no remarkable weight loss from room temperature up to 543 oC. The TGA curve shows the identical onset decomposition temperature as to that of the rGO powder supplied by Graphene Supermarket. This suggests that the former may be rGO, though the melamine and its derivative residues cannot be excluded from the final product at this stage.


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Figure 1. TGA curves of (a) 5 mg GO in air, (b) melamine in air, (c) GO:MA in nitrogen, (d) GO:MA in air, (e) 0.4 mg GO in air,(f) rGO in air and (g) NrGO430 in air.

(a)

(b)

Figure 2. (a) TEM and (b) HRTEM images of NrGO430. The product, NrGO430, was characterized by TEM (Figure 2). Sheets of micrometer size were observed, and the electron diffraction pattern revealed that they had several layers. The lattice structure appears on the HRTEM image (Figure 2b). The lattice spacing is 0.34 nm, which is


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consistent with the lattice spacing of graphene in literatures.37 The HRTEM image also indicates crystallites of a few nanometers. The product was further characterized by UV-Vis absorption spectroscopy, XRD, Raman spectroscopy and FTIR (Figure 3). As shown in Figure 3a, GO exhibits a strong absorption peak at 230 nm and a shoulder at 305 nm. They are assigned to the π-π* and n-π* transition, respectively.38 The π-π* transition band shifts to 271 nm for NrGO430, and the absorbance in the whole visible range increased significantly. This indicated the electronic conjugation is restored after the removal of oxygen groups in the graphene sheets upon thermal reduction in air.39

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NrGO430

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Figure 3. (a) UV-Vis absorption spectra of GO and NrGO430. (b) XRD spectra of GO, melamine, melon, melem and GO:MA and NrGO430. (c) Raman spectra of pristine GO and NrGO430. (d) FTIR spectra of GO, rGO and NrGO430.

The XRD patterns of pristine GO and melamine powder are consistent with that reported by other groups (Figure 3b).40,41 The XRD patterns of GO:MA is just a simple summation of that of GO and melamine. The broadening of the XRD pattern below 15o can be attributed to the separation of the GO sheets by melamine. This also evidences the good mixture of GO with melamine. After heating at 430 °C in air for 3 hours, the XRD undergoes significant changes. Only one broad pattern appears at 2θ = 26°. This corresponds to a d-spacing of 0.34 nm originating from the (002) plane of graphene, which is also consistent to the HRTEM result.42,43 The average crystallite size is estimated to be 2.4 nm by analysing the width of this band with the Scherrer equation (Support information Equation (1)).44 Melamine would produce melem or melon under thermal annealing at 400-500°C.45 The XRD spectra of melem and melon had been


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demonstrated in Figure 3b. None of the XRD patterns of melem or melon appeared in NrGO430. Thus, there may be little or no melamine residue in NrGO430. Figure 3c presents the Raman spectra of GO and NrGO430. Two characteristic bands, D and G bands, were observed. The D band corresponds to the breathing modes of six-atom rings at the presence of defects, whereas the G band is the E2g phonon at the Brillouin zone centre.46,47 The integrated intensity ratio (ID/IG) is 0.97 for GO. It increases to 1.25 for NrGO430. The ID/IG ratio usually increases after the GO reduction.48–50 The increase in the ID/IG ratio is indicative of the sp2 domains produced by heating. Tuinstra and Koenig found that the ID/IG ratio varies inversely with the crystal size (L), ID/IG = C(λ)/L with the coefficient C (514 nm) is about 4.4 nm.46,47 The average sp2 domain size of NrGO430 was estimated in terms of the ID/IG ratio. It is 3.5 nm, agreeing well with the size estimated based on HRTEM image (a few nanometers) and XRD result (2.4 nm). There are many oxygen-containing groups existing on GO sheets, such as hydroxyl, epoxy and carboxyl groups. Most of the oxygen-containing groups will be removed after reduction. The chemical structures of GO, rGO and NrGO430 samples were characterized by FTIR (Figure 3d). FTIR bands at 1050, 1220, 1405, 1627 and 1730 cm-1 were observed for GO. These bands correspond to C-O stretching, C-O-C/C-OH stretching, O-H deformation vibration, skeletal vibrations, and C=O carbonyl stretching.37 Only two FTIR bands at 1250 cm-1 and 1570 cm-1 were observed for NrGO430, and the C=O band at 1730 cm-1 completely disappeared. The bands located at 1180 cm-1 and 1565 cm-1 are assigned to the stretching vibrations of C-N and the stretching vibration of C=C, respectively.51 The FTIR spectra suggest the simultaneous reduction and N-doping of NrGO430. Presumably, the nitrogen element originates from melamine.


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The reduction and nitrogen doping were also studied by X-ray photoelectron spectroscopy (XPS) (Figure 4). The survey spectrum showed that the oxygen content in GO:MA mixture before thermal treatment was 20.7 at.% (Figure S2a), whereas it decreased to 5.52 at.% after thermal annealing (Figure 4a). The C/O ratio of NrGO430 is 14.96, which is higher than that of rGO prepared by chemical reduction of GO with hydrazine (10.3:1), NaBH4 (8.6:1), HI acid (12:1) and most of thermal reduction of GO under vacuum, inert gas or inert/reducing mixed gas protection (10:1~14:1).10,11,17,18,39,52,53 As seen from C1s spectrum (Figure S2b), the GO used in GO:MA mixture was highly oxidized. The XPS band at 286.5 eV is attributed by -C-N/-C-O bond, and the band at 287.8 eV is due to the -C=O or amide bond.54,55 The mixing process allowed the formation of amide linkage between the carboxyl group on GO and the amine group on melamine. However, after 3 hours of thermal annealing at 430°C in air, the -C=O/amide band at 287.8 eV completely disappeared and the -C-N/-C-O band at 286.5 eV was significantly decreased (Figure 4b). Furthermore, the nitrogen content also decreased from 18.29 at.% (Figure S2a) to 11.87 at.% (Figure 4a) after thermal annealing. The reduction in nitrogen content might be attributed to the sublimation and decomposition of melamine and release of ammonia gas during the thermal treatment. The existence of both unreacted melamine and melamine derivatives had been excluded from the final product as earlier discussed, thus the remaining nitrogen was doped in rGO. After the thermal treatment, three new XPS bands appear at 399.3, 401.1 and 403.0 eV (Figure S2c and Figure 4c). These bands correspond to the pyrrolic nitrogen, graphitic nitrogen and nitrogen oxides, respectively.24


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(a)

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399.3eV

Graphitic N (24.5%) 402.5eV

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Figure 4. XPS (a) survey, (b) C1s and (c) N1s spectra of NrGO430. The effect of melamine on the thermal treatment of GO can be attributed to the decomposition of melamine. Melamine and its derivatives are well-known flame retardants.56 In the semi-closed system, both sublimation and decomposition of melamine can occur.36 The melamine vapor and ammonia gas released from thermal decomposition of melamine protects GO from air and provides nitrogen atoms to dope the final product. The release of ammonia was proved by testing the pH of the gas inside the crucible containing GO:MA (1:1) mixture at 430


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°C. The pH of melamine solution was 6, yet the pH of the gas inside the crucible was 8-9 as indicated by a pH test paper. Although the exact mechanism of nitrogen doping during thermal reduction is still not clear, we believe that upon thermal reduction the removal of oxygen containing groups on graphene sheets provides active sites to allocate nitrogen species originated from the decomposition of melamine. Sheng and et al. also suggested a similar mechanism for nitrogen doping.24 Melamine derivatives including melem and melon were also used to replace melamine in the thermal treatment of GO in air. In terms of the XPS spectra (Figure S4 and S5) and FTIR spectra (Figure S6), melem can assist the reduction of GO similar to melamine, whereas melon does not work well. The deamination temperatures of melamine (~305 °C) and melem (~450 °C) are lower than the reduction temperature of GO. GO can thus be reduced in the atmosphere of ammonia. However, the deamination temperature of thermally stable melon (above 500 °C) is much higher than the reduction temperature of GO.57 3.2. NrGO as electrocatalyst for oxygen reduction reaction. The electrocatalysis of NrGO430 towards ORR was studied (Figure 5a). Control experiments were also conducted on N-doped rGO samples (NrGO900) prepared by thermal reduction and nitrogen doping of GO at 900 °C in inert gas.24 The ORR onset potential for NrGO430 is remarkably more positive than that of rGO without nitrogen doping. It is more positive or comparable to that of most of metal-free catalysts reported in literature.25–27,58–61 The ORR current density with NrGO430 is even higher than that with NrGO900. The linear sweep voltammograms (LSVs) of NrGO430 from 500 to 2500 rpm are presented in Figure 5b, and the Koutecky-Levich (K-L) plots at different potentials are illustrated in Figure 5c. The K-L plots indicate the first-order reaction kinetics of the ORR. Calculated with the Levich equation (Support information Equation (3)), the electron transfer


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number (n) is about 3.8 at the range from -0.7 to -0.9V (Figure S7), indicating a 4 electron transfer pathway.

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Figure 5. (a) RDE LSV of ORR on rGO powder, NrGO900, and NrGO430 at a rotating speed of 2500 rpm and scan rate of 10 mV s-1. (b) RDE LSV of ORR on NrGO430 electrode at different rotating speeds in O2 saturated 0.1 M KOH solution with a scan rate of 10 mV s-1. (c) KouteckyLevich plots of NrGO430 derived from RDE LSV at different potentials.


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4. Conclusion A safe and facile method is developed to thermally reduce GO and dope the product with nitrogen in air. GO is reduced to nitrogen doped rGO by mixing it with melamine and heating at 430 oC in air. The reduction and N doping of the product are confirmed by UV-Vis absorption spectroscopy, XPS, Raman spectroscopy and FTIR spectroscopy. NrGO430 prepared by this method has a high C/O ratio of 15 and a nitrogen content of 11.87 at.%. It demonstrates high catalytic activity towards ORR with a 4 electron transfer pathway. Therefore, without the need of inert gas protection and high reaction temperature, our method provides a low-cost and practical way to produce NrGO in large scale, and it will catalyse the commercial application of graphene.

ASSOCIATED CONTENT Supporting Information Supporting Information has been provided on the following topics: calculation of the average grain size and electron transfer number towards ORR; XPS spectra of GO:MA, GO:Melem and GO: Melon under thermal treatment; FTIR spectra of rGO:Melem and rGO:Melon; electrochemical test of NrGO430. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by a research grant from the Ministry of Education, Singapore (R-284-000-113-112).

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Table of Contents By mixing graphene oxide with melamine, simultaneous thermal reduction and nitrogen doping of graphene oxide can be achieved in air atmosphere at 430 °C. The method is facile, safe, low cost and suitable for large scale production.

430°C in air NrGO

GO:MA mixture Melamin e

C O N H


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Nitrogen-Doped Reduced Graphene Oxide Prepared by Simultaneous

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