Low-Temperature Synthesis of Mesoporous Cobalt(II) Carbide Using

A simple and efficient synthesis of Co2C using graphene oxide (GO) as a carbon ..... Anderson , R. B.; Hall , W. K.; Krieg , A.; Seligman , B. Fischer...
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Low-Temperature Synthesis of Mesoporous Cobalt(II) Carbide Using Graphene Oxide as a Carbon Source Panitat Hasin* Department of Chemistry, Faculty of Science, Kasetsart University, Chatuchak, Bangkok 10900, Thailand S Supporting Information *

ABSTRACT: A simple and efficient synthesis of Co2C using graphene oxide (GO) as a carbon source has been established. The procedure consists of two steps: (1) formation of a GO/ Co3O4 nanocomposite via the ammonia-evaporation-induced method and (2) conversion of Co3O4 to Co2C under a H2/N2 mixture at a low temperature (200 °C). Transmission electron microscopy (TEM) analysis showed that Co2C has a crystallite size of around 5 nm and a mesoporous structure with a pore size of ca. 3−5 nm. The amphiphilic behavior of GO contributes to the high porosity, large specific surface area, and narrow pore size distribution of the Co2C. Tungsten carbide has also been successfully obtained using GO as a carbon source at a much lower temperature than that of the traditional carbothermal synthesis. Therefore, this method could be extended to the production of other important carbides with desired mesoporous features at low temperatures.

1. INTRODUCTION Metal carbides represent a large family of compounds with important industrial applications. For example, cobalt(II) carbide (Co2C) has been studied as a permanent magnet1 and heterogeneous catalyst2 because of its high coercivity1 and high surface area3 combined with high activity,4 respectively. Tungsten carbide (WC) has excited much interest recently as a non-noble anode electrocatalyst for acidic low-temperature fuel cells1,5,6 due to its Pt-like surface properties7−9 and ability to withstand high temperatures1 as well as to resist against CO poisoning.10 Moreover, WC has also demonstrated exceptionally high hardness11 and toughness.12 A direct combination of metals or metal oxides with carbon is a common method for the synthesis of metal carbides, and this is called carbothermal reduction.13 Graphite or synthetic pitch is commonly used as a carbon source.14,15 For this type of solid state reaction, the mass transport to the reaction interface is the rate-limiting step. A high reaction temperature (typically >1000 °C) is therefore necessary to facilitate the diffusion of ions/ atoms through the solid phases. In this paper, we report the use of graphene oxide (GO) as a carbon precursor for the synthesis of Co2C. In comparison with other carbon sources, GO contains merely a few layers of carbon,16 and thus it could facilitate the diffusion of carbon atoms.17 Another advantage is that the oxygen-containing functional groups (−COOH, −OH, and −CO groups)18−20 in GO can facilitate direct nucleation of metal oxide nanocrystals on its surface, resulting in an intimate contact between the two phases.21−23 These properties coupled with high monodispersity24 make GO an attractive choice as a carbon source to synthesize Co2C. © 2014 American Chemical Society

In this study, we have put forward a simple ammoniaevaporation-induced method for the synthesis of the GO/ Co3O4 nanocomposite, which is then converted to Co2C via a thermal reduction in the H2/N2 atmosphere. Compared to the traditional high-temperature synthesis of metal carbides, our novel use of GO as a carbon source provides the lowest reported temperature (200 °C) for a metal carbide synthesis so far. The end product is mesoporous Co2C with a large surface area. We also show that the product exhibits superparamagnetic behavior.

2. EXPERIMENTAL SECTION 2.1. Preparation of GO. All materials were obtained from commercial sources. GO was prepared by a modified Hummers method.18,25 In a typical synthesis procedure, 621 g of concentrated H2SO4 (97.2% pure, Fisher Scientific) was added into 10 g of graphite flakes (SP-1, 99% pure, Bay Carbon, Inc.) and 7.5 g of NaNO3 (99% pure, Mallinckrodt Chemicals), and then 45 g of KMnO4 (99% pure, SigmaAldrich) was added gradually. The mixture was stirred continually for an hour in an ice water bath to keep the temperature at approximately 20 °C. It was then allowed to stand for five days with slow stirring to acquire a highly viscous liquid. Then, 5 wt % aqueous solution of H2SO4 (1000 mL) was added, initially in 20−30 mL aliquots, into the above mixture over approximately 1 h with stirring. As the temperature of the resultant mixture increases rapidly during Received: December 3, 2013 Revised: February 17, 2014 Published: February 17, 2014 4726

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Figure 1. Scanning electron micrographs (with different resolutions) of GO/Co3O4 before thermal reduction. The white spots are Co3O4 NPs.

423 K to 20 μTorr for 12 h. Its specific surface area was calculated by the BET method, and pore size was calculated using the Barrett, Joyner, and Halenda (BJH) method on the basis of the adsorption branch of the nitrogen sorption isotherms. X-ray photoelectron spectroscopy (XPS) analysis (Kratos Axis Ultra XPS) was carried out to study the surface composition. Photoelectrons were excited using an Al Kα source with photon energy of 1486.6 eV. The vacuum in the analysis chamber was maintained at 10−9 Torr. Kratos Vision 2 software was used to perform curve fitting. The C 1s peak was taken as the reference for XPS. The magnetization curve was obtained at room temperature in applied magnetic fields between ±20 kOe fields with a Quantum Design superconducting quantum interference device (SQUID) magnetometer. To determine the coercivity, the sample was first cooled in zero-applied magnetic field from room temperature to 35 K, and then the hysteresis loop was measured in the presence of a magnetic field.

the addition of the H2SO4 solution, this addition was performed in an ice bath so that the temperature would not exceed 50 °C. After this step, 30 wt % H2O2 (30% pure, Fisher Scientific) was added into the solution slowly while stirring until the color of the suspension turned a brilliant yellow, indicating full oxidation of graphite. The resultant mixture was purified by centrifugation with the mixture of 0.5 wt % H2O2 and 3 wt % H2SO4 (4.6 L) followed by discarding the supernatant liquid. This washing cycle was repeated 15 times. 2.2. Preparation of the GO/Co3O4 Nanocomposite. In a typical synthesis to prepare the GO/Co3 O4 hybrid nanostructure by the ammonia-evaporation-induced method,26,27 20 mg of GO, 0.25 mmol of Co(NO3)2·6H2O (98.2% pure, Fisher Scientific), and 0.125 mmol of NH4NO3 (99.5% pure, Sigma-Aldrich) were first dispersed in 30 mL of nanopure water under magnetic stirring. An amount of 25 mL of concentrated ammonia aqueous solution (29% pure, Fisher Scientific) was slowly added, and the solution was continuously stirred for 10 min before it was transferred to a covered Petri dish. Then the Petri dish was kept in an oven at 80 °C for 14 h to allow for the evaporation of ammonia. Nanostructured GO/ Co3O4 formed in the solution as precipitate. The precipitate was washed with deionized water five times followed by ethanol and dried in air at 80 °C in an oven overnight. 2.3. Preparation of Co2C. The thermal reduction of the GO/Co3O4 nanocomposite was performed by exposing it to 5% of the H2/N2 gas mixture. After the furnace tube was flushed by the gas mixture for 1 h, the temperature was increased to 200 °C (heating rate: 15 °C/min), and the furnace was maintained at this temperature for 5 h with H2/N2 flow at 110 cc/min. 2.4. Material Characterization. X-ray diffraction patterns were recorded on a Rigaku (Japan) X-ray diffractometer equipped with graphite-monochromatized Cu Kα1 radiation, operated at 40 kV and 25 mA. The crystallite size was calculated by the Scherrer formula, eq 128 L = Kλ /(β cos θ )

3. RESULTS AND DISCUSSION 3.1. Physical and Structural Characterization. After the GO/Co(NO3)2 aqueous solution was heated at 80 °C for 14 h, the representative FESEM images of the composite (Figure 1) were recorded, where Co3O4 NPs were observed. The oxygenated functional groups (epoxide, hydroxyl, and carboxylic groups) present on the surface of GO sheets19,29 are negatively charged, so the surface of GO sheets can attract the positive cobalt ions through electrostatic attraction.30 In other words, the oxygenated functional groups attached to the carbon sheets can provide nucleation sites31 for the Co3O4 NPs deposited on GO to grow. The intimacy between GO and Co3O4 makes GO as an ideal carbon source to synthesize Co2C (Figure 1). The XRD pattern of the obtained powder (Figure 2a) before annealing was perfectly indexed to the cubic spinel Co3O4 (JCPDS No. 43-1003), whereas the XRD pattern of the annealed GO/Co3O4 composite (Figure 2(b)) indicates the presence of the Co2C phase only (JCPDS No. 65-1457). The broad XRD peaks in Figure 1b could be due to the small crystallite size of Co2C which was estimated to be 5.5 nm based upon the Scherrer equation by using the fwhm of the Co2C(111) reflection. For comparison, conventional carbothermal reduction of Co3O4 by graphite was carried out.13 Amounts of 1.67 mmol of graphite and 0.083 mmol of Co3O4 nanopowder in isopropanol were milled for 90 min using two hardened stainless steel balls. The milling was carried out under high-energy conditions using a specially designed ball mill allowing the magnet to control the ball movement. Then, the powder was

(1)

where L is the crystallite size, λ is the wavelength of the X-ray radiation (Cu Kα1 = 0.15418 nm), K is taken as 0.89, and β is the full width at half-maximum height (fwhm). The scanning electron microscope (SEM) images of the products were taken by a field emission SEM (FEI Sirion, 15 kV). Elemental analysis was carried out by Galbraith Laboratories, Inc. The transmission electron microscope (TEM) images and high-resolution transmission electron microscope (HRTEM) images were recorded on a Tecnai F20 operated at 200 kV. The Brunauer, Emmett, and Teller (BET) nitrogen sorption surface area measurement was carried out at 77 K on Micromeritics ASAP 2010 after the sample was degassed at 4727

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pyrolysis at 1500 °C. Such high-temperature results in the restricting the use of Co2C due to the difficulty in its production. Therefore, using GO as a carbon source is a more efficient method to synthesize Co2C from Co3O4 as compared to a conventional carbothermal reduction method. The FESEM images (Figure 4) of Co2C obtained by annealing GO/Co3O4 at 200 °C for 5 h under H2/N2 show that the particles are around 200 nm in size. Graphene sheets seem to wrap/cover the Co2C particles preventing the agglomeration/aggregation of particles. Compared to 200 nm for metal carbide prepared with our method, the synthesized metal carbide powders prepared with the conventional carbothermal reduction method typically have a spherical morphology with an average particle size of about 1−50 μm.13 The microstructure analysis shows that these spherical particles consist of loosely aggregated fine particles with a submicrometer size leading to obtaining the carbide materials with undesired properties. Elemental analysis of the pyrolyzed product shows a content of Co, 55.2%; C, 28.3%; O, 15.4%; and H, 1.1%. Considering that pure Co2C contains 9.2% carbon by mass, the actual carbon associated with Co2C in the sample is estimated to be 5.6%. This suggests that some carbon of graphene remains in the sample, which is in agreement with the FESEM images (Figure 4) where graphene sheets were observed on the surface of Co2C particles. TEM images (Figure 5) indicate that Co2C has a good mesoporous structure with many small and disordered pores of 3−5 nm (Figure 5(a)), and this can be confirmed by the pore size distribution from BET measurement. To further investigate the microstructure of Co2C, the HRTEM image (Figure 5(b)) has been taken. Mesopores of Co2C were interconnected closely forming a random network, and the shapes of the pores were irregular. The average crystallite size was found to be 5−6 nm, which is in good agreement with the value obtained from the Scherrer equation. Li et al. have reported an ammonia-evaporation-induced method to synthesize Co3O4 nanowire arrays.26,27 Surprisingly, introducing GO into the Co(NO3)2 aqueous solution and heating under the same reaction condition, we observed the mesoporous GO/Co3O4 nanocomposite. Xu et al. observed that Co3O4 NPs randomly decorated on exfoliated GO30 when Co3O4 was deposited in situ onto the surface of GO nanosheets under ultrasonication. Thus, a mechanism for the formation of nanoporous GO/Co3O4 composite could be elaborated as follows: GO acts as an amphiphilic surfactant32 as it comprises pristine graphene sheets which are intrinsically hydrophobic, decorated by oxygenated functional groups which are intrinsically hydrophilic. During the ammonia evaporation, GO aggregates into micelles which isolate cobalt species effectively.

Figure 2. XRD patterns of obtained powder (a) before annealing and (b) after annealing at 200 °C for 5 h under a H2/N2 atmosphere.

pressed into a pellet and pyrolyzed in an alumina boat placed into a tube furnace at 200 °C under 5% H2/N2 for 5 h. The XRD pattern (Figure 3) of the heat-treated sample showed the

Figure 3. XRD pattern of the mixture of Co3O4 and graphite after annealing at 200 °C for 5 h under H2/N2 atmosphere; * = Graphite.

mixture of the starting materials (Co3O4 and graphite) without any diffraction peaks of Co2C. Increasing the pyrolyzing temperature to 1000 °C, a XRD pattern similar to Figure 3 was obtained (not shown here). According to the literature,13 the conversion from Co3O4 to Co2C was observed after the

Figure 4. Scanning electron micrographs (with different resolution) of Co2C obtained by annealing GO/Co3O4 at 200 °C for 5 h under H2/N2. 4728

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Figure 5. (a) TEM and (b) HRTEM images of Co2C obtained by annealing GO/Co3O4 at 200 °C for 5 h under H2/N2.

This enables the rapid production of the nanoporous GO/ Co3O4 composite. The nitrogen adsorption−desorption isotherms (Figure 6) indicate that Co2C was mesoporous (type IV adsorption

Figure 7. XRD patterns of the obtained powder (a) before annealing and after annealing under H2/N2 atmosphere at 200 °C for (b) 3 h and (c) 5 h; * = Co2C.

This new synthesis for Co2C has some apparent advantages: (1) the starting materials are cheap; (2) the required temperature is significantly low; (3) the procedure is facile and simple; and (4) the end product has a high surface area. Figure 8 shows the deconvoluted C(1s) XPS spectra of the as-prepared sample and that after the reduction process. The deconvoluted peak centered at the binding energy of 284.8 eV was attributed to the C−C, CC, and C−H bonds.33−35 The binding energies of 286.5 and 287.5 eV were assigned to the C−OH and CO functional groups, respectively.33−35 To trace the change in the concentration of the oxygenated functional groups, the peak area ratios of the C−C, CC, and C−H bonds to C−OH and CO bonds were examined. Before the reduction process (Figure 8(a)), considerable contributions of the oxygen-containing functional groups were observed (C−C/C−O ∼ 1.3). After thermal reduction, the relative concentrations of the oxygen-containing bonds showed about 41% reduction (C−C/C−O ∼ 2.2) (Figure 8b). This indicates that during the reduction under H2/N2, the oxygenated functional groups of GO were removed, and graphene formed. However, the disappearance of the C 1s XPS peak for the carbide in Co2C was observed, and this could be due to the appearance of more than 10 nm thick graphene sheets covering the surface of Co2C particles which can be confirmed by FESEM images (Figure 4). At such thickness, the

Figure 6. Typical nitrogen adsorption−desorption isotherms at 77 K and the derived PSD (the inset) for the Co2C.

isotherm) with a narrow pore size distribution (PSD). The pore size was estimated to be ca. 4.0 nm from the PSD maximum, which is in agreement with the pore size obtained from the HRTEM image (Figure 5(b)). The measured specific surface area was 68 m2/g, and its large surface area could be attributed to the formation of gaseous products such as CO, CO2, and H2O from pyrolyzing oxygenated functional groups on the GO surface. On the contrary, the metal carbides prepared with the conventional carbothermal reduction method normally present low surface areas of around 1 m2/g because of their high temperature synthesis.13 3.2. Study of the Mechanism of Cobalt(II) Carbide Formation. To investigate the process of Co2C formation, a series of XRD patterns have been obtained. The XRD pattern of the sample annealed for 3 h (Figure 7(b)) shows broad reflections that are indicative of the existence of both Co3O4 and Co2C. A similar mixture was observed after 4 h (XRD pattern not shown here). After 5 h, the intensity of the detectable peaks of the Co2C increased, and the Co3O4 peaks totally disappeared (Figure 7(c)). 4729

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with Co3O4 and form Co2C. Furthermore, the H2 can also reduce Co3+ in Co3O4 to Co2+ to form Co2C. A scheme (Figure 9) has been proposed to explain the formation of mesoporous Co2C. First, positively charged cobalt ions are attracted to the surface of the GO sheet due to the electrostatic attraction from the negatively charged functional groups (epoxide, hydroxyl, and carboxylic groups) present on the surface of the GO sheet.19,29,30 This intimate contact facilitates the nucleation of metal oxide during the ammonia evaporation at 80 °C, and the resultant Co3O4 NPs are more thoroughly wrapped by GO sheets. At a low annealing temperature (200 °C) under a reducing atmosphere, graphene is formed and simultaneously reacts with Co3O4 to form Co2C. The single carbon atomic layer is believed to facilitate the diffusion of carbon atoms, resulting in such a low synthesis temperature. The successful synthesis of Co2C using GO as a carbon source suggests that this novel method can be extended to other carbides. Tungsten carbide was prepared by a sol−gel process, using a colloidal solution of tungstic chloride stabilized by poly(alkylene oxide) triblock copolymer P123 (EO20PO70EO20) and GO (synthesis is detailed in Supporting Information). After the resulting solution was dried in an open Petri dish at 60 °C in air, the obtained powder was annealed at 850 °C for 5 h under H2/N2 atmosphere. The XRD pattern (Figure S1 in the Supporting Information) shows the mixture of W2C and WC with a small amount of W metal. This is in stark contrast with the traditional synthesis where tungsten oxide was reduced by carbon at temperatures as high as 1400 °C.36 Thus, using GO as a carbon source to synthesize carbides could reduce the cost significantly. It is envisaged that this method can be applicable to other carbide systems. 3.3. Magnetic Property. The magnetization vs field measurements of the mesoporous Co2C were performed using SQUID operating at ambient temperature, with the field sweeping from −20 to 20 kOe (Figure 10). The mesoporous Co2C behaved as a weak ferromagnet displaying a small coercivity of 300 Oe and remanent magnetization of 0.034 emu/g. Moreover, magnetic behavior characteristic of the mesoporous Co2C was also close to a superparamagnetic material with no saturation magnetization which is a phenomenon that has been ascribed to the presence of small magnetic particles37 in the Co2C sample (∼5−6 nm, see Figure

Figure 8. XPS analysis of (a) GO/Co3 O4 and (b) Co2 C. Deconvolution reveals the presence of C−C (∼284.8 eV), C−O (∼286.5 eV), and CO (∼287.5 eV) species.

XPS cannot detect the carbon of Co2C which is deep beneath the graphene sheet. Graphene acts as a carbon source, reacting with Co3O4 to form Co2C. Overall, the reaction process is based on the reduction of GO and carburization/reduction of Co3O4 according to the following scheme GO/Co3O4 → G/Co3O4 → Co2C

(2)

The CO, CO2, and H2O generated from the reduction/ decomposition of GO escape from the sample and leave cavities in it. This is responsible for the mesoporosity obtained during the heat treatment step as shown in Figure 5. In addition, it was found that the formation of Co2C depends not only on the duration of annealing but also on the atmosphere. When the heat treatment of GO/Co3O4 was carried out at 200 °C under Ar atmosphere, the dominant phase of the sample is Co3O4 only with no observation of Co2C diffraction peaks in XRD (not shown here). To form Co2C under Ar atmosphere, a higher annealing temperature (450 °C) was required. Therefore, H2 can act as an effective reducing agent for GO reduction providing a higher amount of restored C−C bonds to react

Figure 9. Scheme showing a proposed formation route to prepare Co2C. 4730

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Networks”, Thailand, is gratefully acknowledged. In addition, P.H. gratefully acknowledges Dr. Yiying Wu (Department of Chemistry and Biochemistry, The Ohio State University) for helpful discussions.

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(1) Harris, V. G.; Chen, Y.; Yang, A.; Yoon, S.; Chen, Z.; Geiler, A. L.; Gao, J.; Chinnasamy, C. N.; Lewis, L. H.; Vittoria, C.; Carpenter, E. E.; Carroll, K. J.; Goswami, R.; Willard, M. A.; Kurihara, L.; Gjoka, M.; Kalogirou, O. High coercivity cobalt carbide nanoparticles processed via polyol reaction: a new permanent magnet material. J. Phys. D: Appl. Phys. 2010, 43, 16. (2) Xiao, T.; Qian, Y. Promoted carbide-based Fischer−Tropsch catalyst, method for its preparation and uses thereof. 2008-GB703 2008104793, 20080229, 2008. (3) Anderson, R. B.; Hall, W. K.; Krieg, A.; Seligman, B. Fischer− Tropsch synthesis. V. Activities and surface areas of reduced and carburized cobalt catalysts. J. Am. Chem. Soc. 1949, 71, 183−8. (4) Ding, Y.; Pei, Y.; Zhu, H.; Song, X.; Zang, J.; Dong, W.; Lu, Y. Role of cobalt carbide in high alcohols (C1-C18) synthesis from syngas over activated carbon supported cobalt catalysts. AIChE Spring Meet. 6th Global Congr. Process Saf., Conf. Proc., ding1/1-ding1/2. (5) Rees, E. J.; Essaki, K.; Brady, C. D. A.; Burstein, G. T. Hydrogen electrocatalysts from microwave-synthesized nanoparticulate carbides. J. Power Sources 2009, 188 (1), 75−81. (6) Brady, C. D. A.; Rees, E. J.; Burstein, G. T. Electrocatalysis by nanocrystalline tungsten carbides and the effects of codeposited silver. J. Power Sources 2008, 179 (1), 17−26. (7) Levy, R. B.; Boudart, M. Platinum-like behavior of tungsten carbide in surface catalysis. Science 1973, 181 (4099), 547−9. (8) Hwu, H. H.; Chen, J. G.; Kourtakis, K.; Lavin, J. G. Potential Application of Tungsten Carbides as Electrocatalysts. 1. Decomposition of Methanol over Carbide-Modified W(111). J. Phys. Chem. B 2001, 105 (41), 10037−10044. (9) Zellner, M. B.; Chen, J. G. Surface science and electrochemical studies of stability of WC and W2C PVD films as potential electrocatalysts. Catal. Today 2005, 99 (3−4), 299−307. (10) Hwu, H. H.; Chen, J. G. Surface Chemistry of Transition Metal Carbides. Chem. Rev. (Washington, DC, U. S.) 2005, 105 (1), 185−212. (11) Rolison, D. R. Catalytic nanoarchitectures - The importance of nothing and the unimportance of periodicity. Science 2003, 299 (5613), 1698−1701. (12) Roebuck, B.; Gant, A. J.; Gee, M. G. Abrasion and toughness property maps for WC/Co hardmetals. Powder Metall. 2007, 50 (2), 111−114. (13) Eick, B. M.; Youngblood, J. P. Carbothermal reduction of metaloxide powders by synthetic pitch to carbide and nitride ceramics. J. Mater. Sci. 2009, 44 (5), 1159−1171. (14) Berger, L. M.; Gruner, W.; Langholf, E.; Stolle, S. On the mechanism of carbothermal reduction processes of TiO2 and ZrO2. Int. J. Refract. Met. Hard Mater. 1999, 17 (1−3), 235−243. (15) Lee, J.; Kim, J.; Hyeon, T. Recent progress in the synthesis of porous carbon materials. Adv. Mater. 2006, 18 (16), 2073−2094. (16) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Single sheet functionalized graphene by oxidation and thermal expansion of graphite. Chem. Mater. 2007, 19 (18), 4396−4404. (17) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films. Los Alamos Natl. Lab., Prepr. Arch., Condens. Matter 2004, 1−5 arXiv:cond-mat/0410550. (18) Hummers, W. S., Jr.; Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339. (19) Lerf, A.; He, H. Y.; Forster, M.; Klinowski, J. Structure of graphite oxide revisited. J. Phys. Chem. B 1998, 102 (23), 4477−4482.

Figure 10. Room-temperature hysteresis loop of Co2C.

5). This phenomenon is not observed in the metal carbides prepared with the conventional carbothermal reduction due to their large crystallite sizes.1 The superparamagnetic property found in Co2C prepared with our method makes it important for weak magnetic applications such as data storage.38,39

4. CONCLUSION Cobalt(II) carbide (Co2C) with high specific surface area of 68 m2/g can be prepared by the ammonia-evaporation-induced method between the cobalt(II) salt and graphene oxide (GO) followed by thermal reduction under H2−N2 mixture at low annealing temperature (200 °C). Using GO as a carbon source, the transformation of Co3O4 to Co2C occurs in two steps, the first being a GO reduction leading to graphene formation and the second carburization/ reduction of Co3O4 to Co2C. TEM study indicates that the obtained Co2C has a crystallite size of around 5 nm and possesses disordered mesoporous structure with diameter of ca. 3−5 nm. The possibility of using GO to produce other carbides at low temperatures has been tested, and tungsten carbide was successfully synthesized using GO as a carbon source at a much lower temperature compared with the traditional carbothermal reduction.

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ASSOCIATED CONTENT

* Supporting Information S

Preparation of tungsten carbide using GO as a carbon precursor and XRD pattern of the resulting product after annealing at 850 °C for 5 h under H2/N2 atmosphere obtained from the mixture of P123, GO, and WCl6. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (662) 562-5444. Ext. 2186. Fax: (662) 579-3955. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Commission on Higher Education “The Strategic Scholarships Fellowships Frontier Research 4731

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp411844a | J. Phys. Chem. C 2014, 118, 4726−4732