Preparation of Carbon Nano-Onions and Their

ARTICLE pubs.acs.org/JPCC

Preparation of Carbon Nano-Onions and Their Application as Anode Materials for Rechargeable Lithium-Ion Batteries Fu-Dong Han, Bin Yao, and Yu-Jun Bai* Key Laboratory for Liquid
Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, P. R. China

bS Supporting Information ABSTRACT: Carbon nano-onions (CNOs) were prepared at 600 °C by a simple reaction between copper dichloride hydrate (CuCl2 3 2H2O) and calcium carbide (CaC2). The morphology and structure of the obtained products were investigated by field-emission scanning electron microscope, high-resolution transmission electron microscopy, X-ray diffraction, Raman spectrum, and nitrogen adsorption. Large quantities of CNOs consisting of quasi-spherically concentric graphitic shells with high purity and uniform size distribution (about 30 nm) were obtained. The crystal water in CuCl2 3 2H2O plays an important role in the formation of CNOs. The CNOs as-obtained exhibit high capacity and excellent cycling performance as anode materials for lithium-ion batteries, which can deliver a reversible capacity of 391 mAh g
1 up to 60 cycles.

’ INTRODUCTION Carbon nano-onions (CNOs), which consist of concentric graphitic shells, represent another new allotropic nanophase of carbon materials. CNOs have already been shown to offer a variety of potential applications such as solid lubrication,1 electromagnetic shielding,2 fuel cells,3 heterogeneous catalysis,4 gas and energy storage,5 and electro-optical devices6 owing to their outstanding chemical and physical properties. According to a recent study, CNOs can also be used to produce ultrahighpower micrometer-sized supercapacitors due to their accessible external surface area for ion adsorption.7 Since the first observation of the onion-like structure by Iijima in vacuum deposited amorphous carbon films in 1980,8 several methods have been reported to prepare CNOs. For example, Ugarte examined the formation of CNOs in a transmission electron microscope (TEM) by irradiating carbon material consisting of different carbon nanostructures (fullerenes, nanotubes, amorphous carbon).9 Kuznetsov et al. obtained CNOs by high-temperature annealing of diamond nanoparticles under vacuum.10 Cabioc’h et al. prepared CNOs by high-dose carbon ion implantation into copper and silver.11 Sano et al. fabricated CNOs by arc discharge between two graphite rods immersed in water.12 Recently, Hou et al. reported the high-yield synthesis of CNOs in counterflow diffusion flames.13,14 In addition, chemical vapor deposition was also considered as a viable method to synthesize CNOs by lots of scientists.15
18 Other ways have also been proposed to synthesize CNOs, such as laser irradiation,19 shock compression,20 and high-energy ball-milling.21 However, most of the methods above-mentioned require high energy input, and CNOs are sometimes of low yield or just as a r 2011 American Chemical Society

byproduct.16,22 For applications such as fuel-cell electrodes, large quantities (kilograms) of the material are desired. Nevertheless, CNOs can only be produced in minute quantities by lots of methods such as electron beam irradiation of carbon materials. Moreover, it is not practical to use the CNOs obtained by some methods requiring high investment and running costs for ordinary applications such as solid lubrication. Therefore, how to economically synthesize CNOs in large scale is of great importance for their wide applications. In this article, we reported a simple, efficient, and economical route for large-scale preparation of CNOs with uniform morphology and size. The CNOs were obtained via the reaction between CaC2 and CuCl2 3 2H2O at 600 °C. In addition, the electrochemical properties of the as-obtained CNOs as anode materials for lithium-ion battery have also been evaluated.

’ EXPERIMENTAL SECTION Materials and Synthesis. The raw materials used are chemically pure copper dichloride hydrate (CuCl2 3 2H2O) and calcium carbide (CaC2). In a typical procedure, 10 g (0.059 mol) CuCl2 3 2H2O and 3.2 g (0.05 mol) CaC2 were put into a stainless steel autoclave of 30 mL capacity. Because the removal of CuCl2 3 2H2O is easier than that of CaC2, the amount of CuCl2 3 2H2O was slightly in excess to ensure the complete reaction of CaC2. The autoclave containing the raw materials was sealed Received: January 24, 2011 Revised: March 29, 2011 Published: April 14, 2011 8923

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The Journal of Physical Chemistry C tightly and heated in an electric oven to 600 °C and maintained for 10 h at this temperature. When the autoclave was cooled naturally to ambient temperature, the products in the autoclave were collected and primarily washed several times with a mixed solution of ammonia (NH3 3 H2O) and carbon tetrachloride (CCl4) to remove the byproduct copper23 and then rinsed successively with dilute hydrochloric acid and deionized water until all soluble materials were removed. After drying at 60 °C for 10 h, black powders were ultimately obtained. Characterization. The morphology of the product was examined using a Hitachi SU-70 field-emission scanning electron microscope (FESEM) with an energy-dispersive X-ray spectrometer (EDX) and a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM). X-ray powder diffraction (XRD) patterns were obtained on a Rigaku Dmax-rc diffractometer with Ni-filtered Cu KR radiation (V = 40 kV, I = 50 mA) at a scanning rate of 4°/min. Raman spectra were collected on a Renishaw confocal Raman microspectroscopy (Renishaw Co. Ltd., Gloucestershire, U.K.) with a laser excitation wavelength of 780 nm. Nitrogen adsorption and desorption isotherms were carried out at 77 K on a Quadrasorb (Supporting Information) sorption analyzer. The samples were outgassed at 300 °C for 3 h under a vacuum in the degas port of the analyzer. The specific surface area was calculated with the Brunauer
Emmett
Teller (BET) model, and the pore-size distribution was calculated from the adsorption/desorption data by using the DFT method. Electrochemical Measurement. Electrochemical experiments were carried out in 2025 coin-type cells. The working electrodes were prepared by coating the slurry of CNOs (85 wt %), carbon black (5 wt %), and polyvinylidene fluoride (PVDF) (10 wt %) dissolved in n-methyl pyrrolidinone (NMP) onto a Cu foil substrate and dried in a vacuum oven at 120 °C for 12 h. Lithium metal foil was utilized as the counter electrode, and Celgard 2300 was used as the separator. The electrolyte was a mixture of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume). Half-cells were assembled in an argon-filled glovebox. The cell performance was estimated galvanostatically at a current density of C/10 (one Li per six formula units (LiC6) in 10 h)) for both charge (Li extraction) and discharge (Li insertion) at room temperature. The cells were cycled in the voltage range of 0.02
3 V (versus Li/Liþ).

’ RESULTS AND DISCUSSION The morphology of the as
obtained product was observed by FESEM, as shown in part a of Figure 1. The product is comprised of numerous quasi-spheres about 30 nm in diameter, and no other carbon nanostructures such as carbon nanotubes can be detected. Part b of Figure 1 is the EDX spectrum taken from the spheres, from which there is no detectable metal catalyst (Cu) or other impurities except for C and a little O resulting from absorption, indicating the high purity of the spheres. The examination by HRTEM (parts c
e of Figure 1) demonstrates that the spheres have an onion-like structure. The interplanar spacing of 0.34 nm corresponds to that of the (002) plane of graphite. The short-range order or turbostratic graphite structure gives rise to concentric graphite layers around a hollow and irregular core about 5 nm in diameter. The corresponding selected area electron diffraction (SAED) pattern (insert in part d of Figure 1) reveals the poor graphitization of the CNOs. It is worthwhile to note that Cu-encapsulating CNOs can hardly be observed in the product even by careful TEM examination,

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Figure 1. FESEM image (a), EDX spectrum (b), and HRTEM images (c
e) of the as-obtained CNOs. The insert in (d) is the corresponding SAED pattern.

Figure 2. XRD pattern (a) and Raman spectrum (b) of the as-obtained CNOs.

further confirming the high purity of CNOs, which may imply that the formation of CNOs was not by virtue of encapsulating Cu. In addition, a little amorphous carbon can be found on the surface of CNOs, which may result from the deposition of vaporous carbon atoms around the CNOs, and this will be discussed in detail. To determine the structure of the as-obtained product, XRD measurements were conducted, as shown in part a of Figure 2. The main peak at 2θ = 25.8° can be attributed to the (002) diffraction of hexagonal graphite. The broadening nature of the peak is indicative of the long-range disorder structure of the 8924

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the reaction between CaC2 and CuCl2. According to the molar ratio of approximately 5.9:5 for CuCl2 3 2H2O to CaC2, the H2O generated from the CuCl2 3 2H2O is excess enough to consume the CaC2 thoroughly, so no CaC2 could be left for the further reaction between CaC2 and CuCl2 under the experimental conditions. With the increase of temperature, a redox reaction between C2H2 and CuCl2 will happen, resulting in the formation of carbon and Cu. The above-mentioned process can be expressed by the following reactions: Figure 3. Nitrogen adsorption/desorption isotherms (a) and the DFT pore-size distributions (b) of the as-obtained CNOs.

as-obtained CNOs due to their small sizes. Moreover, the disorder structure can be also reflected by the indistinguishability of the (100) and (101) peaks centered at 2θ = 43.7°.24 Raman spectra were measured to identify the bonding and structure of the CNOs as-obtained, as shown in part b of Figure 2. The two broad peaks centered at about 1303 and 1571 cm
1 are ascribed to the D peak for disordered carbon and the G peak for graphite carbon, respectively.25 The intensity ratio of ID to IG in Raman spectra is a typical parameter to quantify the disorder degree of carbon materials, that is a greater value of ID/IG ratio means a higher disorder degree for graphite. As to the as-obtained CNOs, the calculated value of 1.10 for ID/IG implies a low graphitization degree resulted from the long-range disorder structure of the CNOs, in good agreement with the HRTEM and XRD results. Nitrogen adsorption/desorption isotherms and DFT pore size distribution of the as-obtained CNOs are displayed in Figure 3. From part a of Figure 3, the isotherms exhibit a typical IV-type curve with a small hysteresis loop at higher relative pressure, suggesting the diversity of pores and the presence of micropores and mesopores in the product. The specific surface area and pore volume are 101 m2 g
1 and 0.16 cm3 g
1, respectively. The pore-size distribution shown in part b of Figure 3 indicates that the size of pores is primarily between 1 and 5 nm. According to previous reports, CaC2 can be used as carbon source to synthesize carbon spheres under appropriate conditions.26
29 CuCl2 is sometimes used as a promoter in the formation of carbon nanostructures.30 In our experiment, however, CuCl2 3 2H2O acts as a reactant to form CNOs because Cu can be detected in the products washed just by deionized water, as confirmed by XRD analysis (Figure S1 of the Supporting Information). For insight into the formation mechanism of the CNOs, a comparative experiment was performed using CuCl2 without crystal water as the reactant. In contrast, few CNOs were obtained; instead, some hollow structures with various sizes and a few nanoparticles could be observed (Figure S2 of the Supporting Information). Although the reaction between CuCl2 and
1 CaC2 is both thermodynamically (ΔrGΘ m =
508.2 kJ mol , Θ
1 ΔrHm =
515.5 kJ mol ) and experimentally feasible, as expressed by eq 1, the formation of the CNOs cannot be just ascribed to the simple redox reaction between CaC2 and CuCl2. CuCl2 þ CaC2 ¼ Cu þ 2C þ CaCl2

ð1Þ

Thus, the crystal water in CuCl2 3 2H2O plays an important role in the formation of the CNOs. On the basis of the information acquired, a possible chemical reaction mechanism is proposed as follows. The crystal water in CuCl2 3 2H2O will be lost at 110 °C during heating and will immediately react with CaC2 to produce acetylene (C2H2). The reaction between water and CaC2 occurs quite readily even under ambient atmosphere and thus is prior to

CuCl2 3 2H2 O ¼ CuCl2 þ 2H2 O

ð2Þ

CaC2 þ 2H2 O ¼ CaðOHÞ2 þ C2 H2

ð3Þ

CuCl2 þ C2 H2 ¼ Cu þ 2C þ 2HCl

ð4Þ

CaðOHÞ2 þ 2HCl ¼ CaCl2 þ 2H2 O

ð5Þ

The overall reaction can be concluded according to the above reactions: CuCl2 3 2H2 O þ CaC2 ¼ CaCl2 þ Cu þ 2C þ 2H2 O

ð6Þ

As has been known, the chemical reactions described by eqs 2, 3, and 5 can occur readily. According to free energy calculations, eq 4 is also thermodynamically spontaneous (ΔrGΘ m =
223.9
1 kJ mol
1) and highly exothermic (ΔrHΘ m =
191.92 kJ mol ), indicating the feasibility of the above-mentioned chemical processes. Moreover, the thermodynamic calculation for the overall
1 Θ reaction of eq 6 (ΔrGΘ m =
631.393 kJ mol , ΔrHm =
353.539
1 kJ mol ) also demonstrates that the reaction between CuCl2 3 2H2O and CaC2 could happen under the reaction conditions. The heat generated further prompts the progress of the reactions, during which large quantities of carbon and Cu atoms are simultaneously produced. It is well-known that Cu has a very strong effect on graphitization of carbon.31,32 Accordingly, the carbon atoms generated will grow into graphitic flakes under the catalysis of Cu. These graphitic flakes obtained are the combination of pentagonal and heptagonal structures together with the planar hexagonal structure of graphene so that they can accommodate the curvature of the CNOs.33 With continuous heating treatment, the graphitic flakes aggregate into quasi-spherical graphitic shells, giving rise to the formation of CNOs. The driving force for the graphitic flake aggregation has been discussed in terms of a free energy minimization theory,34,35 that is the highly stable spherical shape of carbon species and the multiple shell structure are more energetically favorable than a single or tubular shell. Meanwhile, this process can also result in the elimination of dangling bonds at the edge of the flakes, which can undoubtedly lower the energy of the system.9,36 However, some defects such as dangling bonds also exist on the surface of the CNOs. At the end of the growth of CNOs, these defects would possibly act as nucleation sites for further deposition of vaporous carbon atoms on the surface of CNOs, which could account for the amorphous carbon observed on the surface of CNOs under HRTEM. Galvanostatic discharge/charge experiments were carried out to evaluate the electrochemical performance of the as-obtained CNOs. Part a of Figure 4 shows the discharge/charge curves of the as-obtained CNOs for the first four cycles at a rate of C/10. Surprisingly, at this current density, a first reversible capacity as high as 501 mAh g
1 can be observed, which is significantly higher than the theoretical capacity of graphite (372 mAh g
1). 8925

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], tel: þ8653188392677, fax: þ8653188392315.

Figure 4. Galvanostatic discharge/charge curves (a) and cycling performance (b) of the as-obtained CNOs cycled at a rate of C/10.

Meanwhile, a large irreversible capacity of about 381 mAh g
1 was also observed for the CNOs electrode during the first discharge and charge process; however, after the fifth cycle, the Coulombic efficiency is above 95%. In fact, the initial irreversible capacity is an expected phenomenon in carbonaceous electrodes in lithium-ion batteries.37 The irreversible capacity loss could originate from the decomposition of the electrolyte, which leads to the formation of solid electrolyte interphase films at the electrode/electrolyte interface and/or from the irreversible lithium insertion into special positions such as in the vicinity of residual H atoms in the carbonaceous material.38
40 The cycling performance of the CNO electrode was examined under longterm cycling over 60 cycles, which demonstrated a good cyclic performance and reversibility (part b of Figure 4). After 60 cycles, the CNO anode still maintained a specific reversible capacity of 391 mAh g
1, which also represents a more enhanced performance than that of graphite anodes. From these results, it can be concluded that CNOs are very promising for use as anode electrodes in lithium-ion batteries. In the case of the as-obtained CNOs, the enhanced specific capacity may be originated from the following reasons. (1) The nanoscale carbon onions can provide a high electrode
electrolyte contact area (101 m2 g
1) and a short pathway both for electron and Liþ transport. (2) The unique graphitic multilayer structure of CNOs can afford more lithium storage sites such as cavities and nanopores,37 leading to a higher capacity. (3) The amorphous carbon around the CNOs may also contribute to the enhanced specific capacity due to the fact that the disordered carbon exhibits a higher capacity than that of graphite.41

’ CONCLUSIONS In conclusion, large quantities of carbon nano-onions have been successfully prepared by a simple and economical method via the reaction between CuCl2 3 2H2O and CaC2. The formation of the CNOs is greatly associated with the crystal water in CuCl2 3 2H2O. The electrochemical performance testing gives some evidence that the prepared CNOs with a unique graphitic multilayer structure have intensive potential as a candidate of anode materials with high reversible capacity and good cycling performance for rechargeable lithium-ion batteries. ’ ASSOCIATED CONTENT

bS

Supporting Information. XRD pattern of the product rinsed just by deionized water and without further purification by the mixed solution of NH3 3 H2O and CCl4, TEM image of the product obtained by the reaction of CaC2 and CuCl2 without crystal water at 600 °C. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 50972076, 50872072, and 50772061), Shandong Provincial Natural Science Foundation, China (Y2008F26 and Y2008F40), Science and Technology Development Project of Shandong Province (2009GG10003001, 2009GG10003003), and Special Fund for Postdoctoral Innovative Project of Shandong Province (200702024). ’ REFERENCES (1) Hirata, A.; Igarashi, M.; Kaito, T. Tribol. Int. 2004, 37, 899–905. (2) Shenderova, O.; Grishko, V.; Cunningham, G.; Moseenkov, S.; McGuire, G.; Kuznetsov, V. Diamond Relat. Mater. 2008, 17, 462–466. (3) Xu, B. S.; Yang, X. W.; Wang, X. M.; Guo, J. J.; Liu, X. G. J. Power Sources 2006, 162, 160–164. (4) Keller, N.; Maksimova, N. I.; Roddatis, V. V.; Schur, M.; Mestl, G.; Butenko, Y. V.; Kuznetsov, V. L.; Schlogl, R. Angew. Chem., Int. Ed. 2002, 41, 1885–1887. (5) Sano, N.; Wang, H.; Alexandrou, I.; Chhowalla, M.; Teo, K. B. K.; Amaratunga, G. A. J.; Iimura, K. J. Appl. Phys. 2002, 92, 2783–2788. (6) Koudoumas, E.; Kokkinaki, O.; Konstantaki, M.; Couris, S.; Korovin, S.; Detkov, P.; Kuznetsov, V.; Pimenov, S.; Pustovoi, V. Chem. Phys. Lett. 2002, 357, 336–340. (7) Pech, D.; Brunet, M.; Durou, H.; Huang, P. H.; Mochalin, V.; Gogotsi, Y.; Taberna, P. L.; Simon, P. Nat. Nanotechnol. 2010, 5, 651–654. (8) Iijima, S. J. Cryst. Growth 1980, 50, 675–683. (9) Ugarte, D. Nature 1992, 359, 707–709. (10) Kuznetsov, V. L.; Chuvilin, A. L.; Butenko, Y. V.; Mal’Kov, I. Y.; Titov, V. M. Chem. Phys. Lett. 1994, 222, 343–348. (11) Cabioc’h, T.; Jaouen, M.; Thune, E.; Guerin, P.; Fayoux, C.; Denanot, M. F. Surf. Coat. Technol. 2000, 128, 43–50. (12) Sano, N.; Wang, H.; Chhowalla, M.; Alexandrou, I.; Amaratunga, G. A. J. Nature 2001, 414, 506–507. (13) Hou, S. S.; Chung, D. H.; Lin, T. H. Carbon 2009, 47, 938–947. (14) Chung, D. H.; Lin, T. H.; Hou, S. S. Nanotechnology 2010, 21, 435604. (15) Chen, X. H.; Deng, F. M.; Wang, J. X.; Yang, H. S.; Wu, G. T.; Zhang, X. B.; Peng, J. C.; Li, W. Z. Chem. Phys. Lett. 2001, 336, 201–204. (16) Nasibulin, A. G.; Moisala, A.; Brown, D. P.; Kauppinen, E. I. Carbon 2003, 41, 2711–2724. (17) Yang, Y. Z.; Liu, X. G.; Xu, B. S. J. Mater. Res. 2008, 23, 1393–1397. (18) Zhang, C. G.; Li, J. J.; Shi, C. S.; Liu, E. Z.; Du, X. W.; Zhao, N. Q. Carbon 2011, 49, 1151–1158. (19) Gorelik, T.; Urban, S.; Falk, F.; Kaiser, U.; Glatzel, U. Chem. Phys. Lett. 2003, 373, 642–645. (20) Kobayashi, T.; Sekine, T.; He, H. L. Chem. Mater. 2003, 15, 2681–2683. (21) Huang, J. Y.; Yasuda, H.; Mori, H. Chem. Phys. Lett. 1999, 303, 130–134. (22) Kang, J. L.; Li, J. J.; Du, X. W.; Shi, C. S.; Zhao, N. Q.; Nash, P. Mater. Sci. Eng., A 2008, 475, 136–140. (23) Yanagihara, N.; Nakayama, M.; Tai, H. D. Chem. Lett. 2003, 32, 640–641. (24) Houska, C. R.; Warren, B. E. J. Appl. Phys. 1954, 25, 1503–1509. (25) Robertson, J. Mater. Sci. Eng., R 2002, 37, 129–281. 8926

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