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Large-scale preparation of hollow graphitic carbon nanospheres
Materials Chemistry and Physics 137 (2013) 904e909
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Large-scale preparation of hollow graphitic carbon nanospheres Jun Feng a, Fu Li a, Yu-Jun Bai a, b, *, Fu-Dong Han a, Yong-Xin Qi a, Ning Lun a, Xi-Feng Lu c a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China State Key laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China c Lunan Institute of Coal Chemical Engineering, Jining 272000, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Hollow graphitic carbon nanospheres (HGCNSs) were prepared on large scale at 550 C < The preparation is simple, effective and eco-friendly. < The in situ yielded MgO nanocrystals promote the graphitization. < The HGCNSs exhibit superior electrochemical performance to graphite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2012 Received in revised form 28 October 2012 Accepted 30 October 2012
Hollow graphitic carbon nanospheres (HGCNSs) were synthesized on large scale by a simple reaction between glucose and Mg at 550 C in an autoclave. Characterization by X-ray diffraction, Raman spectroscopy and transmission electron microscopy demonstrates the formation of HGCNSs with an average diameter of 10 nm or so and a wall thickness of a few graphenes. The HGCNSs exhibit a reversible capacity of 391 mAh g1 after 60 cycles when used as anode materials for Li-ion batteries. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Electron diffraction Electrochemical properties
1. Introduction Carbon nanomaterials, such as fullerenes, nanotubes, onions, fibers, graphene and spheres, have wide applications in the field of quantum wires [1], gas storage media [2], semiconductor devices [3], high-strength composites [4] and anodes for Li-ion batteries [5], thus attracting considerable attention. Carbon spheres with hollow cores could endow them unique properties such as low density, large specific surface area, thermal insulation, and electronic properties [6]. Up to date, a lot of researches have reported
* Corresponding author. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ86 531 88392315. E-mail addresses: [email protected], [email protected] (Y.-J. Bai). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.10.032
the synthesis of hollow carbon spheres (HCSs). For example, the HCSs could be fabricated by the reaction between metallic Mg powder, NaCO3 and CCl4 at 450 C [7], the chlorination of ferrocene at 900 C under a flow of pure chlorine gas [8], the reaction of ferrocene and ammonium chloride at 700 C under an Ar atmosphere [9], the carbonization of phenolic resin using ferric nitrate as a catalyst precursor at 1000 C [10], or a detonation route using negative-oxygen balance explosive trinitrotoluene as starting material, nickel powder as catalyst, and inorganic acid as solvent [11]. Thus obtained HCSs usually have a low degree of graphitization (DG), thick walls, and a low yield or just as a byproduct during the synthesis of other carbon nanomaterials. As has been known, graphene is commonly a two-dimensional monolayer graphite sheet, which has exhibited remarkable and exciting properties [12], such as extreme thinness and lightness, almost complete transparency, unusual electronic properties [13],
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MgO nanocrystals could promote the graphitization of the simultaneously yielded carbon even at comparatively low temperatures. 2. Experimental 2.1. Materials and synthesis The raw materials used are analytical pure without further purification. In a typical procedure, 7.8 g glucose and 6.6 g Mg powders were mixed uniformly and were put in an autoclave of 50 mL in capacity. The tightly sealed autoclave was heated to 550 C and maintained at the temperature for 5 h. Then the autoclave was cooled to ambient temperature naturally. The product is a mixture of black and white powders, which was washed with dilute hydrochloric acid and deionized water thoroughly. After drying at 50 C for 12 h in an oven, 1.6 g black product was ultimately obtained, corresponding to an actual carbon yield of 51.3%. 2.2. Characterization Fig. 1. XRD patterns of the products before (a) and after (b) washing with dilute hydrochloric acid and deionized water.
super-strength and stiffness [14], excellent conductor of electricity and heat [15,16], and is currently being actively explored as materials for conducting electrodes, batteries, solar cells, supercapacitors, fuel cells and sensors [16e19]. However, only a few reports have so far been related to hollow graphitic carbon nanospheres (HGCNSs) with a few graphenes. Recently, hollow capsules of graphene were synthesized through layer-by-layer assembly of surface-functionalized reduced graphene oxide nanosheets of opposite charges onto polystyrene colloidal particles, followed by the removal of the sacrificial templates [19,20]. Three-dimensional hollow structures typically made up of two graphene layers were observed in graphite when subjected to arc-discharge [21,22]. Graphene nanospheres were produced by annealing graphene oxide solution at high-temperature with the assistance of sparks [23]. Apparently, the methods available for HGCNSs preparation are either somewhat complex in manipulation or time-and energyconsuming. In this work, we propose an eco-friendly approach to effectively prepare HGCNSs on large scale by the simple reaction of glucose and Mg at 550 C or so, and measured the electrochemical performance using the HGCNSs as anode materials for Li-ion batteries. More interestingly, it is found that the newly generated
X-ray powder diffraction (XRD) patterns were obtained on a Rigaku Dmax-rc diffractometer with Ni filtered Cu Ka radiation (V ¼ 40 kV, I ¼ 50 mA) at a scanning rate of 4 min1. Raman spectra were collected on a Renishaw confocal Raman microspectroscopy (Renishaw Co. Ltd., Gloucestershire, U.K.) with a laser excitation wavelength of 780 nm. The morphology of the products was examined using a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM). Nitrogen adsorption/desorption isotherms were carried out at 196 C on a Quadrasorb SI sorption analyzer. The samples were degassed at 100 C for 8 h under a vacuum in the degas port of the analyzer. The specific surface area was calculated with the BrunauereEmmetteTeller (BET) model and the pore-size distribution was determined from the adsorption/desorption data by using the density functional theory (DFT) method. Electrochemical performance was measured in 2025 coin-type cells. The slurry of HGCNSs (85 wt%), carbon black (5 wt%), and polyvinylidene fluoride (10 wt%) dissolved in n-methyl pyrrolidinone was coated on a Cu foil substrate and dried in a vacuum oven at 120 C for 12 h to fabricate working electrodes. Lithium metal foil was utilized as counter electrode, and Celgard 2300 was used as separator. The electrolyte was a mixture of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 by volume). Half-cells were assembled in a glovebox filled with argon, whose performance was evaluated galvanostatically in a voltage range of 0.02e3 V at different current densities at room temperature.
Fig. 2. (A) Raman spectrum and (b) nitrogen adsorption/desorption isotherms of the resulting product after washing with dilute hydrochloric acid and deionized water. The inset in (b) is the corresponding pore-size distribution curve.
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Fig. 3. TEM images of the products before (aeb) and after (ced) washing with dilute hydrochloric acid and deionized water. The inset in (c) is the magnified image of the rectangular frame.
3. Results and discussion 3.1. Characterization of the product The XRD patterns of the products before and after washing with dilute hydrochloric acid and deionized water are shown in Fig. 1. It can be found that the product without washing is comprised of hexagonal carbon (JCPDS No.41-1487) and MgO
(JCPDS No.45-0946). Though subjected to repeatedly washing with dilute hydrochloric acid, a little MgO was still remained in the resulting product, indicating that some MgO might be encapsulated in carbon shells. The strong and narrow diffraction at 2q ¼ 26.3 suggests the high DG of the carbon material. In order to acquire more structural information, Raman spectrum of the resulting product was measured, as shown in Fig. 2a. Two characteristic peaks for carbon material are clearly present in
Fig. 4. XRD pattern (a) and TEM image (b) of the resulting product obtained by the reaction of glucose and Mg at 300 C for 5 h.
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Fig. 7. Illustration of the formation of the HGCNSs.
Fig. 5. XRD patterns of the resulting products obtained by the reaction of sucrose and Mg at 450 and 200 C for 5 h.
the spectrum. The peak around 1325 cm1 is D-band corresponding to disorder carbon structure, and the one around 1573 cm1 is G-band associated with crystalline graphite. The shoulder located at 1600e1614 cm1 corresponds to the D0 peak which often superimposes with the broad G peak in disordered carbon [24]. The full-width at half-maximum is 23.9 cm1 for the G-band and 49 cm1 for the D-band, and the intensity ratio (IG/ID) of the two bands is about 2.68, further demonstrating the high DG of the resulting product [25]. The nitrogen absorption/desorption isotherms of the resulting product exhibit a typical IV-type curve (Fig. 2b). The specific surface area calculated with the BET model is about 66 m2 g1, and the pore size is dominantly around 4.0 nm. The morphologies of the products before and after washing with dilute hydrochloric acid and deionized water are shown in Fig. 3. Before washing, a lot of squares about tens of nanometers in side length can be observed in the product (Fig. 3a). The squares are surrounded by a large number of HCSs only several nanometers in diameter. From the lattice fringe image in Fig. 3b, the squares consist of two parts. The interplanar spacing of 0.15 nm for the core corresponds to the (220) plane of MgO. Around the core is a few graphenes with a spacing of 0.33 nm, corresponding to the (002) plane of graphite. After washing, the squares were washed away
entirely, and numerous HCSs were remained, as displayed in Fig. 3c, however, the square appearance could still be distinguished. The spherical morphology of the HCSs is visible especially along the edges of the squares, as marked by an arrow and rectangular frame in Fig. 3c (The inset is the magnified image of the rectangular part.). In the high resolution image shown in Fig. 3d, the HCSs are about 10 nm in diameter with a wall thickness of only a few graphenes, namely, the HCSs are actually HGCNSs. The clear lattice fringes demonstrate the high DG of the HGCNSs. In our previous work [26,27], HCSs were prepared by the reaction between Zn powders and glucose or sucrose also at 550 C, however, the HCSs exhibit a low DG and an average wall thickness of about 10 nm. When Mg was replaced by Na to react with glucose, amorphous carbon sheets with several micrometers in size were achieved. Thus it can be seen that MgO plays a critical role in synthesizing the HGCNSs. As has been reported in the literature [28], MgO nanocrystals could promote ordered carbon (graphene) growth under chemical vapor deposition (CVD) conditions for carbon nanotube synthesis. In this work, although the reaction temperature (550 C) is much lower than that of the CVD (850 C), the newly generated MgO nanocrystals are in the absence of surface passivation and could exhibit a strong graphitization behavior. The direct and intense interaction of carbon atoms with MgO nanocrystals promotes the graphitization. Furthermore, the hollow graphitic shells bounded by curved planes appear to be due to the presence of a small number of pentagons and other non-hexagonal rings distributed in a hexagonal graphene network [21,22]. For further verifying the promotion of the renascent MgO nanocrystals on graphitization, we designed and performed some other experiments. The reaction of glucose and Mg could happen at 300 C with the resulting product also exhibiting a high DG (Fig. 4a), though besides the HGCNSs, other morphologies such as nanotubes and nanosheets also present in the product (Fig. 4b). Meanwhile, some weak diffractions resulted from MgO could also be detected in the XRD pattern, and TEM examination confirms the presence of some MgO nanocrystals encapsulated in carbon shells (dark particles marked by arrows in the TEM image). When glucose was substituted by sucrose to react with Mg, an analogous promotion on graphitization also occurred. The XRD patterns of the resulting products obtained from the reaction of sucrose and Mg at
Fig. 6. XRD pattern (a) and FESEM image (b) of the product obtained by directly heating sucrose to 550 C and maintained for 8 h.
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Fig. 8. Galvanostatic charge/discharge curves (a) and cycling performance at a rate of 0.1 C (b), of the HGCNS electrodes.
450 and 200 C for 5 h are shown in Fig. 5. The strong diffractions resulted from carbon suggest the high DG of the resulting products. As a result, the newly formed MgO nanocrystals could certainly promote the graphitization of the simultaneously yielded carbon even at comparatively low temperatures. In terms of the formation of MgO, a redox reaction happens between Mg and glucose or sucrose, which could be simply expressed as follows. 6 Mg þ C6H12O6 ¼ 6MgO þ 6C þ 6H2 Q ¼ 2337.68 kJ mol1) (DrGQ ¼ 2507.02 kJ mol1, DrHm m ¼ 11MgO þ 12C þ 11H2 11 Mg þ C12H22O11 1 1 Q (DrGQ m ¼ 4720.57 kJ mol , DrHm ¼ 4394.03 kJ mol ) Similarly, the reaction between Zn and glucose could be expressed by the following equation. 6Zn þ C6H12O6 ¼ 6ZnO þ 6C þ 6H2 1 1 Q (DrGQ m ¼ 998.74 kJ mol , DrHm ¼ 814.58 kJ mol ) It is apparent that the reaction between Mg and sucrose is more spontaneous with larger heat release than that between Mg and glucose, and both of them easily happen at lower temperatures than that between Zn and glucose, i.e. 200 C for the reaction of Mg and sucrose, 300 C for the reaction of Mg and glucose, and higher than 400 C for the reaction of Zn and glucose. Once the reaction between Mg and glucose or sucrose initiates, the rapidly released heat energy could result in a temperature rise in autoclaves and the simultaneous formation of numerous MgO nanocrystals and H2 bubbles. As discussed in our previous work [26] and other literature [29e31], gas bubbles could act as templates for hollow spheres. According to TEM examination on the products (Fig. 3), besides the template of MgO nanocrystals, the H2 bubbles are the dominant templates for the formation of the HGCNSs. Directly heating glucose to 550 C and maintained for 8 h produced irregular carbon blocks and platelets other than the HCSs (Fig. 6), demonstrating that the reaction between Mg and glucose to yield MgO nanocrystals is crucial for the formation of the HGCNSs. Based on the above discussion, the formation of the HGCNSs could be simply depicted by the schematic in Fig. 7. 3.2. Electrochemical performance of the HGCNSs According to some recent reports [32e34], graphene could exhibit excellent performance as anode materials for Li-ion batteries due to the unique structure. The electrochemical performance of the HGCNSs as anode materials was also measured. Fig. 8(a) shows the discharge/charge curves of the HGCNS electrode for the first four cycles at a rate of 0.1 C. The HGCNSs exhibit a reversible capacity of 423 mAh g1 in the first cycle, higher than the theoretical capacity of graphite (372 mAh g1). Though an irreversible capacity of 311 mAh g1 occurs during the first discharge and charge process, the coulombic efficiency is above 95% after 5 cycles. The initial irreversible capacity loss has been ascribed to the decomposition of the electrolyte and the formation of a solid
electrolyte interface film in carbon anode materials [35,36]. Fig. 8(b) is the cycling performance of the HGCNS electrode during 60 cycles. It can be seen that the capacity hardly decreases after 10 cycles, demonstrating an excellent cyclic performance and reversibility of the HGCNS electrode. After 60 cycles, the electrode still maintains a reversible capacity of 391 mAh g1 which is still higher than the theoretic capacity of graphite, and the efficiency reaches 98% or so. As a consequence, the HGCNSs as anode materials for Li-ion batteries could exhibit superior performance to graphite [37]. The enhanced performance associates significantly with the unique structure of the HGCNSs. The hollow structure could buffer the volume change during charging/discharging, the thin wall of a few graphenes could shorten the pathway both for electron and Liþ transport, and the high DG could endow the HGCNSs excellent electric conductivity. 4. Conclusions HGCNSs could be prepared on large scale by the simple reaction between glucose and Mg at 550 C. The renascent MgO nanocrystals promote the graphitization of the simultaneously yielded carbon. The HGCNSs as anode materials for Li-ion batteries could exhibit superior electrochemical performance to graphite owing to the unique hollow structure with a high DG and a wall thickness of a few graphenes. This simple, effective and eco-friendly approach could offer an important alternative to produce HGCNSs for other applications, such as catalyst supports, gas storage media, supercapacitors. Acknowledgments This work was supported by Independent Innovation Foundation of Shandong University, IIFSDU (2012ZD004), Open Project from State Key Laboratory of Crystal Materials (KF1105), the National Natural Science Foundation of China (no. 50972076 and 50872072), the Shandong Provincial Natural Science Foundation, China (Y2008F26 and Y2008F40), and Shandong Provincial Doctoral Foundation, China (BS2010CL013). References [1] S.J. Tans, M.H. Devoret, H.J. Dai, A. Thess, R.E. Smalley, L.J. Geerligs, Nature 386 (1997) 474e477. [2] O.O. Adisa, B.J. Cox, J.M. Hill, J. Phys. Chem. C 115 (2011) 24528e24533. [3] L.M. Gomez, A. Kumar, Y. Zhang, K. Ryu, A. Badmaev, C.W. Zhou, Nano Lett. 9 (2009) 3592e3598. [4] K. Liu, Y.H. Sun, X.Y. Lin, R.F. Zhou, J.P. Wang, S.S. Fan, ACS Nano 4 (2010) 5827e5834. [5] S. Flandrois, B. Simon, Carbon 37 (1999) 165e180.
J. Feng et al. / Materials Chemistry and Physics 137 (2013) 904e909 [6] A.H. Lu, W.C. Li, G.P. Hao, B. Spliethoff, H.J. Bongard, B.B. Schaack, Angew. Chem. Int. Edit. 49 (2010) 1615e1618. [7] J.W. Liu, M.W. Shao, Q. Tang, X.Y. Chen, Z.P. Liu, Y.T. Qian, Carbon 41 (2002) 1645e1687. [8] N.A. Katcho, E. Urones-Garrote, D. Avila-Brande, A. Gomez-Herrero, S. Urbonaite, S. Csillag, Chem. Mater. 19 (2007) 2304e2309. [9] B.Y. Liu, D.C. Jia, Q.C. Meng, J.C. Rao, Carbon 45 (2007) 668e670. [10] M. Zhao, H.H. Song, X.H. Chen, W.T. Lian, Acta Mater. 55 (2007) 6144e6150. [11] Z.S. Wang, F.S. Li, Mater. Lett. 63 (2009) 58e60. [12] D.A.C. Brownson, D.K. Kampouris, C.E. Banks, J. Power Sources 196 (2011) 4873e4885. [13] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, Science 306 (2004) 666e669. [14] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva, Nature 438 (2005) 197e200. [15] Z.S. Wu, W.C. Ren, L.B. Gao, J.P. Zhao, Z.P. Chen, B.L. Liu, ACS Nano 3 (2009) 411e417. [16] T.K. Hong, D.W. Lee, H.J. Choi, H.S. Shin, B.S. Kim, ACS Nano 4 (2010) 3861e3868. [17] C.A. Merchant, K. Healy, M. Wanunu, V. Ray, N. Peterman, L.M.K. Vandersypen, Nano Lett. 10 (2010) 2915e2921. [18] J. Schrier, J. Phys. Chem. Lett. 1 (2010) 2284e2287. [19] J. Hong, J.Y. Han, H. Yoon, P. Joo, T. Lee, E. Seo, Nanoscale 3 (2011) 4515e4531. [20] J. Hong, K. Char, B.S. Kim, J. Phys.Chem. Lett. 1 (2010) 3442e3445. [21] P.J.F. Harris, Carbon 50 (2012) 3195e3199.
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Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Large-scale preparation of hollow graphitic carbon nanospheres Jun Feng a, Fu Li a, Yu-Jun Bai a, b, *, Fu-Dong Han a, Yong-Xin Qi a, Ning Lun a, Xi-Feng Lu c a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China State Key laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China c Lunan Institute of Coal Chemical Engineering, Jining 272000, PR China b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Hollow graphitic carbon nanospheres (HGCNSs) were prepared on large scale at 550 C < The preparation is simple, effective and eco-friendly. < The in situ yielded MgO nanocrystals promote the graphitization. < The HGCNSs exhibit superior electrochemical performance to graphite.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2012 Received in revised form 28 October 2012 Accepted 30 October 2012
Hollow graphitic carbon nanospheres (HGCNSs) were synthesized on large scale by a simple reaction between glucose and Mg at 550 C in an autoclave. Characterization by X-ray diffraction, Raman spectroscopy and transmission electron microscopy demonstrates the formation of HGCNSs with an average diameter of 10 nm or so and a wall thickness of a few graphenes. The HGCNSs exhibit a reversible capacity of 391 mAh g1 after 60 cycles when used as anode materials for Li-ion batteries. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Electron diffraction Electrochemical properties
1. Introduction Carbon nanomaterials, such as fullerenes, nanotubes, onions, fibers, graphene and spheres, have wide applications in the field of quantum wires [1], gas storage media [2], semiconductor devices [3], high-strength composites [4] and anodes for Li-ion batteries [5], thus attracting considerable attention. Carbon spheres with hollow cores could endow them unique properties such as low density, large specific surface area, thermal insulation, and electronic properties [6]. Up to date, a lot of researches have reported
* Corresponding author. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China. Tel./fax: þ86 531 88392315. E-mail addresses: [email protected], [email protected] (Y.-J. Bai). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.10.032
the synthesis of hollow carbon spheres (HCSs). For example, the HCSs could be fabricated by the reaction between metallic Mg powder, NaCO3 and CCl4 at 450 C [7], the chlorination of ferrocene at 900 C under a flow of pure chlorine gas [8], the reaction of ferrocene and ammonium chloride at 700 C under an Ar atmosphere [9], the carbonization of phenolic resin using ferric nitrate as a catalyst precursor at 1000 C [10], or a detonation route using negative-oxygen balance explosive trinitrotoluene as starting material, nickel powder as catalyst, and inorganic acid as solvent [11]. Thus obtained HCSs usually have a low degree of graphitization (DG), thick walls, and a low yield or just as a byproduct during the synthesis of other carbon nanomaterials. As has been known, graphene is commonly a two-dimensional monolayer graphite sheet, which has exhibited remarkable and exciting properties [12], such as extreme thinness and lightness, almost complete transparency, unusual electronic properties [13],
J. Feng et al. / Materials Chemistry and Physics 137 (2013) 904e909
905
MgO nanocrystals could promote the graphitization of the simultaneously yielded carbon even at comparatively low temperatures. 2. Experimental 2.1. Materials and synthesis The raw materials used are analytical pure without further purification. In a typical procedure, 7.8 g glucose and 6.6 g Mg powders were mixed uniformly and were put in an autoclave of 50 mL in capacity. The tightly sealed autoclave was heated to 550 C and maintained at the temperature for 5 h. Then the autoclave was cooled to ambient temperature naturally. The product is a mixture of black and white powders, which was washed with dilute hydrochloric acid and deionized water thoroughly. After drying at 50 C for 12 h in an oven, 1.6 g black product was ultimately obtained, corresponding to an actual carbon yield of 51.3%. 2.2. Characterization Fig. 1. XRD patterns of the products before (a) and after (b) washing with dilute hydrochloric acid and deionized water.
super-strength and stiffness [14], excellent conductor of electricity and heat [15,16], and is currently being actively explored as materials for conducting electrodes, batteries, solar cells, supercapacitors, fuel cells and sensors [16e19]. However, only a few reports have so far been related to hollow graphitic carbon nanospheres (HGCNSs) with a few graphenes. Recently, hollow capsules of graphene were synthesized through layer-by-layer assembly of surface-functionalized reduced graphene oxide nanosheets of opposite charges onto polystyrene colloidal particles, followed by the removal of the sacrificial templates [19,20]. Three-dimensional hollow structures typically made up of two graphene layers were observed in graphite when subjected to arc-discharge [21,22]. Graphene nanospheres were produced by annealing graphene oxide solution at high-temperature with the assistance of sparks [23]. Apparently, the methods available for HGCNSs preparation are either somewhat complex in manipulation or time-and energyconsuming. In this work, we propose an eco-friendly approach to effectively prepare HGCNSs on large scale by the simple reaction of glucose and Mg at 550 C or so, and measured the electrochemical performance using the HGCNSs as anode materials for Li-ion batteries. More interestingly, it is found that the newly generated
X-ray powder diffraction (XRD) patterns were obtained on a Rigaku Dmax-rc diffractometer with Ni filtered Cu Ka radiation (V ¼ 40 kV, I ¼ 50 mA) at a scanning rate of 4 min1. Raman spectra were collected on a Renishaw confocal Raman microspectroscopy (Renishaw Co. Ltd., Gloucestershire, U.K.) with a laser excitation wavelength of 780 nm. The morphology of the products was examined using a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM). Nitrogen adsorption/desorption isotherms were carried out at 196 C on a Quadrasorb SI sorption analyzer. The samples were degassed at 100 C for 8 h under a vacuum in the degas port of the analyzer. The specific surface area was calculated with the BrunauereEmmetteTeller (BET) model and the pore-size distribution was determined from the adsorption/desorption data by using the density functional theory (DFT) method. Electrochemical performance was measured in 2025 coin-type cells. The slurry of HGCNSs (85 wt%), carbon black (5 wt%), and polyvinylidene fluoride (10 wt%) dissolved in n-methyl pyrrolidinone was coated on a Cu foil substrate and dried in a vacuum oven at 120 C for 12 h to fabricate working electrodes. Lithium metal foil was utilized as counter electrode, and Celgard 2300 was used as separator. The electrolyte was a mixture of 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1 by volume). Half-cells were assembled in a glovebox filled with argon, whose performance was evaluated galvanostatically in a voltage range of 0.02e3 V at different current densities at room temperature.
Fig. 2. (A) Raman spectrum and (b) nitrogen adsorption/desorption isotherms of the resulting product after washing with dilute hydrochloric acid and deionized water. The inset in (b) is the corresponding pore-size distribution curve.
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J. Feng et al. / Materials Chemistry and Physics 137 (2013) 904e909
Fig. 3. TEM images of the products before (aeb) and after (ced) washing with dilute hydrochloric acid and deionized water. The inset in (c) is the magnified image of the rectangular frame.
3. Results and discussion 3.1. Characterization of the product The XRD patterns of the products before and after washing with dilute hydrochloric acid and deionized water are shown in Fig. 1. It can be found that the product without washing is comprised of hexagonal carbon (JCPDS No.41-1487) and MgO
(JCPDS No.45-0946). Though subjected to repeatedly washing with dilute hydrochloric acid, a little MgO was still remained in the resulting product, indicating that some MgO might be encapsulated in carbon shells. The strong and narrow diffraction at 2q ¼ 26.3 suggests the high DG of the carbon material. In order to acquire more structural information, Raman spectrum of the resulting product was measured, as shown in Fig. 2a. Two characteristic peaks for carbon material are clearly present in
Fig. 4. XRD pattern (a) and TEM image (b) of the resulting product obtained by the reaction of glucose and Mg at 300 C for 5 h.
J. Feng et al. / Materials Chemistry and Physics 137 (2013) 904e909
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Fig. 7. Illustration of the formation of the HGCNSs.
Fig. 5. XRD patterns of the resulting products obtained by the reaction of sucrose and Mg at 450 and 200 C for 5 h.
the spectrum. The peak around 1325 cm1 is D-band corresponding to disorder carbon structure, and the one around 1573 cm1 is G-band associated with crystalline graphite. The shoulder located at 1600e1614 cm1 corresponds to the D0 peak which often superimposes with the broad G peak in disordered carbon [24]. The full-width at half-maximum is 23.9 cm1 for the G-band and 49 cm1 for the D-band, and the intensity ratio (IG/ID) of the two bands is about 2.68, further demonstrating the high DG of the resulting product [25]. The nitrogen absorption/desorption isotherms of the resulting product exhibit a typical IV-type curve (Fig. 2b). The specific surface area calculated with the BET model is about 66 m2 g1, and the pore size is dominantly around 4.0 nm. The morphologies of the products before and after washing with dilute hydrochloric acid and deionized water are shown in Fig. 3. Before washing, a lot of squares about tens of nanometers in side length can be observed in the product (Fig. 3a). The squares are surrounded by a large number of HCSs only several nanometers in diameter. From the lattice fringe image in Fig. 3b, the squares consist of two parts. The interplanar spacing of 0.15 nm for the core corresponds to the (220) plane of MgO. Around the core is a few graphenes with a spacing of 0.33 nm, corresponding to the (002) plane of graphite. After washing, the squares were washed away
entirely, and numerous HCSs were remained, as displayed in Fig. 3c, however, the square appearance could still be distinguished. The spherical morphology of the HCSs is visible especially along the edges of the squares, as marked by an arrow and rectangular frame in Fig. 3c (The inset is the magnified image of the rectangular part.). In the high resolution image shown in Fig. 3d, the HCSs are about 10 nm in diameter with a wall thickness of only a few graphenes, namely, the HCSs are actually HGCNSs. The clear lattice fringes demonstrate the high DG of the HGCNSs. In our previous work [26,27], HCSs were prepared by the reaction between Zn powders and glucose or sucrose also at 550 C, however, the HCSs exhibit a low DG and an average wall thickness of about 10 nm. When Mg was replaced by Na to react with glucose, amorphous carbon sheets with several micrometers in size were achieved. Thus it can be seen that MgO plays a critical role in synthesizing the HGCNSs. As has been reported in the literature [28], MgO nanocrystals could promote ordered carbon (graphene) growth under chemical vapor deposition (CVD) conditions for carbon nanotube synthesis. In this work, although the reaction temperature (550 C) is much lower than that of the CVD (850 C), the newly generated MgO nanocrystals are in the absence of surface passivation and could exhibit a strong graphitization behavior. The direct and intense interaction of carbon atoms with MgO nanocrystals promotes the graphitization. Furthermore, the hollow graphitic shells bounded by curved planes appear to be due to the presence of a small number of pentagons and other non-hexagonal rings distributed in a hexagonal graphene network [21,22]. For further verifying the promotion of the renascent MgO nanocrystals on graphitization, we designed and performed some other experiments. The reaction of glucose and Mg could happen at 300 C with the resulting product also exhibiting a high DG (Fig. 4a), though besides the HGCNSs, other morphologies such as nanotubes and nanosheets also present in the product (Fig. 4b). Meanwhile, some weak diffractions resulted from MgO could also be detected in the XRD pattern, and TEM examination confirms the presence of some MgO nanocrystals encapsulated in carbon shells (dark particles marked by arrows in the TEM image). When glucose was substituted by sucrose to react with Mg, an analogous promotion on graphitization also occurred. The XRD patterns of the resulting products obtained from the reaction of sucrose and Mg at
Fig. 6. XRD pattern (a) and FESEM image (b) of the product obtained by directly heating sucrose to 550 C and maintained for 8 h.
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Fig. 8. Galvanostatic charge/discharge curves (a) and cycling performance at a rate of 0.1 C (b), of the HGCNS electrodes.
450 and 200 C for 5 h are shown in Fig. 5. The strong diffractions resulted from carbon suggest the high DG of the resulting products. As a result, the newly formed MgO nanocrystals could certainly promote the graphitization of the simultaneously yielded carbon even at comparatively low temperatures. In terms of the formation of MgO, a redox reaction happens between Mg and glucose or sucrose, which could be simply expressed as follows. 6 Mg þ C6H12O6 ¼ 6MgO þ 6C þ 6H2 Q ¼ 2337.68 kJ mol1) (DrGQ ¼ 2507.02 kJ mol1, DrHm m ¼ 11MgO þ 12C þ 11H2 11 Mg þ C12H22O11 1 1 Q (DrGQ m ¼ 4720.57 kJ mol , DrHm ¼ 4394.03 kJ mol ) Similarly, the reaction between Zn and glucose could be expressed by the following equation. 6Zn þ C6H12O6 ¼ 6ZnO þ 6C þ 6H2 1 1 Q (DrGQ m ¼ 998.74 kJ mol , DrHm ¼ 814.58 kJ mol ) It is apparent that the reaction between Mg and sucrose is more spontaneous with larger heat release than that between Mg and glucose, and both of them easily happen at lower temperatures than that between Zn and glucose, i.e. 200 C for the reaction of Mg and sucrose, 300 C for the reaction of Mg and glucose, and higher than 400 C for the reaction of Zn and glucose. Once the reaction between Mg and glucose or sucrose initiates, the rapidly released heat energy could result in a temperature rise in autoclaves and the simultaneous formation of numerous MgO nanocrystals and H2 bubbles. As discussed in our previous work [26] and other literature [29e31], gas bubbles could act as templates for hollow spheres. According to TEM examination on the products (Fig. 3), besides the template of MgO nanocrystals, the H2 bubbles are the dominant templates for the formation of the HGCNSs. Directly heating glucose to 550 C and maintained for 8 h produced irregular carbon blocks and platelets other than the HCSs (Fig. 6), demonstrating that the reaction between Mg and glucose to yield MgO nanocrystals is crucial for the formation of the HGCNSs. Based on the above discussion, the formation of the HGCNSs could be simply depicted by the schematic in Fig. 7. 3.2. Electrochemical performance of the HGCNSs According to some recent reports [32e34], graphene could exhibit excellent performance as anode materials for Li-ion batteries due to the unique structure. The electrochemical performance of the HGCNSs as anode materials was also measured. Fig. 8(a) shows the discharge/charge curves of the HGCNS electrode for the first four cycles at a rate of 0.1 C. The HGCNSs exhibit a reversible capacity of 423 mAh g1 in the first cycle, higher than the theoretical capacity of graphite (372 mAh g1). Though an irreversible capacity of 311 mAh g1 occurs during the first discharge and charge process, the coulombic efficiency is above 95% after 5 cycles. The initial irreversible capacity loss has been ascribed to the decomposition of the electrolyte and the formation of a solid
electrolyte interface film in carbon anode materials [35,36]. Fig. 8(b) is the cycling performance of the HGCNS electrode during 60 cycles. It can be seen that the capacity hardly decreases after 10 cycles, demonstrating an excellent cyclic performance and reversibility of the HGCNS electrode. After 60 cycles, the electrode still maintains a reversible capacity of 391 mAh g1 which is still higher than the theoretic capacity of graphite, and the efficiency reaches 98% or so. As a consequence, the HGCNSs as anode materials for Li-ion batteries could exhibit superior performance to graphite [37]. The enhanced performance associates significantly with the unique structure of the HGCNSs. The hollow structure could buffer the volume change during charging/discharging, the thin wall of a few graphenes could shorten the pathway both for electron and Liþ transport, and the high DG could endow the HGCNSs excellent electric conductivity. 4. Conclusions HGCNSs could be prepared on large scale by the simple reaction between glucose and Mg at 550 C. The renascent MgO nanocrystals promote the graphitization of the simultaneously yielded carbon. The HGCNSs as anode materials for Li-ion batteries could exhibit superior electrochemical performance to graphite owing to the unique hollow structure with a high DG and a wall thickness of a few graphenes. This simple, effective and eco-friendly approach could offer an important alternative to produce HGCNSs for other applications, such as catalyst supports, gas storage media, supercapacitors. Acknowledgments This work was supported by Independent Innovation Foundation of Shandong University, IIFSDU (2012ZD004), Open Project from State Key Laboratory of Crystal Materials (KF1105), the National Natural Science Foundation of China (no. 50972076 and 50872072), the Shandong Provincial Natural Science Foundation, China (Y2008F26 and Y2008F40), and Shandong Provincial Doctoral Foundation, China (BS2010CL013). References [1] S.J. Tans, M.H. Devoret, H.J. Dai, A. Thess, R.E. Smalley, L.J. Geerligs, Nature 386 (1997) 474e477. [2] O.O. Adisa, B.J. Cox, J.M. Hill, J. Phys. Chem. C 115 (2011) 24528e24533. [3] L.M. Gomez, A. Kumar, Y. Zhang, K. Ryu, A. Badmaev, C.W. Zhou, Nano Lett. 9 (2009) 3592e3598. [4] K. Liu, Y.H. Sun, X.Y. Lin, R.F. Zhou, J.P. Wang, S.S. Fan, ACS Nano 4 (2010) 5827e5834. [5] S. Flandrois, B. Simon, Carbon 37 (1999) 165e180.
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