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Preparation of hollow carbon nanospheres via explosive detonation
Materials Letters 63 (2009) 58–60
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of hollow carbon nanospheres via explosive detonation Zuo-Shan Wang a,⁎, Feng-sheng Li b a b
College of Material Engineering, Suzhou University, Suzhou, 215021, China National Special Superfine Powder Research Center of Engineering and Technology, Nanjing University of Sci. and Tech., Nanjing, 21009, China
a r t i c l e
i n f o
Article history: Received 10 June 2008 Accepted 1 September 2008 Available online 9 September 2008 Keywords: Nanomaterials Hollow carbon nanosphere Powder technology Negative-oxygen balance Detonation
a b s t r a c t Hollow carbon nanospheres were prepared via a rapid detonation technique, by using negative-oxygen balance explosive trinitrotoluene and nickel powder as starting materials and inorganic acid as solvent. The carbon/metal nanocomposite particles precursor with core–shell structure was engendered firstly during detonation, and then the metal nickel core was dissolved through inorganic acid to attain the hollow carbon nanospheres. High-Resolution Transmission Electron Microscope, X-ray diffraction and Raman spectrum were used to characterize the precursor and the as-synthesized samples respectively. The results show that the external diameter of the hollow carbon nanospheres is 25–150 nm and the thickness of the wall is about 2–10 nm. The surface of hollow carbon nanosphere displays multilayer wall in structure with 0.35 nm space between the layers. Based on the experimental results, possible formation mechanism was also proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the past several years, there has been an intense interest in the novel carbon nanostructures such as fullerenes, carbon nanotubes (CNTs), graphitic nanocones and hollow carbon nanospheres (HCSs) because of their unique physical and chemical properties and various potential applications [1,2]. Among these novel carbon nanostructures, HCSs have been applied widely in many fields due to their low density, large surface area, stability, and surface permeability [3,4]. Different methods have been proposed for the synthesis of HCSs, including layer-by-layer (LbL) coating template process [5], reductioncatalysis [6], hydrothermal method via high temperature [7], selfassembly template process [8], one-step hydrolysis process [9], etc. However, templates, catalysts, organic solvent and longer reaction time are always required for most of these methods. Especially in selfgenerated template route [8] and one-step hydrolysis process [9], elevated temperature above 700 °C is also indispensable. Therefore, it is essential to develop more effective and more controllable method to synthesize HCSs. Detonation technology is well-known as an effective technology for producing nano-diamond and graphite nanosphere in a short time [10,11]. Recently detonation technology has also been reported to prepare some common nanomaterials such as ferric oxide, alumina, silica, etc [12,13]. However, there have been few reports about the preparation of HCSs via detonation technology so far [14,15], in which the C60 was used as raw material. In this paper, HCSs were synthesized rapidly using metallic Ni powder and negative-oxygen balance
⁎ Corresponding author. Tel.: +86 512 67244493; fax: +86 512 67246786. E-mail address: [email protected] (Z.-S. Wang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.09.006
explosive as raw material and inorganic acid as solvent. The carbon/ metal nanocomposite particles precursor with core–shell structure was engendered firstly through detonation in a very short time (several micro seconds), and then followed by treatment in concentrated inorganic acid to attain the HCSs with diameter of 25–150 nm and wall thickness of 2–10 nm. Based on the experimental results, the possible formation mechanism of the HCSs was also proposed. In addition, it is very important that the detonation process must be executed in a steel chamber resisting high temperature (3000–3500 K) and high pressure (18–25 GPa) and the blast shelter must be collocated. 2. Experimental All reagents were commercially available and used without further purification. In a typical experiment, 76.25 g TNT powder and 12.15 g metallic Ni powder were mixed uniformly and the mixture was loaded into a mould to be pressed into a column shape with 1.60 g/cm3 density. Then the column mixture was put into a 1 dm3 stainless autoclave, which was specially-designed according to the temperature and pressure during detonation. After filling 500 cm3 water, the autoclave was sealed and the column mixture was ignited through the detonator outside. The detonation resultant was collected and filtered with 230 mesh screen to remove the coarse impurity. The filtrate was filtered again after depositing for 5 h to obtain the dark precursor. After treatment in concentrated inorganic acid for 1 h at 80 °C, 20.25 g dark solid was obtained by filtrating, washing with absolute ethanol, NaOH solution (about 3 mol/dm3) and distilled water, and drying at 60 °C for 5 h. Morphological feature and particle size of the products were examined by JEM-2100 High-Resolution Transmission Electron Microscope (HRTEM), using an accelerating voltage of 200 kV. Crystalline phase was determined by Shimadzu XRD-6000 with CuKα radiation
Z.-S. Wang, F. Li / Materials Letters 63 (2009) 58–60
59
Fig. 3. Raman Spectrum of HCSs.
Fig. 1. X-ray diffractions: (a) XRD of the precursor; (b) XRD of the as-synthesized HCSs.
(λ = 0.15418 nm) and the scanning rate of 0.05° s− 1 was set up to record the pattern in the 2θ range of 20–80°. Laser Common Focus Raman Spectrum Instrument Renishaw-1000 was used to analyze the graphite crystal degree and purity under ambient temperature, employing an argon ion laser at an excitation wavelength of 514.5 nm. 3. Results and discussion Fig. 1 illustrates the XRD patterns of the precursors and the HCSs samples. The strong diffraction peaks in Fig. 1a can be indexed as 111 and 200 of a standard metallic Ni crystal (JCPDS). Although the crystallographic structure of the carbon shell cannot be identified obviously because of the effect of the instrument background noise, fortunately, the XRD pattern of the HCSs as shown in Fig. 1b displays the presence of
reflections characteristic of carbon crystal phase. Two broad diffraction peaks at about 26.2° and 43.0° can be related to inter-layer stacking and correlations within an individual layer respectively, the strong 002 for inter-layer correlations and the weak 101 typical for two-dimensional diffraction of the turbostratic type. Compared with the reported data of graphite (JCPDS, No. 41–1487), the positions of 002 and 101 were inconsistent, which proves further that the HCSs are turbostratic carbons but not graphite. The other small peaks like sawtooth spread all over the whole diffraction curve, which can be attributed to the effect of the instrument background noise and the special nano-sized effect. The d-spacing of 002 of the as-synthesized HCSs is about 9.71 nm according to Scherrer's equation (seen Fig. 1b). The small size resulting to the weak diffraction peak intensity and the broaden effect is obvious. Therefore from the diffraction pattern shown in Fig. 1b, it is suggested about a very disordered structure of the carbon spheres in the investigated objects. The typical HRTEM micrographs of the as-synthesized HCSs are presented in Fig. 2. From Fig. 2(a) we can see that the external diameter of the HCSs is about 25–150 nm and the thickness of the wall is about 2–10 nm. The dark/light contrast clearly reveals the hollow nanosphere nature. Based on large numbers of TEM observation, the proportion of integrate HCSs in the samples is estimated to be about 80% and there are some carbon nanocapsules and cracked HCSs coexisting with the HCSs. The typical HRTEM image of the HCSs in high magnification shown in Fig. 2(b) indicates that the wall of the HCS is a kind of multilayer structure and the estimated value of the inter-layer spacing was about 0.35 nm, apparently larger than that of the graphite (0.335 nm). Such a value is typical for turbostratic carbons or carbon nanotubes [16]. In the graphite structure carbon atoms are arranged in the perfect hexagonal network within individual layers, which are stacked according to the –ABAB– sequence. It is well-known that the formation of any nanosphere requires the presence of pentagons as in the case of fullerenes. So the difference of the inter-layer spacing between HCSs and graphites is understandable. Fig. 2(c) reveals the further amplificatory surface structure of HCSs, on which some parallel crystal lattice stripes are very evident, indicating the carbon is well crystallized. The subsequent Laser Common Focus Raman Spectrum in Fig. 3 approves the crystallographic structure and purity of the HCSs. Fig. 3 shows that there exist two wide peaks at 1363.64 cm− 1 (D-band) and 1579.04 cm− 1 (G-band). The peak at 1579.04 cm− 1 corresponds to an E2g mode of graphite, related to the vibration of sp2-bonded carbon
Fig. 2. HRTEM micrographs of the as-synthesized HCSs: (a) HRTEM of the HCSs; (b) higher magnification HRTEM of individual HCS; (c) HRTEM of the wall of HCSs.
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Z.-S. Wang, F. Li / Materials Letters 63 (2009) 58–60 nickel micro-liquid drops surface. Further decreasing temperature makes nickel microliquid drops transform into solid and the precursors with shell–core structure form. After metal nickel is dissolved away in inorganic acid, well-dispersed HCSs are attained. Fig. 4 illustrates the schematic diagram.
4. Conclusions In conclusion, HCSs were synthesized by an abbreviated technology route in a very short time and possible formation mechanism was proposed based on the experimental results. The as-synthesized HCS had small size and perfect disperse stability. There was no need for additional catalyst and organic solvent and there was no need heating from the external environment in the whole process. The proportion of the nickel powder to negative-oxygen balance explosive has important effect on the formation of well-dispersed HCSs. This technique can also be extended to produce other nanomaterials. Fig. 4. The sketch map of formation mechanism of HCSs. atoms in a two-dimensional hexagonal lattice, such as in a graphite layer, which is consistent with the reported data [7]. The D-peak at 1363.64 cm− 1, related to a resonant Raman process, is indication of heavy disorder that is in agreement with the observed diffraction pattern (Fig. 1(b)). Two broad features could be seen at about 305 cm− 1 and 785 cm− 1 in Fig. 3, which can be related to fullerene-like modes, for graphite is silent in this range [17]. The presented Raman data provides further evidence for the presence of very disordered structure in the investigated material. It is well-known that the detonation of explosive is a very complex and rapid chemistry process (about 2–3 ms), accompanying with the production of violent shock wave, super-high temperature (3000–3500 K) and super-high pressure (18–25 GPa) [18]. At the beginning of the reaction, the starting materials including explosive and Ni powder are fragmented to free atoms or ions. Then, these freshly formed free atoms or ions are rebuilt according to the combining intensity between different atoms and ions. According to H.J.T. Ellingham plot [19], metal Ni can not be oxidized under super-high temperature (3000–3500 K) because ΔrGθm value is negative. Therefore, no metal Ni takes part in reaction process practically and only phase change occurs between solid, gas and solid. So the whole detonation reaction equation can be shown as the following: C7 H5 O6 N3 →2:5H2 O þ 3:5CO þ 3:5C þ 1:5N2 Contrasting the detonation temperature (3246 K) and the boiling point of metal nickel (3073 K) and carbon (4273 K), it is easy to find that metal nickel is in gas state and carbon is in “similar gas state” at the beginning of detonation. When temperature decreases under cooling medium, metal Ni atoms transform from gas state to liquid state and form into micro-liquid drops. At the same time, “similar gas state” dissociative carbons transform into solid carbons and are adsorbed on the surface of nickel microliquid drops layer upon layer to prevent the agglomeration of nickel micro-liquid drops. Subsequently, the adsorbed solid carbons form into small crystal core and cover the
Acknowledgement This work was supported by the postdoctoral Science Foundation of Jiangsu Province. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Yan CM, Luo YJ, Zhao XP. Funct Mater 2006;37:342–7. Maser WK, Benito AM, Martinez MT. Carbon 2002;40:168–74. Shim M, Javey A, Kam NWS, Dai HJ. J Am Chem Soc 2001;123:11512–7. Tamai H, Sumi T, Yasuda H. J Colloid Interface 1996;177:325–31. Liu JW, Shao MW, Tang Q, Chen XY, Liu ZP, Qian YT. Carbon 2003;41:1682–9. Du JM, Kang DJ. Mater Res Bull 2006;41:1785–9. Ni YB, Shao MW, Tong YH, Qian GX, Wei XW. J Solid State Chem 2005;178:908–14. Zou GF, Yu DB, Lu J, Wang DB, Jiang CL, Qian YT. J Solid State Commun 2004;131:749–55. Song YY, Li Y, Xia XH. Electrochem Commun 2007;9:201–7. Yamada K, Swaaoka AB. Carbon 1994;34:21–8. Lu Y, Zhu ZP, Liu ZY. New Carbon Mater 2004;19:1–7. Zheng M, Wang ZS. J Chin Silicate Soc 2005;33:930–6. Li XJ, Qu YD, Xie XH, Wang ZL, Li RY. Mater Lett 2006;60:3673–7. Niwase K, Homae T, Nakamura b KG, Kondo K. Chem Phys Lett 2002;362:47–50. Homae T, Nakamura KG, Kondo K, Niwase K. Solid State Commun 2002;122:69–71. Pimenta MA, Dresselhaus G, Dresselhaus MS, Endo M. Phys Rev B 1999;59:6585–9. Ferrari AC, Robertson J. Phys Rev B 2000;61:14095–9. Zhu YZ, Gu D, Hei SC. Modern Basic Chemistry, Beijing: Chemistry Industry Publishing Company; 2004. p. 433-410. [19] Dean JA, editor. Lange's Handbook of Chemistry. 15th ed. New York: McGraw-Hill; 1998. p. 429–31.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of hollow carbon nanospheres via explosive detonation Zuo-Shan Wang a,⁎, Feng-sheng Li b a b
College of Material Engineering, Suzhou University, Suzhou, 215021, China National Special Superfine Powder Research Center of Engineering and Technology, Nanjing University of Sci. and Tech., Nanjing, 21009, China
a r t i c l e
i n f o
Article history: Received 10 June 2008 Accepted 1 September 2008 Available online 9 September 2008 Keywords: Nanomaterials Hollow carbon nanosphere Powder technology Negative-oxygen balance Detonation
a b s t r a c t Hollow carbon nanospheres were prepared via a rapid detonation technique, by using negative-oxygen balance explosive trinitrotoluene and nickel powder as starting materials and inorganic acid as solvent. The carbon/metal nanocomposite particles precursor with core–shell structure was engendered firstly during detonation, and then the metal nickel core was dissolved through inorganic acid to attain the hollow carbon nanospheres. High-Resolution Transmission Electron Microscope, X-ray diffraction and Raman spectrum were used to characterize the precursor and the as-synthesized samples respectively. The results show that the external diameter of the hollow carbon nanospheres is 25–150 nm and the thickness of the wall is about 2–10 nm. The surface of hollow carbon nanosphere displays multilayer wall in structure with 0.35 nm space between the layers. Based on the experimental results, possible formation mechanism was also proposed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the past several years, there has been an intense interest in the novel carbon nanostructures such as fullerenes, carbon nanotubes (CNTs), graphitic nanocones and hollow carbon nanospheres (HCSs) because of their unique physical and chemical properties and various potential applications [1,2]. Among these novel carbon nanostructures, HCSs have been applied widely in many fields due to their low density, large surface area, stability, and surface permeability [3,4]. Different methods have been proposed for the synthesis of HCSs, including layer-by-layer (LbL) coating template process [5], reductioncatalysis [6], hydrothermal method via high temperature [7], selfassembly template process [8], one-step hydrolysis process [9], etc. However, templates, catalysts, organic solvent and longer reaction time are always required for most of these methods. Especially in selfgenerated template route [8] and one-step hydrolysis process [9], elevated temperature above 700 °C is also indispensable. Therefore, it is essential to develop more effective and more controllable method to synthesize HCSs. Detonation technology is well-known as an effective technology for producing nano-diamond and graphite nanosphere in a short time [10,11]. Recently detonation technology has also been reported to prepare some common nanomaterials such as ferric oxide, alumina, silica, etc [12,13]. However, there have been few reports about the preparation of HCSs via detonation technology so far [14,15], in which the C60 was used as raw material. In this paper, HCSs were synthesized rapidly using metallic Ni powder and negative-oxygen balance
⁎ Corresponding author. Tel.: +86 512 67244493; fax: +86 512 67246786. E-mail address: [email protected] (Z.-S. Wang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.09.006
explosive as raw material and inorganic acid as solvent. The carbon/ metal nanocomposite particles precursor with core–shell structure was engendered firstly through detonation in a very short time (several micro seconds), and then followed by treatment in concentrated inorganic acid to attain the HCSs with diameter of 25–150 nm and wall thickness of 2–10 nm. Based on the experimental results, the possible formation mechanism of the HCSs was also proposed. In addition, it is very important that the detonation process must be executed in a steel chamber resisting high temperature (3000–3500 K) and high pressure (18–25 GPa) and the blast shelter must be collocated. 2. Experimental All reagents were commercially available and used without further purification. In a typical experiment, 76.25 g TNT powder and 12.15 g metallic Ni powder were mixed uniformly and the mixture was loaded into a mould to be pressed into a column shape with 1.60 g/cm3 density. Then the column mixture was put into a 1 dm3 stainless autoclave, which was specially-designed according to the temperature and pressure during detonation. After filling 500 cm3 water, the autoclave was sealed and the column mixture was ignited through the detonator outside. The detonation resultant was collected and filtered with 230 mesh screen to remove the coarse impurity. The filtrate was filtered again after depositing for 5 h to obtain the dark precursor. After treatment in concentrated inorganic acid for 1 h at 80 °C, 20.25 g dark solid was obtained by filtrating, washing with absolute ethanol, NaOH solution (about 3 mol/dm3) and distilled water, and drying at 60 °C for 5 h. Morphological feature and particle size of the products were examined by JEM-2100 High-Resolution Transmission Electron Microscope (HRTEM), using an accelerating voltage of 200 kV. Crystalline phase was determined by Shimadzu XRD-6000 with CuKα radiation
Z.-S. Wang, F. Li / Materials Letters 63 (2009) 58–60
59
Fig. 3. Raman Spectrum of HCSs.
Fig. 1. X-ray diffractions: (a) XRD of the precursor; (b) XRD of the as-synthesized HCSs.
(λ = 0.15418 nm) and the scanning rate of 0.05° s− 1 was set up to record the pattern in the 2θ range of 20–80°. Laser Common Focus Raman Spectrum Instrument Renishaw-1000 was used to analyze the graphite crystal degree and purity under ambient temperature, employing an argon ion laser at an excitation wavelength of 514.5 nm. 3. Results and discussion Fig. 1 illustrates the XRD patterns of the precursors and the HCSs samples. The strong diffraction peaks in Fig. 1a can be indexed as 111 and 200 of a standard metallic Ni crystal (JCPDS). Although the crystallographic structure of the carbon shell cannot be identified obviously because of the effect of the instrument background noise, fortunately, the XRD pattern of the HCSs as shown in Fig. 1b displays the presence of
reflections characteristic of carbon crystal phase. Two broad diffraction peaks at about 26.2° and 43.0° can be related to inter-layer stacking and correlations within an individual layer respectively, the strong 002 for inter-layer correlations and the weak 101 typical for two-dimensional diffraction of the turbostratic type. Compared with the reported data of graphite (JCPDS, No. 41–1487), the positions of 002 and 101 were inconsistent, which proves further that the HCSs are turbostratic carbons but not graphite. The other small peaks like sawtooth spread all over the whole diffraction curve, which can be attributed to the effect of the instrument background noise and the special nano-sized effect. The d-spacing of 002 of the as-synthesized HCSs is about 9.71 nm according to Scherrer's equation (seen Fig. 1b). The small size resulting to the weak diffraction peak intensity and the broaden effect is obvious. Therefore from the diffraction pattern shown in Fig. 1b, it is suggested about a very disordered structure of the carbon spheres in the investigated objects. The typical HRTEM micrographs of the as-synthesized HCSs are presented in Fig. 2. From Fig. 2(a) we can see that the external diameter of the HCSs is about 25–150 nm and the thickness of the wall is about 2–10 nm. The dark/light contrast clearly reveals the hollow nanosphere nature. Based on large numbers of TEM observation, the proportion of integrate HCSs in the samples is estimated to be about 80% and there are some carbon nanocapsules and cracked HCSs coexisting with the HCSs. The typical HRTEM image of the HCSs in high magnification shown in Fig. 2(b) indicates that the wall of the HCS is a kind of multilayer structure and the estimated value of the inter-layer spacing was about 0.35 nm, apparently larger than that of the graphite (0.335 nm). Such a value is typical for turbostratic carbons or carbon nanotubes [16]. In the graphite structure carbon atoms are arranged in the perfect hexagonal network within individual layers, which are stacked according to the –ABAB– sequence. It is well-known that the formation of any nanosphere requires the presence of pentagons as in the case of fullerenes. So the difference of the inter-layer spacing between HCSs and graphites is understandable. Fig. 2(c) reveals the further amplificatory surface structure of HCSs, on which some parallel crystal lattice stripes are very evident, indicating the carbon is well crystallized. The subsequent Laser Common Focus Raman Spectrum in Fig. 3 approves the crystallographic structure and purity of the HCSs. Fig. 3 shows that there exist two wide peaks at 1363.64 cm− 1 (D-band) and 1579.04 cm− 1 (G-band). The peak at 1579.04 cm− 1 corresponds to an E2g mode of graphite, related to the vibration of sp2-bonded carbon
Fig. 2. HRTEM micrographs of the as-synthesized HCSs: (a) HRTEM of the HCSs; (b) higher magnification HRTEM of individual HCS; (c) HRTEM of the wall of HCSs.
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Z.-S. Wang, F. Li / Materials Letters 63 (2009) 58–60 nickel micro-liquid drops surface. Further decreasing temperature makes nickel microliquid drops transform into solid and the precursors with shell–core structure form. After metal nickel is dissolved away in inorganic acid, well-dispersed HCSs are attained. Fig. 4 illustrates the schematic diagram.
4. Conclusions In conclusion, HCSs were synthesized by an abbreviated technology route in a very short time and possible formation mechanism was proposed based on the experimental results. The as-synthesized HCS had small size and perfect disperse stability. There was no need for additional catalyst and organic solvent and there was no need heating from the external environment in the whole process. The proportion of the nickel powder to negative-oxygen balance explosive has important effect on the formation of well-dispersed HCSs. This technique can also be extended to produce other nanomaterials. Fig. 4. The sketch map of formation mechanism of HCSs. atoms in a two-dimensional hexagonal lattice, such as in a graphite layer, which is consistent with the reported data [7]. The D-peak at 1363.64 cm− 1, related to a resonant Raman process, is indication of heavy disorder that is in agreement with the observed diffraction pattern (Fig. 1(b)). Two broad features could be seen at about 305 cm− 1 and 785 cm− 1 in Fig. 3, which can be related to fullerene-like modes, for graphite is silent in this range [17]. The presented Raman data provides further evidence for the presence of very disordered structure in the investigated material. It is well-known that the detonation of explosive is a very complex and rapid chemistry process (about 2–3 ms), accompanying with the production of violent shock wave, super-high temperature (3000–3500 K) and super-high pressure (18–25 GPa) [18]. At the beginning of the reaction, the starting materials including explosive and Ni powder are fragmented to free atoms or ions. Then, these freshly formed free atoms or ions are rebuilt according to the combining intensity between different atoms and ions. According to H.J.T. Ellingham plot [19], metal Ni can not be oxidized under super-high temperature (3000–3500 K) because ΔrGθm value is negative. Therefore, no metal Ni takes part in reaction process practically and only phase change occurs between solid, gas and solid. So the whole detonation reaction equation can be shown as the following: C7 H5 O6 N3 →2:5H2 O þ 3:5CO þ 3:5C þ 1:5N2 Contrasting the detonation temperature (3246 K) and the boiling point of metal nickel (3073 K) and carbon (4273 K), it is easy to find that metal nickel is in gas state and carbon is in “similar gas state” at the beginning of detonation. When temperature decreases under cooling medium, metal Ni atoms transform from gas state to liquid state and form into micro-liquid drops. At the same time, “similar gas state” dissociative carbons transform into solid carbons and are adsorbed on the surface of nickel microliquid drops layer upon layer to prevent the agglomeration of nickel micro-liquid drops. Subsequently, the adsorbed solid carbons form into small crystal core and cover the
Acknowledgement This work was supported by the postdoctoral Science Foundation of Jiangsu Province. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
Yan CM, Luo YJ, Zhao XP. Funct Mater 2006;37:342–7. Maser WK, Benito AM, Martinez MT. Carbon 2002;40:168–74. Shim M, Javey A, Kam NWS, Dai HJ. J Am Chem Soc 2001;123:11512–7. Tamai H, Sumi T, Yasuda H. J Colloid Interface 1996;177:325–31. Liu JW, Shao MW, Tang Q, Chen XY, Liu ZP, Qian YT. Carbon 2003;41:1682–9. Du JM, Kang DJ. Mater Res Bull 2006;41:1785–9. Ni YB, Shao MW, Tong YH, Qian GX, Wei XW. J Solid State Chem 2005;178:908–14. Zou GF, Yu DB, Lu J, Wang DB, Jiang CL, Qian YT. J Solid State Commun 2004;131:749–55. Song YY, Li Y, Xia XH. Electrochem Commun 2007;9:201–7. Yamada K, Swaaoka AB. Carbon 1994;34:21–8. Lu Y, Zhu ZP, Liu ZY. New Carbon Mater 2004;19:1–7. Zheng M, Wang ZS. J Chin Silicate Soc 2005;33:930–6. Li XJ, Qu YD, Xie XH, Wang ZL, Li RY. Mater Lett 2006;60:3673–7. Niwase K, Homae T, Nakamura b KG, Kondo K. Chem Phys Lett 2002;362:47–50. Homae T, Nakamura KG, Kondo K, Niwase K. Solid State Commun 2002;122:69–71. Pimenta MA, Dresselhaus G, Dresselhaus MS, Endo M. Phys Rev B 1999;59:6585–9. Ferrari AC, Robertson J. Phys Rev B 2000;61:14095–9. Zhu YZ, Gu D, Hei SC. Modern Basic Chemistry, Beijing: Chemistry Industry Publishing Company; 2004. p. 433-410. [19] Dean JA, editor. Lange's Handbook of Chemistry. 15th ed. New York: McGraw-Hill; 1998. p. 429–31.