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SiO2 core–shell nanocables
Journal of Alloys and Compounds 462 (2008) 446–451
Simultaneous growth of SiC nanowires, SiC nanotubes, and SiC/SiO2 core–shell nanocables Baosheng Li, Renbing Wu, Yi Pan ∗ , Lingling Wu, Guangyi Yang, Jianjun Chen, Qimaio Zhu Department of Materials Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People’s Republic of China Received 23 July 2007; received in revised form 21 August 2007; accepted 26 August 2007 Available online 4 September 2007
Abstract SiC nanowires, SiC/SiO2 core–shell nanocables, and SiC nanotubes have been synthesized simultaneously by direct heating Si powders and multiwall carbon nanotubes (MWCNTs). The as-obtained SiC nanowires are generally 100 nm in diameter and several tens of micrometers in length, the nanocables consist of a 20–30 nm diameters single-crystalline SiC core covered by a uniform layer of about 20 nm thick amorphous SiO2 , and the nanotubes with very narrow hollow channel have outer diameters of about 20 nm. The characteristics of the products are analyzed by various methods, results of which indicating that temperature and ambience are two key factors for the formation of the three different products; their possible growth mechanisms are also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Gas–solid reactions; Microstructure; SEM; TEM
1. Introduction Fabrication of one-dimensional (1D) nanoscale materials with specific size, morphology, and structure has attracted considerable and intensive research interests due to their importance in understanding the fundamental properties of 1D nanomaterial. They are potential building blocks and devices with designed functions in areas as diverse as electronics, optics, catalysis, and ceramics [1–3]. The synthesis of SiC nanomaterials is of particular interest since they could be promising candidates for fabricating electronic, optic, and nanocomposites that can be operated at extreme environment such as high temperature, high power and radiation, and corrosive conditions [4–6]. Many synthetic approaches for preparing various SiC nanostructures such as nanowires [7,8], nanotubes [9,10], hollow nanospheres [11], nanoflowers [12], nanosprings [13], etc. have been reported, including chemical vapor deposition (CVD) [14], carbon thermal reduction [15], laser ablation [16], arc discharge [17], combustion [18] and so on. However, simultaneous growth
∗
Corresponding author. Tel.: +86 571 87953008; fax: +86 571 87953008. E-mail address: [email protected] (Y. Pan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.076
of two or more kinds of SiC nanostructures in a single processing step has rarely been reported. In this paper, we demonstrate a simple way to grow SiC nanowires, SiC/SiO2 nanocables, and SiC nanotubes simultaneously through a one-step direct heating method. The obtained SiC nanostructures were characterized by various methods. The characterizations reveal that ambience and temperature are key factors for the formation of different structures. Furthermore, based on the reaction conditions and characterizations, the formation mechanisms of these nanostructures are also proposed. 2. Experimental SiC nanowires, SiC/SiO2 core–shell nanocables, and SiC nanotubes were synthesized via thermal chemical vapor transportation and condensation in a conventional vertical graphite furnace. High-purity silicon powder (99.999%, Aldrich) was first put into a graphite crucible (outer diameter 80 mm, inner diameter 64 mm, length 30 mm), then an alumina grid was put on the crucible, over which a certain amount of multiwall carbon nanotubes (MWCNTS) was loosely placed, and a graphite ring with the same diameter was covered on the alumina grid, at last an alumina top was put on the graphite ring. The tube furnace was first evacuated to 10−2 Torr by a mechanical rotary pump, whereafter Ar (99.99%) was introduced till the positive pressure of ∼10 Torr. The furnace was heated to 1450 ◦ C at 30 ◦ C/min and kept at this temperature for 1 h. The whole
B. Li et al. / Journal of Alloys and Compounds 462 (2008) 446–451 heating and cooling process was program-controlled. After the reaction, gray product was formed on the surface of the alumina grid; meanwhile, blue-white cotton like product was also obtained on the inner surface of the graphite ring and the crucible. The collected product was characterized by a field-emission scanning electron microscopy (FE-SEM, FEI-SIRION100, operated at 5 kV), equipped with energy dispersive spectrometry (EDS). The X-ray diffraction patterns of the products were measured on a Rigaku, Geigerflex/D using Cu K␣ ˚ FI-TR transmittance spectra measurements were perradiation (λ = 1.5406 A). formed using Bruker Vector 22. Transmission electron microscope (HRTEM) imaging and selected area electron diffraction (SAED) (JEM-2010, HR, operated at 300 kV accelerating voltage) was used to characterize the internal structures of SiC nanostructures. Samples for the TEM were prepared by ultrasonically dispersing the products into absolute ethanol for 5 min, and then a drop of the suspension containing the products was dropped on a copper grid coated with an amorphous carbon film and then dried in the air.
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3. Results and discussion 3.1. Digital photo, FE-SEM, and EDS investigation A top-view digital photograph of as-synthesized grayish product formed on the alumina substrate is shown in Fig. 1a, evidencing that the black CNTS reacted with Si and the reaction was thorough. Fig. 1b shows the FE-SEM image of original MWCNTs, demonstrating that the random curve and tubular structures have diameters in range of 20–30 nm and lengths up to several micrometers. Fig. 1c presents a typical FE-SEM image of the products illustrating that the formed straight wire-like structures are about 50–100 nm in diameter and lengths up to several tens micrometers. The high magnification FE-SEM image (inset in
Fig. 1. (a) Top-view digital photograph of as-synthesized pale grayish product formed on the alumina substrate; (b) SEM image of original MWCNTs; (c) SEM image of the nanowires obtained on the alumina grid and (d) is the corresponding EDS spectra; (e) SEM image of the worm-like structures found on the alumina grid and (f) shows the corresponding EDS spectra.
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Fig. 2. (a) Top-view digital photograph of the wool-like products on the inner wall of the graphite crucible; (b) SEM image of these products and (c) shows the high-magnification SEM image; (d) is the corresponding EDS spectra.
Fig. 1c) suggests that the obtained nanowires have a smooth and clean surface. EDS spectra of these nanowires is given in Fig. 1d. The EDS spectra indicate that these wires are composed of Si and C elements; the weak Au peak comes from the supporting Au film. The atomic ratio of Si:C is about 48.4:51.6, and this composition is very close to stoichiometric SiC. It was surprisingly found that there are also worm-like structures in the grayish products as shown in Fig. 1e. Compared to MWCNTs, the diameter of these worm-like structures is larger and the surface is rougher. Magnified FE-SEM image (inset in Fig. 1e) indicates that the tube-like structures with diameter of about 50 nm and 1 m in length have both ends closed. Fig. 1f is a typical EDX spectrum taken from a single nanotube. Only C and Si are present in the spectrum and their atomic ratio is 45.5:54.5 ≈ 1:1, suggesting that this tuber-like structure is, most possibly, SiC nanotube. Besides the products synthesized on the alumina substrate, wool-like product was also obtained on the inner wall of the graphite crucible, simultaneously. Fig. 2a shows a top-view digital photograph of the as-grown blue-white products covered on the inner wall of the graphite crucible. FE-SEM image of this product is shown in Fig. 2b. Bulk quantity and high-purity straight wire structures with lengths up to tens even hundreds of micrometers are observed clearly. The high-magnification FESEM image (Fig. 2c) suggest that the wire-like structures with diameters in the range of 50–80 nm have very clean and smooth
surface. No obvious particles could be found attached on the surface. A typical EDS spectrum taken from these nanowires, as shown in Fig. 2d, suggest that the products contain Si, C, and O in a 47.3:40.2:12.5 atomic ratio. The three elements coexisting strongly suggest that the as-prepared samples may be formed SiC/SiO2 core–shell nanocables, which is also proved by later FI-TR and TEM observations.
3.2. FI-TR and XRD analysis Fig. 3a is the FTIR spectrum of SiC/SiO2 core–shell nanocables, which shows several absorption bands from Si–O stretching vibrations (1103 cm−1 ), Si–O–Si bending vibrations (471 cm−1 ), transversal optic (TO) mode Si–C vibrations (793 cm−1 ), and longitudinal optic (LO) mode Si–C vibrations (945 cm−1 ), respectively, indicating that nanocables are composed of SiC and amorphous SiO2 . X-ray diffraction analysis confirms the as-grown SiC nanowires are 3C–SiC, as illustrated in Fig. 3b; there are five peaks in the spectrum agreeing well with (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) diffraction peaks of 3C–SiC. A small peak marked SF, corresponding to stacking faults, is also shown in the spectrum [19]. XRD pattern of the synthesized SiC/SiO2 nanocables shows no difference except broad diffraction peak compared to SiC nanowires, indicating the amorphous structure of the SiO2 sheath.
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image of original MWCNTs and Fig. 4f is the TEM image of some SiC nanotube found in the product of the reacted MWCNTs. It clearly demonstrate that compared to original MWCNTs, the diameters of SiC nanotubes is larger and the surface is rougher. 3.4. Growth process and mechanism
Fig. 3. (a) FTIR spectrum of SiC/SiO2 core–shell nanocables; (b) XRD pattern taken from the synthesized SiCNWs.
3.3. TEM, SAED and EDS characterization To have more details about the structure and crystallinity of the SiC nanostructures, TEM and SAED measurements were performed and the results are shown in Fig. 4. Fig. 4a shows that the SiC nanowires converted from MWCNTs have a metal catalyst particle end, indicating the SiC nanowires growth via VLS mechanism; Ni, Si and C are detected in the spherical end by EDS analysis. Also, we can see that the SiC nanowire possess a high density of planner defects and stacking faults which are perpendicular to the wire axis. The SAED pattern along [0 1 1] zone axis further demonstrates that it is a phase pure 3C–SiC having defects causing the spots streaking. Fig. 4c shows the TEM image and SAED pattern of the nanocable grown on the crucible wall, from which a structure characterized with SiC crystalline core and amorphous SiO2 shell is clearly demonstrated. The SAED patterns also shows that the crystalline SiC core has stacking faults and twins similar to that from SiC nanowires as shown in Fig. 4a. The core is about 30 nm in diameter and the amorphous SiO2 sheath is approximately 25 nm in thickness. Fig. 4d displays the HRTEM image of a SiC nanowire, in which well-defined fringe separation of 0.25 nm is consistent with the d-spacing of (1 1 1) plane, suggesting that the growth direction is [1 1 1]. Fig. 4e is the TEM
By such a simple heating method, SiC nanotubes, SiC nanowires and SiC/SiO2 core–shell nanocables were synthesized simultaneously. However, the formation mechanisms should be different. SiC nanotubes and SiC nanowires were produced on the alumina grid over which MWCNTs were originally placed. Si in the graphite crucible was melted at 1450 ◦ C. It is believed that at the same temperature and the same ambience, the difference of the product obtained on the alumina grid is because of the difference in the original MWCNTs. That is, some of the original MWCNTs contain residue nickel particles as catalyst for their growth, and others contain no nickel particles. Consequently, we propose two different growth mechanisms for the transformation of the MWCNTs on the alumina grid. One is metal-catalyzed VLS mechanism for the transformation to SiC nanowires. In this process, the residue nickel particles could play a catalyst role again. As the temperature increases, the nickel particle would melt and provide an energetically favored site for the absorption of Si vapor, which acted as nucleation sites. In this way, SiC nanowires were obtained at last. The other is VS mechanism for the transformation to SiC nanotubes. In this process, Si atoms first reacted with C atoms on the surface of MWCNTs, and then formed SiC layer by layer, the diffusion mechanism supply Si resources [9], thus result in the worm-like tubular structures. We also believe this structure is only an intermediate [20], and if heated with a higher temperature or for a longer time, it would transform to SiC nanowires or nanorods [20–23]. For the formation of SiC/SiO2 core–shell nanocables on the inner wall of the graphite crucible, a double-VS mechanism was proposed, the first VS step is the growth of SiC nanowires, Si vapor deposits on the inner surface of the graphite crucible and generate many nucleation sites, some of these nucleation sites grow up to SiC nanowires in the end. This procedure occurs during the calefactive and holding stage. The second VS step is the deposition of SiC and SiO2 on the surface of the as-grown SiC nanowires; this procedure takes place in the cooling stage. With temperature decrease, reaction (1), (2) would occur and lead to a decrease in enthalpy and Gibbs energy [24–26], which is thermodynamically favorable at low temperature. Consequently, SiO2 and SiC deposits onto the surface of the grown-up SiC nanowires, by reason of the difference of density and the good fluidity of SiO2 at this temperature, these two materials can be separated easily during growth and thus form SiC/SiO2 nanocables in the end. 3SiO(g) + CO(g) → SiC(s) + 2SiO2 (s)
(1)
SiO(g) + 1/2O2 (g) → SiO2 (s)
(2)
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Fig. 4. (a) TEM of a SiC nanowires and its SAED pattern; (b) EDS spectra of the spherical end; (c) TEM of a SiC/SiO2 nanocable and its SAED pattern; (d) HRTEM of a SiC nanowire; (e) TEM of the original MWCNTs; (f) TEM of a SiC nanotube.
A question is why only the products on the inner wall grown to SiC/SiO2 composites but the products on the alumina grid remained SiC nanowires. Considering the temperature distribution, we can find out the answer; detailed description is given as follows. Fig. 5 displays the cooling process of the furnace. In the cooling stage, the temperature distribution in the furnace room is not uniform due to the cooling water flowing to surround the furnace room. The inner wall of the graphite crucible has a faster cooling rate than elsewhere and could firstly reach the reaction temperature for the deposition of SiO2 and then always be prior to form SiC/SiO2 nanocables. Consequently, SiC/SiO2 nanocables were obtained on the inner wall of the graphite crucible while there were only SiC nanowires and SiC nanotubes on the alumina grid.
Fig. 5. Illustration of the cooling process.
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4. Conclusion In summary, by a simple method of direct heating Si powders and MWCNTs, we obtained SiC nanotubes and SiC nanowires on the alumina grid, but SiC/SiO2 core–shell nanocables on the inner wall of the graphite crucible. Temperature and ambience are two key factors for the formation of different structures. How to choose suitable conditions to actualize the controlled growth of various 1D SiC nanostructures will be further investigated in the future. References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] X.F. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 13 (2001) 526. [3] M.H. Huang, S. Mao, H. Feick, Y. Wu, H. Kind, E. Weiber, R. Russo, P. Yang, Science 292 (2001) 1897. [4] W.Q. Han, S.S. Fan, Q.Q. Li, B.L. Gu, D.P. Yu, Chem. Phys. Lett. 265 (1997) 374. [5] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [6] K.W. Wong, X.T. Zhou, F.C.K. Au, H.L. Lai, C.S. Yu, Chem. Phys. Lett. 265 (1997) 374. [7] Z. Pan, H.L. Lai, F.C.K. Au, X. Duan, W. Zhou, W. Shi, Adv. Mater. 12 (2000) 1186. [8] Y. Baek, Y.H. Ryu, K. Yong, Mater. Sci. Eng. C 26 (2006) 805. [9] Y. Zhang, T. Ichihashi, E. Landree, S. Nihey, Iijima, Science 285 (1999) 1719.
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[10] E. Borowiak-Palen, M.H. Ruemmeli, T. Gemming, M. Knupfer, K. Biedermann, A. Leonhardt, J. Appl. Phys. 97 (2005) 056102. [11] G.Z. Shen, D. Chen, K.B. Tang, Y.T. Qian, S.Y. Zhang, Chem. Phys. Lett. 375 (2003) 177. [12] D. Zhang, A. Alkhateeb, H. Han, H. Mahmood, D.N. Mcllroy, M.G. Norton, Nano Lett. 3 (2003) 983. [13] G.W. Ho, A.S.W. Wong, D.J. Kang, M.E. Welland, Nanotechnology 15 (2004) 996. [14] H.E. Dai, E.W. Wong, Z.H. Lu, H.S.C. Fan, C.M. Liber, Nature 375 (1995) 769. [15] G.W. Meng, L.D. Zhang, C.M. Mo, S.Y. Zhang, Y. Qin, S.P. Feng, H.J. Li, J. Mater. Res. 13 (1998) 2533. [16] Y. Zhang, K. Suenaga, C. Colliex, S. Lijima, Science 281 (1998) 973. [17] T. Seeger, P. Kohler-Redlich, M. Ruhle, Adv. Mater. 12 (2000) 279. [18] A. Huczko, M. Bystrzejewski, H.A. Lange, S. Cudzilo, A. Panas, M. Szala, J. Phys. Chem. B 109 (2005) 16244. [19] G.C. Xi, Y.Y. Peng, S.G. Wan, T.W. Li, W.C. Yu, Y.T. Qian, J. Phys. Chem. B 108 (2004) 20102. [20] X.H. Sun, C.P. Li, W.K. Wong, N.B. Wong, C.S. Lee, S.T. Lee, B.K. Teo, J. Am. Chem. Soc. 124 (2002) 14464. [21] D. Zhou, S. Seraphin, Chem. Phys. Lett. 222 (1994) 233. [22] T. Tomitsugu, I. Naoki, Y. Hiroyuki, S. Shin-ichi, J. Shiro, Physica E 28 (2005) 431. [23] R.B. Wu, Y. Pan, G.Y. Yang, M.X. Gao, L.L. Wu, J.J. Chen, R. Zhai, J. Lin, J. Phys. Chem. C 111 (2007) 6233. [24] C. Vix-Guterl, P. Ehrburger, Carbon 35 (1997) 1587. [25] G.Y. Yang, R.B. Wu, J.J. Chen, Y. Pan, R. Zhai, L.L. Wu, J. Lin, Nanotechnology 18 (2007) 155601. [26] C.H. Liang, G.W. Meng, L.D. Zhang, Y.C. Wu, Z. Cui, Chem. Phys. Lett. 329 (2000) 323.
Simultaneous growth of SiC nanowires, SiC nanotubes, and SiC/SiO2 core–shell nanocables Baosheng Li, Renbing Wu, Yi Pan ∗ , Lingling Wu, Guangyi Yang, Jianjun Chen, Qimaio Zhu Department of Materials Science and Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People’s Republic of China Received 23 July 2007; received in revised form 21 August 2007; accepted 26 August 2007 Available online 4 September 2007
Abstract SiC nanowires, SiC/SiO2 core–shell nanocables, and SiC nanotubes have been synthesized simultaneously by direct heating Si powders and multiwall carbon nanotubes (MWCNTs). The as-obtained SiC nanowires are generally 100 nm in diameter and several tens of micrometers in length, the nanocables consist of a 20–30 nm diameters single-crystalline SiC core covered by a uniform layer of about 20 nm thick amorphous SiO2 , and the nanotubes with very narrow hollow channel have outer diameters of about 20 nm. The characteristics of the products are analyzed by various methods, results of which indicating that temperature and ambience are two key factors for the formation of the three different products; their possible growth mechanisms are also discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanostructured materials; Gas–solid reactions; Microstructure; SEM; TEM
1. Introduction Fabrication of one-dimensional (1D) nanoscale materials with specific size, morphology, and structure has attracted considerable and intensive research interests due to their importance in understanding the fundamental properties of 1D nanomaterial. They are potential building blocks and devices with designed functions in areas as diverse as electronics, optics, catalysis, and ceramics [1–3]. The synthesis of SiC nanomaterials is of particular interest since they could be promising candidates for fabricating electronic, optic, and nanocomposites that can be operated at extreme environment such as high temperature, high power and radiation, and corrosive conditions [4–6]. Many synthetic approaches for preparing various SiC nanostructures such as nanowires [7,8], nanotubes [9,10], hollow nanospheres [11], nanoflowers [12], nanosprings [13], etc. have been reported, including chemical vapor deposition (CVD) [14], carbon thermal reduction [15], laser ablation [16], arc discharge [17], combustion [18] and so on. However, simultaneous growth
∗
Corresponding author. Tel.: +86 571 87953008; fax: +86 571 87953008. E-mail address: [email protected] (Y. Pan).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.08.076
of two or more kinds of SiC nanostructures in a single processing step has rarely been reported. In this paper, we demonstrate a simple way to grow SiC nanowires, SiC/SiO2 nanocables, and SiC nanotubes simultaneously through a one-step direct heating method. The obtained SiC nanostructures were characterized by various methods. The characterizations reveal that ambience and temperature are key factors for the formation of different structures. Furthermore, based on the reaction conditions and characterizations, the formation mechanisms of these nanostructures are also proposed. 2. Experimental SiC nanowires, SiC/SiO2 core–shell nanocables, and SiC nanotubes were synthesized via thermal chemical vapor transportation and condensation in a conventional vertical graphite furnace. High-purity silicon powder (99.999%, Aldrich) was first put into a graphite crucible (outer diameter 80 mm, inner diameter 64 mm, length 30 mm), then an alumina grid was put on the crucible, over which a certain amount of multiwall carbon nanotubes (MWCNTS) was loosely placed, and a graphite ring with the same diameter was covered on the alumina grid, at last an alumina top was put on the graphite ring. The tube furnace was first evacuated to 10−2 Torr by a mechanical rotary pump, whereafter Ar (99.99%) was introduced till the positive pressure of ∼10 Torr. The furnace was heated to 1450 ◦ C at 30 ◦ C/min and kept at this temperature for 1 h. The whole
B. Li et al. / Journal of Alloys and Compounds 462 (2008) 446–451 heating and cooling process was program-controlled. After the reaction, gray product was formed on the surface of the alumina grid; meanwhile, blue-white cotton like product was also obtained on the inner surface of the graphite ring and the crucible. The collected product was characterized by a field-emission scanning electron microscopy (FE-SEM, FEI-SIRION100, operated at 5 kV), equipped with energy dispersive spectrometry (EDS). The X-ray diffraction patterns of the products were measured on a Rigaku, Geigerflex/D using Cu K␣ ˚ FI-TR transmittance spectra measurements were perradiation (λ = 1.5406 A). formed using Bruker Vector 22. Transmission electron microscope (HRTEM) imaging and selected area electron diffraction (SAED) (JEM-2010, HR, operated at 300 kV accelerating voltage) was used to characterize the internal structures of SiC nanostructures. Samples for the TEM were prepared by ultrasonically dispersing the products into absolute ethanol for 5 min, and then a drop of the suspension containing the products was dropped on a copper grid coated with an amorphous carbon film and then dried in the air.
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3. Results and discussion 3.1. Digital photo, FE-SEM, and EDS investigation A top-view digital photograph of as-synthesized grayish product formed on the alumina substrate is shown in Fig. 1a, evidencing that the black CNTS reacted with Si and the reaction was thorough. Fig. 1b shows the FE-SEM image of original MWCNTs, demonstrating that the random curve and tubular structures have diameters in range of 20–30 nm and lengths up to several micrometers. Fig. 1c presents a typical FE-SEM image of the products illustrating that the formed straight wire-like structures are about 50–100 nm in diameter and lengths up to several tens micrometers. The high magnification FE-SEM image (inset in
Fig. 1. (a) Top-view digital photograph of as-synthesized pale grayish product formed on the alumina substrate; (b) SEM image of original MWCNTs; (c) SEM image of the nanowires obtained on the alumina grid and (d) is the corresponding EDS spectra; (e) SEM image of the worm-like structures found on the alumina grid and (f) shows the corresponding EDS spectra.
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Fig. 2. (a) Top-view digital photograph of the wool-like products on the inner wall of the graphite crucible; (b) SEM image of these products and (c) shows the high-magnification SEM image; (d) is the corresponding EDS spectra.
Fig. 1c) suggests that the obtained nanowires have a smooth and clean surface. EDS spectra of these nanowires is given in Fig. 1d. The EDS spectra indicate that these wires are composed of Si and C elements; the weak Au peak comes from the supporting Au film. The atomic ratio of Si:C is about 48.4:51.6, and this composition is very close to stoichiometric SiC. It was surprisingly found that there are also worm-like structures in the grayish products as shown in Fig. 1e. Compared to MWCNTs, the diameter of these worm-like structures is larger and the surface is rougher. Magnified FE-SEM image (inset in Fig. 1e) indicates that the tube-like structures with diameter of about 50 nm and 1 m in length have both ends closed. Fig. 1f is a typical EDX spectrum taken from a single nanotube. Only C and Si are present in the spectrum and their atomic ratio is 45.5:54.5 ≈ 1:1, suggesting that this tuber-like structure is, most possibly, SiC nanotube. Besides the products synthesized on the alumina substrate, wool-like product was also obtained on the inner wall of the graphite crucible, simultaneously. Fig. 2a shows a top-view digital photograph of the as-grown blue-white products covered on the inner wall of the graphite crucible. FE-SEM image of this product is shown in Fig. 2b. Bulk quantity and high-purity straight wire structures with lengths up to tens even hundreds of micrometers are observed clearly. The high-magnification FESEM image (Fig. 2c) suggest that the wire-like structures with diameters in the range of 50–80 nm have very clean and smooth
surface. No obvious particles could be found attached on the surface. A typical EDS spectrum taken from these nanowires, as shown in Fig. 2d, suggest that the products contain Si, C, and O in a 47.3:40.2:12.5 atomic ratio. The three elements coexisting strongly suggest that the as-prepared samples may be formed SiC/SiO2 core–shell nanocables, which is also proved by later FI-TR and TEM observations.
3.2. FI-TR and XRD analysis Fig. 3a is the FTIR spectrum of SiC/SiO2 core–shell nanocables, which shows several absorption bands from Si–O stretching vibrations (1103 cm−1 ), Si–O–Si bending vibrations (471 cm−1 ), transversal optic (TO) mode Si–C vibrations (793 cm−1 ), and longitudinal optic (LO) mode Si–C vibrations (945 cm−1 ), respectively, indicating that nanocables are composed of SiC and amorphous SiO2 . X-ray diffraction analysis confirms the as-grown SiC nanowires are 3C–SiC, as illustrated in Fig. 3b; there are five peaks in the spectrum agreeing well with (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) diffraction peaks of 3C–SiC. A small peak marked SF, corresponding to stacking faults, is also shown in the spectrum [19]. XRD pattern of the synthesized SiC/SiO2 nanocables shows no difference except broad diffraction peak compared to SiC nanowires, indicating the amorphous structure of the SiO2 sheath.
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image of original MWCNTs and Fig. 4f is the TEM image of some SiC nanotube found in the product of the reacted MWCNTs. It clearly demonstrate that compared to original MWCNTs, the diameters of SiC nanotubes is larger and the surface is rougher. 3.4. Growth process and mechanism
Fig. 3. (a) FTIR spectrum of SiC/SiO2 core–shell nanocables; (b) XRD pattern taken from the synthesized SiCNWs.
3.3. TEM, SAED and EDS characterization To have more details about the structure and crystallinity of the SiC nanostructures, TEM and SAED measurements were performed and the results are shown in Fig. 4. Fig. 4a shows that the SiC nanowires converted from MWCNTs have a metal catalyst particle end, indicating the SiC nanowires growth via VLS mechanism; Ni, Si and C are detected in the spherical end by EDS analysis. Also, we can see that the SiC nanowire possess a high density of planner defects and stacking faults which are perpendicular to the wire axis. The SAED pattern along [0 1 1] zone axis further demonstrates that it is a phase pure 3C–SiC having defects causing the spots streaking. Fig. 4c shows the TEM image and SAED pattern of the nanocable grown on the crucible wall, from which a structure characterized with SiC crystalline core and amorphous SiO2 shell is clearly demonstrated. The SAED patterns also shows that the crystalline SiC core has stacking faults and twins similar to that from SiC nanowires as shown in Fig. 4a. The core is about 30 nm in diameter and the amorphous SiO2 sheath is approximately 25 nm in thickness. Fig. 4d displays the HRTEM image of a SiC nanowire, in which well-defined fringe separation of 0.25 nm is consistent with the d-spacing of (1 1 1) plane, suggesting that the growth direction is [1 1 1]. Fig. 4e is the TEM
By such a simple heating method, SiC nanotubes, SiC nanowires and SiC/SiO2 core–shell nanocables were synthesized simultaneously. However, the formation mechanisms should be different. SiC nanotubes and SiC nanowires were produced on the alumina grid over which MWCNTs were originally placed. Si in the graphite crucible was melted at 1450 ◦ C. It is believed that at the same temperature and the same ambience, the difference of the product obtained on the alumina grid is because of the difference in the original MWCNTs. That is, some of the original MWCNTs contain residue nickel particles as catalyst for their growth, and others contain no nickel particles. Consequently, we propose two different growth mechanisms for the transformation of the MWCNTs on the alumina grid. One is metal-catalyzed VLS mechanism for the transformation to SiC nanowires. In this process, the residue nickel particles could play a catalyst role again. As the temperature increases, the nickel particle would melt and provide an energetically favored site for the absorption of Si vapor, which acted as nucleation sites. In this way, SiC nanowires were obtained at last. The other is VS mechanism for the transformation to SiC nanotubes. In this process, Si atoms first reacted with C atoms on the surface of MWCNTs, and then formed SiC layer by layer, the diffusion mechanism supply Si resources [9], thus result in the worm-like tubular structures. We also believe this structure is only an intermediate [20], and if heated with a higher temperature or for a longer time, it would transform to SiC nanowires or nanorods [20–23]. For the formation of SiC/SiO2 core–shell nanocables on the inner wall of the graphite crucible, a double-VS mechanism was proposed, the first VS step is the growth of SiC nanowires, Si vapor deposits on the inner surface of the graphite crucible and generate many nucleation sites, some of these nucleation sites grow up to SiC nanowires in the end. This procedure occurs during the calefactive and holding stage. The second VS step is the deposition of SiC and SiO2 on the surface of the as-grown SiC nanowires; this procedure takes place in the cooling stage. With temperature decrease, reaction (1), (2) would occur and lead to a decrease in enthalpy and Gibbs energy [24–26], which is thermodynamically favorable at low temperature. Consequently, SiO2 and SiC deposits onto the surface of the grown-up SiC nanowires, by reason of the difference of density and the good fluidity of SiO2 at this temperature, these two materials can be separated easily during growth and thus form SiC/SiO2 nanocables in the end. 3SiO(g) + CO(g) → SiC(s) + 2SiO2 (s)
(1)
SiO(g) + 1/2O2 (g) → SiO2 (s)
(2)
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Fig. 4. (a) TEM of a SiC nanowires and its SAED pattern; (b) EDS spectra of the spherical end; (c) TEM of a SiC/SiO2 nanocable and its SAED pattern; (d) HRTEM of a SiC nanowire; (e) TEM of the original MWCNTs; (f) TEM of a SiC nanotube.
A question is why only the products on the inner wall grown to SiC/SiO2 composites but the products on the alumina grid remained SiC nanowires. Considering the temperature distribution, we can find out the answer; detailed description is given as follows. Fig. 5 displays the cooling process of the furnace. In the cooling stage, the temperature distribution in the furnace room is not uniform due to the cooling water flowing to surround the furnace room. The inner wall of the graphite crucible has a faster cooling rate than elsewhere and could firstly reach the reaction temperature for the deposition of SiO2 and then always be prior to form SiC/SiO2 nanocables. Consequently, SiC/SiO2 nanocables were obtained on the inner wall of the graphite crucible while there were only SiC nanowires and SiC nanotubes on the alumina grid.
Fig. 5. Illustration of the cooling process.
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4. Conclusion In summary, by a simple method of direct heating Si powders and MWCNTs, we obtained SiC nanotubes and SiC nanowires on the alumina grid, but SiC/SiO2 core–shell nanocables on the inner wall of the graphite crucible. Temperature and ambience are two key factors for the formation of different structures. How to choose suitable conditions to actualize the controlled growth of various 1D SiC nanostructures will be further investigated in the future. References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] X.F. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 13 (2001) 526. [3] M.H. Huang, S. Mao, H. Feick, Y. Wu, H. Kind, E. Weiber, R. Russo, P. Yang, Science 292 (2001) 1897. [4] W.Q. Han, S.S. Fan, Q.Q. Li, B.L. Gu, D.P. Yu, Chem. Phys. Lett. 265 (1997) 374. [5] E.W. Wong, P.E. Sheehan, C.M. Lieber, Science 277 (1997) 1971. [6] K.W. Wong, X.T. Zhou, F.C.K. Au, H.L. Lai, C.S. Yu, Chem. Phys. Lett. 265 (1997) 374. [7] Z. Pan, H.L. Lai, F.C.K. Au, X. Duan, W. Zhou, W. Shi, Adv. Mater. 12 (2000) 1186. [8] Y. Baek, Y.H. Ryu, K. Yong, Mater. Sci. Eng. C 26 (2006) 805. [9] Y. Zhang, T. Ichihashi, E. Landree, S. Nihey, Iijima, Science 285 (1999) 1719.
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