J. Phys. Chem. C 2007, 111, 12517-12521
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ARTICLES Vapor-Solid Reaction for Silicon Carbide Hollow Spherical Nanocrystals Zhenyu Liu,* Lijie Ci,† N. Y. Jin-Phillipp, and M. Ru1 hle Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany ReceiVed: April 18, 2007; In Final Form: June 4, 2007
We report the synthesis of silicon carbide (SiC) hollow spherical nanocrystals using a vapor-solid reaction between carbon nanoparticles and silicon monoxide vapor generated from a mixture of silicon and silica. The hollow spherical nanocrystal diameters are related to the diameters of the pristine carbon nanoparticles, showing a shape memory effect. The thickness of the hollow spherical nanocrystals can be controlled by the siliconization degree and carbon nanoparticle multilayer structures. Different analysis techniques recorded the phase conversion of the amorphous carbon into graphitic carbon in the structure of the sp2 bond state and further into SiC nanocrystal in the structure of the sp3 diamond-like carbon bond state. The hollow spherical nanocrystals might find applications in catalysis, controlled delivery, gas storage, low-dielectric nanomaterials, acoustic insulation, and nanocomposites.
Introduction Nanostructured materials have received steadily growing interest as a result of their peculiar and fascinating properties and possible applications superior to their bulk counterparts. Many of their properties exhibit a strong dependence on size, shape, and surface composition.1-3 Silicon carbide (SiC), an important group IV-IV wide-band-gap semiconductor, exhibits unique physical and electronic properties, including high breakdown field, high thermal conductivity, high electron mobility, chemical inertness, strong mechanical properties even at high temperature, low thermal expansion, and high thermal shock resistance, which will lead to many potential applications. It is well-suited for the fabrication of electronic devices operating at high temperature, high power, high frequency, and in harsh environments.4-6 Considerable efforts have been made toward the fabrication of SiC nanotubes, nanorods, nanocones, and nanowires from different routes, such as carbon nanotubeconverted reactions, carbothermal reduction, chemical vapor deposition, and chemical solution approaches, among others.5,7-10 SiC nanostructures are being actively pursued as components for nanoelectromechanical sensors, nanocatalytic elements, and nano-optical circuits able to operate in harsh environments.11,12 Such structures have been shown to exhibit properties (greater elasticity and strength) superior to those of bulk SiC.13 The excellent mechanical and chemical properties of SiC have also made this material a natural candidate for microsensor and microactuator applications in microelectromechanical systems (MEMS). * To whom correspondence should be addressed: Present address: 848 Benedum Hall, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261. Tel.: +1-412624-9095. Fax: +1-412-624-8069. E-mail:
[email protected]. † Present address: Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180.
Recently, hollow spheres have attracted increasing interest because of their potential applications in drug delivery,14-15 cell and enzyme transplantation,16 gene therapy, separations in biomedicine, and contrast agents in diagnostics.17 They are also useful as protectors for photosensitive compounds and as heterogeneous catalysts.18 Hollow spheres of various materials, such as glass, ceramics, metals, semiconductors, magnetic materials, and biomaterials, have been synthesized using removable templates including polymer latex spheres, silica sol, microemulsion droplets, liquid crystals, polymer micelles, and polymer-surfactant complex micelles.18-21 Beyond the template approach, a variety of novel processes have been developed to synthesize hollow spheres by employing the Kirkendall diffusion effect,22 a low-temperature hydrothermal method,23 and other techniques. Owing to the useful properties and potential importance in nanotechnology of inorganic hollow nanospheres, we present herein the synthesis and characterization of SiC hollow spherical nanocrystals obtained by a vapor-solid reaction between carbon nanoparticles and silicon monoxide vapor. Experimental Section SiC Hollow Spherical Nanocrystal Synthesis. In our approach, carbon nanoparticles were generated from the selfpyrolysis of toluene at 800-1000 °C. The amorphous carbon nanoparticles formed at atmospheric pressure when toluene vapor was introduced into a 1000 °C horizontal tubular quartz reactor by bubbling an argon (Ar) flow through toluene at room temperature, with another Ar flow as the carrier gas. Typically, the resulting nanoparticles were collected from the wall of the quartz reactor after the furnace had cooled to room temperature in the Ar ambient. The carbon nanoparticles and a mixture of silicon and silica powders in a molar ratio of 1:1 were separately added to an alumina crucible with the carbon nanoparticles located in the gas outlet direction. The SiC hollow spherical
10.1021/jp073012g CCC: $37.00 © 2007 American Chemical Society Published on Web 08/07/2007
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Figure 1. Morphologies and microstructures of the starting carbon nanoparticles and the resulting SiC hollow nanocrystals: (a) SEM image of the staring carbon nanoparticles. (b) TEM image of the carbon nanoparticles. (Inset) SAED diffraction pattern showing amorphous feature. (c) SEM image of the resulting SiC hollow nanocrystals. The arrows indicate broken nanoparticles, showing the hollow structure. (d) Low-magnification TEM image of the SiC hollow nanocrystals. (Inset) SAED pattern of the hollow nanocrystals. (e) Carbon nanoparticles after heat treatment at 1300 °C, showing multilayer graphene structure. (Inset) HRTEM image of a carbon nanoparticle with a graphene fringe of 0.34-nm spacing and voids between multilayers as indicated by arrows. (f) HRTEM image of a SiC hollow nanocrystal shell. (Inset) Fast Fourier transform (FFT) pattern, showing the crystalline orientation of the SiC nanoshell in the 〈111〉 direction.
nanocrystals were prepared by a vapor-solid reaction between the carbon nanoparticles and SiO vapor in an inert atmosphere of Ar. The SiO vapor was generated by heating a mixture of silicon and silica in the temperature range of 1300 °C according to the reaction Si + SiO2 T 2SiO. Then, the SiO vapor reacted with carbon to convert the nanoparticles to SiC at the same temperature by the reaction
3C + 2SiO T 2SiC + CO2 Sample Characterization. The sample morphology and microstructure were observed by field-emission scanning electron microscopy (SEM) (JEOL JSM-6300F). Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained on a JEOL 2000FX instrument at an accelerating voltage at 200 kV. High-resolution TEM was performed on a JEOL JEM-4000 EX microscope with
a point-to-point resolution of 0.16 nm at 400 kV. Electron energy loss spectroscopy (EELS) analyses were performed using a Zeiss 912 TEM instrument equipped with a Gatan 766 2D-DigiPEELS electron energy loss spectrometer operated at 120 kV. X-ray diffraction (XRD) patterns were recorded to analyze the crystal structure using a powder X-ray diffractometer (Philips 1830) operated at a step size of 0.02° and with Co KR radiation at a 40 kV accelerating voltage and 100 mA anode current. Diffraction profiles were obtained by the usual θ-2θ scan. Results and Discussion In a typical synthesis, the process employed resulted in a SiC hollow spherical nanocrystal structure; the macroscopic morphologies and microstructures of the starting carbon nanoparticles and the resultant SiC hollow spherical nanocrystals are shown in Figure 1. Self-pyrolysis of toluene at 800-1000 °C
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Figure 2. Microstructure of composite SiC-C nanoparticles after siliconization: (a) TEM image of SiC-C composite nanoparticles. (b) HRTEM image clearly showing crystalline SiC shell and graphite core.
resulted in carbon nanoparticles, and they were used as the starting materials to synthesize SiC hollow spherical nanocrystals by reaction with SiO vapor generated from a mixture of silicon and silica at 1300 °C. The starting toluene-pyrolyzed spherical nanoparticles had a uniform size about 100-200 nm with an amorphous phase, as shown in Figure 1a,b. After heat treatment, the amorphous phase changed to a multilayer graphene structure (Figure 1e), which reveals that the amorphous carbon evolved into graphite-like multilayer structure. The presence of voids between different graphite-like layers implies a change in density and the reconstruction of the carbon atoms during the crystallization process from amorphous carbon to graphitelike carbon. After the siliconization reaction at 1300 °C, SiCsheathed carbon composite nanospheres formed. The TEM and HRTEM images shown in Figure 2 confirm the composite structure. We then left the SiC sheath intact and removed the carbon core by combustion of the resultant SiC-C composite nanospheres at 600 °C in air to create SiC hollow spherical nanocrystals with diameters similar to those of the starting carbon spherical nanoparticles. Some broken SiC spherical nanocrystals in Figure 1c indicate the obviously hollow structure. The TEM image of the SiC spherical nanocrystals shows a pale region in the center in contrast to dark edges, indicating that the crystals are hollow spheres, as shown in Figure 1d. The SiC hollow spherical nanocrystals have a uniform shell thickness of 30-50 nm. The diameters of the resulting nanospheres depend on those of the pristine carbon nanoparticles, which implies that the transformation from carbon to SiC maintains the original carbon skeleton and reflects a shape memory.24 The hollow layer thickness can be related to the thickness of the continuous layer after the heat treatment before the siliconization reaction and the degree of siliconization. The corresponding SAED pattern, shown in Figure 1d, suggests that the SiC nanocrystals are multicrystalline. The HRTEM image of the hollow shell part (Figure 3) confirms the multicrystalline structure, which can provide tunnels for small molecules to penetrate the shell through the grain boundaries.22 That might be a possible explanation for air to pass through the SiC sheath layer to remove the unreacted carbon to form the hollow SiC structure. The powder X-ray diffraction (XRD) pattern of the resulting product was used to verify the change of species through the process, as shown in Figure 4. It indicates that the resulting hollow spherical nanocrystals are mainly in the SiC phase and
Figure 3. HRTEM images of the microstructure of heat-treated carbon nanoparticles and the wall of the resulting SiC hollow nanocrystals: (a) Carbon nanoparticles after heat treatment at 1300 °C, showing multilayer graphene structure with fringes at 0.34 nm and voids between graphitic layers. (b). Wall portion of the resulting SiC hollow nanocrystals, showing the multiple types of SiC nanocrystals, grain boundaries, and twin structures.
with some SiO2 caused by exposed to air and/or deposited by oversaturation with SiO vapor. Three characteristics peaks can
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Figure 4. X-ray diffraction pattern of the resulting SiC hollow nanocrystals (Co KR radiation, λ ) 0.1788 nm).
Figure 5. Core electron energy loss spectroscopy (EELS) results for SiC hollow nanocrystals. (Inset) C-K edges of raw amorphous carbon nanospheres, heat-treated graphitic multilayer nanospheres, and resulting SiC hollow nanocrystals.
be indexed as the face-centered cubic (fcc) β-SiC structure, in accordance with the reference data (JCPDS file no. 21-2129). HRTEM reveals that hollow SiC nanocrystals with the 0.25-nm spacing of the (111) planes of β-SiC are dominantly obtained and that the thin amorphous layer on the surface of the hollow SiC nanocrystals corresponds to a SiOx layer, as shown in Figure 1f. Carbon materials are very interesting materials showing various hybridized bonding states including sp1, sp2, and sp3. Generally, carbon exists in the sp2 form, as in graphite and carbon nanotubes, and in the sp3 structure as in diamond and silicon carbide. Knowledge about the changing of the bonding character from sp2 to sp3, which is expected to occur not only through the curvature of the shells but also through a decrease in the c lattice spacing, requires a detailed study by electron energy loss spectroscopy (EELS). Characteristic near-edge finestructure (ELNES) shapes can be used to distinguish between different types of graphitic or diamond-like bonding. The core EELS spectra taken from the studied materials are shown in Figure 5. The EELS spectrum for the SiC hollow spherical nanocrystals exhibits three main features at ∼100, ∼290, and ∼530 eV, corresponding to the Si-L, C-K, and O-K edges, respectively. The EELS spectrum for the carbon nanoparticles shows one feature, corresponding to the C-K edge. The C-K edge of the carbon nanoparticles has a shoulder-like feature at
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Figure 6. Calculated specific surface area as a function of the diameter and shell thickness of SiC hollow nanocrystals.
285 eV that indicates the 1s-π* peak caused by the graphitic structure of carbon and some σ character corresponding to the sp3 bonding. The greatly increased intensity for the heat-treated multilayer graphitic nanostructures shows the phase transformation from the amorphous structure to the graphitic structure. The C-K edge of the SiC hollow spherical nanocrystals has no shoulder-like feature at 285 eV. This result suggests that nearly all carbon atoms in the SiC hollow spherical nanocrystals are in the diamond-like sp3 structure. Because the hollow SiC/C nanoparticles were heated at 600 °C in air to remove the carbon core, a few SiOx phases were produced, giving rise to the O-K edge at ∼530 eV. The materials used as catalyst supports must have reasonable surface areas of g10 m2 g-1, which allows for exposure of the active phase to the reaction mixture, but at the same time, the materials must contain as few micropores as possible.25,26 We found that the calculated specific surface areas of the as-prepared SiC hollow spherical nanocrystals depended strongly on the shell thickness and diameter of the nanoparticles, as shown in Figure 6. The specific surface area for hollow nanocrystals with an average outer diameter of 100-200 nm and a shell thickness at 30-50 nm is about 50 m2 g-1, which is similar to other reported values.25,27 The specific surface area increases by about a factor of 10 as the nanoparticle size decrease from 100 to 5 nm. When the nanoparticle size is smaller than 20 nm, the specific surface area is over 3 times that of 60-nm nanoparticles. This fact suggests that we should control both the shell thickness and the nanosphere size of the hollow nanospheres to obtain SiC hollow nanospheres with high specific surface areas. Recent density functional theory calculations reveal that the SiC shell is full of point charges. This happens because there is a continuous charge transfer of more than one-half of an electron from Si to C.28 As a result, the binding energy between the SiC shell and hydrogen can be enhanced, hence leading to improved hydrogen storage. This might imply that the synthesized SiC hollow spherical nanocrystals might potentially be used as gas storage media. The SiC hollow spherical nanocrystal formation process is illustrated in Figure 7. The starting nanoparticles with amorphous phase were obtained by self-pyrolysis of toluene at 8001000 °C. After heat treatment at high temperature of 12501300 °C, the original amorphous carbon phase crystallized into ordered multilayer graphene structures with carbon atom rearrangement and some void formation between multiple graphene layers, which revealed a possible density change during the transformation from amorphous carbon to graphitic carbon. Simultaneously, the formed carbon multilayer structures reacted
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J. Phys. Chem. C, Vol. 111, No. 34, 2007 12521 conversion of the hybridization state. The hollow nanocrystals might find applications in catalysis, controlled delivery, hydrogen storage, low-dielectric nanomaterials, acoustic insulation, and nanocomposites. Acknowledgment. L.C. and Z.L. thank the Alexander von Humboldt Foundation for fellowships. References and Notes
Figure 7. Schematic illustration of the procedure for preparing SiC hollow nanocrystals.
with SiO vapor generated from a mixture of Si and silica to convert sp2 graphitic carbon to sp3 diamond-like silicon carbide. The resulting SiC and carbon nanocomposites maintained the original size of the starting amorphous carbon nanoparticles, indicating that shape memory was retained during the whole conversion process from carbon to SiC. The reaction occurred gradually, limited by SiO diffusion from the nanoparticle surface to the bulk, and then, under controlled conditions and at a lower reaction temperature, SiC-encapsulated carbon core-shell structures could be obtained as intermediate products. The SiC shells were kept intact while the carbon core was removed by oxidation at 600 °C in air. The carbon nanoparticles acted as a template during the whole conversion procedure, so that SiC hollow spherical nanocrystals were synthesized. Conclusions In conclusion, SiC hollow spherical nanocrystals were successfully synthesized using a vapor-solid reaction between carbon spherical nanoparticles and silicon monoxide vapor generated from a mixture of silicon and silica. The diameters of the hollow spherical nanocrystals are related to the diameters of the pristine carbon nanoparticles, indicating a shape memory. The thickness of the hollow spherical nanocrystals can be controlled by the siliconization degree and the carbon nanoparticle multilayer structures. The results demonstrated the conversion from sp2 carbon to sp3 diamond-like carbon bond states. The thermal, mechanical, and electric properties of the SiC hollow spherical nanocrystals are modified greatly compared to those of the original carbon nanoparticles as a result of the
(1) Edelsstein A. S.; Cammarata, R. C. Nano-materials: Synthesis, Properties, and Applications; Institute of Physics: Philadelphia, PA, 1996. (2) Thiaville, A.; Miltat, J. Science 1997, 284, 1939. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769. (5) Sun, X.; Li, C.; Wong, W.; Wong, N.; Lee, C.; Lee, S.; Teo, B. J. Am. Chem. Soc. 2002, 124, 14464. (6) Pan, Z.; Lai, H.; Au, F. C. K.; Duan, X.; Zhou, W.; Shi, W.; Wang, N.; Lee, C.; Wong, N.; Lee, S.; Xie, S. AdV. Mater. 2000, 12, 1186. (7) Seeger, T.; Kohler-Redlich, P.; Ru¨hle, M. AdV. Mater. 2000, 12, 279. (8) Zhou, X.; Wang, N.; Lai, H.; Peng, H.; Bello, I.; Wong, N.; Lee, C.; Lee, S. Appl. Phys. Lett. 1999, 74, 3942. (9) Kim, H. Y.; Park, J.; Yang, H. Chem. Commun. 2003, 2, 256. (10) Hu, J. Q.; Lu, Q. Y.; Tang, K. B.; Deng, B.; Jiang, R. R.; Yu, W. C.; Zhou, G. E.; Liu, X. M.; Wu, J. X. J. Phys. Chem. B 2000, 104, 5251. (11) Nakamura, D.; Gunjishima, I.; Yamaguchi, S.; Ito, T.; Okamoto, A.; Kondo, H.; Onda, S.; Takatori, K. Nature 2004, 430, 1009. (12) Bauer, M.; Kador, L. J. Phys. Chem. B 2003, 107, 14301. (13) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (14) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (15) Ding, Y.; Hu, Y.; JIang, X.; Zhang, L.; Yang, C. Angew. Chem. 2004, 116, 6529. (16) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (17) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872. (18) Chen, Y.; Kang, E. T.; Neoh, K. G.; Greiner, A. AdV. Funct. Mater. 2005, 15, 113. (19) Wang, D.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364. (20) Mulinowski, K. M.; Harsha, V.; Colvin, V. L. AdV. Mater. 2000, 12, 833. (21) Han, S.; Sohn, K.; Hyeon, T. Chem. Mater. 2000, 12, 3337. (22) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (23) Liu, Q.; Liu, H.; Han, M.; Zhu, J.; Liang, Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995. (24) Ledoux, J. M.; Hantzer, J.; Huu, C. P.; Guille, J.; Desaneeaux, M. P. J. Cata. 1988, 114, 176. (25) Keller, N.; Oham-Huu, C.; Roy, S.; Ledoux, M. J.; Estournes, C.; Guille, J. J. Mater. Sci. 1999, 34, 3189. (26) Keller, N.; Oham-Huu, C.; Roy, S.; Ledoux, M. J.; Estournes, C.; Ehret, G. Appl. Catal. A: Gen. 1999, 187, 255. (27) Moene, R.; Kramer, L. F.; Schoonman, J.; Makkee, M.; Moulijin, J. A. Appl. Catal. A: Gen. 1997, 162, 181. (28) Mpourmpakis, G.; Froudakis, G. E. Nano Lett. 2006, 6, 1581.
