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J. Phys. Chem. C 2007, 111, 636-641
Role of Pores in the Carbothermal Reduction of Carbon-Silica Nanocomposites into Silicon Carbide Nanostructures Jianfeng Yao,† Huanting Wang,*,† Xinyi Zhang,† Wei Zhu,† Jinping Wei,† and Yi-Bing Cheng‡ Departments of Chemical Engineering and of Materials Engineering, Monash UniVersity, Clayton, VIC3800, Australia ReceiVed: August 7, 2006; In Final Form: October 16, 2006
Silicon carbide nanostructures were synthesized by carbothermal reduction of carbon-silica (C-SiO2) nanocomposites. C-SiO2 nanocomposites with C/SiO2 molar ratios of 1.90-4.21 were used in this study. Thermogravimetric analysis, nitrogen sorption, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy were used to characterize C-SiO2 nanocomposites and SiC products. Highly crystalline SiC nanoparticles and nanofibers (CS-3.28 and CS-4.21) were obtained from mesoporous C-SiO2 nanocomposites with the carbon content greater than a C/SiO2 stoichiometric ratio of 3 (e.g., C/SiO2 ) 3.28 and 4.21 by mole), and they had a BET surface area of 83.0-76.7 m2/g after unreacted silica and carbon were removed. In contrast, SiC was not formed in the mesoporous C-SiO2 nanocomposites with a lower carbon content (e.g., C/SiO2 ) 1.91 and 2.51 by mole). However, when such mesoporous nanocomposites were infiltrated with a small amount of carbon, SiC nanoparticles and nanofibers with high crystallinity were produced. The major phase formed from the mesoporous C-SiO2 nanocomposite with C/SiO2 ) 2.51 was nanofibers, and only less than 15% nanoparticles was observed in this sample. The carbothermal reduction of dense C-SiO2 nanocomposites resulted in mesoporous SiC with low crystallinity under the heating conditions used. As a result, the pores play a vital role in the carbothermal reduction of C-SiO2 nanocomposites. SiC with controlled nanostructures can be synthesized by simply varying the porosity, C/SiO2 ratio, and structure of the precursors.
Introduction Silicon carbide (SiC) has found many important applications such as in catalysis, materials reinforcement, and electronic devices because it is a wide-band-gap semiconductor and exhibits excellent thermal conductivity, thermal stability, mechanical strength, and chemical inertness. To date, several techniques involving chemical vapor deposition (CVD),1-5 spinning,6 nanocasting,7-15 autogenic-pressure reactions,16-18 and sol-gel19-23 have been developed for synthesizing crystalline SiC material. Many different pairs of silicon and carbon precursors have been used in such syntheses. On the basis of the types of reactions, the pairs of silicon and carbon precursors can be divided into the following categories: (1) Si carbonization, e.g., a mixture of milled Si and SiC powder and C3H6,1 silicon-methane,4,5,24 and silicon-carbon nanotubes,25 (2) pyrolysis and crystallization of preceramic precursors, e.g., poly(carbosilane),6,12,26 poly(methylsilane),27,28 poly(silaethylene),29 and allylhydropoly(carbosilane),30 (3) carbothermal reduction such as organosilanes (trimethylsilane,2 silacyclobutane,3 triethylsilane,16 and phenyltrimethoxysilane),21 silica solwood,13,14,31 carbonaceous silica aerogels,19,20 mesoporous silica and carbon (propylene7 or poly(furfuryl alcohol)),9 mesoporous carbon-silicon powder,32 phenolic resin/silica sol,15 carbon nanotubes-SiO,33-35 freeze gel silica-carbon artifact,36 SiO2 xerogel-carbon (saccharose),22 silica sol-carbon fibers,23 and silicon-carbon powder,37 and (4) reduction-carbonization such * To whom correspondence should be addressed. Phone: +61 3 9905 3449. Fax: +61 3 9905 5686. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Materials Engineering.
as SiCl4-Na-C,17 Si-containing compounds, and halocarbons.18 Among these reactions, the carbothermal reduction is widely used to form SiC with desired structures due to broad availability of silica-carbon precursors. SiC nanostructures such as nanofibers, nanoparticles, and nanoporous monoliths have been produced by the carbothermal reduction reaction. In particular, SiC nanofibers are commonly observed on the surface of bulk silica-carbon precursors through the gas-phase reaction between SiO and CO intermediates.14,36 Therefore, the structures and morphologies of SiC products could be substantially changed by adjusting the conditions in relation to the partial pressure of gaseous SiO and CO species. Some strategies for controlling SiC nanostructures have been addressed in the literature.22,36,38,39 Specifically, SiC nanofibers were synthesized from sol-gelderived silica-carbon materials, while mesoporous SiC structures were obtained by adding Ni catalyst into the sol-gelderived silica-carbon.38,39 The effect of gas environments has also been studied.22,36 Under flowing argon gas, SiC nanowires (nanofibers) were grown from silica xerogel-carbon; in the absence of flowing argon, SiC nanoparticles were obtained from the same silica-carbon precursor.22 However, the opposite effect was observed in the conversion of a freeze gel silica-carbon into SiC; porous SiC was produced from such a precursor in dynamic argon conditions, whereas SiC nanofibers were obtained in static conditions.36 The chemical reactivity of silicacarbon precursors may be another factor affecting the resulting SiC structures. For instance, mesoporous SiC was synthesized by using mesoporous MCM-48 as the silica precursor and gaseous propylene carried by argon as the carbon precursor;7 SiC nanofibers were produced by infiltration of mesoporous
10.1021/jp0650984 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/07/2006
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Figure 1. Nitrogen adsorption-desorption isotherms (left) and pore size distributions (right) of mesoporous carbon-silica nanocomposites with different carbon/silica ratios. These samples were prepared using P123 and calcined at 550 °C for 5 h under a nitrogen atmosphere.
silica (SBA-15) with sucrose and heat treatment under flowing argon. Obviously, a comprehensive understanding of the correlations of the SiC nanostructure, nature of its precursor, and synthetic conditions is still lacking. The aim of the present study is to understand the role of pores in the carbothermal reduction by varying the porosity and chemistry of the precursors and thus control SiC nanostructures in the SiC synthesis. Here, we report our findings that SiC material with controllable nanostructures can be synthesized by tailoring the C/SiO2 ratio and porous structure of the precursors. Interpenetrating silica-carbon nanocomposites40,41 were used as the SiC precursors, and the heat treatment conditions (flowing argon) were maintained unchanged. Mixtures of nanofibers and nanoparticles were synthesized from mesoporous silica-carbon nanocomposites, whereas mesoporous SiC structures were obtained from dense silica-carbon nanocomposites. By infiltrating a small amount of carbon into mesoporous silica-carbon nanocomposites, the proportion of nanofibers was significantly raised. Experimental Section Synthesis of SiC. Mesoporous silica-carbon composites were prepared by using our reported method.40 First, 5 g of H2O, 3.28 g of ethanol (anhydrous, Aldrich), and 0.5 g of 1 M HCl (Merck) were mixed in a capped polypropylene bottle with a magnetic stirrer. To this solution was added 4.1 g of P123 (EO20PO70EO20; MW 5800, Sigma-Aldrich) under continuous agitation to obtain P123 solution. Then 10 g of TEOS (99%, Sigma-Aldrich) and a given amount (2.83, 3.77, 4.71, or 5.65 g) of furfuryl alcohol (FA; 99%, Aldrich) were added to the P123 solution. The resulting mixtures were rigorously stirred at room temperature for 3 h, followed by aging at room temperature for 4 days and drying at 80-90 °C for 2-3 days. The black monoliths obtained were carbonized at 550 °C for 5 h with flowing nitrogen gas, leading to mesoporous C-SiO2 composites. To determine the C/SiO2 ratios of C-SiO2 composites, thermogravimetric analysis (TGA) was used to record the mass loss of the samples at a heating rate of 5 °C/min under flowing air. Up to 800 °C all carbon was burned off. The C/SiO2 ratio was then calculated by taking the total mass loss as the mass of carbon and the residual mass as the mass of SiO2. The mesoporous C-SiO2 composites had C/SiO2 molar ratios of 1.90/1, 2.51/1, 3.28/1, and 4.21/1 (denoted P-CS-1.90, P-CS2.51, P-CS-3.28, and P-CS-4.21, respectively). To investigate the effect of the porosity of the C-SiO2 composites on SiC nanostructures, dense C-SiO2 nanocomposites with a C/SiO2 molar ratio of 4.21/1 (denoted P-CS-4.21D) were prepared without addition of P123.
Another batch of P-CS-1.90 and another batch of P-CS-2.51 were infiltrated with poly(furfuryl alcohol) (PFA) by immersing the samples in FA overnight, followed by washing with ethanol, polymerization of FA under 4 M HCl acid at 90 °C, and carbonization under N2 at 550 °C for 5 h. The resulting samples were denoted as P-CS-1.90C and P-CS-2.51C. The C-SiO2 nanocomposites were transferred into a sealed tube furnace equipped with a vacuum pump. Before heating, the furnace was vacuumed to evacuate air for 10 min. To produce SiC, the C-SiO2 composites were heated under an argon atmosphere with a flow rate of 10 mL/min and at a heating rate of 2 °C/min up to 1450 °C and kept at this temperature for 5 h. The as-synthesized SiC samples were calcined in air at 600 °C for 5 h to eliminate residual carbon and washed with 10% NaOH to remove the excessive silica. Characterization. Thermogravimetric analysis (TGA) (Perkin-Elmer, Pyris 1 thermogravimetric analyzer) was conducted at a heating rate of 5 °C/min to 800 °C. Scanning electron microscopy (SEM) images were taken with a JSM-6300F microscope (JEOL). Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) images were taken with a Philips EM 420 microscope. X-ray diffraction (XRD) patterns were recorded on a Philips PW1140/90 diffractometer with Cu KR radiation at a scan rate of 2 deg/min and a step size of 0.02°. Nitrogen adsorption-desorption experiments were performed at 77 K with a Micromeritics ASAP 2020MC. The samples were degassed at 250 °C for 5 h prior to the measurement. The pore volume was estimated from the desorption branch of the isotherm at P/Po ) 0.98 assuming complete pore saturation. The pore size distribution was calculated from the desorption branch of the isotherm by using the Barrett-Joyner-Halenda (BJH) method. Fourier transform infrared (FT-IR) spectra were recorded on samples embedded in KBr pellets with a GX spectrophotometer (Perkin-Elmer). Results and Discussion Effect of Carbon/Silica Ratios. Figure 1 shows the pore size distributions of mesoporous C-SiO2 nanocomposites with different C/SiO2 ratios. When the C/SiO2 molar ratio ranges from 1.90/1 to 4.21/1, the pore window sizes for these samples are almost the same by comparing the hysteresis loops of the sorption isotherms (Figure 1a) and pore size distributions (Figure 1b). The peak pore size slightly varies from 4.5 to 3.9 nm, which indicates a minor difference in their pore structures. This is consistent with the result reported in our previous work.40 The BET surface area and pore volume of these samples are summarized in Table 1. Mesoporous C-SiO2 nanocomposites were heated at 1450 °C for 5 h under flowing argon to obtain silicon carbide. The
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TABLE 1: C/SiO2 Molar Ratio and Nitrogen Sorption Results of Mesoporous C-SiO2 Nanocomposites C-SiO2 nanocomposite P-CS-1.90 P-CS-2.51 P-CS-3.28 P-CS-4.21 P-CS-1.90C P-CS-2.51C
C/SiO2 (mol/mol)
BET surface area (m2/g)
pore vol (cm3/g)
peak pore size (nm)
1.90 2.51 3.28 4.21 2.13 2.55
534.7 533.7 561.8 487.8 477.7 479.0
0.57 0.53 0.53 0.44 0.52 0.49
4.3 4.4 4.5 3.9 4.3 4.4
products from P-CS-1.90, P-CS-2.51, P-CS-3.28, and P-CS4.21 were denoted CS-1.90, CS-2.51, CS-3.28, and CS-4.21, respectively. XRD patterns shown in Figure 2 reveal that both CS-1.90 and CS-2.51 remain amorphous, whereas CS-3.28 and CS-4.21 exhibit crystalline structures of β-SiC as the diffraction peaks are indexed. Meanwhile, CS-3.28 and CS-4.21 exhibit a broad peak centered at about 2θ ) 23°, which resulted from the residue amorphous carbon and silica.8 Figure 3 shows the SEM images of CS-1.90, CS-2.51, CS3.28, and CS-4.21. Both CS-1.90 and CS-2.51 retain their original shapes of mesoporous C-SiO2 nanocomposites micrometers in size. CS-3.28 and CS-4.21 are composed of nanofibers and nanoparticles. These nanofibers are 20-50 nm in diameter and several micrometers long. It is found that CS3.28 and CS-4.21 can be easily dispersed in water or ethanol under ultrasonication. TEM images of the nanofibers and nanoparticles from CS-4.21 are shown in Figure 4. The SiC nanofibers possess a high density of planar defects, stacking faults, which have been observed in a previous study.34 The nanoparticles (Figure 4b) exhibit irregular shapes 60-100 nm in size. SAED patterns inset in Figure 4 confirm that both SiC nanofibers and nanoparticles are highly crystalline. N2 sorption measurements show that CS-1.90 and CS-2.51 have a very low BET surface area (3.1 m2/g for CS-1.90, 18.2 m2/g for CS2.51) and they are almost nonporous, whereas CS-3.28 and CS4.21 have BET surface areas of 87.2 and 87.4 m2/g, respectively. The adsorption-desorption isotherms and pore size distributions of CS-3.28 and CS-4.21 are plotted in Figure 5. These two samples possess significantly lower N2 adsorption capacity as compared with their precursors (Figure 1). The mesopores with bimodal size distribution arise from the unconverted mesoporous C-SiO2 nanocomposites and the packing pores of SiC nanoparticles. The pore volume is calculated from nitrogen sorption results to be 0.19 cm3/g for CS-3.28 and 0.26 cm3/g for CS4.21. After treatment with 10% NaOH and calcination at 600 °C, their BET surface area somewhat decreases (e.g., 83.0 m2/g for treated CS-3.28 and 76.6 m2/g for treated CS-4.21). This
Figure 2. XRD patterns of the products obtained from mesoporous C-SiO2 nanocomposites (P-CS-1.90, P-CS-2.51, P-CS-3.28, and P-CS4.21): CS-1.90 (a), CS-2.51 (b), CS-3.28(c), and CS-4.21 (d).
Figure 3. SEM images of CS-1.90 (a), CS- 2.51(b), CS-3.28 (c), and CS-4.21 (d).
Figure 4. TEM images and selected-area electron diffraction patterns (inset) of CS-4.21: nanofibers (a) and nanoparticles (b).
Figure 5. Nitrogen adsorption-desorption isotherm and the pore size distribution (inset) of CS-3.28 and CS-4.21.
confirms that the mesoporous C-SiO2 nanocomposites are mostly converted into SiC nanofibers and nanoparticles. Carbon Infiltration in Mesoporous C-SiO2 Nanocomposites. The mesoporous carbon-silica nanocomposites with a low carbon content (P-CS-1.90 and P-CS-2.51) were infiltrated with a small amount of PFA and then carbonized again. The resulting nanocomposites were denoted P-CS-1.90C and P-CS2.51C. TGA results indicated that the molar ratio of carbon to silica slightly increased from 1.90/1 to 2.13/1 for P-CS-1.90C and from 2.51/1 to 2.55/1 for P-CS-2.51C (Table 1). Figure 6 shows the nitrogen adsorption-desorption isotherms and pore size distributions of P-CS-1.90 and P-CS-1.90C and P-CS-2.51 and P-CS-2.51C. When carbon was infiltrated into the mesoporous C-SiO2 composites, the overall nitrogen sorption capacity (P-CS-1.90C and P-CS-2.51C) slightly decreased
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Figure 6. Nitrogen adsorption-desorption isotherms and pore size distributions (inset) of mesoporous C-SiO2 nanocomposites before and after infiltration with a small amount of carbon: P-CS-1.90 and P-CS-1.90C (a) and P-CS-2.51 and P-CS-2.51C (b).
Figure 7. SEM images of CS-1.90C (a) and CS-2.51C (b) after they were heated at 1450 °C under argon. The mesoporous C-SiO2 nanocomposites infiltrated with a small amount of carbon result in highly crystalline SiC with nanofibers and nanoparticles.
(Figure 6). The pore volume decreased from 0.57 cm3/g for P-CS-1.90 to 0.52 cm3/g for P-CS-1.90C and from 0.53 cm3/g for P-CS-2.51 to 0.49 cm3/g for P-CS-2.51C (Table 1). However, the adsorption-desorption hysteresis loops do not shift, suggesting that their pore window sizes (Figure 6) remain almost unchanged. This can be clearly seen in the pore size distributions inset in Figure 6. P-CS-1.90C and P-CS-2.51C were heated at 1450 °C for 5 h; the resulting samples (denoted CS-1.90C and CS-2.51C) exhibit a highly crystalline β-SiC phase (XRD patterns not shown). SEM images (Figure 7) show mixtures of nanofibers and nanoparticles in both CS-1.90C and CS-2.51C. Obviously, nanoparticles dominate in CS-1.90C (less than 30% nanofibers), whereas nanofibers dominate in CS-2.51C (less than 15% nanoparticles). Dense Carbon-Silica Nanocomposites. C-SiO2 composites (P-CS-4.21D) prepared without addition of P123 had a BET surface area of 51.1 m2/g and a pore volume of 0.02 cm3/g. This indicates that the interpenetrating C-SiO2 structure was nearly dense.41,42 It is expected that such an interpenetrating C-SiO2 showed an extremely low permeability to gaseous species SiO, CO, and CO2. Figure 8a shows the XRD patterns of the product (CS-4.21D) prepared by heating P-CS-4.21D at 1450 °C for 5 h. A broad peak centered at about 2θ ) 23° suggested the presence of amorphous silica and carbon, and low XRD peak intensities of SiC indicated the CS-4.21D possessed low crystallinity. The SEM image (Figure 8b) shows that CS-4.21D is made up of microsized particles. The TEM image of CS-4.21D shows a typical wormhole pore structure (Figure 8c). The IR spectra of P-CS-4.21D and CS-4.21D are shown in Figure 9. The absorption band at 1080 cm-1 in P-CS-4.21D corresponds to the asymmetric Si-O stretching vibration. After heat treatment, the band at 1080 cm-1 is significantly weakened
Figure 8. XRD pattern (a) and SEM (b) and TEM (C) images of the product (CS-4.21D) prepared by heating dense C-SiO2 composites (PCS-4.21D) at 1450 °C for 5 h.
Figure 9. FT-IR spectra of the dense C-SiO2 nanocomposite (PCS-4.21D) (a) and the resulting mesoporous SiC (CS-4.21D) (b).
whereas the band at around 830 cm-1 ascribed to the Si-C vibrations appears in CS-4.21D.12 This confirms that the Si-C structure was formed and some SiO2 was still present in CS4.21D. The BET surface area of as-prepared CS-4.21D is 234.7 m2/g. The nitrogen adsorption-desorption isotherm (Figure 10) indicates CS-4.21D is mesoporous, and the peak pore size (Figure 10, inset) is 7.6 nm. This result is consistent with the TEM observations. After residual SiO2 and carbon were removed, the BET surface area of treated CS-4.21D significantly decreased to 24.5 m2/g. This is because the mesoporous structures collapsed after removal of residual SiO2 and carbon due to a low extent of SiC crystallization.
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Figure 10. Nitrogen adsorption-desorption isotherm and pore size distribution (inset) of CS-4.21D.
Formation Mechanism of SiC in Carbon-Silica Nanocomposites. The carbothermal reduction reaction between silica and carbon yields SiC through the following steps:23,36,43
SiO2(s) + C(s) f SiO(g) + CO(g)
(1)
SiO2(s) + CO(g) f SiO(g) + CO2(g)
(2)
SiO(g) + C(s) f SiC(s) + CO(g)
(3)
SiO(g) + 2CO(g) f SiC(s) + CO2(g)
(4)
CO2(g) + C(s) f 2CO(g)
(5)
(6)
Figure 11. Schematic diagram of the formation of SiC nanostructures: (a) SiC nanofibers and nanoparticles from mesoporous C-SiO2 nanocomposites with high C/SiO2 ratios (e.g., CS-3.28 and CS-4.21), (b) SiC nanofibers and nanoparticles from mesoporous C-SiO2 nanocomposites with low C/SiO2 ratios and infiltrated with a small amount of carbon (e.g., CS-1.91C and CS-2.51C), and (c) mesoporous SiC from dense C/SiO2 nanocomposites (e.g., CS-4.21D).
During the carbothermal reduction reaction, SiO2 reacts with carbon, leading to gaseous silicon monoxide (SiO). SiC is then produced by the reactions between SiO and C through eq 3 or between SiO and CO through eq 4. The equilibrium conditions at each step depend on the temperature and partial pressures of SiO (pSiO) and CO (pCO). In our case, when the C/SiO2 ratio is low (e.g., 1.90 and 2.51, less than the C/SiO2 stoichiometric ratio in eq 6), the rate of SiO production through eqs 1 and 2 is very small. Therefore, no SiC can be detected by XRD in both CS-1.90 and CS-2.51. Instead mesopores in the two samples collapsed, and dense structures were obtained. When the C/SiO2 is raised to 3.28, the high carbon content in mesoporous pore walls gives higher partial pressures of SiO and CO. Flowing argon gas may carry away very little SiO and CO from the mesoporous C-SiO2 samples because of small pore channels. SiC may nucleate through eq 3 throughout the mesoporous precursors. As soon as SiC nuclei form, SiC nanofibers grow via gas-phase reaction between SiO and CO on the surface of mesoporous C-SiO2 particles14 whereas SiC nanoparticles form inside the precursor particles since gas products generated from interpenetrating C-SiO2 pore walls diffuse through mesoporous channels to accelerate the reaction kinetically. After nanoparticles form, internal mesopores of the precursors collapse; free space is thus generated, allowing gasphase reaction to form SiC nanofibers (Figures 3c,d and 7). The formation of mixtures of SiC nanofibers and nanoparticles is illustrated in Figure 11a. While the mesoporous C-SiO2 nanocomposites with low C/SiO2 ratios (i.e., 1.90 and 2.51) were infiltrated with a small amount of carbon, the interface between C/SiO2 pore walls and infiltrated carbon significantly enhances the local concentrations of SiO and CO and lowers the oxygen partial pressure. Locally concentrated SiO species readily react with carbon, resulting in a large number of SiC nuclei throughout the mesoporous
C-SiO2 nanocomposites, and nanofibers and nanoparticles form by the process similar to that described above (Figure 11b). In the case of dense C-SiO2 nanocomposites with high C/SiO2 ratios (i.e., P-CS-4.21D), the reaction between SiO2 and carbon is suppressed inside the dense C-SiO2 nanocomposites because there are no porous channels for diffusion of the gaseous products. The carbothermal reduction reaction takes place only starting from the surface of the dense C-SiO2 nanocomposites. Owing to diffusion limitation, the interpenetrating C-SiO2 is slowly transformed into SiC domains, and the consumption of carbon and silica facilitates the formation of pores on the surfaces. As the carbothermal reduction reaction proceeds, SiC pore walls develop at the interface between exterior porous SiC and interior C-SiO2 due to concurrent gas transport and solid SiC formation. It is worth mentioning that, from a thermodynamic point of view, the driving force for SiC domain growth is quite small because of the relatively low reaction temperature (1450 °C), and the porous structure therefore remains almost unchanged once formed. The pore channels eventually develop uniformly throughout the dense C-SiO2 nanocomposites, leading to mesoporous SiC (Figure 11c). To better understand the role of diffusion channels in the formation of SiC from the C-SiO2 nanocomposites, the PFASiO2-P123 nanocomposites were coated with a PFA layer and then carbonized to form mesoporous C-SiO2 nanocomposites (C/SiO2 ) 4.21) coated with a carbon layer. Such carbon-coated mesoporous C-SiO2 nanocomposites having the same internal mesoporous structures as P-CS-4.21 were heated under the same conditions (1450 °C for 5 h and flowing argon). XRD measurement showed that no crystalline SiC phase was formed. It was reported that PFA derived carbon layer became very dense at high temperature and was almost impermeable to gases.42 As a result, gaseous SiO, CO, and CO2 products were trapped inside the mesopores, and the carbothermal reduction reaction could
The overall reaction is
SiO2(s) + 3C(s) f SiC(s) + 2CO(g)
Carbothermal Reduction of C-SiO2 Nanocomposites not proceed. This strongly suggests that the paths for gaseous diffusion play a vital role in SiC formation through carbothermal reduction. Conclusions We have shown that the pores of C-SiO2 nanocomposites play a key role in the carbothermal reduction reaction, and the C/SiO2 ratio and C-SiO2 carbon structure are among other factors affecting the reaction kinetics and thus the nanostructures of resulting SiC. Mixed SiC nanofibers and nanoparticles were produced from mesoporous C-SiO2 nanocomposites; the proportion of nanofibers (or nanoparticles) was tunable by changing the C/SiO2 ratio and using carbon infiltration. Mesoporous SiC was obtained from dense C-SiO2 nanocomposites with a high C/SiO2 ratio (e.g., 4.21). This is because the carbothermal reduction rate can be promoted by a high C/SiO2 ratio and a C-SiO2 carbon interfacial structure, and mesoporous channels provide diffusion paths for the gaseous SiO, CO, and CO2 products and confine SiC growth inside the mesoporous C-SiO2 nanocomposites. The results presented here provide new insight into the formation of SiC nanostructures through carbothermal reduction. It should be possible to synthesize pure SiC nanofibers, nanoparticles, and mesoporous structures by carefully designing structures of their precursors. Acknowledgment. This work was supported by the Australian Research Council and Monash University. The technical assistance from staff at the Electron Microscopy and Microanalysis Facility of Monash University is gratefully acknowledged. H.W. thanks the Australian Research Council for the QEII Fellowship (Grant DP0559724). References and Notes (1) Li, H. J.; Li, Z. J.; Meng, A. L.; Li, K. Z.; Zhang, X. N.; Xu, Y. P. J. Alloys Compd. 2003, 352, 279-282. (2) Madapura, S.; Steckl, A. J.; Loboda, M. J. Electrochem. Soc. 1999, 146, 1197-1202. (3) Yuan, C.; Steckl, A. J.; Loboda, M. J. Appl. Phys. Lett. 1994, 64, 3000-3002. (4) Kim, H. Y.; Bae, S. Y.; Kim, N. S.; Park, J. Chem. Commun. 2003, 2634-2635. (5) Ho, G. W.; Wong, A. S. W.; Kang, D. J.; Welland, M. E. Nanotechnology 2004, 15, 996-999. (6) Patel, N.; Kawai, R.; Oya, A. J. Mater. Sci. 2004, 39, 691-693. (7) Parmentier, J.; Patarin, J.; Dentzer, J.; Vix-Guterl, C. Ceram. Int. 2002, 28, 1-7. (8) Yang, Z. X.; Xia, Y. D.; Mokaya, R. Chem. Mater. 2004, 16, 38773884. (9) Lu, A. H.; Schmidt, W.; Kiefer, W.; Schuth, F. J. Mater. Sci. 2005, 40, 5091-5093. (10) Krawiec, P.; Weidenthaler, C.; Kaskel, S. Chem. Mater. 2004, 16, 2869-2880. (11) Yan, J.; Wang, A. J.; Kim, D. P. J. Phys. Chem. B 2006, 110, 5429-5433.
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