Selective Synthesis of 3C-SiC Hollow Nanospheres and Nanowires Peng Li,† Liqiang Xu,*,† and Yitai Qian†,‡ Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan, Shandong, 250100, P.R. China, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, P.R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2431–2436
ReceiVed January 4, 2008; ReVised Manuscript ReceiVed April 20, 2008
ABSTRACT: 3C-SiC hollow nanospheres with a high yield (∼80%) were prepared by using SiCl4, CBr3H, and Na-K alloy at 130 °C for 15 h and a subsequent HClO4 treatment process at 180 °C. These SiC hollow nanospheres have diameters in the range of 80-120 nm and an average shell thickness of ∼15 nm. High resolution transmission electron microscopy investigation reveals that these hollow spherical nanocrystals have rough surfaces, indicating they are composed of nanoparticles. When Na-K alloy was substituted by Na (or K) and in the mean time the temperature was set at 240 °C while keeping other conditions unchanged, a large quantity of randomly distributed and highly crystalline SiC nanowires with diameters ranging from 30 to 50 nm and lengths up to several tens of micrometers also can be produced. The possible formation mechanisms of the products with distinct dimensions were briefly discussed. The method used here generally could be used to synthesize other carbides at low temperature.
1. Introduction SiC has attracted extensively attention due to its excellent properties, including high breakdown electric field strength, high saturated drift velocity of electrons, wide band gap, high thermal conductivity, high mechanical strength, high chemical stability, and low induced activity;1 therefore, SiC-based devices could be used at higher temperature and in harsh conditions as a functional ceramic or as a high temperature semiconductor.2 SiC with different nanostructures including nanowires,3 nanotubes,4 nanorods,5 nanobelts,6 solid nanospheres,7 nanocages,8 etc. have been synthesized, and many indications show that their electrical and optical properties exhibit strong dependences on shape-structure and size.9 In recent years, SiC hollow spheres have attracted increasing attention because they can be potentially used as sensing devices, controllable release catalysts, drug delivery cells, lightweight fillers, and shape-selective disorbents.10 However, compared with the above-mentioned SiC structures, there are few reports about the synthesis of SiC with hollow spherical morphology. For example, spherical 3C-SiC powders of a few micrometers in size have been prepared by heating a mixture of phenolic resin powder and fine-grained fumed silica at 1600 °C in argon,11 or have been derived from the carbothermal reduction of phenolic resin-ethylsilicate heterogeneous precursors at 1800 °C.12 SiC hollow nanospheres and nanowires were also selectively synthesized through the sodium reduction of silicon tetrachloride and hexachlorobenzene in an autoclave at 600-700 °C.13 Recently, hollow and monodispersed SiC spheres were prepared by the solid-gas reactions of carbon spheres with silicon14 or silicon monoxide15 above 1000 °C, the sizes and shells of these SiC hollow spheres can be controlled by properly adjusting the reaction conditions. Silicon carbide (SiC) nanowires possess high elasticity and strength, which may be utilized as good candidates for making various types of composites. Recent results16 showed that the superplasticity of single-crystalline 3C-SiC nanowires with higher than 200% elongation compared with bulk SiC was * Author to whom correspondence should be addressed. E-mail: xulq@ sdu.edu.cn. Fax: 86-531-88366280. † Shandong University. ‡ University of Science and Technology of China.
directly found by in situ axial-tensile experiments; these asproduced 3C-SiC nanowires with superplasticity make them promising for use as the reinforcing element in ceramic-, metal-, and polymer-matrix composites. Several techniques have been developed for the synthesis of SiC nanowires; for instance, SiC nanowires were synthesized by the reaction between carbon nanotubes (CNTs) and SiO (or SiI2) in a sealed tube under vacuum in 1300-1400 °C (or in 1100-1200 °C),5c or by using generated SiO vapor with CNTs at 1400 °C in Ar atmosphere.17 Lu et al.5b also fabricated SiC nanorods by using SiCl4, CCl4 and Na as raw materials at 400 °C through a self-catalyzed vapor-liquid-solid (VLS) growth process. Very recently, SiC nanowires of high purity were prepared through a catalyst-free arc-discharge process using silicon dioxide powders as filler in a graphite anode.18 To our knowledge, there have been few reports about the synthesis of SiC hollow nanospheres and nanowires below 250 °C. In this study, we report a metal coreduction route for the selective synthesis of 3C-SiC hollow nanospheres and nanowires in 130-240 °C, respectively. High resolution transmission electron microscopy (HRTEM) observations indicate that the particles have rough surfaces. These SiC hollow nanospheres have diameters in the range of 80-120 nm and an average wall thickness of ∼15 nm. The yield of the hollow nanospheres is estimated to be about 80%. When Na-K alloy was substituted by Na (or K) in the mean time the temperature was set at 240 °C, 3C-SiC nanowires with diameters ranging from 30 to 50 nm and lengths of mainly several tens of micrometers also were produced. The overall reactions in this experiment can be described as follows:
2SiCl4 + 2CHBr3 + 14M f 2SiC + 14MX + H2 (M ) Na-K alloy or Na or K; X ) Cl or Br) 2. Experimental Section All the reagents (Shanghai Chemical Reagents Company) used in these experiments were analytically pure and were used without further purification. [Warning: All of the manipulations (before or after the reaction) were performed in a glovebox with flowing nitrogen gas, and these procedures should be done carefully and quickly.] In a typical procedure, Na-K alloy (made up of mixed 1.5 g of Na and 1.5 g of K) was loaded into a 15-mL stainless steel autoclave, and then CHBr3 (1.5 mL) and SiCl4 (1.5 mL) were added into the same autoclave. After
10.1021/cg800008f CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
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Figure 1. (a) A typical XRD pattern, and (b) FTIR spectrum of Sample 1. the autoclave was sealed, it was maintained at 130 °C for 15 h and was cooled to room temperature naturally. The products in the autoclave were collected and washed with 1 M HCl, 40% HF solution, and hot concentrated 80% HClO4 (at 180 °C), respectively, to remove alkali metal halides, Si, and amorphous carbon. Finally, the gray product (labeled as “Sample 1”) was washed with distilled water and absolute alcohol several times and was dried in a vacuum at 60 °C for 3 h. When Na-K alloy was substituted by Na (or K) and in the mean time the temperature was set at 240 °C, the as-obtained product was labeled as “Sample 2” (or “Sample 3”). X-ray powder diffraction (XRD) measurements were determined using a Bruker D8 advanced X-ray diffractmeter equipped with graphite monochromatized Cu KR radiation (λ ) 1.5418 Å) and Ni filter from 15° to 80° (2θ). The morphology and structure of the products were investigated by transmission electron microscopy (TEM, Hitachi H7000), and high-resolution TEM (HRTEM, JEOL-2100, 200kV). The Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Bruker Alpha-T. The Raman spectra were obtained from a NEXUS 670 FT-IR Raman spectrometer. Photoluminescence (PL) spectra were measured by a FL9920.
3. Results and Discussion The phases of the final products were examined by XRD. Figure 1a shows a typical XRD pattern of Sample 1. The distinct diffraction peaks in Figure 1a can be indexed as 3C-SiC with lattice constant of a ) 4.349 Å, which is close to the reported value of 3C-SiC (a ) 4.359 Å, JCPDS card no. 29-1129). The appearance of the peak that centered at d ) 2.653 Å (marked with “SF”) with low diffraction intensity may originate from the stacking faults.19 No noticeable diffraction peaks of other impurities such as Si, SiO2, and C are detected in this pattern. A typical FTIR spectrum of Sample 1 is shown in Figure 1b. The weak absorption vibration peak which centered at about 1640 cm-1 was indexed as the H-O-H bending vibration, and its appearance was due to the absorption of water on this sample. The intense absorption peak centered at ∼870 cm-1 could be assigned to the transverse optical (TO) photon vibration mode of the Si-C bond.20 Detailed structure and morphology analysis of the products was further carried out with TEM, electron diffraction (ED), and HRTEM. As can be seen from the low-magnification TEM images (Figure 2a,b), Sample 1 is mainly composed of SiC spherical nanocrystals, and a few randomly distributed onedimensional nanowires with uniform diameters were found coexisting with them. Analysis results of a number of products produced by the same process as Sample 1 indicate that the proportion of the nanospheres (most of them have diameters ranging from 80 to 120 nm) in the product is approximately 80%. The nanowires that coexisted with the nanospheres have a diameter of ∼30-50 nm and lengths up to several tens of micrometers. The high-magnification TEM image of a
randomly selected nanosphere (Figure 2c) shows that the nanosphere has a rough surface, and its corresponding HRTEM image (Figure 2d) reveals that the nanostructure has an average interplanar spacing of 0.25 nm, which corresponds to (111) spacing of 3C-SiC. Meanwhile, they existed in different orientations, which confirm the polycrystalline structure of the shell. This result is consonant with the ED pattern presented above (inset in Figure 2b). A typical Raman spectrum of Sample 1 is shown in Figure 3a. The peaks centered at 786 and 930 cm-1 are usually associated with the TO and LO phonon at the Γ point of the cubic SiC, respectively. But the LO phonon line is not only broadened but also shifted by 30-50 cm-1 toward high energy direction in the present spectrum, which is strikingly different from other reports in the literature.13,17 The reasons for these exceptions may be attributed to the reduction of grain sizes and polycrystalline structure of the nanospheres.21 Figure 3b shows the room temperature PL spectrum (with the excitation wavelength of 375 nm and filter wavelength of 420 nm) of Sample 1. It is clearly seen that an intense peak is observed with a maximum at ∼443 nm. The strong light emission is similar to the previous report of 3C-SiC hollow spheres.13 Compared with the photoluminescence spectra of SiC films,22 nanotubes,4a and nanowires,3d the present emission peak of SiC nanospheres is obviously blue-shifted, and the appearance of this phenomenon may be due to the morphology and size confinement effects;9a,23 however, detailed interpretation of the Raman and PL spectra is needed. To substantially study the formation process of the hollow nanospheres under the present synthesis route, we have systematically surveyed their growth process by changing the reaction time. It is found that in the first 7 h (at 130 °C), nearly all the CBr3H reacts and converts into black solids. Meanwhile, most SiCl4 and part of the Na-K alloy remain unreacted (which were removed carefully before the dilute acid treatment process). The TEM image in Figure 4b shows that the product is composed of aggregated and uniform particles with diameters of 50-150 nm. Some dark spots dispersed in the light particles with size of several nanometers also can be seen, and these particles are sheathed by a thin shell on the surface. The result of the XRD pattern in Figure 4a (I) confirms that these particles are mainly composed of amorphous graphite. The XRD pattern (Figure 4a (II)) of another sample (obtained after heating at 130 °C for 10 h process) reveals a little Si (JCPDS card no. 27-1402) coexisted with SiC. The TEM image (Figure 4c) shows a solid sphere with a residual core. So it is reasonable to speculate that the SiC hollow nanospheres hold the shape and diameter of carbon nanoparticles, which implies that the final
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Figure 2. (a) and (b) Typical TEM images of Sample 1, and the inset in panel (b) is the corresponding ED pattern of a SiC nanosphere. (c) A high magnification TEM image of a nanosphere; (d) corresponding HRTEM image of the edge of the sphere that marked with a white square in (c).
Figure 3. (a) Raman spectrum and (b) room temperature PL spectrum of Sample 1.
shape of spherical SiC partilces maintains the original carbon sphere skeleton and reflects an in situ shape memory.5c,15,24 In the present system, it is found that the reaction temperature and metal are vital factors for the shape controlled synthesis of SiC, and the results are listed in Table 1. To investigate the
influence of temperature, the reactions were carried out at different temperatures but for the same reaction time (15 h). It is found that no solid powders were obtained when the temperature was below 100 °C. If the temperature was set at 240 °C while keeping other parameters constant, hollow SiC
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Li et al. Table 1. As-Obtained Final Products Obtained with Different Metals As Reductants or at Different Reaction Temperatures metal
Figure 4. (a) XRD patterns of the products produced after heating at 130 °C for 7 h (I) and 10 h (II); (b) a typical TEM image of the sample obtained after heating at 130 °C for 7 h, revealing the uniform morphology of the product; (c) representative TEM image of a SiC hollow nanosphere with a residual core (obtained after heating at 130 °C for 10 h). All the products were purified by 1 M HCl.
nanospheres with smooth surfaces (inset in left corner of Figure 5b) could also be produced but in a trace amount; the rest of the products are mainly composed of SiC hexagonal flakes and nanowires (Figure 5b).
temperature (°C) phase
Na-K alloy Na-K alloy
100 130
Na-K alloy
240
Na (or K) Na (or K) Na (or K)
160 180 240
morphology
figures
SiC hollow nanospheres Figure 2a-d (∼80%), and nanowires SiC nanowires and hexagonal Figure 5b nanoparticles, SiC amorphous (low yield) SiC nanowires
Figure 5c-e
To investigate the effect of Na-K alloy on the formation of SiC product, metallic Na (or K) was used instead of it as the reductant. It is found that the final morphology (Figure 5c-e) of the as-obtained crystalline SiC product was distinct from that of using the Na-K alloy. Figure 5d shows the TEM image of the product derived from Na (2.50 g), SiCl4 (1.5 mL), and CBr3H (1.5 mL) at 240 °C. The sample is composed of a large quantity of randomly distributed nanowires with uniform diameters. Analysis of a number of the nanowires shows that most of them have diameters ranging from 30 to 50 nm and lengths of several tens of micrometers. The HRTEM image shown in Figure 5e was obtained from the middle part of a straight nanowire (in the bottom left-hand corner of Figure 5e). The clearly resolved fringes separated by 0.25 and 0.22 nm correspond to the (111) and (200) lattice spacing of 3C-SiC, respectively. Its corresponding selected-area electron diffraction (SAED) pattern is depicted in the inset of Figure 5e, indicating that the nanowire is single crystalline with the growth direction along [111]. When the temperature is below 160 °C, almost no solid powders even amorphous or poorly crystalline SiC (obtained at 180 °C) can be collected. When K (1.50 g) was used instead of Na at different temperatures, similar results were obtained (see Figure 5c). In this experiment, CBr3H and SiCl4 act not only as reactants but also as solvents (boiling point of CBr3H: b.p. ) 146-151 °C; SiCl4: b.p. ) 57.6 °C) during the formation process of SiC. As the bond energy of C-Br (∆cHmo ) 284.5 kJ/mol) was lower than that of Si-Cl (∆cHmo ) 380.7 kJ/ mol), it is reasonable that CBr3H could be first reduced by Na-K alloy (b.p. ∼ -11 °C) to produce graphite clusters at 130 °C, and then these graphite clusters would aggregate into larger spherical partilces. The subsequently produced Si tends to adhere onto these spherical graphite partilces in order to reduce the total surface energy of the reaction system.13 It is worth noting that the longer reaction time (at least 7 h) at 130 °C is necessary for the gradual and complete decomposition of CBr3H or/and SiCl4 with Na-K alloy (see Figure 4b,c); this phenomenon was similar to that of the previously reported polysilyne synthesis procedure.8 The calculated results of Gibbs free energy show that the overall reaction occurred at 130 °C is thermodynamically spontaneous and highly exothermic (∆G ) -4.209 × 103 kJ/mol and ∆H ) -4.523 × 103 kJ/mol, which enables the reaction to proceed continuously. After the prolonged reacion time (15 h) and the HClO4 solution treatment process, hollow spherical SiC powders with high crystallinity were finally obtained. The above process could be partly evidenced by the observation of C/SiC core/shell structure without the HClO4 treatment process (see Figure 4c, Figure 2a-d). When the heating temperature was directly set at 240 °C, the reactants would turn into gas-state rapidly, while the metallic Na (m.p. ) 97.8 °C; or K, m.p. ) 63.2 °C) was in liquid-state. It is
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Figure 5. (a) XRD patterns of the samples obtained by using different metals as reductants at 240 °C for 15 h: (I) Na-K alloy, (II) K, and (III) Na; (b) TEM image of the final product using Na-K alloy; (c) typical TEM image of Sample 3; (d) typical TEM image of Sample 2; (e) HRTEM image of the middle part of a straight nanowire. The inset shows its selected area ED pattern (upper) and the low magnification TEM image of the nanowires (bottom).
considered that a similar self-catalyzed vapor-liquid-solid (VLS) growth process was largely responsible for the
formation of SiC nanowires.5b However, much work still needs to be carried out to explore the exact formation
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mechanisms of the as-obtained highly crystalline SiC nanospheres and nanowires.
4. Conclusions In summary, we have demonstrated a metal (Na-K alloy, Na, or K) coreduction route for the controlled synthesis of 3C-SiC nanospheres and nanowires at 180 and 240 °C, respectively. Studies show that the low reaction temperature and appropriate metal reductants are the key factors for the formation of SiC hollow nanospheres. These SiC hollow nanospheres are expected to be applied in many fields such as catalysis, hydrogen storage, and ceramic composites et al. In addition, 3C-SiC nanowires, which mainly have diameters ranging from 30 to 50 nm and lengths of up to several tens of micrometers, also could be synthesized by a Na reduction route at 240 °C. Their formation mechanisms are briefly discussed. The method used here can be generally extended to synthesize other carbides at low temperature. Acknowledgment. This work was supported by National Natural Science Found of China (Nos. 20671058, 20701026), the 973 Project of China (No. 2005CB623601) and Tai-Shan Scholar Project of Shandong Province.
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CG800008F
