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Synthesis and characterization of 3C and 2H-SiC nanocrystals starting from SiO2, C2H5OH and metallic Mg
Journal of Alloys and Compounds 484 (2009) 341–346
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Synthesis and characterization of 3C and 2H-SiC nanocrystals starting from SiO2 , C2 H5 OH and metallic Mg Ting Li, Liqiang Xu ∗ , Liancheng Wang, Lishan Yang, Yitai Qian ∗ Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, PR China
a r t i c l e
i n f o
Article history: Received 3 February 2009 Received in revised form 8 April 2009 Accepted 19 April 2009 Available online 24 April 2009 Keywords: Silicon carbide Nanostructured materials Transmission electron microscopy X-ray diffraction
a b s t r a c t Silicon carbide (3C-SiC) nanocrystals were prepared starting from SiO2 , C2 H5 OH, and metallic Mg in an autoclave at 200 ◦ C. X-ray diffraction patterns of the sample can be indexed as the cubic phase of SiC with the lattice constant a = 4.357 Å, in good agreement with the reported value (JCPDS card no. 291129; a = 4.359 Å). Transmission electron microscopy images show that the product mainly composed of nanowires with diameters in the range of 10–30 nm and lengths up to tens of micrometers; Highresolution transmission electron microscopy images reveal that these 3C-SiC nanowires grow along [1 1 1] direction; As polyvinylpyrrolidine was added into the above reactant system, the final products obtained at 180 ◦ C were mixed 3C and 2H-SiC flakes. Thermal gravimetric analysis curves reveal that these two samples have thermal stability below 800 ◦ C, and room-temperature photoluminescence spectrum of the 3C-SiC sample show a strong emission peak centered at 403 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is a semiconducting material with wide band gap (2.39 ev for 3C-SiC and 3.33 ev for 2H-SiC at room temperature) [1], which has high mechanical strength, high thermal conductivity, high breakdown electric field [2]. These unique physical and electronic properties make SiC a suitable material for the fabrication of electronic devices operating at high-temperature, high power, high frequency and in harsh environments [3]. Various methods have been developed for the synthesis of SiC nanocrystals, such as the carbothermal reduction reaction [4,5], chemical vapor reaction [6], sol–gel [7,8], self-propagating high temperature synthesis [9], autoclave route [10–15] and so on. Among these methods, prepare SiC in an autoclave is one of the effective routes at low temperature, such as the reactant systems: SiCl4 -Na-C [10] or SiCl4 -Na-C6 Cl6 [11] at 600 ◦ C, SiCl4 -Na-CCl4 at 400 ◦ C [12], SiCl4 -Na-K-CBr3 H at 130 ◦ C [13], or sulfur-assisted reduction route (Si-S-Na-C2 Cl4 ) at 130 ◦ C [14]. Moreover, 2H-SiC nanoflakes also have been prepared in an autoclave at 180 ◦ C [15]. In the previous reports [13–15], SiCl4 and Si were chosen as silicon sources, C2 Cl4 and CBr3 H were used as carbon sources, Na–K alloy and Na were used as reductant. Among the silicon precursors used to synthesize SiC, SiO2 is widely used due to the low cost, such as SiO2 -carbon black at 1200–1600 ◦ C [4,5], Si-SiO2 -C3 H6 at 1250 ◦ C [6], and so on. However, there are fewer reports about the
∗ Corresponding authors. Tel.: +86 531 8836 6280; fax: +86 531 8836 6280. E-mail address: [email protected] (L. Xu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.096
synthesis of SiC by using SiO2 as silicon sources at relative low temperature (below 1000 ◦ C). In the current study, SiO2 was used as silicon source, C2 H5 OH as carbon source, and metallic Mg substitute Na–K alloy and Na was used as reductant. The advantage of this route is that parlous or corrosive reagents have been avoided. In this study, 3C-SiC nanocrystals were prepared starting from SiO2 , C2 H5 OH, and Mg at 200 ◦ C; As polyvinylpyrrolidine (PVP) was added into the above reactant system, the final product obtained at 180–200 ◦ C were mixed 3C and 2H-SiC. The yield of 3C-SiC sample prepared at 200 ◦ C was about 23%, calculated based on the amount of SiO2 . The yield of mixed 3C and 2H-SiC sample prepared at 180 ◦ C was about 57%. Low temperature and cheap raw materials make it possible for large scale synthesis of SiC nanocrystals. The reaction can be described as follows: SiO2 + C2 H5 OH + 3Mg = SiC + 3MgO + C + 3H2
(1)
Thermal gravimetric analysis (TGA) curves of the product reveal that the as-prepared samples have high thermal stability below 800 ◦ C. Room-temperature photoluminescence (PL) spectrum of the 3C-SiC sample exhibit a strong emission peak centered at 403 nm. 2. Experimental 2.1. Preparation of 3C-SiC nanocrystals In a typical process, 10 ml C2 H5 OH (analytical grade), 3.6 g SiO2 (analytical grade, Shanghai Chemical Reagents Co.), and 1.5 g Mg (Tianjin damao Chemical Reagents Co., 99%) were loaded into a stainless steel autoclave with a capacity of about 20 ml. The autoclave was sealed and maintained at 200 ◦ C for 10 h, then cooled to room temperature naturally. The raw products in the autoclave were collected and washed
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with dilute HCl to remove the MgO byproducts, then treated with the mixture of HNO3 and HF with the volume ratio of 1:1 to remove the unreacted SiO2 and the intermediate Si. In order to eliminate the residual carbon, the sample were treated by HClO4 (70%) at 150–180 ◦ C (calcine the products at 600 ◦ C under air atmosphere can achieve the same result), then treated with HF again, finally they were washed with distilled water and anhydrous ethanol. After drying in vacuum at 70 ◦ C for 6 h, a gray-white sample was finally obtained. 2.2. Preparation of mixed 3C and 2H-SiC nanocrystals In a typical procedure, 10 ml C2 H5 OH (analytical grade), 1.2 g SiO2 (analytical grade, Shanghai Chemical Reagents Co.), 1.5 g Mg (Tianjin damao Chemical Reagents Co., 99%) and 0.5 g polyvinylpyrrolidine (PVP, Shanghai Chemical Reagents Co.) were loaded into an autoclave with a capacity about 20 ml. The autoclave was sealed and maintained at 180 ◦ C for 10 h, then cooled to room temperature naturally. The product was collected and treated via similar processes as mentioned above. After dried in vacuum, the final sample was obtained. 2.3. Characterization The X-ray powder diffraction (XRD) patterns of the products were recorded on a Bruker D8 advanced X-ray diffractometer with Cu K␣ radiation ( = 1.5418 Å). Transmission electron microscope (TEM) images were taken on a Hitachi H-7000 transmission electron microscope, using an accelerating voltage of 100 kV. The highresolution transmission electron microscope (HRTEM) images were performed on a JEOL 2100 transmission electron microscope operated at 200 kV. The infrared (IR) spectra were recorded on a Bruker VERTEX 70, using a KBr wafer. Thermal gravimetric analysis was taken on a SDT Q600 V8.0 Build 95 thermal analyzer apparatus under air flow. Photoluminescence spectrum measurement was performed in an Edinburgh instruments FLS920 fluorescence spectrophotometer with a Xe lamp at room temperature.
3. Results and discussion Phase identification of the as-obtained products was carried out using the XRD pattern. Fig. 1a shows XRD pattern of the 3C-SiC sample obtained before the HClO4 treatment, revealing the main phase of the products is crystalline 3C-SiC, besides 3C-SiC, the diffraction peaks of residual carbon was observed. Fig. 1b shows the XRD pattern of the same sample after treated by hot HClO4 solution. The peaks with strong diffraction intensity indicate that the sample was well crystalline. It is found that the peaks centered at 2 = 35.66◦ , 41.42◦ , 59.99◦ , 71.76◦ and 75.51◦ can be indexed as the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) reflections of 3C-SiC, respectively. The appearance of low-intensity peak near 2 = 33.5◦ (marked with SF) is due to the stacking faults [16]. The calculated lattice constant a = 4.357 Å agrees well with the reported values of -SiC (JCPDS card no. 29-1129). No other impurities have been detected. The remarkable enhanced (1 1 1) peak suggests the preferential growth orientation of the product. This result was further proved by the following HRTEM investigation. Fig. 2a shows the XRD pattern of the product prepared by using SiO2 , C2 H5 OH and Mg at 180 ◦ C for 10 h. All the peaks in this pattern can be indexed as 3C-SiC (JCPDS card no. 29-1129). Compared with
Fig. 1. XRD patterns of the 3C-SiC sample prepared at 200 ◦ C. (a) Before treated by HClO4 ; (b) after treated by HClO4 .
Fig. 2. XRD patterns of the samples prepared by using (a) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg at 180 ◦ C for 10 h; (b) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVP at 180 ◦ C for 10 h; (c) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVP at 200 ◦ C for 10 h.
that of Fig. 2a, the result of Fig. 2b reveals that the product with mixed phases of 3C and 2H-SiC has been obtained when PVP was additionally used as the reactant. The peaks in Fig. 2b that signed by black dots can be indexed as 2H-SiC, the calculated lattice constants of a = 3.079 Å and c = 5.037 Å agree well with those of the 2H-SiC (JCPDS card no. 29-1126), while the peaks that marked with black rectangle can be assigned to 3C-SiC, the calculated lattice constant a = 4.357 Å is close to the reported value of -SiC (JCPDS card no. 29-1129). The typical IR spectra of the as-synthesized samples are shown in Fig. 3. The obvious absorption peaks centered at ∼817 cm−1 can be assigned to the transverse optical (TO) photon vibration mode of the Si-C bond [17]. The weak absorption peak (shown in Fig. 3b) at ∼1110 cm−1 resulted from the asymmetry stretching vibration of SiO [18], which indicates that the sample still has very small amount of SiO2 , the weaker absorption peak at ∼1630 cm−1 corresponds to the H-O-H bending vibration could be attributed to the absorption of water on the sample [19]. The above XRD patterns and IR spectra indicate that the as-synthesized samples through the present synthesis route were crystalline SiC. The morphology and structure of the final products were determined by TEM and HRTEM. The TEM image shown in Fig. 4a reveals that the 3C-SiC sample was mainly consists of nanowires (∼60%) with diameters ranging from 10 to 30 nm and lengths up to several tens of micrometers. The HRTEM image of part of a single SiC nanowire (in Fig. 4b) reveals that the interplanar spacing of the two adjacent fringes is about 0.25 nm, which corresponds to the
Fig. 3. IR spectra of the samples. (a) 3C-SiC obtained at 200 ◦ C; (b) mixed phases of 3C and 2H-SiC obtained at 180 ◦ C.
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(1 1 1) spacing of 3C-SiC. In addition, the [1 1 1] direction is parallel to the axis of the nanowire, indicating that the nanowire grows along the [1 1 1] direction. Fig. 4d shows the high-resolution TEM image of the area of an individual nanowire (shown in Fig. 4c), in which the interfringe distances of 0.25 nm and 0.21 nm are consistent with (1 1 1) and (2 0 0) planes of 3C-SiC (JCPDS card no. 29-1129), respectively, and the stacking faults could be observed clearly as magnified in right corner of Fig. 4d. The SAED (inset in Fig. 4d) indicates a streaking diffraction pattern characteristic of stacking faults. Combined with the XRD pattern (Fig. 1b) of the 3CSiC sample obtained at 200 ◦ C, the appearance of low-intensity peak near 2 = 33.5◦ (marked with SF) is due to stacking faults of 3C-SiC, which might belong to deformation fault [16]. Fig. 4e depicts a typical TEM image of the sample with mixed phases, displaying the products are mainly composed of irregular shaped nanoflakes and hexagonal flakes. The corresponding HRTEM image of a hexagonal flake (in Fig. 4f) shows the interfringe distances are 0.23 nm and
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0.26 nm, which are consistent with (1 0 1) and (1 0 0) planes of 2HSiC (JCPDS card no. 29-1126), respectively. HRTEM image (Fig. 4g) of an irregular flake signed by a black arrow reveals that the clearly resolved interplanar distance is 0.25 nm, which coincides with the (1 1 1) lattice spacing of 3C-SiC (JCPDS card no. 29-1129). The regular arranged lattice fringes can be clearly seen from the above HRTEM images, indicating the well crystalline of the as-prepared samples. The thermal stability of the SiC samples was assessed by thermogravimetric analysis (TGA). Fig. 5 shows the TGA curves carried out under air in the temperature range of 30–1100 ◦ C. Initially, it is observed from the TGA curve (Fig. 5a) that there is a slight weight loss (∼1%), which might be attributed to the loss of water that absorbed on the SiC sample surface. An obvious weight gain is observed above 800 ◦ C, suggesting SiC was oxidized in air atmosphere. This result is in agreement with the previous reports [20,21]. Compared to curve (a), curve (b) has a slight weight gain starts at about 400 ◦ C, probably because that smaller particle sizes
Fig. 4. (a) A typical TEM image of the 3C-SiC nanowires. (b) HRTEM image of a part of an individual 3C-SiC nanowire. (c) TEM image of a single 3C-SiC nanowire. (d) The corresponding HRTEM image of the SiC nanowire shown in (c), revealing the presence of stacking faults. Inset shows a streaking diffraction pattern (SAED) characteristic of stacking faults. (e) TEM images of the sample with mixed phases. (f) The corresponding HRTEM image of a hexagonal flake (as arrowed in C). (g) The HRTEM image of a nanoflake (as arrowed in C).
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Fig. 4. (Continued ).
accordingly have greater surface areas, which might result in their vulnerable character to be oxidized [14]. However, almost no drastic gain or loss of weight in the SiC samples with the temperature in the range of 30–1000 ◦ C. When the temperature was above 1000 ◦ C, the TGA curves correspondingly have a drastic weight gain, suggesting that the samples were accelerated oxidized. The room-temperature photoluminescence spectrum of the asobtained 3C-SiC sample is depicted in Fig. 6, which was obtained with an excitation wavelength of 329 nm. It is clearly observed that a strong emission peak centered at about 403 nm. Compared with the previously reported PL spectra of the 3C-SiC nanowires [22] or films [23], the emission peak for the 3C-SiC product is obviously blue shifted. Recently, various emission wavelengths from 3C-SiC nanostructures have been reported [22,24,25], suggesting that the luminescence characteristics depend strongly on the dif-
Fig. 5. TGA curves of the SiC samples obtained under air, at a heating rate of 10 ◦ C/min. (a) Sample of 3C-SiC. (b) Sample of mixed 3C and 2H-SiC.
ferent SiC nanostructures (usually obtained via different synthesis conditions). Therefore, it is considered that the different optical performances of the present sample and other reported 3C-SiC nanostructures might be attributed to their different shapes and sizes [22,26–28]. In order to study the optimal conditions for the synthesis of SiC nanocrystals, a series of contrast experiments were further carried out. In the experimental system, when SiO2 was replaced by Si, no SiC was obtained. This result indicates that the reactive Si produced from SiO2 was prone to react with C (carbon atoms) to form SiC. Moreover, the amount of SiO2 influence the morphology of the as-prepared 3C-SiC, when its amount was reduced to 2.4 g or 1.2 g while other experimental parameters remain unchanged, mainly SiC nanoparticles were obtained; when its amount was increased to 3.6 g, 3C-SiC nanowires were obtained besides irregular particles. It is worth noting that when metallic Mg was replaced by Na,
Fig. 6. Room-temperature photoluminescence (PL) spectrum of the 3C-SiC sample.
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Table 1 Experimental results of the final products produced at 180 ◦ C for 10 h. Sign
Reaction system
Quantity of reactants
Final products
1 2 3 4 5 6 7
SiO2 + Mg + C2 H5 OH SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + PVP SiO2 + Mg + C2 H5 OH + polyethylene glycol SiO2 + Mg + C2 H5 OH + PVA
1.2 g + 1.5 g + 10 ml 1.2 g + 1.5 g + 10 ml + 0.2 g 1.2 g + 1.5 g + 10 ml + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.8 g 1.2 g + 1.5 g + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.5 g
3C-SiC 3C-SiC and 2H-SiC 3C-SiC and 2H-SiC 3C-SiC and 2H-SiC No SiC 3C-SiC and 2H-SiC Mainly 3C-SiC
Al, Zn or Ni, no SiC could be obtained even the temperature was increased to 400 ◦ C, therefore, it is obvious that metallic Mg powder was crucial for the low temperature synthesis of SiC. During the reaction process, C2 H5 OH not only acts as a reactant but also as a solvent. Owing to its low boiling point (about 78.5 ◦ C), the presence of appropriate quantity of C2 H5 OH produces high pressure in the autoclave, which was important for the formation of SiC. Appropriate pressure generated by the vaporization of C2 H5 OH and other produced molecules is necessary for the formation of the SiC. For example, when the volume of C2 H5 OH used was less than 10 ml, no SiC was obtained. The influences of other parameters such as the reaction temperature and time on the formation of SiC were also studied. The results indicate that the reaction could proceed when the temperature was above 180 ◦ C, however, higher temperature and longer reaction time had no significant influences on the yield augment of SiC nanowires, but resulted in the production of larger SiC particles. In this experiment, it is found that the accession of PVP (polyvinylpyrrolidine, 0.2 g) usually leads to the production of the final products with mixed phases (see Table 1). The PVP used in our experiment is in solid state, which dissolves in the C2 H5 OH. TGA analysis (as shown in Fig. 7) of the polyvinylpyrrolidine shows that there is a weight loss when the temperature was raised above 200 ◦ C, which might be attributed to its decomposition. C2 H5 OH and Mg can react with each other to form MgO, graphite and H2 at 180 ◦ C. With the reaction proceeded, the temperature of the reaction system will be higher than the maintained temperature due to the exothermic reaction. Therefore, PVP might also as carbon source in this experiment. Similar result was obtained after the addition of polyvinylpyrrolidine at 200 ◦ C (see the XRD pattern in Fig. 2c). If polyvinylpyrrolidine was substituted by other organic reagents, such as polyethylene glycol and polyvinyl alcohol (PVA), different results were obtained (as shown in Table 1 and Fig. 8). The exact reason why the assistance of PVP is favourable for the formation of 2H-SiC is presently not clear and needs further investigation.
Fig. 7. TGA curve of the polyvinylpyrrolidine (PVP) obtained under ambient atmosphere with a heating rate of 10 ◦ C/min.
In order to study the possible reaction mechanism, only 10 ml C2 H5 OH and 1.5 g Mg were loaded into an autoclave with a capacity of ∼20 ml, and then maintained at 180 ◦ C for 10 h. It was found that crystalline MgO and graphite were obtained. According to the result of free energy calculation, the reaction between C2 H5 OH and Mg is thermodynamically spontaneous and highly exothermic (G = −446.97 kJ mol−1 , H = −366.20 kJ mol−1 ) [29]. Therefore, combined this experimental results with the previous report [30,31], the chemical equations can be formulated as follows: C2 H5 OH(l) + Mg(s) = 2C(s) + MgO(s) + 3H2 (g)
(2)
SiO2 + C2 H5 OH + 3Mg = SiC + 3MgO + C + 3H2
(1)
SiO2 (s) + 2Mg(s) = Si(s) + 2MgO(s)
(3)
The calculated thermodynamic factor values of the reaction (1) is thermodynamically spontaneous (G = −761.91 kJ mol−1 ) and exothermic (H = −716.13 kJ mol−1 ) [29]; while the values calculated from reaction (3) (without C2 H5 OH) are G = −276.43 kJ mol−1 ; H = −292.52 kJ mol−1 [29], the values are smaller than those calculated from reaction (1). It is obvious that the reaction in Eq. (1) is more prone to occur. Furthermore, a large quantity of caloric released by the reaction (2) (reaction between C2 H5 OH and Mg) may raise the temperature in autoclaves higher than the maintained temperature, and appropriate pressure in the system may all in favour of the reaction (1) to occur. The XRD pattern (Fig. 9) of the product without any post treatments can be indexed as mixtures of SiC, MgO, Si, SiO2 and residual carbon. The generation of Si suggests that SiO2 may have been reduced by Mg or the newly formed C. The newly formed Si and C were active to react with each other to form SiC through reaction (4). Si(s) + C(s) = SiC(s)
(4)
Up to now, the VLS mechanism is a well-accepted mechanism for explaining the growth process of SiC nanowires [22,32]. In which
Fig. 8. XRD patterns of the samples prepared at 180 ◦ C for 10 h using (a) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg and 0.5 g polyethylene glycol; (b) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVA as reactants.
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low temperature synthesis method could be used in principle to produce many other carbides. Acknowledgements This work was supported by National Natural Science Found of China (No. 20671058, 20871075), the 973 Project of China (No. 2005CB23601), the Natural Science Foundation (no. 11190004010664) and Tai Shan scholar ship of Shandong province. References
Fig. 9. XRD patterns of the synthesized SiC without any treatment process.
a metal particle is located at the tip of the wire (or tube) to act as the nucleation active site. In this experiment, it is obvious that SiO2 could be reduced to form Si, while the freshly formed carbon atoms and silicon atoms are not stable because of high-energy, they react and create SiC nuclei. At the beginning, SiC may appeared in the form of nanoparticles, as the continues of the reactions, more and more silicon and carbon atoms adhere to the surface of the nanoparticles, most of them moved on to the lowest energy plane, (1 1 1) plane for cubic SiC [33–35], to lower the surface energy of the nanoparticles, while growth along other directions were depressed, therefore, the SiC nanowires were gradually formed along the [1 1 1] direction. It is occasionally found that Mg powder coexisted with MgO were remained in the raw product, however, they were completely removed after the acid treatment process, which can be evidenced by the XRD patterns (see Fig. 1). Combined this result with the observation of the curved SiC nanowires under TEM observations (Fig. 4a), a VLS process might be plausible involved in the growth of SiC nanowires though metal particles have not been found existed at the tips of the nanowires after acid treatment process. As it is difficult to monitor the reaction once the autoclave was sealed, much work still needs to be carried out to explore the exact formation mechanisms of the SiC nanowires. 4. Conclusions Cubic SiC nanowires with diameters of 10–30 nm and lengths up to tens of micrometers have been prepared by using SiO2 , C2 H5 OH, and metallic Mg at 200 ◦ C. Mixed phases of cubic SiC (3C-SiC) and hexagonal SiC (2H-SiC) also were obtained with the assistance of PVP at 180 ◦ C. Thermal gravimetric analysis curves reveal that these two samples have high thermal stability. A large blue shift of the 3C-SiC nanowires was observed in the visible photoluminescence, which might have potential applications in optical devices. This
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of 3C and 2H-SiC nanocrystals starting from SiO2 , C2 H5 OH and metallic Mg Ting Li, Liqiang Xu ∗ , Liancheng Wang, Lishan Yang, Yitai Qian ∗ Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, PR China
a r t i c l e
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Article history: Received 3 February 2009 Received in revised form 8 April 2009 Accepted 19 April 2009 Available online 24 April 2009 Keywords: Silicon carbide Nanostructured materials Transmission electron microscopy X-ray diffraction
a b s t r a c t Silicon carbide (3C-SiC) nanocrystals were prepared starting from SiO2 , C2 H5 OH, and metallic Mg in an autoclave at 200 ◦ C. X-ray diffraction patterns of the sample can be indexed as the cubic phase of SiC with the lattice constant a = 4.357 Å, in good agreement with the reported value (JCPDS card no. 291129; a = 4.359 Å). Transmission electron microscopy images show that the product mainly composed of nanowires with diameters in the range of 10–30 nm and lengths up to tens of micrometers; Highresolution transmission electron microscopy images reveal that these 3C-SiC nanowires grow along [1 1 1] direction; As polyvinylpyrrolidine was added into the above reactant system, the final products obtained at 180 ◦ C were mixed 3C and 2H-SiC flakes. Thermal gravimetric analysis curves reveal that these two samples have thermal stability below 800 ◦ C, and room-temperature photoluminescence spectrum of the 3C-SiC sample show a strong emission peak centered at 403 nm. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silicon carbide (SiC) is a semiconducting material with wide band gap (2.39 ev for 3C-SiC and 3.33 ev for 2H-SiC at room temperature) [1], which has high mechanical strength, high thermal conductivity, high breakdown electric field [2]. These unique physical and electronic properties make SiC a suitable material for the fabrication of electronic devices operating at high-temperature, high power, high frequency and in harsh environments [3]. Various methods have been developed for the synthesis of SiC nanocrystals, such as the carbothermal reduction reaction [4,5], chemical vapor reaction [6], sol–gel [7,8], self-propagating high temperature synthesis [9], autoclave route [10–15] and so on. Among these methods, prepare SiC in an autoclave is one of the effective routes at low temperature, such as the reactant systems: SiCl4 -Na-C [10] or SiCl4 -Na-C6 Cl6 [11] at 600 ◦ C, SiCl4 -Na-CCl4 at 400 ◦ C [12], SiCl4 -Na-K-CBr3 H at 130 ◦ C [13], or sulfur-assisted reduction route (Si-S-Na-C2 Cl4 ) at 130 ◦ C [14]. Moreover, 2H-SiC nanoflakes also have been prepared in an autoclave at 180 ◦ C [15]. In the previous reports [13–15], SiCl4 and Si were chosen as silicon sources, C2 Cl4 and CBr3 H were used as carbon sources, Na–K alloy and Na were used as reductant. Among the silicon precursors used to synthesize SiC, SiO2 is widely used due to the low cost, such as SiO2 -carbon black at 1200–1600 ◦ C [4,5], Si-SiO2 -C3 H6 at 1250 ◦ C [6], and so on. However, there are fewer reports about the
∗ Corresponding authors. Tel.: +86 531 8836 6280; fax: +86 531 8836 6280. E-mail address: [email protected] (L. Xu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.096
synthesis of SiC by using SiO2 as silicon sources at relative low temperature (below 1000 ◦ C). In the current study, SiO2 was used as silicon source, C2 H5 OH as carbon source, and metallic Mg substitute Na–K alloy and Na was used as reductant. The advantage of this route is that parlous or corrosive reagents have been avoided. In this study, 3C-SiC nanocrystals were prepared starting from SiO2 , C2 H5 OH, and Mg at 200 ◦ C; As polyvinylpyrrolidine (PVP) was added into the above reactant system, the final product obtained at 180–200 ◦ C were mixed 3C and 2H-SiC. The yield of 3C-SiC sample prepared at 200 ◦ C was about 23%, calculated based on the amount of SiO2 . The yield of mixed 3C and 2H-SiC sample prepared at 180 ◦ C was about 57%. Low temperature and cheap raw materials make it possible for large scale synthesis of SiC nanocrystals. The reaction can be described as follows: SiO2 + C2 H5 OH + 3Mg = SiC + 3MgO + C + 3H2
(1)
Thermal gravimetric analysis (TGA) curves of the product reveal that the as-prepared samples have high thermal stability below 800 ◦ C. Room-temperature photoluminescence (PL) spectrum of the 3C-SiC sample exhibit a strong emission peak centered at 403 nm. 2. Experimental 2.1. Preparation of 3C-SiC nanocrystals In a typical process, 10 ml C2 H5 OH (analytical grade), 3.6 g SiO2 (analytical grade, Shanghai Chemical Reagents Co.), and 1.5 g Mg (Tianjin damao Chemical Reagents Co., 99%) were loaded into a stainless steel autoclave with a capacity of about 20 ml. The autoclave was sealed and maintained at 200 ◦ C for 10 h, then cooled to room temperature naturally. The raw products in the autoclave were collected and washed
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with dilute HCl to remove the MgO byproducts, then treated with the mixture of HNO3 and HF with the volume ratio of 1:1 to remove the unreacted SiO2 and the intermediate Si. In order to eliminate the residual carbon, the sample were treated by HClO4 (70%) at 150–180 ◦ C (calcine the products at 600 ◦ C under air atmosphere can achieve the same result), then treated with HF again, finally they were washed with distilled water and anhydrous ethanol. After drying in vacuum at 70 ◦ C for 6 h, a gray-white sample was finally obtained. 2.2. Preparation of mixed 3C and 2H-SiC nanocrystals In a typical procedure, 10 ml C2 H5 OH (analytical grade), 1.2 g SiO2 (analytical grade, Shanghai Chemical Reagents Co.), 1.5 g Mg (Tianjin damao Chemical Reagents Co., 99%) and 0.5 g polyvinylpyrrolidine (PVP, Shanghai Chemical Reagents Co.) were loaded into an autoclave with a capacity about 20 ml. The autoclave was sealed and maintained at 180 ◦ C for 10 h, then cooled to room temperature naturally. The product was collected and treated via similar processes as mentioned above. After dried in vacuum, the final sample was obtained. 2.3. Characterization The X-ray powder diffraction (XRD) patterns of the products were recorded on a Bruker D8 advanced X-ray diffractometer with Cu K␣ radiation ( = 1.5418 Å). Transmission electron microscope (TEM) images were taken on a Hitachi H-7000 transmission electron microscope, using an accelerating voltage of 100 kV. The highresolution transmission electron microscope (HRTEM) images were performed on a JEOL 2100 transmission electron microscope operated at 200 kV. The infrared (IR) spectra were recorded on a Bruker VERTEX 70, using a KBr wafer. Thermal gravimetric analysis was taken on a SDT Q600 V8.0 Build 95 thermal analyzer apparatus under air flow. Photoluminescence spectrum measurement was performed in an Edinburgh instruments FLS920 fluorescence spectrophotometer with a Xe lamp at room temperature.
3. Results and discussion Phase identification of the as-obtained products was carried out using the XRD pattern. Fig. 1a shows XRD pattern of the 3C-SiC sample obtained before the HClO4 treatment, revealing the main phase of the products is crystalline 3C-SiC, besides 3C-SiC, the diffraction peaks of residual carbon was observed. Fig. 1b shows the XRD pattern of the same sample after treated by hot HClO4 solution. The peaks with strong diffraction intensity indicate that the sample was well crystalline. It is found that the peaks centered at 2 = 35.66◦ , 41.42◦ , 59.99◦ , 71.76◦ and 75.51◦ can be indexed as the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) reflections of 3C-SiC, respectively. The appearance of low-intensity peak near 2 = 33.5◦ (marked with SF) is due to the stacking faults [16]. The calculated lattice constant a = 4.357 Å agrees well with the reported values of -SiC (JCPDS card no. 29-1129). No other impurities have been detected. The remarkable enhanced (1 1 1) peak suggests the preferential growth orientation of the product. This result was further proved by the following HRTEM investigation. Fig. 2a shows the XRD pattern of the product prepared by using SiO2 , C2 H5 OH and Mg at 180 ◦ C for 10 h. All the peaks in this pattern can be indexed as 3C-SiC (JCPDS card no. 29-1129). Compared with
Fig. 1. XRD patterns of the 3C-SiC sample prepared at 200 ◦ C. (a) Before treated by HClO4 ; (b) after treated by HClO4 .
Fig. 2. XRD patterns of the samples prepared by using (a) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg at 180 ◦ C for 10 h; (b) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVP at 180 ◦ C for 10 h; (c) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVP at 200 ◦ C for 10 h.
that of Fig. 2a, the result of Fig. 2b reveals that the product with mixed phases of 3C and 2H-SiC has been obtained when PVP was additionally used as the reactant. The peaks in Fig. 2b that signed by black dots can be indexed as 2H-SiC, the calculated lattice constants of a = 3.079 Å and c = 5.037 Å agree well with those of the 2H-SiC (JCPDS card no. 29-1126), while the peaks that marked with black rectangle can be assigned to 3C-SiC, the calculated lattice constant a = 4.357 Å is close to the reported value of -SiC (JCPDS card no. 29-1129). The typical IR spectra of the as-synthesized samples are shown in Fig. 3. The obvious absorption peaks centered at ∼817 cm−1 can be assigned to the transverse optical (TO) photon vibration mode of the Si-C bond [17]. The weak absorption peak (shown in Fig. 3b) at ∼1110 cm−1 resulted from the asymmetry stretching vibration of SiO [18], which indicates that the sample still has very small amount of SiO2 , the weaker absorption peak at ∼1630 cm−1 corresponds to the H-O-H bending vibration could be attributed to the absorption of water on the sample [19]. The above XRD patterns and IR spectra indicate that the as-synthesized samples through the present synthesis route were crystalline SiC. The morphology and structure of the final products were determined by TEM and HRTEM. The TEM image shown in Fig. 4a reveals that the 3C-SiC sample was mainly consists of nanowires (∼60%) with diameters ranging from 10 to 30 nm and lengths up to several tens of micrometers. The HRTEM image of part of a single SiC nanowire (in Fig. 4b) reveals that the interplanar spacing of the two adjacent fringes is about 0.25 nm, which corresponds to the
Fig. 3. IR spectra of the samples. (a) 3C-SiC obtained at 200 ◦ C; (b) mixed phases of 3C and 2H-SiC obtained at 180 ◦ C.
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(1 1 1) spacing of 3C-SiC. In addition, the [1 1 1] direction is parallel to the axis of the nanowire, indicating that the nanowire grows along the [1 1 1] direction. Fig. 4d shows the high-resolution TEM image of the area of an individual nanowire (shown in Fig. 4c), in which the interfringe distances of 0.25 nm and 0.21 nm are consistent with (1 1 1) and (2 0 0) planes of 3C-SiC (JCPDS card no. 29-1129), respectively, and the stacking faults could be observed clearly as magnified in right corner of Fig. 4d. The SAED (inset in Fig. 4d) indicates a streaking diffraction pattern characteristic of stacking faults. Combined with the XRD pattern (Fig. 1b) of the 3CSiC sample obtained at 200 ◦ C, the appearance of low-intensity peak near 2 = 33.5◦ (marked with SF) is due to stacking faults of 3C-SiC, which might belong to deformation fault [16]. Fig. 4e depicts a typical TEM image of the sample with mixed phases, displaying the products are mainly composed of irregular shaped nanoflakes and hexagonal flakes. The corresponding HRTEM image of a hexagonal flake (in Fig. 4f) shows the interfringe distances are 0.23 nm and
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0.26 nm, which are consistent with (1 0 1) and (1 0 0) planes of 2HSiC (JCPDS card no. 29-1126), respectively. HRTEM image (Fig. 4g) of an irregular flake signed by a black arrow reveals that the clearly resolved interplanar distance is 0.25 nm, which coincides with the (1 1 1) lattice spacing of 3C-SiC (JCPDS card no. 29-1129). The regular arranged lattice fringes can be clearly seen from the above HRTEM images, indicating the well crystalline of the as-prepared samples. The thermal stability of the SiC samples was assessed by thermogravimetric analysis (TGA). Fig. 5 shows the TGA curves carried out under air in the temperature range of 30–1100 ◦ C. Initially, it is observed from the TGA curve (Fig. 5a) that there is a slight weight loss (∼1%), which might be attributed to the loss of water that absorbed on the SiC sample surface. An obvious weight gain is observed above 800 ◦ C, suggesting SiC was oxidized in air atmosphere. This result is in agreement with the previous reports [20,21]. Compared to curve (a), curve (b) has a slight weight gain starts at about 400 ◦ C, probably because that smaller particle sizes
Fig. 4. (a) A typical TEM image of the 3C-SiC nanowires. (b) HRTEM image of a part of an individual 3C-SiC nanowire. (c) TEM image of a single 3C-SiC nanowire. (d) The corresponding HRTEM image of the SiC nanowire shown in (c), revealing the presence of stacking faults. Inset shows a streaking diffraction pattern (SAED) characteristic of stacking faults. (e) TEM images of the sample with mixed phases. (f) The corresponding HRTEM image of a hexagonal flake (as arrowed in C). (g) The HRTEM image of a nanoflake (as arrowed in C).
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Fig. 4. (Continued ).
accordingly have greater surface areas, which might result in their vulnerable character to be oxidized [14]. However, almost no drastic gain or loss of weight in the SiC samples with the temperature in the range of 30–1000 ◦ C. When the temperature was above 1000 ◦ C, the TGA curves correspondingly have a drastic weight gain, suggesting that the samples were accelerated oxidized. The room-temperature photoluminescence spectrum of the asobtained 3C-SiC sample is depicted in Fig. 6, which was obtained with an excitation wavelength of 329 nm. It is clearly observed that a strong emission peak centered at about 403 nm. Compared with the previously reported PL spectra of the 3C-SiC nanowires [22] or films [23], the emission peak for the 3C-SiC product is obviously blue shifted. Recently, various emission wavelengths from 3C-SiC nanostructures have been reported [22,24,25], suggesting that the luminescence characteristics depend strongly on the dif-
Fig. 5. TGA curves of the SiC samples obtained under air, at a heating rate of 10 ◦ C/min. (a) Sample of 3C-SiC. (b) Sample of mixed 3C and 2H-SiC.
ferent SiC nanostructures (usually obtained via different synthesis conditions). Therefore, it is considered that the different optical performances of the present sample and other reported 3C-SiC nanostructures might be attributed to their different shapes and sizes [22,26–28]. In order to study the optimal conditions for the synthesis of SiC nanocrystals, a series of contrast experiments were further carried out. In the experimental system, when SiO2 was replaced by Si, no SiC was obtained. This result indicates that the reactive Si produced from SiO2 was prone to react with C (carbon atoms) to form SiC. Moreover, the amount of SiO2 influence the morphology of the as-prepared 3C-SiC, when its amount was reduced to 2.4 g or 1.2 g while other experimental parameters remain unchanged, mainly SiC nanoparticles were obtained; when its amount was increased to 3.6 g, 3C-SiC nanowires were obtained besides irregular particles. It is worth noting that when metallic Mg was replaced by Na,
Fig. 6. Room-temperature photoluminescence (PL) spectrum of the 3C-SiC sample.
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Table 1 Experimental results of the final products produced at 180 ◦ C for 10 h. Sign
Reaction system
Quantity of reactants
Final products
1 2 3 4 5 6 7
SiO2 + Mg + C2 H5 OH SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + C2 H5 OH + PVP SiO2 + Mg + PVP SiO2 + Mg + C2 H5 OH + polyethylene glycol SiO2 + Mg + C2 H5 OH + PVA
1.2 g + 1.5 g + 10 ml 1.2 g + 1.5 g + 10 ml + 0.2 g 1.2 g + 1.5 g + 10 ml + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.8 g 1.2 g + 1.5 g + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.5 g 1.2 g + 1.5 g + 10 ml + 0.5 g
3C-SiC 3C-SiC and 2H-SiC 3C-SiC and 2H-SiC 3C-SiC and 2H-SiC No SiC 3C-SiC and 2H-SiC Mainly 3C-SiC
Al, Zn or Ni, no SiC could be obtained even the temperature was increased to 400 ◦ C, therefore, it is obvious that metallic Mg powder was crucial for the low temperature synthesis of SiC. During the reaction process, C2 H5 OH not only acts as a reactant but also as a solvent. Owing to its low boiling point (about 78.5 ◦ C), the presence of appropriate quantity of C2 H5 OH produces high pressure in the autoclave, which was important for the formation of SiC. Appropriate pressure generated by the vaporization of C2 H5 OH and other produced molecules is necessary for the formation of the SiC. For example, when the volume of C2 H5 OH used was less than 10 ml, no SiC was obtained. The influences of other parameters such as the reaction temperature and time on the formation of SiC were also studied. The results indicate that the reaction could proceed when the temperature was above 180 ◦ C, however, higher temperature and longer reaction time had no significant influences on the yield augment of SiC nanowires, but resulted in the production of larger SiC particles. In this experiment, it is found that the accession of PVP (polyvinylpyrrolidine, 0.2 g) usually leads to the production of the final products with mixed phases (see Table 1). The PVP used in our experiment is in solid state, which dissolves in the C2 H5 OH. TGA analysis (as shown in Fig. 7) of the polyvinylpyrrolidine shows that there is a weight loss when the temperature was raised above 200 ◦ C, which might be attributed to its decomposition. C2 H5 OH and Mg can react with each other to form MgO, graphite and H2 at 180 ◦ C. With the reaction proceeded, the temperature of the reaction system will be higher than the maintained temperature due to the exothermic reaction. Therefore, PVP might also as carbon source in this experiment. Similar result was obtained after the addition of polyvinylpyrrolidine at 200 ◦ C (see the XRD pattern in Fig. 2c). If polyvinylpyrrolidine was substituted by other organic reagents, such as polyethylene glycol and polyvinyl alcohol (PVA), different results were obtained (as shown in Table 1 and Fig. 8). The exact reason why the assistance of PVP is favourable for the formation of 2H-SiC is presently not clear and needs further investigation.
Fig. 7. TGA curve of the polyvinylpyrrolidine (PVP) obtained under ambient atmosphere with a heating rate of 10 ◦ C/min.
In order to study the possible reaction mechanism, only 10 ml C2 H5 OH and 1.5 g Mg were loaded into an autoclave with a capacity of ∼20 ml, and then maintained at 180 ◦ C for 10 h. It was found that crystalline MgO and graphite were obtained. According to the result of free energy calculation, the reaction between C2 H5 OH and Mg is thermodynamically spontaneous and highly exothermic (G = −446.97 kJ mol−1 , H = −366.20 kJ mol−1 ) [29]. Therefore, combined this experimental results with the previous report [30,31], the chemical equations can be formulated as follows: C2 H5 OH(l) + Mg(s) = 2C(s) + MgO(s) + 3H2 (g)
(2)
SiO2 + C2 H5 OH + 3Mg = SiC + 3MgO + C + 3H2
(1)
SiO2 (s) + 2Mg(s) = Si(s) + 2MgO(s)
(3)
The calculated thermodynamic factor values of the reaction (1) is thermodynamically spontaneous (G = −761.91 kJ mol−1 ) and exothermic (H = −716.13 kJ mol−1 ) [29]; while the values calculated from reaction (3) (without C2 H5 OH) are G = −276.43 kJ mol−1 ; H = −292.52 kJ mol−1 [29], the values are smaller than those calculated from reaction (1). It is obvious that the reaction in Eq. (1) is more prone to occur. Furthermore, a large quantity of caloric released by the reaction (2) (reaction between C2 H5 OH and Mg) may raise the temperature in autoclaves higher than the maintained temperature, and appropriate pressure in the system may all in favour of the reaction (1) to occur. The XRD pattern (Fig. 9) of the product without any post treatments can be indexed as mixtures of SiC, MgO, Si, SiO2 and residual carbon. The generation of Si suggests that SiO2 may have been reduced by Mg or the newly formed C. The newly formed Si and C were active to react with each other to form SiC through reaction (4). Si(s) + C(s) = SiC(s)
(4)
Up to now, the VLS mechanism is a well-accepted mechanism for explaining the growth process of SiC nanowires [22,32]. In which
Fig. 8. XRD patterns of the samples prepared at 180 ◦ C for 10 h using (a) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg and 0.5 g polyethylene glycol; (b) 1.2 g SiO2 , 10 ml C2 H5 OH, 1.5 g Mg, and 0.5 g PVA as reactants.
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low temperature synthesis method could be used in principle to produce many other carbides. Acknowledgements This work was supported by National Natural Science Found of China (No. 20671058, 20871075), the 973 Project of China (No. 2005CB23601), the Natural Science Foundation (no. 11190004010664) and Tai Shan scholar ship of Shandong province. References
Fig. 9. XRD patterns of the synthesized SiC without any treatment process.
a metal particle is located at the tip of the wire (or tube) to act as the nucleation active site. In this experiment, it is obvious that SiO2 could be reduced to form Si, while the freshly formed carbon atoms and silicon atoms are not stable because of high-energy, they react and create SiC nuclei. At the beginning, SiC may appeared in the form of nanoparticles, as the continues of the reactions, more and more silicon and carbon atoms adhere to the surface of the nanoparticles, most of them moved on to the lowest energy plane, (1 1 1) plane for cubic SiC [33–35], to lower the surface energy of the nanoparticles, while growth along other directions were depressed, therefore, the SiC nanowires were gradually formed along the [1 1 1] direction. It is occasionally found that Mg powder coexisted with MgO were remained in the raw product, however, they were completely removed after the acid treatment process, which can be evidenced by the XRD patterns (see Fig. 1). Combined this result with the observation of the curved SiC nanowires under TEM observations (Fig. 4a), a VLS process might be plausible involved in the growth of SiC nanowires though metal particles have not been found existed at the tips of the nanowires after acid treatment process. As it is difficult to monitor the reaction once the autoclave was sealed, much work still needs to be carried out to explore the exact formation mechanisms of the SiC nanowires. 4. Conclusions Cubic SiC nanowires with diameters of 10–30 nm and lengths up to tens of micrometers have been prepared by using SiO2 , C2 H5 OH, and metallic Mg at 200 ◦ C. Mixed phases of cubic SiC (3C-SiC) and hexagonal SiC (2H-SiC) also were obtained with the assistance of PVP at 180 ◦ C. Thermal gravimetric analysis curves reveal that these two samples have high thermal stability. A large blue shift of the 3C-SiC nanowires was observed in the visible photoluminescence, which might have potential applications in optical devices. This
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