- Email: [email protected]
SiO2 composite nanorods
Materials Chemistry and Physics 243 (2020) 122573
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Preparation and characterization of mesoporous SiC/SiO2 composite nanorods Ye Zhang a, **, Qiang Wang a, Zhigang Ren a, Yongxing Yang b, * a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27#, Taiyuan, 030001, PR China School of Chemistry and Chemical Engineering, Shanxi University, Wucheng Road 92, Taiyuan, 030006, People’s Republic of China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Mesoporous rod-like SiC coated with amorphous silica has been prepared through a vapor-solid mechanism. � SBA-15 act as hard template and sucrose act as carbon source. � The diameter of the SiC nanorods are ranged from 8 to 60 nm, and the silica shell has a thickness of about 4 nm.
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticles Cast SiC
Mesoporous rod-like SiC has been prepared by using SBA-15 as hard template and sucrose as carbon source through a vapor-solid mechanism. The material was characterized by X-ray diffraction (XRD), high resolution transmission electronmicroscopy (HRTEM) and thermalgravimetric (TG) analysis. The obtained material exhibits different morphologies, and consists of a rod-like SiC core and amorphous silica shell which is due to the excessive silica during the reaction between silica and carbon to form SiC. The diameter of the SiC nanorods are ranged from 8 to 60 nm, and the silica shell has a thickness of about 4 nm. The silica coating SiC nanorods possess more porous structure than pure SiC and in turn make it more useful in catalytic fields.
1. Introduction Silicon carbide (SiC), due to its extremely high hardness, high ther mal stability, superb oxidation and thermal stock resistance, has attracted considerable interests in many fields such as abrasive, varistor, catalyst supports, microwave dielectrics and semiconductor ceramic
materials [1–7]. Although SiC is generally produced and used in powdered form, the form of one-dimensional morphology (whiskers, nanotubes, nanowires) has obtained increasing attention due to their characteristics suited for applications as reinforcement in composite materials [4,5]. In particular, SiC nanowires possess high mechanical strength and oxidation resistance at elevated temperature, and this
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Yang). https://doi.org/10.1016/j.matchemphys.2019.122573 Received 13 September 2019; Received in revised form 16 December 2019; Accepted 23 December 2019 Available online 23 December 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
provides a perfect alternative for the development of matrix composites where carbon nanotubes may not be used. What’s more, SiC nanowires are effective not only for applications as additive of reinforcement but for field emission and nanoelectrical devices. One-dimensional SiC nanostructures can be produced by many pro cesses [8–12]. Commercially, they are prepared by chemical vapor deposition (CVD) through thermal decomposition of silianes and ther mal reduction of silicon oxides. Cetinkaya [8] synthesized SiC parti cle/whiskers using CVD process from CH4 and SiO2 particles and subsequently carbothermal reduction at temperature between 1700 K and 1800 K. Li et al. [9] prepared SiC/SiO2 nanochains from a mixtures (Si/SiO2) and activated carbon fibers by CVD. Koc [11] used a high-temperature-cracking process to coat silica with carbon, multicycle carbon coating process and the use of different reactors and atmosphere are necessary to achieve the amount of carbon required for carbothermal process. Another method [10,12,13] to synthesize one-dimensional SiC is carbothermal process, which is an extremely versatile and is one of the most promising techniques for the synthesis of one-dimensional SiC nanostructures. The approaches recently focus on the diversity of the precursors with different silica and carbon sources. Lu et al. [2] used SBA-15 as silica source and furfuryl alcohol as carbon sources to prepare mesoporous SiC with needle-like and irregular morphology incorpo rated with a sol-gel route. Zhang [13] prepared nano-SiC single crystal wires and SiC-cored nanocables using silica xerogels and carbon parti cles from sucrose at 1650 and 1800 � C. Guo et al. [10] prepared worm-like SiC nanofibers with diameters of 20–110 nm via carbo thermal reduction technology using water glass and phenolic resin as raw materials. In present study, rod-like SiC was prepared by carbothermal reduc tion between assembled SBA-15 and carbon. The phase composition, morphology, and microstructure of the resulted SiC/SiO2 composites were characterized in details. 2. Experimental SBA-15 was prepared using the triblock copolymer, EO20PO70EO20 (Pluronic P123, Aldrich), as the surfactant and tetraethyl orthosilicate (TEOS, 98%, Acros) as the silica source, following the synthesis pro cedure reported by Zhao et al. [14]. Typically, the raw materials were mixed with the molar ratio as P123: H2O: TEOS: HCl ¼ 0.00166: 14: 1: 0.6. The mixture was stirred at 40 � C for 20 h then transferred to 80 � C for 48 h. The obtained solid was dried at 100 � C and calcined at 550 � C for 6 h to obtain SBA-15. The rod-like silicon carbide was prepared by the following procedure: First, the C/SiO2 composite powders were prepared by carbonizing sucrose inside the pores of SBA-15 following the synthesis procedure of CMK-3 except for the start composition. Typically, the start composition was 0.06 g H2SO4: 5 g H2O: 0.5 g su crose: 0.5 g SBA-15. After incorporation of the carbon precursor, the sample was dried at 150 � C, labeling as C/SiO2 composite. The molar ratio of C/SiO2 was further determined by thermal gravimetric analysis (TG). Then, the C/SiO2 composite powders were treated in a tubular furnace under argon atmosphere at 1300 � C for 2 h following by calci nation at 700 � C in air flow to remove the carbon residue. The product was donated as T13-2.
Fig. 1. TG and DTG results of the C/SiO2 composites (a), XRD pattern of T13-2 (b) and small angle XRD patterns (inset) and N2-sorption isotherms of T13-2 (c).
Fig. 1, the residual carbon can be completely removed when the tem perature goes beyond 700 � C. XRD result of T13-2 sample is shown in Fig. 1b. The reflection at approximately 35.8� is attributed to the (111) reflection of β-SiC (JCPDS Card no. 0029–1129) [4,5]. Previous study [4] has proven that a longer reaction time is necessary to promote the carbothermal reaction between silica and carbon species. In the current study, 2 h are not sufficient to fully convert the silica template to SiC. The broad diffraction hump at the (2θ) range of 21–23� was caused by the unreacted amorphous silica [4,5]. The small angle XRD pattern inserted in Fig. 1b shows a shoulder, suggesting the decreasing of mes oporous regularity. N2 adsorption-desorption curve of T13-2 sample in Fig. 1c shows an obvious H1 hysteresis with a varying degree of tailing at the closing of the hysteresis, which is consistent with the result of small
3. Results and discussion DTG and TG were depicted in Fig. 1a. The carbon percentage (28.48 wt%) and the C/SiO2 molar ratio (1.99) can be calculated by the TG results considering the main weight loss was contributed by the com bustion of the carbon filled in the mesopores of SBA-15 [4]. The TGA curve presents three platforms, with the first one at 29–113 � C attributed to the evaporation of H2O. The second weight loss ranged at 113–393 � C is the dehydration condensation of sucrose to form caramel and the third platform between 393 and 700 � C belongs to the oxidative combustion of caramel to carbon dioxde and water. Based on the results shown in 2
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
angle XRD pattern. These results suggest that the regularity of meso pores in SBA-15 has been destroyed to some extent. The microstructures of the product T13-2 was observed via HRTEM as shown in Fig. 2. The crystalline SiC exhibits one-dimensional rod-like morphology. The lat tice fringe of 0.25 nm in the crystalline domains is consistent with the lattice spacing of SiC (111), as indicated in Fig. 2e. From Fig. 2d, the rod-like silicon carbide was coated homogeneously with a layer of silica. The nanorods have an outer diameter range of 15–70 nm and a length ranged between 1 and 10 μm. The thickness of the silica shell is about 4 nm. In addition, amorphous bulk silica was also found in the sample, in which the original regular channels were destroyed seriously. This is in accordance with the small angleXRD result. However, the N2 adsorption-desorption curve shows an obvious H1 hysteresis, indicating the mesopores still existed to some extent in the final product. In gen eral, the product obtained at the final holding time exhibits morphol ogies consisting of nanostructured rod-like SiC coated with silica and bulk silica with irregular pores. Rod-like SiC structure has an amorphous silica shell and crystalline SiC core, providing an effective way to broaden applications of SiC as catalyst support by reason of facilitating the modification and grafting on the silica surface. Two different mechanisms, vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms are generally proposed for the formation and growth of one-dimensional SiC [8–10,12]. The VS mechanism has been employed to explain the formation of one-dimensional SiC for a system without the introduction of any catalysts or impurities, whilst the VLS mechanism is used when the catalysts or impurities exist. There is no doubt that the current experiment follows the VS mechanism. Briefly, the VS reactions are considered in the SiO–C system with a total reaction as [5]: SiO2 þ 3C⇌SiC þ 2CO
Fig. 2. HRTEM images of the T13-2, (a) the bulk-like silica, (b) (c) the mixture of coated SiC and the bulk-like silica, (d) (e) the HRTEM of the rod-like SiC coated by amorphous silica. The inset FFT images in (a) (e) belongs to the bulklike silica and the coated rod-like SiC.
4. Conclusion Rod-like SiC has been synthesized from assembled SBA-15 and su crose at 1300 � C through vapor-solid mechanism. The SiC nanorods have a SiC core with diameters from 8 to 60 nm, and an armorphous silica shell with a thickness of about 4 nm. The product exhibits different morphologies consisting of nanostructured rod-like SiC and bulk silica. The bulk silica still remains mesoporous to some extent. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1)
Firstly, the gas phase of SiO is generated by reaction (2) or direct reduction of silica by the CO already produced in (3) when physical contact between silica and carbon particles is no longer possible, and acts as an intermediate.
Acknowledgements
CðsÞþSiO2 ðsÞ⇌SiOðgÞ þ COðgÞ
(2)
This work was supported by National Natural Science Foundation of China (U1610108).
SiO2 ðsÞ þ COðgÞ⇌ SiOðgÞþCO2 ðgÞ
(3)
References
Carbon monoxide was generated through the Boudouard reaction CO2 ðgÞ þ CðsÞ⇌2COðgÞ
[1] J. G, J.L. Liu, Effects of source materials and container on growth process of SiC crystal, Cryst. Growth Des. 6 (2006) 2166–2168. [2] W. S, A.H. Lu, W. Kiefer, High surface area mesoporous SiC synthesized via nanocasting and carbothermal reduction process, J. Mater. Sci. 40 (2005) 5091–5093. [3] H. Z, P. Lv, J. Wang, X. Liu, Facile preparation and electrochemical properties of amorphous SiO2/C composite, J. Power Sources 237 (2013) 291–294. [4] S. A, S.T. Selvan, S.M. Zaidi, Morphological control of porous SiC templated by Assynthesized form of mesoporous silica, J. Nanosci. Nanotechnol. 11 (2011) 6823–6829. [5] J. F, P.C. Silva, Production of SiC and Si3N4 whiskers in CþSiO2 solid mixtures, Mater. Chem. Phys. 72 (2001) 326–331. [6] J. R, A. Vital, R. Figi, et al., One-step flame synthesis of ultrafine SiO2 C nanocomposite particles with high carbon loading and their carbothermal conversion, Ind. Eng. Chem. Res. 46 (2007) 4273–4281. [7] J. Z, H. Zhou, Q. Yan, Facile preparation and high microwave absorption of C/SiO2 composites with an ordered inter-filled string mesostructure, Mater. Lett. 85 (2012) 117–119. [8] S. E, S. Cetinkaya, Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers, Int J Refract Met H 29 (2011) 566–572. [9] O. H, L. Cuiyan, H. Jianfeng, Z. Xierong, C. Liyun, F. Jie, X. Xinbo, Synthesis and visible-light photocatalytic activity of SiC/SiO2 nanochain heterojunctions, Mater. Lett. 122 (2014) 125–128. [10] Y. W, J.Y. Hao, G.Q. Jin, X.Y. Guo, Design of new coatings using magnetron sputtering – numerical estimations, Optoelectron Adv. Mat 7 (2013) 334–338. [11] S. C, R. Koc, J Mater Sci, A new approach to fabricate SiC nanowire-embedded dense SiC 33 (1998) 52–55.
(4)
SiC can be prepared mainly by the reactions (5) and (6) SiOðgÞ þ 2CðsÞ⇌SiCðsÞ þ COðgÞ
(5)
2SiOðgÞ þ COðgÞ⇌2SiCðsÞþCO2 ðgÞ
(6)
SiOðgÞ þ 3COðgÞ⇌SiCðsÞþ2CO2 ðgÞ
(7)
3SiOðgÞ þ COðgÞ⇌SiCðsÞþ2CO2 ðgÞ
(8)
The reactions (7) and (8) are thermodynamically unfavorable in comparison to reactions (5) and (6). In our experiment, the generated SiO reacts with the solid carbon and lead to the formation of rod-like SiC through the VS mechanism. The molar ratio of the C/SiO2 composite is 1.99, lower than the stoichiometric ratio of 3. The residual vapor phase of SiO can react with the oxidation gases and resulted in the formation of a layer of viscous liquid SiO2 and further coated on the surface of the asgrown one-dimensional SiC nanostructure [9]. In addition, due to the Rayleigh instability and poor wettability between SiC and SiO2, some excessive SiO2 could be assembled into spherical nanodroplets to minimize its surface energy [9]. This is why some spherical SiO2 droplets were found in Fig. 2c.
3
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
[12] K. D, J. Prakash, B.M. Tripathi, J. Bahadur, S.K. Ghosh, A new approach to fabricate SiC nanowire-embedded dense SiC matrix/carbon fiber composite, J. Mater. Sci. 49 (2014) 6784–6792.
[13] G. M, L.D. Zhang, F. Phillipp, Synthesis and characterization of nanowires and nanocables, Mater. Sci. Eng. 286 (2000) 34–38. [14] J. F, D. Zhao, Q. Huo, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Sci 279 (1998) 548–552.
4
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Materials science communication
Preparation and characterization of mesoporous SiC/SiO2 composite nanorods Ye Zhang a, **, Qiang Wang a, Zhigang Ren a, Yongxing Yang b, * a b
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, South Taoyuan Road 27#, Taiyuan, 030001, PR China School of Chemistry and Chemical Engineering, Shanxi University, Wucheng Road 92, Taiyuan, 030006, People’s Republic of China
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Mesoporous rod-like SiC coated with amorphous silica has been prepared through a vapor-solid mechanism. � SBA-15 act as hard template and sucrose act as carbon source. � The diameter of the SiC nanorods are ranged from 8 to 60 nm, and the silica shell has a thickness of about 4 nm.
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanoparticles Cast SiC
Mesoporous rod-like SiC has been prepared by using SBA-15 as hard template and sucrose as carbon source through a vapor-solid mechanism. The material was characterized by X-ray diffraction (XRD), high resolution transmission electronmicroscopy (HRTEM) and thermalgravimetric (TG) analysis. The obtained material exhibits different morphologies, and consists of a rod-like SiC core and amorphous silica shell which is due to the excessive silica during the reaction between silica and carbon to form SiC. The diameter of the SiC nanorods are ranged from 8 to 60 nm, and the silica shell has a thickness of about 4 nm. The silica coating SiC nanorods possess more porous structure than pure SiC and in turn make it more useful in catalytic fields.
1. Introduction Silicon carbide (SiC), due to its extremely high hardness, high ther mal stability, superb oxidation and thermal stock resistance, has attracted considerable interests in many fields such as abrasive, varistor, catalyst supports, microwave dielectrics and semiconductor ceramic
materials [1–7]. Although SiC is generally produced and used in powdered form, the form of one-dimensional morphology (whiskers, nanotubes, nanowires) has obtained increasing attention due to their characteristics suited for applications as reinforcement in composite materials [4,5]. In particular, SiC nanowires possess high mechanical strength and oxidation resistance at elevated temperature, and this
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Zhang), [email protected] (Y. Yang). https://doi.org/10.1016/j.matchemphys.2019.122573 Received 13 September 2019; Received in revised form 16 December 2019; Accepted 23 December 2019 Available online 23 December 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
provides a perfect alternative for the development of matrix composites where carbon nanotubes may not be used. What’s more, SiC nanowires are effective not only for applications as additive of reinforcement but for field emission and nanoelectrical devices. One-dimensional SiC nanostructures can be produced by many pro cesses [8–12]. Commercially, they are prepared by chemical vapor deposition (CVD) through thermal decomposition of silianes and ther mal reduction of silicon oxides. Cetinkaya [8] synthesized SiC parti cle/whiskers using CVD process from CH4 and SiO2 particles and subsequently carbothermal reduction at temperature between 1700 K and 1800 K. Li et al. [9] prepared SiC/SiO2 nanochains from a mixtures (Si/SiO2) and activated carbon fibers by CVD. Koc [11] used a high-temperature-cracking process to coat silica with carbon, multicycle carbon coating process and the use of different reactors and atmosphere are necessary to achieve the amount of carbon required for carbothermal process. Another method [10,12,13] to synthesize one-dimensional SiC is carbothermal process, which is an extremely versatile and is one of the most promising techniques for the synthesis of one-dimensional SiC nanostructures. The approaches recently focus on the diversity of the precursors with different silica and carbon sources. Lu et al. [2] used SBA-15 as silica source and furfuryl alcohol as carbon sources to prepare mesoporous SiC with needle-like and irregular morphology incorpo rated with a sol-gel route. Zhang [13] prepared nano-SiC single crystal wires and SiC-cored nanocables using silica xerogels and carbon parti cles from sucrose at 1650 and 1800 � C. Guo et al. [10] prepared worm-like SiC nanofibers with diameters of 20–110 nm via carbo thermal reduction technology using water glass and phenolic resin as raw materials. In present study, rod-like SiC was prepared by carbothermal reduc tion between assembled SBA-15 and carbon. The phase composition, morphology, and microstructure of the resulted SiC/SiO2 composites were characterized in details. 2. Experimental SBA-15 was prepared using the triblock copolymer, EO20PO70EO20 (Pluronic P123, Aldrich), as the surfactant and tetraethyl orthosilicate (TEOS, 98%, Acros) as the silica source, following the synthesis pro cedure reported by Zhao et al. [14]. Typically, the raw materials were mixed with the molar ratio as P123: H2O: TEOS: HCl ¼ 0.00166: 14: 1: 0.6. The mixture was stirred at 40 � C for 20 h then transferred to 80 � C for 48 h. The obtained solid was dried at 100 � C and calcined at 550 � C for 6 h to obtain SBA-15. The rod-like silicon carbide was prepared by the following procedure: First, the C/SiO2 composite powders were prepared by carbonizing sucrose inside the pores of SBA-15 following the synthesis procedure of CMK-3 except for the start composition. Typically, the start composition was 0.06 g H2SO4: 5 g H2O: 0.5 g su crose: 0.5 g SBA-15. After incorporation of the carbon precursor, the sample was dried at 150 � C, labeling as C/SiO2 composite. The molar ratio of C/SiO2 was further determined by thermal gravimetric analysis (TG). Then, the C/SiO2 composite powders were treated in a tubular furnace under argon atmosphere at 1300 � C for 2 h following by calci nation at 700 � C in air flow to remove the carbon residue. The product was donated as T13-2.
Fig. 1. TG and DTG results of the C/SiO2 composites (a), XRD pattern of T13-2 (b) and small angle XRD patterns (inset) and N2-sorption isotherms of T13-2 (c).
Fig. 1, the residual carbon can be completely removed when the tem perature goes beyond 700 � C. XRD result of T13-2 sample is shown in Fig. 1b. The reflection at approximately 35.8� is attributed to the (111) reflection of β-SiC (JCPDS Card no. 0029–1129) [4,5]. Previous study [4] has proven that a longer reaction time is necessary to promote the carbothermal reaction between silica and carbon species. In the current study, 2 h are not sufficient to fully convert the silica template to SiC. The broad diffraction hump at the (2θ) range of 21–23� was caused by the unreacted amorphous silica [4,5]. The small angle XRD pattern inserted in Fig. 1b shows a shoulder, suggesting the decreasing of mes oporous regularity. N2 adsorption-desorption curve of T13-2 sample in Fig. 1c shows an obvious H1 hysteresis with a varying degree of tailing at the closing of the hysteresis, which is consistent with the result of small
3. Results and discussion DTG and TG were depicted in Fig. 1a. The carbon percentage (28.48 wt%) and the C/SiO2 molar ratio (1.99) can be calculated by the TG results considering the main weight loss was contributed by the com bustion of the carbon filled in the mesopores of SBA-15 [4]. The TGA curve presents three platforms, with the first one at 29–113 � C attributed to the evaporation of H2O. The second weight loss ranged at 113–393 � C is the dehydration condensation of sucrose to form caramel and the third platform between 393 and 700 � C belongs to the oxidative combustion of caramel to carbon dioxde and water. Based on the results shown in 2
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
angle XRD pattern. These results suggest that the regularity of meso pores in SBA-15 has been destroyed to some extent. The microstructures of the product T13-2 was observed via HRTEM as shown in Fig. 2. The crystalline SiC exhibits one-dimensional rod-like morphology. The lat tice fringe of 0.25 nm in the crystalline domains is consistent with the lattice spacing of SiC (111), as indicated in Fig. 2e. From Fig. 2d, the rod-like silicon carbide was coated homogeneously with a layer of silica. The nanorods have an outer diameter range of 15–70 nm and a length ranged between 1 and 10 μm. The thickness of the silica shell is about 4 nm. In addition, amorphous bulk silica was also found in the sample, in which the original regular channels were destroyed seriously. This is in accordance with the small angleXRD result. However, the N2 adsorption-desorption curve shows an obvious H1 hysteresis, indicating the mesopores still existed to some extent in the final product. In gen eral, the product obtained at the final holding time exhibits morphol ogies consisting of nanostructured rod-like SiC coated with silica and bulk silica with irregular pores. Rod-like SiC structure has an amorphous silica shell and crystalline SiC core, providing an effective way to broaden applications of SiC as catalyst support by reason of facilitating the modification and grafting on the silica surface. Two different mechanisms, vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms are generally proposed for the formation and growth of one-dimensional SiC [8–10,12]. The VS mechanism has been employed to explain the formation of one-dimensional SiC for a system without the introduction of any catalysts or impurities, whilst the VLS mechanism is used when the catalysts or impurities exist. There is no doubt that the current experiment follows the VS mechanism. Briefly, the VS reactions are considered in the SiO–C system with a total reaction as [5]: SiO2 þ 3C⇌SiC þ 2CO
Fig. 2. HRTEM images of the T13-2, (a) the bulk-like silica, (b) (c) the mixture of coated SiC and the bulk-like silica, (d) (e) the HRTEM of the rod-like SiC coated by amorphous silica. The inset FFT images in (a) (e) belongs to the bulklike silica and the coated rod-like SiC.
4. Conclusion Rod-like SiC has been synthesized from assembled SBA-15 and su crose at 1300 � C through vapor-solid mechanism. The SiC nanorods have a SiC core with diameters from 8 to 60 nm, and an armorphous silica shell with a thickness of about 4 nm. The product exhibits different morphologies consisting of nanostructured rod-like SiC and bulk silica. The bulk silica still remains mesoporous to some extent. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1)
Firstly, the gas phase of SiO is generated by reaction (2) or direct reduction of silica by the CO already produced in (3) when physical contact between silica and carbon particles is no longer possible, and acts as an intermediate.
Acknowledgements
CðsÞþSiO2 ðsÞ⇌SiOðgÞ þ COðgÞ
(2)
This work was supported by National Natural Science Foundation of China (U1610108).
SiO2 ðsÞ þ COðgÞ⇌ SiOðgÞþCO2 ðgÞ
(3)
References
Carbon monoxide was generated through the Boudouard reaction CO2 ðgÞ þ CðsÞ⇌2COðgÞ
[1] J. G, J.L. Liu, Effects of source materials and container on growth process of SiC crystal, Cryst. Growth Des. 6 (2006) 2166–2168. [2] W. S, A.H. Lu, W. Kiefer, High surface area mesoporous SiC synthesized via nanocasting and carbothermal reduction process, J. Mater. Sci. 40 (2005) 5091–5093. [3] H. Z, P. Lv, J. Wang, X. Liu, Facile preparation and electrochemical properties of amorphous SiO2/C composite, J. Power Sources 237 (2013) 291–294. [4] S. A, S.T. Selvan, S.M. Zaidi, Morphological control of porous SiC templated by Assynthesized form of mesoporous silica, J. Nanosci. Nanotechnol. 11 (2011) 6823–6829. [5] J. F, P.C. Silva, Production of SiC and Si3N4 whiskers in CþSiO2 solid mixtures, Mater. Chem. Phys. 72 (2001) 326–331. [6] J. R, A. Vital, R. Figi, et al., One-step flame synthesis of ultrafine SiO2 C nanocomposite particles with high carbon loading and their carbothermal conversion, Ind. Eng. Chem. Res. 46 (2007) 4273–4281. [7] J. Z, H. Zhou, Q. Yan, Facile preparation and high microwave absorption of C/SiO2 composites with an ordered inter-filled string mesostructure, Mater. Lett. 85 (2012) 117–119. [8] S. E, S. Cetinkaya, Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers, Int J Refract Met H 29 (2011) 566–572. [9] O. H, L. Cuiyan, H. Jianfeng, Z. Xierong, C. Liyun, F. Jie, X. Xinbo, Synthesis and visible-light photocatalytic activity of SiC/SiO2 nanochain heterojunctions, Mater. Lett. 122 (2014) 125–128. [10] Y. W, J.Y. Hao, G.Q. Jin, X.Y. Guo, Design of new coatings using magnetron sputtering – numerical estimations, Optoelectron Adv. Mat 7 (2013) 334–338. [11] S. C, R. Koc, J Mater Sci, A new approach to fabricate SiC nanowire-embedded dense SiC 33 (1998) 52–55.
(4)
SiC can be prepared mainly by the reactions (5) and (6) SiOðgÞ þ 2CðsÞ⇌SiCðsÞ þ COðgÞ
(5)
2SiOðgÞ þ COðgÞ⇌2SiCðsÞþCO2 ðgÞ
(6)
SiOðgÞ þ 3COðgÞ⇌SiCðsÞþ2CO2 ðgÞ
(7)
3SiOðgÞ þ COðgÞ⇌SiCðsÞþ2CO2 ðgÞ
(8)
The reactions (7) and (8) are thermodynamically unfavorable in comparison to reactions (5) and (6). In our experiment, the generated SiO reacts with the solid carbon and lead to the formation of rod-like SiC through the VS mechanism. The molar ratio of the C/SiO2 composite is 1.99, lower than the stoichiometric ratio of 3. The residual vapor phase of SiO can react with the oxidation gases and resulted in the formation of a layer of viscous liquid SiO2 and further coated on the surface of the asgrown one-dimensional SiC nanostructure [9]. In addition, due to the Rayleigh instability and poor wettability between SiC and SiO2, some excessive SiO2 could be assembled into spherical nanodroplets to minimize its surface energy [9]. This is why some spherical SiO2 droplets were found in Fig. 2c.
3
Y. Zhang et al.
Materials Chemistry and Physics 243 (2020) 122573
[12] K. D, J. Prakash, B.M. Tripathi, J. Bahadur, S.K. Ghosh, A new approach to fabricate SiC nanowire-embedded dense SiC matrix/carbon fiber composite, J. Mater. Sci. 49 (2014) 6784–6792.
[13] G. M, L.D. Zhang, F. Phillipp, Synthesis and characterization of nanowires and nanocables, Mater. Sci. Eng. 286 (2000) 34–38. [14] J. F, D. Zhao, Q. Huo, G.D. Stucky, Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores, Sci 279 (1998) 548–552.
4