Received: 1 July 2017
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Accepted: 12 July 2017
DOI: 10.1111/ijac.12757
ORIGINAL ARTICLE
Morphological features of porous silicon carbide obtained via a carbothermal method Nataliya Shcherban1
| Svitlana Filonenko1 | Sergii Sergiienko2
| Pavel Yaremov1 |
Mykola Skoryk3,4 | Volodymir Ilyin1 | Dmitry Murzin5 1 L.V. Pisarzhevsky Institute of Physical Chemistry, NAS of Ukraine, Kyiv, Ukraine 2
National University of Science and Technology MISiS, Moscow, Russia 3
NanoMedTech LLC, Kyiv, Ukraine
4
G.V. Kurdyumov Institute for Metal Physics, NAS of Ukraine, Kyiv, Ukraine 5 Johan Gadolin Process Chemistry Centre, Abo Akademi University, Turku/ Abo, Finland
Correspondence Nataliya Shcherban Email address:
[email protected]
Abstract Samples of porous silicon carbide were obtained using sucrose or carbon and aerosil or silica mesoporous molecular sieves (SBA-3, SBA-15, KIT-6 and MCF). Fibers content in silicon carbide samples is higher when the mesopore surface area of carbon materials derived from carbon-silica composites is lower. Based on the found correlation between the morphology and porosity of SiC and mesopore surface area of the carbon component in the composites, a templating action of carbon in carbothermal reduction was suggested. KEYWORDS carbothermal reduction, fiber, porosity, silicon carbide, template
Funding information Ministry of Education and Science of the Russian Federation, Grant/Award Number: К4-2016-059
| INTRODUCTION
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Silicon carbide is one of the few materials which being semiconductors are characterized by high thermal, mechanical and chemical stability, as well as high thermal conductivity.1,2 Since many catalytic reactions occur at high temperatures and in aggressive environments synthesis of catalyst supports bearing high thermal stability and chemical inertness is important. Taking this into account, silicon carbide seems to be a suitable support for development of efficient catalysts required for high temperature processes of exothermic nature.1,3,4 For synthesis of porous silicon carbide several methods were developed including a direct reaction between silicon and carbon, chemical vapor deposition (CVD), sol-gel synthesis, carbothermal reduction of silica, magnesiothermic reduction etc.5-7 For example, porous SiC was obtained as a result of sol-gel synthesis using as reagents—tetraethyl orthosilicate 36
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© 2017 The American Ceramic Society
and polymer of phenol and formaldehyde, followed by carbothermal reduction of as-synthesized SiO2/C composite at 1250°C.8 SiC nanorods were formed during heat treatment of multiwalled carbon nanotubes and silicon powder in a mixture of sodium chloride and sodium fluoride at 11001200°C in an inert atmosphere.9 Carbothermal reduction in silicon oxide can be considered as one of the most effective and controlled methods of silicon carbide synthesis.10,11 So-called matrix synthesis of silicon carbide involves the use of carbon-silica composites containing silica MMS as the silicon source and carbonized organic matter in the pores of the silica matrix as the carbon component. Thus carbothermal reduction of the carbon-silica composites (from SBA-15 and sucrose) leads to formation of silicon carbide which has a developed surface area (120-190 m2/g) and morphology of whiskers or nanotubes.12 Nanocrystalline silicon carbide b-modification with substantial surface area (up to 147 m2/g) and mesoporosity
wileyonlinelibrary.com/journal/ijac
Int J Appl Ceram Technol. 2018;15:36–41.
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(pores of 5-40 nm in diameter) was formed after heating of spatially ordered mesoporous carbons (carbon replicas of MCM-48, SBA-15, KIT-6 silica matrices) with silicon powder at 1200-1300°C.5 In our previous work it was shown that b-SiC with a developed surface area (up to 410 m2/g) and high pore volume (up to 1.0 cm3/g) can be obtained via carbothermal reduction using carbon-silica composites based on various types of silica MMS (SBA15, KIT-6, MCF, SBA-3).13,14 It should be, however, stated that mechanistic details of silicon carbide formation with a specific morphology (particles, fibers) in particular in the case of carbothermal reduction of silica remain unclear and should be further explored. The aim of the current work was to determine the influence of synthesis conditions on silicon carbide morphology. Silica mesoporous molecular sieves of different types as well as bulk reagents (silica and carbon precursors) were used for comparison in order to obtain SiC. The present paper demonstrates that porosity of the carbon component in the initial carbon-silica composites, as well as the number of contact zones between silica and carbon phases are the main factors determining morphology (in particular, fibers presence) and consequently textural parameters of the resulting silicon carbide.
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| EXPERIMENTAL
Silicon carbide samples were synthesized as described elsewhere.13,14 The most pertinent details are provided below. SiC-1 was generated from a mixture of sucrose (Ecolab, Kyiv, Ukraine) and silica (Aerosil A-175, Chlorovinyl Co, Kalush, Ukraine), sucrose/silica wt. ratio was 3. SiC-2 was obtained from a mixture of carbon (carbonized sucrose at 900°C for 2.5 hours) and silica (Aerosil 175), carbon/silica wt. ratio was 1. SiC-SBA-3, SiC-MCF, SiC-SBA-15 and SiC-KIT-6 were obtained from carbon-silica composites based on SBA-3, MCF, SBA-15 and KIT-6, respectively. A weighed quantity of initial silica MMS was impregnated with an aqueous solution containing sucrose and sulfuric acid (estimated as 1.25 g and 0.14 g of sucrose and H2SO4 respectively, for 1 g MMS with pore volume 1.3 cm3/g), dried primarily at 100°C for 6 hours followed by treatment for another 6 hours at 160°C. The resulting composites were again mixed with an aqueous solution of sucrose and sulfuric acid (0.8 g and 0.08 g of sucrose and H2SO4 respectively, for 1 g MMS with pore volume 1.3 cm3/g). Resulting powders were heated (heating rate 5°/min) in an inert atmosphere to 900°C and kept at this temperature for 2.5 h. Thereafter the synthesized carbon-silica composites as well as mixtures sucrose/silica and carbon/silica were heated at 1400°C (heating rate 2°C/
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min) in an inert atmosphere for 9 hours. The obtained powders were purified by heating to 700°C in air and washing in HF solution in order to remove residual carbon and silica, respectively. The phase composition of the obtained samples was determined by powder X-ray diffraction. The computerized Bruker D8 Advance diffractometer was equipped with Cu Ka (k = 0.15406 nm) X-ray source (step 0.03°, 3 seconds per step). Morphology of the samples was investigated using scanning electron microscopy (SEM) with MIRA3 TESCAN microscope at accelerating voltage of 5-20 kV.
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| RESULTS AND DISCUSSION
According to our previous studies the obtained samples are almost pure b-modification of silicon carbide (b-SiC).13,14 Silicon carbide obtained as a result of high temperature treatment of silica (Aerosil 175) and sucrose (sample SiC-1) predominantly has fibers morphology with a length of a few micrometers and a diameter of ca. 50 nm (Figure 1A). Heating of silica with carbon (pore volume 0.3 cm3/g, surface area 745 m2/g,) leads to formation of a relatively dense structure consisting of particles ranging in the size from 50 to 200 nm (sample SiC-2, Figure 1B). Formation of a significant proportion of fibers with a length of 3-4 micron and diameter of 50-200 nm takes place as a result of carbothermal reduction of carbonsilica composite C-SBA-3 (Figure 1C). Content of the fibers in the product of carbothermal reduction of C-MCF composite is somewhat smaller (length 2-3 microns, diameter 30-80 nm, Figure 1D). The structure of SiC-MCF consists mainly of globular and worm-like particles. Thermal treatment of C-SBA-15 leads to formation of the particles with size of 150-200 nm, while generation of fibers was barely observed (Figure 1E). Material with a loose packing of wormlike particles size of 100-250 nm and containing only single fibers (Figure 1F) is a result of carbothermal reduction of carbon-silica composite C-KIT-6. Obviously such morphology of the obtained samples is due to the mechanism of carbothermal reduction and the features of the structure of used carbon-silica composites. Carbothermal reduction in silica and carbon system involves several stages:15-17 SiO2ðsÞ þ CðsÞ ! SiOðgÞ þ COðgÞ
(1)
SiO2ðsÞ þ COðgÞ ! SiOðgÞ þ CO2ðgÞ
(2)
SiOðgÞ þ CðsÞ ! SiCðsÞ þ COðgÞ
(3)
SiOðgÞ þ 3COðgÞ ! SiCðsÞ þ 2CO2ðgÞ
(4)
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(A)
(B)
(C)
(D)
(E)
(F)
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F I G U R E 1 SEM-images of the obtained silicon carbide samples: (A) SiC-1, (B) SiC-2, (C) SiC-SBA-3, (D) SiC-MCF, (E) SiC-SBA-15, (F) SiC-KIT-6 [Color figure can be viewed at wileyonlinelibrary.com]
CO2ðgÞ þ CðsÞ ! 2COðgÞ
(5)
SiO2ðsÞ þ 3CðsÞ ! SiCðsÞ þ 2COðgÞ
(6)
Gaseous silicon monoxide formed in the (Equation 1) is apparently reacting with carbon particles resulting in generation of silicon carbide crystallites according to (Equation 3). Formed SiC particles can further serve as nuclei for growth of the fibers, thus determining their diameter. The growth of the fibers is possible in conditions of supersaturation by carbon monoxide vapors (Equation 4). Carbon dioxide produced by the (Equation 4) can be converted into carbon monoxide according to the (Equation 5). Finally the aforementioned reactions lead to local supersaturation of CO. The (Equation 6) corresponds to the overall reaction between silica and carbon. Thus, growth of silicon carbide fibers occurs as a result of silicon monoxide interactions with carbon in the areas of a significant local concentration increase. An investigation of silicon carbide formation at different temperatures can confirm the indicated sequence of reactions. The X-ray diffraction patterns of the samples obtained at 1200-1400°C are presented in Figure 2. It can be seen that the silicon carbide phase is already formed at
1300°C in both cases corresponding to bulk and template synthesis. Heat treatment of the reaction mixture at 1200°C doesn’t lead to SiC formation (Figure 2). The obtained samples are amorphous (obviously contain amorphous silica and carbon) indirectly testifying the silicon carbide formation through amorphous and possible gaseous phases rather crystalline intermediate products. The formation of SiC nuclei and further fibers growth take place predominantly on carbon particles, and a significant increase in CO concentration is only possible when reactions involve carbon (Equations 1 and 5). Based on those facts it can be suggested that morphology and consequently textural parameters of the resulting product are likely to be determined essentially by the porous structure of the carbon component in the initial composites, and by the number of contact zones of silica and carbon phases. In particular, in the case of silicon carbide formation from silica and sucrose (sample SiC-1) primarily pyrolysis of the organic substance with the formation of carbon occurs. Carbonization of sucrose in the presence of silica provides local interactions of intermediate carbon-containing oligomers with active sites on the surface, such as silanol groups, to form uniformly distributed toward the surface
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F I G U R E 2 XRD patterns of synthesized silicon carbide samples from the mixture corresponding to sample SiC-1 (A) and from carbon-silica composite based on KIT-6 corresponding to SiC-KIT-6 (B) [Color figure can be viewed at wileyonlinelibrary.com]
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carbonization products, which may also include nuclei of the silicon carbide phase. In distributed across the surface areas of carbon-silica contacts and carbon nanoparticles when temperature further increases, because of local interactions between CO and SiO (reaction 4), formation (reaction 3) and further growth of SiC fibers occur. Slow formation of silicon carbide in the form of nanofibers likely implies existence of dispersed over the surface active (“oligomeric,” not crosslinked) forms of carbon and starts at lower temperatures. Since sucrose initially acts as C-containing component, and carbonization takes place in the presence of silica, formation of carbon mainly in the form of isolated individual particles should be expected. Further growth of silicon carbide fibers occurs on such not interconnected carbon particles. Use of a preliminary prepared carbon, which has a crosslinked structure, did not lead practically to formation of fibers (sample SiC-2) due to absence of contact zones between mentioned above active carbon forms and carbon-silica. As a result, carbothermal reduction is shifted toward higher temperatures with a significantly increased rate. This subsequently leads to an increase in the concentration of silicon carbide nuclei and a rapid growth of their size and formation of nanoparticles. Presumably, in these conditions the presence of continuous carbon structure provides a relatively uniform and fast formation of SiC particles because of the presence of carboncarbon and carbon-silica phase contacts. The particles of SiC-2 sample are smaller than SiC-1 (40 nm and 70 nm, respectively, calculations according to the Scherrer’s equation). In the case of silica MMS the contact area of carbon and silica components is obviously determined by their surface areas. Given the fact that silicon dioxide during the reaction is transformed into gaseous silicon monoxide
T A B L E 1 Sorption characteristics (N2 ad(de)sorption, 77 K) of silicon carbide and carbon samples obtained from carbon-silica composite Sample
Vmicroa, cm3/g
Vmeso, cm3/g
Smesoa, m2/g
SiC-1
–
ND
15
19
SiC-2
–
0.31
45
> 20
SiC-SBA-3
–
0.16
40
21
SiC-SBA-15
–
0.23
45
38
SiC-KIT-6
–
ND
ND
> 50
SiC-MCF
–
ND
ND
C-SBA-3
0.30d
0.26
–
C-SBA-15
0.11
1.01
730
C-KIT-6
0.05
1.99
1210
ND, not determined. a calculations using t-plot method. b mesopore diameter calculated from adsorption isotherms branches using BJH method. c total pore volume calculated at p/p0 = 1.0. d micropore parameters by the Dubinin-Radushkevich equation.
Dmaxb, nm
Sexta, m2/g 20 10
SBET, m2/g 35
V∑c, cm3/g 0.085
55
0.31
40
0.16
5
50
0.23
210
410
1.02
–
> 50
90
105
0.42
–
35
1025
0.57
3.60.3
130
1125
1.12
3.1 10.9
120
1570
2.04
40
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and formation of SiC particles occurs on carbon surface, the contact area of C and SiO2 phases should depend primarily on the mesopore specific surface area of the carbon component, the micropores located within carbon particles are less involved in the process. As evidenced by SEM (Figure 1) and nitrogen physisorption (Table 1) 11,12 the content of silicon carbide fibers is inversely proportional to the mesopore specific surface area of carbon materials after removing the silica component from carbon-silica composites. Thus our current results show that utilization of carbon component C-SBA-3 with the smallest mesopore specific surface area (~ 400 m2/g) leads to formation of silicon carbide with the highest fiber content among the investigated silica MMS. On the contrary for the carbon-silica KIT-6 based composite with the carbon component characterized by the highest mesopore specific surface area (1210 m2/g) SiC containing a relatively small amount of fibers was formed, possessing therefore the highest porosity among the synthesized samples. Accordingly, the indicated material has the smallest particle size (17 nm). Furthermore, C/SiO2 ratio is an additional factor affecting the formation mechanism of silicon carbide of a specific morphology (C/SiO2 = 3 according to the overall reaction (Equation 6). In the case of SBA-3 (1.4) it is much smaller than the stoichiometric one, limiting formation of silicon carbide nuclei due to a lack of carbon and thus promoting SiC fibers growth. Additionally, carbonization of the carbon-containing precursor in the presence of silica MMS with a developed surface at the stage of carbon-silica composites formation elevated the number of active contact zones between reagents. This in turn increases the amount of silicon carbide crystal seeds, reducing thus a probability of fibers formation and resulting in a decrease of nanoparticles size and increase of the specific surface area. The found correlation between the type of silicon carbide structure and mesopore specific surface area of carbon component of the composites (in the case of silica MMS) or the nature of carbon component (in the case of aerosil revealed in the current work) may indicate the templating role of carbon in carbothermal reduction as also noted in.18
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| CONCLUSIONS
In the current work b-SiC samples possessing high structural and sorption characteristics (SBET up to 410 m2/g, Vpore up to 1.0 cm3/g) were obtained using carbon-silica composites as precursors. Morphology (fibers presence) and consequently textural parameters of SiC are largely defined by porosity of the carbon component in the initial carbon-silica composites, as well as the number of contact zones between silica and
ET AL
carbon phases. In the case of silica MMS a contact area of silica and carbon is determined by the specific surface of mesopores. As evidenced by SEM and our previous data on nitrogen physisorption an amount of silicon carbide fibers depends on the mesopore specific surface area of carbon materials derived from carbon-silica composites showing an inverse relationship. Such dependence between silicon carbide structure and mesopore specific surface area of the carbon component in the composite indicates a templating role of carbon in carbothermal reduction.
ACKNOWLEDGMENT The authors gratefully acknowledge the financial support of the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST «MISiS» (№ К4-2016-059), implemented by a governmental decree dated 16th of March 2013, N 211.
CONFLICT OF INTEREST The authors declare that they have no conflict of interest. REFERENCES 1. Moene R, Makkee M, Moulijn JA. High surface area silicon carbide as catalyst support characterization and stability. Appl Catal A. 1998;167:321-330. 2. Krawiec P, Kaskel S. Thermal stability of high surface area silicon carbide materials. J Solid State Chem. 2006;179:22812289. 3. Ledoux MJ, Pham-Huu C. Silicon carbide: a novel catalyst support for heterogeneous catalysis. Cattech. 2001;5:226-246. 4. Pham-Huu C, Del Gallo P, Peschiera E, et al. n-Hexane and nheptane isomerization at atmospheric and medium pressure on MoO3-carbon-modified supported on SiC and c-Al2O3. App Catal A. 1995;132:77-96. 5. Liu Z, Shen W, Bu W, et al. Low-temperature formation of nanocrystalline b-SiC with high surface area and mesoporosity via reaction of mesoporous carbon and silicon powder. Micropor Mesopor Mater. 2005;82:137-145. 6. Guo XY, Jin GQ. Pore-size control in the sol–gel synthesis of mesoporous silicon carbide. J Mater Sci. 2005;40:1301-1303. 7. Zhao B, Zhang H, Tao H, et al. Low temperature synthesis of mesoporous silicon carbide via magnesiothermic reduction. Mater Lett. 2011;65:1552-1555. 8. Jin GQ, Guo XY. Synthesis and characterization of mesoporous silicon carbide. Micropor Mesopor Mater. 2003;60:207-212. 9. Xie W, M€ obus G, Zhang S. Molten salt synthesis of silicon carbide nanorods using carbon nanotubes as templates. J Mater Chem. 2011;21:18325-18330. 10. Kim YW, Kim SH, Song IH, et al. Fabrication of open-cell, microcellular silicon carbide ceramics by carbothermal reduction. J Am Ceram Soc. 2005;88:2949-2951. 11. Kim Y-W, Eom JH, Wang C, et al. Processing of porous silicon carbide ceramics from carbon-filled polysiloxane by extrusion
SHCHERBAN
12.
13.
14.
15.
16.
ET AL
and carbothermal reduction. J Am Ceram Soc. 2008;91:13611364. Zhuxian Y, Xia Y, Mokaya R. High surface area silicon carbide whiskers and nanotubes nanocast using mesoporous silica. Chem Mater. 2004;16:3877-3884. Shcherban ND, Filonenko SM, Sergiienko SA, et al. Structure and porosity of silicon carbide produced by matrix method. Theor Exp Chem. 2015;51:320-326. Shcherban ND, Filonenko SM, Yaremov PS, et al. Carbothermal synthesis of porous silicon carbide using mesoporous silicas. J Mater Sci. 2017;52:3917-3926. Vix-Guterl C, Ehrburger P. Effect of the properties of a carbon substrate on its reaction with silica for silicon carbide formation. Carbon. 1997;35:1587-1592. Liang CH, Meng GW, Zhang LD, et al. Large-scale synthesis of b-SiC nanowires by using mesoporous silica embedded with Fe nanoparticles. Chem Phys Lett. 2000;329:323-328.
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17. Yao J, Wang H, Zhang X, et al. Role of pores in the carbothermal reduction of carbon-silica nanocomposites into silicon carbide nanostructures. J Phys Chem C. 2007;111:636-641. 18. Pan Z, Lai HL, Au FC, et al. Oriented silicon carbide nanowires: synthesis and field emission properties. Adv Mater. 2000;12:1186-1190.
How to cite this article: Shcherban N, Filonenko S, Sergiienko S, et al. Morphological features of porous silicon carbide obtained via a carbothermal method. Int J Appl Ceram Technol. 2018;15:36-41. https://doi.org/10.1111/ijac.12757
