Materials Letters 63 (2009) 2361–2364 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t Electrospinning of beta silicon carbide nanoﬁbers Harvey A. Liu, Kenneth J. Balkus Jr. ⁎ The University of Texas at Dallas, Department of Chemistry and the Alan G. MacDiarmid NanoTech Institute, Richardson, TX 75083-0688, USA a r t i c l e i n f o Article history: Received 13 May 2009 Accepted 4 August 2009 Available online 9 August 2009 Keywords: Silicon carbide Nanoﬁbers Electrospinning a b s t r a c t Silicon carbide exhibits many unique properties such as its mechanical robustness, chemical inertness, and thermal stability, which make the material appealing for many applications. Some of these applications include its use as a support for nanocomposites or as a high temperature ﬁlter material. The ability to fabricate nanoﬁbers of SiC could enhance its utility in these applications. In the current study, nanoﬁbers of β-SiC have been fabricated through the technique of concentric electrospinning. This method demonstrates the ability to fabricate uniform SiC nanoﬁbers with a diameter ranging from 1 to 2 nm, the smallest to date. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Silicon carbide has been acclaimed for many of its characteristics such as its mechanical robustness, chemical inertness, and thermal stability. Due to the many unique properties of silicon carbide, different morphologies of this material have been fabricated, including: thin ﬁlms [1,2], rods [3,4], and ﬁbers [5,6]. For example, ﬁbers with a diameter of 1–3 μm were fabricated through wet-type electrospinning . A more recent study in the electrospinning of silicon carbide by Eick et al. demonstrated the ability to fabricate composite ﬁbers with diameters as small as 20 nm, though the majority of the ﬁbers observed were much larger, some of which exceeded 100 nm in diameter. In addition to the wide variation in the diameter of these ﬁbers, a silica shell formed with a SiC core caused by the atmospheric oxygen and impurities in the furnace . A ﬁbrous morphology provides many unique favorable properties; the most prevalent is the increase in surface to volume ratio, which expands the utility of a material to various applications. Nanoﬁbers of silicon carbide in particular offer potential applications as a support material for nanocomposites , substrates for catalysis [8–10], and as well as high temperature ﬁlters . Electrospinning of the polycarbosilane followed by curing and pyrolysis has proven to be a successful approach to fabricating SiC ﬁbers [5,6]. However, the resulting SiC ﬁbers exhibited a wide range of ﬁber diameters as well as nonuniform and impure structures. In the present study, we utilized a polycarbosilane as a precursor, but employ the technique of concentric, or core–shell, electrospinning to fabricate uniform nanoﬁbers. By employing concentric electrospinning we demonstrated the ﬁrst fabrication method to attain consistently uniform silicon carbide ﬁbers with diameters in the range of 1–2 nm. There has been growing interest in the method of electrospinning, especially in the area of ceramic materials. This method has been utilized in the fabrication of submicron to nanosized ﬁbers composed of various materials including polymers , ceramics [12–18], and composites, which have found application in areas such as tissue engineering [20,21], catalysis , composite materials , supercapacitors [23,24], drug delivery , as well as self-healing materials . In the past few years, a variation of conventional electrospinning has been developed, known as coaxial or concentric electrospinning, and has resulted in the fabrication of ﬁbrous materials that previously could not have been achieved [27–29]. A schematic of concentric electrospinning is shown in Fig. 1. Similar to conventional electrospinning, concentric electrospinning utilizes an external electrostatic force, which is applied to a polymer melt . Upon excess electrostatic force, the polymer melt forms a Taylor cone and undergoes a whipping action, stretching the polymer solution and forming a nonwoven mat . The technique of coaxial electrospinning allows for the formation of ﬁbers for materials that cannot be electrospun through conventional spinning. Typically, a polymer that is easily electrospun is fed through the outer sheath and a material that is difﬁcult to spin is fed through the core. The outer sheath material serves to contain the inner core and form the ﬁber morphology. After the fabrication of these coaxial ﬁbers, post-fabrication methods can be performed on the inner material which may include either annealing and/or washing away of the outer sheath to reveal the inner ﬁbrous material. 2. Experimental ⁎ Corresponding author. Tel.: +1 972 883 2659; fax: +1 972 883 2925. E-mail address: [email protected] (K.J. Balkus). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.08.009 In the present study, high molecular weight polystyrene was used for the fabrication of the outer sheath while polycarbosilane 2362 H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364 Fig. 1. Schematic of concentric electrospinning setup. (SMP-10, Starﬁre Systems Inc.) (Fig. 2a) was fed through the core as the silicon carbide precursor. The concentric electrospinning setup was assembled through the insertion of a 22 gauge needle through an 18 gauge needle. The ﬂow rate through the outer and inner needle was independently regulated with a syringe pump (7802CO, KD Scientiﬁc Inc., USA). In a typical synthesis, a 15 wt.% polystyrene in chloroform melt was fed through the outer sheath at a rate of 7× 10− 2–1×10− 1 mL/min and the silicon carbide precursor, SMP-10, was used as received and fed through the inner core at a rate of 8× 10− 3 mL/min. A 10 kV charge was applied to the needle tip, which was adjusted to be 25 cm from the grounded rotating drum wrapped with foil as the collecting surface. 3. Results and discussion Scanning electron micrographs (SEM) of the as-spun core shell ﬁbers are shown in Fig. 2b–c. The average diameter of the core–shell ﬁbers typically ranged from 10 to 20 μm (Fig. 2b), however, ﬁbers as small as 2 μm were observed as well (Fig. 2c). The ﬁbers were spun into a freestanding sheet and cured in an oven overnight at 100 °C for 12 h followed by heating to 200 °C for 24 h to cure the polycarbosilane. The as-spun core–shell ﬁbers as a free-standing sheet exhibited a slight yellow tint from the polycarbosilane precursor embedded in the core (Fig. 2d). It can be seen in the SEM micrograph (Fig. 2c) that some ﬁbers Fig. 2. (a) Molecular structure of polycarbosilane (b,c) SEM micrographs of the as-spun core–shell ﬁbers (d) digital image of free-standing sheet of ﬁbers. H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364 2363 Fig. 3. XRD of crystallized β-SiC nanoﬁbers showing miller indices. Fig. 5. TEM of crystallized β-SiC. were fractured, due to the brittle nature of polystyrene, though the ﬁbers were robust enough to handle (Fig. 2d). SEM imaging of the cured ﬁbers revealed no changes in the morphology of the ﬁbers from the curing process. The ﬁlm was then washed in chloroform and centrifuged repeatedly to wash away the polystyrene outer sheath. The resulting cured ﬁbers were then transferred to a graphite crucible and subjected to pyrolysis in a high temperature vacuum furnace under an inert gas atmosphere (Red Devil™, R. D. Webb Company, Inc.). The pyrolysis procedure was executed under argon gas according to published processing procedures . The ﬁbers were heated from room temperature to 650 °C at a rate of 1 °C/min, then from 650 °C to 850 °C at 3 °C/min. The oven was then held at 850 °C for 1 h whereupon heating was stopped and cooled to below 100 °C before the sample was recovered. The pyrolysis treatment results in the recovery of amorphous (glassy) silicon carbide, which was conﬁrmed by X-ray diffraction. Further heat treatment was used in order to convert the glassy silicon carbide to polycrystalline β-SiC. The crystallization was conducted in a high temperature vacuum oven under argon where the sample was heated from room temperature to 1600–1650 °C at an approximate rate of 2–3 °C/min. The sample was held at this temperature for 6 h to crystallize the ﬁbers. The X-ray diffraction pattern of the crystallized SiC nanoﬁbers is shown in Fig. 3. Trace amounts of the hexagonal α-SiC are seen accompanied by a strong diffraction pattern for β-SiC (Fig. 3) In addition to SiC, trace amounts of graphite were also seen, represented by a peak appearing at approximately 26°. The appearance of graphite is likely from the graphite crucible and/or the graphitic insulation of the furnace. SEM micrographs of the crystalline β-SiC reveal uniform nanoﬁbers with diameters approximately 10 nm in size (Fig. 4). Comparisons of the ﬁbers before the washing and pyrolysis of the electrospun composite ﬁbers show a large difference in the diameter size. This is expected since the polystyrene was fed through the needle at a much higher rate than the silicon carbide precursor, and thus a smaller volume of the precursor was actually present in the core of the composite ﬁbers. Transmission electron microscopy (TEM) of the crystallized silicon carbide is shown in Fig. 5, and is in good agreement with SEM micrographs. Sample preparation was carried out by physically grinding the large crystals and sonication followed by ﬁshing with a lacey carbon TEM grids. The TEM images revealed similar structures as the SEM micrographs. Bundles were seen throughout the lacey carbon grid consisting of entangled nanoﬁbers less than 1–2 nm in diameter. The discrepancy between the ﬁber diameter observed in the SEM and TEM was attributed to the Pd/Au coating for SEM imaging. In a typical Pd/Au coating, approximately 10 nm of the conductive metal was sputtered, accounting for the increased diameter observed in the SEM micrographs. In addition to the electrospinning of β-SiC with the SMP-10 precursor, we have also attempted to electrospin the hexagonal α-SiC. This was done by seeding the polycarbosilane precursor polymer with crystals of the hexagonal α-SiC during the electrospinning process. The curing, pyrolysis, and crystallization were performed in the same manner as the non-seeded precursor. However, there were no signiﬁcant change in ratio of the α-SiC to the β-SiC nanoﬁbers. 4. Conclusion The ability to fabricate uniform nanoﬁbers of β-SiC 1–2 nm in diameter has been demonstrated via the technique of concentric Fig. 4. SEM micrographs of crystallized β-SiC. 2364 H.A. Liu, K.J. Balkus Jr. / Materials Letters 63 (2009) 2361–2364 electrospinning. These represent the smallest SiC nanoﬁbers produced to date. The strategy demonstrated here to form nanoﬁbers of silicon carbide can be applied to the preparation of other carbide ﬁbers. Future work is focused on electrospinning the hexagonal α-SiC ﬁbers, which may have more versatile applications. Acknowledgement Financial support for this work was provided by the Robert A. 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