Synthesis and Characterization of Crystalline Silicon Carbide Nanoribbons
© The Author(s) 2010
Received: 22 February 2010
Accepted: 5 May 2010
Published: 22 May 2010
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© The Author(s) 2010
Received: 22 February 2010
Accepted: 5 May 2010
Published: 22 May 2010
In this paper, a simple method to synthesize silicon carbide (SiC) nanoribbons is presented. Silicon powder and carbon black powder placed in a horizontal tube furnace were exposed to temperatures ranging from 1,250 to 1,500°C for 5–12 h in an argon atmosphere at atmospheric pressure. The resulting SiC nanoribbons were tens to hundreds of microns in length, a few microns in width and tens of nanometers in thickness. The nanoribbons were characterized with electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy, and were found to be hexagonal wurtzite–type SiC (2H-SiC) with a growth direction of . The influence of the synthesis conditions such as the reaction temperature, reaction duration and chamber pressure on the growth of the SiC nanomaterial was investigated. A vapor–solid reaction dominated nanoribbon growth mechanism was discussed.
Silicon carbide (SiC) is a material with outstanding physical and mechanical properties. It has high mechanical strength, high hardness, low density, high thermal conductivity, low thermal expansion coefficient, large band-gap, and excellent oxidation and corrosion resistances [1–3]. It is a leading material for components and devices operating at high temperature, high power and under harsh environments [4, 5]. Micro-sized SiC particles and whiskers are commonly used as reinforcement materials for ceramics, metals and alloys for various structural and tribological applications [6, 7].
In the past decade, one-dimensional (1D) SiC nanostructures have been successfully synthesized. In 1995, Dai et al.  reported the first SiC nanorod synthesis by the reaction of carbon nanotubes with either silicon monoxide or silicon and iodine vapor. After that, a number of techniques have been developed to synthesize SiC nanowires, including the sol–gel , vapor–liquid–solid , vapor–solid , laser ablation  and chemical vapor deposition (CVD)  methods. More recently, SiC micro-/nanoribbons have been successfully synthesized by several research groups [14–16]. For instance, Xi et al.  reported the growth of cubic SiC (3C-SiC) nanobelts via the reaction of tetrachlorosilane, ethanol and lithium powder in an autoclave at low temperature (600°C) and suggested a lithium-assisted mechanism of SiC nanostructure growth. Yushin et al.  synthesized α-SiC micro-ribbons by a carbothermal reaction of silicon dioxide and graphite at high temperature (1,800–1,900°C). Wu et al.  reported the synthesis of bicrystalline SiC nanobelts via a thermal evaporation and condensation process with silicon powder and multi-wall carbon nanotubes as the raw materials at 1,250°C.
Compared with the bulk and micro-sized SiC structures, SiC nanostructures have several novel mechanical, electrical and optical properties as a result of their reduced size [17–21]. SiC nanowhiskers have been shown to have a much higher mechanical strength than SiC microwhiskers and bulk SiC. According to Wong et al. , SiC nanorod has an estimated yield strength of over 50 GPa, substantially higher than that of bulk SiC. Pan et al.  reported that SiC nanowires have a very low electron emission threshold and are thus promising for vacuum microelectronics application.
In this paper, we report a simple catalyst-free growth of crystalline SiC nanoribbons from powders of silicon and carbon black at high temperature (1,500°C). Detailed analyses by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) show that the nanoribbons are crystalline SiC with a hexagonal wurtzite structure (2H-SiC or α-SiC). Compared with other reported SiC nanobelt/ribbon synthesis methods, the main advantages of the current approach are the relatively low growth temperature and the catalyst-free synthesis.
Silicon powder (200 nm–2 μm in diameter, 99.9985% purity, Alfa Aesar, Ward Hill, MA) and carbon black powder (~50 nm in diameter, 99.9% purity, Alfa Aesar, Ward Hill, MA) were used as the starting materials. The silicon powder was placed in the upstream and the carbon black powder in the downstream of an alumina boat (100 × 20 × 20 mm) with a silicon and carbon molar ratio of about 1:1. The alumina boat was then placed in the center of the reaction chamber (horizontal alumina tube). Before heating, the chamber was first evacuated to ~10 mTorr by a rotary vacuum pump. Then, the chamber pressure was adjusted to the desired pressure level with ultra pure argon (Ar) gas (99.999%, Airgas, Radnor, PA). After that, the chamber was ramped from room temperature to 1,500°C (center position temperature) at a rate of 10°C/min and was maintained at this temperature for desired reaction duration. Finally, the chamber was cooled down to room temperature at 10°C/min rate. A continuous flow of 200 sccm (standard cubic centimeters per minute) Ar gas was maintained during the entire process. After the reaction, grey colored powder was found in the carbon black powder section of the alumina boat.
The morphology of the as-synthesized SiC nanomaterial was observed under a SEM (7400F, JEOL, Tokyo, Japan) at an acceleration voltage of 15 kV. The chemical composition of the material was analyzed with EDX (NORAN System Six, Thermo Scientific, Waltham, MA) in the SEM. The nanomaterial was also observed under a HRTEM (CM200 FEG, Philips, Eindhoven, Netherlands) at 200 kV acceleration voltage for crystal structure determination and selected area electron diffraction (SAED) analysis. To prepare specimens for TEM observation, SiC nanomaterial was first dispersed in acetone by ultrasonication for 5 min. Then, a drop of the suspension was dipped onto a carbon-coated copper TEM grid. The crystallinity of the nanomaterial was characterized with XRD (AXS D8 FOCUS, Bruker, Germany) using a copper Kα1 (λ = 1.54 Å) radiation source. The Raman spectrum of the SiC nanomaterial was taken with a Raman spectrometer (514.5 nm excitation, Ar+ ion laser source, Renishaw invia, Gloucestershire, UK) at room temperature. XPS (PHI VersaProbe, Physical Electronics, MN) analysis was also performed on the SiC nanomaterial to characterize its chemical composition.
Atomic percentages of silicon and carbon at three regions of a SiC nanoribbon
Figure 5b shows a typical Raman spectrum (200–1,100 cm−1) of the SiC nanoribbons. Raman peaks at around 260, 752, 786 and 946 cm−1 are observed that correspond to the peaks of 2H-SiC. The peak at 260 cm−1 is attributed to the transverse acoustic (TA) mode, the peaks at 752 and 786 cm−1 correspond to the transverse optical (TO) mode, and the peak at 946 cm−1 is the characteristic of longitudinal optical (LO) mode. The reported Raman peaks of bulk 2H-SiC are 264, 764, 799 and 968 cm−1. The SiC nanoribbon sample has a frequency shift of ~4–22 cm−1, which could be the result of size confinement and/or the presence of structural defects [29, 30].
Reaction duration effect: A series of synthesis trials were carried out with the reaction time of 5, 6, 9, 10.5 and 12 h at 1,500°C reaction temperature and 1 atm chamber pressure with a 200 sccm continuous flow of Ar gas. Figure 8a–8d show the SEM images of the nanostructures synthesized with 5-, 6-, 9- and 10.5-h reaction duration, respectively. For 5-h reaction duration, a number of SiC micro-/nanocrystals were observed on the surface of the carbon black powder. For 6-h reaction duration, small clusters of SiC nanoribbons were observed, which appeared to grow from the SiC micro-/nanocrystals on the carbon black powder surface. With increasing reaction duration, the length of the nanoribbons increased while their width remained roughly the same. To quantify the dependence of nanoribbon length on the reaction duration, we randomly picked a number of ribbons in the recorded SEM images and measured their lengths. The resulting relationship between the nanoribbon length and the reaction duration appeared to be linear, with a projected nanoribbon growth initiation at ~5.5 h reaction duration.
Chamber pressure effect: We briefly explored the influence of the chamber pressure on the SiC nanostructure growth by performing synthesis trials at 1.5 torr and 1 atm pressure at 1,500°C for 9 h with a 200 sccm continuous flow of Ar gas. At 1.5 torr pressure, micro-sized silicon needles were observed in the region where silicon powder was located, but no SiC nanomaterial was observed in the carbon black region. Such observation indicates that the chamber pressure is a critical parameter for the reaction of silicon vapor with carbon black. However, due to the limitation of our current home-built CVD system, we were not able to explore the chamber pressure effect over a wide range of pressure levels.
The catalyst-free thermal evaporation growth has been previously reported by several researchers for the synthesis of SiC nanowhiskers and nanobelts [14, 34–36]. Because the carbon black powder is physically separated from the silicon powder in the alumina boat, the growth of the SiC nanoribbon in carbon black powder region is believed to be dominated by the reaction of the solid carbon with silicon vapor (vapor–solid mechanism) rather than the reaction of solid carbon with liquid silicon (vapor–liquid–solid mechanism). Synthesis trials were carried out with mixed silicon and carbon black powders under the same reaction condition (1,500°C, 9 h, 1 atm, 200 sccm continuous Ar flow). Randomly oriented submicron- and micron-sized SiC polycrystals were observed in the resulting material without any evidence of nanoribbon growth.
In summary, SiC nanoribbons were synthesized by a reaction of silicon vapor and carbon black powder at 1,500°C in Ar atmosphere at atmospheric pressure. The nanoribbons were typically tens to hundreds of microns in length, several microns in width and tens of nanometers in thickness. The nanoribbons were characterized with SEM, HRTEM, EDX, XRD, Raman spectroscopy and XPS, and were found to be hexagonal 2H-SiC with a growth direction of . The influence of synthesis conditions including the reaction temperature, reaction duration and chamber pressure on the growth of the SiC nanomaterials was investigated. The reaction temperature was varied from 1,250 to 1,500°C, and the nanoribbon growth was observed for reactions above 1,400°C. The reaction duration was varied from 5 to 12 h, and the nanoribbons were found to start growing at ~5.5-h reaction duration, with their lengths increasing almost linearly with the reaction duration. Reactions were carried out at 1.5 torr and 1 atm chamber pressures, and no nanoribbon growth was observed at 1.5 torr pressure. Based on the synthesis condition study, a vapor–solid reaction dominated growth mechanism was proposed for the SiC nanoribbon growth.
W. Ding appreciates the support of the start-up fund at Clarkson University. We are grateful to the Center for Advanced Materials Processing (CAMP) at Clarkson, the John M. Cowley Center for High Resolution Electron Microscopy at Arizona State University and the Nanosystem Engineering System Facility and Equipment Resources at West Virginia University for supplying multi-user facilities used in this work.
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