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Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Silicon carbide nanowires synthesized with phenolic resin and silicon powders Hongsheng Zhao à , Limin Shi, Ziqiang Li, Chunhe Tang Division of New Materials, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 102201, China article info Article history: Received 13 October 2008 Received in revised form 10 December 2008 Accepted 11 December 2008 Available online 24 December 2008 PACS: 61.46.Àw 81.05.Je Keywords: Silicon carbide Nanostructure Scanning electron microscopy High-resolution electron microscopy abstract Large-scale silicon carbide nanowires with the lengths up to several m illimeters were synthesized by a coat-mix, moulding, carbonization, and high-temperature sintering process, using silicon powder and phenolic resin as the starting materials. Ordinary SiC nanowires, bamboo-like SiC nanowires, and spindle SiC nanochains are found in the fabricated samples. The ordinary SiC nanowire is a single- crystal SiC phase with a fringe spacing of 0.252 nm along the [111] growth direction. Both of the bamboo-like SiC nanowires and spindle SiC nanochains exhibit uniform periodic structures. The bamboo-like SiC nanowires consist of amorphous stem and single-crystal knots, while the spindle SiC nanochains consist of uniform spindles which grow uniformly on the entire nanowires. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991, increasing attention has been given to nanometer-scaled one- dimensional materials because of their unique properties and wide variety of potential applications [1–5]. Silicon carbide (SiC) possesses a range of excellent physical, chemical, mechanical, and electronic properties. These properties make SiC nanowires an attractive candidate material for many applications, such as reinforcement material, catalysis supports, and next-generation high-temperature, high-power, and high-frequency electronic devices [6–8]. Various methods have been reported to fabricate SiC nano- wires. One-dimension SiC nanotubes and nanowires with various shapes and structures have been synthesized at different temperatures using carbon nanotubes as templates [9]. Pan et al. [10] have successfully synthesized oriented SiC nanowires by reacting aligned carbon nanotubes template with SiO. These oriented nanowires may have promising applications for vacuum microelectronic devices due to their excellent field emission performances. Core–shell SiC/SiO 2 nanowires have been success- fully fabricated by directly heating the NiO-catalysed silicon substrate under reductive environments using the carbothermal reduction [11]. Helical crystalline core–shell SiC/SiO 2 nanowires and amorphous SiC nanosprings have also been synthesized by a chemical vapor deposition technique [12,13]. A screw-dislocation- induced growth mechanism was proposed for the formation of the novel structure. Hao et al. [14] have prepared beaded SiC nanochains, which consist of a stem and uniform beads. A simple chemical vapor deposition route has been employed to fabricate BN-nanotube-encapsulated SiC nanowires which possess an unusual gap between the BN-nanotube sheath and the SiC core [15,16]. They have also developed an efficient route to synthesize coaxial nanocables composed of a single-crystalline SiC core, an amorphous SiO 2 intermediate layer, and a graphitic carbon outer sheath [17]. Carbon-rich SiC nanowires have been synthesized by a method, which is based on the ability to form and manipulate the properties of ethyl alcohol nanometer-sized bridges [18]. Electrospun PAN fibers have been employed as templates to fabricate SiC nanowires with a uniform cross-section, well- ordered structure, and a very low concentration of stacking faults [19]. Niu and Wang [20,21] synthesized scales of high-quality crystalline silicon carbide nanowires with small diameters by direct thermal evaporation of ferrocene or ZnS onto a silicon wafer at high temperature. This kind of SiC nanowires possessed a uniform size, crystalline structure, and a thin oxide layer. A tentative growth model according to the vapor–liquid–solid (VLS) mechanism was also proposed. b -SiC nanowires can also be directly synthesized by heating single-crystal silicon wafer and graphite without metal catalysts [22]. The diameter of SiC ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.12.005 à Corresponding author. Tel./fax: +8610 89796090. E-mail address: hshzhao@tsinghua.edu.cn (H. Zhao). Physica E 41 (2009) 753–756 nanowires is in the range of 10–30 nm, and the length is up to a few millimeters. The protective gas, the high temperature, the closed space, and the slow-cooling rate are key factors for the formation of b -SiC nanowires. This paper proposed a method to synthesize SiC nanowires with phenolic resin and silicon powders without using any metal catalysts. The advantages of this method are as follows: (1) The process is simple and the starting materials are commercially available. Therefore, SiC nanowires can be fabricated at a low cost. (2) SiC nanowires can be produced in a relatively high quantity. (3) Three types of SiC nanowires can be obtained. (4) The lengths of the SiC nanowires can be up to several millimeters. (5) No metal catalysts are required during the fabrication of SiC nanowires. 2. Experimental The preparation of SiC nanowires includes the following steps: (1) As is described in our previous work [23], precursor powders, core/shell silicon/phenolic resin powders, were fabricated by coating each silicon powder with a homogeneous phenolic resin shell. (2) The obtained precursor powders were pressed to form cylindrical green compacts. (3) The as-prepared green compacts were subsequently heated at 800 1C in Ar atmosphere to get carbonized compacts. (4) The carbonized compacts, which were held in a graphite crucible with a graphite lid, were sintered in a graphite furnace in Ar atmosphere. Here, the graphite lid is used as the substrate to grow SiC nanowires. The sintering temperature was 1500 1C. A soaking time of 2 h was sufficient to synthesize a large quantity of SiC nanowires. X-ray diffraction (XRD) patterns of the prepared nanowires were recorded on a Japan D/max-IIIA X-ray diffractometer using standard Cu K a radiation. A Hitachi S-3000N SEM operated at an accelerating voltage of 10 KV was employed to investigate the morphology of the products. Further structure characterization and selected area electron diffraction (SAED) patterns were performed on a Tecnai TF20 high-resolution transmission electron microscopy (HRTEM) operated at 200 KV. During the HRTEM sample preparation, the nanowires were first dispersed in ethanol under ultrasonic vibration over 15 min and then placed onto standard carbon-coated copper grids. 3. Results and discussion It can be observed obviously that the as-synthesized sample on the graphite lid is a light green fluffy material and the wires have the length of up to several millimeters. SEM image shown in Fig. 1 reveals that most of the materials are wire-like structures with diameters in the nanometer range. At the same time, it can be found that the nanowires have several types of structure. This can be further evidenced by the TEM analysis. The XRD pattern shown in Fig. 2 verifies that the fabricated sample is cubic b -SiC. Moreover, the stronger (111) diffraction peak indicates that the [111] is the dominant growth direction of the SiC nanowires. Using HRTEM, we found that the fabricated SiC nanowires can be grouped into three types of one-dimension structures. They are described as follows: (1) Ordinary SiC nanowires, as is indicated by (1) in Fig. 3a: this figure presents a typical morphology of the ordinary SiC nanowires. Most of the synthesized nanowires belong to this structure. They are uniform in diameter along their entire length. The HRTEM lattice image and SAED pattern shown in Fig. 4 indicate that the single-crystal SiC nanowire with a fringe spacing of 0.252 nm grows along the [111] direction. It is known that, among the SiC surfaces, the {111} surfaces have the lowest surface energy [24]. Thus, when the SiC ARTICLE IN PRESS Fig. 1. A typical SEM image of SiC nanowires. Fig. 2. XRD pattern of SiC nanowires. Fig. 3. A typical morphology of the ordinary SiC nanowires: (a) bamboo-like structure and (b) spindle nanochain structure. H. Zhao et al. / Physica E 41 (2009) 753–756754 nanowires grow parallel to the {111} surfaces, the system’s energy can be reduced significantly. It also shows that the SiC nanowire is covered with a continuous SiO 2 amorphous layer with a thickness of 1 nm, which is probably formed during the TEM sample preparation. (2) Bamboo-like SiC nanowires as is indicated by (2) in Fig. 3a: this figure presents a TEM image of the bamboo-like SiC nanowire with a stem diameter of about 40 nm. Larger diameter knots, like bandages wrapped around the nanowires, grow periodically along the entire length of the bamboo-like SiC nanowire. The knots possess a high density of stacking faults which are perpendicular to the axis of the bamboo-like SiC nanowire. The formation of these stacking faults is generally thought to be attributed to the thermal stress during the nanowire growth [25]. However, it is interesting to find that the stacking faults only exist in the knots instead of the whole bamboo-like SiC nanowire. (3) Fig. 5a and b shows the HRTEM lattice images taken from the area marked with circles (S and K) in Fig. 3a, corresponding to the stem and knot of the bamboo-like SiC nanowire, respectively. Fig. 5a suggests that the stem of the bamboo-like SiC nanowire is amorphous SiC without stacking faults and planar defects. This can be confirmed by the corresponding SAED pattern shown in Fig. 5c. It can be found from Fig. 5b that the knots are single-crystalline b -SiC with a fringe spacing of 0.252nm, which corresponds well to the {111} plane of cubic b -SiC. Stacking faults exist in the whole knot. This can be further evidenced by the SAED pattern of the knot (recorded from the area marked with circled K in Fig. 3a) shown in Fig. 5d. The SAED pattern of the knot shows bright spots and streaks, indicating that defects exist in the knot area. (4) Spindle SiC nanochains: this type of SiC nanowires also has a uniform periodic structure. As is indicated by (3) in Fig. 3b, the spindle nanochain possesses a thinner stem with a diameter of 20 nm and uniform spindles with a diameter of 50 nm. The distance between two neighboring spindles is about 250 nm. Therefore, the period of the spindle nanochain can be regarded as 250 nm. Similar to the beaded SiC nanochains reported in Ref. [14], the mechanical interlocking between spindles can enhance the interfacial adhesion between the nanochains and the matrix. Thus, the unusual morphology of spindle SiC nanochains may endow them with an excellent reinforcing effect. By simply laying the spindle SiC nanochains on a flat metal surface, a high density of semiconductor–metal junctions can be easily fabricated. 4. Conclusions In summary, we have demonstrated a process for the fabrication of SiC nanowires with three distinct structures, by using commercially available phenolic resin and silicon powders as starting materials. The first type of SiC nanowire, regarded as ordinary SiC nanowire, is single-crystal SiC phase with a fringe spacing of 0.252 nm along the [111] growth direction. The second type of SiC nanowire, namely bamboo-like SiC nanowire, possesses amorphous SiC stem decorated periodi- cally by larger diameter single-crystal SiC knots along its whole length. The third type of SiC nanowire, spindle SiC nanochain, consists of uniform spindles which grow evenly on its entire length. These nanowires may find applications in composite materials or electronic devices. Given the simplicity of the procedures and the unique morphology of the synthesized materials, the method described here would attract a great deal of attention. 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