Self-Catalytic Growth of Tin Oxide Nanowires by Chemical Vapor Deposition Process

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 712361, 7 pages http://dx.doi.org/10.1155/2013/712361 Research Article Self-Catalytic Growth of Tin Oxide Nanowires by Chemical Vapor Deposition Process Bongani S. Thabethe,1,2 Gerald F. Malgas,1,2 David E. Motaung,1 Thomas Malwela,1 and Christopher J. Arendse2 1 DST/CSIR Nanotechnology Innovation Centre, National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa 2 Department of Physics, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa Correspondence should be addressed to Gerald F. Malgas; and David E. Motaung; Received 20 February 2013; Accepted 10 June 2013 Academic Editor: Rakesh Joshi Copyright © 2013 Bongani S. Thabethe et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We report on the synthesis of tin oxide (SnO2 ) nanowires by a chemical vapor deposition (CVD) process. Commercially bought SnO nanopowders were vaporized at 1050∘ C for 30 minutes with argon gas continuously passing through the system. The assynthesized products were characterized using UV-visible absorption spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The band gap of the nanowires determined from UV-visible absorption was around 3.7 eV. The SEM micrographs revealed “wool-like” structure which contains nanoribbons and nanowires with liquid droplets at the tips. Nanowires typically have diameter in the range of 50–200 nm and length 10–100 𝜇m. These nanowires followed the vapor-liquid-solid (VLS) growth mechanism. 1. Introduction One-dimensional (1D) metal oxides semiconductor nanomaterials have attracted great interest as CO gas sensors due to their novel electronic and optical properties in nanodevices and because of their active sites to adsorb gas molecules and catalytic reactions [1–7]. These sensors play an essential role in the fields of industrial processing, environmental protection, and medical treatment. However, many problems need to be solved to further improve the selectivity, sensitivity, and stability of the sensors. Among them SnO2 , an n-type semiconductor with a wide direct band gap energy of 3.6 eV, is a particular material of interest because of its unique electrical properties, optical properties, small size, high sensitivity, good chemical stability, high electron mobility, fast response, and recovery speed [4, 8–10]. Tin oxide thin films find the best use in chemical sensors which are commercially available for detecting fuel gas, carbon monoxide, combustible gases, ammonia, water vapour, and numerous other gases and vapors [11]. Due to the enhanced surface-to-volume ratio of 1D structures, tin oxide nanowires have been shown to have excellent sensing performance which is comparable to or even better than the thin film sensors [12–14]. These structures with a high aspect ratio (i.e., size confinement in two coordinates) offer better crystallinity, higher integration density, and lower power consumption [15]. It has been reported by several authors that the performances of gas sensors can be enhanced by increasing the specific surface area by way of achieving nanoparticles [16, 17] and by incorporation of noble metals [18], such as platinum or gold. SnO2 nanowires have been synthesized by different methods, such as laser ablation [19], solution [20], template-based method [21], chemical vapor deposition [22], and thermal evaporation [23]. In the last year, a great effort has been put into understanding and controlling the growth process for the preparation of high quality quasi one-dimensional nanostructures. In this study, we report on the self-catalytic growth of SnO2 nanowires using a chemical vapor deposition process. The optical and structural 2 Journal of Nanomaterials properties of the as-grown nanowires at room temperature are also studied. 2. Experimental Details Tin oxide nanowires were grown on a silicon substrate and at the end of the tube by a CVD process using about one gram of monotin oxide (SnO) (purity 90%) at a growth temperature of 1050∘ C. The SnO was placed into a quartz boat and inserted into the centre of a quartz tube (radius = 5 cm, length = 20 cm) at a distance of about 1-2 cm from the Si substrate. The furnace temperature was increased from room temperature (25∘ C) to 1050∘ C for 30 min. The annealing was made in an argon atmosphere at atmospheric pressure. After cooling down, a fluffy layer of deposition could be collected from inside the tube wall ends in the lower end temperature zone or on the Silicon substrate. UV-visible absorption measurements were carried out on the SnO nanopowders and the SnO2 nanowires using a Perkin Elmer spectrophotometer in a range between 200 and 900 nm. The morphology of the product was examined by field-emission scanning electron microscopy (Auriga Zeiss SEM) with a beam energy of 3 keV. The crystal structure of the as-synthesized product was analysed by X-ray diffraction (XRD) analysis using a Phillips X-ray diffractometer. A high-resolution transmission electron microscope (HRTEMJEOL-2000) was used to examine the internal structure of the as-synthesized product. The chemical compositions were determined using an energy dispersive X-ray spectrometer (EDS) attached to the HRTEM instrument. 3. Results and Discussion The optical band gap of commercially bought SnO and SnO2 nanowires was investigated using the UV-visible absorption spectrometer. Figure 1 shows the UV-vis spectra of the commercially bought SnO powder and as-synthesized SnO2 nanowires at 1050∘ C for 30 min in argon gas. Both samples were dissolved in isopropanol for UV-vis analysis. A weak band edge absorption in the spectrum is observed around the 375 nm wavelength. The optical transition of SnO2 crystals is known to be a direct type [24], where the absorption coefficient 𝛼 can be expressed as [25] (𝛼ℎ])2 ∝ (ℎ] − 𝐸𝑔 ) , (1) where ℎ] is the photon energy, 𝐸𝑔 is the apparent optical band gap, and 𝛼 is the absorption coefficient. Plots of (𝛼ℎ])2 versus ℎ] can be derived from the absorption data in Figure 1 as shown in the inset. Therefore, the band gap can be obtained by extrapolation of the previous relation in the inset of Figure 1. The intercept of the tangent to the plot gives a good approximation of the band gap energy of direct band gap materials. The calculated energy band gap values are ∼3.7 eV for SnO2 nanowires and ∼3.64 eV for commercially bought SnO powder, respectively. These values are slightly larger than that of bulk SnO2 (3.62 eV) and might be due to the quantum size effect [26–28]. Table 1: Quantitative analysis of the SnO2 nanowires. Element O Sn Cu Zn Atomic% 64.5330.42 ± 0.27 2.7 (...truncated)