Synthesis of Tin Oxide Nanoparticles Using a Mini-arc Plasma Source
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Synthesis of Tin Oxide Nanoparticles Using a Mini-arc Plasma Source Ganhua Lu1, Junhong Chen1, and Marija Gajdardziska-Josifovska2 Department of Mechanical Engineering and Laboratory for Surface Studies 2 Department of Physics and Laboratory for Surface Studies University of Wisconsin-Milwaukee Milwaukee, WI 53211 1
ABSTRACT Miniaturized electronic noses to rapidly detect and differentiate trace amount of chemical agents are extremely attractive. Use of tin oxide nanoparticles as sensing elements has been proved to significantly improve both the response time and the sensitivity of gas sensors or electronic noses. In this paper, we report the synthesis of pure tin oxide nanoparticles using a simple, convenient, and low-cost mini-arc plasma source. The nanoparticle size distribution is measured online using a scanning electrical mobility spectrometer (SEMS). The product nanoparticles are analyzed ex-situ by high resolution transmission electron microscopy (HRTEM) for morphology, crystal structure, and defects. Non-agglomerated rutile tin oxide (SnO2) nanoparticles as small as a few nm have been produced, with rounded shapes and some faceting on the lowest energy surfaces. INTRODUCTION Rutile tin oxide (SnO2), a wide band gap (3.6 eV at 300K [1]) n-type semiconductor material, is widely used as sensing elements in gas sensors [2]. The sensing mechanism is based on the fact that the adsorption of oxygen on the semiconductor surface can cause a significant change in the electrical resistance of the material [3]. The formation of oxygen adsorbates (O2- or O-) results in an electron-depletion surface layer due to electron transfer from the oxide surface to oxygen [4]. Recent studies have shown that use of tin oxide nanocrystals as sensing elements significantly improves the response and the sensitivity of sensors since the space charge region develops in the whole crystallite [5, 6]. Tin oxide nanoparticles have been produced by both colloidal and aerosol routes [5]. The colloidal synthesis route affords considerable control over particle size and structure since the surface chemistry can be manipulated through adjustment of the solution properties [7, 8]. However, colloidal synthesis introduces contaminants that may be deleterious for sensor applications. Aerosol routes can minimize contamination, provide more flexibility in process control [9, 10], and improve the compatibility of the nanoparticle-based sensor fabrication process with current microelectronics fabrication facilities. Moreover, the higher processing temperature employed in aerosol synthesis facilitates production of stable phases that are difficult to achieve in colloidal synthesis [11]. This paper introduces a simple, convenient and low-cost mini-arc plasma source to synthesize tin oxide nanoparticles at atmospheric pressure. The new source shows great potential in producing high-quality tin oxide nanoparticles for sensor applications. Because of the small crystallite size, high resolution transmission electron microscopy (HRTEM) becomes a power
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