Formation of Highly Porous Gas-sensing Films by In-situ Thermophoretic Deposition of Nanoparticles from Aerosol Phase
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0915-R07-03
Formation of Highly Porous Gas-sensing Films by In-situ Thermophoretic Deposition of Nanoparticles from Aerosol Phase Thorsten Sahm1, Weizhi Rong2, Nicolae Barsan1, Lutz Mädler2, Sheldon K. Friedlander2, and Udo Weimar1 1 Institute of Physical Chemistry, University of Tuebingen, Tuebingen, Germany 2 Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California, CA 90095-1592 ABSTRACT Gas sensors based on tin dioxide nanoparticles show high sensitivity to reducing and oxidizing gases. Dry aerosol synthesis applying the flame spray pyrolysis was used for manufacture and directly (in-situ) deposit nanoparticles on sensor substrates. For the first time this technique has been used to synthesize a combination of two stacked porous layers for gas sensor fabrication. Compared to state-of-the-art techniques, aerosol technology provides a direct and versatile method to produce homogeneous nanoparticle films. Two different sensing layers were deposited directly on interdigital ceramic substrates. These porous bottom layers consisted either of pure tin dioxide or palladium doped tin dioxide. The top layer was a palladium doped alumina nanoparticle film which served as a chemical filter. The fabricated gas sensors were tested with methane, CO and ethanol. In case of CH4 detection, the pure tin dioxide sensor with the Pd/Al2O3 filter layer showed higher sensor signals and significantly improved analyte selectivity with respect to water vapor compared to single tin dioxide films. At temperatures up to 250°C the Pd-doping of the tin dioxide strongly increased the sensitivity to all gases. At higher temperatures the sensor signal significantly decreased for the Pd/SnO2 sensor with a Pd/Al2O3 filter on top indicating high catalytic activity. INTRODUCTION Soon after discovering that gases may alter the properties of metal oxides [1-3], industry has realized the potential of the latter as chemical gas sensors and initiated the development of a new kind of user-friendly, robust and cheap gas sensing devices [4]. Today, metal oxide based gas sensors are widely used for detection of toxic gases as well as for comfort applications such as air quality control in buildings, vehicles and airplanes [5-7]. Well-known gas sensitive materials are SnO2, ZnO, TiO2, WO3 and In2O3. Among these SnO2 is most commonly used and often doped with noble metals such as Pt and Pd to enhance sensing performance. Various film preparations have been tested and applied during the last decades in order to develop sensors with optimized features, i.e. high selectivity, low cross sensitivity, fast response [8]. Only few of these techniques were able to successfully overcome the barriers into industrial implementation. Most of the commercially available sensors are fabricated by screen printing of pre-processed powders to obtain a porous thick film with a large accessible surface for gas interactions [9]. However, screen printed sensors have several drawbacks with regard to both manufacturing and sensor performan
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