Formation of multilayer films for gas sensing by in situ thermophoretic deposition of nanoparticles from aerosol phase
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Weizhi Rong Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095-1592
Nicolae Bârsan Institute of Physical Chemistry, University of Tuebingen, Tuebingen 72076, Germany
Lutz Mädler and Sheldon K. Friedlander Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095-1592
Udo Weimar Institute of Physical Chemistry, University of Tuebingen, Tuebingen 72076, Germany (Received 3 November 2006; accepted 5 January 2007)
Dry aerosol synthesis applying the flame spray pyrolysis was used to manufacture and directly (in situ) deposit tin dioxide nanoparticles on sensor substrates. For the first time this technique was used to synthesize a combination of two porous layers for gas-sensor fabrication. Two different sensing layers were deposited on ceramic substrates, i.e., pure tin dioxide and palladium-doped tin dioxide. The top layer was a palladium-doped alumina as a filter. The fabricated sensors were tested with methane, CO, and ethanol. In the case of CH4, the pure tin dioxide sensor with the Pd/Al2O3 filter showed higher sensor signals and improved 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, indicating high catalytic activity.
I. INTRODUCTION
Soon after discovering that gases may alter the properties of metal oxides,1–3 industry realized the potential of the latter as chemical gas sensors and initiated the development of a new kind of user-friendly, robust, and inexpensive gas-sensing device.4 Today, metal-oxidebased 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
a)
Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2006 MRS Spring Meeting Symposium R Proceedings, Vol. 915. DOI: 10.1557/JMR.2007.0106 850
J. Mater. Res., Vol. 22, No. 4, Apr 2007 http://journals.cambridge.org Downloaded: 12 Mar 2015
and Pd to enhance sensing performance. Various film preparations have been tested and applied during the last decades to develop sensors with optimized features, i.e., high selectivity, low cross sensitivity, and fast response.8 Only a 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 preprocessed 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 manufac
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