Gold-Doped Oxide Nanocomposites Prepared by Two Solution Methods and Their Gas-Sensing Response
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Gold-Doped Oxide Nanocomposites Prepared by Two Solution Methods and Their Gas-Sensing Response Chien-Tsung Wang*, Huan-Yu Chen and Yu-Chung Chen Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliou, Yunlin, 640, Taiwan ABSTRACT Gold species on an oxide support possess variable electronic structures via charge transition so as to increase their chemical redox activity. They are also viably promising for use to enhance gas-sensing response when being exploited in a solid state gas sensor. The synthesis method of the gold-loaded materials plays a crucial role in the functionality. In this paper, we report two types of gold/tin oxide based nanopowders prepared by co-precipitation method and by deposition-precipitation method, respectively. They were evaluated as sensing elements in a semiconductor carbon monoxide (CO) gas sensor. Effects of the material type and CO concentration on sensor response were investigated. Their structural characterizations were done by X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy. Results demonstrate the surface gold species effective to facilitate CO oxidation in gas atmosphere and promote low-temperature sensor performance. INTRODUCTION Semi-conducting metal oxides have been widely exploited as sensing elements in solid-state sensors, because they possess active sites to chemisorb molecules and catalyze reactions. For example, tin dioxide (SnO2) is most frequently used in gas sensors to detect hazardous species such as carbon monoxide (CO), ethanol, ..etc. This n-type semiconductor adsorbs oxygen in air and CO molecules on the surface, and the oxidation of both species produces carbon dioxide and electrons. A decrease in the electrical resistance represents the sensor response. However, the SnO2 sensors often operate at higher temperature (e.g., up to 400 °C) in order to achieve a higher sensing response. Unfortunately, this thermal effect frequently causes material instability and high power consumption [1]. Great progress has been made by many research teams with invention of noble metal/oxide nanocomposite materials in powder form. Among the examples, gold nanoparticles or clusters deposited on an oxide support can promote sensor performance, by reducing operating temperature or increasing response magnitude [2,3]. This kinetic effect arises from a high catalytic activity over the gold particles (in form of sphere or semi-sphere) located on the oxide support for CO oxidation [4]. The catalytic properties of the small gold clusters or nanoparticles supported are related to their capability to chemisorb small molecules at defect sites on the gold surface or at the hetero-junction between the gold and the oxide support [5]. The synthesis approach of the gold/oxide composite materials plays a crucial role to influence their structural and chemical properties. For example, a previous study using deposition-precipitation (DP) and traditional co-precipitation (
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