Application of CuBr Ion Conductor Thin-Films for Ammonia Gas Detection
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Application of CuBr Ion Conductor Thin-Films for Ammonia Gas Detection Pascal LAUQUE, Marc BENDAHAN, Jean Luc SEGUIN and Philippe KNAUTH* L2MP, Université Paul Cézanne-CNRS, Centre St Jérôme, Service 142, 13397 Marseille Cedex 20, France *MADIREL, Université de Provence-CNRS, Centre St Jérôme, 13397 Marseille Cedex 20 ABSTRACT CuBr films are used to design an ammonia gas microsensor operating at ambient temperature, with high sensitivity and much improved selectivity in comparison with usual conductometric microsensors. The specific detection mechanism relies on adsorption of ammonia molecules and interactions with mobile Cu+ ions in the ionic conductor film. The CuBr films can be prepared by r.f. sputtering, but more simply and cost efficiently by chemical reaction of copper metal with bromine ions. Photolithographic techniques are applied to miniaturize the sensor.
INTRODUCTION Copper(I) bromide CuBr is a prominent solid cation conductor. Classic investigations, including early work by Carl Wagner [1], show that besides Cu+ ion conduction, there is a certain amount of electron hole conduction in this solid. Around ambient temperature, CuBr is thus best characterized as a mixed electronic-ionic conductor [2, 3]. The intrinsic disorder in copper halides is of cation Frenkel type, corresponding to cation interstitial-cation vacancy pair formation. The Frenkel reaction can be represented in level diagrams, like those used in solid state physics [4]. Measurement and control systems for pollutant and toxic gases gain increasing importance for a sustainable and ecologically responsible development and for industrial process control and safety. The growing trend to portable devices further triggers the development of miniaturized and cheap microsystems, including gas microsensors. These requests led to conductometric gas sensors using thin films or porous ceramics of n-type semiconductor oxides, such as ZnO [5] and SnO2 [6]. The surface conductivity of the sample is modified by adsorption of gas species and related space charge effects. In oxidizing atmosphere, the oxide surface is covered by negatively charged oxygen adsorbates and the adjacent space charge region is electron-depleted: the SnO2 layer presents therefore a high resistance. Under reducing conditions, the oxygen adsorbates are removed by reaction with the reducing gas and the electrons are re-injected into the space charge regions: as a result, the SnO2 layer resistance decreases. [7] Semiconductor gas sensors present an inherent lack of selectivity, because the gas detection mechanism is rather unspecific and more or less any type of reducing gas is detected, giving very large cross-effects. The most promising approach for the development of more selective microsensors is based on “molecular recognition”. The basic idea is to use specific interactions between a molecule to be detected and a mobile ion in an ionic or mixed conductor. The first resistive gas sensor using a solid ion conductor was published for ammonia gas in the 1990’s. A variation
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