High Temperature Phases of Nanostructured Tungsten Oxide for Gas Sensing Applications
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0915-R07-07
High Temperature Phases of Nanostructured Tungsten Oxide for Gas Sensing Applications Andrea Ponzoni, Elisabetta Comini, Matteo Ferroni, Guido Faglia, and Giorgio Sberveglieri Chem. and Phys. Dept., CNR-INFM SENSOR Lab, Brescia University, Via Valotti 9, BRESCIA, ITALY, 25133, Italy
ABSTRACT WO3 layers have been synthesized by thermal evaporation at high temperature in order to induce the crystallization of stable films. Phase stability has been proved by annealing treatments carried out at different temperatures. Annealing effects on gas sensing performances have been explained in terms of crystallinity degree and grain coarsening phenomena.
INTRODUCTION Metal oxide based gas sensors are a promising solution for gas detection because of their high sensitivity towards a wide rang of gases, reduced production cost, reduced size, low power consumption, suitability for portable instrumentation realization. The working principle relies on the interaction between gaseous molecules and active species, such as O-, O2-, OH- that populate the material surface according to temperature, pressure and atmospheric composition (humidity, oxygen partial pressure) conditions. Such an interaction is usually characterized by an activation energy which is usually provided by heating the device at temperatures ranging from 200-400°C. As a consequence of such an interaction, charge carriers are injected to or extracted from the conduction band of the material. Polycrystalline materials are usually employed in this field because they offer a large surface area suitable for solid-gas interaction and the possibility to tailor grain size down to the space charge region dimension, so that the whole grain is influenced by surface reactions. So far, synthesis works have been dedicated to enhance the material surface and to reduce grain size. Despite their promising features, commercial diffusion of metal oxides based gas sensors is still limited. The drawbacks are lack of long time stability occurring during high temperature operation and lack of selectivity. The latter can be overcome by the use of an array of different metal oxides handled by means of pattern recognition software [1]. The former has been the subject of several studies and one of the major causes of this drawback has been identified with grain coarsening phenomena. So far, the main objective in synthesis of sensitive materials for such an application is the deposition of nanostructured layers able to maintain their nanostructure along time. The material phase is another important parameter to control. For example, in the case of titanium dioxide it has been reported that the anatase and rutile phases exhibit different gas sensing properties. In this work we focus on tungsten trioxide (WO3). It is a widely used material for the development of solid-state gas sensors. It exhibits high sensing performances towards different gases such as NO2 [2-4], O3 [5] and H2S [1]. As far as the WO3 phase is concerned, it has been shown that bulk samples heated up to the melting
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