Mo-W-O thin films for CO sensing
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Mo-W-O thin films for CO sensing Elisabetta Comini, Matteo Ferroni1, Vincenzo Guidi1,3, Giuliano Martinelli1, Michele Sacerdoti2, Gianantonio Della Mea4, Alberto Vomiero4 , Girolamo Di Francia5 and Giorgio Sberveglieri INFM and Department of Chemistry and Physics for Engineering and Materials, Brescia University, via Valotti 9, 25133 Brescia, Italy 1 Università di Ferrara, Dipartimento di Fisica and INFM, Via Paradiso 12, I-44100 Ferrara, ITALY. 2 Università di Ferrara, Istituto di Mineralogia, Corso Ercole I d’Este, I-44100 Ferrara, ITALY. 3 Istituto Nazionale di Fisica Nucleare - Sezione di Ferrara, Via Paradiso 12, I-44100 Ferrara, ITALY. 4 Laboratori Nazionali Legnaro, INFN, Via Romea 4- 35020, Legnaro (Padova), Italy 5 ENEA PORTICI, Via Vecchio Macello, Loc. Granatello, 80055 Portici
ABSTRACT
The Mo-W-O thin films were deposited by RF reactive sputtering from composite target of W and Mo (20:80 weight ratio). Structural characterisation was carried out by X-ray diffraction spectroscopy and the composition of the film was obtained by Rutherford backscattering analysis. The layers were investigated by volt-amperometric technique for electrical and gassensing properties. The films were capable of sensing CO. No effect of poisoning of the surface was recorded and recovery of the resistance was complete. A concentration of CO as low as 15 ppm produced a relative variation in the conductance of 390% with response and recovery times of about 2 minutes at a working temperature of 200°C.
INTRODUCTION
Research on gas sensors using semiconductor-based devices seeks for new materials: the response of these materials can be remarkably different from that of the most studied oxides and this feature can be exploited in electronic noses facilitating the deconvolution of the signal. Nanostructured highly-compact materials are optimal candidate for gas sensing due to the high ratio surface-volume, to the high surface exposed for gas adsorption and to the low grain dimension. SnO2 is the most investigated material among metal-oxide-based gas sensors for detection of a wide spectrum of gases [1]. Besides, some other materials are currently being studied for gas sensing applications in the temperature range of 300-500°C, e.g. In2O3 [2], MoO3 [3], TiO2-WO3 [4], WO3 [5], and porous silicon [6]. WO3 and MoO3 based sensors for NOx and CO have good response owing to exaggerated grain growth. Mo and W were chosen because its oxides exhibit
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promising gas-sensing capability. Addition of another element may result in a finer grain of the layer, we have chosen a combination of two metal oxides. In this paper, we present some recent results on a promising material like Mo-W-O with a discussion on the correlation between structural and electrical properties. Radio-frequency reactive sputtering assisted by annealing was found to be a methodology for the preparation of nanosized Ti:WO3 thin films [7], which are fast and reliable NO2 sensors, and MoO3 which is sensitive to CO and NO2[3]. It has already been proven that the nanostru
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