Sensors and Actuators B 95 (2003) 157–161 CO sensing properties of W–Mo and tin oxide RGTO multiple layers structures E. Comini a,∗ , M. Ferroni b , V. Guidi b,c , G. Martinelli b , G. Sberveglieri a a Dipartimento di Chimica e fisica, INFM and University of Brescia, via Valotti 9, 25133 Brescia, Italy b INFM and University of Ferrara, via Paradiso 12, Ferrara, Italy c INFN Sezione di Ferrara, Ferrara, Italy Abstract The possibility to increase the surface-to-volume ratio of tin di-oxide thin films deposited through the RGTO technique is investigated. A novel strategy was used to increase the surface roughness and to exploit the catalytic and sensing properties of a W–Mo layer. Deposition consists in four steps: deposition by sputtering of W–Mo layer, first annealing process, deposition of metallic Sn, second annealing process. Electrical characterization shows a considerable increase in the conductivity of the combined layers and also an increase in the interactivity of the double layer compared to either pure SnO 2 or W–Mo oxide layers. This can be ascribed both to the increase in the surface to volume ratio and to the catalytic effect of W and Mo oxide. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Tin oxide; W–Mo oxide; Gas sensing 1. Introduction Multi-metal oxide materials for gas-sensing applications have recently stimulated the interest of the researchers working in the field. The aim is to fabricate stable nano- sized grain morphologies that possess all the attributes for high-performance gas sensing thin films, in particular an increased surface-to-volume ratio with respect to the case of a single-metal oxide layer. Thereby, various multi- component metal oxides have been explored, in particular incorporating SnO 2 , MoO 3 , WO 3 , ZnO, TiO 2 , Fe 2 O 3 and In 2 O 3 compounds [1–3]. In the past, we investigated the gas-sensing properties of different mixed oxide compounds such as TiO 2 –WO 3 , TiO–WO 3 , MoO 3 –TiO 2 , WO 3 –MoO 3 thin films [4–6]. WO 3 and MoO 3 have been recognized as sensors for NO x and CO with good response, but some problems for long-term stability appeared because of exaggerated grain growth. It has also been demonstrated that addition of a foreign element or phase, constituting a binary oxide compound, hindered grain coalescence and stabilized the nanostructure [7,8]. In this work, we investigated the dispersion of tin diox- ide particles over a porous Mo–W–O layer and studied the ∗ Corresponding author. Tel.: +39-030-3715706; fax: +39-030-2091271. E-mail address: comini@tflab.ing.unibs.it (E. Comini). advantages offered by such a technique in terms of gas re- sponse toward CO. 2. Experimental Deposition of a layer of W–Mo mixed oxide proved pos- sible through rf sputtering from a target of W/Mo 80%/20% in weight in a reactive atmosphere. After deposition, the layers underwent an annealing treatment in a furnace at temperature ranging within 400 and 800 ◦ C under flux of synthetic humidified air. As the annealing temperature reached 600 ◦ C, partial sublimation of the film was ob- served resulting in a very porous layer, with topologically non-connected agglomerates. Such characteristics debar the layer for direct gas sensing application due to an elevate resistivity level of such film, but appears to be a good candidate as a template to host a dispersion of another sensing material. With this aim, we deposited a wealth of tin droplets over the surface of the W–Mo oxide layer (at 400 ◦ C), then imparted an annealing procedure at 600 ◦ C in humidified synthetic air, according to the well-established RGTO technique [1]. Such a two-step process was de- signed to increase the surface-to-volume ratio with respect to a traditionally-achieved RGTO-based SnO 2 thin films. Control over the deposition of tin droplets would lead to trimming of the thickness of the deposited tin dioxide layer. 0925-4005/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00401-5