Sensors and Actuators B 204 (2014) 588–595 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical jo u r nal homep age: www.elsevier.com/locate/snb Gas sensing properties of the nanostructured anodic Zr–W oxide film Rosa M. Vázquez a,∗ , Alexander Mozalev b , Raul Calavia a , Francesc Gispert-Guirado c , Xavier Vilanova a , Hiroki Habazaki d , Eduard Llobet a,∗∗ a MINOS-EMaS, Universitat Rovira i Virgili, Avda. Països Catalans 26, 43007 Tarragona, Spain b Central European Institute of Technology (CEITEC), Brno University of Technology, Technická 10, 616 00 Brno, Czech Republic c Scientific Service, Universitat Rovira i Virgil, Avda. Països Catalans 26, 43007 Tarragona, Spain d Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8628, Japan a r t i c l e i n f o Article history: Received 2 June 2014 Received in revised form 23 July 2014 Accepted 5 August 2014 Available online 13 August 2014 Keywords: Zr–W alloy Anodizing Porous anodic alumina Nanostructure Mixed metal oxide Gas sensor a b s t r a c t The sensing properties of nanostructured Zr–W mixed oxide film have been investigated. The film was prepared via anodizing a sputter-deposited Zr–W alloy layer through nanopores in an anodic alumina layer superimposed on the alloy. The morphology, structure and chemical composition of the film were examined by SEM and XRD. The film consists of an array of self-ordered nanocolumns protruding from a continuous thin oxide layer. The initially amorphous film material crystallizes to monoclinic WO 3 and orthorhombic ZrO 2 due to a high temperature annealing in air. A sensor employing the ZrO 2 –WO 3 oxide film as active layer was fabricated and used for detecting various concentrations (1–1000 ppm) of H 2 , CO, C 2 H 5 OH and NO 2 at temperatures up to 300 ◦ C. In hydrogen detection experiments, the sensor was very fast, with a response time of 19 s, and highly sensitive to hydrogen, with a response value of up to 50, while showing incomparably weaker and slower responses to carbon monoxide, ethanol and nitrogen dioxide. The features of the films revealed to date are of importance for improving the chemical, structural and exploitation stability of nanostructured tungsten-oxide-based films and their selectivity in hydrogen gas detection. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Crystalline nanostructured oxides of refractory metals are attracting growing interest as active layers for chemical sensors. In this niche, tungsten and zirconium metal oxide have shown to be effective in detecting a wide range of gases [1–5]. In the past few years, new preparation techniques for obtain- ing nanostructured gas sensing films have appeared with the aim of increasing significantly surface-to-volume ratio leading to an improvement in their gas sensing features [6–11]. Among these technologies, anodizing of sputter-deposited tungsten layers has the advantages of achieving tailored morphology and being well compatible with Si microtechnology [6–8]. The major drawback associated with nanostructured anodic WO 3 films is their chem- ical instability, which may cause degradation of film morphology ∗ Corresponding author: Tel.: +34 977256572. ∗∗ Corresponding author: Tel.: +34 977558502. E-mail addresses: rosamaria.vazquez@urv.cat, rosamaria.vazquez@estudiants.urv.cat (R.M. Vázquez), alexander.mozalev@ceitec.vutbr.cz (A. Mozalev), raul.calavia@urv.cat (R. Calavia), francesc.guirado@urv.cat (F. Gispert-Guirado), xavier.vilanova@urv.cat (X. Vilanova), habazaki@eng.hokudai.ac.jp (H. Habazaki), eduard.llobet@urv.cat (E. Llobet). and properties already during film growth [7]. As recently reported for the case of porous-alumina-assisted titanium oxide nanostruc- tured anodic films [12], nanoscale mixing of TiO 2 with a dissimilar metal oxide (Al 2 O 3 ) under an electric field restrains and stabilizes the growth of TiO 2 phase, improving substantially the post-formed properties of the mixed oxide, suppressing the temperature stim- ulated grain expansion and oxygen out-diffusion, which are the well-known drawbacks accompanying gas sensing application of thin titanium oxide layers. In the present work, we have synthesized a stable and well-reproducible film of tungsten oxide regularly mixed at the nanoscale with zirconium oxide via porous-alumina-assisted anodizing of a sputter-deposited Zr–W alloy layer, and gained insight into its gas sensing properties. 2. Experimental 2.1. Film preparation A procedure technically similar to that described in our previ- ous work [6] was followed in order to prepare the nanostructured ZrO 2 –WO 3 gas sensing film. Briefly, a four-inch p-type Si wafer with (100) crystal orientation was employed as a substrate. Then a layer of Zr–W alloy (50/50 at%), which will be used for http://dx.doi.org/10.1016/j.snb.2014.08.014 0925-4005/© 2014 Elsevier B.V. All rights reserved.