Sensors and Actuators B 118 (2006) 215–220 Synthesis and characterisation of gas sensor materials obtained from Pt/Zn/Al layered double hydroxides S. Morandi a , F. Prinetto a , M. Di Martino a , G. Ghiotti a,∗ , O. Lorret b , D. Tichit b , C. Malag ` u c , B. Vendemiati c , M.C. Carotta c a Dipartimento di Chimica IFM and INSTM Consortium, Universit` a di Torino, Via P. Giuria 7, 10125 Torino, Italy b Laboratoire de Mat´ eriaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM-UM1, Institut C. Gerhardt FR 1878, 8 rue Ecole Normale, 34296 Montpellier Cedex 5, France c Dipartimento di Fisica, Universit` a di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy Available online 13 June 2006 Abstract We reported about the possibility to employ Zn(Al)O and Pt–Zn(Al)O mixed oxides obtained from layered double hydroxides (LDH) as gas sensor materials. We prepared the samples following two different routes: a classical co-precipitation and a sol–gel method. After structural and textural characterisation, the powders were studied by FT-IR spectroscopy under different controlled atmospheres at different temperatures. Eventually, conductance measurements were performed on thick films of these materials for testing their response to CO, CH 4 and benzene. FT-IR spectra recorded after reducing treatments in the three different gases show an increase in the absorbance for all the samples studied. For the co-precipitated samples the absorption increases in intensity on decreasing the wavenumbers (free-carrier absorption). For the sol–gel samples the absorption shows a maximum (absorption due to trapped electrons). Concerning the conductance measurements, it was interesting to note that Pt markedly increases the electrical responses and that the presence of humidity affects significantly the responses towards CO and CH 4 , but not the responses towards benzene. Moreover, sol–gel samples show high stability of the performances on varying working temperature. © 2006 Elsevier B.V. All rights reserved. Keywords: Layered double hydroxides; FT-IR spectroscopy; Gas sensors 1. Introduction Layered double hydroxides (LDH or anionic clays) can be described by the generic formula [M 1-x 2+ M x 3+ (OH) 2 ] x+ [A x/n ] n- ·mH 2 O and their structure is formed by layers contain- ing divalent cations (M 2+ : Mg, Zn, Ni, Co, ...) and trivalent cations (M 3+ : Al, Ga, Cr, ...) with an octahedral coordination [1,2]. The structure of the layers is similar to that of brucite (Mg(OH) 2 ), in which the partial substitution of divalent cations with trivalent ones induces an excess of positive charge compen- sated by anions (A n- : CO 3 2- , NO 3 - , Cl - , OH - , ...) located in the interlayer spaces along with water molecules. Controlled thermal decomposition allows to obtain, with retention of the original structure, mixed oxides presenting high specific sur- face areas and homogeneous dispersion between the elements. Moreover, they exhibit a “memory effect”, i.e. the ability to ∗ Corresponding author. Tel.: +39 011 670 7539; fax: +39 011 670 7855. E-mail address: giovanna.ghiotti@unito.it (G. Ghiotti). reconstruct the LDH lamellar network upon re-hydration and to re-obtain mixed oxide after calcination, without losing surface area [1]. Recently, we prepared Zn(Al)O and Pt–Zn(Al)O mixed oxides, obtained by calcination of Zn/Al and Pt/Zn/Al LDH materials, that showed, in a preliminary spectroscopic charac- terisation, n-type semiconductor properties. Pure or Al-doped ZnO is an n-type semiconducting oxide that finds applications in chemo-resistive gas sensor devices [3–6]. However, the fabrication of nano-crystalline powders stable at high temperature and in humid atmospheres is still a matter of research. The “memory effect” characteristic of this type of mixed oxides should be considered a reason for their instabil- ity in humid atmospheres. However, for Zn-containing mixed oxides previously calcined at 450–550 ◦ C, as in this work, it has been shown that the reconstruction cannot be achieved, even under hydrothermal conditions [7]. Their electronic properties and their changes passing from an oxidising to a reducing atmosphere were investigated by FT-IR spectroscopy. Moreover, we prepared thick films with Zn(Al)O 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.04.025