Selective impedance based gas sensors for hydrocarbons using ZSM-5 zeolite films with chromium(III)oxide interface

Abstract Highly sensitive and selective hydrocarbon gas sensors were obtained by covering gold interdigital capacitor (IDC) electrodes with platinum-doped Na-ZSM-5 zeolite thick films. A chromium(III) oxide layer between gold and zeolite is responsible for the new outstanding sensor effect. Such Au/Cr 2 O 3 /zeolite interfaces might be formed by diffusion of the chromium bonding agent through the gold electrode during annealing of the zeolite layer and subsequent oxidation or depositing of additional Cr 2 O 3 layers on top of the electrodes by thin-film techniques. Impedance spectra consist of high-frequency semicircles and low-frequency Warburg-like contributions. The impedance at a fixed frequency in the low-frequency region depends very selectively on the hydrocarbon concentration in wet oxygen/nitrogen mixtures. Very low or zero cross-sensitivities were found towards CO, H 2 , NO, and CO 2 gases. Different substrate materials and thicknesses of the Cr 2 O 3 layer were compared. A preliminary simple equivalent circuit model is presented.