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

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/Cr2O3/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 Cr2O3 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, H2, NO, and CO2 gases. Different substrate materials and thicknesses of the Cr2O3 layer were compared. A preliminary simple equivalent circuit model is presented.