THE INFLUENCE OF MORPHOLOGY ON THE RESPONSE OF IRON-OXIDE GAS SENSORS

Microgranular layers of iron oxide have been prepared by the deposition of dried aerosol droplets of iron oxalate and subsequent decomposition to investigate the gas-sensing properties of this special morphology. For comparison, compact iron-oxide films have been prepared by sputtering of iron and successive oxidation. Several different granular gas detectors have been produced consisting of spherical particles with sizes between 0.2 and 1.2 mu m in narrow size distributions. Photoelectron spectroscopy and secondary mass spectrometry are employed for depth-resolved chemical analysis, showing Fe2O3 to be the major compound of both the compact and the granular layers. Methane, propane, benzene or toluene contained in dry air are used to test the response of the gas-sensitive films. No significant difference in the average sensitivity is found for the two sensor types. However, the selectivity and the response time of the gas detectors show a considerable dependence on the morphology of the oxide layers. The compact films exhibit a pronounced sensitivity increase with molecular weight of the vapours. In contrast, the granular layers detect all gases with similar sensitivities and react faster than the compact layers.