ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe2O3 nanorods and Fe2O3-ZnO core-shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core-shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5-200 ppm) at different temperatures (150-250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe2O3-ZnO core-shell nanorods, as well as their uniformity in terms of shape and size. The Fe2O3-ZnO core-shell nanorod sensor showed a stronger response to ethanol than the pristine Fe2O3 nanorod sensor. The response (i.e., the relative change in electrical resistance Ra/Rg) of the core-shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe2O3-ZnO core-shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core-shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe2O3-ZnO core-shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe2O3-ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe2O3-ZnO interface.