Ethanol sensing mechanism of ZnO nanorods revealed by DRIFT spectroscopy and DFT calculations

Abstract

Semiconductor gas sensors operate based on the changes in their resistivity in response to adsorption and reaction of gases on their surface, and understanding this mechanism is important in the design of sensitive and selective devices. In this study, the ethanol sensing mechanism of ZnO nanorods was investigated. Under hydrothermal conditions, the nanorods were grown vertically on an alumina substrate at random angles to expose the (10−10) plane of ZnO. The nanorods showed approximately twice the response to 12 ppm ethanol at 300 °C compared to commercial nanocrystals, which have the typical wurtzite structure of ZnO. This performance was attributed to the higher ethanol adsorption capacity of the nanorods and the reactivity of the (10−10) surface, which were confirmed from TPD analyses and control experiments with ZnO (10−10) single crystals, respectively. The reaction pathway for the conversion of ethanol was then determined using in situ DRIFT spectroscopy. Increasing the detection temperature resulted in the emergence of the peaks of acetate and the corresponding disappearance of those of ethoxide. Concurrent resistance measurements also showed a decrease in the sensor response with increasing temperature. Moreover, an analysis of the exhaust gas revealed that acetaldehyde was a major product at high temperatures. Therefore, the dehydrogenation of ethanol to acetaldehyde and its oxidation to acetate are proposed to be key reactions in detection of ethanol using the ZnO nanorods. DFT calculations also revealed that the adsorption of acetaldehyde and acetate causes a decrease in the resistivity of ZnO, which is the basis for its gas sensing performance.