Gas Sensing Characteristics of ZnO Thin Films Exposed to Ethanol
Authors: Lin Chun-Yu , Wang Jeng-Han , Cheng I-Kai , Pan Fu Ming 
Published: February 2020

There are hundreds of nature and man-made volatile organic compounds (VOC) in atmosphere. Many of them have adverse effects on environments and human health. Ethanol (EtOH) is a frequently encountered VOC in our living environments, and an important interfering gas in breath analyses. Effective monitoring of EtOH is, therefore, beneficial to air pollution control and disease diagnosis. ZnO is an n-type semiconductor, and has been widely studied as a chemoresistive sensor for EtOH detection. In this work, we studied the sensing property of ZnO toward EtOH, and characteristic response features were examined for selectivity improvement.
The ZnO sensor was prepared by sputter-depositing a 50 nm-thick ZnO thin film on the Al electrodes, which were thermal evaporated on a thermal oxide substrate and patterned by a stainless steel (SS) hard mask. During the gas sensing test, the ZnO sensor was exposed to a dry air gas mixture of EtOH of various concentrations at different temperatures.
The ZnO sensor responses weakly to the EtOH gas mixture below 150oC. However, the sensor has a quick conductance rise upon the exposure to EtOH at 150oC and above, manifesting the nature of n-type oxides according to the oxygen ionosorption model. Figure 1a shows the electrical response of the sensor exposed to 0.15 ppm EtOH at different temperatures. Note that a conductance drops right after the early conductance rise and the characteristic response feature is more apparent at higher temperatures and lower EtOH concentrations. In our previous work, we found that PdO, when exposed to EtOH between 100-250oC, exhibits a similar response feature but in a reverse manner, i.e. a quick conductance drop followed by a rise (see Fig.1b). Since PdO is a p-type oxide, the opposite response behavior suggests that the n-type ZnO sensor has a sensing mechanism likely analogous to the PdO sensor when exposed to EtOH. On the basis of the previous study on the PdO sensor, we ascribe the formation of the characteristic response feature to the combined effects of the reduction of preadsorbed oxygen anions and the ZnO substrate by EtOH and the subsequent adsorption of hydrogen and oxygen adatoms on reduced ZnO nanodomains. Chemical and material characterizations, such as XPS, TDS and DRIFTS have been carried out to examine surface and gaseous species produced in the EtOH sensing reaction
In summary, we studied the gas sensing behavior of the sputter-deposited ZnO thin film toward ethanol. A characteristic electrical response feature is observed and its shape greatly depends on the sensing temperature and the EtOH concentration. The response feature is a result of various surface processes, including reduction of oxygen anions and the ZnO substrate, which induces the conductance rise, and adsorption of hydrogen and oxygen adatoms on reduced ZnO nanodomains, which reduces the conductance. The distinct response feature is useful for characteristic feature extraction in pattern recognition algorithms to improve the selectivity of the sensor.

Figure 1