Chiang Mai Journal of Science

Heterocontact gas sensors are promising candidates for high temperature sensor applications with their inherent corrosion resistance, environmental stability, and smart sensor properties. Heterocontact sensors based on p-type CuO and n-type ZnO ceramics have been shown to exhibit sensitivity and selectivity towards reducing gas species. The sensor is smart in that it can be tuned to detect CO and/or H2 gases in air by selecting the measurement frequency and the applied bias using an impedance analyzer. In this work, heterocontact sensors were doped in an attempt to maximize the hydrogen sensitivity and the sensor response. CuO was doped with various monovalent (Li, Na) and isovalent (Ca, Sr, Ni) dopants at different compositions to form both single phase and two phase samples. ZnO was also doped with acceptor (Li) and donor dopants (Ga). The effects of the dopants on the hydrogen sensitivity and sensor response through conductivity and heterogeneous microstructure were studied using dc current-voltage, dc current-time, and impedance spectroscopy measurements. Through analysis of the time dependent current data, it was shown that the sensor response of the heterocontact can be modeled via a two-site Langmuir adsorption model. While one of the sites showed a significant decrease in response time when the p-side was doped with Ni, the response time of the other site changed only slightly. The highest sensitivity was obtained by doping the n-side with Ga. The fastest response times were achieved when both sides of the heterocontact were doped. This suggests that carrier density may play a significant role in the sensor response.