Oxygen Vacancy-Controlled Sensing Characteristics and Stability of In₂O₃ Nanorod NO₂ Sensors

This work investigated the effects and influences of oxygen vacancy on the characteristics and stability of NO₂ gas sensors using the sensing membranes of hydrothermally synthesized In₂O₃ nanorods. The amount of oxygen vacancy was changed by dehydrating In(OH)₃ nanorods into In₂O₃ nanorods in a hydrogen ambience at different temperatures and times. More oxygen vacancies could provide more gas adsorption sites to increase the adsorbed NO₂ gas molecules and the chemisorbed oxygen. Therefore, the response of the NO₂ gas sensors was increased. However, more oxygen vacancies also created more electron concentrations. Consequently, the response of the NO₂ gas sensors was degraded because the changed depletion width caused by various NO₂ concentrations was decreased by the created more electrons. The optimal response of 228.4 occurred at an oxygen vacancy content of 37.08%. Both the activation energy required for the reaction with the NO₂ gas and the speed of gas adsorption and desorption were reduced and accelerated, respectively, due to the presence of more gas adsorption sites. Therefore, the optimal operating temperature decreased as the amount of oxygen vacancy increased, and the operating speed with shortened response time and recovery time increased. To study the stability of humidity-dependent characteristics, the NO₂ gas sensors with various amounts of oxygen vacancy were exposed in different relative humidity (RH) atmospheres. As the RH increased, the response degradation became more severe. After measuring 20 cycles, the repeatability and short-term stability of the NO₂ gas sensors were verified due to the response degradation of only 2.86%.