Adjustment of oxygen vacancy states in ZnO and its application in ppb-level NO2 gas sensor

Oxygen vacancy (VO) is long believed as a key factor influencing the gas sensing properties. However, the concentration of VO is generally focused while the VO state is neglected, which masks the inherent mechanism of gas sensor. Using a post annealing process, the influence of VO states on the response of ZnO nanofilm to NO2 gas is investigated in this study. The systematical analysis of the results obtained by different methods indicates a transformation of VO from the neutral to the doubly ionized state during post annealing treatment. The results also imply that the gas sensing properties is not directly correlated with the VO concentration. And due to the competitive adsorption of ambient O2, the neutral VO is majorly occupied by the adsorbed O2 while the VO in doubly ionized state can promote the adsorption of NO2. Consequently, the transition of VO from the neutral to the doubly ionized state can lead to a dramatic increase of the response to NO2, from 733 to 3.34 × 104 for 100 ppm NO2. Guided by this mechanism, NO2 gas sensing in ppb-level is also achieved: the response reaches 165% to 25 ppb (0.025 ppm) NO2 with a good repeatability.