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

Oxygen vacancy (V-O) is long believed as a key factor influencing the gas sensing properties. However, the concentration of V-O is generally focused while the V-O state is neglected, which masks the inherent mech-anism of gas sensor. Using a post annealing process, the influence of V-O states on the response of ZnO nanofilm to NO2 gas is investigated in this study. The systematical analysis of the results obtained by dif-ferent methods indicates a transformation of V-O 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 V-O concentration. And due to the competitive adsorption of ambient O-2, the neutral V-O is majorly occupied by the adsorbed O-2 while the V-O in doubly ionized state can promote the adsorption of NO2. Consequently, the transition of V-O from the neutral to the doubly ionized state can lead to a dramatic increase of the response to NO2, from 733 to 3.34 x 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. (C) 2020 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.