It is well known that ZnO is a typical n-type semiconductor and the sensing mechanism is dominated by the surface process. In an air atmosphere, oxygen molecules are firstly absorbed on the surface of ZnO and capture free electrons to produce ionic species such as O, O2 or O2. When ZnO materials are exposed to acetone gas at higher temperature, the acetone gas molecules react with these ionic species, which leads to more free electrons liberated and an increase in the resistance of
the ZnO semiconductor. The reaction kinematics is as follows