Absorbed oxygen plays a key role in gas sensing process of ZnO nanomaterials. In this work, the transformation of absorbed oxygen on ZnO (101̅0) and its effects on gas sensing properties to ethanol are studied by a novel thermal pulse method and density functional theory (DFT) simulation. Thermal pulse results reveal that the absorbed O2 molecule dissociates into two individual oxygen adatoms by extracting electrons from ZnO surface layers when temperature is above 443 K. The temperature at which absorbed O2 molecule begins to dissociate is the lowest working temperature for gas sensing. DFT simulation demonstrates the dissociation process of O2 at ZnO (101̅0) surface, and the activation energy (Ea) of dissociation is calculated to be 351.71 kJ/mol, which suggests that the absorbed O2 molecule is not likely to dissociate at room temperature. The reactions between ethanol and absorbed O2 molecule, as well as reactions between ethanol and O adatom, are also simulated. The results indicate that ethanol cannot react with absorbed O2 molecule, while it can be oxidized by O adatom to acetaldehyde and then to acetic acid spontaneously. Mulliken charge analysis suggests electrons extracted by O adatom return to ZnO after the oxidation of ethanol.
Keywords: DFT; ZnO; absorbed oxygen; gas sensing; thermal pulse method.