Effect of surface defects on the interaction of the oxygen molecule with the ZnO(100) surface

Abstract

We study the interactions of the O₂ molecule with stoichiometric and reduced ZnO(100) surfaces by means of density functional theory calculations. We find that the O₂ molecule can strongly interact with ZnO(100) surfaces that have either O or Zn–O dimer vacancy defects. A strong ZnO–O₂ interaction leads to the elongation of the O–O bond of the adsorbed O₂ molecule. The trend of the O₂ dissociation energy is the following: stoichiometric surface > Zn vacancy > Zn–O dimer vacancy > O vacancy. The results from the kinetic simulation are consistent with the trend of the O₂ dissociation energy. The concentration of adsorbed atomic oxygen in the system with lower O₂ dissociation energy will start to rise in a lower temperature region than in the system with higher O₂ dissociation energy. The presence of adsorbed atomic oxygen is very important for the sensing mechanism of a ZnO-based resistive gas sensor. Based on our results, we propose a strategy to increase the concentration of adsorbed atomic oxygen on the ZnO(100) surface. Theoretically, our results suggest that the sensing performance of ZnO-based sensors could be improved if one could increase the amount of not only the surface O vacancies but also the surface Zn–O dimer vacancies, while at the same time suppressing the presence of surface Zn vacancies. We find that the required criteria could be achieved by forming the surface under Zn-rich conditions. In such conditions, the formation energy of a Zn vacancy becomes relatively higher than the formation energies of an O vacancy and Zn–O dimer vacancy.