Adsorption of NO2 on Oxygen Deficient ZnO(21̅1̅0) for Gas Sensing Applications: A DFT Study

The adsorption of NO2 onto oxygen vacancy sites, which naturally exist on the ZnO(21̅1̅0) surface, is widely believed to be one of the most important factors affecting gas sensor responses for this system. In this work we have examined surface reconstruction and relaxation, charge transfer, Bader charges, density of states, vibrational frequencies, and binding energies of the stable structures of NO2 adsorbed on the defect ZnO(21̅1̅0) surface containing oxygen vacancies (VO••). Multiple minimum energy structures were found with binding energies of the order of ∼1 eV, indicating chemisorption on the surface. Significant post-adsorption reconstruction was observed, accompanied by minor surface relaxation. Adsorption in the most stable site gave rise to an impurity state within the band gap of the clean defect surface and was found to induce a magnetic moment on the most stable structure only. For all minimum energy structures, NO2 behaves as a charge acceptor, withdrawing charge from the surface, which was calculated to be approximately six times greater on the ZnO(21̅1̅0)−VO•• surface than on the stoichiometric surface, suggesting that the defect surface may prove to be more sensitive. A comparison between theoretically obtained properties of defective and stoichiometrically balanced surfaces and experimental sensing observations is given.