ZnO nanostructures for gas sensing: Interaction of NO2, NO, O, and N with the ZnO(10(1)over-bar0) Surface

The adsorption of NO2, NO, and atomic N and O on the clean ZnO(100) surface is examined using density functional theory and ab initio molecular dynamics simulations. Adsorption of NO2 is also investigated on a defect surface containing an O vacancy. Calculated binding energy values, Bader charges, vibrational frequencies, density of states, planar averaged charge density differences, and electron localization plots are presented. Both the molecular and atomic adsorbates are found to adsorb in multiple minimum-energy sites with NO2 and NO preferring to weakly adsorb on surface Zn sites, while O and N chemisorb in highly coordinated surface sites forming an O2 and NO surface species, respectively. The presence of atomic O or N on the surface causes significant structural changes to the surface geometry. All adsorbates can induce magnetism on the surface, with the distribution of the magnetic moment being highly dependent on the adsorption geometry. The adsorbates behave as electron acceptors, withdrawing charge from the surface. The calculations also indicate that dissociation of NO2 and NO is exothermally unfavorable. Ab initio molecular dynamics simulations show that NO2 and NO readily desorb from the surface at 500 or 700 K, while O and N are more stable, remaining on the surface after 2 ps at 700 K, poisoning the surface. On the defect surface with an O vacancy, NO2 readily dissociates over the vacancy site to yield O and NO. The dissociated O atom fills the vacancy site to re-form the stoichiometric surface, while the NO desorbs from the surface. At a lower NO2 coverage, adsorption is stronger, with no spontaneous dissociation observed at 0 K. Ab initio MD simulations, however, show that NO2 dissociation is facile at temperatures as low as 248 K, indicating that the barrier to dissociation is low.