Adsorption of NO and NO2 on the ZnO(2 (1)over-bar (1)over-bar 0) surface: A DFT study

The adsorption of NO and NO2 was studied on the (2 (1) over bar (1) over bar 0) crystal face of zinc oxide (ZnO). Binding energies, workfunction changes, vibrational frequencies, electron localisation functions, charge density differences and Bader charges were calculated. More than one stable structure was found for both molecules on this surface and it was elucidated that adsorbed NO was more stable than NO2. The relatively small binding energy values indicate that the interaction of NO and NO2 with the surface is weak and was found to cause minimal distortion to the surface geometry. The most stable configurations of NO were those with the nitrogen atom of the adsorbate interacting with surface oxygen and zinc atoms. Similarly, stable NO2 configurations were hallmarked by interactions between the adsorbate oxygen atoms and surface zinc atoms. The calculated workfunction changes were negative for all NO systems, and most NO2 Systems. Positive workfunction changes were calculated for geometries where the nitrogen atom of the NO2 adsorbate was positioned closer to the surface than the oxygen atoms which were both pointing away from the surface. The Bader charges and charge density differences indicate that the interaction between the adsorbate molecule and the respective surface atoms is weak. Despite showing a weak interaction, there is still a calculated transfer of charge from the surface to the adsorbate for the most stable NO2 and NO structures. Even though this transfer of charge is opposite to what is predicted by some of the calculated negative work function changes, small features in the planar averaged charge density differences may explain this discrepancy. (C) 2009 Elsevier B.V. All rights reserved.