Dresden 2009 – wissenschaftliches Programm

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O: Fachverband Oberflächenphysik

O 17: Oxides and insulators III

O 17.2: Vortrag

Dienstag, 24. März 2009, 10:45–11:00, SCH A01

Adsorption of benzoic acids on NaCl surface — •Wei Chen1, Christoph Tegenkamp1, Herbert Pfnür1, and Thomas Bredow21Institut fuer Festkörperphysik, Leibniz Universität Hannover, Hannover, Germany — 2Institut für Physikalische und Theoretische Chemie, Universität Bonn, Bonn, Germany

The adsorptions of benzoic acid (BA) and its OH-substituted derivatives, salicylic acid (SA) and para-salicylic acid (p-SA) on NaCl surfaces have been investigated by periodic DFT. On NaCl(100) these molecules bond to the surface weakly with binding energies of around 0.4 eV (in the absence of van der Waals interaction), through the electrostatic interaction between oxygen and surface sodium. The band gap is reduced to the HOMO-LUMO separation of the adsorbate (4.86 eV for SA). In existence of surface anion vacancy, the unpaired electron localized in the vacancy transfers to the molecules after adsorption. As a result the LUMO of the adsorbate is partially occupied. Larger increase of adsorption energies have been found due to stronger bonding between oxygen and sodium, as well as the interaction between the transferred unpaired electron over the benzene ring and surface sodium. On (001)-like step site, it shows a stronger binding than on NaCl (100) surface, with adsorption energies of around 0.8 eV, whereas the electronic structure remains similar. On (011)-like step site the adsorption energy further increases to about 1.0 eV. Interestingly, due to the 1.5 eV upward shift of 2p state from step edge Cl atoms, the band gap is effectively reduced (2.97 eV for SA). Implication of these results to the contact charging effect will be discussed.

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