D2o Interaction With Planar Zno(0001) Bilayer Supported On Au(111): Structures, Energetics And Influence Of Hydroxyls

We investigate the interaction between D2O and the planar ZnO(0001) bilayer grown on Au(111) with temperature programmed desorption (TPD), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. We show that D2O molecules adsorbed on this planar surface form two ordered overlayers, a (3 X 3) and a (root 3 X root 3)R30 degrees, not seen before on any of the bulk ZnO single crystal surfaces. The apparent activation energies of desorption (E-d) estimated from TPD peaks are 15.2 and 16.7-17.3 kcal/mol for (3 X 3) and (root 3 X root 3)R30 degrees, respectively, which agree well with the adsorption energy values calculated from DFT (14.9-15.6 kcal/mol and 16.8-16.9 kcal/mol, respectively). The DFT calculations reveal that the formation of the overlayers takes place at different packing densities and is mediated by extensive hydrogen bonding among the molecules. The hydroxyl groups, which accumulate very slowly on the ZnO(0001) bilayer surface under the standard ultrahigh vacuum (UHV) environment, strongly suppress the formation of the (root 3 X root 3)R30 degrees overlayer but have less impact on the (3 X 3) overlayer. These findings are explained based on the difference in packing densities of the overlayers such that only the (3 X 3) overlayer with a more open structure can accommodate small amounts of the adsorbed hydroxyl groups.