Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101̄0), (0001)Zn and (0001̄)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001̄)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001̄)O face and are compared with the multilayer ice emission at 1̄1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101̄0) face the 1b₁ emission is very broad, indicating some overlap between different states.