Structures and Mechanisms of Water Adsorption on ZnO(0001) and GaN(0001) Surface

The adsorption structures and mechanisms of water adsorption on ZnO(0001) and GaN(0001) surface are investigated by using the first-principles methods. It is found that the stable adsorption structure at full monolayer (ML) coverage is (2 x 1) reconstructed. A (2 x 1) molecular adsorption is definite for ZnO, and a (2 x 1) dissociative adsorption is also possible for GaN. For these structures the hydrogen bonds between adsorbates are significant besides the covalent interaction with substrate. For the coverage below 0.5 ML for GaN and 0.25 ML for ZnO, the individually adsorbed H2O can easily decompose to OH and H. Both covalent and electrostatic attractions contribute to the stability of dissociative adsorption. For the coverage between the above two cases, molecular adsorption is found to be stable in theory, but the real structure may be greatly dependent on the chemical condition. These results give a detailed description of the interaction between the first water adlayer and ZnO(GaN)(0001) surface.