Stabilization of Zinc-Terminated ZnO(0001) by a Modified Surface Stoichiometry

The stabilization of polar oxide surfaces is of fundamental interest for understanding many processes in surface chemistry. We study the zinc-terminated Zn-ZnO(0001), a surface important in heterogeneous catalysis, by highest resolution scanning force microscopy (SFM) operated in the noncontact mode (NC-AFM). While most of the surface morphology is dominated by a phase consisting of triangular shaped nanostructures, we observe a coexisting (1 x 3) reconstructed phase where the reconstruction is ascribed to the formation of missing Zn-rows. Our findings provide evidence that the electrostatic instability of the polar Zn-ZnO(0001) surface can be canceled by a reduction of the surface charge by 1/3 which is considerably larger than the value of 1/4 derived from a simple ionic model for polar stabilization. Within the presented model for the (1 x 3) reconstruction, the role of point defects tentatively ascribed to hydrogen adsorbed on top of double Zn-rows forming zinc-hydride is discussed.