In Situ Study of the Polar ZnO(0001)-Zn Surface in Alkaline Electrolytes

We discuss how kinetic effects can be utilized to prepare polar ZnO(0001)-Zn surfaces as very well defined and single-crystalline surfaces by hydroxide stabilization of the polar face via a wet chemical etching process in 3N NaOH. An in situ AFM imaging study of the etching process is presented. In addition, measurement and analyses of grazing incidence X-ray diffraction experiments, reflectivity, and crystal truncation rods (CTRs) of the resulting ZnO(0001) surface structures in both dry and humid atmospheres are discussed. Analysis of the CTRs shows that these surfaces are topographically extremely flat, Zn-terminated, but covered with a defect-containing hydroxide/oxygen adlayer, which is adsorbed at hcp-hollow sites. This result is fully consistent with a stabilization of the polar surface by means of an adlayer of disordered hydroxides, which is adsorbed at hcp positions. Moreover, these studies indicate that the water structure at the solid/liquid interface is ordered within the first few layers, but no evidence for an icelike structure was found. Also, the pH-dependent stability of these hydroxide-stabilized ZnO(0001) surfaces within electrolyte solutions was investigated by means of an ex situ LEED approach. Hydroxides effectively stabilize the (0001) surface within a wide range of pH values between 11 and 4. In acidic solutions below pH 3.8, the formation of deep hexagonal etching pits is observed, whereas a crystalline structure with triangular reconstructions evolves between these etching pits. The origin of the hexagonal etching pits is discussed as a result of faster dissolution kinetics at dislocation sites.