Selective adsorption to particular crystal faces of ZnO.

We examine the hypothesis that selective adsorption to a particular face of ZnO is responsible for the ability of small organic molecules to control the aspect ratio of ZnO crystals during hydrothermal synthesis. Large, single crystals of ZnO were prepared such that the vast majority of a surface consisted of a single crystal plane, as shown by atomic force microscopy, and the adsorption to a single crystal plane was determined by attenuated total reflectance spectroscopy. The results show that citrate strongly and selectively adsorbs to the (0001) face. Similarly, results show that ethylenediamine selectively adsorbs to the (1010) face. Each of these results separately shows a correlation between selective adsorption to and growth of large areas of a particular face, and thus, each result is consistent with the proposed hypothesis.

Differential etching of ZnO native planes under basic conditions.

The in situ dissolution of polished (0001), (101(-)0), and (0001(-)) surfaces of ZnO was studied using Atomic Force Microscopy under alkaline conditions. In aqueous NaOH solution the (0001) plane forms a stepped surface whereas the (0001(-)) plane converts into more stable {101(-)1(-)} planes. Dissolution of the (101(-)0) plane leaves a combination of (0001) and (101(-)1(-)) planes. Dissolution in solutions containing both NaOH and Na(3)citrate causes the (0001) plane steps to increase in number and reduce in height, and cause an overall increase in the rate of dissolution in the [101(-)0] directions. These observations are explained using a mechanism based on edgewise dissolution where the etching rate depends on the number of surface oxygen atoms per zinc atom. Large areas of single index faces (over 50 µm(2)) of (0001) and (0001(-)), suitable for surface chemistry studies, were also generated by chemical dissolution.