Control of ZnO crystal morphology through face specific adsorption

© 2011 Dr. Nathan Johann NicholasMany of the potential applications of ZnO are dependent upon either its overall shape (e.g. piezoelectric devices or LEDs), or the relative ratio of the different ZnO faces (e.g. catalysis). These applications have stimulated renewed interest in controlling the aspect ratio of ZnO crystals during their growth. One promising way to control the aspect ratio of ZnO crystals during hydrothermal growth is via the introduction of small organic molecules. Whilst there have been many studies showing that particular molecules affect ZnO crystal shape, few studies explain exactly how particular molecules influence growth. The overriding goal of the work contained in this thesis is to enhance understanding of how small organic molecules affect ZnO growth surfaces. The first data section of this thesis investigates the mechanism for hydrothermal growth of ZnO in an ammonium hydroxide solution (a common synthesis condition). The products formed at 20−90°C and ambient pressure were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photon Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectroscopy (SIMS). Under these conditions, the growth of ZnO occurs via the initial precipitation of ε-Zn(OH)2 (Wülfingite), which subsequently dehydrates, to form Würtzite ZnO. Isotope tracking experiments show that most of the oxygen atoms do not mix with water during the conversion from Wülfingite to ZnO, and thus that the reaction proceeds via conversion in the solid phase, as well as dissolution and re-precipitation. The dissolution of ZnO, which is the complementary process to growth, was then studied in situ using Atomic Force Microscopy (AFM) under alkaline conditions. The etching of the three native surfaces of ZnO,(0001), (000-1), and (10-10) was investigated. In aqueous NaOH solution the (0001) plane forms a stepped surface, whereas the (000-1) plane converts into more stable {10-1-1} planes. Dissolution of the (10-10) plane leaves a combination of {10-1-1} and (0001) planes with {10-11} risers. Dissolution in solutions containing both NaOH and trisodium 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 directions. The stability of the (0001) and {10-1-1} planes is attributed to their lower average number of surface oxygens per zinc tetrahedra (1 and 1.5 respectively) than the (000-1) and {10-10} planes (both having 3). This is because Zn tetrahedra with a lower number of exposed surface oxygen are more difficult to convert into a soluble Zn species as the Zn-O-Zn bonds of the solid are not as exposed to solution. The formation of the meta-stable surfaces is caused by the dissolution of the edge of the plane. The final section of this work large explored the adsorption of small organic molecules to single crystals of ZnO. Single crystals of ZnO were prepared such that the vast majority of a surface consisted of a single crystal plane, as shown by AFM, yet still retained the native hydroxide layer. The presence of the hydroxide layer was confirmed by XPS. The beam path of an infrared spectrometer was modified such that attenuated total internal reflectance spectroscopy could be performed on the faces of these crystals to determine adsorption to a single crystal plane. The results show that citrate strongly and selectively adsorbs to the face. Studies on the derivatives of citrate indicate that two carboxylate groups separated by three carbon atoms is the key structural component for adsorption to (0001) ZnO. Finally, these results show that ethylenediamine strongly and selectively adsorbs to the (10-10) surface. These findings show that small molecules strongly and specifically adsorb to different ZnO surfaces based on their chemical functionality and molecular shape. Furthermore by selectively adsorbing to a particular ZnO surface, a molecule encourages the preferential growth of that surface