Sachiko Maeda

Growth of large tetrapod-like ZnO crystals: I. Experimental considerations on kinetics of growth

Abstract A new method to grow large and uniform tetrapod-like crystals has been developed and the kinetics of the growth has been analyzed based on the reaction curves obtained experimentally. The formation of such crystals, having a leg length of 30 to 200 μm and an edge size of centering nucleus of 1 to 10 μm, is achieved by using ZnO/Zn particles prepared by oxidation of the surface of Zn powder, by making use of a smouldering reaction controlling the reaction rate, and by introducing zeolite as a reaction catalyst. The plot of the amount of O 2 absorbed during the reaction showed that the oxidation process proceeded parabolically and the reaction could be divided into two elementary reactions: the growth reaction of the nucleus and that of the succeeding legs, which can be controlled independently by the amount of zeolite added or the air-flow rate. The condition for reducing the secondary growth crystals like platelets between legs was elucidated by connecting the amounts of Zn reacted with the reaction rates.

Growth of large tetrapod-like ZnO crystals II. Morphological considerations on growth mechanism

Abstract A newly developed method to grow large tetrapod-like ZnO crystals, described in part I, has made possible morphological observations of the products formed at each stage of the growth process. The octahedral nuclei are formed and grown in the oxide scales accompanying the generation of stresses and these crystals are expelled out of the grain boundary when they become unable to resist the internal stress. The emerged octahedral crystals begin to develop their legs on their alternative faces to form large and uniform tetrapod-like crystals in the vapor of zinc and oxygen. The octahedral nucleus as well as the accurate shape of the fourlings can be understopd by supposing that at the initial stages of growth a nucleus was formed with cubic symmetry, corresponding to zinc-blende type ZnO.

Morphology and growth mechanism of new-shaped ZnO crystals

We report new-shaped ZnO crystals grown by using the similar method reported previously for growing four-legs-form tetrapod-like crystals with adding Sn Zn alloy powder to ZnO/Zn powder. The crystal has four long and four short legs which are united at a same junction and make an equal angle of 70.5° to each other. From the determination of the crystallographic polarity of the faces of octahedral nuclei, we consider that the additional short legs grow on the four oxygen faces of an octahedral nucleus by VLS mechanism where liquid Sn ZnO alloy acts as an agent for the formation of the stable liquid-solid interface.