Motoi Kitano

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.

Bénard convection ZnO/resin lacquer coating — a new approach to electrostatic dissipative coating

Abstract Coating of ZnO/resin composite films has been investigated by optical microscopy, scanning electron microscopy and electromeasurement. Poly(vinyl chloride)/poly(methyl methacrylate) lacquer mixed with either needle-like (N-) or tetrapod-like (T-) ZnO particles is spread in a 200 μm thick layer on a glass sheet. Benard convection occurs in the coating. N-ZnO particles flow along the convection streams, and are then dispersively fixed within the cells but not at the boundaries. On the other hand, T-ZnO particles aggregate in rings along the boundaries of Benard cells. While the lacquer dries and solidifies, the rings of the T-ZnO particles are broken down into two or three pieces, and form a network structure by meeting and parting among them. The T-ZnO/resin composite films have a surface resistivity of 106 Ω/square or less, and provide a good electrostatic dissipative coating.

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.

Growth of electrocrystallized ZnO particles by reaction of vacuum-deposited Zn films with distilled water

Products by electrochemical reaction of vacuum-deposited Zn films with distilled water at 20°C are investigated by scanning electron microscopy (SEM) and an X-ray diffraction method. The time dependence of the pH of the water indicates three stages of the reaction; the initial stage I at pH = 7 for about 20 h, the middle stage II at pH > 7 (Max = 7.7) for about 100 h and the final stage III at pH = 7. In stage I etching pits occur as a result of the local cell formation. Each pit is enclosed by a ZnO bank on the ring margin of 10–100 |gmm in diameter. Stage II is characterized by the growth of many spindle-like ZnO particles on the speckles which are traces of the pits of stage I. The ZnO banks dissolve and dendritic |Gb-Zn(OH)|2 crystals also appear near the margins. The spindle crystals disappear in stage III. The formation and morphology of the spindle crystals are described.

Morphology and growth mechanism of ZnO particles electro-crystallized on Zn in aqueous solution

Products made by the electrochemical reaction of 500 μm thick Zn sheets and 25 μm thick Zn foils with distilled water at 20°C are investigated by scanning electron microscopy, X-ray diffraction, and pH measurements of the water. The reaction goes through the following processes; (A) the formation of formless ZnO layers; (B) the growth of spindle-like ZnO particles and the precipitation of octahedral β-Zn(OH) 2 particles, with increasing pH (Max = 8.4) that is caused by the dissolution of the ZnO; and (C) the reduction of spindle growth with decreasing pH, caused by the precipitation of Zn(OH) 2 . The processes are repeated, and the reaction ceases when the metal is completely consumed. These cyclic processes appear as peaks in the pH/time-dependence curve.

Observation of ZnO Particles Grown by Electrochemical Reaction of Zn

The electrochemical reaction of Zn is investigated by scanning electron microscopy and transmission electron microscopy. Zn films are used as anodes, loaded at DC voltages of 10-200 V in distilled water at 20 degrees C. Zn ions diffuse from the anode and are coupled with OH ions to form spindle-like ZnO particles of 1-2 microns on the cathode or on a permeable paper barrier placed between the anode and cathode. Formless layers, which are composed of fine ZnO crystallites of several nanometers or less in size, form on the cathode in the water at lower OH ion density before the growth of the spindles. They become spindle-shaped after only a few minutes of water bath treatment at 60 degrees C.