Wen-Jun Li

Growth mechanism and growth habit of oxide crystals

In this paper, the growth mechanism and growth habit of oxide crystals are investigated. Firstly, from the kinetics viewpoint, the growth mechanism of ZnO powders under hydrothermal condition is disclosed starting from the hypothesis of growth unit. It is concluded that the growth mechanism of oxide crystals contains the formation of growth units and the incorporation of growth units into the crystal lattice by a dehydration reaction. Then, a new growth interface model of oxide crystals in solution is established on the basis of an ideal growth mechanism of oxide crystals, which considers the interface structure of the crystal as the stacking order of coordination polyhedrons with OH− ligands. Finally, a new rule concerning the growth habit is deduced considering the relation between the growth rate and the orientation of the coordination polyhedron at the interface. It is concluded that the direction of the crystal face with the corner of the coordination polyhedron occurring at the interface has the fastest growth rate; the direction of the crystal face with the edge of the coordination polyhedron occurring at the interface has the second fastest growth rate; the direction of the crystal face with the face of the coordination polyhedron occurring at the interface has the slowest growth rate. In terms of this rule, the growth habit of ZnO crystal particles and AlO(OH) crystal particles, and the effect of reaction medium on the growth habit are successfully explained.

Hydrothermal synthesis and optical property of nano-sized CoAl2O4 pigment

Abstract Nano-sized CoAl 2 O 4 was synthesized by hydrothermal method. The powder was characterized by XRD, DTA–TG, TEM, BET, IR, XPS, and UV–Vis techniques. The particle size was ca. 70 nm, and the particle size distribution was narrow. The BET surface area was 29.22 m 2 g −1 . It was thermally stable. The maximum absorption was ∼600 nm.

Synthesis of mono-dispersed ZnAl2O4 powders under hydrothermal conditions

Abstract Mono-dispersed ZnAl 2 O 4 was successfully synthesized by a hydrothermal method at 260 °C for 20 h. The powder was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential thermal analysis–thermal gravimetric (DTA–TG), and BET. The mean particle size was 482 nm with narrow distribution.

Hydrothermal preparation of nanometer ZnO powders

Zinc oxide powder is an important material for applications such as sensors, varistors, pigments, electrography, medical materials, etc. [1]. Various methods adopted for the preparation of zinc oxide crystallites include the sol-gel method [2], evaporative decomposition of solution (EDS) [3], wet chemical synthesis [4] and gas-phase reaction [5], etc. The hydrothermal synthesis is an ideal method which is widely used to synthesize the oxide powders with high quality. However, it is difficult to synthesize nanometer ZnO powders by the traditional hydrothermal route. In 1999, our groups first reported the hydrothermal discharging-gas method [6] which was used to synthesize the acicular ZnO particles. The letter describes our successful attempt in the preparation of nanometer ZnO powders using Zn(CH3COO)2 solution as precursor by the hydrothermal discharging-gas method. The reaction vessel adopted in the experiments is a silver-lined tube-type stainless steel autoclaves with 30 mm inner diameter and about 215 ml capacity. In the upper part of the autoclave are located the discharge valve for discharging gas and a gas manometer. The Zn(CH3COO)2 solution with the concentration ranges of 0.5–1.5 mol/l is used as precursor. The percentage of fill is 70% in the experiment. The reaction temperature ranges from 160 ◦C–250 ◦C, and no temperature gradient is applied. After the temperature of autoclave is elevated to a desired value and remains stable for 1 h in a furnace with a constant temperature-controlling device, the discharge valve is immediately opened to complete reaction. Meanwhile, the power is cut off to make the autoclave cool to room temperature. After the preparation process is completed, the obtained solid products are washed out with water into a glass beaker, then each is filtered. The as-dried products are studied by X-ray diffractometer, and electron microscope. The results are shown in Table I. From Table I, it can be seen that the crystallite size of ZnO powders, which are prepared using 1 mol/l Zn(CH3COO)2 solution as precursor at different temperatures is greatly different; when reaction tempera-

Formation mechanism of barium titanate nanocrystals under hydrothermal conditions

Growth units and the crystallization habit of BaTiO3 nanocrystals have been investigated. It has been proposed that the growth units of BaTiO3 were surface hydroxylated Ti(OH)62− octahedra. The relationship between crystal morphology and the variation of the current intensity in the solution has been obtained through the measurement of the OH− in the solution and the hydrothermal experiment with superimposed direct electric field. Based on the computation of the stability energy of the growth units, the relationship between the crystallized morphology of crystallites and the most favorable growth units under different conditions has been derived. It has been found that there is structure similarity between anatase (TiO2) and BaTiO3 from the crystal chemistry viewpoint, so they are soluble in each other, which can serve as a reasonable explanation for the abnormal phenomenon of the retention of cubic substable phase at room temperature.

Synthesis of ZnO fibers and nanometer powders by hydrothermal method

A new hydrothermal process—hydrothermal salt solution pressure-relief—was introduced, and ZnO powder with the size of 15 nm and ZnO fiber with length/diameter=16∶1 were prepared by this process. The change of particle size of ZnO powder and the formation mechanism of ZnO fiber were investigated. It is proposed that the main factor affecting the particle size of powders is nucleation rate, that is, the bigger the nucleation rate is, the smaller the particle is: the main factors affecting the formation of fiber include whether hydrothermal reaction proceeds under the condition of pressure-relief or not and the crystallization degree of powders before pressure-relief.

Hydrothermal preparation and formation mechanism of titanium dioxide microcrystallites

Hydrothermal preparation experiments of TiO2 microcrystallites were carried out. The formation of TiO2 microcrystallites of polymorphic forms was related to the precursor types and the basicity or acidity of reaction mediums under hydrothermal conditions. Based on the “growth unit” model, the space lattice graphs of growth units responding to different polymorphs were established. The stable energy of growth unit was calculated for different basicity or acidity of reaction mediums. The formation of polymorphic TiO2 microcrystallites under hydrothermal conditions was discussed based on the experimental results.

Growth habit of polar crystals

Using coordination polyhedron rule, growth habit of polar crystals such as ZnO, ZnS and SiO2 is investigated. It shows that the growth rates in the positive and negative polar axis directions are different. The theoretical growth habit of ZnO crystal is hexagonal prism and the growth rates of its various faces are: V{0001} > V{0111} >V{0110} > V{0111} > V{0001}. The growth habit of ZnS crystal is tetrahedron and its growth rates of different crystal faces are: V{111}>V{001}>V{001}=V{100} = V{010}>V{111}. The growth rate relationship between positive and negative polar axis directions of SiO2 crystal is V[1120] > V[1120]. These results are in agreement with the growth habits observed under hydrothermal conditions. The different growth rates between positive and negative polar axis directions cannot be explained by PBC theory.

Characterization of GaN grown by RF plasma MBE

Abstract GaN (Gallium nitride) has been grown successfully on sapphire by RF plasma MBE through optimizing the growth condition. Two dimensional triple axis mapping (TDTAM) was used to characterize the GaN epilayers. The dislocation density in the epilayers was estimated to be about 7.8×108 cm−2 according to the mosaic model. The 77K photoluminescence (PL) of the Si-doped GaN with different concentration show that Si can act both donor and acceptor at very high concentration. Transmission spectrum has also been measured for further understanding of the sample.

Solvothermal preparations of α-alumina and boehmite crystallites and investigations on the formation of their morphologies

The solvothermal preparations of α-alumina and boehmite crystallites have been investigated, and the effects of solvothermal conditions such as reactive medium and temperature on the phase, crystallization morphology and size of the crystallites have been discussed. The calculation of the stability energies of the growth units of the crystallites has been performed based on the model of anion coordinate polyhedrons growth unit. The results could explain the formation mechanisms of the crystallites.