Er-Wei Shi

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.

High-temperature ferromagnetism in (Co, Al)-codoped ZnO powders

Zn0.95−xCo0.05AlxO (x=0,0.01,0.03) powders were prepared from the acetate-derived precursor by the sol-gel route. The structural and magnetic properties of the powders were investigated. X-ray absorption spectroscopy and high-resolution transmission electron microscope analyses indicate that Co2+ substitute for Zn2+ without changing the wurtzite structure. The powder shows paramagnetic behavior at 5K for x=0 sample. For x=0.01 and 0.03, the powders exhibit ferromagnetic behavior at 360K. It was demonstrated experimentally that high-temperature ferromagnetism in Co-doped ZnO powders can be obtained through increasing the carrier concentration which was realized by doping a few percent of Al.

Twinning Morphologies and Mechanisms of ZnO Crystallites under Hydrothermal Conditions

Two kinds of twinning morphologies of ZnO crystallites prepared under hydrothermal conditions were obtained. The twin relations of the crystallites could be influenced by additives. The twinned crystallites in the pure H 2 O or weak basic solutions (1 N KOH) are bipyramidal and take (0001) as the twin and composition plane, whereas the twinning morphology of the crystallites obtained from 4 N KBr or 3 N NaNO 2 solutions is dumbbell-like and takes (0001) as the composition plane. Various twinning mechanisms based on the linkage of the growth units are suggested. The formation of twin morphologies of ZnO crystallites clearly demonstrate that twinning of crystals is either a result of differences between symmetrical and energetic most favourable structure arrangements or due to the consequence of oriented intergrowths.

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-

Large-scale fabrication of silicon carbide hollow spheres

The synthesis of SiC hollow spheres is reported. The shell of the spheres is formed through a solid–gas reaction rather than traditional chemical methods. The sphere size can be modulated from the microscale to the nanoscale, and the shell thickness can be finely tuned by adjusting the experimental parameters. This technique presents a new paradigm in the preparation of hollow spheres. Based on this technique, other carbide hollow spheres could also be synthesized.

Hydrothermal synthesis of BaTiO3 nano/microcrystals

The results of the hydrothermal preparation of BaTiO 3 fine powders are reported. The effects of the reaction temperature, the molar ratio of Ba/Ti in the precursors, the chemical form of the precursors on the phase composition, and the size and morphology of the products have been determined. Perovskite-type BaTiO 3 crystallite forms more easily as the temperature, basicity, and Ba:Ti ratio in the precursor increase. BaTiO 3 microcrystals (150 to 300 nm) were synthesized through the hydrothermal reaction of commercial TiO 2 with a Ba(OH) 2 aqueous solution. The hydrothermal reaction of a newly prepared Ti(OH) 4 gel with the Ba(OH) 2 solution produced highly crystallized, well-dispersed perovskite-type BaTiO 3 crystallites with very fine (< 100 nm) particles. The newly prepared Ti(OH) 4 gel turned out to be a suitable precursor for the hydrothermal preparation of BaTiO 3 fine powders. X-ray diffraction (XRD) of the hydrothermal BaTiO 3 powders reveals a simple cubic perovskite structure. The lattice constant, a, decreased with an increase in the reaction temperature. These abnormal crystallographic features are assumed to result from lattice defects caused by OH incorporated in the perovskite lattice.

Origin of ferromagnetism of (Co,Al)-codoped ZnO from first-principles calculations

Magnetic mechanisms for the (Co,Al)-codoped ZnO, based on the first-principles calculations, are evaluated. Additional electrons induced by Al doping can stabilize the ferromagnetic state in Co-doped ZnO. The Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction may be the dominant mechanism when the distance between Co and Al is far away, while the double-exchange interaction may gain an advantage over the RKKY when the distance is near. The multiple mechanisms can explain the relationship between the magnetic performance and the relative substitutional site of Al. The study yields insight into the origin of the ferromagnetism of ZnO-based materials.

Oxygen enhanced ferromagnetism in Cr-doped ZnO films

Cr-doped ZnO films have been prepared by inductively coupled plasma enhanced physical vapor deposition, and an in-depth study is performed on the chromium doping and oxygen partial pressure dependence of ferromagnetism. The x-ray diffraction and photoluminescence results indicate that the moderate oxygen can relax the lattice strain thus enhancing ferromagnetism which, as confirmed by soft x-ray absorption spectroscopy, is mainly attributed to the Cr valence state transition from Cr3+ to Cr6+. However, excessive oxygen suppresses the oxygen vacancies and the ferromagnetic exchange. Furthermore, the parabola-like dependence of ferromagnetism on oxygen partial pressure is consistent with the bound magnetic polaron scenario.