Growth mechanism and growth habit of oxide crystals
Introduction
The hydrothermal method [1] is one of the wet chemical methods for the preparation of metal oxide powders of high quality. The powders prepared by the hydrothermal method have good crystallinity, good dispersity, and do not show macroscopic agglomeration. Because the supersaturation of the solution during hydrothermal reaction is low, the particles prepared by the hydrothermal method tend to have a regular polyhedral crystal face. So the hydrothermal method is also an ideal method that is used to investigate the growth habit of crystals.
The growth habit of crystals is mainly determined by the internal structure of a given crystal, and affected by external conditions such as temperature, supersaturation, pH value of the solution etc. So the study of the crystal growth habit can reveal the growth mechanism of the crystal and vice versa. The theoretical models concerning crystal growth habit mainly include the BFDH law [2] and the PBC theory [3]. The BFDH law starting from planar mesh density, and considering the effects of screw axis and glide plane on crystal growth habit, provides a predicted theoretical growth habit of the crystal. The PBC theory provides an ideal growth habit of crystals in terms of bond chain types between molecules and attachment energy. But the above mentioned models have some shortages in explaining or predicting the crystal growth habit. For example, they cannot reasonably explain the growth habit of polar crystal and the change of growth habit of oxide crystal in different crystallization conditions. Moreover, from the angle of crystal growth, it can be found that the change of crystal growth habit is due to the change of relative growth rate at various crystal faces. So, studying the growth habit of an oxide crystal must start from the growth mechanism of the crystal. Wei-zhuo Zhong [4] was the first to put forward the growth unit model, which hypothesizes that in the crystallization procedure, cations exist in the form of complexes whose ligands are OH− ions, and the complex whose coordination numbers are equal to that of the crystal formed is called a growth unit. The paper will reveal crystal growth mechanisms, and try to advance a new rule of crystal growth habits that is applied to different crystals.
Section snippets
Experimental procedure
The reaction vessel adopted in the experiments is a silver-lined tube-type stainless steel autoclave with a capacity of about 215 ml and an inner diameter of 30 mm. In the upper part of the autoclave, a discharge valve for releasing gas and a gas manometer are located. The Zn(CH3COO)2 solution or the newly-prepared hydroxide colloids Zn(OH)2 by adding the proper quantity of ammonia hydroxide to the chloride solution are used as starting materials. Various solvents involving pure water and
Result and discussion
The ZnO crystal obtained by the hydrothermal method belongs to the hexagonal crystal system. Due to its higher solubility under hydrothermal conditions, the growth rate of the ZnO crystal is smaller, which allows the ZnO particles to have good crystallinity and regular polyhedral crystal faces. So the ZnO crystal tends to be a good material for studying the crystal growth habit. Laudise [5], [6] was the first to describe the idealized growth habit of a ZnO crystal, preparing the ZnO single
Conclusions
The main results of this study may be summarized as follows:
- 1.
In solution, the growth unit of a crystal is the complex that is formed by the connection of cation with OH− ions. The ideal growth procedures of crystal contain the formation procedure of growth unit and the incorporating procedure of the growth unit at the interface. In practical growth procedures, due to the hindrance effect of external conditions on the interface, the replacement reaction of OH− ligands with other ligands at the
Acknowledgements
The authors thank Professor Sam R. Coriell for the detailed revision due to poor English in their manuscript. This project was supported by National Science Foundation of China under grant numbers 59832080 and 59772002.
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