Unconventional Crystallization Of Rutile Crystals

Crystallization is of fundamental importance in solid-state chemistry, material sciences and biology, and is also among the most studied processes in science as well as of great practical significance. It has been nearly one hundred years since establishment of classical crystal growth theory and to date, great successes of classical nucleation and growth models have been achieved in explaining a wide range of crystal growth behaviors. However, there are still a number of phenomena associated with crystal formation that cannot satisfactorily be explained or predicted. Moreover, previous studies on crystallization mainly focused on the external morphology evolution rather than took the inner structure evolution into account, thus failing to enable people to fully understand the underlying growth mechanism.As a result of the advances in focus ion beam (FIB) microfacrication and Cs-corrected transmission electron microscopy (Cs-corrected TEM), it is possible for us now to investigate the inner/outer micro-sized crystal structure at atomic scale at the same time. Here, our work mainly concentrated on crystallization process of rutile crystals under non-equilibrium conditions, surface/inner structure evolution as well as nonclassical crystal growth behaviors in particular. Combining electron micoscopy with FIB techniques enables us to directly observe the inner structure and defect evolution inside the crystal body. Analysing the inner structures together with surface configuration we observed provides us with deeper insight into the unconventional crystallization of rutile crystals. The main innovative resluts are summarized as follows:(1) Rutile TiO2 crystals with novel curved surfaces were successfully synthesized through a facile hydrothermal method under the synergistic effects of HF and HCl and the surface atomic structures of such curved surfaces were revealed. Besides, according to the DFT theoretical calculations and experimental results, we found temperature and synthetic environment played an important role in formation and transition of such curved surfaces.(2) We explored the inner structure of rutile crystals with curved surfaces via FIB and TEM techniques and demonstrated an intriguing crystal growth phenomenon inside rutile crystals. More specifically, there existed two growth mechanisms simutaneously competing in 3D dimension during crystallization. Furthermore, the growth dynamics in longitudinal 2D space, the distributions of two growth sectors in 3D space as well as the causes of such competitive growth phenomenon were carefully investigated. It is revealed that such crystal behaviors could be attributed to both far-from-equilibrium growth conditions and relatively low surface energy of thermodynamically-stable{110} facets. At last, we successfully fabricated similar inner structures in tin dioxide system. Our work, to the best of knowledge, demonstrated for the first time both equilibrium and non-equilibrium growth mechanisms could exist simutaneously in one crystal particle during crystallization and that semi-quantitatively illustrated competing dynamics of both modes.(3) We introduced a simple pre-seeded treatment to explore the unconventional crystallization process of a new type of capsule-shaped rutile TiO2 mesocrystals at early growth stages. It is found that these rutile mesocrystals may be oriented self-assembled by mesoporous rutile nanorod clusters and furthermore, the presence of embryonic form of nanocapsules and the slight misalignment of the primary particles during attachment are also solid evidences of oriented aggregation behaviors.