Читать книгу Amorphous Nanomaterials - Lin Guo - Страница 22

In the traditional growth mechanism, the formation and growth of amorphous structures could not be explained, and a suitable new mechanism is desired. The rapid development of biomineralization research provides a theoretical basis for explaining the formation of amorphous materials in solution.

Biomineralization is a natural synthesis process, which use organic templates to control the growth of the inorganic phase. For example, amorphous calcium carbonate (ACC) have been particularly well studied as precursor in the biomineralization of invertebrates, such as mollusk shells and sea urchin spines (Figure 1.7). The high degree of crystallographic control is achieved from amorphous precursor in the biologically formed crystals. Scientists hope to be able to extrapolate the knowledge gained from such model systems and apply it to other inorganic systems to regulate crystallographic properties for advanced materials applications.

A large number of observations on biomineralization have found that the formation of many biomineral go through an amorphous precursor, which cannot be explained by the classical crystallization theory. Thus, the biomineralization pathway may be inconsistent with the traditional nucleation theory. Laurie B. Gower of the University of Florida and Helmut Cölfen of the University of Constance have proposed a multistep growth mechanism based on biomineralization for this problem.

Figure 1.7 Different organisms likely use the same strategy to generate diverse skeletal parts from crystals that arise from a transient amorphous calcium carbonate phase. Source: Reproduced with permission from Weiner et al. [27]. Copyright 2005, AAAS.

Gower pointed out [28] that the formation of solids in solution requires a multistep process, with multiple metastable states in it. The Ostwald–Lussac rule specifies that if a solution is supersaturated with respect to more than one phase, the more soluble (least stable) phase is often the first phase to form. Therefore, a highly unstable liquid precursor (polymer-induced liquid precursor) is formed first in the solution, and then it transform into an amorphous phase with unstable structure, followed by successively crystalline phases with decreasing energy and gradually increasing stability.

Cölfen proposed the theory of stable prenucleation cluster concept [29]. He pointed out that when the supersaturation of the solution reached a certain threshold, the ions meet in solution based on stochastic collisions and formed a stable pre-nucleation cluster. He experimentally showed that the clusters can be understood as a solute in the solution, without a phase interface. Its structure may not be related to the macroscopic bulk. In this process, the entropy increase caused by the release of ion hydration water is the driving force for the formation of the cluster. Crystalline could be directly nucleated from stable pre-nucleation clusters under certain conditions.

In general, the possible continuous phase transition of the biomineralization process from solution to crystal was shown in Figure 1.8. The process of biomineralization generally involves the participation of amorphous precursors, which makes its research very important for the formation mechanism of amorphous materials. The entire formation process of crystal CaCO₃ is clearly divided into two phases: the termination of the liquid phase and the generation of the solid phase. This process may provide an answer for the former question in the commemoration of the 125th anniversary of Science, where and why does liquid end and amorphous begin? If our target product is amorphous rather than crystalline, the reaction needs to be truncated at some stage in the process.

Figure 1.8 A reported growth mechanism of amorphous nanomaterials in solution.