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.
1.4.3 Complex Growth Process in Solution
Using in-situ imaging techniques (scanning electron microscope, transmission electron microscope, and atomic force microscope) to characterize the growth process, James J. De Yoreo of Pacific Northwest National Laboratory summarized the existing crystal growth modes and proposed the crystallization by particle attachment (CPA) theory [30]. He pointed out that, in addition to the monomer-by-monomer addition described in classical models, crystallization by addition of particles, ranging from multi-ion complexes to fully formed nanocrystals, is now recognized as a common phenomenon. Crystallization can occur by attachment of a wide range of species more complex than simple ions. These higher order species are collectively named as particles. They are broadly defined to include multi-ion complexes, oligomers (or clusters), and nanoparticles, whether crystalline, amorphous, or liquid. Compared to traditional growth models, the growth, assembly, and transformation of these particles seem to be the actual route in the formation of crystal particles.
In a real crystallization process, even if only considering the reaction mechanism of a specific system, multiple growth mechanisms can occur simultaneously. It depends on the values of global parameters such as supersaturation, local factors that include interface curvature, and materials parameters such as phase stability versus particle size. The growth process may include the traditional direct connection of atoms, the connection of crystal clusters, the connection of amorphous particles and the subsequent crystallization or maturation, and the oriented or non-oriented connection of crystal particles and recrystallization. Ostwald ripening can occur in all particles to provide free radicals for the main particles. At the same time, twins, stacking faults, and dislocations can result from the attachment of crystalline particles. It can be found that the predictive understanding of the theory of particle-connected crystallization is helpful for the further development of amorphous nanomaterial design and synthesis.
1.5 Summary and Outlook
Although many studies have confirmed the significance of amorphous nanomaterials, their development is still in their infancy. For example, it is widely believed that amorphous materials have SRO, but their atomic arrangement cannot be accurately defined, even with the help of AC-TEM. Intense debates regarding whether they could exhibit medium-range ordering at the nanometer scale or even LRO are still ongoing. In addition, compared with the highly ordered arrangement of atoms in crystals, the atomic disorder of amorphous materials endows them highly unsaturated centers on the surface. If the unsaturated centers were all active sites, the catalytic performance would be improved by an order of magnitude. However, the reported research has not achieved such an improvement. Therefore, as an emerging discipline, the study of amorphous nanomaterials is of great significance.
At the same time, the synthesis of amorphous nanomaterials is only in the exploration stage. Most of the as-reported synthesis method is incomprehensive and non-systematic, while systematical and universal methods are needed. Limited by the isotropic feature of amorphous structure, the morphology of naturally growing amorphous materials are spherical particles. If 1D materials are expected, an external force must be applied to confine the growth direction, for example, electrospinning and chemical vapor deposition. The preparation of 3D materials with regular morphology can only rely on the hard template method and then etching the template to obtain a polyhedron frame or shell. The template-free self-growth of amorphous materials with specific and programmed morphology is extremely difficult. Thus, researches on the design and synthesis of amorphous nanomaterial should be comprehensively reviewed to summarize the laws. In addition, the study of the relationship between structure and related properties of the amorphous nanomaterials and their potential applications will greatly promote the development of amorphous nanomaterials.
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