Amorphous Nanomaterials. Lin Guo
rapidly below the minimum supersaturation, no additional nucleation will occur and the resulting product will exhibit a monodisperse size. With the continuous supply of atoms through the precursor decomposition, the initial nucleus will grow into nanocrystals with an increasing size, until achieving an equilibrium state between the surface atoms on the nanocrystals and the free atoms in the solution.
Once the seed is formed, its size would increase by continuous addition of free atoms. From the perspective of chemical deposition, when atoms are added to a solid surface, the atoms diffuse on their surface until they encounter a step position where they can be incorporated. At the same time, as particle size increases, the volume free energy reduces (favorable for growth) and surface energy increases (favorable for dissolution). The dynamic interaction of growth and dissolution leads to the formation of anisotropic nanocrystal with specific morphology. With the development of in-situ electron microscopes, the structure and shape of seeds and nanocrystals produced at different stages could be observed. Therefore, a large number of studies have reported to explore the relationship between the initial seed and the final nanocrystal. After the nucleation and growth of the seed, the generated new nanocrystal will still go through complex evolution, such as Ostwald ripening, Kirkendall diffusion, oriented attachment, etc. Finally, nanoparticles with specific structure and morphology are obtained.
A typical synthesis method developed under the guidance of this theory is the hot injection method proposed by A. P. Alivisatos and Peng Xiaogang of the University of California [25]. Cold stock solution is quickly injected into the rapidly stirred, hot solvent, and a large number of crystal nuclei are instantaneously generated. The monomer concentration is rapidly reduced below the supersaturation threshold, and further nucleation is suppressed. This method separates the nucleation from growth, yielding particles of one size. It is a classic method for preparing monodisperse nanoparticles, especially quantum dots.
However, the theory is still an ideal model under extreme conditions, while the actual reactions and processes that occur in solution are far more complicated. Professor David W. Oxtoby of the University of Chicago, author of the famous Chemistry textbook “Principles of Modern Chemistry” in 1998 [26] stated that “Nucleation theory is one of the few areas of science in which agreement of predicted and measured rates to within several orders of magnitude is considered a major success” to comment the deficiencies of the classic nucleation growth theory.
For example, apart from decomposition to metal atoms, the precursors could also firstly assemble to aggregation and directly decompose to clusters. In the growth stage of the nucleus, the atomic adding mode is not the only growth way. The nucleus and nanocrystals can also directly merge into larger particles through attachment.
1.4.2 Multistep Transformation Mechanism with Amorphous Participation
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.
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.
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