Crystallization pathways and interfacial drivers for the formation of hierarchical architectures
Introduction
Hierarchical architectures are common in nature with notable examples including shells [1] and teeth enamel,[2] to name a few. The formation of these architectures is believed to involve multistep processes directed by interfacial interactions between the crystals and aqueous solutions of electrolytes, small organic ligands, and proteins. Some of the complexity of these crystal structures can be mimicked in purely synthetic protein-free systems, known as biomorphs, which couple crystalline particles and amorphous phases to form smooth edges and curved life-like structures [3], [4], [5]. In materials science, hierarchical architectures are particularly attractive as functional materials for various applications because they translate the nanoscale functionality of their building blocks and emergent collective behavior into their unique function and enhanced performance in catalysis, photochemical, photovoltaic, and other device applications [6], [7], [8]. To date, hierarchical nanostructures of metals [9], [10], organic–inorganic hybrid materials [11], and semiconductors [12], [13], [14], have all been successfully synthesized using a variety of methods. The development of hierarchy via branching is of particular interest, because highly-branched structures often exhibit regular patterns and/or self-similarity over many length scales [15], [16] and can exhibit short electron mean free paths [17], and optical heterogeneity.[18].
Hierarchy develops through several crystallization pathways ranging from particle-based assembly of oriented nanocrystals to branching through heterogeneous nucleation. The mechanism of oriented attachment is now better understood through direct in situ observation of particle dynamics and measurements of interfacial structure and interparticle forces.[19] In contrast, the mechanism by which hierarchy develops through branching remains largely unknown. Empirical data point to various factors that may drive branch development including confinement or the effect of structure directing ligands. However, branching can be observed in the absence of such external factors in simple precursor salt solutions and at low temperature [20]. A common feature of branched architecture formation is that crystallization starts from the formation of well-faceted nanocrystals. Subsequent pathway often involves heterogeneous nucleation and twinning to form branches. These observations pose a baffling question of why do new nuclei form twin boundaries with the seed crystal instead of seemingly lower energy ideal crystals? It is also not clear whether branching reflects metastability of the system and fast nucleation kinetics or whether it is a manifestation of the development of the thermodynamically most stable structure. Here, these open questions are discussed based on the combination of experimental evidence and theoretical predictions.
Section snippets
Heterogeneous nucleation pathway for the formation of hierarchical architectures
Heterogeneous nucleation is one of the possible pathways for the formation of branched and hierarchical architectures. In biological system, these processes are directed by proteins, which modify interfacial dynamics by preferential adsorption onto select surface sites, thereby making them inaccessible for precursors. Interestingly, preferential adsorption of proteins can also have an opposite effect of promoting nucleation at the adsorption sites by accumulating ions within their structure
Hierarchical architectures formed via particle-based crystallization
Classical monomer-by-monomer addition mechanisms discussed in the previous section do not exhaust the breadth of possible pathways leading to hierarchical morphologies. Non-classical particle-based crystallization furnish a range of alternative pathways that can result in hierarchical regular architectures,[51] although there are examples when the outcomes of particle-based crystallization and oriented attachment are continuous crystal lattices [52], [53] or irregular architecture with
Hierarchy development in confined volume
Biomineralization often takes place in confined volumes of nanopores, which creates a distinct environment for crystallization characterized by structuring of the precursor solution. The conditions have been emulated in microcapillaries, porous templates, and cross-cylinder architecture to understand how confinement affects crystallization pathways and outcomes. Recent review [82] covers the breadth of unusual phenomena observed in confinement including the stabilization of metastable
Particle assembly through steric / dipolar coupling in external fields
External fields can drive the formation of metastable polymorphs or particles with high-index surfaces [84] and the assembly of hierarchical structures [85], [86]. As discussed above, the formation of particle-based regular architectures through OA, in which particles form single crystal or twinned hierarchical structures, can be driven by macroscopic interparticle forces and/or interactions between opposing EDLs at particle surfaces [59]. Often these forces work in tandem to direct face
Conclusions
The development of hierarchical architectures often relies on specificity of binding of organic molecules or coprecipitates to specific sites of the seed crystal thereby breaking the symmetry and driving subsequent nucleation away from crystallographic matching with the seed. These processes are routinely exploited to drive the formation of complex architectures. However, branching may be also triggered by the intrinsic properties of the seed nanoparticles and their own deviation from the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This material is based upon work supported by the U.S. Department of Energy (DOE) Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering through its Synthesis & Processing Program FWP 12152 at Pacific Northwest National Laboratory, which is a DOE multiprogram national laboratory located in Richland, Washington.
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