Quantifying the Kinetics of Crystal Growth by Oriented Aggregation
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Kinetics of Crystal Growth by Oriented Aggregation
Nathan D. Burrows, Virany M. Yuwono, and R. Lee Penn Abstract Oriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary particles composed of crystallographically aligned primary crystallites. These secondary crystals often have unique and symmetry-defying morphologies, can be twinned, and can contain stacking faults and other significant defects. A wide range of materials, such as titanium dioxide, iron oxides, selenides and sulfides, and metal oxyhydroxides, are known to grow by oriented aggregation under certain conditions. Evidence for oriented aggregation also has been observed in natural materials. Over the last decade, reports of this crystal growth mechanism have appeared with increasing frequency in the scientific literature. The development of kinetic models aimed at improving our fundamental understanding as well as facilitating purposeful control over size, size distribution, and shape has ranged from simple dimer formation models to polymeric models and population balance models. These models have enabled detection and characterization of crystal growth by oriented aggregation using methods such as small-angle x-ray scattering, among others, in addition to transmission electron microscopy. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge. This article presents a summary of some recent results, methods, and models for characterizing crystal growth by oriented aggregation.
Introduction Oriented aggregation, also known as oriented attachment or oriented assembly, is a special case of aggregation in which secondary particles composed of oriented primary crystallites are produced (Figure 1). This growth mechanism can result in the formation of unique, often symmetry-defying morphologies, as well as twins and stacking faults, and can lead to the incorporation of defects, such as edge dislocations.1–4 Morphological features consistent with crystal growth by attachment of oriented nanocrystals were found in transmission electron micrographs.3,5 Oriented aggregation is distinct from coarsening, which is also known as Ostwald ripening. Coarsening is a classical crystal growth mechanism and is
driven by the inverse relationship between particle size and chemical potential.6,7 Larger crystals grow at the expense of smaller crystals by way of diffusion of molecular-scale species through the growth medium or across grain boundaries. Crystals grown by coarsening are characteristically defect-free (except for point defects), faceted, and have morphologies consistent with the crystal structure. Recent efforts have moved toward quantifying kinetics and elucidating the fundamental mechanism of crystal growth by oriented aggregation. Results are expected to lead to improved control over size and shape, understanding of crystal growth in natural environments, and methods for fabricating
MRS BULLETIN • VOLUME 35 • FEBRUARY 2010 • www.mrs.org/bulletin
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