The study of oriented aggregation: a nonclassical nanocrystal growth mechanism

Abstract

Oriented aggregation is a nonclassical crystal growth mechanism resulting in new secondary nanoparticles 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 important 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. However questions remain about what conditions are the most importing in facilitating purposeful control over nanoparticle size, size distribution, and morphology. Kinetic models for oriented aggregation point to important variables such as ionic strength, pH, temperature, and choice of dispersing solvent as being the key or keys to gaining control of this natural phenomenon and moving it towards a tool to be used in designing novel nanomaterials. The main technique used in this research is transmission electron microscopy with temporal resolution to characterize the population of growing nanocrystals. Cryogenic transmission electron microscopy is employed to observe the various stages of crystal growth. With extensive image analysis, it is possible to determine the kinetics of growth and the effects of systematically changing these key growth conditions. Additional complimentary techniques are employed, such as dynamic light scattering as well as various methods of characterization, such as powder X-ray diffraction. As our fundamental understanding of oriented aggregation improves, novel and complex functional materials are expected to emerge.