Epitaxial assembly in aged colloids

Self-assembly of nanoparticles is an important mechanism of particle growth in the solution-phase synthesis of oxides and oxyhydroxides. In this work, particle growth in aqueous colloidal suspensions of anatase (TiO2), hemati te (Fe2O3), feroxyhite (FeOOH), and heterogenite (CoOOH) was observed to oc cur by two primary mechanisms: coarsening and growth by topotactic assembly . Coarsening is governed by the growth of larger particles at the expense o f smaller particles, and topotactic assembly results in single crystals of unique morphology. The hematite nanocrystals are nominally equidimensional crystals that are usually constructed from more than 10 primary building bl ocks. The heterogenite particles are hexagonal plates that are, on average, 0.7 mum across and 20-30 nm thick. These plates are porous and are assembl ies of hundreds of oriented nanocrystalline building blocks. The feroxyhite nanocrystals attach to form similar to 30 nm porous flakes that are severa l nanometers thick. The anatase nanocrystals assemble to form elongated, be nt, or nominally equidimensional single crystals with ultimate morphologies that frequently violate crystal-symmetry rules. Kinetic experiments, using anatase particles, show that the number of isolated primary particles decr eases with time and that the assembly order, which reflects the average num ber of primary particles per secondary particle, increases with time. Growt h by oriented aggregation is highly dependent on solution chemistry and may provide a means by which intricate assemblies can be achieved without the use of organic additives.