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), hematite (Fe₂O₃), feroxyhite (FeOOH), and heterogenite (CoOOH) was observed to occur by two primary mechanisms: coarsening and growth by topotactic assembly. Coarsening is governed by the growth of larger particles at the expense of 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 blocks. The heterogenite particles are hexagonal plates that are, on average, 0.7 μm across and 20-30 nm thick. These plates are porous and are assemblies of hundreds of oriented nanocrystalline building blocks. The feroxyhite nanocrystals attach to form ∼30 nm porous flakes that are several nanometers thick. The anatase nanocrystals assemble to form elongated, bent, 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 decreases with time and that the assembly order, which reflects the average number of primary particles per secondary particle, increases with time. Growth 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.