Anthony Y. Kim

Self-assembly in the synthesis of ceramic materials and composites

Abstract The self-assembly process provides an effective and environmentally benign method for synthesizing novel ceramic and composite materials. Three different approaches are discussed in this article: (1) self-assembly of colloidal crystals, (2) self-assembled monolayers, and (3) three-dimensional self-assembly with amphiphilic molecules. The self-assembly of colloidal crystals allows the synthesis of periodic optical devices and quantum dots from simple monodispersed particles. Self-assembled monolayers provide a molecular template to control the nucleation and growth of ceramic thin films. The emphasis of this paper will be on the self-assembly process with amphiphilic molecules, which represents the latest breakthrough in the design and synthesis of tailored nanoscale materials and composites. In the past few years, self-assembled materials have become a very active research area. This article illustrates how the basic principles of self-assembly can be applied in the synthesis of ceramic materials and discusses the role of various inter-molecular and into-particular forces.

Linear chains and chain-like fractals from electrostatic heteroaggregation.

The internal structure of materials prepared by aggregation of oppositely charged polystyrene spheres (electrostatic heteroaggregation) is investigated by static light scattering, optical microscopy, and Brownian dynamics simulation. Light scattering indicates ultralow mass fractal dimensions, as low as 1.2. Such low fractal dimensions, approaching the theoretical limit of a linear object, imply a chaining mechanism. Optical micrographs reveal linear chains with the particle charge alternating down the chains. Brownian dynamics simulation gives additional support for a chaining mechanism. For the polystyrene system (120-nm primary particle diameters), the fractal dimension is found to increase from 1.2 to 1.7 as the background electrolyte is increased. In terms of electrostatic screening, the results match those reported recently for larger polystyrene spheres. The low fractal dimensions appear to represent a crossover from linear chains to a structure of diffusion-limited aggregates; however, experiments under density-neutral conditions imply that sedimentation plays an important role in the formation of ultralow fractal dimensions. The practical implication is that microcomposites with a locally uniform distribution of starting materials and almost any degree of branching can be prepared from oppositely charged particles.

Hybrid Mesoporous Materials with Functionalized Monolayers

Mesoporous materials have great potential for environmental and industrial processes, but many applications require the materials to exhibit specific surface chemistry and binding sites. A new approach has been developed so that organized functional monolayers are covalently bound to mesoporous supports. The functionalized hybrid materials show exceptional selectivity and capacity for removing heavy metals from waste streams. Tailored hybrid materials have also shown potential to selectively bind anions and radionuclides. Rational design of the surface properties of mesoporous materials will lead to more sophisticated functional composites.

Investigation of the local chemical interactions between Hg and self-assembled monolayers on mesoporous supports.

The synthesis of mesoporous silica has greatly expanded the possibilities for the design of open-pore structures. Because of their large surface area and well-defined pore shape, these materials have great potential in environmental and industrial processes, such as selective extraction of heavy metals from solutions. However, these applications require the materials to have specific attributes such as binding sites and charge density.

AMPHOTERIC SURFACTANT TEMPLATING ROUTE FOR MESOPOROUS ZIRCONIA

An amphoteric surfactant templating route permits the synthesis of hexagonal mesophases of zirconium compounds by self-assembly of the surfactant and soluble zirconium species in aqueous solution.

Preparation of mesoporous spherulites in surfactant solutions

A new mesoporous structure, silicate spherulite, has been produced in dilute surfactant and silicate solutions. The spherulites were formed from radially arranged rod-like micelles. The surfactant molecules in the rod-like micelles can be removed by heat treatment to leave a unique mesoporous material with radially arranged channels for easy access. Although the spherulite morphology has been observed in the nucleation and growth of polymeric crystals for a long time, it has not been reported in surfactant solutions. Similar to the polymeric materials, the surfactant spherulites are most likely the fingerprints of the early nucleation process in the preparation of ordered mesophase silicates.

Synthesis of mesoporous zirconia using an amphoteric surfactant

An amphoteric surfactant, cocamidopropyl betaine, was used for the synthesis of mesoporous zirconia. The carboxylate functionality of the surfactant permitted strong bonding with soluble zirconium species, while the quaternary ammonium group ensured large headgroup area and high solubility under acidic conditions. An amphoteric co-template [betaine, or (carboxymethyl)trimethylammonium hydroxide] improved uniformity of the hexagonal mesophase. Transmission electron microscopy (TEM) of the as-synthesized zirconium sulfate mesophase indicated hexagonal mesostructure, and low-angle X-ray diffraction (XRD) showed a 41 {angstrom} primary d-spacing and two higher order reflections of a hexagonal lattice. High surface area zirconia was produced by controlled base treatment of the hexagonal mesophase with sodium hydroxide, followed by calcination. TEM and XRD indicated that the mesostructure was stable to 350 C.

Heterogeneous nucleation of ordered mesoporous materials

Recently we proposed that heterogenous nucleation is an important phenomenon for the preparation of ordered mesoporous materials. In this paper we further investigate the effect of colloidal particles on the nucleation process of mesoporous materials. Based on the change of the electrical mobilities of the particles in the surfactant solution, we suggest that the adsorption and co-adsorption of surfactant and ceramic precursors changes local structural and chemistry on the particle surfaces, and favors the nucleation events within these regions.