Interactions of Organic Surfactants With Oxide Nanoparticles Grown in Aqueous Environments

Report No. ARL-TR-4047
Authors: Jennifer Synowczynski
Date/Pages: February 2007; 28 pages
Abstract: The goal of this report is to provide a fundamental understanding of how organic surfactants with different functional endgroups and branching structures affect the surface chemistry, organization, and growth of oxide nanoparticles (e.g., TiO2, ZnO) in aqueous solutions. It is generally known that particles grow from solution into shapes that minimize their surface free energy ?. The surface free energy is a complex function of the surface area, crystal anisotropy and defect structure, degree of coordination unsaturation, surface polarity, reconstruction barriers, and stoichiometry gradients. In solution, the situation is further complicated by changes in pH as well as interactions with ions and surfactants. Whether an ion or surfactant adsorbs to a specific facet depends on the competition between electrostatic and hydrophobic forces as well as their ability to form chemical complexes with the surface. By engineering these surface interactions, one can either promote or prevent particle growth along specific crystallographic planes. Using this approach, researchers have succeeded in precipitating particles with atypical shapes such as nanotubes, rods, cubes, and monodispersed spheres. A firm understanding of the relationship between solution chemistry and the complexes that form at the aqueous/TiO2 surface will greatly enhance our ability to control the morphology of TiO2 nanoparticles and assemble these particles into more complicated structures. This work is critical to demonstrating many devices that rely on quantum confinement effects, including photonic bandgap devices and self-cleaning photocatalytically active surfaces.
Distribution: Approved for public release
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Last Update / Reviewed: February 1, 2007