Surface Characterization of Oxides Synthesized by Successive Ionic Layer Deposition.

Abstract

Successive ionic layer deposition (SILD) is an aqueous technique for depositing thin oxide films on a surface in a layer-by-layer fashion through a series of chemical reactions. This dissertation examines empirical aspects of the SILD technique by characterizing thin oxide films synthesized on model planar supports and then extends the SILD technique to synthesize supported oxide nanostructures on three dimensional supports of interest to catalysis.

Atomic force microscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy provided insight into the SILD of zirconia, alumina, and barium oxide thin films on silicon wafers. The SILD conditions that most affected the surface morphology of the thin oxide films were the selection of aqueous metal salt precursors comprising the SILD solutions and the total number of SILD cycles.

Recent studies suggest that a highly dispersed phase of barium oxide supported on alumina interacts differently with NO2 than a bulk-like phase of barium oxide. SILD was used to synthesize disperse nanoislands or rafts of barium oxide on larger rafts of alumina supported on a silicon wafer. The SILD method was then extended to deposit barium oxide on an alumina powder support comprised of dense 150 nm spherical crystallites fused together into 1-2 µm particles.

Equally weight loaded samples of barium oxide on the fused alumina powder were prepared by SILD and wet impregnation. The NO2 storage behavior of the barium oxide, evaluated by thermogravimetric analysis during NO2 temperature programmed desorption (TPD) experiments, provided insight into the dispersion of barium oxide that resulted from each of the loading techniques. The highly dispersed barium oxide rafts synthesized by SILD on fused alumina released NO2 at temperatures below 500ºC during TPD. By comparison, the barium oxide loaded by wet impregnation showed a higher temperature desorption feature above 500ºC indicative of bulk-like barium oxide nanoparticles. The NO2 weight loss curves were also used to calculate the relative percentages of BaO in the dispersed phase and bulk-like phase for each loading technique.

The ability of SILD to synthesize highly disperse and uniform, conformal oxide coatings on three dimensional supports provides fundamental insight into the interactions between catalysts and supports.