CHEMICAL CONTROL OF PHASE, MORPHOLOGY AND SURFACE STRUCTURE OF METAL OXIDE NANOMATERIALS

Gonghua Wang, University of Nebraska-Lincoln

Document Type Article

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Chemistry, Under the Supervision of Professor Chin Li Cheung. Lincoln, Nebraska: April, 2012

Abstract

Metal oxides exhibit many different phases, compositions, morphologies and surface structures. When the dimensions of these oxides fall into the nanoscale region, the nanosize effects endow the materials with structure-dependent physical and chemical properties. This dissertation focuses on the control of phase, morphology and structure of metal oxide nanomaterials using chemical means.

The phase stability of metal oxide nanocrystallites with metastable crystal structures under ambient conditions is usually controlled by the size effect. It remains challenging to maintain the phase integrity of these nanomaterials with crystallite sizes beyond the critical values. In this dissertation, the use of low concentrations of nitrogen dopants are explored in the growth and stabilization of favored metastable cubic zirconium oxide. The implanted nitrogen introduced during ion beam assisted deposition has been proven to reduce the structural symmetry and promote the cubic phase stability of zirconium oxide. Both the nanosize effect and soft-mode hardening mechanisms are found crucial for the cubic phase zirconia made by ion beam assisted deposition technique.

The surface morphologies of metal oxide nanomaterials can be modified by the careful selection of synthetic conditions. Though subtle, the ion beams with different chemical identities may modulate the surface morphology of the as-synthesized oxide materials because of their unique chemical reactivities. This dissertation presents a study of the modulation of titanium oxidefilms’ surface morphology by varying the chemical identity of concurrent ion beams during the film deposition synthesis.

Defect sites in reducible metal oxide nanomaterials such as cerium oxide play an essential role in their catalytic activities. By surface engineering through chemical means, the surface defect sites, especially the oxygen vacancy defect site density, can be increased, leading to improved catalytic performance in various important reactions. This dissertation illustrates surface modifications of cerium oxide nanomaterials and presents an investigation of the effect of surface defects on the catalytic activity of these nanomaterials towards the cyanosilylation of aldehydes.

Adviser: Dr. Chin Li Cheung