Defect Properties of Anodic Oxide Films on Titanium and Impact of Oxygen Vacancy on Oxygen Electrode Reaction
Open Access
- Author:
- Roh, Bumwook
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 06, 2007
- Committee Members:
- Digby D Macdonald, Committee Chair/Co-Chair
Howard W Pickering, Committee Member
Elizabeth C Dickey, Committee Member
Thomas E Mallouk, Committee Member - Keywords:
- PDM
Point Defect Model
Titanium
Anodic Oxide - Abstract:
- The anodic titanium oxide film formed on titanium substrate is studied with the experimental techniques of ellipsometry, Mott-Schottky analysis, electrochemical impedance spectroscopy (EIS), and the EIS modeling with the Point Defect Model (PDM), in order to characterize the defective and semiconductive structure of the oxide film and investigate the impact of the defects of the oxide barrier layer on the oxygen electrode reactions. Ellipsometry is used to measure the thickness of the oxide film, and the model used to analyze the thickness in ellipsometry is applied to determine the structure of the oxide film that it is single-layered. The EIS model with the PDM is proposed to determine whether the oxygen vacancy is the major type of oxide defect. The expression of film thickness/potential from the PDM combined with the solution to the Nernst-Plank equation is proposed to investigate the oxygen vacancy diffusion coefficient. The effect of oxygen vacancies in the anodic oxide film on the kinetics of the oxygen electrode reaction has been studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Oxide films of different donor density are prepared galvano-potentiostatically at various current densities. The semiconductive properties of the oxide films are characterized using EIS and Mott-Schottky analysis. The film thickness is found to be almost constant, but Mott-Schottky analysis shows that the donor (oxygen vacancy) density decreased sharply with increasing oxide film formation rate (current density). With regard to the relationships between the rates of oxygen reduction/evolution and the donor density, the results show that the rates of both the reactions are higher for passive films having higher donor densities. Possible explanation, including enhancement of the conductivity of the film due to the vacancies facilitating charge transfer and enrichment of the reaction sites of the surface oxygen vacancies acting as catalytic sites, are discussed. The possible mechanisms for the oxygen electrode reactions where oxygen vacancies are acting as catalytic reaction sites are proposed. Finally, the oxygen vacancy profile and the surface oxygen vacancy concentration are provided by the advanced EIS modeling with the PDM.