Thesis Format

Integrated Article

Probing the anodic growth mechanism of titanium oxide

Hunter Feltham, Western UniversityFollow

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Noel, J.

2nd Supervisor

Goncharova, L.

Abstract

This thesis describes the study of the oxidation growth mechanism of thin film titanium oxide on the surface of titanium metal under aqueous saline conditions relevant to highly corrosive environments. Complimentary electrochemical and physical methods were applied, both in-situ and ex-situ, to differentiate between High-Field Model (HFM) and Point Defect Model (PDF) mechanisms of titanium oxide growth. These methods included using implanted Kirkendall markers, electrochemistry, and ion beam analysis.

Achieving a low-density gold marker layer without influencing the oxide's growth was a significant challenge. This was addressed using modification of the outer layer (mOO) with ion implantation, followed by chemical etching. This allowed precise control over the implantation energy and dose to ensure minimal impact on the growth process. Our results demonstrate the efficacy of this ultra-shallow ion implantation mOO technique and provide evidence supporting the point defect model as the dominant growth mechanism of TiO₂ passive film growth.

In-situ ion beam analysis like Rutherford backscattering spectroscopy (RBS) to determine oxide growth mechanisms is challenging due to the need for a liquid electrolyte in ultra-high vacuum (UHV). Our research involves using a specially designed in-situ cell with an ion-permeable silicon nitride window to provide a barrier between the UHV needed for RBS and the electrolyte solution required for electrochemical techniques. In-situ RBS results show a significant increase in the oxidation rate of titanium compared to equivalent ex-situ measurements, as well as spontaneous TiO₂ film growth without applied potential. Direct and indirect alpha radiation exposure measurements determine the enhanced titanium oxide thickness generated via radiation and radiolysis effects. Quantification of these effects allows for reliable comparison of in-situ and ex-situ anodization experiments.

The mechanism of titanium electrochemical oxidation was examined for ultra-thin Ti films sputtered onto Si (001) substrates and in-situ exposed to H₂¹⁸O, and then anodized in D₂¹⁶O. The effects of this isotopic labelling procedure were studied using medium energy ion scattering (MEIS) and nuclear reaction profiling (NRP). Both MEIS and NRP results show that the titanium oxide layer is composed of two distinct regions, Ti¹⁶O₂/Ti¹⁸O₂/Ti/Si(001), suggesting that PDM is a likely mechanism of anodic titanium oxidation.

Summary for Lay Audience

Titanium is a metal, a lot like iron. Iron is found in cars, buildings, and everyday life. Like iron, titanium can be used in hip replacements, golf clubs, bikes, and aircraft. The differences between titanium and iron are that titanium is stronger, doesn’t typically degrade in corrosive environments, and is far more expensive. When the iron in your car rusts, it makes the metal weaker, until it eventually completely breaks. The equivalent of rust for titanium is known as an oxide, and instead of making the material more likely to corrode further, protects the material by forming a thin layer of oxide all the way around the surface which acts as a protective coating for the metal underneath. This happens very quickly, even when exposed to air, and prevents the titanium from corroding.

This makes it a useful metal for applications that are prone to corrosion, such as factories that remove salt from water. You can make this oxide grow thicker by applying electricity to the titanium when it is in salt water. Since a small amount of titanium oxide is good, more may be better. This thesis works to understand how electricity can make titanium oxide grow thicker when there is supposed to be a protective shell around the metal. By figuring out where the atoms are moving during this oxidization, we can find ways to make the oxide more beneficial and work even better than natural titanium.

To look at this oxide formation, we use a process called impedance spectroscopy, where you apply electricity at many different frequencies, including the 60 Hz used in North American houses, but also higher frequencies. By doing this, we can measure what frequencies cause a response. This lets us match up a frequency with an event to figure out what is happening in the metal.

The other technique being used is Rutherford Backscattering Spectroscopy, where tiny particles are fired into a piece of titanium, and when these particles bounce off atoms in the material, they lose energy. Seeing how much energy they lose tells you what atom they interact with. This lets us determine how thick the oxide is on the metal.

Recommended Citation

Feltham, Hunter, "Probing the anodic growth mechanism of titanium oxide" (2024). Electronic Thesis and Dissertation Repository. 10564.
https://ir.lib.uwo.ca/etd/10564