This thesis explores the low temperature dynamics of photo-generated charges in titanium dioxide; with particular reference to the rutile polymorph. The phenomenon of charge trapping in anatase has been heavily investigated using electron paramagnetic resonance (EPR) spectroscopy, whereas the rutile polymorph has been given very little attention. An understanding of charge trapping dynamics in semiconductors, such as titanium dioxide, is of key importance to advancing technologies such as photocatalysis. The research contained in this thesis examines charge trapping in a number of rutile systems, nano- and micro-structured, and compares these finding with those of the anatase polymorph. For contrast, a similar EPR study was carried out on a hematite; an iron oxide semiconductor. The data highlights some interesting differences between the rutile and anatase polymorphs. Firstly, the trapped electrons in rutile are particularly sensitive to being re-excited to the conduction band by the further absorption of photons; as is not the case in anatase. This phenomenon has been termed in this work as a dynamic equilibrium between trap site and conduction band. Secondly, the observation of paramagnetic holes is very difficult in rutile systems, leading to the hypothesis of hydroxyl radical dimerisation; producing hydrogen peroxide of the surface. This hypothesis was formed following an extensive study of the effects of surface hydration. Most importantly, a number of distinctly different trapped electron species were observed in the rutile samples. The two main trapped electron species have been assigned to; i) an electron residing on a surface titanium ion associated with a hydroxyl group and ii) a sub-surface titanium ion residing in either a lattice or an interstitial site. Finally, the study conducted on Hematite at low temperature, shows evidence for a transition from ferromagnetism to superparamagnetism during broadband irradiation. An attempt to rationalise this observation is given within.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:582712 |
| Date | January 2013 |
| Creators | Macdonald, Iain |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=201922 |
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