Characterisation and reactivity of thermal and photo-generated surface defects and radical centres over TiO2 : an EPR investigation

• Main Author: Attwood, Anna Lisa
• Published: Cardiff University 2004
• Subjects: 543.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583493

Electron Paramagnetic Resonance Spectroscopy (EPR) has been used to thoroughly characterise the nature of trapped charge carrier pairs (i.e., holes and electrons) generated in different TiC>2 polycrystalline powders (P25 Anatase and two different Rutile materials). The paramagnetic centres were formed by vacuum reduction and UV irradiation of the TiC>2 samples, allowing a comparison of the two methods to be made. The reactivity of these trapped charge carrier pairs have been examined following the formation of oxygen centred radicals via oxygen addition. These generated species can change depending on the pre-treatment conditions and nature of the surface. On thermally reduced Anatase, O2" anions are preferentially formed whilst O" and O3" anions are produced on the Rutile forms of TiO2. Superoxide anions (O2) and trapped holes (O) were identified after photo-irradiation of all dehydrated TiO2 samples. The nature and stability of these oxygen centred radicals have been studied as a function of surface hydration. The ability to generate the radicals and the effects of surface hydration on the stability, and lifetimes have been examined. Nitric oxide has also been employed as a surface probe to study the surface crystal fields of the different TiO2 surfaces. The effect of surface hydration and NO have also been studied, revealing the different heterogeneity of sites responsible for NO binding, which was complimentary to the results obtained from the superoxide experiments. Co-adsorption and subsequent UV irradiation (100K) of acetone and oxygen onto a clean oxidised Anatase surface results in the formation of an unstable peroxy intermediate. The radical decayed irreversibly at temperatures greater than 150K but was regenerated by subsequent low temperature irradiation. Use of isotopically labelled gases (CD3COCD3, 1702) enabled the identification of the radical as the CH3COCH2OO* species generated by hole transfer to adsorbed acetone. The photo dynamics of the thermally generated Ti3+ and medium polarised conduction electrons in the TiO2 samples have also been followed as a function of TiO2 material and temperature. Interestingly, the rate of photoexcitation of surface Ti*1* centres follows opposite trends on anatase samples compared to rutile, and this difference has been discussed within the context of the different trapping sites available for electron stabilisation.