Photocatalysis on Titanium Dioxide Surfaces
The adsorption, photo- and thermal- chemistry of adsorbates, and the photoactivation of TiO2(110) single crystalline surfaces are the central topics investigated and presented in this thesis.
The hole-induced photodesorption of chemisorbed O2 from a defective TiO2(110) surface was studied to monitor the kinetics of electron-hole pair formation, recombination and hole-trapping. Two distinct O2 desorption processes are found which are characteristic of low and high photon fluxes. At a critical photon flux, the slow O2 desorption process converts to a fast process as a result of the saturation of hole trapping sites in the TiO2 crystal. Both the slow and fast O2 desorption processes are governed by the flux of UV radiation, Fhv1/2, indicating that the steady state concentration of photogenerated holes can be described by second-order electron-hole pair recombination kinetics. A hole scavenger is used to probe the role of added hole trap centers on the photodesorption rate.
The photodesorption for O2 from TiO2(110) was found to exhibit fractal kinetic behavior where the photodesorption process is described by a rate coefficient that varies throughout the measurement. A model is proposed in which the electrons associated with O-vacancy defects on the surface percolate from vacancy site to vacancy site via the filled orbitals at these sites to neutralize photo-produced holes. This electron percolation, causing e-h recombination, reduces the efficiency of charge transfer between a photoproduced hole and an O2-(a) species localized at a vacancy defect site, causing the rate of O2 photodesorption to follow a fractal rate law. We postulate that the fractal electron conduction path across the surface is one-dimensional.
The adsorption and thermal desorption of CO2 on TiO2(110) was investigated as a probe of the concentration of oxygen vacancy defects. For the stoichiometric and fully oxidized surface, a single thermal desorption feature (Ed = 48.5 kJ/mol) is measured and is attributed to CO2 bound to regular 5-fold coordinated Ti4+ atoms. For the partially reduced TiO2(110) surface, CO2 binds not only to regular sites, but also to oxygen vacancy sites (Ed = 54.0 kJ/mol), created by thermal annealing. The variation in the characteristic CO2 desorption kinetics was measured as a function of the surface reduction temperature and the systematic production of increasing levels of surface defects is observed in the temperature range of 600K- 1100K.
The effect of impurity doping on the photoactivity of TiO2 rutile single crystals was subjected to a combined surface science and bulk analysis study. Two approaches were taken: First, the incorporation of nitrogen ions, N-, into TiO2 single crystals was achieved by sputtering with N2+/Ar+ mixtures and subsequent annealing to 900 K under ultra high vacuum conditions; Secondly, a chemical doping method where TiO2(110) single crystals were exposed to gaseous NH3 at 870 K was used. The nitrogen implantation method employed in the first study resulted in an unexpected blueshift of the photoexcitation threshold energy for the doped material compared to undoped TiO2. For crystals doped using the second chemical doping method, a decrease in the required photoactivation energy was observed when compared to undoped TiO2 crystals.
Finally, the thermal- and photo- chemistry of the mustard gas simulant molecule, 2-chloroethyl ethyl sulfide (2-CEES), was investigated on both TiO2(110) single crystals and TiO2 powdered materials. 2-CEES decomposition occurs through the incorporation of lattice oxygen from the TiO2 material into the oxidized products.
A review of aspects of photochemistry on TiO2 surfaces forms the concluding chapter of the thesis.
Advisor:Professor John T. Yates, Jr.; Professor Kenneth Jordan; Professor Hvorje Petek; Professor David Waldeck
School:University of Pittsburgh
School Location:USA - Pennsylvania
Source Type:Master's Thesis
Date of Publication:06/30/2006