Electron and Hole Behavior in Powdered TiO2 - Photoluminescence and Infrared Studies

This thesis reports the behavior of electrons and holes in photoexcited TiO2 powder under high vacuum conditions. We studied the effect of UV irradiation and different adsorbates, electron acceptors and electron donor molecules, on the perturbation of the band structure of TiO2. The electron-hole recombination rate in electronically excited TiO2 is influenced by band bending at the TiO2 surface and was studied by photoluminescence (PL) spectroscopy. In addition, we also employed infrared spectroscopy to qualitatively estimate the surface condition and to quantitatively estimate the amount of adsorbates on the surface.
It was found that continuous UV irradiation at 3.88 eV on an n-type TiO2 surface enhances the photoluminescence emission at ~529 nm (2.34 eV) as UV-induced surface potential causes upward bent bands to flatten. In addition, the adsorption of electron donor molecules such as CO and NH3 caused downward band bending in TiO2 and upon their adsorption/desorption, the PL intensity responded reversibly, showing reversible band bending. On the other hand, adsorption of an electron acceptor molecule such as O2 induces upward band bending and does not respond completely reversibly upon desorption.
We measured the kinetics of transport of adsorbed molecules to and from the outer surface of the TiO2 powder, in a depth of ~ 20 nm, compared to the molecular transport kinetics in the interior of a 95,000 nm thick porous TiO2 material. Here, we employed NH3 and CO as representatives of slow and fast diffusing molecules, respectively. Adsorption of NH3 at the outer surface of the TiO2 powder occurs quickly while its distribution in the powder was found to be retarded. In contrast, adsorption of CO molecule shows that the surface coverage in the first ~ 20 nm of the TiO2 powder lags behind the surface coverage in the bulk due to the fast diffusion into the interior of the powder. In addition, we found that adsorption/desorption of CO showed a hysteresis effect when viewed by the two spatially sensitive surface spectroscopies.
This thesis presents a new method for measuring charge transport between TiO2 particles using an optical method, photoluminescence spectroscopy. We found that long continuous UV irradiation of the TiO2 powder caused photoexcited electrons to percolate deep into the powder inducing changes in the surface photovoltage and band flattening as detected by PL enhancement. The main finding was that in the dark after charging by illumination, the negative charge tends to return back to the surface partially restoring upward band bending as detected by the PL intensity decrease. It was found that the relaxation of the negative charge at 140 K is a very slow process, on a minute time scale. The slow process was assign to either electron hopping from a TiO2 surface site to a surface site or from a TiO2 nanocrystallite to a nanocrystallite in the TiO2 bed. This electron hopping process is temperature-dependent with an activation energy of 15 meV. We used the rate of the discharging process to estimate the electron mobility (~ 10-10 m2 V-1 s-1) at room temperature. Further, we studied the effect of single wall carbon nanotubes and gold nanoparticles on charge transport between TiO2 particles. We found that carbon nanotubes act as electron acceptor molecules but do not affect charge transport between TiO2 particles. In contrast to that, ~ 3 nm gold nanoparticles supported on TiO2 nanpoarticls act as strong electron acceptors significantly suppressing PL intensity and the buildup of surface photovoltage as measured by PL intensity change.