Infrared Spectroscopic Studies of Conduction Band and Trapped Electrons in UV-Photoexcited, H-Atom n-Doped, and Thermally Reduced TiO2

Transmission FTIR spectroscopy is used to explore the electronic structure of excited TiO2 nanoparticles. Broad infrared spectral features in UV-photoexcited, n-doped, and thermally reduced titania are found to be well-described by two theoretical models, which independently account for the creation of free conduction band electrons and trapped localized electrons that occupy states within the band gap. The infrared spectra indicate that the trapped electrons reside at shallow donor levels that exist 0.12-0.3 eV below the conduction band minimum. IR excitation, of the trapped electrons is evidenced by a broad feature in the spectra, which exhibits a. maximum that corresponds to the energy of the donor level. These features are well described by a hydrogenic-effective mass model. In addition, free conduction band electrons have a dramatic effect on the infrared spectra by exhibiting a broad featureless absorbance that increases exponentially across the entire mid-IR range. This absorbance is the result of intraconduction band transitions, for which free electron coupling to acoustic phonons is required to conserve momentum. Both localized (within the band gap) and delocalized (within the conduction band) electrons are found to exist in TiO2 when excess electrons (are created by different means: UV photoexcitation in the presence of a hole scavenger (methanol), irradiation with atomic hydrogen, and thermal removal of lattice oxygen.