Electron Hopping through TiO2 Powder: A Study by Photoluminescence Spectroscopy

Photoluminescence spectroscopy was employed to observe electron transport between TiO 2 particles. Ultraviolet (UV) irradiation (3.88 eV) was shown to positively enhance the photovoltage of TiO 2 particles at the powder surface, causing an enhancement of their photoluminescence (PL) at 530 nm. The charging of the TiO 2 particles on the powder surface by UV irradiation partially discharges in the dark, where the displaced bulk negative charge diffuses back toward the TiO 2 surface. This charge flow partially restores upward band bending, causing the PL intensity to decrease. The rate of the discharging process was used to estimate the electron migration mobility (∼10 –10 m 2 V –1 s –1 at 300 K) between TiO 2 particles in the TiO 2 matrix. Electron migration between TiO 2 particles is temperature-dependent with an activation energy of 0.015 ± 0.008 eV. In addition, it was found that the adsorption of an immobile electron-donor molecule (NH 3 ), attracts negative charge on the TiO 2 surface which does not exhibit mobility behavior, in contrast to mobile electrons produced by UV. These measurements were carried out in high vacuum in the absence of oxygen and surface impurities detectable by IR spectroscopy.