Photoluminescence of the Titanium Dioxide-Effect of UV Light and Adsorbed Molecules on Surface Band Structure

The photoluminescence (PL) of powdered titanium dioxide at 529.5 nm (2.34 eV) has been found to be a sensitive indicator of UV-induced surface band structure modification. As UV irradiation occurs, the positive surface potential changes and shifts the depth of the depletion layer. It was found that UV light (3.88 eV) induces a positive surface potential which diminishes band bending in n- type titanium dioxide (TiO₂) and enhances PL. Also, adsorbates modify the PL intensity by exchanging charge with TiO₂, producing a change in the surface band bending structure.

In the work, we employ photoluminescence (PL) spectroscopy to probe the development of adsorbed layers on the very outermost surface sites of a porous solid adsorbent (TiO₂) in a depth of 20 nm where the meso-pores, separating 30-80 nm TiO₂ particles, join the gas phase2. In parallel, we also employ transmission infrared (IR) spectroscopy to gain insight into the extent of adsorption averaged over the entire depth of the diffusion process. The combination of the two surface spectroscopies (PL and IR) allows one to observe the kinetics of transport of adsorbate molecules between the very outermost surface region (where adsorption first occurs) and the interior of the powdered substrate. The transport is governed by the surface mobility of the adsorbate molecules, causing hysteresis effects in adsorption/desorption.

In addition, photoluminescence spectroscopy was employed to observe electron transport between TiO₂ particles. Ultraviolet (UV) irradiation (3.88 eV) was shown to positively enhance the photovoltage of TiO₂ particles at the powder surface causing an enhancement of their photoluminescence (PL) at 530 nm3. The charging of the TiO₂ particles on the powder surface by UV irradiation is observed to partially discharge in the dark where the displaced bulk negative charge diffuses back toward the TiO₂ 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 m2Vs-1 at 300 K) between TiO₂ particles in the TiO₂ matrix. Electron migration between TiO₂ particles is temperature dependent with an activation energy of 0.015 ± 0.008 eV.