The photoluminescence (PL) of powdered TiO₂ at 529.5 nm (2.34 eV) has been found to be a sensitive indicator of UV-induced 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 TiO₂ and enhances PL1. Also, adsorbates modify the PL intensity by exchanging charge with TiO₂, producing a change in the surface band bending structure.

In addition, 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, and hysteresis effects in adsorption/desorption are observed.

References:

1. Stevanovic, A.; Buttner, M.; Zhang, Z.; Yates, J. T., Jr., Photoluminescence of TiO2: effect of UV light and adsorbed molecules on surface band structure. Journal of the American Chemical Society 2012, 134, (1), 324-32.

2. Stevanovic, A.; Yates, J. T., Jr., Probe of NH(3) and CO Adsorption on the Very Outermost Surface of a Porous TiO(2) Adsorbent Using Photoluminescence Spectroscopy. Langmuir : the ACS journal of surfaces and colloids 2012, 28, (13), 5652-9.

Work supported by the Army Research Office.