Photoacoustic Fourier Transform Near- and Mid-Infrared Spectroscopy for Measurement of Energy Levels of Electron Trapping Sites in Titanium(IV) Oxide Photocatalyst Powders

Electron trapping sites in titanium­(IV) oxide photocatalyst powders were investigated by photoacoustic (PA) Fourier transform near-infrared spectroscopy (FT-NIR) and mid-infrared spectroscopy (FT-MIR). PA measurements using FT-NIR and FT-MIR enabled in situ observation of the energy levels of electron trapping sites in wide energy levels (0.1–1.9 eV) below the bottom of the conduction band. During ultraviolet (UV) irradiation, PA intensity increased depending on the wavenumber, and changes in the PA spectra were observed as a result of the trivalent titanium species generated by accumulation of electrons at trapping sites. Moreover, the PA spectral shape during UV irradiation was essentially different between the crystal structures, and it greatly depended on the crystal structure rather than other properties such as specific surface area and particle size. The results for various samples suggest that anatase has shallower energy levels of electron trapping sites than those of rutile and brookite, with the main energy level of the trapping sites being deep in the order of brookite > rutile > anatase. Thus, the PA technique using FT-NIR and FT-MIR is an effective method for measurements of energy levels of electron trapping sites in semiconductor photocatalysts.