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

Abstract

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.

Polaron-Induced Deep Defect Levels in Brookite TiO2: A Many-Body Green’s Function Theory Study

“…A close resemblance in the electron paramagnetic resonance spectrum between anatase and brookite was noticed by Livraghi et al , indicating a similar electron trapping behavior between them . A defect level was detected to lie 1.1 eV below the CBM of brookite by Shinoda and Murakami . Additionally, Vequizo et al found a moderate depth of 0.4 eV below the CBM for the trapped electron in brookite .…”

“…Experiments have detected two defect bands in brookite which are 0.4 and 1.1 eV below the CBM. 67,68 From our GW calculations, it seems that polaronic states of Ov near the surface may all together contribute to the shallow defect band, while the deep defect band may result from the ddσ bonds of bulk Ov.…”

“…66 A defect level was detected to lie 1.1 eV below the CBM of brookite by Shinoda and Murakami. 67 Additionally, Vequizo et al found a moderate depth of 0.4 eV below the CBM for the trapped electron in brookite. 68 These studies reveal the coexistence of shallowly trapped electrons and deeply trapped electrons in brookite.…”

Simultaneous Measurements of Photoabsorption and Photoelectrochemical Performance for Thickness Optimization of a Semiconductor Photoelectrode

“…In contrast, the photoacoustic (PA) technique can be applied to simultaneous measurements, regardless of sample transparency. The PA technique is a method for absorption spectroscopy that is applicable to even opaque and strongly scattering materials because it detects photoabsorption indirectly through photothermal waves generated by relaxation of the photoexcited states. , We have so far set up various in situ measurement systems, including PEC reaction over a photoelectrode and a particulate photocatalyst. , Here, we propose a simple method for thickness optimization by combination of the PA technique and step scanning of focused laser light. To accomplish it, a photoelectrode with a film-thickness gradient was prepared by electrophoretic deposition of tungsten­(VI) oxide (WO 3 ) particles while pulling up the substrate.…”

“…12,13 We have so far set up various in situ measurement systems, including PEC reaction over a photoelectrode 14 and a particulate photocatalyst. 15,16 Here, we propose a simple method for thickness optimization by combination of the PA technique and step scanning of focused laser light. To accomplish it, a photoelectrode with a film-thickness gradient was prepared by electrophoretic deposition of tungsten(VI) oxide (WO 3 ) particles while pulling up the substrate.…”