Defect Sites on TiO₂(110). Detection by O₂ Photodesorption

Abstract

The TiO2(110) surface, prepared with different densities of anion vacancy defect sites, has been investigated by studies of O2 photodesorption at 105 K. Two separate O2 photodesorption processes, α1 and α2, are detected and are postulated to be due to the presence of two different types of defect sites produced by annealing the crystal in vacuum before O2 adsorption. The measured photodesorption cross sections for these two states are Q α 1 -O 2 = (3.1 ± 0.2) × 10-16 cm2 and Q α 2 -O 2 = (4.3 ± 0.3) × 10-17 cm2 for 3.96 ± 0.07 eV photons. Both O2 states photooxidize CO chemisorbed on the reduced TiO2(110) surface. While the yield of the CO2 photoproduct increases with increasing crystal annealing temperature (and increasing defect coverage), the yield of α1-O2 and α2-O2 passes through a maximum at an annealing temperature of ∼600 K.

Probing Defect Sites on TiO2 with [Re3(CO)12H3]: Spectroscopic Characterization of the Surface Species

“…[19] Abstract: Samples of the anatase phase of titania were treated under vacuum to create Ti 3 + surface-defect sites and surface O À and O 2 À species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti 3 + À OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re 3 (CO) 12 (CO) 12 H 3 ] is adsorbed on a titania surface with Ti Titania surfaces incorporating Ti 3 + sites can be monitored with techniques such as X-ray photoelectron spectroscopy, [20] ultraviolet photoelectron spectroscopy, [21] and electron paramagnetic resonance (EPR) spectroscopy. [22] Molecular probes have also been used to characterize defect sites on rutile (1 1 0) by thermal desorption (temperature-programmed desorption), with the probe molecules being O 2 , which is photodesorbed, [12] and CO 2 and CO, which are thermally desorbed.…”

“…EPR spectra showed that the probe molecule [Re 3 (CO) 12 (CO) 12 H 3 ] is adsorbed on a titania surface with Ti Titania surfaces incorporating Ti 3 + sites can be monitored with techniques such as X-ray photoelectron spectroscopy, [20] ultraviolet photoelectron spectroscopy, [21] and electron paramagnetic resonance (EPR) spectroscopy. [22] Molecular probes have also been used to characterize defect sites on rutile (1 1 0) by thermal desorption (temperature-programmed desorption), with the probe molecules being O 2 , which is photodesorbed, [12] and CO 2 and CO, which are thermally desorbed. [13,23] These methods distinguish Ti 3 + from Ti 4 + sites, for example.…”

“…Thus, we chose a metal carbonyl with a relatively high molecular weight, [Re 3 (CO) 12 H 3 ], because it offers the following potential advantages, assessed in the research described here:…”

“…* The n CO spectra are sensitive indicators of the bonding of [Re 3 (CO) 12 H 3 ] to the surface. * As [Re 3 (CO) 12 H 3 ] is a proton donor, its reactions can provide evidence of the basic character of the surface sites on which it adsorbs.…”

“…* As [Re 3 (CO) 12 H 3 ] is a proton donor, its reactions can provide evidence of the basic character of the surface sites on which it adsorbs. * Extended X-ray absorption fine structure (EXAFS) spectra can be used to determine the bond lengths in the adsorbed probe molecule, giving evidence of its interaction with the surface.…”