Interaction of Molecular Oxygen with the Vacuum-Annealed TiO₂(110) Surface:  Molecular and Dissociative Channels

Abstract

We have examined the interaction of molecular oxygen with the TiO 2 (110) surface using temperatureprogrammed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO 2 (110) surface in the temperature range between 100 and 300 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water TPD. Adsorption of O 2 at 120 K on a TiO 2 (110) surface with 8% oxygen vacancies (about 4 × 10 13 sites/cm 2 ) occurs with an initial sticking probability of 0.5-0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no O 2 desorption below a coverage of 4 × 10 13 molecules/cm 2 (the vacancy population), presumably due to dissociative filling of the vacancy sites in a 1:1 ratio. Above a coverage of 4 × 10 13 molecules/cm 2 , a first-order O 2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with other coadsorbed oxygen molecules. Sequential exposures of 16 O 2 and 18 O 2 at 120 K indicate that each adsorbed O 2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that O 2 adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O 2 molecules reside in this precursor state at 120 K. This precursor state may be associated with a weak 145 K O 2 TPD state observed at high O 2 exposures. ELS measurements suggest charge transfer from the surface to the O 2 molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 2.8 eV loss that can be assigned to an adsorbed O 2species based on comparisons with Ti-O 2 inorganic complexes in the literature. Utilizing results from recent spin-polarized DFT calculations in the literature, we propose a model where three O 2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 K, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom, which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO 2 surfaces.

Surface photochemistry probed by two-photon photoemission spectroscopy

“…Various spectroscopic techniques have been used to probe chemical reactions at interfaces. Since surface chemical reactions involve structural changes, many spectroscopic techniques, such as electron energy loss spectroscopy (EELS), [5][6][7][8] infrared (IR) spectroscopy, 9,10 Raman spectroscopy 9,11,12 and sum frequency generation (SFG) spectroscopy [12][13][14] have been widely used to probe vibrational structures and their changes in surface chemical reactions. In addition, scanning tunnelling microscopy (STM) has also been utilized to study surface chemical reactions by exploiting its ability to probe changes in electronic structure and large geometric changes of species reacting at a surface.…”

Effect of Oxygen Vacancies on Photocatalytic Efficiency of TiO2Nanotubes Aggregation

“…25 It is generally accepted that O vacancies enhance the reactivity of the TiO 2 (110) surface. 24,[26][27][28] The correlation between oxygen vacancies and photocatalytic property of TNTs may be attributed to two aspects reasons. One is that oxygen vacancies might have affected results on water molecules adsorption and increase of the hydroxyl concentration.…”

Visible Light Photocatalytic Properties of Modified Titanium Dioxide Nanoparticles via Aluminium Treatment

“…As reported, oxygen deficiencies in TiO2 are reflected in a pronounced color change from transparent to light, and dark blue, which introduce changes in electronic structure (in particular a bandgap feature at ∼0.8 eV below EFermi) [10]. However, upon exposure to molecular oxygen gas even at room temperature, the oxygen defect stated disappears [11,12] due to dissociation of the gaseous oxygen and filling of the oxygen vacancies [6]. In the case of modified TiO2 via Al treatment, oxygen deficiencies was appeared after the process with the color change from white to grayish; and then after a week placed in the open air, the oxygen gas from its surrounding started to enter the oxygen vacancies (color changes were observed).…”

“…Oxygen vacancies caused by Al reduction contribute to the colour change in modified TiO2 and enhancement in the optical absorption properties which lead to the increased amount of excited electrons and holes. The presence of oxygen vacancies were also reported to cause trapped states in the forbidden bandgap, 0.75-1.18 eV away from the conduction band minimum (CBM) [12,21,22]. On the other hand, Al is also found in the samples and it may attached as a dopant.…”