Two-photon photoemission spectroscopy ofTiO2(110)surfaces modified by defects andO2

Abstract

Two-photon photoemission (2PP) spectra of TiO 2 ͑110͒ surfaces are measured for the nearly perfect surface, and surfaces modified by introduction of defects and adsorbed molecules. Defects are generated on nearly perfect surfaces by three methods: electron irradiation, annealing in vacuum, and Ar + sputtering. Nearly perfect or damaged surfaces can be further modified by adsorption of O 2 or H 2 O molecules. 2PP spectroscopy is used to systematically investigate the work function change due to the presence of defects or adsorbates. 2PP spectroscopy detects both surface and bulk oxygen vacancy defects. We find from the results on oxygen adsorption that oxygen vacancies created by electron irradiation are localized on the surface and may be removed by O 2 adsorption at 100 K. The surface defects are substantially different from those created by annealing or by ion sputtering where vacancies in the subsurface region are proposed. We find that O 2 acts as an acceptor molecule on surface defect states whereas H 2 O acts as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of H 2 O adsorbed on TiO 2 surface is derived to be 0.5 D positive outward. We also find an unoccupied electronic state 2.45 eV above the Fermi level that appears at submonolayer coverage of H 2 O, which we tentatively assign to charge transfer from surface titanium ions to the surface-adsorbed H 2 O molecules or OH ligands.

Surface photochemistry probed by two-photon photoemission spectroscopy

“…This feature is not present when the spectrum is taken using s-polarized light. This suggests that the transition moment of the excited electronic state is along the surface normal, 27 implying that the new feature is a surface electronic resonance state. This feature was not observed in the one-photon photoemission spectra, indicating that it arises from an initially unoccupied excited electronic state.…”

“…The model also shows BBO rows exhibiting frequent vacancies, i.e., bridge-bonded oxygen vacancies (BBOv's), which are often preferential adsorption sites on this surface. BBOv's can be easily introduced by thermal annealing, 48,50-52,57 ion sputtering 27,58,59 or electron bombardment. 50,60,61 Besides BBOv's, ''near surface defects'' such as Ti interstitials and subsurface oxygen vacancies also exist.…”

“…[20][21][22][23][24][25][26] Recently, the application of the 2PPE technique has also been extended to the study of excited electronic structure and dynamics of adsorbates on metal oxide surfaces, [27][28][29] which serve as models for practical catalytic or photocatalytic systems. The application of 2PPE in the study of metal oxide systems provides us with some hope that this method could be used to study photocatalytic processes at surfaces.…”

Band-Gap States of TiO2(110): Major Contribution from Surface Defects

“…8 Though the method used in this work for O br vacancy creation, i.e., low energy electron (75 eV) bombardment, has been argued to cause subsurface damage, 9 two-photon photoemission (2PPE) results have revealed that it had no effect on the subsurface structure. 10,11 Whereas the study of hydroxylated surface involved the exposure of oxygen which leads to complicated species on TiO 2 (110). 8,9 A recent resonant photoelectron diffraction (RPED) study shows that O br vacancy is the sole origin of the band gap states on TiO 2 (110).…”

Spectroscopy and control of near-surface defects in conductive thin film ZnO

“…34,35 Beyond these slow electrons, a Since 2PPE simultaneously probes both the occupied and unoccupied levels, the origin of a spectroscopic feature can be identified by measuring the final state energy as a function of photon energy. To this end, the excitation energy was varied over a range of 600 meV.…”