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Title: Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy

Abstract

TiO2 is a wide-band gap semiconductor and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n– clusters for n = 1–10 have been obtained at 193 (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n– clusters for n > 1 appears to be localized in a tricoodinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanisticmore » understanding of TiO2 surface defects and photocatalytic properties.« less

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
918463
Report Number(s):
PNNL-SA-53623
Journal ID: ISSN 0002-7863; JACSAT; 20901; KC0301020; TRN: US0805829
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society, 129(10):3022-3026; Journal Volume: 129; Journal Issue: 10
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; ANIONS; DEFECTS; ELECTRONIC STRUCTURE; ELECTRONS; GEOMETRY; MOLECULAR MODELS; PHOTOCATALYSIS; PHOTOELECTRON SPECTROSCOPY; PHOTONS; SPECTRA; TITANIUM OXIDES; Environmental Molecular Sciences Laboratory

Citation Formats

Zhai, Hua-jin, and Wang, Lai S. Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy. United States: N. p., 2007. Web. doi:10.1021/ja068601z.
Zhai, Hua-jin, & Wang, Lai S. Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy. United States. doi:10.1021/ja068601z.
Zhai, Hua-jin, and Wang, Lai S. Wed . "Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy". United States. doi:10.1021/ja068601z.
@article{osti_918463,
title = {Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n- (n=1-10) Using Photoelectron Spectroscopy},
author = {Zhai, Hua-jin and Wang, Lai S.},
abstractNote = {TiO2 is a wide-band gap semiconductor and it is an important material for photocatalysis. Here we report an experimental investigation of the electronic structure of (TiO2)n clusters and how their band gap evolves as a function of size using anion photoelectron spectroscopy (PES). PES spectra of (TiO2)n– clusters for n = 1–10 have been obtained at 193 (6.424 eV) and 157 nm (7.866 eV). The high photon energy at 157 nm allows the band gap of the TiO2 clusters to be clearly revealed up to n = 10. The band gap is observed to be strongly size-dependent for n < 7, but it rapidly approaches the bulk limit at n = 7 and remains constant up to n = 10. All PES features are observed to be very broad, suggesting large geometry changes between the anions and the neutral clusters due to the localized nature of the extra electron in the anions. The measured electron affinities and the energy gaps are compared with available theoretical calculations. The extra electron in the (TiO2)n– clusters for n > 1 appears to be localized in a tricoodinated Ti atom, creating a single Ti3+ site and making these clusters ideal molecular models for mechanistic understanding of TiO2 surface defects and photocatalytic properties.},
doi = {10.1021/ja068601z},
journal = {Journal of the American Chemical Society, 129(10):3022-3026},
number = 10,
volume = 129,
place = {United States},
year = {Wed Mar 14 00:00:00 EDT 2007},
month = {Wed Mar 14 00:00:00 EDT 2007}
}
  • In an effort to elucidate the variation of the electronic structure as a function of oxidation and composition, we investigated an extensive series of dichromium oxide clusters, Cr2On- (n = 1-7) using photoelectron spectroscopy (PES). Well-resolved PES spectra were obtained at several photon energies. While low photon energy spectra yielded much better spectral resolution, high photon energy data allowed both Cr 3d- and O 2p-dreived detachment features to be observed. The overall spectral evolution of Cr2On– exhibits a behavior of sequential oxidation with increasing oxygen content, where low binding energy Cr 3d-based spectral features diminish in numbers and the spectramore » shift towards higher binding energies as a result of charge transfer from Cr to O. Evidence was obtained for the population of low-lying isomers for Cr2O2-, Cr2O3-, and Cr2O6-. The current data are compared with previous studies and with related studies on W2On- and Mo2On-.« less
  • Photoelectron spectroscopy has been conducted for a series of (CrO3)n- (n = 1-5) clusters and compared with density functional calculations. Well-resolved photoelectron spectra were obtained for (CrO3)n- (n = 1-5) at 193 nm (6.424 eV) and 157 nm (7.866 eV) photon energies, allowing for accurate measurements of the electron binding energies, low-lying electronic excitations for n = 1 and 2, and the energy gaps. Density functional and molecular orbital theory (CCSD(T)) calculations were performed to locate the ground and low-lying excited states for the neutral clusters and to calculate the electron binding energies of the anionic species. The experimental andmore » computational studies firmly establish the unique low-spin, non-planar, cyclic ring structures for (CrO3)n and (CrO3)n- for n ≥ 3. The structural parameters of (CrO3)n are shown to converge rapidly to those of the bulk CrO3 crystal. The extra electron in (CrO3)n- (n ≥ 2) is shown to be largely delocalized over all Cr centers, in accord with the relatively sharp ground state photoelectron bands. The measured energy gaps of (CrO3)n exhibit a sharp increase from n = 1 to n = 3 and approach to the bulk value of 2.25 eV at n = 4 and 5, consistent with the convergence of the structural parameters.« less
  • We report a photoelectron spectroscopic investigation of mono-nitrogen doped aluminum cluster anions Al{sub n}N{sup -} (n = 2-22). Well-resolved spectra were obtained at three photon energies (355, 266, and 193 nm), revealing the structural and electronic evolution as the number of aluminum atoms increases in the doped clusters. For small AlnN (n < 9) clusters, the Al atoms may be viewed to be monovalent, similar to pure aluminum clusters. Even-odd alternation of the electron affinities was observed for Al{sub n}N clusters, suggesting that neutral clusters with odd n are closed shell and those with even n are open shell. Themore » most interesting observation is the similarity between the spectra of Al{sub n}N{sup -} and Al{sub (n-1)}{sup -} for n > 12. This observation suggests that these clusters can be described as (AlN)Al{sub (n-1)}{sup -}, i.e., an AlN unit weakly interacting with AlAl{sub (n-1)}{sup -} clusters. The electronic and atomic structural implications of this observation are discussed.« less
  • We report a photoelectron spectroscopic investigation of a series of monoiron-sulfur clusters FeSn- (n= 1-6) at various photon energies. Vibrationally resolved spectra were measured for FeS- and FeS3-. A wealth of electronic structure information was obtained for FeS and were tentatively assigned, yielding a (5)Delta ground state for FeS and a (7)Sigma(+) and a (5)Delta excited state at 0.675 and 1.106 eV above the ground state, respectively. Franck-Condon factor simulations were performed for the vibrationally resolved (5)Delta ground state and the (5)Delta excited state, yielding an Fe-S bond length of 2.18 and 2.29 Angstrom for the anion ground state andmore » the (5)Delta excited state, respectively, as well as a vibrational temperature of 180 K for the anion. The electron affinities (EA's) of FeSn were measured to be 1.725+/- 0.10, 3.222+/- 0.009, 2.898+/- 0.008, 3.129+/- 0.008, 3.262+/- 0.010, and 3.52+/- 0.02 eV for n= 1-6, respectively. A significant EA increase was only observed from FeS to FeS2, whereas all larger species FeSn (n= 3-6) possess EA's similar to that of FeS2 within+/-0.3 eV. By comparing the trend of EA in FeSn to that of FeOn, we proposed that all the FeSn- (n > 1) species take (S-m(2-))Fe3+(S-n-m(2-)) type structures, in which Fe assumes its favorite+3 formal oxidation state. Preliminary density functional calculations were carried out and the obtained structures support the proposed structural evolution of the FeSn clusters.« less
  • The electronic structure of two series of small titanium oxide clusters, TiO{sub y} (y=1{endash}3) and (TiO{sub 2}){sub n} (n=1{endash}4), is studied using anion photoelectron spectroscopy. Vibrationally resolved spectra are obtained for TiO{sup {minus}} and TiO{sub 2}{sup {minus}}. Six low-lying electronic states for TiO are observed with five of these excited states resulting from multielectron transitions in the photodetachment processes. TiO{sub 2} is found to be closed-shell with a 2 eV highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO-LUMO) gap. The two lowest triplet and singlet excited states of TiO{sub 2} are observed with excitation energies at 1.96 and 2.4 eV,more » respectively. TiO{sub 3} is found to have a very high electron affinity (EA) of 4.2 eV, compared to 1.30 and 1.59 eV for TiO and TiO{sub 2}, respectively. The larger (TiO{sub 2}){sub n} clusters are all closed-shell with HOMO-LUMO gaps similar to that of TiO{sub 2} and with increasing EAs: 2.1 eV for n=2, 2.9 eV for n=3, 3.3 eV for n=4. The small HOMO-LUMO gaps for the clusters compared to that of bulk TiO{sub 2} are discussed in terms of the structure and bonding of these clusters. {copyright} {ital 1997 American Institute of Physics.}« less