Location via proxy:   [ UP ]  
[Report a bug]   [Manage cookies]                

Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2

Chiara Bigi, Zhenkun Tang, Gian Marco Pierantozzi, Pasquale Orgiani, Pranab Kumar Das, Jun Fujii, Ivana Vobornik, Tommaso Pincelli, Alessandro Troglia, Tien-Lin Lee, Regina Ciancio, Goran Drazic, Alberto Verdini, Anna Regoutz, Phil D. C. King, Deepnarayan Biswas, Giorgio Rossi, Giancarlo Panaccione, and Annabella Selloni
Phys. Rev. Materials 4, 025801 – Published 28 February 2020
PDFHTMLExport Citation
Abstract
Authors
Article Text
  • INTRODUCTION
  • EXPERIMENTAL AND COMPUTATIONAL METHODS
  • EXPERIMENTAL RESULTS
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
    • Supplemental Material
    References

    Abstract

    Two-dimensional (2D) metallic states induced by oxygen vacancies (VOs) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of VO-induced states at the (001) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface VOs and remain localized at the surface, where they can promptly react with O2. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with O2 is kinetically hindered on anatase TiO2 (001), thus making them much less sensitive to oxygen dosing.

    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    • Figure
    • Received 30 September 2019
    • Revised 3 February 2020
    • Accepted 10 February 2020

    DOI:https://doi.org/10.1103/PhysRevMaterials.4.025801

    ©2020 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electronic structureSurface & interfacial phenomena
    1. Physical Systems
    2-dimensional systemsThin filmsTransition metal oxides
    1. Techniques
    Density functional theoryPhotoemission spectroscopy
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Chiara Bigi1,2, Zhenkun Tang3,4, Gian Marco Pierantozzi1, Pasquale Orgiani1,5, Pranab Kumar Das1,6, Jun Fujii1, Ivana Vobornik1, Tommaso Pincelli1, Alessandro Troglia1,2, Tien-Lin Lee7, Regina Ciancio1, Goran Drazic8, Alberto Verdini1, Anna Regoutz9, Phil D. C. King10, Deepnarayan Biswas10, Giorgio Rossi1,2, Giancarlo Panaccione1,*, and Annabella Selloni3,†

    • 1Istituto Officina dei Materiali (IOM)-CNR, Laboratorio TASC, in Area Science Park, S.S.14, Km 163.5, I-34149 Trieste, Italy
    • 2Dipartimento di Fisica, University of Milano, Via Celoria 16, I-20133 Milano, Italy
    • 3Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
    • 4Department of Physics, Hengyang Normal University, 16 Heng-Hua Road, Zhu-Hui District, Hengyang 421008, People's Republic of China
    • 5CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, 84084 Fisciano, Italy
    • 6International Centre for Theoretical Physics (ICTP), I-34100 Trieste, Italy
    • 7Diamond Light Source, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom
    • 8Department for Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, SI-1001 Ljubljana, Slovenia
    • 9Department of Materials, Imperial College London, South Kensington, London SW7 2AZ, United Kingdom
    • 10SUPA, School of Physics and Astronomy, University of St. Andrews, St. Andrews KY16 9SS, United Kingdom

    • *giancarlo.panaccione@elettra.eu
    • †aselloni@princeton.edu

    Article Text (Subscription Required)

    Click to Expand

    Supplemental Material (Subscription Required)

    Click to Expand

    References (Subscription Required)

    Click to Expand
    Issue

    Vol. 4, Iss. 2 — February 2020

    Reuse & Permissions
    Access Options
    CHORUS

    Article Available via CHORUS

    Download Accepted Manuscript
    Author publication services for translation and copyediting assistance advertisement

    Authorization Required

    Log In
    Other Options
    ×

    Images

    • Figure 1
      Figure 1

      (a) Representative high-resolution Z-contrast image of the TiO2/LAO interfacial region. (b) LEED pattern (∼110eV) showing the (1×4)−(4×1) surface reconstruction of the anatase thin films. (c) Sketch of the first BZ of anatase. (d) Fermi surface contour, measured at 46 eV photon energy, covering the first BZ, obtained by superimposing the Fermi surfaces measured with different light polarizations. (e), (f) ARPES spectra of the metallic state acquired at hν=46eV photon energy around the Γ¯ point of the second Brillouin zone (Γ¯1): (e) 2DEG of anatase film with high amount of oxygen vacancies after the annealing treatment, (f) 2DEG of as-grown film. (g) MDCs at the Fermi level, in correspondence with the straight lines in panels (e) and (f) for the annealed (red) and as-grown (blue) samples, respectively; the dashed lines highlight the position of the Fermi momenta kF. (h) EDCs extracted at the kF of the outer band for the annealed (red) and the as-grown (blue) samples.

      Reuse & Permissions
    • Figure 2
      Figure 2

      DOS of anatase films deposited on LAO and measured at (a) APE beamline (Elettra, hv=46eV) in the highest beam flux condition available and (b) I09 beamline (Diamond Light Source, hv=120eV); in both panels, arrows indicate the position of the IGs for the as-grown and annealed samples, as discussed in the main text.

      Reuse & Permissions
    • Figure 3
      Figure 3

      ARPES spectra of a pristine as-grown (a) sample and (b) with molecular O2 dosing. (c) MDCs extracted at EF (red and light blue straight lines in panels (a) and (b), respectively, indicate where the MDCs have been extracted). (d) Evolution of the DOS upon oxygen dosing (red line corresponds to partial pressure P=1×10−10 mbar, light blue P=1×10−9 mbar and dark blue P=4×10−9 mbar).

      Reuse & Permissions
    • Figure 4
      Figure 4

      (a) XAS spectra at Ti L2,3 of a pristine (red) and during oxygen dosing (blue); (b) RESPES in the second BZ upon oxygen dosing (color map—central panel—displays the momentum-integrated photoemission intensity; the resonant DOS curves show the different resonating energies for the IG and the 2DEG states (red arrows) are reported on the right.

      Reuse & Permissions
    • Figure 5
      Figure 5

      (a), (b) DOS evolution at the Fermi level as a function of the photon energy before and during oxygen dosing. (c) Resonant angle-resolved-photoemission (RESARPES) spectra acquired while dosing oxygen. (d), (e) Intensity of the IG and 2DEG states across the Ti L3−eg absorption edge, respectively.

      Reuse & Permissions
    • Figure 6
      Figure 6

      (a) Side view of the reconstructed anatase TiO2(001)-1×4 slab model; the investigated oxygen vacancy sites are indicated; Ti and O atoms are light blue and red, respectively. (b) VO formation energies (eV; blue bars) at different surface and subsurface oxygen sites computed using DFT-PBE; (c) Electrostatic potential profile in the surface region, computed from the shift of the Ti3s peak in the different Ti layers of the pristine and reduced slabs with VO1, VO4, or VO7 defects. Here, layer 0 corresponds to the ridge Ti4c sites, layer 1 to the terrace Ti5c sites, and so forth. The yellow shading highlights the region of negative (attractive) potential; (d)–(i) Charge density contours of the excess electron states induced by VO1, VO3, VO4, VO5, VO6, and VO7, respectively; the vacancy positions are indicated by dashed red circles; dashed black lines show the unit cell used for the calculations. Additional density contours are shown in Supplemental Material [31].

      Reuse & Permissions
    • Figure 7
      Figure 7

      (a) O2 adsorption energy as a function of the subsurface (VO4 VO7) or surface (VO3) oxygen vacancy location. Relevant structures with a subsurface VO, denoted VOn+O2* (n=4−7), are shown in panels (c)–(f). For VO3, two nearly degenerate structures are present, as shown in panels (g) and (h), where O* indicates an oxygen adatom and (O2)O a bridging peroxide replacing an O2c. The energy zero corresponds to the adsorption energy of VO7+O2*. (b) Energy barrier for the diffusion of an O vacancy from VO4 to VO3 in the presence of adsorbed oxygen. (c)–(h) Atomic geometries of adsorbed O2 on reduced anatase (001) with a subsurface (VO4–VO7) or surface (VO3) oxygen vacancy, as described in (a). Ti atoms are blue, O atoms are red, adsorbed O2 is orange; dashed red circles indicate the positions of the vacant sites.

      Reuse & Permissions
    ×

    Sign up to receive regular email alerts from Physical Review Materials

    Log In

    Cancel
    ×

    Search

    Article Lookup

    Paste a citation or DOI
    Enter a citation
    ×