Research ArticleChemistry

Evolution of anatase surface active sites probed by in situ sum-frequency phonon spectroscopy

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Science Advances  30 Sep 2016:
Vol. 2, no. 9, e1601162
DOI: 10.1126/sciadv.1601162
  • Fig. 1 Structure and phonon spectra of anatase (101).

    (A) Structure of anatase (101). Ti(5c), O(2c), and O(3c) denote five-coordinated titanium and two- and three-coordinated oxygen ions, respectively. Yellow arrows describe the displacements of Ti(5c) and O(3c) associated with the surface phonon mode according to ab initio calculation. (B) Raman (blue) and SFG (red) spectra of the anatase (101) sample, with all beams being P-polarized. The two Raman modes are the highest-frequency transverse optical (TO) phonon modes of bulk anatase. The 860-cm−1 SFG mode is from the surface. arb. u., arbitrary units.

  • Fig. 2 Symmetry of the surface and phonon spectral anisotropy.

    (A to C) Experimental configurations with the sample placed at various azimuthal angle φ. The beam incident geometry is as described in (A), with green, red, and blue arrows denoting NIR, IR, and SFG beams, respectively, which are either P- or S-polarized. A Ti(5c)–O(3c) bond is highlighted in yellow at each φ. (D to G) SFG spectra at various φ with different beam polarization combinations (labeled in capital letters, referring to the polarization of SFG, NIR, and IR beams from left to right). Black (red) curves in (D) to (F) are spectra taken at φ = 0° (180°), and the blue (orange) curve in (G) is taken at φ = 90° (270°).

  • Fig. 3 Evolution of the surface structure in UV and different ambient gases.

    (A) Surface phonon spectra of anatase (101) in pure nitrogen before (blue) and after (green) UV irradiation. (B) Preferential locations of surface (Embedded Image) and subsurface (Embedded Image) oxygen vacancies on anatase (101), marked by dashed circles. (C) Evolution of the mode intensity in pure oxygen, saturated methanol vapor, and pure nitrogen in response to switching on and off of UV irradiation as well as oxygen purge. (D) Circles indicate energy difference between surface and subsurface oxygen vacancies [Embedded Image] in various ambient gases obtained from ab initio calculation. Vertical columns represent percentage drop of the 860-cm−1 mode after a constant UV irradiance in various ambient gases: Pure hydrogen and nitrogen were at ~1 atm; water and methanol were at saturated vapor pressure mixed with dry nitrogen. (E and F) Stable configurations of methanol and nitrogen molecules adsorbed near a surface oxygen vacancy by ab initio calculation.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1601162/DC1

    S1. Basic theory of the sum-frequency generation

    S2. Calculation of the surface phonon mode of anatase (101)

    S3. Symmetry properties and spectral anistropy of the surface phonon mode on anatase (101)

    S4. Adsorbed methanol on anatase (101)

    S5. Calculation on the stability of oxygen vacancies on anatase (101) exposed to ambient gases

    S6. The removal of hydrocarbon contaminants by UV-ozone treatments

    fig. S1. The calculated in-phase and out-of-phase surface phonon modes near 880 cm−1.

    fig. S2. SFG spectra in the C–H stretching vibration range for adsorbed methanol on anatase (101).

    fig. S3. Optimized surface structures of anatase (101) with various ambient molecules.

    fig. S4. SFG spectra of anatase (101) in the C–H stretching vibration range for hydrocarbon contaminants before and after UV-ozone treatment.

    table S1. Energy difference between Formula and Formula of anatase (101) in different ambient environment.

    References (37, 38)

  • Supplementary Materials

    This PDF file includes:

    • S1. Basic theory of the sum-frequency generation
    • S2. Calculation of the surface phonon mode of anatase (101)
    • S3. Symmetry properties and spectral anistropy of the surface phonon mode onnatase (101)
    • S4. Adsorbed methanol on anatase (101)
    • S5. Calculation on the stability of oxygen vacancies on anatase (101) exposed to ambient gases
    • S6. The removal of hydrocarbon contaminants by UV-ozone treatments
    • fig. S1. The calculated in-phase and out-of-phase surface phonon modes near 880 cm−1
    • fig. S2. SFG spectra in the C–H stretching vibration range for adsorbed methanol on anatase (101).
    • fig. S3. Optimized surface structures of anatase (101) with various ambient molecules.
    • fig. S4. SFG spectra of anatase (101) in the C–H stretching vibration range for hydrocarbon contaminants before and after UV-ozone treatment.
    • table S1. Energy difference between VsurfO and VsubO of anatase (101) in different ambient environment.
    • References (37, 38)

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