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  • Vanadium local structure and photoinduced charge transfer in V-TiO2 by

    X-ray absorption spectroscopy

    Z. El Koura,1 G. Rossi,2 M. Calizzi,2 L. Amidani,3 F. Boscherini,2

    A. Miotello,1 L. Pasquini2

    1Department of Physics, University of Trento 2Department of Physics and Astronomy, University of Bologna 3European Synchrotron Radiation Facility, Grenoble

    • Intro: TiO2-based photocatalysts, 1 st and 2nd generation

    • Deposition and structure of V-TiO2 thin films • X-ray Absorption Spectroscopy: results and simulations • Laser on / off differential RIXS: V→Ti charge transfer • Conclusions

    Outline

  • Semiconductor photocatalysts

    2

    X. Chen, M. Grätzel et al., Chem Soc Rev (2012)

    Photo-electrochemical cell (PEC) with oxide photoanode for water

    splitting

    Photo-generated ●OH, ●O2

    -, H2O2 →mineralization of

    organic contaminants / inactivation of

    microorganisms

    J Mat Chem 20 (2010)

    TiO2 – based photocatalysts

    Photo-catalytic CO2 reduction

    Habisreutinger et al., Angew. Chem (2013)

  • Semiconductor photocatalysts

    3

    X. Chen, M. Grätzel et al., Chem Soc Rev (2012)

    Photo-electrochemical cell (PEC) with oxide photoanode for water

    splitting

    Photo-generated ●OH, ●O2

    -, H2O2 →mineralization of

    organic contaminants / inactivation of

    microorganisms

    J Mat Chem 20 (2010)

    TiO2 – based photocatalysts

    Photo-catalytic CO2 reduction

    Habisreutinger et al., Angew. Chem (2013)

    Fundamental steps involved:

    • Generation of charge carriers by photo-excitation

    • Separation and migration to trapping sites

    • Interfacial charge transfer

    Common requirements:

    • Absorption in the solar spectral range

    • Energy of conduction band and valence band adapted

    to the reduction / oxidation potentials

    • Catalytic efficiency / fast reaction kinetics

  • 1st generation benchmark: TiO2

     long-term photostability and inertness to chemical environments

     earth abundant material, non-toxic

     CB / VB energy suitable for water splitting

     absorbs only a small portion of the solar spectrum

    I41/amdP42/mnm

    3.0 - 3.2 eV

    TiO2 –based photocatalysts 4

  • 2nd generation photocatalysts: «doped» TiO2

    5TiO2 –based photocatalysts

  • Electronic structure of metal-ion doped TiO2

    6

    the position of the dopant’s t2g state in the octahedral field determine which optical transitions occur: from the

    dopant into TiO2 states or from TiO2 states into the dopant. While both types

    of transitions may occur using visible light, both do not have the same

    potential for redox

    T. Umebayashi et al, J Phys Chem Sol (2002)

    TiO2 –based photocatalysts

  • Deposition of NPs and NPs-assembled V-TiO2 films

    Deposition of V-TiO2 thin films 7

    • V / T ratio: from 3 to 8 at.% • Anatase / rutile mixture • Visible-light absorption ↑ and B.G. ↓

    decreases with V ↑

    G. Rossi et al, J. Phys. Chem. C 120 (2016)

    Gas-phase condensation @ DIFA, UniBO + annealing in air

    Convection / forced flow

    Ti-V alloy

  • Structure of NPs and NPs-assembled V-TiO2 films

    Deposition of V-TiO2 thin films 8

    G. Rossi et al, J. Phys. Chem. C 120 (2016)

    A: 10 nm, mainly anatase B: 20 nm, mainly rutilesource T

  • Structure of NPs and NPs-assembled V-TiO2 films

    Deposition of V-TiO2 thin films 9

    G. Rossi et al, J. Phys. Chem. C 120 (2016)

    source T

    rutile

    anatase

    HRTEM of V-TiO2 NPs - courtesy of A. Migliori, IMM CNR BO

  • Deposition of V-(N)-TiO2 compact thin films

    Deposition of V-TiO2 thin films 10

    RF magnetron sputtering of a composite TiO2 / V target @ Dept. Physics, UniTN

    50 °C: amorphous substrateT 350 °C: anatase

    amorphous

    V-N-codoped

    V-doped

    • V / T ratio: from 2.5 to 4.5 at.% • N codoping by reactive sputtering with

    Ar/N2 mixture (N/O ~ 4 at.%)

    El Koura et al, Int. J. Nanotechnol. 11 (2014)

  • Deposition of V-(N)-TiO2 compact thin films

    Deposition of V-TiO2 thin films 11

    Enhanced visible-light absorption and photo-catalytic activity by V and V-N doping

    Z. El Koura et al, Int. J. Nanotechnol. 11 (2014) N. Patel et al, Appl. Catal. B 150-151 (2014)

    Aim: characterize the local structure and oxidation state of V with “bulk” sensitivity

  • X-ray Absorption Fine Structure (XAFS)

    12

    From C. Milne, SwissFEL Conceptual Design Report (2013)

    X-ray Absorption Spectroscopy

  • XAS on V-TiO2 compact films

    13

    Z. El Koura et al, submitted

    Ti K-edge V K-edge

    V-TiO2 crystalline

    V-TiO2 amorphous

    V-TiO2 crystalline

    V-TiO2 amorphous

    X-ray Absorption Spectroscopy

    BM23 @ ESRF

    Main edge: V similar to shifted Ti (anatase)

    V pre-edge: similar to VO2

  • XAS on V-TiO2 compact films

    14X-ray Absorption Spectroscopy

    Z. El Koura et al, submitted

    amorph.

    cryst.

    V-N

    The strong similarity between V and Ti (shifted) K main edges holds for all samples

  • V site: experiment vs ab initio DFT calculations

    15

    electron density maps

    anatase cell

    V OTi

    • substitutional cation • 4+ oxidation state

    X-ray Absorption Spectroscopy

    Quantum Espresso (structure)

    FDMNES (XANES)+

  • XAS on V-TiO2 NPs-assembled films

    16X-ray Absorption Spectroscopy

    G. Rossi et al, J. Phys. Chem. C 120 (2016)

    BM23 @ ESRF: Ti K-edge XANES and EXAFS

    4964 4972

    C ro

    s s s

    e c tio

    n (

    a rb

    . u n

    .)

    Photon energy (eV)

    a

    r

    2

    5

    A 1

    A 2

    A 3

    B

    4960 4980 5000 5020

    C ro

    ss s

    e c ti o

    n (

    a rb

    . u

    n .)

    Photon energy (eV)

    a

    r

    2

    5

    0 1 2 3 4 5 6

    M a

    g . F

    o u .

    T ra

    n s f.

    [  (k

    ) k

    3 ]

    R (Å)

    r

    a

    2

    5

    A A

    A

    B

    B

    B

  • XAS on V-TiO2 NPs-assembled films

    17X-ray Absorption Spectroscopy

    G. Rossi et al, J. Phys. Chem. C 120 (2016)

    BM23 @ ESRF: V K-edge XANES

    5460 5480 5500 5520

    C ro

    s s

    se ct

    io n (

    a rb

    . u

    n .)

    Photon energy (eV)

    2

    5

    r+500 eV

    a + 500 eV

    VO 2

    V 2 O

    5

    V 2 O

    3

    A

    B

  • V oxidation state: NPs-assembled vs compact films

    18X-ray Absorption Spectroscopy

    In NPs-assembled films, both V4+ and V5+ cations, the latter likely located at near-surface sites, contribute to the pre-edge peak

    NPs-assembled

    compact

  • Combining X-ray Absorption and Emission: RIXS

    19Laser on/off differential RIXS

    RIXS plane

    1. high resolution diagonal cut of RIXS plane

    what do we gain combining XAS & XES?

    4968 4972 4976

    2. in TiO2 special sensitivity to localization of final states

    Ti 3d

  • Laser On/Off differential RIXS

    20Laser on/off differential RIXS

    Experiment @ ID26, ESRF

    • Ti and V edges shift in opposite directions! • Evidence for charge transfer: oxidation of V / reduction of Ti • The effect is clearly visible in NPs but quite weak in compact films

  • Conclusions and Outlook

    21Conclusions

    CB

    VB

    V 4+/5+

    Ti trap

    V →Ti e- transfer Long-lived (ms) e- trapping in Ti sites

    Substitutional V V5+ at the surface, V4+ in bulk

    Future activities: N K-edge XANES with soft X-rays

    Time resolved pump/probe experiments

    Thanks for your kind attention

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