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 Trento2Department of Physics and Astronomy, University of Bologna3European Synchrotron Radiation Facility, Grenoble

• Intro: TiO2-based photocatalysts, 1st 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

VOTi

• 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

Cro

ss s

ectio

n (

arb

. un

.)

Photon energy (eV)

a

r

2

5

A1

A2

A3

B

4960 4980 5000 5020

Cro

ss s

ectio

n (

arb

. u

n.)

Photon energy (eV)

a

r

2

5

0 1 2 3 4 5 6

Ma

g. F

ou.

Tra

nsf.

[(k

) k

3]

R (Å)

r

a

2

5

AA

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

Cro

ss

sect

ion (

arb

. u

n.)

Photon energy (eV)

2

5

r+500 eV

a + 500 eV

VO2

V2O

5

V2O

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- transferLong-lived (ms) e- trapping in Ti sites

Substitutional VV5+ 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