vanadium local structure and photoinduced charge …...vanadium local structure and photoinduced...
TRANSCRIPT
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