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AdvancedMaterialsOpticalDiagnostics group
Nonlinear Optics with Nanostructured TiO²
A.GALAS and V.GAYVORONSKY
Institute of Physics NASU, pr. Nauki 46, 03028 Kiev, Ukraine; E-mail: vlad@iop.kiev.ua ; Tel: (380) 44 265 08 14
JASS’04 , S.-Petersburg, Russia, 28 March - 7 April 2004
AMOD group members
From left to right:A.Galas, E.Shepelyavy, V.Kalicev, V.Gayvoronsky
F.KochTechnical University of Munich, Physics Department E16, 85748
Garching, Germany
In collaboration with
V.TimoshenkoMoscow State University, Physics Department, 119992 Moscow, Russia
Outline
1. Introduction
2. Samples characterization • Sol-gel synthesis• Structural characterization• Optical and electron properties
3. Nonlinear optical (NLO) monitoring of anatase nanoparticles
• NLO refraction and absorption• Giant NLO response ((3) ~ 10-5 esu)• Monitoring of photocatalytic activity with NLO response
4. Conclusions
Porous TiO2 applications:
• - dye-sensitized solar cells
• (Graetzel cell) low cost, high efficiency, exceptional stability
• - sensors
• hydrogen, ethanol, humidity, oxygen, combustion fuel sensors
• - photocatalysis
• photocatalytic production of hydrogen and methane from ethanol and water water, air and wastewater treatment
• - thin film capacitors, gate electrodes for MOS devices
• high dielectric constant ~ 90
• - interference filters, optical waveguides
• large refractive index
• - pigment for the paint and plastics
• from house paint to type correction fluid
• - model system for the nanoporous materials research (electron transport, optical properties)
• excellent reproducibility by oxidation/reduction cycle
Applications of TiO2 photocatalyst:
U. Diebold/Surface Science Reports 48 (2003) 53-229
Cell dimensions
rutile a = b = 4.587 Å, c = 2.953 Å
anatase a = b = 3.782 Å, c = 9.502 Å.
Bulk structures of rutile and anatase.
TiO2 (anatase) nanoparticle samples characterization
Nanoparticle TiO2 layers on glass substrate were prepared in Institute of Surface Chemistry NASU (Kiev) with film drawing from viscous solution (precursor). The precursor was prepared with sol-gel technique using Titanium(IV) isopropoxide, acetic acid, -terpineol (to control viscosity). Polyethylenе glycol with molecular weights 300 (PEG 300) and 1000 (PEG 1000) were used as pore and complexing agents.
The drawing layers on glass substrate were treated 1 hour at 5000 C. Multilayer films are annealed at the same conditions after each layer deposition. Thicknesses 100 – 1000 nm, porosity 34-39%
XRD -TiO2 layers contain nanocrystals of only single phase – anatase TEM – nanoparticle mean diameter 16 nm (distribution 5 - 30 nm)
Samples TiO2 TiO2(300) TiO2(1000) Complexing agent any PEG 300 PEG 1000
TEM, HRTEM and Electron Diffraction data for anatase nanoparticle films
TiO2(1000)
TiO2(300)
Size distribution in anatase nanoparticle films
0 5 10 15 20 25 30 350
5
10
15
20
Par
ticle
s nu
mbe
r
particle size, nm0 5 10 15 20 25 30
0
5
10
15
20
Par
ticle
s nu
mbe
rparticle size, nm
5%
TiO2(1000) TiO2(300)
Optical parameters characterization
•Absorption and reflection spectra•Refractive index dispersion•Angular resolved light scattering•Nonlinear refraction
•Photovoltage measurements•Ellipsometry•Photoluminescence•Nonlinear absorption/saturation
Transmission spectraof single and double layers TiO2 films on glass substrate versus light photon energy and refractive index dispersion curves
1 2 3 40.0
0.2
0.4
0.6
0.8
1.0
2.0
2.2
2.4
1064 nm
refractive index
double layer,d=360 nm
single layer,d=180 nm
Tra
nsm
ittan
ce, %
hv, eV
TiO2(1000) n, r
efra
ctiv
e in
dex
1 2 3 40.0
0.2
0.4
0.6
0.8
1.0
2.0
2.2
2.4
TiO2(300)1064 nm
refractive index
double layer,d=240 nm
single layer,d=120 nm
Tra
nsm
ittan
ce, %
hv, eV
n, r
efra
ctiv
e in
dex
Eg
indirect
direct3.4 eV
3.6 eV
Z-scan technique for the nonlinear optical response measurements
Refractive index NLO variation n > 0, n ~ (3)I, I - laser intensity, (3) - cubic nonlinearity
-1 0 1 Z/Z0
Tpv
On-axis transmittance in far field
Z0 – diffrational length at the beam waist
Ultrafast optical nonlinearity in polymethyl-methacrylate-TiO2 nanocomposites
Z-scans performed with 780 nm, 250 fs laser pulses
The two photon coefficient and nonliner refractive index n2 values plotted as a function of the weight percentage of Ti-iP in PMMA
NLO response time ~1.5 ps
NLO absorptionIm((3))=0.8910-9 esu
=1.4103 cm/GW~ 100 for a rutile @ 532 nm
NLO refractionRe((3))=1.710-9 esu n2=2.510-2 cm2/GW
~ 100n2 for rutile @ 1.06 m
H. I. Elim et.al., Applied Physics Letters 28 (2003) 2691-2693
S - sample, A – the beam attenuator, L – focusing lens with focal length f, Sp – beam splitters, D – diaphragm in the far field, P1, P2
and P3 – photodiodes, r – transverse coordinate.Dashed line – laser beam propagation without a sampleSolid line - focused by a sample beam
Setup for the laser beam selfaction effect research
f
Sp
Sp
SDLAr
P3
P2P1
f
Sp
Sp
DLAr
P3
P2P1
Total transmittance and normalized on-axis transmittance in far field
77
78
79
80
81
0 20 40 60 80 10071
72
73
74
75
double layer
single layer
Tot
al tr
ansm
ittan
ce, %
Laser Intensity, MW/cm2
0 20 40 60 80 1001.00
1.02
1.04
1.06
1.08
1.10
On-
axis
tran
smitt
ance
, arb
.un.
single layer
Laser Intensity, MW/cm2
double layer
Single layer d = 180 nmDouble layer d = 360 nm
p= 40 ps
Giant NLO Response
(3) ~ 2 ·10-5 esu
of TiO2(1000) films versus input laser intensity at =1064 nm.
Giant NLO Response
WHY Giant ?Bulk TiO2 - (3) ~ 10-11 esu
Thin TiO2 films - (3) ~ 10-9 esu
Our nanoparticle TiO2 films - (3) ~10-5 esu
(3) ~ 10-5 esu
Total transmittance and normalized on-axis transmittance in far field.
77
78
79
80
81
1 10 100
62
64
66
68
70
TiO2(1000)
TiO2(300)
Tot
al tr
ansm
ittan
ce, %
Laser Intensity, MW/cm2
1 10 1001.0
1.1
1.2
1.3
1.4
1.5
TiO2(1000)
TiO2(300)
On-
axis
tran
smitt
ance
, arb
.un.
Laser Intensity, MW/cm2
TiO2(1000) (3) ~ 2 ·10-5 esu
TiO2(300) (3) ~ 6 ·10-5 esu
Giant NLO Response
TiO2(1000) d = 360 nmTiO2(300) d = 240 nm
p= 40 ps
of TiO2(1000) and TiO2(300) films versus input laser intensity at =1064 nm
200 300 400 500 6000.0
0.5
1.0
1.5
2.0
2.5
initial
5 hours
2 hours
Op
tica
l De
nsi
ty
, nm0 50 100 150 200 250 300 350
0.0
0.2
0.4
0.6
0.8
1.0= 524 nm
Standard P-25
TiO2(1000)
TiO2(300)
OD
/OD
0
t, min
TiO2 + h h+ + e (1)R6G + h R6G* (2)
R6G* +TiO2 R6G+ +TiO2(e-) (3)
Photocatalytic activity of the anatase films
R6G water solution absorption spectra for different UV dose in TiO2 presense
Dynamics of R6G photodestruction with UV light due to the presence of TiO2 films.
R6G* + O2 R6G+ + O-2 (4)
TiO2(e-) + O2 TiO2 + O-2 (5)
R6G++O-2
destruction products (6)
Destruction of Rhodamine (R6G):
Energy band structure of nanoporous anatase.
0
1
2
3
4
180 fs << p=40 ps < 100 ps
electronslocalized
delocalized
ST-ST
ST-DT relaxation ~100 ps
CB-ST relaxation ~180 fs
Hole trapping
TPA
EF
HoleTrap (HT)
DeepTrap (DT)
ShallowTrap (ST)
Conduction Band (CB)
Valence Band (VB)
Ene
rgy,
eV
Laser quantum 1.17 eV, pulse duration~ 40 ps
Schematic diagram of possible water dissociation mechanisms on the vacancy defected TiO2(110) surfaces. Dissociation at a vacancy would result in two equivalent OH groups.
Dark atoms are Ti cations, lighter atoms are in-plane O anions. Models for water and OH are represented with covalent radii.
Physisorbtion of H2O
Chemisorbtion of H2O
Photoemission spectra (h = 35 eV, normal emission) from the valence band region of a sputtered and UHV - annealed, clean TiO2(1 1 0) surface.
U. Diebold / Surface Science Reports 48 (2003) 5-229
Defect state and molecular orbitals of adsorbed H2O
Size distribution in anatase nanoparticle films
0 5 10 15 20 25 30 350
5
10
15
20
Par
ticle
s nu
mbe
r
particle size, nm0 5 10 15 20 25 30
0
5
10
15
20
Par
ticle
s nu
mbe
rparticle size, nm
5%
TiO2(1000) TiO2(300)
(3) ~2·10-5esu (3) ~6·10-5esu
Photocatalytic activity (reference P-25 =1)
1.34
Photocatalytic activity (reference P-25 =1)
2.72
ConclusionsElectron and optical properties (refraction index, absorption, optical band gap) of nanoparticle anatase films slightly vary for the samples prepared with different comlexing agents
Giant NLO susceptibility (3)eff ~10-5 – 10-7 esu ((3) ~ 10-11 esu
for the bulk) which is sensitive to preparation technique have been observed in picosecond range in nanoparticle anatase
The NLO response can be used for the monitoring of surface states and photocatalytic activity of TiO2 based nanocomposites
Sample (3), esu Photocatalytic activity(reference P-25 =1)
TiO2(300) 610-5 2.72
TiO2(1000) 210-5 1.34
1
2
3
The work was partially supported by the grant:
DLR-BMBF UKR01/062.
Acknowledgements
We acknowledge to S.A. Nepijko for HRTEM and ED data, and
to I.Petrik, N.Smirnova, A.Eremenko for the prepared samples.