Advanced
Materials
Optical
Diagnostics group
JASS’04 , S.-Petersburg, Russia,
28 March - 7 April 2004
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
AMOD group members
From left to right:A.Galas, E.Shepelyavy, V.Kalicev, V.Gayvoronsky
In collaboration with
F.Koch
Technical University of Munich, Physics Department E16, 85748
Garching, Germany
V.Timoshenko
Moscow 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:
Bulk structures of rutile and anatase.
Cell dimensions
rutile
a = b = 4.587 Å,
c = 2.953 Å
anatase
a = b = 3.782 Å,
c = 9.502 Å.
U. Diebold/Surface Science Reports 48 (2003) 53-229
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
Complexing agent
TiO2 TiO2(300) TiO2(1000)
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
TiO2(300)
20
20
5%
15
15
10
10
5
0
Particles number
Particles number
TiO2(1000)
0
5
10
15
20
25
30
particle size, nm
35
5
0
0
5
10
15
20
25
particle size, nm
30
Optical parameters characterization
•Absorption and reflection spectra
•Refractive index dispersion
•Angular resolved light scattering
•Nonlinear refraction
•Photovoltage measurements
•Ellipsometry
•Photoluminescence
•Nonlinear absorption/saturation
Transmission spectra
of single and double layers TiO2 films on glass substrate versus light
photon energy and refractive index dispersion curves
single layer,
d=180 nm
double layer,
d=360 nm
2.2
0.4
0.2
0.0
1
1064 nm
refractive index
2
Eg
2.0
TiO2(1000)
hv, eV
3
4
indirect 3.4 eV
direct 3.6 eV
single layer,
d=120 nm
1.0
2.4
0.8
double layer,
d=240 nm
0.6
2.2
0.4
0.2
1064 nm
0.0
1
2.0
TiO2(300)
refractive index
2
hv, eV
3
4
n, refractive index
0.6
2.4
Transmittance, %
0.8
n, refractive index
Transmittance, %
1.0
Z-scan technique for the nonlinear optical
response measurements
Refractive index NLO variation n > 0,
n ~ (3)I, I - laser intensity, (3) - cubic nonlinearity
On-axis transmittance in far field
Tp
v
-1
0
1 Z/Z0
Z0 – diffrational length
at the beam waist
Ultrafast optical nonlinearity in polymethylmethacrylate-TiO2 nanocomposites
H. I. Elim et.al., Applied Physics Letters 28 (2003) 2691-2693
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 absorption
Im((3))=0.8910-9 esu
=1.4103 cm/GW
~ 100 for a rutile @ 532 nm
NLO refraction
Re((3))=1.710-9 esu
n2=2.510-2 cm2/GW
~ 100n2 for rutile @ 1.06 m
NLO response
time ~1.5 ps
Setup for the laser beam selfaction effect research
r
P1
P2
A L
Sp
f
D
S
Sp
P3
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 sample
Solid line - focused by a sample beam
75
81
74
80
Giant NLO Response
73
79
(3) ~ 2 ·10-5 esu
72
71
0
double layer
78
single layer
20
40
On-axis transmittance, arb.un.
Total transmittance, %
Total transmittance and normalized on-axis
transmittance in far field
of TiO2(1000) films versus input laser intensity at =1064 nm.
60
80
Laser Intensity, MW/cm
Single layer d = 180 nm
Double layer d = 360 nm
p= 40 ps
77
100
2
1.10
single layer
1.08
1.06
double layer
1.04
1.02
1.00
0
20
40
60
80
Laser Intensity, MW/cm
100
2
Giant NLO Response
(3) ~ 10-5 esu
WHY Giant ?
Bulk TiO2 - (3) ~ 10-11 esu
Thin TiO2 films - (3) ~ 10-9 esu
Our nanoparticle TiO2 films - (3) ~10-5
esu
81
70
TiO2(1000) d = 360 nm
TiO2(300) d = 240 nm
p= 40 ps
80
68
TiO2(1000)
66
79
64
78
TiO2(300)
62
1
10
100
Laser Intensity, MW/cm
77
2
Giant NLO Response
TiO2(1000) (3) ~ 2 ·10-5 esu
TiO2(300)
(3) ~
6
·10-5
esu
On-axis transmittance, arb.un.
Total transmittance, %
Total transmittance and normalized on-axis
transmittance in far field.
of TiO2(1000) and TiO2(300) films versus input laser
intensity at =1064 nm
1.5
TiO2(300)
1.4
1.3
1.2
TiO2(1000)
1.1
1.0
1
10
100
Laser Intensity, MW/cm
2
Photocatalytic activity of the anatase films
Destruction of Rhodamine (R6G):
TiO2 + h  h+ + e
(1)
R6G + h  R6G*
(2)
R6G* +TiO2  R6G+ +TiO2(e-) (3)
1.0
initial
2.0
1.5
2 hours
1.0
5 hours
0.5
0.0
200
400
Standard P-25
0.8
0.6
TiO2(1000)
0.4
0.2
0.0
300
500
l, nm
600
(4)
(5)
R6G++O-2  destruction products (6)
l= 524 nm
OD/OD0
Optical Density
2.5
R6G* + O2  R6G+ + O-2
TiO2(e-) + O2  TiO2 + O-2
TiO2(300)
0
50
100 150 200 250 300 350
t, min
R6G water solution absorption spectra Dynamics of R6G photodestruction with
for different UV dose in TiO2 presense UV light due to the presence of TiO2
films.
Energy band structure of nanoporous anatase.
CB-ST
4 relaxation
~180 fs
EF
Energy, eV
3
2
TPA
180 fs << tp=40 ps < 100 ps
ST-ST
Conduction Band (CB)
Shallow
delocalized
electrons
Trap (ST)
localized
Deep
ST-DT relaxation
Trap (DT)
~100 ps
1
Hole trapping
0
Hole
Trap (HT)
Valence Band (VB)
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.
Physisorbtion of H2O
Chemisorbtion of H2O
Dark atoms are Ti cations, lighter atoms are inplane O anions. Models for water and OH are
represented with covalent radii.
Defect state and molecular orbitals of adsorbed 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
Size distribution in anatase nanoparticle films
TiO2(300)
20
20
5%
15
15
10
10
5
0
Particles number
Particles number
TiO2(1000)
0
5
10
15
20
25
30
particle size, nm
(3) ~2·10-5esu
Photocatalytic activity
(reference P-25 =1)
1.34
35
5
0
0
5
10
15
20
25
particle size, nm
(3) ~6·10-5esu
Photocatalytic activity
(reference P-25 =1)
2.72
30
Conclusions
1
Electron and optical properties (refraction index,
absorption, optical band gap) of nanoparticle anatase films
slightly vary for the samples prepared with different
comlexing agents
2
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
3
(3), esu
TiO2(300)
610-5
Photocatalytic activity
(reference P-25 =1)
2.72
TiO2(1000)
210-5
1.34
Sample
The NLO response can be used for the monitoring of
surface states and photocatalytic activity of TiO2 based
nanocomposites
Acknowledgements
We acknowledge to S.A. Nepijko for HRTEM and ED data,
and
to I.Petrik, N.Smirnova, A.Eremenko for the prepared
samples.
The work was partially supported by the grant:
DLR-BMBF UKR01/062.