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.