1. Advanced Materials Optical Diagnostics group Nonlinear Optics with Nanostructured TiO ² 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

2. AMOD group members From left to right:A.Galas, E.Shepelyavy, V.Kalicev, V.Gayvoronsky

3. F.Koch Technical University of Munich, Physics Department E16, 85748 Garching, Germany In collaboration with V.Timoshenko Moscow State University, Physics Department, 119992 Moscow, Russia

4. 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

5. Porous TiO 2 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

6. Applications of TiO 2 photocatalyst:

7. 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.

8. TiO 2 (anatase) nanoparticle samples characterization Nanoparticle TiO 2 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 500 0 C. Multilayer films are annealed at the same conditions after each layer deposition. Thicknesses 100 – 1000 nm, porosity 34-39% XRD -TiO 2 layers contain nanocrystals of only single phase – anatase TEM – nanoparticle mean diameter 16 nm (distribution 5 - 30 nm)

9. TEM, HRTEM and Electron Diffraction data for anatase nanoparticle films TiO 2 (1000) TiO 2 (300)

10. Size distribution in anatase nanoparticle films TiO 2 (1000) TiO 2 (300)

11. Optical parameters characterization Absorption and reflection spectra Refractive index dispersion Angular resolved light scattering Nonlinear refraction Photovoltage measurements Ellipsometry Photoluminescence Nonlinear absorption/saturation

12. Transmission spectra of single and double layers TiO 2 films on glass substrate versus light photon energy and refractive index dispersion curves EgEg indirect direct 3.4 eV 3.6 eV

13. 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/Z 0 TpvTpv On-axis transmittance in far field Z 0 – diffrational length at the beam waist

14. 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 n 2 values plotted as a function of the weight percentage of Ti-iP in PMMA NLO response time ~1.5 ps NLO absorption Im(  (3) )=0.89  10 -9 esu  =1.4  10 3 cm/GW ~ 100  for a rutile @ 532 nm NLO refraction Re(  (3) )=1.7  10 -9 esu n 2 =2.5  10 -2 cm 2 /GW ~ 100n 2 for rutile @ 1.06  m H. I. Elim et.al., Applied Physics Letters 28 (2003) 2691-2693

15. 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 Setup for the laser beam selfaction effect research

16. Total transmittance and normalized on-axis transmittance in far field Single layer d = 180 nm Double layer d = 360 nm  p = 40 ps Giant NLO Response  (3) ~ 2 ·10 -5 esu of TiO 2 (1000) films versus input laser intensity at =1064 nm.

17. Giant NLO Response WHY Giant ? Bulk TiO 2 -  (3) ~ 10 -11 esu Thin TiO 2 films -  (3) ~ 10 -9 esu Our nanoparticle TiO 2 films -  (3) ~ 10 -5 esu  (3) ~ 10 -5 esu

18. Total transmittance and normalized on-axis transmittance in far field. TiO 2 (1000)  (3) ~ 2 ·10 -5 esu TiO 2 (300)  (3) ~ 6 ·10 -5 esu Giant NLO Response TiO 2 (1000) d = 360 nm TiO 2 (300) d = 240 nm  p = 40 ps of TiO 2 (1000) and TiO 2 (300) films versus input laser intensity at =1064 nm

19. TiO 2 + h  h + + e (1) R6G + h  R6G* (2) R6G* +TiO 2  R6G + +TiO 2 (e - ) (3) Photocatalytic activity of the anatase films R6G water solution absorption spectra for different UV dose in TiO 2 presense Dynamics of R6G photodestruction with UV light due to the presence of TiO 2 films. R6G* + O 2  R6G + + O - 2 (4) TiO 2 (e - ) + O 2  TiO 2 + O - 2 (5) R6G + +O - 2  destruction products (6) Destruction of Rhodamine (R6G):

20. Energy band structure of nanoporous anatase. Laser quantum 1.17 eV, pulse duration~ 40 ps

21. Schematic diagram of possible water dissociation mechanisms on the vacancy defected TiO 2 (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 H 2 O Chemisorbtion of H 2 O

22. Photoemission spectra (h = 35 eV, normal emission) from the valence band region of a sputtered and UHV - annealed, clean TiO 2 (1 1 0) surface. U. Diebold / Surface Science Reports 48 (2003) 5-229 Defect state and molecular orbitals of adsorbed H 2 O

23. Size distribution in anatase nanoparticle films TiO 2 (1000) TiO 2 (300)  (3) ~2·10 -5 esu  (3) ~6·10 -5 esu Photocatalytic activity (reference P-25 =1) 1.34 Photocatalytic activity (reference P-25 =1) 2.72

24. Conclusions Electron 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 TiO 2 based nanocomposites 1 2 3

25. 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.