Nonlinear Optics of Nanostructured TiO2 for Advanced Optical Diagnostics

1. 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

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

3. 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

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 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

6. Applications of TiO2 photocatalyst:

7. Bulk structures of rutile and anatase. Celldimensions 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

8. 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 molecularweights300 (PEG 300) and1000 (PEG 1000) were used as pore andcomplexing 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)

9. TEM, HRTEM and Electron Diffraction data for anatase nanoparticle films TiO2(1000) TiO2(300)

10. Size distribution in anatase nanoparticle films TiO2(1000) TiO2(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 TiO2 films on glass substrate versus light photon energy and refractive index dispersion curves indirect 3.4 eV Eg direct 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 On-axis transmittance in far field Tpv -1 0 1 Z/Z0 Z0 – diffrational length at the beam waist

14. Ultrafast optical nonlinearity in polymethyl- methacrylate-TiO2nanocomposites 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10-9esu =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 NLOresponse time ~1.5 ps

15. Setup for the laser beam selfaction effect research S - sample, A– the beam attenuator, L – focusing lens with focal lengthf, 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

16. Total transmittance and normalized on-axis transmittance in far field of TiO2(1000) films versus input laser intensity at =1064 nm. Giant NLO Response (3) ~ 2 ·10-5 esu Single layer d = 180 nm Double layer d = 360 nm p= 40 ps

17. 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

18. Total transmittance and normalized on-axis transmittance in far field. of TiO2(1000) and TiO2(300) films versus input laser intensity at =1064 nm TiO2(1000) d = 360 nm TiO2(300) d = 240 nm p= 40 ps Giant NLO Response TiO2(1000) (3) ~ 2 ·10-5 esu TiO2(300) (3) ~ 6 ·10-5 esu

19. Photocatalytic activity of the anatase films Destruction of Rhodamine (R6G): • R6G* + O2  R6G+ + O-2 (4) • TiO2(e-) + O2  TiO2 + O-2 (5) • R6G++O-2 destruction products (6) • TiO2 + h h+ + e (1) • R6G + h  R6G* (2) • R6G* +TiO2  R6G+ +TiO2(e-) (3) 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.

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

21. Physisorbtion of H2O 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. Chemisorbtion of H2O Dark atoms are Ti cations, lighter atoms are in-plane O anions. Models for water and OH are represented with covalent radii.

22. 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

23. Size distribution in anatase nanoparticle films 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

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 1 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 2 The NLO response can be used for the monitoring of surface states and photocatalytic activity of TiO2 based nanocomposites 3

25. 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.