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ICONO/LAT 2005, St. Petersburg

Femtosecond dynamics of photoexcited nanocrystals TiO 2 in aqueous suspension. Yuri E. Lozovik * , Viktor A. Nadtochenko **, Oleg M. Sarkisov ***, Aleksandr S. Sobennikov *** * Inst. of Spectroscopy , Troitsk, Russia ** Inst. of. Prob. of Chem. Phys. , Chernogolovka, Russia

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ICONO/LAT 2005, St. Petersburg

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  1. Femtosecond dynamics of photoexcited nanocrystals TiO2 in aqueous suspension Yuri E. Lozovik * , Viktor A. Nadtochenko **, Oleg M. Sarkisov ***, Aleksandr S. Sobennikov *** *Inst. of Spectroscopy, Troitsk, Russia **Inst. of. Prob. of Chem. Phys., Chernogolovka, Russia ***Inst. Of Chem. Phys., Moscow, Russia ICONO/LAT 2005, St. Petersburg

  2. Scientific interest to Titanium Dioxide ( TiO2) ICONO/LAT 2005, St. Petersburg

  3. Historical overview of photocatalysis on TiO2 • In 1972 Fujishima andHonda discovered photocatalytic splitting of water onTiO2 electrodes. Nature37, 238,1972 2. In 1991 Grätzel demonstrated high-efficiency solar cell elementbased on colloidalTiO2 Nature353, 737,1991 Note: Only Anatase form shows photocatalytic activity ICONO/LAT 2005, St. Petersburg

  4. Key scheme of RedOx reactions driven by light in nanoparticles Nanoparticle acts as an intermediate in reaction of electron transfer recombination In some cases it can be more efficient than direct electron transfer ICONO/LAT 2005, St. Petersburg

  5. Probability of electron transfer (ET) • Due to the large value of S/V ratio the probability of ET • for small particles is significantly higher than for bulk material R when 2. For very small nanoparticles non-radiative recombination enhances R > Rexciton 3. Surface defects trap carriers and increase the ET probability Surface trapsare very important pH serves as an indicator of photocatalytic activity ICONO/LAT 2005, St. Petersburg

  6. Experimental Setup Pump – Probe Spectroscopy ICONO/LAT 2005, St. Petersburg

  7. Information in photoinduced spectra For TiO2 nanoparticles: e •  400-500 nm absorption by trapped holes CB •  600-700 nm absorption by trapped electrons VB h Serpone et al., J.Phys.Chem 99, 16655,1995 Skinner et al., J.Phys.Chem 99, 7853,1995 ICONO/LAT 2005, St. Petersburg

  8. Samples and conditions of experiment Pump pulse: λ = 308 nm τ = 70 fs Supercontinuum probe: 380нм < λ <1000нм Pump TiO2 • pH  2 • pH  10 Probe ICONO/LAT 2005, St. Petersburg

  9. Photoinduced spectra dynamics ICONO/LAT 2005, St. Petersburg

  10. Early time kinetic curves ICONO/LAT 2005, St. Petersburg

  11. Long time kinetic curves ICONO/LAT 2005, St. Petersburg

  12. Mechanism of stretched exponent occurrence Single way of relaxation Multiple ways of relaxation 2 1   4 3   ICONO/LAT 2005, St. Petersburg

  13. Dependence of stretched exponent parameters on pH and  Constant  lead to pH ICONO/LAT 2005, St. Petersburg

  14. Conclusions • It was found that trapped carrier relaxation obeys stretch exponential • decay 2. Parameter  has “structure” nature and is equal to 0.3 3. In acid suspension relaxation is faster than in base due to surface charge ICONO/LAT 2005, St. Petersburg

  15. Thank you for attention ICONO/LAT 2005, St. Petersburg

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