ICONO/LAT 2005, St. Petersburg

ICONO/LAT 2005, St. Petersburg

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


Scientific interest to Titanium Dioxide ( TiO2)


1. In 1972 Fujishima and Honda discovered photocatalytic splitting of water on TiO2 electrodes.

Nature 37, 238,1972

2. In 1991 Grätzel demonstrated high-efficiency solar cell element based on colloidal TiO2

Nature 353, 737,1991

Historical overview of photocatalysis on TiO2

Note: Only Anatase form shows photocatalytic activity


Key scheme of RedOx reactions driven by light in nanoparticles

L ight photon

eh

D

D

A

A

recombination

Nanoparticle acts as anintermediate in reaction of electron transfer

In some cases it can bemore efficient than direct electron transfer


Probability of electron transfer (ET)

1. Due to the large value of S/V ratio the probability of ET for small particles is significantly higher than for bulk material

S

Vwhen R

2. For very small nanoparticles non-radiative recombination enhances

R > Rexciton

3. Surface defects trap carriers and increase the ET probability

Surface traps are very important

pH serves as an indicator of photocatalytic activity


Experimental Setup

Pump – Probe Spectroscopy


Information in photoinduced spectra

VB

CB

h

e

For TiO2 nanoparticles:

400-500 nm absorption by trapped holes

600-700 nm absorption by trapped electrons

Serpone et al., J.Phys.Chem 99, 16655,1995

Skinner et al., J.Phys.Chem 99, 7853,1995


Samples and conditions of experiment

Pump pulse: λ = 308 nm τ = 70 fs

Supercontinuum probe:

380нм < λ <1000нм

TiO2

30 nm

Pump

Probe

- pH 2- pH 10


Photoinduced spectra dynamics


Early time kinetic curves


Long time kinetic curves

( )t

e


Mechanism of stretched exponent occurrence

Single way of relaxation Multiple ways of relaxation

12

34

( )t

e

( )

,0 1t

e


Dependence of stretched exponent parameters on pH and

pH , nm , ps

2 500 1.6 0.3

2 700 0.7 0.3

10 500 4.7 0.3

10 700 3.3 0.3

t

e

Constant

pH lead to


Conclusions

1. It was found that trapped carrier relaxation obeys stretch exponentialdecay

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


Documents

ICONO/LAT 2005, St. Petersburg