Transcript Document

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
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Scientific interest to Titanium Dioxide ( TiO2)
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Historical overview of photocatalysis on 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
Note: Only Anatase form shows photocatalytic activity
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Key scheme of RedOx reactions driven by light in nanoparticles
A
Light photon

A
Nanoparticle acts as an
intermediate in reaction
of electron transfer
e
h
recombination
D

D
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In some cases it can be
more 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
V
when
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
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Experimental Setup
Pump – Probe Spectroscopy
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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
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Samples and conditions of experiment
Pump pulse:
λ = 308 nm
τ = 70 fs
Supercontinuum probe:
380нм < λ <1000нм
Pump
TiO2
Probe
- pH  2
- pH  10
30 nm
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Photoinduced spectra dynamics
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Early time kinetic curves
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Long time kinetic curves
e
t 
( )

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Mechanism of stretched exponent occurrence
Single way of relaxation

2
1


Multiple ways of relaxation
3

t
t
e
( )

e
 ( )

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4
,0    1
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
Constant
pH
lead to 
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e
t
 
 

Conclusions
1.
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
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ICONO/LAT 2005, St. Petersburg