Test slides

1. The Role of the Water Oxidation Catalyst
IrO2 in Shuttling Photogenerated Holes
Across TiO2 Interface
Benjamin H. Meekins and Prashant V. Kamat*
Radiation Laboratory and Departments of Chemistry and Biochemistry, and Chemical and
Biomolecular Engineering
University of Notre Dame, Notre Dame, IN 46556
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

2. KEY FINDINGS
•Iridium oxide, a water oxidation co-catalyst, plays an important role in mediating the hole
transfer process of a UV-irradiated TiO2 system.
•Spectroscopic identification of trapped holes has enabled their characterization in
colloidal TiO2 suspension and monitoring of the transfer of trapped holes to IrO2.
•Titration of trapped holes with potassium iodide yields an estimate of 3 holes per particle
during 7 min of UV-irradiation of TiO2 suspension in ethanol containing 5% acetic acid. An
extinction coefficient of 11,230 M-1 cm-1 at 360 nm, corresponding to the absorbance of
the trapped holes, has been calculated.
•The hole transfer to IrO2 occurs with a rate constant of 6×105 s-1.
•Interestingly, IrO2 also catalyzes the recombination of trapped holes with reduced oxygen
species.

3. (a) (b) (c)
1.00
0.75
0.50
0.25
0.00
b c
350 400 450 500 550 600 650 700 750 800
Absorbance
Wavelength (nm)
a
Trace (a): 16 mM TiO2 colloidal solution (5/95 vol% acetic acid/ethanol) after illumination under
N2 atmosphere
Trace (b): absorbance spectrum of (a) after equilibration with air
Trace (c): 16 mM TiO2 colloidal solution (ethanol solution) after illumination under N2
atmosphere
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

4. 0.5
0.4
0.3
0.2
0.1
0.0
e
0 minutes (0.628 mol IrO
)
350 400 450 500 550 600 650 700 750 800
Absorbance
Wavelength (nm)
2
2 minutes
4 minutes
6 minutes
8 minutes
B
a
350 400 450 500 550 600 650 700 750 800
0.5
0.4
0.3
0.2
0.1
0.0
c
b
Absorbance
Wavelength (nm)
A
a
Absorbance corresponding to the
trapped holes on TiO2 at (a) 2, (b) 6, and
(c) 10 minutes of illumination in oxygen
atmosphere
Decrease in absorbance as the trapped
holes are scavenging by oxygen radicals
in the presence of IrO2!
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

5. TiO2 ?
IrO2
et
ht
hn
hhh
CB
VB
So what is the mechanism, as suggested by experiments?
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

6. IrO2
et
ht
N2 hn
hhh
CB
TiO2
VB
e
h
IrO2
et
ht
eee
hhh
No oxygen available to scavenge electrons, and thus, no scavenging of trapped holes,
even with IrO2 present!
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

7. IrO2
et
ht
O2
hn
hhh
CB
TiO2
VB
e
h
IrO2
et
ht
O2
O2
–
Oxygen is able to scavenge trapped electrons from the TiO2 surface, and IrO2 enables the
newly formed oxygen radicals to scavenge trapped holes as well, regenerating the
oxygen!
Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

8. So what does it mean?

9. So what does it mean?
• Photocatalytic water splitting with IrO2 as the water
oxidation catalyst could be hindered by this
unexpected and undesirable side reaction
• For a pure photo-driven water splitting setup to be
viable, it will be necessary to separate photogenerated
electrons quickly to prevent scavenging by oxygen
• One way to get around this is to use a “reverse” fuel
cell, which separates the working electrode (where O2
is produced) and the counter electrode (where H2 is
produced). This facilitates both charge separation and
removes the gas separation step, as they are generated
in different compartments!

10. Schematic of our “reverse” fuel cell
Brian Seger; Prashant V. Kamat; J. Phys. Chem. C 2009, 113, 18946-18952.

11. Thank you for watching!
This work can be found in the Journal of Physical Chemistry Letters
(DOI: 10.1021/jz200852m)
J. Phys. Chem. Lett., 2011, 2, pp 2304–2310
More information on the Kamat group can be found at:
http://www.nd.edu/~pkamat
Thanks to: