The Role of IrO2 in Mediating Hole Transfer at the TiO2 Interface

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)

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.

(a)
(b)
(c)
Trace (a): 16 mM TiO2 colloidal solution (5/95 vol% acetic acid/ethanol) after illumination under N2atmosphere
Trace (b): absorbance spectrum of (a) after equilibration with air
Trace (c): 16 mM TiO2 colloidal solution (ethanol solution) after illumination under N2 atmosphere

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!

et
ht
CB
IrO2
?
hn
TiO2
VB
hhh
So what is the mechanism, as suggested by experiments?

et
eee
et
ht
ht
e
h
hhh
CB
IrO2
IrO2
N2
hn
TiO2
VB
hhh
No oxygen available to scavenge electrons, and thus, no scavenging of trapped holes, even with IrO2 present!

et
et
ht
ht
e
h
CB
IrO2
IrO2
O2
hn
TiO2
VB
hhh
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!

So what does it mean?

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!

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

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
Thank you for watching!
More information on the Kamat group can be found at:
http://www.nd.edu/~pkamat
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The Role of IrO2 in Mediating Hole Transfer at the TiO2 Interface

by kamatlab on Sep 07, 2011

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