The document summarizes a study on the role of iridium oxide (IrO2) in shuttling photogenerated holes across the TiO2 interface in a UV-irradiated TiO2 system. Spectroscopic analysis found that IrO2 mediates the transfer of trapped holes on TiO2 to itself with a rate constant of 6×105 s-1. Interestingly, IrO2 also catalyzes the recombination of trapped holes with reduced oxygen species, which can hinder photocatalytic water splitting. A "reverse" fuel cell design is proposed to separate the electrons and oxygen/hydrogen products to overcome this issue.

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)

3. Spectroscopic identification of trapped holes has enabled their characterization in colloidal TiO2 suspension and monitoring of the transfer of trapped holes to IrO2.

4. 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.

5. The hole transfer to IrO2 occurs with a rate constant of 6×105 s-1.

7. 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)

8. et ht CB IrO2 ? hn TiO2 VB hhh 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)

9. 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! Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

10. 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! Figure reprinted from J. Phys. Chem. Letters 2011, 2, 2304-2310 with permission from the American Chemical Society (© 2011)

11. So what does it mean?

12. 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!

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

14. 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 Thanks to: