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- 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: