Publication date: 31st March 2013
In recent years, frequency- and time-domain studies have elucidated some details of charge carrier dynamics during water oxidation on metal oxide photoanodes, such as hematite (α-Fe2O3), TiO2 and WO3.[1- 6] A general requirement for long-lived holes (on timescales of milliseconds to seconds) has been observed for water oxidation to occur on such photoanodes. Many studies have focussed on hematite as, despite its relatively small band gap and extreme stability under water oxidation conditions, water oxidation efficiencies remain disappointingly low. Slow charge transfer kinetics at the semiconductor/electrolyte interface and rapid recombination of photogenerated charge carriers are often invoked to account for this. Although several photoelectrochemical impedance, transient absorption spectroscopy and IMPS studies have investigated charge carrier dynamics on microsecond to second timescales,[1, 4, 5] water oxidation efficiency is likely to be limited by recombination and trapping processes that occur on shorter, sub-microsecond timescales. However, very little is known about charge carrier dynamics on such ultrafast timescales under operational water-oxidation conditions.[6]
Using ultrafast (picosecond to nanosecond) transient absorption spectroscopy (TAS), we have compared charge carrier dynamics in various metal oxide semiconductors, including hematite, TiO2 and BiVO4. TAS is a pump-probe technique which allows the dynamics of photogenerated electrons and holes to be directly monitored. Varying the concentration of photogenerated carriers (by varying the pump beam intensity) allows separation of geminate and non-geminate recombination processes. The effect of cobalt oxide surface layers and applied electrical bias on electron/hole recombination in photoanodes in working photoelectrochemical cells is investigated. This elucidates some of the limitations to water oxidation efficiency on various metal oxide photoanode materials.
1. Pendlebury, S. R.; Cowan, A. J.; Barroso, M.; Sivula, K.; Ye, J.; Grätzel, M.; Klug, D. R.; Tang, J.; Durrant, J. R.; Correlating long-lived photogenerated hole populations with photocurrent densities in hematite water oxidation photoanodes, Energy & Environmental Science, 2012, . 2. Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.; Sivula, K.; Grätzel, M.; Durrant, J. R.; Klug, D. R.; Activation energies for the rate-limiting step in water photooxidation by nanostructured α-Fe2O3 and TiO2, Journal of the American Chemical Society, 2011, 133, 10134-10140 3. Pesci, F. M.; Cowan, A. J.; Alexander, B. D.; Durrant, J. R.; Klug, D. R.; Charge Carrier Dynamics on Mesoporous WO3 during Water Splitting, Journal of Physical Chemistry Letters, 2011, 2, 1900-1903. 4. Peter, L.M.; Energetics and kinetics of light-driven oxygen evolution at semiconductor electrodes: the example of hematite; Journal of Solid State Electrochemistry; 2013, 17, 315-326 5. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J.; Water oxidation at hematite photoelectrodes: the role of surface states, Journal of the American Chemical Society, 2012, 134, 4294-4302 6. Huang, Z.; Lin, Y.; Xiang, X.; Rodríguez-Córdoba, W.; McDonald, K. J.; Hagen, K. S.; Choi, K.-S.; Brunschwig, B. S.; Musaev, D. G.; Hill, C. L.; Wang, D.; Lian, T.; In situ probe of photocarrier dynamics in water-splitting hematite (α-Fe2O3) electrodes, Energy & Environmental Science, 2012, 5, 8923-8926
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