Publication date: 31st March 2013
Processes involving photogenerated charges have been scrutinized in hematite photoanodes by transient absorption spectroscopy (TAS) and photo-electrochemical impedance spectroscopy (P-EIS), characterizing the limitations of this material for water oxidation.
Hematite is widely recognized as a promising candidate material to produce hydrogen from the Sun because of its low cost and high chemical stability. However the long absorption depth at long wavelengths (close to band gap) combined with the poor charge carrier conductivity observed in iron oxide has been a major drawback to implement this material in a water photolysis device. Recent advances in film preparation involving addition of dopants, to increase the electron mobility, and nanostructuring strategies have substantially increase the performance of hematite electrodes. This enhancement has often been assigned to the increased quantity of hematite in proximity to the interface with electrolyte, decreasing subsequently the distance between where holes are photogenerated and where they react.
The second main limitation of hematite is the large overpotential required to drive the oxygen evolution reaction (O.E.R.) on its surface. This has been related to the slow reaction kinetics, resulting in positive charge accumulation at the semiconductor electrolyte interface. Additionally to reduce the space charge layer, the accumulation can increase the competition between the reaction of holes and other detrimental processes such as charge trapping and charge recombination.[1]
Transient optical spectroscopy and photo-electrochemical measurements, such as EIS, have been shown to provide useful information on the photoanode performance. [2-4] In particular, events occurring for photogenerated holes at the semiconductor-liquid junction can be determined and characterized at different time scales or excitation frequencies. Addition of bias illumination during the measurements allows investigation of the photoanode in working conditions. The resulting hole accumulation observed at the hematite surface is shown to accelerate all processes, including charge transfer to the electrolyte and recombination, and to modify the competition between them.
The different techniques used herein, in addition to the different time domains scrutinized, help to distinguish several processes involved and to elucidate the origin of the low solar-to-hydrogen energy conversion efficiency.
[1] Le Formal, F.; Sivula, K.; Grätzel, M. The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments. J. Phys. Chem. C, 2012, 116, 26707–26720 [2] 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 Environ. Sci., 2012, 5, 6304-6312 [3] Barroso M.; Mesa C. A.; Pendlebury S. R.; Cowan A. J.; Hisatomi T.; Sivula, K.; Grätzel M., Klug D. R.; Durrant J. R.; Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar water splitting. PNAS, 2012, 109, 15640-1564 [4] Klahr, B.; Gimenez S.; Fabregat-Santiago F.; Hamann T.; Bisquert J. Water oxidation at hematite photoelectrodes: the role of surface states. J. Am. Chem. Soc., 2012, 134, 4294-4302.
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