Publication date: 15th December 2014
Hematite (α-Fe2O3) is an extensively studied model material for photoanodes used in light-driven water splitting, due to its abundance, stability and favourable light absorption properties. Its performance is limited by short charge carrier diffusion lengths but can be increased by bulk doping or by the application of a catalyst to the surface.[1-3] In this study, we prepared hematite thin film electrodes by atomic layer deposition (ALD), resulting in well-de fined and highly uniform films. Catalysts were applied by either coating the fi lms with a single ALD cycle of cobalt oxide or by spin-coating a dispersion of Co3O4 nanoparticles, resulting in cathodic shifts of the water oxidation onset potential by 190 and 140 mV respectively. The kinetics of light-driven water oxidation at these electrodes were probed by photocurrent transient analysis and intensity-modulated photocurrent spectroscopy (IMPS). The latter uses small perturbations of the light intensity to avoid significantly changing conditions inside the semiconductor, especially the band bending. It was found that the charge transfer effi ciency ηtrans derived from both methods follows similar trends, with the catalysed electrodes reaching near 100% ηtrans at lower potentials than electrodes without a catalyst. Furthermore, the transient analysis method systematically underestimates ηtrans, possibly due to non-negligible changes in band bending between the illuminated and dark states. Finally, the rate constants for charge transfer, ktrans, and recombination, krec, were extracted from IMPS results and compared for the different electrodes.
[1] B. Klahr et al., J. Am. Chem. Soc. 2012, 134, 16693-16700.
[2] O. Zandi et al., Energy Environ. Sci. 2013, 6, 634-642.
[3] H. K. Dunn et al., Phys. Chem. Chem. Phys. 2014, 16, 24610-24620.
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