Epitaxial α-Fe2O3 thin films on conducting Ta-doped SnO2 bottom layers are prepared by pulsed-laser deposition in order to elucidate variation of the photoelectrochemical (PEC) properties for the water oxidation reaction on the α-Fe2O3 photoanodes with different orientation and thickness. Cyclic voltammetry under illumination shows that current density (photocurrent onset potential) of the m-axis oriented samples is considerably larger (lower) than that of the c-axis oriented ones. Using electrochemical impedance spectroscopy under illumination, we ascribe these behaviors to difference in surface states between c- and m-planes besides anisotropic conductivity along the individual axes. Our study demonstrates the power of utilizing well-defined epitaxial thin films to investigate the anisotropic PEC properties of visible-light-driven photoelectrodes.
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