Synthesis and Photoelectrochemical Properties of Fe2O3/ZnFe2O4 Composite Photoanodes for Use in Solar Water Oxidation

alpha-Fe2O3/ZnFe2O4 composite electrodes are prepared via a simple surface treatment performed on nanoparticulate alpha-Fe2O3 electrodes. The alpha-Fe2O3 electrodes are uniformly covered with a solution containing Zn2+ ions which react with Fe2O3 upon heating to form ZnFe2O4. As Zn2+ ions do not completely diffuse into the core of Fe2O3 particles under mild heating conditions, the ZnFe2O4 forms only on the surface of Fe2O3 as a shell layer, resulting in the formation of Fe2O3/ZnFe2O4 composite electrodes. Any unreacted ZnO is removed by dissolution in 1 M NaOH, where ZnFe2O4 and Fe2O3 are stable. X-ray diffraction and energy-dispersive spectroscopy studies show that a crystalline ZnFe2O4 phase forms after the heat treatment and the composite electrode with the best photoelectrochemical performances contains a 1:1 mol ratio of ZnFe2O4 and Fe2O3. The Fe2O3/ZnFe2O4 composite electrode shows a significantly enhanced photocurrent response compared to the bare Fe2O3 electrode because ZnFe2O4 has conduction and valence band edges shifted ca. 200 mV from those of Fe2O3 to the negative direction which allows for the efficient separation of electron hole pairs at the Fe2O3/ZnFe2O4 interface. Further improvement in photocurrent is observed when the surface is modified by an Al3+ treatment that forms thin ZnFe2-xAlxO4 or Fe2-xAlxO3 layers that reduce surface states created by Fe3+ ions exposed on the surface having imperfect coordination environments. The formation of ZnFe2O4 and Al3+-containing layers make the surface less catalytic for O-2 evolution, and therefore, introduction of the Co2+ ions as oxygen evolution catalysts further improved the over performance of the composite electrodes.