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

α-Fe 2 O 3 /ZnFe 2 O 4 composite electrodes are prepared via a simple surface treatment performed on nanoparticulate α-Fe 2 O 3 electrodes. The α-Fe 2 O 3 electrodes are uniformly covered with a solution containing Zn 2+ ions which react with Fe 2 O 3 upon heating to form ZnFe 2 O 4 . As Zn 2+ ions do not completely diffuse into the core of Fe 2 O 3 particles under mild heating conditions, the ZnFe 2 O 4 forms only on the surface of Fe 2 O 3 as a shell layer, resulting in the formation of Fe 2 O 3 /ZnFe 2 O 4 composite electrodes. Any unreacted ZnO is removed by dissolution in 1 M NaOH, where ZnFe 2 O 4 and Fe 2 O 3 are stable. X-ray diffraction and energy-dispersive spectroscopy studies show that a crystalline ZnFe 2 O 4 phase forms after the heat treatment and the composite electrode with the best photoelectrochemical performances contains a 1:1 mol ratio of ZnFe 2 O 4 and Fe 2 O 3 . The Fe 2 O 3 /ZnFe 2 O 4 composite electrode shows a significantly enhanced photocurrent response compared to the bare Fe 2 O 3 electrode because ZnFe 2 O 4 has conduction and valence band edges shifted ca. 200 mV from those of Fe 2 O 3 to the negative direction which allows for the efficient separation of electron–hole pairs at the Fe 2 O 3 /ZnFe 2 O 4 interface. Further improvement in photocurrent is observed when the surface is modified by an Al 3+ treatment that forms thin ZnFe 2– x Al x O 4 or Fe 2– x Al x O 3 layers that reduce surface states created by Fe 3+ ions exposed on the surface having imperfect coordination environments. The formation of ZnFe 2 O 4 and Al 3+ -containing layers make the surface less catalytic for O 2 evolution, and therefore, introduction of the Co 2+ ions as oxygen evolution catalysts further improved the over performance of the composite electrodes.