Nanostructured anodic iron oxide film as photoanode for water oxidation

Two different configurations of photoanodes based on anodic iron oxide were investigated for photoelectrochemical water oxidation. A self-ordered and vertically oriented array of iron oxide nanotubes was obtained by anodization of pure iron substrate in an ethylene glycol based electrolyte containing 0.1M NH4F + 3 vol% water (EGWF solution) at 50 V for 15 min. Annealing of the oxide nanotubes in a hydrogen environment at 500 °C for 1 h resulted in a predominantly hematite phase. The second type of photoanode was obtained by a two-step anodization procedure. This process resulted in a two-layered oxide structure, a top layer of nano-dendrite morphology and a bottom layer of nanoporous morphology. This electrode configuration combined the better photocatalytic properties of the nano-dendritic iron oxide and better electron transportation behaviour of vertically oriented nano-channels. Annealing of these double anodized samples in an acetylene environment at 550 °C for 10 min resulted in a mixture of maghemite and hematite phases. Photocurrent densities of 0.74 mA cm−2 at 0.2 VAg/AgCl and 1.8 mA cm−2 at 0.5 VAg/AgCl were obtained under AM 1.5 illumination in 1M KOH solution. The double anodized samples showed high photoconductivity and more negative flat band potential (−0.8 VAg/AgCl), which are the properties required for promising photoanode materials.