Nanostructured hematite photoelectrochemical electrodes prepared by the low temperature thermal oxidation of iron

We report on a facile low temperature method for the preparation of high surface area, nanostructured α-Fe₂O₃ (hematite) thin films and their application as photoelectrochemical (PEC) water splitting electrodes. The hematite films are fabricated by thermal oxidation in air of DC sputter deposited iron films at temperatures as low as 255 °C. This method results in films with a higher surface area than typically obtained by directly sputtering α-Fe₂O₃. It is shown that beyond a minimum iron thickness, α-Fe₂O₃ nanowires result upon thermal treatment in atmospheric conditions. Structural and optical characteristics of the resulting films are analyzed. The oxidation process is studied in detail and correlated to the photoelectrical properties. The Fe films oxidize in stages via Fe-oxide layers of increasing oxidation states. Resulting photoelectrochemical performance of fully oxidized films is a balance between optical absorption and charge collection, which varies with film thickness. The optimum film achieved a net photocurrent density of 0.18 mA/cm² in 1 M NaOH at 1.23 V vs. RHE under simulated AM1.5 sunlight, amongst the highest values reported for undoped hematite films produced at low temperature.