α-Fe2O3 films for photoelectrochemical water oxidation – insights of key performance parameters†
We report the deposition of ultra-thin α-Fe2O3 (hematite) films on fluorine-doped tin oxide (FTO) substrates using radio frequency (RF) sputtering, and the investigation of their photoelectrochemical (PEC) performance towards water oxidation. By varying the deposition pressure and time, the film microstructure and morphology could be optimized. The best hematite films having a thickness of about 50 nm exhibited a photocurrent density of 0.59 mA cm−2 at U = 1.23 V vs. RHE and 1.92 mA cm−2 at U = 1.85 V using a tungsten halogen lamp of 40 mW cm−2 light intensity in the wavelength range from 300 to 600 nm. These values are comparable or even higher than those ever measured hematite films (undoped and having no co-catalyst deposited on top of the electrode). Further measurements were explored to investigate the limiting factors in our films for possibly approaching their predicted PEC properties. A detailed analysis reveals that a slow water oxidation reaction and a trapping of charges on the surface, especially at the potential below 1.4 V, are obviously the reasons for the limited PEC performance.