Ti doped hematite photoanodes: Protective coverage by titania overlayers

In this work, the photoresponse of Ti doped hematite electrodes was optimized by adjusting the thickness and the level of doping. The electrodes were then covered by a TiO2 overlayer by dip coating (DC) or by atomic layer deposition (ALD) to improve chemical stability. Coverage of hematite by a dip-coated sol-gel TiO2 film of thickness up to 135 nm resulted only in a small decrease in photocurrent (by about 30 %). A similar decrease in photocurrent (about 25 %) was observed after the coverage by a very thin (2 nm) ALD TiO2 film while the increase of the thickness to 8 nm led to almost complete suppression of photocurrent. This behaviour can be explained by the different morphology and structure of the overlayers. Although the surface morphology of sol-gel TiO2 films was very smooth and dense, penetration of such films by the electrolyte occurred even for a thickness of 135 nm. On the other hand, much thinner ALD TiO2 films (8 nm) exhibited almost no electrolyte penetration. Such different blocking properties of the two TiO2 overlayers are in direct agreement with the observed dissolution rate in acidic media - the better the blocking properties of an overlayer the better the chemical stability of stratified hematite/titania photoelectrode.