Interplay of thickness and photoelectrochemical properties in nanostructured alpha-Fe2O3 thin films

Nanostructured Fe2O3 thin films were grown by plasma enhanced-chemical vapor deposition (PE-CVD) from Ar/O2 plasmas for photoelectrochemical (PEC) water splitting applications. Iron oxide coatings were deposited on fluorine-doped tin oxide (FTO) substrates at 300°C under optimized conditions, and subsequently annealed ex situ in air at 650°C. Structural and compositional analyses confirmed the formation of pure alpha-Fe2O3 (hematite), free from other crystalline iron oxide phases. Controlled variations of the deposition time enabled tuning of the thickness and nano-aggregate sizes in the resulting deposits and, correspondingly, their current-voltage characteristics. A maximum photocurrent density close to 1mA/cm2 was achieved at 1.23 V versus the reversible hydrogen electrode (RHE), without the need of any oxygen evolution catalyst or over/underlayer. The present findings revealed the key role played by the engineering of Fe2O3-based nanomaterials, resulting ultimately in a lowered carrier diffusion length, and in an optimal diffusion of tin from FTO into thinner layers. These features offer an amenable opportunity for harvesting radiant energy to trigger water photoelectrolysis and produce clean hydrogen in a carbon-neutral fashion.