Water oxidation at photoanodes based on hematite films and nanowire arrays

Highlights: • Photoanodes based on hematite nanowire arrays and hematite films were prepared. • The content of oxygen vacancies is higher for hematite nanowires. • TiO₂ underlayer induced a significant decrease in oxygen vacancy content. • Hematite nanowire arrays grown on TiO₂ substrate show the highest photoactivity. In this paper we report the results of a photoelectrochemical study performed on photoanodes based on hematite nanowire arrays and films prepared on fluoride-doped tin oxide coated glass (FTO) and FTO/TiO₂ substrates, respectively by hydrothermal and spray pyrolysis methods. The hematite nanowires grown on FTO/TiO₂ substrate are more stable mechanically, longer (1 µm) and their density on substrate is higher. Hematite film obtained on FTO substrate has a thickness of 92 nm covering uniformly the substrate. X-ray photoelectron spectroscopy measurements showed that hematite samples synthesized on FTO/TiO₂ substrate have lower content of oxygen vacancies. The photoelectrochemical performances of the prepared photoanodes are in close connection with the presence or absence of the TiO₂ underlayer, with oxygen vacancies content and with their morphological characteristics. Electrochemical impedace spectroscopy was used to investigate the charge transfer kinetics at electrode/electrolyte interface and Mott-Schottky analysis was performed to estimate the flatband potential and the carrier density. TiO₂ underlayer led to the formation of defects on the samples surface which induced a positive shift of the flatband potentials compared to that of the bare hematite film. The results showed that the best density photocurrent was obtained with a photoanode of hematite nanowires grown on FTO/TiO₂ substrate.