Self-motivated, thermally oxidized hematite nanoflake photoanodes: Effects of pre-polishing and ZrO₂ passivation layer

High-temperature thermal oxidation of an Fe foil produces a high-quality, crystalline hematite nanoflake suitable as a photoanode for the photoelectrochemical (PEC) water oxidation. Physical pre-polishing of the foil surface has a profound effect in the formation of a vertically-aligned nanoflakes of hematite phase with extended (110) planes by removing the loosely-bonded oxide layer. When the surface of the photoanode is modified with a ZrO₂ passivation layer and a cobalt phosphate co-catalyst, the charge recombination at the photoanode-electrolyte interface is greatly suppressed to improve its overall PEC activity. As a result, the photocurrent density at 1.10 V_RHE under 1 sun condition is enhanced from 0.22 mA cm⁻² for an unmodified photoanode to 0.59 mA cm⁻² for the fully modified photoanode, and the photocurrent onset potential is shifted cathodically by 400 mV. Moreover, the photoanode demonstrates outstanding stability by showing steady production of H₂ and O₂ gases in the stoichiometric ratio of 2:1 in a continuous PEC operation for 10 h.