α-Fe2O3/NiOOH: An Effective Heterostructure for Photoelectrochemical Water Oxidation

The study of the semiconductor/electrocatalyst interface in electrodes for photoelectrochemical water splitting is of paramount importance to obtain enhanced solar-to-fuel efficiency. Here, we take into consideration the multiple effects that a thin layer of photodeposited amorphous Ni-oxyhydroxide (NiOOH) induces on hematite (alpha-Fe2O3) photo-anodes. The reduction of overpotential produced a photocurrent onset potential advance of 150 mV and an increase of photocurrent of about 50% at 1.23 V vs RHE. To give an interpretation to these phenomena, we carried out deep electrochemical investigations by cyclic voltammetry and electrochemical impedance spectroscopy. The effective charge injection into the electrolyte due to the reduction of the charge transfer resistance at the electrode/electrolyte interface was observed and increased along with the amount of deposited NiOOH. The benefits of NiOOH deposition are ascribable to its ability to scavenge holes from hematite surface traps. This effect is mitigated at a potential higher than 1.25 V, since a fraction of photogenerated holes is consumed into the Ni redox cycle.