Photoelectrochemical Investigation of the Mechanism of Enhancement of Water Oxidation at the Hematite Nanorod Array Modified with "NiBi"

Modifying the surface of a hematite photoanode with NiBi or CoPi oxygen evolution catalysts has been reported to reduce the bias required for photooxidation of water, but the mechanism remains a subject of discussion. In this work, the effect of photochemically modifying an array of hematite nanorods in Ni2+ solution in potassium borate (or NiBi) on water oxidation was studied using cyclic voltammetry in the dark and in the light and with photocurrent transient measurements. A prominent reduction of a photooxidized species is reported here on the reverse scans of cyclic voltammograms under illumination near the photocurrent onset, coinciding with the cathodic shift in water oxidation potential, showing transfer of the hole in a long-lived oxidized state at NiBi-hematite. The reduction peak was still observed at slow scan rate indicating a stable photooxidized state with slow back electron reaction. A reduction was similarly detected albeit with lower magnitude in the dark when the modified electrode was biased to high anodic potential into the water oxidation region. Photocurrent transients revealed slow decay with a median lifetime of ca. 1-3 s at low bias at NiBi-hematite and a slower decay relative to the bare electrode biased 0.6 V more positive. The results are explained by proposing that photogenerated hole transfer occurs to a state at Ni-oxide-modified hematite, forming a photooxidized species where the photohole is sufficiently long lived, with reduced recombination rate allowing water oxidation to occur at low bias. Electrochemical evidence points to this being the same oxidized state formed in the dark at water oxidation potentials at NiBi-hematite, but the chemical identity of the photooxidized state involved in water oxidation is yet to be determined.