A significant cathodic shift in the onset potential of photoelectrochemical water splitting for hematite nanostructures grown from Fe-Si alloys

Thermal oxidation of Fe to nanostructured hematite (wires, flakes) is currently widely investigated to produce efficient photoanodes for photoelectrochemical water splitting. The process carried out on pure iron, however, has the key drawback that not only hematite but a layered structure of Fe2O3-Fe3O4-FeO is formed where the thick suboxide layer underneath the Fe2O3 is highly detrimental to the photoresponse. In the present work, we show that suboxide formation can be largely suppressed if hematite nanowires/nanoflakes are thermally grown on Fe-Si alloys. For hematite structures grown on a Fe-Si alloy with 5 at% Si, a photocurrent onset potential as low as 0.6 V-RHE can be reached (under AM 1.5 illumination and 1 M KOH). We believe that the results represent a key finding towards the formation of optimized hematite nanostructures using a thermal oxidation method.