Controlling phase fraction and crystal orientation via thermal oxidation of iron foils for enhanced photoelectrochemical performance

It has been known that the intrinsic properties of a semiconducting photoanodes significantly influence the overall photoelectrochemical (PEC) performance. Here, we report on the fabrication of layered structure of mixed-phase FeO (wustite), Fe3O4 (magnetite), and alpha-Fe2O3 (hematite) iron oxide nanoflake/nanowire morphologies through the thermal oxidation of pristine Fe foils, and the role of metastable FeO phase on the PEC performance discussed. X-ray diffraction and Raman spectroscopic measurements revealed the variation in phase fraction of wustite, magnetite, and hematite with respect to oxidation temperature. The PEC measurements indicate a dependence of onset potential and photocurrent density on phase proportion. The sample, which contains metastable wustite phase FeO, along with Fe3O4 and alpha-Fe2O3, shows a lower onset and higher photocurrent density, followed by the sample that contains a nearly equal ratio of magnetite to hematite phase (similar to 42:58) than that of relatively higher magnetite phase content samples. It is attributed to the improvement in the intrinsic transport of photogenerated charge carriers from hematite via the magnetite and wustite phases to the back contact of the photoanode. It consequently led to a decrease in bulk charge recombination across the interfaces of multiple phases. We carried out electrochemical impedance (EIS) and light intensity-modulated photocurrent measurements (IMPS) to elucidate the mechanism behind the charge separation across the multiple phases.