Evolution of an oxygen NEXAFS transition in the upper Hubbard band in {\alpha}-Fe2O3 upon electrochemical oxidation

TL;DR

This study investigates how electrochemical oxidation alters the electronic surface states of { extalpha}-Fe2O3, revealing a new transition in the upper Hubbard band linked to surface modifications at specific potentials.

Contribution

It demonstrates the formation of a new surface-related electronic transition in hematite upon electrochemical oxidation, not observed in pristine samples, and links it to surface structural changes.

Findings

New surface state appears at around 600 mV oxidation potential.

Transition correlates with increased dark current and tunneling exchange current.

Surface faceting likely causes the observed electronic structure change.

Abstract

Electrochemical oxidation of hematite ({\alpha}-Fe2O3) nano-particulate films at 600 mV vs. Ag+/AgCl reference in KOH electrolyte forms a species at the hematite surface which causes a new transition in the upper Hubbard band between the Fe(3d)-O(2p) state region and the Fe(4sp)-O(2p) region, as evidenced by oxygen near edge x-ray absorption fine structure (NEXAFS) spectra. The electrochemical origin of this transition suggests that it is related with a surface state. This transition, not known for pristine {\alpha}-Fe2O3 is at about the same x-ray energy, where pristine 1% Si doped Si:Fe2O3 has such transition. Occurrence of this state coincides with the onset of an oxidative dark current wave at around 535 mV - a potential range, where the tunneling exchange current has been previously reported to increase by three orders of magnitude with the valence band and the transfer coefficient by a factor of 10. Oxidation to only 200 mV does not form such extra NEXAFS feature, supporting that a critical electrochemical potential between 200 and 600 mV is necessary to change the electronic structure of the iron oxide at the surface. Decrease of the surface roughness, as suggested by visual inspection, profilometry and x-ray reflectivity, points to faceting as potential structural origin of the surface state.