Evolution of an Oxygen Near-Edge X-ray Absorption Fine Structure Transition in the Upper Hubbard Band in alpha-Fe2O3 upon Electrochemical Oxidation
Electrochemical oxidation of hematite (alpha-Fe2O3) nanoparticulate films at 600 mV vs Ag+/AgCl 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 to a surface state. This transition, not previously observed for pristine alpha-Fe2O3, is at about the same X-ray energy as that of 196 Si-doped Si: Fe2O3. The 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 3 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 an extra NE.XAFS feature, suggesting that a critical electrochemical potential between 200 and 600 mV is necessary to change the electronic structure of the iron oxide at the surface. A decrease of the surface roughness, as suggested by visual inspection, profilometry, and X-ray reflectivity, points to faceting as the potential structural origin of the surface state.