Evolution of an Oxygen Near-Edge X-ray Absorption Fine Structure Transition in the Upper Hubbard Band in 伪-Fe2O3 upon Electrochemical Oxidation

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• 作者：Debajeet K. Bora ; Artur Braun ; Selma Erat ; Ahmad K. Ariffin ; Romy Lo虉hnert ; Kevin Sivula ; Jo虉rg To虉pfer ; Michael Gra虉tzel ; Recardo Manzke ; Thomas Graule ; Edwin C. Constable
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：April 7, 2011
• 年：2011
• 卷：115
• 期：13
• 页码：5619-5625
• 全文大小：920K
• 年卷期：v.115,no.13(April 7, 2011)
• ISSN：1932-7455

文摘

Electrochemical oxidation of hematite (伪-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)鈭扥(2p) state region and the Fe(4sp)鈭扥(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 伪-Fe₂O₃, is at about the same X-ray energy as that of 1% Si-doped Si:Fe₂O₃. The occurrence of this state coincides with the onset of an oxidative dark current wave at around 535 mV鈥攁 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 NEXAFS 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.