Hole Accumulation at the Grain Boundary Enhances Water Oxidation at α-Fe₂O₃ Electrodes under a Microwave Electric Field

Abstract

We studied the effects of surface morphology and carrier distribution of α-Fe2O3 electrodes on the enhancement of water electrolysis under microwave (MW) irradiation. We deposited α-Fe2O3 electrodes with various morphologies on Nb-doped rutile TiO2 (100) substrates. α-Fe2O3 films with rough and flat surfaces were deposited using electrodeposition (ED) and pulsed laser deposition (PLD), respectively. The ED α-Fe2O3 film showed a larger response to the MW electric field applied to the electrodes than did the PLD film. In addition, the response was linearly correlated with the MW electric field intensity. Using scanning MW microscopy, we found that the local MW susceptibility of the α-Fe2O3 electrode was enhanced at the grain boundary of the ED α-Fe2O3 film. Analysis of the surface band structure of both ED and PLD α-Fe2O3 films using electrochemical impedance spectroscopy showed that the ED α-Fe2O3 film had a wider depleted layer, indicating increased accumulation of holes on the surface of the electrode to enhance water oxidation. We concluded that the accumulation of holes at the grain boundary of the ED α-Fe2O3 film determines the enhancement of water oxidation under an MW electric field.

Recent advances on interfacial engineering of hematite photoanodes for viable photo‐electrochemical water splitting

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“…The doping of Cr 4+ ions for substituting Fe 3+ leads to lattice contraction in α‐Fe 2 O 3 , showing the peak shifting to lower angle in the XRD pattern. Matsuhisa and co‐workers 97 deposited α‐Fe 2 O 3 photoanode on Nb‐doped rutile TiO 2 (100) substrates. A broadening deplete layer in the α‐Fe 2 O 3 film was characterized by the electrochemical impedance spectroscopy, indicating the increased holes accumulation on the surface of the electrode to enhance water oxidation.…”

“…Under microwave irradiation, Wada et al found the specific microwave absorption at grain boundaries of α‐Fe 2 O 3 electrode (Figure 18(A)), by which the energy could be effectively injected into localized carriers at grain boundaries. The synergy of the localization of carriers at grain boundaries and accumulation of holes on the electrode surface enhances the electron transfer process in the electrochemical water oxidation reaction (Figure 18(A)), which could effectively increase the photocurrent density 97 . Jia et al proposed a facile chemical bath deposition technology to deposit TiO 2 on Fe 2 O 3 film to improve its grain boundaries 90 .…”

“…(A) SMM contact‐mode mapping images of the amplitude change on α‐Fe 2 O 3 electrode, and schematic hole accumulation processes during the water oxidation reaction using the α‐Fe 2 O 3 electrode. Source: Reprinted with permission from Reference 97 copyright (2020) American Chemical Society. (B) Schematic illustration of band edge positions of U 3 O 8 as underlayer and Fe 2 O 3 .…”

Decrease in the Crystallite Diameter of Solid Crystalline Magnetite around the Curie Temperature by Microwave Magnetic Fields Irradiation

“…Microwave processing exhibits some unique features, such as rapid heating, rapid cooling, and selective heating, which are different from conventional processing using an electric furnace. Thus, microwave heating provides many benefits for material processing, such as decreasing the sintering time, synthesizing functionalized nanomaterials, and promoting catalytic reactions [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ]. Additionally, “non-thermal effects”, phenomena of which cannot be explained by thermal effects, have also been reported [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ].…”