In situ O K-edge X-ray absorption
fine structure (XAFS) spectroscopy
was applied to investigate the electronic and structural change in
the nickel–borate (Ni–Bi) electrocatalyst
during the oxygen evolution reaction (OER). An absorption peak was
observed around 528.7 eV at 1.0 V versus Ag/AgCl in a potassium borate
aqueous solution, which relates with the formation of nanoscale order
domains of edge-sharing NiO6 octahedra in the Ni–Bi electrocatalyst. XAFS spectra were measured with variation
of the electrode potential from 0.3 up to 1.0 V. The measured absorption
peaks suggest that the quantity of NiO6 octahedra increased
in correlation with the OER current; however, when the potential was
changed downward, the XAFS absorption peak assigned to NiO6 octahedra remained constant, even at the electrode potential for
no OER current. This difference implies that the water oxidation catalysis
proceeds at the domain edge of NiO6 octahedra. The XAFS
technique provides the first successful direct probing of the active
species in the Ni–Bi electrocatalyst during electrochemical
reaction.
The Mn-oxide/Nb:SrTiO3 photoelectrode for oxygen evolution reaction was investigated by in situ Mn K-edge XAFS spectroscopy under UV irradiation. The oxidization of the Mn oxide was observed via photoexcited carrier transfer, which results in the positive potential shift of the Mn oxide cocatalyst toward oxygen evolution reaction.
Photoexcited hole transfer to MnO
x
cocatalysts
on SrTiO3 photoelectrodes was examined under controlled
potential conditions during UV irradiation using in situ Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy.
The absorption edges of spectra were found to shift to higher energies
during irradiation, indicating that MnO
x
cocatalysts were oxidized by the migration of photoexcited holes
accompanied by a positive potential shift of the MnO
x
cocatalysts. This oxidation process was promoted by the application
of a positive applied potential, suggesting that the photoexcited
hole transfer was enhanced by upward band bending at the cocatalyst–photoelectrode
interface. Structural changes of the MnO
x
cocatalyst were found to depend on the UV photon intensity; thus,
the observations of photoexcited electron transfer by XAFS are associated
with the photoelectrochemical activity during water splitting.
The electrochromic transition of a nickel borate thin film between colorless and brown was examined by means of in situ XAFS and UV/vis spectroscopy. The XAFS spectra showed that the average valence state of the nickel species in the film changed from +2.1 to +3.8 following the application of an electrode potential. Additionally, a broad peak at 700 nm was observed during in situ UV/vis absorption measurements on the application of a positive potential. These results suggest that the nickel borate film reversibly forms a NiOOH structure with a domain size of several nanometers during the electrochromic reaction.
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