X-ray absorption spectroscopy studies of the local atomic and electronic structure of iron incorporated into electrodeposited hydrous nickel oxide films

We have utilized X-ray absorption fine structure (XAFS) spectroscopy to investigate the local atomic and electronic structure of iron incorporated into electrodeposited nickel hydroxide films. We found that cathodic codeposition from a solution containing Fe(II) and Ni(II) ions results in iron occupying Ni lattice sites in CL-Ni(OH)(2). The X-ray absorption near edge structure (XANES) shows that Fe is present as Fe(III) ions in the cathodically codeposited film. Analysis of the extended X-ray absorption fine structure (EXAFS) shows that Fe is coordinated to oxygen at similar to 2.00 Angstrom and to Ni at similar to 3.11 Angstrom. This Fe-O bond length is smaller than the Fe(II)-O bond distance found in Fe(OH)(2) (similar to 2.10 Angstrom) but is in good agreement with the average Fe(III)-O bond distance found in FeOOH(alpha, gamma). The Fe-Ni bond distance is in agreement with that of the Ni(II)-Ni(II) bond distance found in alpha-Ni(OH)(2). Moreover, the radial structure function (RSF) around Fe shows a distinct peak at similar to 5.8 Angstrom, which is a fingerprint of the brucite (CI-Ni(OH)(2)) structure. On anodic oxidation of the codeposited film in KOH, we found that the Fe ions occupied Ni lattice sites in gamma-NiOOH. The XANES shows that the Fe edge shifts to higher energy values, indicating an increase in the oxidation state of Fe on charging. Analysis of the EXAFS data shows that Fe is coordinated to oxygen at similar to 1.94 Angstrom and to Ni at similar to 2.84 Angstrom. The latter value is in good agreement with the Ni(IV)-Ni(IV) bond length found in gamma-NiOOH. The RSF around Fe in the oxidized film shows a distinct peak at similar to 5.4 Angstrom, just as in the RSF of Ni in gamma-NiOOH. The Fe-O bond distance of similar to 1.94 Angstrom is in good agreement with the Fe(IV)-O bond distance found in SrFeO3. Our results strongly suggest that the Fe ions in the oxidized film is nominally tetravalent but with the Fe-O bond possessing a high degree of covalency.