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 inv estigate the local atomic and electronic structure of iron incorporated int o electrodeposited nickel hydroxide films. We found that cathodic codeposit ion from a solution containing Fe(II) and Ni(II) ions results in iron occup ying Ni lattice sites in CL-Ni(OH)(2). The X-ray absorption near edge struc ture (XANES) shows that Fe is present as Fe(III) ions in the cathodically c odeposited film. Analysis of the extended X-ray absorption fine structure ( EXAFS) shows that Fe is coordinated to oxygen at similar to 2.00 Angstrom a nd 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) b ut is in good agreement with the average Fe(III)-O bond distance found in F eOOH(alpha, gamma). The Fe-Ni bond distance is in agreement with that of th e Ni(II)-Ni(II) bond distance found in alpha-Ni(OH)(2). Moreover, the radia l 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 i ons occupied Ni lattice sites in gamma-NiOOH. The XANES shows that the Fe e dge 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 coord inated to oxygen at similar to 1.94 Angstrom and to Ni at similar to 2.84 A ngstrom. 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 gamm a-NiOOH. The Fe-O bond distance of similar to 1.94 Angstrom is in good agre ement 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.