Langmuir 1987,3, 931
Tunnel Processes in Photoelectrochemical Reactions of Passive Films Ulrich Stimming Electrochemistry Laboratory, Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027 Received September 4, 1986. In Final Form: January 14, 1987 Previous experimental results for photocurrent measurements on passive films on iron, tantalum, and zirconium and on Xe-implanted passive films on hafnium indicate a behavior different from that of bulk cryatalline materials. A discussion is presented that tries to explain cathodic photocurrents that are observed a t potentials above the flatband potential and wavelength-dependent potentials where the photocurrent changes sign. The model emphasizes tunneling processes of photoexcited carriers from the passive film directly into the electrolyte. Depending on conditions such as photon energy and field, a direct competition of tunneling with and against the field which corresponds to anodic and cathodic photocurrents, respectively, is possible. Model calculations are used to support the proposed mechanism. The title, authors, and abstract from a paper originally published on pages 423-428 of the 1987 May/ June issue of Langmuir are reprinted above. This is part of the collection of papers presented at the symposium on Corrosion (191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986).
EXAFS Study of the Nickel Oxide Electrode J. McBreen and W. E. O’Grady* Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973
K. I. Pandya and R.W. Hoffman Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106
D. E. Sayers Department of Physics, North Carolina State University, Raleigh, North Carolina 27650 Received September 25, 1986 Transmission EXAFS has been used, in situ, to follow the changes in the structure of Ni(OHI2 as it is being electrochemically oxidized in concentrated alkali electrolyte. Changes occurring during repeated oxidation and reduction of the electrode were also studied. EXAFS results reveal changes occurring in the xy plane of the crystal lattice. Lack of interplanar interactions indicates a loose disordered stacking of planes in Ni(OH)2. The interatomic distances and coordination numbers for Ni(OH)2are similar to those determined from X-ray diffraction. Oxidation of Ni(OHI2to the trivalent state results in a contraction of the Ni-O and Ni-Ni distances in the xy plane. Reduction of the trivalent material yields similar interatomic distances to unoxidized Ni(OH)2. A single oxidation-reduction cycle causes considerable disorder in the xy plane. This may facilitate subsequent electrochemical oxidation. The cell and techniques reported here are generally applicable to the study of oxidation-reduction processes in other hydrous oxides. Two advantages of the EXAFS technique are the ability to do in situ structure determinations and facility with which short-range order can be probed, even in highly disordered materials. The title, authors, and abstract from a paper originally published on pages 428-433 of the 1987 May/ June issue of Langmuir are reprinted above. This k part of the collection of papers presented at the symposium on Corrosion (191st National Meeting of the American Chemical Society, New York, NY, April 13-18, 1986).
0743-7463/87/2403-0931$01.50/0
0 1987 American Chemical Society
931
