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J. Phys. Chem. 1995,99, 16498
COMMENTS Comment on “Electrochemical Promotion of Oxygen Atom Back-Spillover from Yttria-Stabilized Zirconia onto a Porous Platinum Electrode: Detection of SERS Signals” Ian R. Harkness Department of Chemistry, The University of Edinburgh, Joseph Black Building, King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K. Received: June 23, 1995; In Final Form: September 5, 1995 In their SERS investigation of a PtlZrOn(8 mol % Y203)IAg galvanic cell, Basini et al.’ detected novel Raman signals when oxide ions are electrochemically pumped toward the platinum electrode. These Raman signals were assigned to F‘t-0 vibrations of a negatively charged oxygen species which spills over onto the platinum from the electrolyte when the F’tlZrO2(8 mol % Y2O3) interface is highly polarized. Spillover of such a species under these conditions has been proposed previously and is thought to be responsible for the non-Faradaic electrochemical modification of catalytic activity (NEMCA) phenomena observed on cells of this type.2 The electrochemically induced spillover of two distinct oxygen species on PtlZrO2(8 mol % Y203)IAu cells has been observed in X P S . 3 These species were assigned as being due to normal chemisorbed oxygen and a more highly charged oxygen species. The vibrational frequencies observed by Basini et al. are 113, 125, and 136 cm-I, all of which are very much lower than literature values for F’t-0vibrations. A SERS study of platinum-coated gold gives a band at 450-600 cm-’ for chemisorbed ~ x y g e n which ,~ is in good agreement with two HREELS studies of oxygen adsorbed on F’t( 111) which give frequencies of 4905 and 470 cm-Is6 SERS of an oxidized platinum electrode in H2S04 gives a signal at 560-580 cm-’.’ In light of these values the frequencies of Basini et al. are highly surprising, since the frequency for the proposed Pt-0- species would be expected to be intermediate between those for the chemisorbed and oxidic species mentioned above. Therefore, in the absence of any other evidence it seems highly doubtful to assign the novel features to F’t-0vibrations. This doubt is heightened by the fact that a novel enhancement mechanism has to be invoked to explain the absence of any signal from chemisorbed oxygen which will be as abundant on the surface as the more highly charged specie^.^ This novel enhancement mechanism is also invoked to explain why signals are detectable on the platinum electrode whereas a silver electrode gives no signal under the same conditions, despite silver in general giving
0022-365419512099- 16498$09.00/0
higher SERS intensities than platinum.’ Indeed, Raman signals from oxygen adsorbed on silver films on ZrO2(8 mol % Y203) have been observed previously.* The platinum electrode used by Basini et al. was highly porous, as is usual in such cells,2 and significant Raman signals from the Z r o ~ ( 8mol % Y2O3) were present at all times due to exposed areas of the electrolyte visible through pores in the platinum. The presence of these ZrO2(8 mol % Y2O3) signals means that the source of the novel Raman signals is not necessarily on the platinum but could also be on the ZrO2(8 mol % Y2O3). This proposition is given further weight by the fact that the silver electrode, which showed no ZrO2(8 mol % Y2O3) Raman features (due to a more continuous electrode), showed no cun-ent-induced Raman signals. The results of Basini et ai. could therefore be satisfactorily reinterpreted as a novel species on the electrolyte rather than on the electrode, thus explaining the seemingly anomalous “Pt0 ’vibrational frequency. This proposal could be investigated by using a range electrode materials. If the species is on the electrode, the frequency of the current induced features should shift reflecting the variation in the bonding of the oxygen species to the electrode material; however if, as is proposed above, the species responsible is on the electrolyte, the observed frequency should be relatively constant. The time dependent behavior of the novel features shows that the species responsible is clearly related to electrochemical oxide ion transport toward the face of the sample being probed by SERS, but a definitive assignment is impossible. The role, if any, of the novel species in the NEMCA catalytic phenomena is unclear.
Acknowledgment. The author thanks Dr. A. Khodakov for valuable discussions. References and Notes (1) Basini, L.; Cavalca, C. A.; Haller, G. L. J. Phys. Chem. 1994, 98, 10853. ( 2 ) Vayenas, C. G . ;Bebelis, S.; Yenetekakis, I. V.; Lintz, H.-G. Curul. Today 1992, 11, 303. (3) Ladas, S.;Kennou, S.; Bebelis, S.; Vayenas, C. G. J. Phys. Chem. 1993, 97, 8845. (4)Wilke, T.; Gao, X.; Takoudis, G.; Weaver, M. J. Langmuir 1991, 7, 714. (5) Avery, N. R. Appl. Surf: Sci. 1982, 11/12, 774. (6) Gland, J. L.; Sexton, B. A,; Fisher, G. B. Surf: Sci. 1980, 95, 587. (7) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. SOC.1987, 109, 5113. (8) Kondarides, D. I.; Papatheandrou, G. N.; Vayenas, C. G.; Verykios, X. E. Ber. Bunsen-Ges. P h y . Chem. 1993, 97, 709.
JP95 1749C
0 1995 American Chemical Society
