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The preparation of Pt-group metal films on roughened gold electrodes by utilizing spontaneous redox replacement of an underpotential-deposited (upd) copper or lead monolayer with a Pt-group metal cation solute is described. The resulting films display intense surface-enhanced Raman scattering (SERS) for adsorbates bound to the overlayer and free from substrate interferences. This strategy provides a useful alternative, at least for platinum, to the constant-current electrodeposition method commonly utilized to prepare SERS-active Pt-group metal films (Zou, S.; Weaver, M. J. Anal. Chem. 1988, 70. 2387). Similarly to related earlier studies, the film uniformity (specifically, the absence or otherwise of residual Au "pinhole" sites) was tested by employing carbon monoxide, and also ethylene, as "probe" chemisorbates, since they yield vibrational frequencies on Au that are blue-shifted from the corresponding bands for adsorbate bound to Pt-group metal sites. While a single redox replacement of upd Cu with Pt(IV) yielded incomplete surface coverage, as expected, the use of multiple (up to eight) replacement cycles produced Pt films displaying remarkably intense CO vibrational bands as well as apparently "pinhole-free" properties, although such imperfections were detected with the ethylene probe. A single upd Cu replacement with Pt(II), however, yielded a remarkably uniform Pt layer, as indicated by pinhole-free characteristics using both the CO and ethylene probes along with the voltammetric behavior. The use of additional redox replacement cycles yielded marked progressive attenuation in the SERS signals. Comparable, although less optimal, SERS behavior was obtained for Pd films prepared similarly from Pd(II). The value of the strategy for exploring catalytic as well as equilibrium adsorptive chemistry on Pt surfaces is also illustrated.

Procedures are outlined for the electrodeposition of ultrathin films of Pt-group transition metals onto gold that provide intense surface-enhanced Raman scattering (SERS) for adsorbates bound to the overlayers yet (unlike earlier reports) are sufficiently "pinhole-free" to avoid significant spectral and chemical interferences from the underlying substrate. Constant-current electrodeposition of Pd, Rh, Pt, and Ir from perchloric acid and/or phosphate electrolytes yields essentially layer-by-layer growth, so that near-ideal pinhole-free electrochemical and spectral characteristics are achieved for film thickness of ∼2 monolayers or more. The desired film uniformity is diagnosed from the voltammetric oxide formation-removal behavior and, especially, from the absence of the characteristic C-O stretching (ν(CO)) SERS band at 2110-2120 cm(-)(1) due to CO binding to Au surface sites. Carbon monoxide is also employed as a surface environment-sensitive adsorbate to probe the electrochemical SERS characteristics as a function of the transition-metal film thickness. The Raman enhancement was observed to decrease by 2-fold every 10-20 Å or so, exhibiting a "spacer distance" dependence that is weaker than (but functionally similar to) recently reported organic insulator films. The practical value of the present films for obtaining rich vibrational spectra for diverse adsorbates on transition metals is pointed out and briefly illustrated for benzonitrile on a Pt film electrode. The more general promise of this overlayer film SERS strategy as a versatile vibrational technique for characterizing other types of chemically important surface materials is also noted.