Surface-Enhanced Raman Scattering as a Versatile Vibrational Probe of Transition-Metal Interfaces: Thiocyanate Coordination Modes on Platinum-Group versus Coinage-Metal Electrodes

The surface-enhanced Raman spectroscopy (SERS) properties of thiocyanate adsorbed on platinum, palladium, rhodium, and iridium electrodes, formed by pinhole-free ultrathin film deposition onto SERS-active gold, are compared with corresponding data obtained on copper, silver, and gold, and on underpotential (upd) silver and mercury layers on gold, to assess the sensitivity of the anion coordination modes to the chemical nature of the metal surface. A crucial virtue of SERS for this purpose lies in the observability of each of the intramolecular adsorbate modes plus the metal surface−adsorbate stretching vibration, ν m - a . The latter modes, inaccessible to other in-situ vibrational methods, provide particularly straightforward insight into metal-thiocyanate binding. In contrast to gold and upd mercury surfaces, which favor N-coordination only at far-negative potentials, both S- and N-binding modes are fingerprinted on Pd, Rh, and Pt surfaces from the characteristic ν m - a bands. These bands, whose relative frequencies are sensitive primarily to the lead-in atomic mass, are observed throughout the range of potentials (typically −0.4 to 0.4 V vs the saturated calomel electrode) where “reversible” potential-dependent spectral behavior was obtained. Such coexisting modes of surface coordination are also indicated from the appearance of a pair of C−S stretching (ν CS ) vibrations indicative of S- and N-binding from the known behavior of bulk-phase metal complexes. Less diagnostic in this regard is the C−N stretching (ν CN ) vibration, which exhibits only a single broad band at ca. 2115−2135 cm - 1 over the above-mentioned range of potential on the Pt-group surfaces. The virtually potential-independent ν CN frequencies observed on these metals, (dν CN /d E ) ≤ 10 cm - 1 V - 1, contrast with the larger “Stark-tuning” slopes found on silver and copper. The potential-insensitive adsorbate frequencies and coordination modes, together with the higher ν m - a frequencies compared with those on the coinage metals, are consistent with the occurrence of largely covalent transition-metal surface bonding.