Periodic trends in monoatomic chemisorbate bonding on platinum-group and other noble-metal electrodes as probed by surface-enhanced raman spectroscopy

Metal-adsorbate vibrations evaluated by surface-enhanced Raman spectroscopy (SERS) are reported for chloride, bromide, and sulfide on four Pt-group electrode materials (platinum, palladium, rhodium, and iridium) in comparison with Group IB surfaces (copper, silver, and gold) to ascertain periodic trends in monoatomic chemisorbate bonding at electrochemical interfaces in comparison with theoretical predictions and related vibrational behavior at metal-vacuum interfaces. The measurements utilize our recently developed strategies for preparing ultrathin yet pinhole-free Pt-group films on gold, yielding near-optimal SER spectra essentially free from substrate interferences. The metal-adsorbate stretching frequencies, v(M-X), and derived force constants, f(M-X), for all three chemisorbates display the same periodic trends, the f(M-X) values decreasing from left to right across the 4d and 5d series, increasing from 4d to 5d within a given group. While the VM-X values for bromide and especially chloride on silver and gold blueshift significantly with increasing electrode potential, symptomatic of partial ionic bonding, this potential dependence is smaller on the Pt-group electrodes, indicative of more covalent bonding, i.e., with greater charge transfer. The f(M-X) Values for chloride increase more sharply toward the left of the 4d and 5d series than for bromide or sulfide, also indicative of metal-dependent covalency. The observed metal-dependent f(M-X) values can be understood at least qualitatively on the basis of simplified theoretical treatments invoking d-band occupancy, as enunciated earlier to describe surface bonding for oxygen and related adsorbates. The present electrode metal-dependent bonding trends, however, are not dissimilar to those evident from the Raman spectra of symmetric mononuclear metal halide complexes. Related periodic trends are also briefly considered for corresponding vibrational data for chemisorbed carbon monoxide on Pt-group electrodes.