Potential-Dependent Chemisorption of Carbon Monoxide at a Gold Core−Platinum Shell Nanoparticle Electrode: A Combined Study by Electrochemical in Situ Surface-Enhanced Raman Spectroscopy and Density Functional Theory

The potential ( E )-dependent spectral behavior of both C−O and Pt−CO stretching vibrations from a saturated CO ad layer at a Au core−Pt shell nanoparticle film electrode has been examined in a wide potential window by surface-enhanced Raman spectroscopy (SERS). The Stark slopes of C−O stretching for linear-(CO L ) and bridge-(CO B ) binding CO adsorbates are positive and increase toward more negative potentials, which is explained by a synergetic effect from the potential-induced site conversion, the consequent changes in dipole—dipole coupling interactions among the nearby CO oscillators in addition to the potential-dependent chemical bonding changes. The Stark slopes of the Pt−CO stretching of CO L and CO B are negative and roughly constant (ca. −5 and −20 cm −1 /V) throughout the potential regime examined, which is mainly attributed to the potential-dependent changes in the Pt−CO chemical bonding. From the measured potential-dependent frequencies of the Pt−CO stretching, the potential-induced changes in Pt−CO bond length are estimated to be 0.005 (0.01) Å/V for CO L (CO B ) and the changes in CO binding energies (Δ E b /Δ E ) are ca. 0.20 eV/V (for CO L ) and 0.37 eV/V (for CO B ), respectively. The higher Δ E b /Δ E for CO B than that for CO L reveals that the chemical bonding of Pt−CO B is more sensitive to the changes in the interfacial electric field, as is consistent with theoretical predictions. From the DFT calculations using two different (2 × 2)-3CO slab models with Pt(111), we found that the overall trends of the potential-dependent frequencies of Pt−CO and C−O stretching predicted by the DFT slab model are in good agreement with the observation by SERS. However, great discrepancies in the Stark slopes between the calculations and experimental results exist; possible origins for such differences have been discussed.