Nonlinear Stark effect observed for carbon monoxide chemisorbed on gold core/palladium shell nanoparticle film electrodes, using in situ surface-enhanced Raman spectroscopy

Abstract

The potential (E)-dependent vibrational behavior of a saturated CO adlayer on Au-core Pd-shell nanoparticle film electrodes was investigated over a wide potential range, in acidic, neutral, and basic solutions, using in situ surface-enhanced Raman spectroscopy (SERS). Over the whole of the examined potential region (–1.5 to 0.55 V vs. NHE), the peak frequencies of both the C–OM and the Pd–COM band (here, M denotes the multiply-bonded configuration) displayed three distinct linear regions: dνC–OM/dE decreased from ∼185–207 (from –1.5 to –1.2 V) to ∼83–84 cm−1/V (–1.2 to –0.15 V), and then to 43 cm−1/V (–0.2 to 0.55 V); on the other hand, dνPd–COM/dE changed from ∼–10 to –8 cm−1/V (from –1.5 to –1.2 V) to ∼–31 to –30 cm−1/V (–1.2 to –0.15 V), and then to –15 cm−1/V (–0.2 to 0.55 V). The simultaneously recorded cyclic voltammograms revealed that at E < –1.2 V, a hydrogen evolution reaction (HER) occurred. With the help of periodic density functional theory calculations using two different (2 × 2)-3CO slab models with Pd(111), the unusually high dνC–OM/dE and the small dνPd–COM/dE in the HER region were explained as being due to the conversion of COad from bridge to hollow sites, which was induced by the co-adsorbed hydrogen atoms formed from dissociated water at negative potentials.