Theoretical Approaches for Modeling the Effect of the Electrode Potential in the SERS Vibrational Wavenumbers of Pyridine Adsorbed on a Charged Silver Surface

Daniel Aranda,Juan C Otero,Juan Soto,Isabel López-Tocón,Samuel Valdivia,Francisco J Avila

doi:10.3389/fchem.2019.00423

Abstract

Vibrational wavenumbers of pyridine adsorbed on a silver electrode have been correlated to the calculated ones from different theoretical approaches based on DFT methods. The vibrational tuning caused by the electrode potential has been simulated by means of pyridine-silver clusters with different densities of charge or, alternatively, under applied external electric fields. Both methodologies predict correctly a qualitative red-shift of the vibrational wavenumbers at negative potentials. As a result, harmonic frequency calculations performed at the B3LYP/LanL2DZ level of theory by using a linear [AgnPy]q complex model with different densities of charge (qeff = q/n) have exhibited the best agreement with the experimental observations although the tuning amplitudes are overestimated. Electric fields calculations are unable to account for subtle details observed in the spectra related to the differentiated chemical nature of the metal-molecule bond at positive or negative potentials with respect to the potential of zero charge of the electrode.

Highlights

Surface-enhanced Raman Scattering (SERS) is one of the most powerful techniques to get insight into the complex electronic structure of molecules bonded to charged metals (Weaver et al, 1985; Wolkow and Moskovits, 1992; Szekeres and Kneipp, 2019)
The main characteristic of SERS is the huge enhancement of the Raman signal, but the spectra show very often changes in the relative intensities as well as in the vibrational wavenumbers of the bands which are modified by the electric potential in electrochemical SERS experiments
To avoid selecting arbitrarily one of them we have considered the simplest case where pyridine is bonded to a terminal atom of the linear cluster through the nitrogen atom

Summary

Introduction

Surface-enhanced Raman Scattering (SERS) is one of the most powerful techniques to get insight into the complex electronic structure of molecules bonded to charged metals (Weaver et al, 1985; Wolkow and Moskovits, 1992; Szekeres and Kneipp, 2019) These metal-molecule hybrid systems are involved in many scientific fields like electrochemistry or heterogeneous catalysis but they are related to the electrical transport through single molecule junctions or are part of electronic devices for solar energy conversion. All of these scientific areas are controlled by the overall electronic structure of the metal-adsorbate complex whose chemical and physical properties can be, in many cases, tuned continuously by applying electric potentials. Changes in the intensities are related to the enhancement mechanism or mechanisms acting in each particular experiment (plasmonic, charge-transfer resonances, etc.) (Aroca, 2006; Kneipp et al, 2006; Le Ru and Etchegoin, 2008).

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