Structural and spectroscopic study of adsorption of naphthalene on silver

Abstract

Adsorption characteristics of naphthalene on silver are investigated using methods based on Density Functional Theory (DFT) and Surface Enhanced Raman Spectroscopy (SERS). Variations in bond angles and dihedral angles of the optimized geometry of naphthalene after adsorption on silver indicate distorted hexagonal structure of the ring nearer to Ag atoms and deviations in co-planarity of carbon atoms. Theoretical computations establish binding interactions through π electrons as natural bond orbital analysis confirms intramolecular charge transfers originating from the orbital overlap between π(CC) to π∗(CC) and π(CC) to σ∗(AgAg) orbitals. Higher polarization values resulting from charge transfers on adsorption, indicated by DFT calculations, account for Raman enhancement of selective vibrational modes and band shifts. Silver nanoparticles (Ag NPs) were prepared using solution combustion method and were characterized by X-ray diffraction (XRD) and High Resolution Transmission Electron Microscopy (HRTEM). Surface plasmon resonance peak observed around 412nm in the optical absorption spectrum of Ag NPs after adsorption of naphthalene is in good agreement with the theoretically simulated UV spectra derived using Time-Dependent Density Functional Theory (TDDFT) calculations. Theoretical and experimental SERS are correlating well, strongly confirming the process of adsorption, the tilted orientation of naphthalene on silver surface and the adsorption mechanism reported. Localization of electron density resulting from redistribution of electrostatic potential after adsorption on silver together with the reduction in bandgap of naphthalene suggests its utility in the design of electro active organic molecular devices.