Surface sensitive infrared spectroelectrochemistry using palladium electrodeposited on ITO-modified internal reflection elements

Abstract

Palladium nanoparticles have been electrodeposited on the surfaces of conductive indium tin oxide (ITO) modified silicon internal reflection elements. The resulting films are shown to be excellent platforms for attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) studies of palladium surfaces. Monitoring the mid-infrared reflectivity of the interface during the constant potential electrodepostion of a Pd²⁺ precursor reveals a distinct and reproducible minimum that corresponds to the onset of the electronic percolation threshold of the deposited metal islands as confirmed by scanning electron microscopy. Effective medium theory (EMT) is used to model the reflectivity of the Si/ITO/Pd interface as a function of the volume fraction of the deposited metal and to calculate the ATR-SEIRA spectra of an adsorbed monolayer of organic molecules. EMT calculations are in qualitative agreement with most aspects of the experimental spectra which show that the intensities and spectral line shapes are highly dependent on the amount of deposited palladium. The methodology is applied to the potential dependent adsorption of 4-methoxypyridine on palladium. The experimental results show that the pyridine derivative adopts an edge-tilted orientation on oxide covered Pd and undergoes an orientation change in the hydrogen adsorption region that increases both the degree of edge-tilt and the extent of π-bonding between the pyridine ring and the metal.