Revealing Formic Acid Adsorption Geometries on Magnetite (001) and (111) Surfaces by IRAAS Line Shape Analysis

Abstract

On oxide surfaces, the sign of an Infrared band depends to a large extent on the polarization of light and the geometry of the adsorbed species. In this work, we demonstrate that single crystalline magnetite (001) and (111) being neither a perfect metal nor a perfect insulator not only present Fano shape lines but also result in IR bands which can be surprisingly seen by both s- and p-polarized light. We conducted formic acid adsorption on two surfaces of Fe₃O₄(001) and (111) in both s- and p- polarized light and demonstrated distinguished IR bands and even new bands which were not observed before with unpolarized light or only in p-polarization experiments. The experimental results were compared with spectra obtained from density functional theory (DFT) calculations for further insights on the geometry of the adsorbates and the vibrational motions corresponding to the observed bands. On the Fe₃O₄(001) surface, we found a new band at 1555 cm⁻¹ at p-polarized light assigned to the asymmetric stretching vibration of OCO on octahedrally Fe sites with a Fe_tet1 underneath. On the magnetite (111) surface, we observed a new IR band at 1730 cm₋₁ in both non-polarized and s-polarized light originating from a stretching vibration of C=O in molecularly adsorbed formic acid, in good agreement with the DFT calculations. This study shows the importance of different s- and p-polarized light in Infrared Reflection-Absorption Spectroscopy (IRRAS) on detecting and assigning of the IR bands, adsorption sites and electronic nature of the magnetite (001) and (111) surfaces.