Revealing formic acid adsorption geometries on magnetite (001) and (111) surfaces by IRRAS line shape analysis

The sign and intensity of infrared (IR) bands on oxide surfaces strongly depend on light polarization and the adsorption geometry of surface species. In this study, we investigate formic acid adsorption on single crystalline magnetite Fe₃O₄(001) and Fe₃O₄(111) surfaces, which being neither perfect metals nor insulators exhibit characteristic Fano shaped IR line profiles. Using both s- and p-polarized infrared reflection absorption spectroscopy (IRRAS), we identify distinct spectral features and reveal vibrational bands that were previously unobserved in experiments employing unpolarized or solely p-polarized light. Complementary density functional theory (DFT) calculations provide structural and vibrational insights into the adsorbed species. On Fe₃O₄(001), a new band at 1555 cm^-1 observed with p-polarized light is attributed to the asymmetric OCO stretching vibration of formate bound to octahedral Fe sites with tetrahedral Fe_tet1 underneath. On Fe₃O₄(111), a band at 1730 cm^-1 detected in both unpolarized and s-polarized light corresponds to the CO stretch of a monodentate species. These results highlight the crucial role of light polarization in IRRAS for elucidating adsorption geometries and electronic properties of oxide surfaces.