Organic linkers on oxide surfaces: Adsorption and chemical bonding of phthalic anhydride on MgO(100)

Abstract

To elucidate the adsorption behavior and interaction mechanisms of organic linker units on oxide surfaces, we have performed a model study under ultrahigh vacuum (UHV) conditions. We apply infrared reflection absorption spectroscopy (IRAS) in combination with density-functional theory (DFT), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). Phthalic anhydride (PAA) was deposited at temperatures between 100 and 300K by physical vapor deposition (PVD) onto an ordered MgO(100) film grown on Ag(100). At 100K, the first monolayer adsorbs molecularly with the molecular plane aligned parallel to the surface. Subsequent growth of a multilayer film at low temperature also occurs with preferential molecular alignment parallel to the surface. At 240K, the multilayer desorbs without decomposition. At 300K, a mixed monolayer of chemically modified ring-opened and intact phthalic anhydride exists on the surface. The chemically modified species binds in a strongly tilted geometry via opening of the anhydride ring to form a bis-carboxylate species. This species additionally stabilizes the coadsorbed molecular PAA via intermolecular interactions. Finally, surface defects and hydroxyl groups are found to increase the amount of surface bis-carboxylate at 300K, whereas the relative amount of coadsorbed molecular PAA decreases.