Assembly of octadecyl phosphonic acid on the α-Al2O3 (0 0 0 1) surface of air annealed alumina: Evidence for termination dependent adsorption

Self-assembly of octadecyl phosphonic acid (ODPA) from solution to the (0 0 0 1) plane surface of single crystal alumina is studied using atomic force microscopy (AFM), wettability measurements, transmission infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). The crystals were annealed in air at 1300 °C to produce a surface with terraces of 0.2 nm height. This resulted in a surface comprising a mixture of terraces terminated by either oxygen or aluminium atoms. After exposure of this surface to a solution of ODPA, the XPS C1s core level reveals partial displacement of adventitious carbonaceous material by the ODPA. The phase contrast image from tapping mode AFM acquired during assembly reveals that the surface chemistries of terraces differ significantly, suggesting along with the topography images that ODPA molecules adsorbed preferentially to certain terraces. We suggest that the ODPA adsorbs preferentially to the aluminium terminated terraces that are hydroxylated in air. The adsorption to only Al-terminated terraces at the micron scale is consistent with the apparent low coverage of ODPA estimated using techniques sampling the surface on a millimetre scale including XPS and wettability measurements. This behaviour is compared to ODPA close-packed monolayer coverages formed on the amorphous oxide/hydroxide surface of aluminium metal.