Experimental characterization of hydroxyl groups and acid-base properties on flat oxide surfaces

The knowledge about oxide surfaces mostly refers to finely divided samples, instead of flat surfaces which, because of their technological relevance, deserve a specific research effort. In the present work, three experimental approaches on such surfaces are presented. For well-defined MgO and Al2O3 crystals surfaces, the ability of oxide substrates to become hydroxylated is discussed by comparing experimental results to quantum calculations. A connection between the OH stretching frequency and the basicity of the surface oxygen atoms is established in the case of the magnesium oxide, in agreement with theory. A direct determination of the point of zero charge of oxide surfaces is then achieved by probing the surfaces by either adhesion of colloidal particles, or force measurements which are performed by atomic force microscopy. These methods open a way for a local characterization of surfaces. Finally, the role of OH groups in grafting and adhesion to oxide substrates is examined by in situ infrared spectroscopy study. On silica surfaces, the carbonyl groups of poly(methyl methacrylate) are shown to be hydrogen-bonded to the surface silanol groups, while organosilanes are shown to graft via siloxane Si-O-Si covalent bonds.