Characterization of alumina surfaces by fluorescence spectroscopy : Part 2. Photophysics of a bound pyrene derivative as a probe of the spatial distribution of reactive hydroxyl groups

In order to characterize and compare the spatial distribution of hydroxyl groups on gamma- and delta-alumina surfaces, a pyrene derivative was covalently grafted on these groups and studied by steady-state and time-resolved fluorescence spectroscopy. When the pyrene probes are close enough, they can form excimers with a characteristic emission band. The relative intensities of the monomer and excimer bands provide a global information on the proximity of pyrene probes, whereas time-resolved fluorescence experiments offer a more detailed characterization of the surface in terms of heterogeneity. The fluorescence decay curves were satisfactorily analyzed in terms of stretched exponentials that account for the distribution of decay times resulting from the distribution of distances between the pyrene probes that can form excimer, and the distribution of the energies of interaction between the alumina surface and the pyrene monomers or excimers. Comparison of samples having the same surface coverage of pyrene probes allowed us to conclude that there are indeed significant differences in the characteristics of gamma- and delta-alumina surfaces. In particular, the reactive hydroxyl groups are not homogeneously distributed on alumina surfaces but they are rather clustered into regions of high density. The fraction of these reactive groups belonging to the clustered regions is more than four times larger for delta-alumina than for for gamma-alumina.