Speciation of Europium (III) Surface Species on Monocrystalline Alumina Using Time-Resolved Laser-Induced Fluorescence—Scanning Near-Field Optical Microscopy

Abstract

The aim of this work was to perform highly localized spectroscopic surface measurements by combining time-resolved laser spectroscopy and scanning near-field optical microscopy. The final purpose of that was to study surface sorption at the molecular level of trivalent ions in the framework of nuclear waste disposal assessment. Time-resolved laser spectroscopy presents the advantages of being selective, sensitive, and noninvasive and scanning near-field optical microscopy is a promising technique for high resolution surface speciation. Investigation of the interaction between trivalent europium and a monocrystalline alumina (11̄02) surface was made using different conditions of concentration and pH. We found that the distribution of sorbed europium was always homogeneous with a decay time of europium (III) equal to 350 μs ± 15 μs. On the other hand, carbonate species with a decay time of 210 μs ± 10 μs or other hydroxide species with a decay time of 180 μs ± 10 μs were detected on the surface when a higher concentration or a higher pH solution, respectively, were used. Distribution of these species was heterogeneous and their associated fluorescence signal was relatively high, evoking a precipitated form. X-ray photoelectron spectroscopy (XPS) was also used on the same samples as a complementary technique. A binding energy of 1135.1 eV was obtained for the sorbed europium and another binding energy of 1134.4 eV was obtained for the hydroxide species, thus confirming the presence of two kinds of species on the surface.