Surface chemical analysis and electrokinetic properties of spherical hematite particles coated with yttrium compounds

We describe in this work the chemical and electrokinetic surface characterization of core-shell particles consisting of a practically spherical hematite nucleus coated by a layer of yttrium basic carbonate or yttrium oxide (obtained after calcination of the carbonate-coated particles, following the method of E. Matijevic and B. Aiken (J. Colloid Interface Sci. 126, 645 (1988))). The morphological and surface characteristics of the particles were controlled by modifying the initial yttrium nitrate concentration and the growing time. A total of 14 samples of hematite-yttrium basic carbonate composites were obtained, and three of them (obtained by keeping at 90 degrees C solutions containing 6.5 x 10(-4) M alpha-Fe2O3, 1.8 M urea, and 1.1, 3, and 4.9 mM Y(NO3)(3), respectively) were then converted into hematite-Y2O3 particles. Transmission electron microscopy was used to ascertain the shape and size of the particles. The spherical geometry of the core hematite is found, as a rule, on the core-shell particles; in general, carbonate samples obtained with intermediate initial concentration of Y(NO3)(3) have the maximum coating thickness, whereas increasing that concentration does not lead to thicker coatings. Hence, formation of individual yttrium basic carbonate, together with coated hematite, cannot be completely ruled out under such conditions. Two techniques were employed for the elucidation of the surface composition of the particles, namely EDX and XPS (or ESCA). In particular, XPS data show that the coating of hematite by yttrium carbonate is almost complete in the case of particles obtained with 3 mM Y(NO3)(3) concentration and 9-h heating time. The oxide samples obtained after calcination show high contents of yttrium and low iron surface concentration for initial [Y(NO3)(3)] = 1.1 mM (sample OB9) and 3 mM. According to XPS analysis, both types of particles have a quite similar surface composition and structure. For all types of particles but the carbonate-coated ones obtained at the shortest reaction times, the pH(iep) was found to be above that of pure hematite, approaching that of yttrium basic carbonate or oxide. In particular, among the oxide-coated particles, it is sample OB9 the one that most closely approaches its pH(iep) to that of Y2O3, in good agreement with the surface chemical analysis performed with XPS. (C) 1997 Academic Press.