SURFACE CHEMICAL-ANALYSIS AND ELECTROKINETIC PROPERTIES OF SPHERICAL HEMATITE PARTICLES COATED WITH YTTRIUM COMPOUNDS

We describe in this work the chemical and electrokinetic surface chara cterization of core-shell particles consisting of a practically spheri cal hematite nucleus coated by a layer of yttrium basic carbonate or y ttrium oxide (obtained after calcination of the carbonate-coated parti cles, following the method of E. Matijevic and B. Aiken (J. Colloid In terface Sci. 126, 645 (1988))). The morphological and surface characte ristics of the particles were controlled by modifying the initial yttr ium nitrate concentration and the growing time. A total of 14 samples of hematite-yttrium basic carbonate composites were obtained, and thre e 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. Tran smission 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 sample s obtained with intermediate initial concentration of Y(NO3)(3) have t he maximum coating thickness, whereas increasing that concentration do es not lead to thicker coatings. Hence, formation of individual yttriu m 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 a nd XPS (or ESCA). In particular, XPS data show that the coating of hem atite 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 particl es have a quite similar surface composition and structure. For all typ es of particles but the carbonate-coated ones obtained at the shortest reaction times, the pH(iep) was found to be above that of pure hemati te, approaching that of yttrium basic carbonate or oxide. In particula r, among the oxide-coated particles, it is sample OB9 the one that mos t closely approaches its pH(iep) to that of Y2O3, in good agreement wi th the surface chemical analysis performed with XPS. (C) 1997 Academic Press.