Stability of dispersions of colloidal hematite/yttrium oxide core-shell particles.

The colloidal stability of suspensions of hematite/yttria core/shell particles is investigated in this work and compared with that of the pure hematite cores. The different electrical surface characteristics of yttrium and iron oxides, as well as the diameters of both types of spherical particles, dominate the overall process of particle aggregation. The aggregation kinetics of the suspensions was followed by measuring their optical absorbance as a function of time. By previously calculating the extinction cross section of particle doublets, it was demonstrated that for both core and core/shell particles the turbidity of the suspensions should increase on aggregation. Such an increase was in fact found in the systems in spite of the ever-present tendency of the particles to settle under gravity. The authors used the initial slope of the turbidity increment time plots as a measure of the ease of aggregation between particles. Thus, they found that the essential role played by pH on the charge generation on the two oxides and the shift of one pH unit between the isoelectric points of hematite and yttria manifest in two features: (i) the stability decreases on approaching the isoelectric point from either the acid or basic side and (ii) the maximum instability is found for hematite at pH 7 and for hematite/yttria at pH 8, that is, close to the isoelectric points of alpha-Fe(2)O(3) and Y(2)O(3), respectively. The role of added electrolyte is simply to yield the suspensions of either type more unstable. Using the surface free energy of the particles, the authors could estimate their Hamaker constants in water. From these and their zeta potentials, the DLVO theory of stability was used to quantitatively explain their results.

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 90degreesC 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 pHiep 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 pHiep to that of Y2O3, in good agreement with the surface chemical analysis performed with XPS. Copyright 1997 Academic Press. Copyright 1997Academic Press