RESUMO
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
RESUMO
In this work, we analyze the surface and bulk chemical composition, as well as the crystal structure, of colloidal spherical particles of Zn-Cd mixed sulfides of different Zn/Cd ratios. The particles were obtained by precipitation from solution according to the method described by Wilhelmy and Matijevic [Colloids Surfaces 16, 1 (1985)]. Transmision electron microscopy of the particles show that their average diameter ranges from 50-60 nm (when the synthesis is carried out at 50degreesC) up to 150-200 nm (for a temperature of 70degreesC). Atomic absorption analysis of the twelve samples obtained indicated that the bulk Zn/Cd ratio increases with aging temperature; the same behavior is found when the concentration of Cd(NO3)2 used in the synthesis is decreased. Similarly, the bulk proportion of Zn in the particles is higher the longer the growth time. EDX microanalysis was also performed on all the samples; although this technique is not a bulk (but rather surface) analytical tool, the fact that it gives information down to a depth of approximately 500 A from the surface makes the type of information obtained with EDX comparable to atomic absorption. Although the overall Zn/Cd trends are reproduced by EDX data, these are not as sensitive as atomic absorption. The surface composition of three selected samples (M3, 50 min growth time, 50degreesC, 0.52 mM Cd2+ in the growing solution; M8, 100 min, 60degreesC, 0.52 mM; M12, 100 min, 70degreesC, 0.52 mM) was determined by XPS spectra od Cd 3d5/2, Zn 2p3/2, and O 1s electrons, for the three samples. The sequence of variation of the Zn/Cd ratio of M3, M8, and M12 particles agrees qualitatively with that found by atomic absorption or EDX; the fact that no detectable Cd is found in sample M12 suggests that the particles have a nonhomogeneous composition that changes from the core to the surface layer. The analysis of O 1s electrons allows to reach the conclusion that the surface oxidation changes in the order M3 > M8 > M12, i.e., the particles are more oxidized the larger the amount of cadmium on their surface. This is confirmed by electric conductivity determinations in aqueous suspensions of the samples, both in the presence of natural light and in the dark, as a function of time. These data, together with crystal structure determinations by XRD, suggest that, when the growth temperature is 50-60degreesC, the particles contain a ZnS (sphalerite) nucleus covered by a layer of mixed, hexagonal Zn-CdS and a surface layer of cubic ZnS. When the aging temperature is 70degreesC, the ZnS core is surrounded by a shell containing cubic ZnS and amorphous CdS. The surface electrical properties of the particles in aqueous suspensions were analyzed by electrophoresis: the effect of pH on the electrophoretic mobility, and in particular the pH value at which the mobility is zero (isoelectric point or pHiep) confirms the conclusions obtained from our previous surface chemical analysis concerning the surface oxidation of the particles. The effect of lattice ions (Zn2+, Cd2+, S2-) in solution on the mobility (and hence on the surface charge) of the particles is very significant: the latter ions are able to find easily their way to the surface of the sulfides and change to a large extent the overall pH-dependence of the mobility and specifically the values of the pHiep. Copyright 1997 Academic Press. Copyright 1997Academic Press
