Surface Chemical Analysis and Electrokinetic Properties of Synthetic Spherical Mixed Zinc–Cadmium Sulfides

Abstract

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 Surfaces16,1 (1985)]. Transmision electron microscopy of the particles show that their average diameter ranges from 50–60 nm (when the synthesis is carried out at 50°C) up to 150–200 nm (for a temperature of 70°C). 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)2used 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 ∼500 Å 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, 50°C, 0.52 mMCd2+in the growing solution; M8, 100 min, 60°C, 0.52 mM;M12, 100 min, 70°C, 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 50–60°C, 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 70°C, 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.