Surface Chemical Analysis and Electrokinetic Properties of Synthetic Spherical Mixed Zinc–Cadmium Sulfides
✍ Scribed by J.D.G. Durán; M.C. Guindo; A.V. Delgado; F. González-Caballero
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 440 KB
- Volume
- 193
- Category
- Article
- ISSN
- 0021-9797
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✦ Synopsis
properties of the particles in aqueous suspensions were analyzed In this work, we analyze the surface and bulk chemical composiby electrophoresis: the effect of pH on the electrophoretic mobility, tion, as well as the crystal structure, of colloidal spherical particles and in particular the pH value at which the mobility is zero (isoof Zn-Cd mixed sulfides of different Zn/Cd ratios. The particles electric point or pH iep ) confirms the conclusions obtained from our were obtained by precipitation from solution according to the previous surface chemical analysis concerning the surface oxidamethod described by Wilhelmy and Matijevic [Colloids Surfaces tion of the particles. The effect of lattice ions (Zn 2/ , Cd 2/ , S 20 ) 16, 1 (1985)]. Transmision electron microscopy of the particles in solution on the mobility (and hence on the surface charge) of show that their average diameter ranges from 50-60 nm (when the particles is very significant: the latter ions are able to find the synthesis is carried out at 50ЊC) up to 150-200 nm (for a easily their way to the surface of the sulfides and change to a large temperature of 70ЊC). Atomic absorption analysis of the twelve extent the overall pH-dependence of the mobility and specifically samples obtained indicated that the bulk Zn/Cd ratio increases the values of the pH iep . ᭧ 1997 Academic Press with aging temperature; the same behavior is found when the Key Words: mixed cadmium-zinc colloidal sulfides; XPS specconcentration of Cd(NO 3 ) 2 used in the synthesis is decreased. troscopy; electrophoretic mobility. 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 INTRODUCTION rather surface) analytical tool, the fact that it gives information down to a depth of Ç500 A ˚from the surface makes the type of Zinc, cadmium, and mixed zinc-cadmium sulfides are information obtained with EDX comparable to atomic absorption. compounds with a large variety of technological applica-Although the overall Zn/Cd trends are reproduced by EDX data, tions, including pigment manufacture, because of their high these are not as sensitive as atomic absorption. The surface compoopacity, phospholuminescent screens, and photovoltaic solar sition of three selected samples (M3, 50 min growth time, 50ЊC,
cells because they are semiconductors (1, 2). In many of 0.52 mM Cd 2/ in the growing solution; M8, 100 min, 60ЊC, 0.52 mM; M12, 100 min, 70ЊC, 0.52 mM) was determined by XPS these applications, those sulfides are deposited on some spespectra od Cd 3d 5/2 , Zn 2p 3/2 , and O 1s electrons, for the three cific substrate; this can be done by physical high-vacuum samples. The sequence of variation of the Zn/Cd ratio of M3, M8, techniques, but it is also possible to attach a thin film by and M12 particles agrees qualitatively with that found by atomic precipitation from solution (the so-called ''chemical bath absorption or EDX; the fact that no detectable Cd is found in deposition'') as fine colloidal particles. In such a case, both sample M12 suggests that the particles have a nonhomogeneous the shape and the particle size distribution play an important composition that changes from the core to the surface layer. The role in the material processing as well as in the final product analysis of O 1s electrons allows to reach the conclusion that the properties. For instance, it has been found that particle size surface oxidation changes in the order M3 ú M8 ú M12, i.e., the affects the brightness and the voltage in screens and solar particles are more oxidized the larger the amount of cadmium on cells, respectively (3, 4).
their surface. This is confirmed by electric conductivity determina-In this work we will focus on mixed compounds, which tions in aqueous suspensions of the samples, both in the presence of natural light and in the dark, as a function of time. These data, have received much attention recently; in particular, methods together with crystal structure determinations by XRD, suggest have been devised for their obtention as colloidal particles that, when the growth temperature is 50-60ЊC, the particles conhomogeneous in shape and size (5). Let us mention, for tain a ZnS (sphalerite) nucleus covered by a layer of mixed, hexagtheir relevance for this article, Zn/Cd, Cd/Pb (6), or Zn/ onal Zn-CdS and a surface layer of cubic ZnS. When the aging Ag (3) sulfides and Cd/Ni phosphates (7).
temperature is 70ЊC, the ZnS core is surrounded by a shell con-
The synthesis procedure most often followed is chemical taining cubic ZnS and amorphous CdS. The surface electrical precipitation, starting from homogeneous solutions of salts of the metals desired (5). The characteristics of the particles