Colloidal cadmium sulfide preparation

Matijevic, E. and Wilhehny. D.M., Preparation and properties of monodispersed spherical colloidal particles of cadmium sulfide. J. Colloid Intetf. Sci., 86, 476, 1982. 

Dutta P, Fendler JH (2002) Preparation of cadmium sulfide nanoparticles in self-reproducing reversed micelles. J Colloid Interface Sci 247 47-53... 

Hydrazone cyclization and hydroalkylation [138-140] are rare examples of reactions conducted on a preparative scale, since the products were isolated in milligram amounts and not just identified in solution. As already mentioned in Section 6.2.5, photocorrosion of the semiconductor photocatalyst often prevents its use in preparative chemistry. This is very true also for colloidal semiconductors although the pseudo-homogeneous nature of their solutions allows one to conduct classical mechanistic investigations, until now they were too labile to be used in preparative chemistry [107, 141, 142]. In contrast to the above-mentioned reactions, in recent years we have isolated novel compounds on a gram-scale employing photostable zinc and cadmium sulfide powders as photocatalysts [97, 107, 143-145]. During this work we found also a new reaction type which was classified as semiconductor photocatalysis type B [45]. In contrast to type A reactions, where at least one oxidized and one reduced product is formed, type B reactions afford only one unique product, i.e., the semiconductor catalyzes a photoaddition reaction (see below). 

Preparation of Colloidal CdS. Colloidal CdS samples were prepared by precipitation from an aqueous surfactant solution. Aqueous sodium sulfide was slowly added to a stirred solution of cadmium chloride plus surfactant, which produced a clear, yellow-orange colloidal sample of cadmium sulfide. The particle sizes of the colloids were on the order of 250-300 A in radius. There was no observable change in the particle radius upon addition of MV to the CdS colloids. Furthermore, in the absence of surfactant CdS rapidly precipitates from solution. 

Photochemistry of Titanium Dioxide Colloids. Another semiconductor colloid used in our studies is titanium dioxide which has a band gap of 3.2 eV. As in the case of cadmium sulfide, excitation of aqueous suspensions of this particle leads to electron-hole pair separation which can be intercepted with suitable redox reagents. In the absence of externally added solutes, the photogenerated electron-hole pair recombines to give the starting material and the light energy is dissipated to the medium as heat. Two types of TiOj samples are used in this study. TiOj prepared at high temperature (80°C) which behaves very similarly to commercial samples, and TlOj prepared at low temperature (35°C) which has a particle size of 300 100 A radius and shows different properties. 

Chang, S.Y., Liu, L., and Asher, S.A., Preparation and properties of tailored morphology, monodisperse colloidal silica-cadmium sulfide nanocomposites, J. Am. Chem. Soc., 116, 6739, 1994. 

S.-Y. Chang, L. Liu and S. A. Asher, Preparation and Processing ofMonodisperse Colloidal Silica-Cadmium Sulfide Nanocomposites, Mat. Res. Symp. Proc. 346, 875-880 (1994). 

The luminescence of macrocrystalline cadmium and zinc sulfides has been studied very thoroughly The colloidal solutions of these compounds also fluoresce, the intensity and wavelengths of emission depending on how the colloids were prepared. We will divide the description of the fluorescence phenomena into two parts. In this section we will discuss the fluorescence of larger colloidal particles, i.e. of CdS particles which are yellow as the macrocrystalline material, and of ZnS particles whose absorption spectrum also resembles that of the macrocrystals. These colloids are obtained by precipitating CdS or ZnS in the presence of the silicon dioxide stabilizer mentioned in Sect. 3.2, or in the presence of 10 M sodium polyphosphate , or surfactants such as sodium dodecyl sulfate and cetyldimethylbenzyl-ammonium... 

Fig. 2.2 shows the adsorption spectra of a colloidal CdS solution prepared by the above method in the presence of cadmium complexones of various nature. The position of the colloids adsorption band indicates that the equilibrium size of the colloidal particles decreases as the stability constant of the complex increases. This may relate to the fact that it is precisely the decay rate of the cadmium complex that determines the number of nuclei N and, hence, the size of the forming particles. This is supported by the fact that with the fixed initial (before the addition of the sulfide anion and after the addition of the ligand) concentration of activated Cd2+ (lg[CdL]/[Cd2+]) = const and [Cd°] = const), for complexones of various nature, the sizes of colloidal particles differ the stronger the initial complex, the smaller the particle size. 

One of the other early investigations came from Lianos and Thomas [346] who used AOT micelles in heptane for the synthesis. Two solutions, one containing cadmium perchlorate hexahydrate and another containing sodium sulfide nonahydrate, were prepared. Mixing of the two solutions in the reverse micelles produced small particles of CdS. The solutions were bubbled with nitrogen gas for deoxygenation and avoiding colloidal sulfur formation. The same authors used... 

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