Colloidal growth

Controlled reduction of cadmium (or lead) ions on surfaces of nanosized silver (or gold) metallic particles results in the formation of double-layer colloids [532-534]. Depending on the coverage, the second layer can vary from being non-metallic clusters to quasi-metallic and metallic colloids. Growth of the second-layer particles can be monitored by absorption spectrophotometry. For... 

In this chapter we consider the feasibility of easily controlled and reproducible synthesis of CdS colloids. To provide control and restrain the growth rate of the CdS nanoparticles, we used the complex salt of a colloid-forming component (Cd2+) instead of its diluted solution actually, in this case the rate of colloid growth may be limited by the decay rate of the initial cadmium complex. 

In aqueous solutions, the most common method used to stabilize nanostructures is the use of organic capping ligands. For instance, the Turkevich process, which dates back to early colloidal growth of the 1950s, uses sodium citrate (I) to entrain the reduced gold nuclei. 

The frequency with which two reactive species encounter one another in solution represents an upper bound on the bimolecular reaction rate. When this encounter frequency is rate limiting, the reaction is said to be diffusion controlled. Diffusion controlled reactions play an important role in a number of areas of chemistry, including nucleation, polymer and colloid growth, ionic and free radical reactions, DNA recognition and binding, and enzyme catalysis. 

In this paper we have tried to present the chemical and mechanistic aspects of chemical bath deposition of chalcogenide compounds as they appear both in the recent literature and also in older studies dealing with hydrolysis of chalcogenide precursors. A better account of these aspects gives clues to understanding the properties of the films such as the dependence of composition on solution composition and competitive precipitation processes, and the dependence of structure on competition between atom-by-atom and colloidal growth deposition mechanisms. 

Table I. Experimental Conditions for Colloid Growth Involving the UV-Vis... 

Colloid growth under different conditions was monitored using photon correlation spectroscopy, which measures the size of submicron particles. The size of colloid particles during the growdi process was measured with 90 Plus Particle Sizer (Brookhaven Instrument Co., Holtsville, NY). The reactions were conducted inside the cuvettes supplied with the instrument The experiments were run imder conditions listed in Table n. Measurements were made automatically by the instruments at pre-set times. KCl was added to the reaction solution to provide the same ionic strength for all runs. 

Another explanation of the phosphate effect is possible phosphate-Mn(IV) interactions in aqueous phase. When chlorinated ethylenes are oxidized by Mn04, soluble Mn(IV) forms before any colloids. The existence of the soluble Mn(IV) has been reported by many researchers 16, 17). Phosphate ion can react with soluble Mn(IV) species and reduce the formation of die colloid. The process is probably involved with the formation of a phosphate-Mn(IV) conqilex. As the conqilex forms, it keeps the Mn(IV) in the aqueous phase without forming colloids. Eventually, colloids and prec itates will be produced when the capacity of the phosphate effect has reached its limit This mechanism is in agreement widi our observation in the colloid growth experiments. 

Under these assumptions these structures can be explained by mechanisms of colloidal growth processes. In other words, a self-organization process on the mesoscale through the assembly of calcium carbonate nanoparticles is assumed [36]. The polymorphic phase of these primary particles could not be determined in these experiments due to the small amount of substance available. 

Burshtain, D., L. Zeiri, and S. Efrima. 1999. Control of colloid growth and size distribution by adsorption-silver nanoparticles and adsorbed anisate. Langmuir 15 (9) 3050-3055. 

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