Colloid properties particle structure

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... 

Because of its heterophase nature, emulsion polymerization is generally more complicated than simple solution polymerization in which monomers and polymers are soluble in a suitably chosen solvent. In emulsion polymerization the different relative solubilities of monomers in water and in the polymer particles lead to different reaction locales and to different particle structures. Another complicating factor is the need to achieve and maintain colloidal stability throughout the polymerization and subsequent handling of the dispersions. Emulsion polymers can properly be called products by process since the process details exert such a powerful effect on the properties of the particles and resultant films. Consequently, an emulsion polymer is far more than a product defined by a simple polymer composition. 

More recently Bouzek et al. investigated the effect of the preparation conditions of Pt-modified polypyrrole films on their electrocatalytic properties for the HOR [26]. Three methods were considered (1) cathodic deposition of Pt from H2PtCl6 in the previously synthesized film, (2) incorporation of colloidal Pt particles during the electropolymerization of polypyrrole (3) incorporation of [PtCU] as a counter-ion during the electropolymerization process and its subsequent reduction. Only the first two methods lead to active electrocatalytic films, whereas the last one gives very poor catalysts, maybe because the Pt particles are embedded in the PPy structure and therefore are not accessible to the reactant. 

The measurement of the NMR shift of a metal in small particulate form addresses quite directly the important question of at what size a metal particle begins to develop metdlic properties. Particles of copper (less than 8 nm) have been observed to have a diminished Cu NMR Kni t shift at low temperature [235] as a result of quantum size effects (i. e. the loss of the metallic band structure). This will have a clear relevance to the study of colloidal metal partides. 

In principle, then, through equation (45), light-scattering studies of concentrated colloidal dispersions yield structural information in the form of h(r). In practice, however, the usefulness of equation (45) is severely limited by the fact that it is valid only in the absence of multiple scattering. With concentrated aqueous dispersions of latex particles, this means studying very small sample volumes in which structure could be influenced by the surface properties of the container. Even so, no measurements of this sort have yet been reported due to the experimental difficulties involved. 

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