Nanocrystalline particles, colloidal

This review is a discussion of the kinetic modelling of the photoelectrochemistry of colloidal semiconductor systems. This area is currently attracting significant attention from the scientific community due to the applications of colloidal semiconductors within two rapidly advancing research fronts heterogeneous photocatalysis and nanocrystalline particle technology. 

Nanocrystalline Ti02 surfaces are prepared by coating conducting glass with a paste containing colloidal semiconductor particles, followed by a sintering process. For the solar cell type applications of nanocrystalline surfaces under discussion here,... 

The mechanism of the charge separation in the colloidal Ti02 particles was studied at low temperature by direct time-resolved and light-modulated EPR techniques [91]. The recombination kinetics in the nanocrystalline semiconductor particles usually is very fast, on... 

Usually there is a lot of effort required to make nanomaterials by electrochemical means. In aqueous solutions the electrodeposition of nanocrystalline metals requires pulsed electrodeposition and the addition of additives whose reaction mechanism hitherto has only been partly understood (see Chapter 8). A further shortcoming is that usually a compact bulk material is obtained instead of isolated particles. The chemical synthesis of metal or metal oxide nanoparticles in aqueous or organic solutions by colloidal chemistry, for example, also requires additives and often the desired product is only obtained under quite limited chemical conditions. Changing one parameter can lead to a different product. 

As shown in [31], after curing at 120 °C these composites contain nanocrystalline monoclinic zirconia particles. IR and NMR analysis (as shown in Fig. 16) show that the carbon CO frequency obtained by complexation is still maintained after hydrolysis of the colloid and after polymerization of the double bonds together with methaciyloxy silanes, no change in the carbonyl frequency can be observed. Even after using these systems as coatings, after two weeks exposure to water no change is observed. Similar results are also obtained by NMR analysis of the carboxylic carbon atom, as shown in Fig. 16. 

Figure 8. a) Energy level position in nanocrystalline Ti02 particles of flat band [41], (Reprinted with permission from N. M. Dimitrijevic, D. Savic, O. I. Micic, A. J. Nozik, J. Phys. Chem., 1984, 88, 4278. Copyright (2000) American Chemical Society), b) Energy level position of surface-trapped holes [42]. (From O. I. Micic, T. Rajh, M. V. Comor, in Electrochemistry in Colloids and Dispersions. (Eds. R. A. Mackay, J. Texter), VCH, New York, 1992, p. 457. Reprinted by permission of John Wiley Sons, Inc.). 

During the last 15 years, many investigations have been performed with semiconductor particles or nanocrystals, either dissolved as colloids or used as suspensions in aqueous solutions. Recently films of nanocrystalline layers have also been produced which were used as electrodes in photoelectrochemical systems. Essential results have already been summarized in various reviews [1-9]. All kinds of systems, containing small or large particles have been used in various investigations. In this chapter, the essential properties of semiconductor particles will be described. Small particles, i.e. nanocrystals, are of special interest because of quantum size effects. 

Fig. 31. SEM showing a porous nanocrystalline electrode consisting of an interconnected assembly of 30 nm colloidal Ti02 particles. Electrodes of this type are used, for example, in dye sensitised solar cells.

Fig. 2 shows the XRD pattern of ZnS nanoparticles. The particles showed the peaks from (111), (220) and (311) planes of the cubic crystal structure. The broadening of the peaks indicates the nanocrystalline nature of the samples. The XRD peaks became sharper with increasing precursor concentration. The absorption spectrum of ZnS colloids is shown in Fig. 3 (left). 

The pol5mier nanocomposite field has been studied heavily in the past decade. However, polymier nanocomposite technology has been around for quite some time in the form of latex paints, carbon-black filled tires, and other pol5mier systems filled with nanoscale particles. However, the nanoscale interface nature of these materials was not truly understood and elucidated until recently [2 7]. Today, there are excellent works that cover the entire field of polymer nanocomposite research, including applications, with a wide range of nanofillers such as layered silicates (clays), carbon nanotubes/nanofibers, colloidal oxides, double-layered hydroxides, quantum dots, nanocrystalline metals, and so on. The majority of the research conducted to date has been with organically treated, layered silicates or organoclays. 

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