Nanocrystalline dispersions

Mattoussi H efa/1996 Characterization of CdSe nanocrystalline dispersions by small angle x-ray scattering J. Chem. Phys. 105 9890... 

Thin film coatings of nanocrystalline semiconductors, as collections of quantum dots (QD or Q-dot) attached to a solid surface, resemble in many ways semiconductor colloids dispersed in a liquid or solid phase and can be considered as a subsection of the latter category. The first 3D quantum size effect, on small Agl and CdS colloids, was observed and correctly explained, back in 1967 [109]. However, systematic studies in this field only began in the 1980s. 

XRD patterns of the prepared samples V-Mo-Zeolite are similar to that of zeolites which suggests that the metal species (i.e. oxide, cations,...) are well dispersed through the zeolites structure and the absence of bulk phases in the XRD patterns implies that for these samples the molybdenum and vanadium oxides are present in either a nanocrystalline state or as a small crystallites which measured less than 4 nm in diameter. Furthermore, XRD and FTIR (1500-400 cm 1) showed no significant damage of the zeolite host structure after exchange and thermal treatment except for the sample V2MoMor. 

This technique yields a catalyst composed entirely of metal nanoparticles or nanocrystalline thin film, and it allows for control of size and distribution while eliminating the need for a dispersing and supporting medium. The obtained electrodes contained as little as 0.017 mg Pt/cm and performed as well as standard E-TEK electrodes (Pt loading 0.4 mg/cm ). The PLD technique may be of special interest as an alternative to the sputtering process in the production of micro fuel cells. 

Figure 4.10 SEM image of nanocrystalline zeolite taken with field emission SEM operated at 0.5 kV. Sample prep was by dispersing powder in isopropyl alcohol and placing a droplet of the suspended powder on a standard TEM grid. Sample is uncoated.

J. Klabunde, Carbon Dispersion and Morphology in Carbon-Coated Nanocrystalline MgO, Langmuir 19(24), 10426-10433 (2003). 

The aerogel-prepared metal oxide nanoparticles constitute a new class of porous inorganic materials because of their unique morphological features such as crystal shape, pore structure, high pore volume, and surface areas. Also, it is possible to load catalytic metals such as Fe or Cu at very high dispersions on these oxide supports and hence the nanocrystalline oxide materials can also function as unusual catalyst supports. Furthermore, these oxides can be tailored for desired Lewis base/Lewis acid strengths by incorporation of thin layers of other oxide materials or by preparation of mixed metal oxides. 

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.). 

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