Amorphous particle size effect

Numerous physical techniques have been used to characterize the bulk and the surface of amorphous phases, including X-ray, AES, SEM, XPS, Mossbauer spectroscopy (MOS) and STM. MOS has been able to highlight the state of Fe in a elec-trodeposited FeP alloy [573]. Three different non-equivalent positions of Fe with different Fe-P distances have been identified. In Feg3P7, 16% of Fe is in the amorphous state. This indicates the extent by which the properties of these phases can be controlled. STM of FeCo alloys has shown that the surface may possess properties depending on the size of the homogeneous zones (cf. particle size effect) [588]. 

Many varieties of the so-called red phosphorus (P , n 4), both amorphous and crystalline, are now known. All are polymeric with colours desaibed as brown-red, yellowish-red, pale-red, bright-red, dark-red, pnrple-red, violet and so forth. While some of these variations can be associated with differences of crystalline form, other factors inflnencing colour include degree of crystallinity, degree of polymerisation, presence of impurities, and particle size effects. Many of these factors have been unrecognised when precise measurements of physical properties have been reported, and variations in published data will be found. Black phosphorus, the high-pressure form, is also highly polymeric, but has (historically) been considered as a form separate from the white or red varieties (see Chapter 4.1). 

The particle size effect is probably also involved in a phenomenon described by Baumann (173). Amorphous silica powder condensed from a flame consists of fine spherical particles generally less than 150 A diameter. When such a powder is placed in water, a supersaturated solution of silica is obtained, no doubt owing to a very small percentage of more soluble particles under 50 A diameter. The dissolved monomeric silica then rapidly polymerizes to polysilicic acids, but these later disappear as supersaturation is relieved by deposition of silica on the larger amorphous particles in suspension. 

Infrared spectra suggested that a sulfate ion coordinates to two titanium atoms as a bidentate in particles. The maximum particle size was found at Aerosol OT mole fraction of 0.35 in the mixtures. The particle size increased linearly with increasing the concentration of sulfuric acid at any Wo, but with increasing Wo the effect was the opposite at any sulfuric acid concentration. These effects on the particle size can be explained qualitatively in relation with the extent of number of sulfate ions per micelle droplet. These precursor particles yield amorphous and nanosized TiO particles, reduced by 15% in volume by washing of ammonia water. The Ti02 particles transformed from amorphous to anatase form at 400°C and from anatase form to rutile form about at 800°C. In Triton X-100-n-hexanol-cyclohexane systems, however, spherical and amorphous titanium hydroxide precursor were precipitated by hydrolysis of TiCl4 (30). When the precursor particles were calcinated,... 

The terms nanocrystals and quantum dots are often used interchangeably. Quantum dots, as used here, are invariably nanocrystals (amorphous materials could, in principle, also exhibit quantum size effects as long as some electronic separation between different particles occurs) that show quantum effects, while nanocrystals may or may not be small enough to exhibit such effects. 

The most obvious choice to determine phases that may be present in the molybdena catalyst is XRD. Matching of diffraction lines obtained for the catalyst with those of pure bulk compounds gives unequivocal identification of phases present. This is one of the few techniques that yields positive results. The absence of matching diffraction lines, however, is not proof that the phase in question is not present in the catalyst. The XRD technique is limited to particle sizes of above approximately 40 A for oxides or sulfides, lower sized particles giving no discernible pattern over that of the broad alumina pattern. Thus, the presence of a highly dispersed phase, either as small crystallites or as a surface compound of several layers thickness will not be detected. Also, if the phase is highly disordered (amorphous), a sharp pattern will not be obtained, although some broad structure above that of the alumina may be detected. It is a moot point as to whether such a case is considered as a separate phase or a perturbation of the alumina structure. Ratnasamy et al. (11) have examined their CoMo/Al catalyst from the latter point of view, with particular emphasis on the effect of calcination temperature. 

Surfactants are employed in nanoparticle suspensions. Chen et al. (2002) evaluated the pre paration of amorphous nanoparticle suspensions containing cyclosporine A using the evaporative precipitation into aqueous solution (ERAS) system. The effect of particle size was studied varying the drug surfactant ratios, type of surfactants, temperature, drug load, and solvent. Acceptable particle sizes suitable for both oral and parenteral administration were also studied. Additional articles in the nanoparticle delivery of poorly water-soluble drugs include Kipp (2004), Perkins et al. (2000), Young et al. (2000), and Tyner et al. (2004). 

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

Masson, A., Bellamy, B., Romdhane, Y. H., Che, M., Roulet, H., and Dufour, G., Intrinsic size effect of platinum particles supported on plasma-grown amorphous alumina in the hydrogenation of ethylene. Surf. Sci. 173, 479 (1986). 

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