Nanoparticles dispersion

Noble metal nanoparticles dispersed in insulating matrices have attracted the interest of many researchers fromboth applied and theoretical points of view [34]. The incorporation of metallic nanoparticles into easily processable polymer matrices offers a pathway for better exploitation of their characteristic optical, electronic and catalytic properties. On the other hand, the host polymers can influence the growth and spatial arrangement of the nanoparticles during the in situ synthesis, which makes them convenient templates for the preparation of nanoparticles of different morphologies. Furthermore, by selecting the polymer with certain favorable properties such as biocompatibiHty [35], conductivity [36] or photoluminescence [37], it is possible to obtain the nanocomposite materials for various technological purposes. 

Let us come back to the sample preparation A drop of solution containing silver nanoparticles dispersed in hexane is deposited on the substrate. The nanocrystals can be removed by washing the substrate and collected in hexane. The absorption spectrum of silver particles recorded before and after deposition remains the same. This indicates that coalescence does not take place. Similar behavior was observed by using HOPG as a substrate [6,35]. 

Another method is based on the evaporation of a w/o microemulsion carrying a water-soluble solubilizate inside the micellar core [221,222], The contemporaneous evaporation of the volatile components (water and organic solvent) leads to an increase in the concentration of micelles and of the solubilizate in the micellar core. Above a threshold value of the solubilizate concentration, it starts to crystallize in confined space. Nanoparticle coalescence could be hindered by surfactant adsorption and nanoparticle dispersion within the surfactant matrix. 

The preparation and study of metal nanoparticles constitutes an important area of current research. Such materials display fascinating chemical and physical properties due to their size [62, 63]. In order to prevent aggregation, metal nanoparticles are often synthesized in the presence of ligands, functionalized polymers and surfactants. In this regard, much effort has focused on the properties of nanoparticles dispersed into LCs. In contrast, the number of nanoparticles reported that display liquid crystal behavior themselves is low. Most of them are based on alkanethiolate stabilized gold nanoparticles. 

Principally purification and characterization methods of monometallic nanoparticles are directly applied to those of bimetallic nanoparticles. Purification of metal nanoparticles dispersed in solution is not so easy. So, in classical colloid chemistry, contamination is carefully avoided. For example, people used pure water, distilled three times, and glass vessels, cleaned by steam, for preparation of colloidal dispersions. In addition, the reagents which could not byproduce contaminates were used for the preparation. Recently, however, various kinds of reagents were used for the reaction and protection. Thus, the special purification is often required especially when the nanoparticles are prepared by chemical methods. 

Metal Nanoparticles Dispersed in Solution Tests to Identify the Catalyst Nature... 

FIG. 20 23 Normalized photoluminescence spectra of 3.1-um ( excitation = 320 nm) and4.2-nm (Xexdtation = 340 nm) Ge nanoparticles dispersed in chloroform at 25 C with quantum yields of 6.6 and 4.6 percent, respectively. [Reprinted with permission from Lu et al. Nano Lett, 4(5), 969-974 (2004). Copyright 2004 American Chemical Society. ]... 

An apparatus for measuring the UV-visible spectrum of a nanoparticle dispersion under pressurized conditions is shown in Figure 2.2 [16, 19, 20]. This apparatus... 

Figure 2.7 Effect of various factors on nanoparticle dispersion (as determined by tracking the absorbance oTIspr) as a function of applied C02 pressure, (a) effect of temperature on DDT-stabilized gold nanoparticles dispersed in hexane (b) effect of solvent on DDT-stabilized gold nanoparticles at room temperature (c) effect of ligand on gold...

Figure 2.8 SANS measurement of ligand shell thickness as a function of C02 composition for differently sized DDT-stabilized silver nanoparticles dispersed in n-hexane-t/14 [40].

One method of overcoming the detrimental solvent dewetting effects is to use liquid C02 as the solvent for nanoparticle dispersions [52], since C02 does not experience the dewetting instabilities due to its extremely low surface tension [53]. In this case, nanoparticles must be stabilized with fluorinated ligands [30, 33, 54—65] or other C02-philic ligands [60,66-76], such that they will disperse in the C02 prior to dropcasting. These fluorinated ligands tend be toxic and environmentally persistent and, typically, only very small nanoparticles can be dispersed at low concentrations. 

Saunders, S.R., Anand, M., You, S.S. and Roberts, C.B. (2010) Total interaction energy model to predict nanoparticle dispersibility in C02-expanded solvents, in Computer Aided Chemical Engineering, vol. 28, Elsevier, Amsterdam,... 

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