Nanoparticle preparation via RESOLV

Figure 28 Absorption spectra of Ag nanoparticles prepared via RESOLV with N2H4 (dashed Une) and NaBH4 (solid Une) reduction.

Figure 29 TEM images of the Ag nanoparticles prepared via RESOLV with (top) N2E14 and (bottom) NaBEl4 reduction.

Figure 31 X-ray powder diffraction pattern of the Cu nanoparticles prepared via RESOLV with NaBILj reduction. The pattern for bulk Cu in the JCPDS database is also shown for comparison.

The Ag nanoparticles prepared via RESOLV with the microemulsion having a Wo value of 12 had an average particle size of 10.4 nm and a size distribution standard deviation of 3.8 nm (263). 

Figure 51 Optical limiting responses of the Ag2S nanoparticles prepared via RESOLV with the rapid expansion of a supercritical/ammonia solution (narrow particle size distribution) ( ) and a water-in-C02 microemulsion (broader particle size distribution) (A).

Nanocrystalline metal (silver and copper) and metal sulfide (silver sulfide, cadmium sulfide, and lead sulfide) particles were prepared via RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) with water-in-carbon dioxide microemulsion as solvent for the rapid expansion. The nanoparticles were characterized using UV/vis absorption. X-ray powder diffraction, and transmission electron microscopy methods. The results of the different nanoparticles are compared and discussed in reference to those of the same nanoparticles produced via RESOLV with the use of conventional supercritical solvents. 

Nickel nanoparticles were also prepared via RESOLV with NaBH4 reduction (258). The preparation involved rapid expansion of a NiC solution in near-critical ethanol at 230°C into a room-temperature solution of NaBH4 in DMF. The DMF solution, which also contained PVP polymer for particle protection, was deoxygenated before the expansion to avoid oxidation of the... 

Figure 34 UV-Vis absorption spectra of the Ag nanoparticles in PVP polymer-protected suspension prepared via RESOLV. Solid line, as-prepared dashed line, after dialysis against freshwater.

CdS nanoparticles were prepared via RESOLV by rapidly expanding a supercritical ammonia solution of Cd(N03)2 into a room-temperature aqueous or ethanol solution of Na2S (256,260). The CdS nanoparticles produced in the process formed a yellowish suspension under the protection of PVP polymer. The absorption spectrum of the suspension was typical of the quantum-confined CdS, with a shoulder at about 370 nm (Figure 35). The nanoparticles were characterized using x-ray powder diffraction and TEM, from which an average particle size of about 3.3 nm was estimated (256). 

The properties of the PbS nanoparticles produced via RESOLV were also found to be insensitive to selection of the supercritical solvents. For example, methanol was used in place of ammonia for preparation of a homogeneous methanol solution of Pb(N03)2 in the syringe pump. After the solution was heated to the preset temperature in the heating unit of the experimental apparatus (Figure 27), the supercritical methanol/Pb(N03)2 solution at 250°C and... 

Water-in-C02 microemulsion was used to dissolve metal salts in the production of nanoparticles via RESOLV. In order to evaluate the solubility of Cu(N03)2, for example, the same microemulsion as that used in the rapid expansion was prepared in a high-pressure optical cell. With Cu(N03>2 in the water phase, which exhibited the distinctive blue color of aqueous Cu (70), the microemulsion appeared homogenous. According to the observed absorbance (the band centered at 740 nm), the Cu(N03)2 salt was completely dissolved in the PFPE-NH4-stabilized water-in-C02 microemulsion. The other metal salts were similarly soluble, resulting in microemulsions that appeared equally homogeneous. 

The formation of silver (Ag) nanoparticles in the rapid expansion of PFPE-NH4-stabilized aqueous AgN03-in-C02 microemulsion into a room-temperature reductant solution has been reported (9). In order to use Ag as a reference in this study, the same preparation of Ag nanoparticles via RESOLV was repeated. 

Since CO2 is widely considered to be the desirable SCF because of its environmentally benign characteristics and ambient critical temperature, the use of supercritical CO2 for the preparation and processing of nanomaterials has naturally received considerable attention. However, as discussed in previous sections, the poor solubility of most solutes in supercritical CO2 represents a major limitation. Surfactants containing both C02-soluble and hydrophilic moieties are often added to CO2 to form reverse micelles. Such water-in-C02 microemulsions offer a convenient means of dissolving hydrophilic compounds, as demonstrated by the in situ methods for the preparation of nanoparticles (240,247-249). For the production of nanoscale metals and semiconductors via RESOLV, on the other... 

The same microemulsions in CO2 were used to dissolve other metal ions, such as Cu(N03)2 and Pb(N03)2, for the preparation of nanoparticles (Cu and PbS) via RESOLV (263). The nanoparticles produced were similar to those obtained via RESOLV with ammonia as the supercritical solvent. 

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