Nanoparticles arrested precipitation

Figure 2.11 TEM images and nanoparticle size distributions of nanopartides synthesized from a modified Brust arrested precipitation technique (a) under ambient conditions and (b) at an applied C02 pressure of 33.0 bar.

Supercritical fluids may be utilized for the synthesis of metal nanoparticles. Perfluorodecanethiol-stabilized silver nanocrystals were synthesized in supercritical CO2 through arrested precipitation, by reducing silver acetylacet-onate with hydrogen in the presence of fluorinated thiol (Figure 8). The CO2 density used during synthesis controls the particle size and polydispersity. At... 

Arrested precipitation denotes a technique where a poorly soluble product is precipitated within a template by mixing solutions of the respective ions. The template might be a microemulsion, surface ligand solution, mesoporous material (e.g., anodised alumina), polymer or dendrimer, or any other system that provides a confined space for the precipitation. Historically, arrested precipitation was the first method used to synthesise semiconductor nanoparticles that were used to study quantum size effects systematically [33] (and thus paved the way for the whole field of nanosciences). 

Arrested precipitation can be performed in many different ways most simply, two solutions containing ions of a poorly soluble salt are mixed within a template. Alternatively, ions (or reactants) can be released slowly as described in stage 1 of the LaMer mechanism. Subsequently, nucleation and growth take place within the template. Furthermore, arrested precipitation can be combined with other methods of synthesis such as the sol-gel method. In that case, a gel is precipitated within the template and condensed in a second step. AU of these variations have been used to synthesise upconverting nanoparticles. 

The groups of Song and Zhang et al. have used a combination of a sol-gel method and arrested precipitation to prepare nanocrystalline Y203-based upconverters [15,16]. They first prepared a gel from yttrium and lanthanide nitrates and ammonia hydroxide that was arrested by citrate ligands. Then, they calcinated the dried gel at 800°C and obtained upconverting nanocrystalline material. It has to be stated that methods that involve annealing/calcination always result in nanocrystalline material rather than colloidally stable nanoparticles. 

Another method of synthesis mentioned in [39] is tte direct mechanical mixing of a polymer solution (aqueous or non-aqueous) with initially synthesized highly dispersive particles of nanosize with vaporization of water from the solution. In this case the inhomogeneity of filler distribution is high and coagulation often takes place [39]. The nanopartides are usually synthesized by this arrested precipitation . The phosphides and arsenides of Zn and Cd [12], CuCl [12], semiconductor Pbl2 and Fc302 were reported to form nanoparticles in this way. 

The major advantage of precipitation synthesis such as the type described above is that large quantities of nanoparticles can be made. However, it can be difficult to tailor the size since only kinetic factors are available to arrest growth. The addition of complexing agents or surfactants can help control particle size. Following the example of gold nanoparticles, it is possible to synthesized... 

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