Ligand-stabilized particles

Figure 3 Different types of steric stabilization of metal nanoparticles. (a) polymer molecules on the particles surface (b) micelle (c) inverse micelle (d) ligand stabilized particle...

The curve follows the fit of the model describing the size dependence. The three experimental points match the curve quite well and we can now understand why even 15 nm palladium colloids show magnetic behavior still far from that of the bulk. Even for a 1 micron sample, one can expect a reduction in x of about 1 % Values for A and X obtained from different experiments for similarly sized particles agree quite well with the results discussed here. [61-64] Finally, we see that the magnetic properties of ligand stabilized particles, even if they are very huge particles like colloids, differ considerably from those found in the bulk. The... 

Traditional aqueous metal colloids were formed by reducing a metal salt in the presence of protective polymer such as polyvinyl alcohol (PVA), which absorbs on the surface. More recently large ligand-stabilized particles have been prepared that are intermediate between clusters and nanoparticles. 

Figure 12. Schematic representation of the setup for single particle measurements by electrostatic trapping (ET). Pt denotes two freestanding Pt electrodes (dashed region). A ligand-stabilized Pd cluster is polarized by the applied voltage and attracted to the gap between the Pt electrodes. (Reprinted with permission from Ref. [29], 1997, American Institute of Physics.)...

Purification yields 200-300 mg of triphenylphosphine-stabilized gold nanoparticles, which should be stored cold (—20°C) in the sofid state, or immediately converted to thiol- or amine-stabilized nanoparticles through subsequent reaction with the appropriate ligand. The particles decompose in solution thus manipulation of the product in solution should be minimized. 

The 1.5-nm nanoparticles readily react with thiol or amine-terminated ligands under mild conditions to yield thiol- or amine-stabilized nanoparticles. Triphenylphosphine-stabilized particles thermally decompose with the production of (PPh3)AuCl and metallic gold. 

Near monodisperse Au NPs in the size range of 1—4nm can be obtained using dodecylthioether end-functionalized PMMA as stabilizer. Particle size is controlled by varying the concentration of the stabilizing polymer, which can be readily displaced by thiol ligands to yield MPCs of the usual type [97]. 

Metal clusters formed in this way are always contaminated by adsorbed host solvent and fragments of this solvent. (29) In one way this is an advantage since this adsorption tends to "ligand stabilize" the cluster and stop further growth — thus nano-scale particles are isolable. However, the complex nature of these fragments do complicate study of these particles. Another frequent problem is that growth is less controllable than desirable. 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective maimer. Very stable covalent metal-phosphorus or metal sulfur bonds lead to such strong ligand shells that in some cases the protected particles can even be isolated in solid state, which can never be done with electrostatically stabilized particles. The chemical nature of the protecting ligand molecules is responsible for the solubility of the particles. Thus, the use of organic solvents has become very useful for several reasons. Figme 3 shows a sketch of the three types of steric stabihzations of metal nanoparticles. 

Ligand-stabilized species can be described as colloids, according to the definition given above. Colloids used in polymer matrices or other stabilizing liquid media are not considered here. Continuing the principle of ligating metal particles of cluster size, only such metal particles are taken into account which exist as individuals outside the liquids in which they are produced. 

Many nanopartide preparations lack suffident stability (above. Fig. 11.5) to allow the ordered assembly of two-dimensional or three-dimensional materials and structures, in which the particles are dosely packed, without the onset of uncontrolled aggregation (agglomeration). To overcome this problem, the partides must be rendered chemically stable, for example by ligand stabilization, also to avoid degradation processes such as partial oxidation or undesired sintering of particles [11.6]. 

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