Metallic nanoparticles colloidal dispersion formation

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

Coreduction of Au and Pt ions by refluxing alcohol in the presence of PVP gives the colloidal dispersions of Au-core/Pt-shell structured bimetallic nanoparticles, as mentioned before. The formation of this bimetallic nanoparticles was traced by in situ UV-Vis spectra (68). The spectral change is shown in Figure 9.1.15, in which the peaks ascertained to be the metal ions disappear at first, and then the broad tailing peaks due to the colloidal dispersions appear. More precisely speaking, the tetrachloroauric(III) acid (at —320 nm) is reduced first, followed by reduction of hexachloroplatinic(IV) acid (at —265 nm). This order of reduction is consistent with the standard redox potential of the two metal ions. After the reduction of two... 

Colloidal dispersions of fine metal particles have a long history. Metal nanoparticles are now in the spotlight because of recent developments in nanometer-scale science and technology. Especially the precise structure of the monodispersed bimetallic nanoparticles has become clear quite recently, thanks to the development of EXAFS technology. The mechanism of formation, growth, and structure control is not completely clear yet. In some parts, especially in Section 9.1.4, the discussion may be speculative but is based on the experience of the present author for over 20 years. 

IPECs are of considerable interest because of their numerous promising (potential) applications in agriculture, water treatment, biotechnology, and medicine. Some examples include effective and available binders for dispersed systems and flocculants of colloidal dispersions [2], biocompatible coatings [3, 4], components of membranes [5-11], carriers of biologically active compounds (including enzymes and DNA) [12-16], matrices for metal ions and metal nanoparticles [17-22], and the formation of multilayered PE films and capsules using layer-by-layer techniques [23-32]. 

I C Colloidal Dispersion of Metallic Nanoparticles Formation and Functional Properties... 

The environmentally benign, nontoxic and nonflammable fluids water and carbon dioxide (CO2) are the two most abundant and inexpensive solvents on earth. Vater-in-CO2 (W/C) or C02-in-water (C/W) dispersions in the form of microemulsions and emulsions offer new possibilities in waste minimization for the replacement of organic solvents in fields including chemical processing, pharmaceuticals, and microlectronics for solubilization and separations (e.g., proteins, ions, heavy metals), particle formation, enzymatic catalysis, organometallic catalysis, and synthesis of polymer colloids and inorganic nanoparticles (2,13,11). 

In a similar manner, several nanoparticles have been produced in the presence of block copolymers in selective solvents so as to form micelles that encapsulate particles such as metal salts. Consequently, these micelles are chemically converted to finely disperse colloidal hybrid polymer/metal particles with interesting catalytic, non-linear optic, semiconductor and magnetic properties [1, 20]. Finally, another area of potential application of amphiphilic block copolymers is that involving surface modification through the adsorption of block copolymer micelles or film formation. The use of a suitable micellar system allows for the alteration of specific surface characteristics, such as wetting and biocompatibility, or even enables the dispersion and stabilisation of solid pigment particles in a liquid or solid phase [1, 178]. 

The simulation protocol for the formation of Pt-decorated primary C particles (PPCs) mimics catalyst dispersions obtained by pertinent fabrication techniques. In practice, two methods are used to obtain PPCs (1) impregnation of carbon nanoparticles with Pt precursor or (2) adsorption of Pt oxide or Pt metal colloids onto the carbon surface (Antolini, 2003 Antolini et al., 2002). In the case of impregnation with Pt precursor, diffusion into the pores of each individual support particle can occur. For the second mechanism, colloidal Pt or Pt oxide particles adsorb on the external surface of the support particles as a result of size exclusion, the accessibility of the inner pores is limited and, therefore, Pt particles are mostly formed on the surface of CNPs. 

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