Noble metal colloids

Sakka S., Kozuka H., Sol-gel preparation of coating fdms containing noble metal colloids, J. Sol-Gel Sci. Technol. 1998 13 701-705. 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

Fig. 17 a, b. Photosensitized H2-evolution systems a) basic configuration including a photosensitizer, S, an electron acceptor, A, and electron donor, D, and H2-evolution catalyst, b) H2-evolution system including MV2+ as electron acceptor and a noble metal colloid as heterogeneous catalyst... 

Antonietti M, Groehn F, Hartmann J, Bronstein L (1997) Nonclassical shapes of noble-metal colloids by synthesis in microgel nanoreactors. Angew Chem Int Ed Engl 36 2080-2083... 

The excellent electron-transfer mediator properties of nanoparticles find special use in the different oxidation [126] and reduction [143,144] reactions catalyzed by noble metal colloids. Recently, Ung et al. [145] showed how Ag particles coated with a thin layer of silica act as redox catalysts, and how the control of the rate of the catalyzed hydrogen evolution reaction was possible by tuning the silica shell thickness. It was concluded that the shell acts as a size-selective membrane, which can be used to alter the chemical yields for competing catalytic reactions. This kind of tailoring of the catalyst properties opens up very interesting prospects in future catalyst planning. 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

Liu and coworkers have also shown the incorporation influence of some metal cations such as Co on the catalytic performances of the noble metal colloidal nanocatalysts [64]. In particular, Co " significantly increases both the activity and the selectivity of the Ru nanosized catalyst The yield reached 97.8% and the selectivity increased to 98.8% when Co was introduced into the catalytic system. 

Polymer-protected noble metallic colloids can be prepared by the one-step radiation-induced reduction of aqueous metallic ions with 2-propanol using y-irradiator or e-beam irradiator (see. Figures 19.1 and 19.2). In an aqueous solution, water molecules absorb the irradiation energy and generate many reactive species, such as solvated electrons (e q"), H, and OH" Equation 19.1... 

There exists a method of forming a thin metal film on a base material by using a noble metal colloid of gold or silver (34). Stable gold colloidal particles have a particle size of about 10-20 nm, and the color of a colloid dispersion is purple in this state. In this case, in order to obtain a thin film of gold, it is necessary to heat at 150 C or more. 

Antonietti M, Wenz E, Bronstein LM, Seregina MS (1995) Synthesis and characterization of noble metal colloids in block copolymer micelles. Adv Mater 7 1000-1005... 

Sakka S, Kozuka H, Zhao G. Sol-gel preparation of metal particle/oxide nanocomposites. Sol-gel optics, vol. ni, J.D. Mackenzie, ed. SPIE Proc. 1994 2288 108-119 Sakka S., Kozuka H. Sol-gel preparation ofcoating films containing noble metal colloids. J. Sol-Gel Sci. Technol. 1998 13 701-705... 

PREPARATION OF NOBLE METAL COLLOIDS AND SELECTED STRUCTURES... 

In this chapter, we intend to introduce to the reader some of the most popular preparation methods of noble metal colloids, but do not intend by any means to thoroughly review the scientific literature on metal colloid synthesis. We have chosen examples of synthesis in aqueous and organic solvents, and differentiate between spherical and anisotropic nanoparticles (nanorods and nanoprisms). Additionally, in section 3 we shall describe one of the most popular recent procedures to assemble metal colloids into nanostructured materials (layer-by-layer assembly), as well as the properties of the resulting structures. 

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