Metal colloid synthesis preparation

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. 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

In the chemical preparation of unprotected metal colloids, the metal concentration usually has a significant influence on the particle size of obtained metal nanoclusters. For example, when increasing Pd concentration from 0.1 to 1.0 mM in the preparation of Pd metal colloids by the thermal decomposition of Pd acetate in methyl isobutyl ketone, the average Pd particle size increased from 8 to 140nm [6,7]. However, in the alkaline EG synthesis method, the size of metal nanoclusters was only slightly dependent on the metal concentration of the colloidal solution. The colloidal Pt particles prepared with a metal concentration of 3.7 g/1 had an average diameter of... 

The emphases of future investigation on these unprotected metal nanoclusters should be mainly placed on (1) further controlling the size, composition and shape of the unprotected metal or alloy nanoclusters (2) better understanding the stabilizing mechanism of the unprotected metal nanoclusters in colloidal solutions prepared by the alkaline EG synthesis method (3) developing novel catalytic and other functional systems for real applications. 

The approaches used for preparation of inorganic nanomaterials can be divided into two broad categories solution-phase colloidal synthesis and gas-phase synthesis. Metal and semiconductor nanoparticles are usually synthesized via solution-phase colloidal techniques,4,913 whereas high-temperature gas-phase processes like chemical vapor deposition (CVD), pulsed laser deposition (PLD), and vapor transfer are widely used for synthesis of high-quality semiconductor nanowires and carbon nanotubes.6,7 Such division reflects only the current research bias, as promising routes to metallic nanoparticles are also available based on vapor condensation14 and colloidal syntheses of high-quality semiconductor nanowires.15... 

A promising strategy towards stable and catalyticaUy active metal colloids is their preparation inside the core of micelles formed by amphiphilic block copolymers. This strategy offers a number of advantages (i) micelles represent a nano-structured environment which can be exactly tailored by block copolymer synthesis (ii) polymers act as effective steric stabilizer ]36] (iii) metal leaching might be avoided (iv) micelles allow control over particle size, size distribution and particle solubility [37] and (v) micelles are also supposed to effect catalytic activity and selectivity [38]. 

Common methods for the fabrication of metallic nanoparticle arrays are electron beam lithography, photolithography, laser ablation, colloidal synthesis, electrodeposition and, in recent time, nanosphere lithography for which a monodisperse nanosphere template acts as deposition mask. A review on advances in preparation of nanomaterials with localized plasmon resonance is given in [15]. 

In general, metal colloid sols are referred to as monodisperse when the particle size deviates by less than 15% from the average value, and histograms with a standard deviation a from the mean particle size of approximately 20% are described as showing a narrow size distribution. The kinetics of the particle nucleation from atomic units and of the subsequent growth process cannot be observed directly by current physical methods. The tools of the preparative chemist to control the particle size in practice are size-selective separation [50c,52] and size-selective synthesis [42,12a,50,53]. 

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. 

The metal colloid preparations arc mostly based on reduction of metal ions in the presence of bulky anions (e.g. citrate and mercaptoethanesulfonatc), but also quaternary ammonium salts (e.g. tetraoctylammonium bromide) or polymers (e.g. poly(iV-vinylpyr-rolidone (PVP)) are used as stabilizers (Figure 7.2). Ionic species involved in the NP synthesis play a dual role of stabilizers for the metal particles and dopants for the polymer material. In some cases the synthesis of the metal NPs occurs under y-irradiation in the presence of the monomer, which stabilizes the metal particles [64—66,68]. A two-phase approach for the preparation of the monomer or eventually oUgomer-stabUized metal particles has been also suggested in the case of octylthiophen [57] and 3,4-ethylenediox-ythiophene (EDOT) [70,71]. 

The synthetic methods which have been used include modern versions of established methods of metal colloid preparation such as the mild chemical reduction of solutions of transition metal salts and complexes and newer methods such as radiolysis and photochemical reduction, metal atom extrusion from labile organometallics. And the use of metal vapor synthesis techniques. Some of these reactions have been in use for many years, and some are the results of research stimulated by the current resurgence in metal colloid chemistry. The list of preparative methods is being extended daily, and, as examples of these methods are described below, the reader will quickly be made aware that almost any organometallic reaction or physical process which results in the deposition of a metal is in fact a resource for the metal colloid chemist. The acquisition of new methods requires only the opportunism of the synthetic chemist in turning a previously negative result into a synthetic possibility. 

Dispersions of metallic nanoparticles can be obtained by two main methods (i) mechanic subdivision of metallic aggregates (physical method) or (ii) nucleation and growth of metallic atoms (chemical method). The physical method yields dispersions where the particle size distribution is very broad. Traditional colloids are typically larger (>10nm) and not reproducibly prepared, giving irreproducible catalytic activity. Chemical methods such as the reduction of metal salts is the most convenient way to control the size of the particles. Today, the key goal in the metal colloid area is the development of reproducible nanoparticle (or modem nanocluster) syntheses in opposition to traditional colloids. As previously reported, nanoclusters should be or have at least (i) specific size (1-10 nm), (ii) well-defined surface composition, (iii) reproducible synthesis and properties, and (iv) be isolable and redissolvable ( bottleable )- ... 

The synthesis methods used for the preparation of carbon supported PtRuMo nanoparticles could be classified as adsorption of metal colloids onto the carbon surface, or impregnation of carbon support with metals precursor solution. Additionally, the incorporation of the metals has been carried out in a (1) one step method or with simultaneous incorporation of the three metals, and in (2) two step methods or sequential incorporation of Mo and PtRu nanoparticles ... 

The reduction of metal salts with borohydride or trialkylborohydride is a widely used colloid synthesis method. The preparation of platinum microcrystals having a mean diameter of 28 A by the reduction of chloroplatinic add with sodium borohydride has been reported as a reprodudble standardized preparation. [52] PVP stabilized copper sols have been prepared by borohydride reduction of copper salts. [53, 54] In some cases, however, the formation of metal... 

Since organometallic complexes are often photolabile, they could serve as sources of metal colloids under photolytic conditions. Although this method has been used for the preparation of supported metal partides in heterogeneous catalysis, the method has not found application in collcnd synthesis. 

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