Nanometal colloid

After a period of hibernation following the publication of Turkevich s seminal papers in the 1950s [44,46], a whole plethora of preparative protocols has been published since Henglein and coworkers resumed this work in the late 1970s [107,108], The goal of this chapter is not to present a comprehensive directory of all synthetic papers on nanometal colloids, but to provide the reader with a general survey of a number of modern versions of established preparative methodologies that have been tested and found effective in a number of practical applications. 

Using long-chain alkylsulfobetaines as the stabilizer, a number of highly water soluble nanometal colloids have been isolated in excellent yields (see Figure 8). The core particle size can be tailored between 1 and 10 nm. TEM examinations have shown that the resulting materials are generally mono-disperse. Further, a combination of spectroscopic methods confirmed the zerovalent nature of the metal cores [200]. 

As alternatives to amphiphilic betaines, a wide range of cationic, anionic, and non-ionic surfactants including environmentally benign sugar soaps have been successfully used as colloidal stabilizers [201]. Electrochemical reduction of the metal salts provides a very clean access to water soluble nanometal colloids [192]. 

The formation of nanometal colloid via reductive stabilization using aluminum organic reagent operates with a different mechanism as is depicted in Figure 3.2. The mechanism has been elucidated based on various physical and analytical data [37],... 

A synthetic alternative to this is the chemical reduction of metal salts in the presence of extremely hydrophilic surfactants have yielded isolable nanometal colloids having at least 100 mg of metal per litre of water [105], The wide range of surfactants conveniently used to prepare hydrosols with very good redispersibility properties include amphiphilic betaines A1-A4, cationic, anionic, nonionic and even environmentally benign sugar soaps. Table 3.1 presents the list of hydrophilic stabilizers used for the preparation of nanostructured colloidal metal particles, and Table 3.2 shows the wide variety of transition metal mono- and bi-metallic hydrosols formed by this method [105,120],... 

Heterogeneous catalysts are readily obtained when pre-prepared nanometal colloids are deposited on supports [20], The so-called precursor concept for manufacture of heterogeneous... 

Bonnemann, H. et al., The reductive stabilization of nanometal colloids by organo-aluminum compounds, Rev. Roum. Chim., 44,1003, 1999. 

Bonnemann, H., Brij oux, W., and Brinkmann, R., Method for modifying the dispersion characteristics of metal-organic-prestabilized or pre-treated nanometal colloids, WO 99/59713, (to Studiengesellschaft Kohle), November 25,1999. 

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

A suspension of tiny particles in some medium is called a colloidal dispersion, or a colloid. The suspended particles can be single, large molecules or aggregates of molecules or ions ranging in size from 1 to 1000 nanometers. Colloids are classified according to the states of the dispersed phase and the dispersing medium. Table 17.7 summarizes various types of colloids. 

In the following section we will present the various inorganic systems that have been synthesized in the last few years, and we will then try to emphasize the unique features of sonochemistry, or what can be described by the famous song anything you can do I (sonochemistry) can do better . In this section metals will serve as a demonstration of what can be done sonochemically. We will discuss the synthesis of nanometals, colloidal metallic solutions, formation of alloys, the coat-... 

The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called precursor concept ll has attracted industrial inter-est.l l An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). 

Another example of the application of supported nanometal colloids in line chemicals catalysis is the cw-selective partial hydrogenation of 3-hexyn-l-ol to leaf alcohol, a valuable fragrance in 1996 the amount produced was 400 tons, including esters (see Eq. 2). 

A broad variety of hydrophilic surfactants may also be used as alternatives to amphiphilic sulfobetaines. Isolable nanometal colloids soluble in water with at least 100 mg of metal 1 have been obtained with a wide range of cationic, anionic, and nonionic surfactants. Even, environmentally benign sugar soaps have been successfully applied [see Table 2 in Ref. [11]]. 

Table 2.3 Starting materials organometallic-prestablilized nanometal colloids. 

The AFM was invented in 1986 by Binnig, Quate, and Gerber. In this technique a sharp tip at the end of a cantilever is used to probe and raster-scan the sample of interest (Fig. 1.10). Probes are typically composed of silicon or silicon nitride with typical radii of curvature from 2 to 50 nanometers. Colloidal microspheres can... 

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