Metal salts Nanometer

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]. 

PVP, a water soluble amine-based pol5mer, was found to be an optimum protective agent because the reduction of noble metal salts by polyols in the presence of other surfactants often resulted in non-homogenous colloidal dispersions. PVP was the first material to be used for generating silver and silver-palladium stabilized particles by the polyol method [231-233]. By reducing the precur-sor/PVP ratio, it is even possible to reduce the size of the metal particles to few nanometers. These colloidal particles are isolable but surface contaminations are easily recognized because samples washed with the solvent and dried in the air are subsquently not any more pyrophoric [231,234 236]. 

Summary Materials containing uniform, nanometer-sized metal particles homogeneously dispersed in a Si02 matrix, with a variable metal loading, were prepared by the sol-gel processing starting from metal salts, alkoxysilanes of the type X(CH2)nSi(OR)3 and, optionally, Si(OR)4. 

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],... 

The sol is made of a stable suspended solution of metal salts or solvated metal precursors containing solid particles of nanometer diameter. Polycondensation or polyesterification results in the appearance of particles in a new phase called the gel . Aging, drying and dehydration are steps required to achieve solid-form ultra-fine particles. Coarsening and phase transformation occur simultaneously with aging. Gel drying is associated with the... 

Among the synthetic methods, the reverse micelle technique shall only briefly be mentioned. It is based on the use of water droplets in an organic phase of a surfactant. bi Metal salts, dissolved in the water droplets, are reduced inside the nanometer-sized water volume, to result in nanoparticles trapped in the micelles. Alloy-like particles as well as bimetallic core-shell particles thus become available. Size distributions up to over 20% have to be put up with this technique. Another disadvantage of the reverse micelle procedure is the lack of crystallinity due to the low synthesis temperature (<100°C) requiring subsequent aimealing at 200-300 Fe, Co, and Ni... 

During steady state deactivation, the smaller pores are blocked (initial coke deposition) and further adsorption on acidic sites is unlikely. As a result, the accumulation of coke and metal salts must be considered relative to the catalyst surface area in the larger pores. It is known, however, that metals deposit as very long crystallises (some tens of nanometers in length) originating from fixed nucleation sites [32]. As a result, consideration of metal deposits even on large pore surface area may not be accurate. 

Reversed micelles stabilize a nanometer-sized aqueous environment within the surfactant aggregate, which is dispersed in a fully nonhydrous or a semipolar solvent. In this micellar core environment we may dissolve metal salt solutions that react to the extent of the available reactants. 

In the standard chemical preparation methods, the properties, especially the size and size distribution of the nanoparticles, are defined by the choice of the reaction conditions, reactant concentrations, etc. The use of electrochemical techniques to generate nuclei has the advantage that the supersaturation is determined by the applied potential or current density. Thus, the size of the particles can be controlled by electrochemical instrumentation rather than by changing the experimental conditions. Reetz and Helbig [115] demonstrated how electrochemical methods can be used to produce metal colloids of nanometer size and more importantly how particle size can be controlled in a simple manner by adjusting the current density [159]. First, a sacrificial anode was used as the source of the metal ions, which were then reduced at the cathode. Later, a more general approach was introduced, where metal salts were used as the starting material [160]. The particles were stabilized by alkylammonium or betaine salts. With a suitable choice of surfactants, the electrochemical method can be applied in the preparation of different shapes of particles, e.g., nanorods [161]. 

Ultrathin films in the nanometer range of poly(metal tetrathiooxalates) (M = Cu(II), Ni(II), Pd(II), Co(II)) have been prepared by sequential adsorption of divalent metal cations and tetrathiooxalate dianions on charged substrates (quartz slides with an upper layer of poly(allylamine hydrochloride) as shown in Eq. 7-11 [70]. Non-modified ITO glass or ZnSe substrates have also been also used. The modified quartz slide was dipped alternately into aqueous solutions of the tetrathiooxalate and a metal salt, and cleaned in between. The dark-brownish layers show a very broad absorption extending from the UV to the NIR. The electrical conductivities are in the order of 10 S cm" . 

Many technically important materials prepared by sol-gel processing, such as perovskites, contain more than one kind of metal atom. The first approach is to employ precursor mixtures, which can be either mixtures of metal alkoxides or mixtures of one or more metal alkoxides and metal salts. The co-hydrolysis and co-condensation of different precursors may result in inhomogeneous materials because of their different reaction rates and the fact that the formation of a gel is a kinetically controlled process. The inhomogeneity can be on any length scale from the nanometer range up to macroscopic phase separation. Common methods to solve this problem and to obtain homogeneous (mixed on a nanometer scale) heterometallic gels from precursor... 

In the case of solution methods, metal salts and additives dissolved in solution are heated. Additives are often chelating reagents such as citric acid and ethylene glycol (Pechini method) that ensure homogeneous mixing of metals. Lower temperature compared with the solid-state reaction is enough to produce the desired perovsldte mixed metal oxides, and production of nanometer-sized particles is possible. 

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