Preparation metal sols

Solution Deposition of Thin Films. Chemical methods of preparation may also be used for the fabrication of ceramic thin films (qv). MetaHo-organic precursors, notably metal alkoxides (see Alkoxides, metal) and metal carboxylates, are most frequently used for film preparation by sol-gel or metallo-organic decomposition (MOD) solution deposition processes (see Sol-GEL technology). These methods involve dissolution of the precursors in a mutual solvent control of solution characteristics such as viscosity and concentration, film deposition by spin-casting or dip-coating, and heat treatment to remove volatile organic species and induce crystaHhation of the as-deposited amorphous film into the desired stmcture. 

The basic structure of Ti02 wasn t changed by platinization and metallization under this preparation conditions. Particle size of modified TiOa catalysts were about 30nm bigger than P25 based catalyst. Also the surface areas of P25 based catalysts were larger than those of TiOi based catalysts prepared by sol-gel method. 

After few minutes of gold sol preparation, metal particles have to be immobilized on a suitable support for practical purpose. Normally this step is simply performed by dipping the support in the sol. The metal particles are... 

A review of preparative methods for metal sols (colloidal metal particles) suspended in solution is given. The problems involved with the preparation and stabilization of non-aqueous metal colloidal particles are noted. A new method is described for preparing non-aqueous metal sols based on the clustering of solvated metal atoms (from metal vaporization) in cold organic solvents. Gold-acetone colloidal solutions are discussed in detail, especially their preparation, control of particle size (2-9 nm), electrophoresis measurements, electron microscopy, GC-MS, resistivity, and related studies. Particle stabilization involves both electrostatic and steric mechanisms and these are discussed in comparison with aqueous systems. 

More direct and successful methods for the preparation of non-aqueous metal sols are desirable. Especially valuable would be a method that avoids the metal salt reduction step (and thus avoids contamination by other reagents), avoids electrical discharge methods which decompose organic solvents, and avoids macromolecule stabilization. Such a method would provide pure, non-aqueous metal colloids and should make efficient use of precious metals employed. Such colloids would be valuable technologically in many ways. They would also be valuable to study so that more could be learned about particle stabilization mechanisms in non-aqueous media, of which little is known at the present time. 

Fig. 5.18 Schematic and TEM image of reaction scheme to prepare metal nanoparticles encapsulated within metal oxide coating on oxidized MWCNTs. Metal NPs are added to developing metal alkoxide sol followed by addition of oxidized MWCNTs and water for hydrolysis. Adapted with permission from [228], (2012) American Chemical Society.

F. Porta, L. Prati, M. Rossi, S. Colluccia, and G. Marta, Metal sols as a useful tool for heterogeneous gold catalyst preparation reinvestigation of aUquid phase oxidation, Catal. Today 61(1 ), 165-172 (2000). 

Silica-based monolithic columns (Figure 9) are generally prepared using sol-gel technology. This involves the preparation of a sol solution and the gelation of the sol to form a network in a continuous liquid phase within the capillary. The precursors for the synthesis of these monoliths are normally metal alkoxides that react readily with water. The most widely used are alkoxysilanes such as tetramethoxysilane (TMOS) and TEOS. 

Metal particles. Chemical preparation of metal colloids was initiated by Michael Faraday long ago. A number of procedures have been employed for the preparation of metal sols. In general, the preparation involves the treatment of a metal salt solution with a suitable reducing agent (e.g., NaBH, hydroxylamine) in the... 

Sol-gel processing involves the use of a hydrolysis reaction to obtain a cross-linked network, which results in the formation of a gel. When preparing metal-ceramic composites, both components may be obtained in this way, or alternatively the metal reinforcement can be introduced by adding, for example, metal nitrates.1718 The gel properties may be controlled by adjusting the pH level, water to metal ratio, and temperature. 

Some occlusion of the metal particles is not necessarily bad. In fact, unlike other methods such as impregnation and ion exchange, sol-gel preparation actually allows a certain amount of control over what fraction of a metal particle is accessible at the support surface. As long as there are exposed metal atoms available for catalysis, a partially buried metal particle may better resist deactivation due to sintering. For example, Lopez et al. [44] found that a Ru/SiC>2 sample prepared by sol-gel is more stable than one prepared by ion exchange. The sol-gel sample, which contains partially occluded Ru particles, does not sinter or volatilize under oxygen treatment at 723 K, whereas the ion-exchange sample does. 

The various steps in the sol-gel technique described above may or may not be strictly followed in practice. Thus, many complex metal oxides are prepared by a modified sol-gel route without actually preparing metal alkoxides. For example, a transition metal salt solution is converted into a gel by the addition of an appropriate organic reagent. In the case of cuprate superconductors, an equimolar proportion of citric acid is added to the solution of metal nitrates, followed by ethylene-diamine until the solution attains a pH of 6-6.5. The blue sol is concentrated to obtain the gel. The xerogel is obtained by heating at approximately 420 K. The xerogel is decomposed at an appropriate temperature to obtain the cuprate. 

Yu, J.G. and X.J. Zhao (2000). Ag-doped Ti02 composite thin films prepared by sol-gel and its photocatalytic activity. Rare Metal Materials and Engineering, 29(6), 390-393. 

Holzwarth et al. (51) reported the synthesis and IR thermographic-imaging screening of a 37-member, focused discrete heterogeneous catalyst library L6 for oxidations and reductions. The library was prepared using sol-gel solution synthetic protocols (47, 51) to produce the library individuals as amorphous microporous mixed oxides (AMMs), which have previously shown heterogeneous catalytic properties (54, 55). The scaffolding metal oxides contained either Ti (subset 1, Fig. 11.7) or Si (subset 2), and many active metal components were used. The complete structure of L6 is reported... 

Sol-gel processing describes a type of solid materials synthesis procedure, performed in a liquid and at low temperature (typically T< 100 °C). The development of sol-gel techniques has long been known for preparations of metal oxides and has been described many times [30-38, 40-46, 65]. The process is typically used to prepare metal oxides via the hydrolysis of reactive precursors, usually alkoxides in an alcoholic solution, resulting in the corresponding hydroxide. It is usually easy to maintain such hydroxide in a dispersed state in the solvent. Condensation of the hydroxide molecules with loss of water leads to the formation of a network When hydroxide species undergo polymerization by condensation of the hydroxy... 

Examples of common metal oxide membranes prepared by sol-gel process... 

Spin-casting techniques have also been used to prepare imprinted thin films. Makote and Collinson recently prepared metal oxide thin films imprinted with recognition sites for dopamine [18], The dopamine template was loaded at 4 mole % in a sol with a 10 1 ratio of tetramethoxysilane and phenyltrimethoxysilane. The prepared film had a thickness of ca. 450 nm. CV analysis found that 90% of the templates could be removed by washing the films with pH 7 phosphate buffer. The opened receptor sites offered selective binding for related molecules containing catechol amines, such as dopamine, epinephrine and norepinephrine, as determined by CV. The use of phenyltrimethoxysilane turns out to be an essential ingredient for the gel matrix and is believed to provide some complementary affinity for the catechol amines via hydrophobic and/or 7i-stacking interactions. 

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