Hydrosols of metals

In 1994, thiols were firstly used as stabilizers of gold nanoparticles [6a]. Thiols form monolayer on gold surface [18] and highly stable nanoparticles could be obtained. Purification of nanoparticles can be carried out, which makes chemical method of metal nanoparticles a real process for nanomaterial preparation. Various thiol derivatives have been used to functionalize metal nanoparticles [6b, 19]. Cationic and anionic thiol compounds were used to obtain hydrosols of metal nanoparticles. Quaternary ammonium-thiol compounds make the nanoparticle surface highly positively charged [20]. In such cases, cationic nanoparticles were densely adsorbed onto oppositely charged surfaces. DNA or other biomolecule-attached gold nanoparticles have been proposed for biosensors [21]. 

A novel method of thiol-derivatizing hydrosols of metals has been recently developed by Sarathy et al. [25, 26], The procedure involves mixing a hydrosol... 

L. M. Liz-Matzdn and A. P. Philipse, Stable hydrosols of metallic and bimetallic nanoparticles immobilized on imogolite fibers, /. Phys. Chem. 99,15120-15128 (1995). 

While there are extensive reviews of organosols and the catalysts therefrom in the literature, hy-drosols are relatively unknown in spite of the promising electrocatalysts that can emerge from them. Hydrosols of mono-, bi- and multimetallic nanoparticles as isolable precursors for producing supported metal catalyst are an economically beneficial alternative to the traditional wet impregnation of active metal components on carrier surfaces [25],... 

Although this method gives hydrosols in isolable form and redispersible in water in high concentration of metal, a scale-up of this is not possible. 

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],... 

Stable hydrosols may be obtained similarly by reduction of arsenious oxide, dissolved in aqueous sodium hydroxide containing some other protective colloid such as gelatin or egg-albumin, by means of alkaline pyrogallol.6 Salts of metallic acids, such as sodium antimonate or calcium plumbate,-with or without the addition of protalbic acid, may also be employed as protective colloids.1... 

The hydrosols arc more active than tlieir respective metals in finely divided condition this is attributable to t.licir increased surface and to the more intimate contact between the particles of metal and the molecules to be catalytically acted upon. 

By measuring the volumes of hydrogen utilised in this way in unit time per unit volume of metallic hydrosol, a measure of the relative efficiencies of the colloidal metals is obtained. The results obtained by Paal and his co-workcrs arc as follow ... 

Colloidal osmium readily undergoes oxidation it catalvtieally assists the oxidation of unsaturated compounds by gaseous hydrogen. In this respect it is stated to be even more effective than the finely divided metal, but less active than either platinum or iridium. Carbon monoxide combines with oxygen, yielding the dioxide, when shaken at ordinary temperatures with the hydrosol of osmium.7... 

An impure hydrosol of copper was first prepared by Lottermoser 2 as an adsorption-compound with stannic oxide by heating a slightly alkaline solution of a cupric salt with a similar solution of stannous chloride in presence of an alkali-metal citrate or tartrate. With water the black precipitate yields a reddish-brown liquid, rapidly oxidized by atmospheric oxygen, with production of a greenish coloration which soon becomes yellow. 

In some other cases, the noble metals were deposited onto ceria from colloidal suspensions. Tlius, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(i%)/Ce02 catalyst (182). Likewise, a series of Pd/Ce02 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofiiran (78). 

Silver hydrosols (10 ml) with chloride anions were filtered by using ANODISC alumina filters (0.02 mm diameter) and a membrane filter holder supplied by Whatman Intern. Ltd. (England). After two successive filtrations, the liquid was free of metal particles, as detected by the total absence of plasmon absorption bands in the UV-vis region. The filter surface resulted coated by a layer of colloidal silver. 

Ayral et al. [75] showed that the template method can also be applied to hydrosols of colloidal metal oxide particles in view of the preparation of tailor-made porous material. In order to produce macroporous silica materials... 

Gaseous hydrogen is another long-standing and very effective reducing agent for metal salts. In this case, hydrosols of palladium, platinum, rhodium, and iridium, stabilized with PVA, were prepared by a hydrogen reduction of the metal hydroxides [70-74]. 

Hydrogen has been used effectively as a redudng agent for metal salts in electrostatically stabilized metal sol syntheses. [45] Polymer stabilized hydrosols of palladium, platinum, [46-48] rhodium, [49] and iridium [50] stabilized with polyvinyl alcohol were prepared by hydrogen reduction of the metal hydroxides. Sols were produced with a broad distribution of particle sizes. 

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