Colloid metal vapor synthesis

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. 

Figure 6-6. The first metal vapor synthesis reactor for preparing colloidal metals. Metal is cocondensed with organic vapors from the liquid reservoirs onto the cold finger, and the frozen matrix then allowed to melt into the collection tube (adapted from ref. [117]).

Figure 6-7. Schematic of rotary metal vapor synthesis reactor (adapted from refs. [119, 120]). Used for colloid preparations by both cocondensation and metal vapor-into-liquid processes.

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... 

The synthesis of silica membranes has only recently been described. Silica forms sols and gels very easily both by the colloidal suspension and by the polymeric gel route. Its chemical resistance and its thermal stability in the presence of water vapor or metal impurities are not very good however. Larbot et al. (1989) have described the synthesis of silica membranes starting with a commercially available silica sol (Cecasol Sobret) in an aqueous solution at pH 8. 

The advantages in tuning many physical and chemical properties using a bimetallic combination has triggered special interest in the synthesis and stabilization of bimetallic particles over monometallic particles. Here, bimetal refers to particles containing two different kinds of metals, which has either a core-shell or an alloy structure and the kind of structure is decided by the method of preparation. Bimetals can be prepared by either physical or chemical routes. Physical routes mainly consist of vapor deposition of one metal on top of the other, whereas chemical bonds involve simultaneous reduction of two metal ions or reduction of one after another in presence of a suitable stabilizer [238]. Additionally, bimetals generate properties that are different from monometallic components. After preparation of the desired colloid, the microdomains can be reloaded with precursor materials, which can subsequently be reacted to obtain intermetallic nanocolloids, sometimes in the form of onion-type clusters. 

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