Precipitation, nanoscale

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

Folk, R. L. Chafetz, H. S. (2000). Bacterially induced microscale and nanoscale carbonate precipitates. In Microbial Sediments, ed. R. E. Riding S. M. Awramik. Berlin Springer-Verlag, pp. 41-9. 

Because the colloidal nucleus was in nanoscale, the specific surface area was very large and the specific surface free enthalpy was very high, the system was thermomechanically unstable. Thus the small colloidal nuclei could spontaneously aggregate each other to form big colloidal nuclei and to precipitate followed by separation from the dispersed medium. However, due to the high specific surface free enthalpy, each colloidal nucleus could also spontaneously adsorb some groups or... 

Where might this be important As discussed above, biological activity can result in the simultaneous precipitation of mixtures of nanoscale sulfide minerals under certain conditions. Each mineral will exhibit a particular particle size distribution, dependent on the solution composition, bacterial activity, rate of crystal growth, and the nature of electrochemical interactions between the particles. These electrochemical reactions could lead to oxidation of one type of nanophase sulfide mineral of a certain size, and reduction of another type of nanophase sulfide particle or other species in the solution. In this way, a tremendous number of mineral-solution-mineral galvanic cells could develop, with potentially significant impact on dissolution kinetics, growth kinetics, and the mixture of phases observed. In addition to environmental relevance, these processes may shape the mineralogy of low-temperature ore deposits. 

All precipitated products formed during the microwave synthesis of CdSe in presence of glycine were identified as large conglomerates of Cd(OH)x nanowires decorated by CdSe NCs. The formation of nanowires was attributed to the hydrolysis of cadmium. The morphology of the samples could be controlled on the nanoscale through the adjustment of initial reaction conditions. PL QY of the produced samples did not exceed 1 %. Such decorated nanowires may be used as components of photonic devices like solar cells. 

The cooperative self-assembly route is the most commonly used synthesis procedure for surfactant templated materials. It uses aqueous solutions at a much lower initial surfactant concentration than for the true liquid templating route, reducing the required amounts of expensive surfactant template. In these solutions, the surfactants are at a high enough concentration to form micelles, which may be spherical, elliptical, rod-like or vesicular, but do not form the ordered aggregates found in the final templated silica-surfactant composites. The solutions are in thermodynamic equilibrium so are stable at a given temperature until the silica precursors are added. Once added, a series of interactions between the inorganic species and the surfactant micelles occur, which involve simultaneous association of all components, hence the name, cooperative self-assembly. The result of the interactions is formation of the silica-surfactant composite, usually a precipitate, with an ordered nanoscale structure similar to those found in concentrated surfactant solutions. 

These results have important implications for surface complexation and reactive transport models as co-precipitation of nanoscale multicomponent phases may be a significant mode of sorption, particularly under conditions where the concentration of metal ions in the aqueous solution is fairly high. Such phases also provide new surfaces on which further sorption of aqueous ions can occur. However, detection of these phases and distinguishing them from the sorbate metal hydroxide phase requires very careful EXAFS analysis and HRTEM studies of sorption samples. 

Chemically bonded ceramics constitute ceramics that are being formed due to chemical reactions. Often the precursor material is a ceramic powder (e.g. Ca-silicate or Ca-aluminate), which is "activated in a water-based liquid. A chemical reaction takes place in which the initial powder is partly or completely dissolved and new phases precipitate. The precipitated phases are composed of species from both the liquid and the precursor powder. The precipitates can be formed in situ in vivo, often in the nanoscale due to low solubility of the phases formed. The nanostructural chemically bonded bioceramics are especially found within the Ca-phosphate, Ca-aluminate and Ca-silicate systems. 

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