Stober process

At large length scales restructuring is manifested as ripening (24). In order to minimize interfacial area, small and/or weakly polymerized species depolymerize and preferentially condense with larger, more highly polymerized species. A dramatic example of this phenomenon occurs in the Stober process (21) for preparing... 

Figure 5.9 Synthesis of gold nanoparticles within Zr02. A gold colloid is first prepared then coated with silica using a modified Stober process [75]. The silica is removed using NaOH. Reproduced with permission from [74],...

Fluorescent silica nanoparticles, called FloDots, were created by Yao et al. (2006) by two synthetic routes. Hydrophilic particles were produced using a reverse micro-emulsion process, wherein detergent micelles formed in a water-in-oil system form discrete nanodroplets in which the silica particles are formed. The addition of water-soluble fluorescent dyes resulted in the entrapment of dye molecules in the silica nanoparticle. In an alternative method, dye molecules were entrapped in silica using the Stober process, which typically results in hydrophobic particles. Either process resulted in luminescent particles that then can be surface modified with... 

Von Werne and Patten, and Matyjaszewski et al., respectively, prepared si-loxane-based nanoparticles via the base-catalyzed hydrolysis and condensation of tetralkoxysilanes (i.e.,the Stober process) [336,337] or using a microemulsion... 

However, if we use the expressions for the classical nucleation rate, we also do not find the large increase in radius that is possible. Furthermore, Weres et al. (57) state that addition of NaCl to a homogeneous nucleating solution of silicic acid in water has only a moderate effect if the concentrations are below 1 M. Addition of salt to the reaction mixture of the Stober process has, however, very large effects. Clearly, classical nucleation cannot explain all the features of the Stober synthesis either. 

Fig. 3.19. (a) Calibrated silica particles of diameter 500 nm obtained by the Stober process, (b) Titanium oxide particles adapted to paint applications (mean diameter 200 nm). (c) Titanium oxide fibres of thickness 200 nm and length several microns, usable as a catalyst. The objects in (b) and (c) were obtained from the same titanium salt precursor solution, but under different conditions (photo courtesy of Rhone Poulenc)... 

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

Hydrolysis of metal-organic compounds (e.g., metal alkoxides) in alcoholic solution, generally referred to as the Stober process and... 

In the preparation of Ti02 powder by the Stober process, a student starts out with a solution containing 20 vol% of titanium isopropoxide in isopropanol. Assuming that the reaction is stoichiometric, how much water must be added ... 

The first route is based on generating sol—gel particles in an hydrophobic medium. Since the Stober process is based oti water or alcohol solvents and uses agents (ammonium hydroxide) that are not miscible with the polymerization medium of toluene, adaption was necessary. Higher alcohol (butanol, pentanol, octanol), toluene and toluene-ethanol mixtures were screened and, in addition to TEOS, TBOS (tetrabutoxy silane) was used as precursor [85]. It was found that the formation of defined and spherical particles is not easily achieved. For example, in a... 

The standard Stober process employs tetraethoxysilane (TEOS) in a mixture of water, ammonia, and ethanol. The use of other alkoxysilanes as well as replacing ethanol with other solvents are possible modifications of the process. The specific conditions will have a pronounced effect on the particle size, uniformity, reaction... 

The Stober process was also studied by several research groups with respect to the formation and reaction mechanisms. Further details can be found in those publications. For example the overall particle growth reaction was described as a function of TEOS, water, and anunonia concentrations, as well as reaction temperatures by the following equation ... 

Exceedingly monodispersed nanometer-sized silica particles can also be produced by a so-called microemulsion technique, which employs essentially the same chemicals as the Stober process. However, the presence of a surfactant and the oil-phase provide means of creating small (nanometer sized) water droplets (microemulsion), within which the particles are formed. These droplets can be seen as size-limiting reaction vessels, producing very small but rather uniform nanoparticles. Since this technique is not limited to sUica, it has been applied to the synthesis of many different nanopowders. 

Lee K.T., Look J.L., Harris M. T., McCormick A.V. Assessing extreme models of the Stober ssmthesis Using transients under a range of initial composition. J. Colloid Interface Sci. 1997 194 78-88 Lee K.T., Sathyagal A.N., McCormick A.V. A Closer look at an aggregation model of the Stober process. CoUoids Surf. A 1998 144(1-3) 115-125 LeNeveu D.M., Rand R.P., Parsegian VA. Measurement of forces between lecithin bilayers. Nature 1976 259 601-603... 

In situ SAXS investigations of particles formed by the Stober process yield Porod slopes ranging from -3 to -4 [140] indicative of more compact structures either surface fractals or uniform (nonfractal) species. Solvent evaporation results in Porod slopes of -4 [141], which indicates that surface roughness, if any, is collapsed by the capillary pressure during drying. 

The rare earth tetrakis P-diketonate complex functionalized silica spheres are conveniently prepared by a one-pot synthesis method which is based on the modified Stober process [56]. The resulted luminescent nanoparticles are shown schematically in Fig. 8.9 (top). Because the introduction of siloxy-bearing rare earth complex precursor can result in coagulation, a step-by-step approach is adopted to implement the synthesis of uniform silica sphere. The rare earth complex precursors added into the reaction system in the second step can ensure the size uniformity of the nanoparticles furthest. As a result, the rare earth chelate mainly lies in the outer layer of the silica sphere, which has been shown schematically in Fig. 8.9 [55]. As shown in Fig. 8.9 (bottom), the nanoparticles obtained are uniform spheres, approximately 61 5 nm in diameter. And there is no obviously change in the particle size or morphology. All nanoparticles show relatively high luminescent lifetimes. Among the quantum efficiencies, the experiment values of Eu-TTA-SS (34.8 %)... 

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