Preparation of monodisperse silica sols

SILICA NUCLEATION, POLYMERIZATION, AND GROWTH PREPARATION OF MONODISPERSE SILICA SOLS... 

Sol-gel condensation of encapsulated tetrame-thoxysilane guest molecules within the spherical cavities of coordination capsules 529 and 530 has been used in [4] for preparation of monodisperse silica nanoparticles by Scheme 5.4. 

B. A. Keiser s contribution to this book (the introduction to the section Preparation and Stability of Sols ) constitutes an excellent introduction to silica nucleation, polymerization, and growth in both aqueous and alcoholic systems for the preparation of silica sols. Yoshida s chapter (Chapter 2) focuses on industrial development in the preparation of monodisperse sols from sodium silicate and predicts further progress in the development of silica sols that have shapes other than spherical, such as elongated, fibrous, and platelet. Colloidal silica particles with these shapes show novel properties and open the possibility of new industrial applications. 

Silica Nucleation, Polymerization, and Growth Preparation of Monodispersed Sols... 

The industrial development of silica sol manufacturing methods is reviewed. Primary attention is focused on the preparation of monodispersed sols from water glass by the ion-exchange method. Details are given for variations of manufacturing process and for the characteristics of both the processes and sols obtained. Furthermore, the following surface modifications of particles are demonstrated silica sols stabilized with ammonia, amine, and quaternary ammonium hydroxide aluminum-modified or cation-coated silica sol and lithium silicate. Finally, future trends in silica sol manufacturing are discussed from the viewpoint of not only raw materials and improvement of the procedures but also the function of the silica sols and their particle shape. 

Various raw materials can be used in the manufacturing of monodis-persed sol. Examples of these materials include silicon metals (6), silicon tetrachloride (7), ethyl silicate (8), water glass (2), and silica powder (9). In this chapter, I focus attention on the preparation of monodispersed sols from water glass, a raw material that is presently used in large amounts industrially for the inexpensive production of silica sols. 

The work of Mallouk et al. (39) offers an interesting extension of the microemulsion sol-gel technique. In this case, microemulsion-derived silica nanoparticles were used as templates for preparing ordered mesoporous polymers with tailored pore sizes. Utilizing the Triton N-101/cyclohexane/hexanol/water/ammonia microemulsion, monodisperse silica nanoparticles were first synthesized. The silica product... 

Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse that their size-dependent physical properties can be studied in detail [6j. It is also possible to organize these monodisperse nanoparticles via slow evaporation to yield superlattices [11-13]. Superlattices of nanocrystals can rightly be described as a new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals are crystals of amorphous silica spheres [15]. 

In the present study, amorphous silica-alumina nanomaterials with controlled mesoporous distribution have been synthesized by two templateless approaches (1) vacuum-sol process, and (2) ultrasonic-sol process. It is found that the preparation method affects the precursor sol properties and the specific surface area and pore volume of the final materials. Ultrasonic-sol method favors the formation of monodispersed sol particles with narrow size distribution. Because of several base-exchange cycles and absence of drying process prior to heat treatment, the gel derived fiom ultrasonic-sol method may have enough stiffiiess to protect the network fiom pore collapse by capillary force, thus, leading to produce the materials with... 

As for the preparation of colloidal crystals using polymer spheres, the monodispersity of the cells strongly influences the order of the material. Hence yeast cells were carefully grown to form spherical cells of similar diameter. These cells were dip coated with a silica sol on a microscope slide [39]. A mono-layer of the cells arranged in a hexagonal close packing form on the microscope slide. The interstitial sites between the cells contained silica, an SEM image of the film is shown in Fig. 4. The cells remain alive and such films have potential applications in catalysis and as sensors. 

Valentin C., Munoz M.C., Alarcdn J. Synthesis and characterization of vanadium-containing ZrSi04 solid solutions from gels. J. Sol-Gel Sci. Technol. 1999 15 221-230 Van Helden A.K., Jansen J.W., Vrij A. Preparation and characterization of spherical monodisperse silica dispersions in nonaqueous solvents. J. Colloid Interf. Sci. 1981 81 354-368 Woodhead J.L. Sol-gel processes to ceramic particles using inorganic precursors. J. Mater. Educ. 1984 6 887-925... 

Yoldas B.E. Alumina sol preparation from alkoxides. Am. Ceram. Soc. Bull. 1975 54 289-290 Yoshida M., Lai M., Kumar N.D., Prasad P.N. Ti02 nano-particle-dispersed polyimide composite optical waveguide materials through reverse micelles. J. Mater. Sd. 1997 32 4047-4051 Yoshizawa K., Sugoh Y., Ochi Y. Controlled growth of monodispersed spherical silica by a new synthetic process. In Science ofCeramics Vol. 14, D. Taylor, ed. Stoke-on-Trent, UK The Institute of Ceramics, 1988... 

Using silica sols with the addition of PAA, films of ca. 25 fim in thickness are prepared on a stainless steel sheet with no cracks. SEM photographs of the surface of the silica thick films prepared without and with addition of 0.11 mass% PPA are shown in Figure 14-5. Both films are prepared under an applied voltage of 10 V for 10 mill. Micro-cracks are observed in the films prepared from the sol without PAA (Fig. 14-5(a)), whereas the films prepared with PAA have no cracks (Fig. 14-5(b)). SEM photographs of silica particles prepared with different amounts of PAA added are shown in Figure 14-6. Monodispersed spherical particles are prepared at all concentrations of PAA. The particles prepared with PAA are much smaller than those of the particles prepared without PAA. The addition... 

The films prepared by the electrophoretic sol-gel deposition are basically composed of monodispersed spherical particles, and have a lot of open spaces among these particles. If the open spaces are filled with some organic polymers, new type of inorganic-organic composite films with unique characteristics are expected to be obtained (Hasegawa, 1999). Silica particles are modified with 3-aminopropyltriethoxysilane (APS) and vinyltriethoxysilane (VTES). Smooth and crack-free films ca. 15 fim thick are obtainable when ASP-modified silica particles are used for cathodic deposition with addition of PEI. Thick films with reduced open spaces are obtained when VTES modified silica particles are co-deposited with polyethylene maleate. 

Nishimori H., Tatsumisago M., Minami T. Growth mechanism of large monodispersed silica particles prepared from tetraethoxysilane in the presence of sodium dodecyl sulfate. J. Sol-Gel Sci. Tech. 1997 9 25-31... 

Some more examples with interesting multifunctional applications can be mentioned here. The richness of the SGM allows obtaining nanoparticles with different morphologies. The preparation of silica spheres with controllable and monodisperse sizes was indeed reported in the classical paper of Stober et al. [56]. Sol-gel siUca spheres find application in several different domains of photonics and biophotonics. In oiu- research group, for example, functionalized silica spheres have been used in labeling for biological systems [57-59]. Figure 30.8, adapted... 

Most of 3DOM metal oxides are fabricated by an alkoxide-based sol-gel process. The typical procedures include (i) preparation of a colloidal crystal template by ordering monodispersive microspheres (e.g., PS, PMMA or silica) into a face-centered close-packed array (ii) infiltration of a metal alkoxide solution into voids in the colloidal crystals, and in-situ solidification of the precimsor via a sol-gel process and (iii) template removal via a calcination or extraction pathway [99,217-219], Using the alkoxide-based sol-gel method, 3DOM oxides of Si, Ti, Zr, Al, Sb, W, Fe, and 3DOM mixed oxides of some of them can synthesized due to the moderate reactivity of their alkoxide preciuTsors [99], However, most of the other metal alkoxides exhibit high reactivity and their reactions are too quick to be... 

It can now be said that the microemulsion-mediated silicon alkoxide sol-gel process has come of age. The ability to form monodisperse spherical silica particles (20-39) and monolithic gels (40-53) by this method has been amply demonstrated. Recipes are available to prepare materials with predetermined characteristics, especially particle size and polydispersity. Potential applications of these microemulsion-derived... 

Rare earth silicates exhibit potential applications as stable luminescent materials for phosphors, scintillators, and detectors. Silica and silicon substrates are frequently used for thin films fabrication, and their nanostructures including monodisperse sphere, NWs are also reliable templates and substrates. However, the composition, structure, and phase of rare earth silicates are rather complex, for example, there are many phases like silicate R2SiOs, disilicate R2Si207 (A-type, tetragonal), hexagonal Rx(Si04)602 oxyapatite, etc. The controlled synthesis of single-phase rare earth silicate nanomateriais can only be reached with precisely controlled experimental conditions. A number of heat treatment based routes, such as solid state reaction of rare earth oxides with silica/silicon substrate, sol-gel methods, and combustion method, as well as physical routes like pulsed laser ablation, have been applied to prepare various rare earth silicate powders and films. The optical properties of rare earth silicate nanocrystalline films and powders have been studied. 

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