Hollow silica particles

Among hollow inorganic particles, silica-based particles are the most extensively studied because of their ease of synthesis, structural and compositional diversities, and wide range of applications. Hollow silica particles can be synthesized by either templating or template-free methods. 

Several reports have shown that hollow silica particles can be prepared by hydrolysis and polycondensation of tetraalkoxysilane in the presence of aggregates of polymers such as polyamines [5], polylysine [6], poly(acrylic acid) (PAA) [7], and poly(JV-isoprop)dacrylamide) [8]. Although this method often results in broad particle size distributions (100 nm to several micrometers), relatively fine control of the size (in the range of 25-400 nm) and shell thickness was achieved with PAA [7]. The synthesis route involves (i) the formation of spherical aggregates of PAA in an ethanol solution, (ii) the formation of silica particles from tetraethoxy-silane (TEOS) by the modified Stober method [9], and (iii) subsequent removal of PAA by washing. 

A more reliable method to control the dimensions of hollow silica particles can be achieved using molecular assemblies, such as micelles and vesicles, as soft templates. For example, hollow silica particles of a controlled diameter (60-120 nm) were obtained using unilamellar vesicles consisting of mixtures of cationic and anionic surfactants [10]. The key point is that these vesicles are stable under the conditions for acid-catalyzed hydrolysis and polycondensation of 

Alkoxysilanes can also act as oil droplets. Hollow silica microspheres of diameters 0.3-65 pm were prepared in a CTAB-stabilized emulsion system consisting of water and the mfacture of TEOS and aminopropyltriethoxysilane (10 mol%) as the oil phase [18]. Organo-functionalization of the microspheres was achieved by replacing TEOS with organotriethoxysilane, and encapsulation of TEOS-soluble additives within the silica shdls was demonstrated by incorporation into the ethoxysilane droplets. 

Colloidal silica particles can be used as shell constituent for emulsion tern-plating [19]. By adding an organic solvent (isopentyl acetate) and 3-methacryl-oxypropyltrimethoxysilane into an aqueous dispersion of colloidal silica (7, 12, and 25 nm in diameter), a particle-stabilized emulsion, called Pickering emulsion [20], with small droplet sizes was formed due to the low interfacial tension between the colloidal silica particles and monomers. After polymerization of organotrialkoxysilane, the organic solvent in the core was evaporated to form hollow particles with a porous shell. These hollow particles have a hydrophobic interior, which is potentially useftil for adsorbing hydrophobic contaminants in water. 

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Butts, M. D. Genovese, S. E. Glaser, P. B. Williams, D. S., Hollow silica particles and methods for making same, US Patent Application 20070036705 2007... 

In Figure 51.21, we show an electron micrograph of hollow silica particles obtained after exposure of the coated particles to 1 mM cyanide ion at pH 10.5. 

Figure 11.3 Scanning electron microscopy (SEM) and transmission eiectron microscopy (TEM) images of hollow silica particles prepared using hematite templates with various shapes. (Reproduced with permission from Ref. [29]. Copyright 2013, American Chemical Society.)...

Du et al. demonstrated the utihty of hollow sihca nanoparticles in fabricating AR coatings on a poly(methyl methacrylate) (PMMA) substrate [93]. Hollow silica particles, prepared using PAA template [7], were coated on the substrate... 

Bayu, A., Nandiyanto, D., Akane, Y., Ogi, T., and Okuyama, K. (2012) Mesopore-free hollow silica particles with controllable diameter and shell thickness via additive-free synthesis. Langmuir, 28, 8616-8624. 

Tan, B., Lehmler, H.-J., Vyas, S.M., Knutson, B.L., and Rankin, S.E. (2005) Fluorinated-surfactant-templated synthesis of hollow silica particles with a single layer of mesopores in their shells. Adv. Mater, 17, 2368-2371. 

The sacrificial core approach entails depositing a coating on the surface of particles by either the controlled surface precipitation of inorganic molecular precursors from solution or by direct surface reactions [2,3,5,6,8,9,33-35,38], followed by removal of the core by thermal or chemical means. Using this approach, micron-size hollow capsules of yttrium compounds [2], silica spheres [38], and monodisperse hollow silica nanoparticles [3,35] have been generated. 

FIG. 10 SEM micrographs of (a) sUica nanoparticle/polymer [Si02/PDADMAC)3]-coated PS lat-ices and (b) hollow silica capsules. The hollow sUica capsules were obtained by calcining coated particles as shown in (a). The calcination process removes the PS core and the polymer bridging the silica nanoparticles, while at the same time fusing the silica nanoparticles together. Some of the silica capsules were deliberately broken to demonstrate that they were hollow (b). (From Ref. 106.)... 

FIG. 11 TEM images of (a) a [(Si02/PDADMAC)2]-coated PS particle and hollow silica capsules produced from PS latices coated with (b) one, (c) two, or (d) three Si02 layers. The hollow silica capsules maintain the shape of the original PS particle template. Removal of the core by calcination is confirmed by the reduced electron density in the interior of the capsules (compare b-d with a). The images of the hollow silica capsules show the nanoscale control that can be exerted over the wall thickness and their outer diameter. (From Ref. 106.)... 

Fig. 2.18 Vesicle structures with silica wall (A) mesostructured silica vesicle (B) hollow capsule composed of silica particles prepared by LbL assembly.

Fig. 4.2 TEM images of fabricated nanoparticles, (a) Isolated composite core/shell submicron particles, (b) Hollow silica submicron particles prepared by removing the polystyrene core to demonstrate the high quality of the formed sol gel shell of the composite nanospheres employed to prepare sensing colloidal crystal films...

Fig. 4 a, b. TEM micrographs of hollow silica spheres produced by calcining PS particles coated with (a) one and (b) three Si02 nanoparticle/PDADMAC layer pairs at 450°C. The wall thickness of the hollow capsules is approximately three times greater for those shown in (b) compared with those shown in (a), c, d Cross-sections of the hollow silica spheres of the same composition as those shown in (b). The hollow silica spheres retain the spherical shape of the original PS particle templates (see Fig. 3). (Adapted from [22,62] by permission of the American Association for the Advancement of Science and the American Chemical Society)...

Fillers may decrease thermal conductivity. The best insulation properties of composites are obtained with hollow spherical particles as a filler. Conversely, metal powders and other thermally conductive materials substantially increase the dissipation of thermal energy. Volume resistivity, static dissipation and other electrical properties can be influenced by the choice of filler. Conductive fillers in powder or fiber form, metal coated plastics and metal coated ceramics will increase the conductivity. Many fillers increase the electric resistivity. These are used in electric cable insulations. Ionic conductivity can be modified by silica fillers. 

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