Hollow inorganic 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. 

Hollow inorganic particles have various potential applications such as in coatings catalysis, adsorption, and drug dehvery system (DDS) due to their imique structural features [90]. The porous shell provides diffusion pathways to their interior voids, which is particularly important for catalysis and drug dehvery. Fiuther-more, the yolk-shell structured particles containing functional cores inside the hollow interior are useful as nanoreactors for various catalytic reactions and multifunctional carriers for drug dehvery [91]. 

Individual submicron entities can be cast to form monolithic structures with voids that maintain the initial organic shape (as was discussed above), or they can be coated to give individual hybrid entities that form hollow inorganic structures on removal of the template. The particles, tubes, and crystals are generally dispersed in solution for templating. 

This is achieved either by chemical or thermal means. If a solvent is used, only the core is dissolved which results in hollow polymer, (4) in Fig. 12.7, or composite, (6) in Fig. 12.7, capsules. Heat treatment (= calcination) of the coated particles, (5) in Fig. 12.7, removes both the colloidal core and the bridging polymer, thereby producing hollow inorganic spheres. 

Heat treatment (calcination) of the coated particles, (5) in Fig. 11.11, removes both the colloidal core and the bridging polymer, thereby producing hollow inorganic spheres. 

The formation of calcium carbonate particles can be carried out in W/O calcium sulfosuccinate microemulsions [149]. Hollow porous shells of calcium carbonate or reticulated calcium phosphate skeletons made under similar conditions are likely candidates for high technology applications such as the production of lightweight ceramics, catalysts, biomedical implants, and extremely strong membranes [150,151]. Other nanosized inorganic particles can be prepared in W/O microemulsions as well [152-154]. 

Fuji, M Shin, T., Watanabe, H., and Takei, T. (2012) Shape-controlled hollow silica nanoparticles synthesized by an inorganic particle template method. Adv. Powder TechnoL, 23, 562-565. 

In this regard, there is an excellent review article on MMMs for gas separation, with a detailed discussion on the morphology of the interface between the inorganic particles and the polymer matrix (Chung et al. 2007). Unlike many other articles, this deals with asymmetric membranes for both flat sheets and hollow fibers aimed at the formation of an ultrathin defect-free mixed-matrix skin layer. 

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. 

A broad variety of sacrificial colloidal cores have been used for hollow capsule fabrication. They are inorganic or organic particles from tens of nanometers and up to tens of micrometerss, like melamine formaldehyde (MF), polysterene spheres, CaCC>3 and MgCQ3 particles, protein and DNA aggregates, small dye... 

Inorganic nanoparticles themselves can be assembled into mesoscopic structures. Dinsmore et al. proposed an approach for the fabrication of solid capsules from colloidal particles with precise control of size, permeability, mechanical strength, and compatibility (Fig. 2.9).44 This unusual mesoscopic structure is called colloidosome and is prepared through emulsion droplets at a water-oil interface. Following the locking together of the particles to form elastic shells, the emulsion droplets were transferred to a fresh continuous-phase fluid identical to that contained inside the droplets. The resultant structures are hollow, elastic shells whose permeability and elasticity can be precisely controlled. 

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