Hollow silica spheres, formation

Following a related approach, Castelvetro et al. reported the formation and properties of hybrid latex films resulting from the coalescence of low 7 poly(BA-co-MMA-co-MPTMS) terpolymer latex particles coated by a silica shell [78], The latex was synthesized at neutral pH by semi-continuous emulsion polymerization under starved-feed conditions in order to protect the MPTMS monomer from premature hydrolysis and condensation reactions. A substantial amount of free silanols were therefore available for further reaction with the silica precursor. In order to avoid the formation of a densely crosslinked silica network around the latex core, which may significantly alter film formation, the pH was kept at around 2 (at this pH, hydrolysis is promoted and condensation is significantly retarded). TEM and AFM studies of the nanocomposite film indicated that the silica shell formed a continuous percolating network throughout the polymer matrix. A porous film of interconnected hollow silica spheres was next elaborated by thermo-oxidative decomposition of the organic phase. 

The hard template method is as effective as the soft template method in the synthesis of hollow silica spheres. PS particles are the most widely used template [21]. The general procedure involves two steps the formation of core-shell particles and the subsequent removal of the template, either by selective dissolution or by... 

Figure 11.2 A scheme for the formation of and larger (c) amounts of ammonia result in hollow silica spheres using PS template Spon- no dissolution of the core and deterioration of taneous dissolution of the core occurs with a the core before shell formation, respectively, suitable amount of ammonia (b). Lower (a) (Reproduced with permission from Ref. [26].)...

Figure 11.4 (a) A scheme for the formation of hollow silica spheres by simultaneous dissolution and redeposition of siiica. TEM images of (b) Stober siiica spheres and (c and d)... 

Formation of hollow silica spheres with ordered mesoporous structure by treatment with dimethyl carbonate for selective decomposition of mesoporous silica core. Micropor. Mesopor. Mater.,... 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

HCSs with smooth single shells, deformed single shells, double shells, and N-doped shells have been prepared using silica templates and FcH/benzene mixtures via the CVD process. The morphology of the HCSs was found to depend on the CVD reaction time and temperature. Results showed that relatively large silica spheres favored the formation of HCSs with a very smooth surface. However, a short CVD time led to a thin carbon shell and deformed HCSs [25]. Spherical carbons (solid and hollow) with a spongy appearance and controlled sizes were produced by CVD from camphor and FcH. The spheres were seen to be fused and interconnected [25]. 

Recently, a selective etching method has been developed as a simple and effective strategy to fabricate hollow mesoporous silica particles. Fang et al. reported the formation of hollow mesoporous sihca spheres with either a wormhole-like or an oriented pore structure by treatment of sohd silica spheres in a sodium... 

Preformed metal oxide nanoparticles have been successfully coated on polymer spheres by the use of the layer-by-layer method. This involves the coating of the template spheres with polyelectrolyte layers, which are oppositely charged to the metal oxide nanoparticles to be deposited. Alternating the polyelectrolyte and nanoparticle deposition has led to the successful formation of silica [67,68] and titania [69] coated PS spheres. Using this approach preformed crystalline nanoparticles can be deposited on the organic spheres and crystalline hollow spheres can be obtained without the need of calcination. On removal of the template and the polymer interlayers by heating, hollow spheres of the inorganic material can be obtained [68-70]. This procedure is described in detail in the chapter by Dr Frank Caruso. 

Binks and Murakami report unique behavior in NP-induced phase transformation in particle-stabilized air-water systems that is not demonstrated by surfactants. It was seen that by altering silica-NP (20-30 nm) hydrophobicity at constant air water ratio or by changing the air water ratio at fixed NP wettability, phase inversion could be induced from air-in-water to water-in-air foams (Fig. 12) [36]. This investigation thus demonstrates that control over interfacial assembly of NPs leads to the formation of stable NP-shelled hollow spheres, thus resulting in the formation of stable foams, dispersions, and powders with far reaching consequences in opening new avenues for advanced encapsulation (Fig. 13). 

Scheme 8 Formation of cross-linked hollow spheres by (a) silica nanopartide templated layer-by-layer (LbL) assembly of polyelectrolytes, (b) cross-linking of polymer multilayer assembly by TEC, (c) functionalization with PEG using residual thiol, and (d) removal of silica core by hydrofluoric acid (HF) etch. ...

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