Hollow silica structures, formation

Figure 1.27 Schematic representation of the microemulsion formation leading to hollow silica structures. Reproduced with permission from Ref [77] 2008, National Academy of Sciences, USA.

The product obtained under acid conditions was an ellipsoid nanoparticle, and that obtained under alkaline condition was hollow silica ensembles. The study revealed that [bmimJBF played a critical role in the process of formation of the mesoporous silica hollow structure. It was thus evident that nonaqueous IL microemulsions can contribute to the synthesis of materials with various structures by providing a reaction environment that is able to control the size and morphology of the materials. 

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

Use of the LbL technique is not restricted to the preparation of planar thin films. One of the most outstanding strategy modifications of the LbL technique involves assembly on colloidal particles followed by hollow capsule formation. For example, Caruso and co-workers reported the formation of hollow silica vesicles through LbL assembly on colloidal nanoparticle templates (Fig. 14). Polyelectrolytes and smaller silica particles were initially formed on a larger colloidal core, which was subsequently selectively destroyed. Calcination of the hybrid vesicles resulted in a hollow vesicle composed of silica. Formation of controlled organic-inorganic layer structures on colloidal particles by LbL assembly also provides media appropriate for investigation of fundamental phenomena. 

Careful control of the surfactant-water content and the rate of condensation of silica at high alkalinity resulted in hollow tubules 0.3 to 3 pm in diameter.[292] The wall of the tubules consisted of coaxial cylindrical pores, nanometers in size, that are characteristic of those of MCM-41. The formation of this higher-order structure may take place through a liquid-crystal-phase transformation mechanism involving an anisotropic membrane-to-tubule phase change. 

The formation of hierarchical pore structures in the silica has also been achieved by the use of a gelator 2,3-di- -decyloxyanthracene in methanol [61]. Hollow fibers with micron-sized diameters are obtained on removal of the gelator, which contain mesopores from smaUer gelator aggregates. Changes in the mesopore diameter (5-12 nm) and shape (ink-bottle or cyHndrical) occur for different gelator concentrations. 

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. 

Table 1 summarizes some microstructural and electrochemical properties of porous Si anode materials, as pertaining to the second approach mentioned above, collected from the literature published since 2005. Several synthesis methods have been identified for preparing the porous Si anode materials (column 1, Table 1). One of the two most adopted methods is known as the metal-assisted chemical etching (MACE denoted as E in Table 1). The fundamental principle of this method can be found in the handbook chapter Porous Silicon Formation by Metal Nanoparticle Assisted Etching. Figure 2 shows an example of the MACE-derived porous Si particle. The other most adopted method is magnesiothermic reduction (denoted as M in Table 1). In this method (see handbook chapter Porous Silicon Formation by Porous Silica Reduction ), porous Si oxide materials are reduced by magnesium vapor under high-temperature thermal treatment. The porous Si oxide precursors may be synthesized via the conventional sol-gel processes. Porous Si particles with unique pore structures, such as hollow interior and ordered mesoporosity, may be obtained from Si oxides having the same pore structures which are achieved by using proper templates.

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... 

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