Templating colloidal mesoporous silica particles

The materials which have been mentioned here so far are predominantly shaped in planar films of hierarchical order. However, the synthesis of hierarchically structured particles is also highly desirable, as they might be further processed and used for the preparation of composite porous materials. Wu et al. showed the synthesis of raspberry-like hollow silica spheres with a hierarchically structured, porous shell, using individual PS particles as sacrificial template [134]. In another intriguing approach by Li et al. [135], mesoporous cubes and near-spherical particles (Fig. 10) were formed by controlled disassembly of a hierarchically structured colloidal crystal, which itself was fabricated via PMMA latex and nonionic surfactant templating. The two different particle types concurrently generated by this method derive from the shape of the octahedral and tetrahedral voids, which are present in the template crystal with fee lattice symmetry. 

To synthesise mesoporous carbons with larger pore size, colloid silica particles and silica gels have been explored as hard templates. Hyeon s group pioneered the synthesis of mesoporous carbon using colloidal silica particles as hard templates. Initially, they synthesised mesoporous carbon using a silica sol solution with silica particle size of 12 nm as template and resorcinol/formaldehyde as carbon source. It was found that the... 

Making use of constrained polymerisation of divinylbenzene on surfactant-modified colloid silica, Jang and Lim prepared carbon nanocapsules and mesocellular foams. Later, they reported that mesoporous carbons with highly uniform and tunable mesopores were fabricated by one-step vapour deposition polymerisation using colloidal silica nanoparticles as template and polyacrylonitrile as carbon precursor. Hampsey et al. recently reported the synthesis of spherical mesoporous carbons via an aerosol-based, one-step approach using colloidal silica particles and/or silicate clusters as template. ... 

Continuous mesoporous carbon thin films were fabricated by direct carbonization of sucrose-silica nanocomposite films and subsequent removal of the silica [236]. The mesoporous carbon film with uniform and interconnected pores had a surface area of 2603 mVg and a pore volume of 1.39 cmVg. Subsequently, nanoporous carbons with bimodal PSD centered at about 2 and 27 nm in diameter were prepared by using both the TEOS-derived silica network and the colloidal silica particles as templates [237]. Figure 2.33 illustrates the preparation pathway. The pore sizes of the carbon are determined by the sizes of the added silica particles and the silica network. As the colloidal silica particles are commercially available with different diameters (e.g., 20 to 500 nm), this dual template synthesis process provides an efficient route to preparing nanoporous carbons with a controllable hierarchical pore structure. 

Kim et al. synthesized mesoporous earbon xerogels by sol-gel polymerization of resorcinol and formaldehyde using colloidal silica particles of 12 nm diameter as templates [275]. The main pore size range of the resulting carbon xerogels was between about 30—40 mn, and the BET surface areas varied as a function of the preparation pH between 321 m g" (at pH 1.5) and 654 m g (at pH 9). The PtRu nanoparticles synthesized and deposited on the support had diameters between 2.2 and 2.8 nm (the larger the support surface area, the smaller the catalyst particle size). 

Templates can be defined as hard or soft templates. " " The former refers to solid matter having interconnected pore systems, such as some zeolites, ordered mesoporous silica (OMS), ° or colloidal silica particles, whereas, soft templates relate to soft matter, such as surfactant micelles, polymers, and other organic molecules, exhibiting liquid crystalline properties. 

Dispersed colloidal silica particles of various sizes and colloidal silica crystals (opals) " have been used as templates of porous carbons, with spherical pores having narrow pore size distributions (PSDs). By coating monodisperse colloidal silica particles or crystals with a suitable carbon precursor, followed by carbonization and etching of the sUica, porous carbon particles can be obtained. The diameters of the mesopores are determined by the size of the silica particles. Because... 

Preparing carbons with hierarchical pore structure is done by impregnation of preformed macropo-rous structures, such as silica colloids, with the carbon precursor gels, followed by precursor carbonization and macropore template dissolution. Although in hard-templated carbons, two particle sizes of the hard template are simultaneously required for the colloidal imprinting method the soft-templating method makes this procedure much simpler and broadens the selection of templates for the larger mesopores and macropores. 

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