Hierarchical colloidal crystals

Several approaches towards the synthesis of hierarchical meso- and macro-porous materials have been described. For instance, a mixture that comprised a block co-polymer and polymer latex spheres was utilized to obtain large pore silicas with a bimodal pore size distribution [84]. Rather than pre-organizing latex spheres into an ordered structure they were instead mixed with block-copolymer precursor sols and the resulting structures were disordered. A similar approach that utilized a latex colloidal crystal template was used to assemble a macroporous crystal with amesoporous silica framework [67]. 

Complex templates combine soft and hard template techniques. This methodology is used for synthesizing hierarchically bimodal and trimodal meso-macroporous materials with interconnected pore channels combining a surfactant template with a colloidal crystal template (Yuan and Su, 2004). 

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. 

Sen T, Tiddy GJT, Casci JL, Anderson MW (2004) Synthesis and characterization of hierarchically ordered porous silica materials. Chem Mater 16 2044 Deng Y, Liu C, Yu T, Liu F, Zhang F, Wan Y, Zhang L, Wang C, Tu B, Webley PA, Wang H, Zhao D (2007) Facile synthesis of hierarchically porous carbons from dual colloidal crystal/block copolymer template approach. Chem Mater 19 3271 Luo Q, Li L, Yang B, Zhao D (2000) Three-dimensional ordered macroporous structures with mesoporous silica walls. Chem Lett 29 378... 

Colloidal crystals. Combined with an amphiphilic triblock copolymer, for the synthesis of hierarchical macro-/mesoporous Ti-Si oxide photonic crystals (i-Ti-Si PC) with highly efficient photocatalytic activity [135]. 

Stein, A., Li, F., and Denny, N.R. (2008) Morphological control in colloidal crystal templating of inverse opals, hierarchical structures, and shaped particles. Chem, Mater., 20, 649-666. 

Another family of synthesis strategies uses small solid particles as additional templates to micellar templating to create hierarchical pore structures. These small solid particles as additional templates consist of colloidal crystals, biomaterials, macroporous polymers, salts, and ice crystals. This combination of surfactant and small solid particles offers an efficient way for the generation of ordered and interconnected mesoporous-macroporous architectures, small solid particles creating macropores and surfactant micelles creating mesopores. 

Hierarchically Structured Porous Materials Colloidal crystal... 

Xia, X. Tu, J. Wang, X. Gu, C. Zhao, X. Hierarchically porous NiO film grown by chemical bath deposition via a colloidal crystal template as an electrochemical pseudocapacitor material. J. Mater. Chem. 2010, 21, 671-679. 

Doherty CM, Caruso RA, Smarsly BM, Drummond CJ (2009) Colloidal crystal templating to produce hierarchically porous LiFeP04 electrode materials for high power lithium ion batteries. Chem Mater 21 2895-2903... 

Windows or external surfaces that are self-cleaning are possible with superhydrophobic surfaces. Based on a monolayer polystyrene (PS) colloidal crystal, large-scale two-dimensional (2D) hierarchical porous silica (orderly arranged macropores and disordered mesopores in its skeleton) with a high specific surface area have been fabricated by the sol-gel technique. Such material has demonstrated reversible superhydrophobicity (16). 

Finally, zeolite nanoparticles have been used as building blocks to construct hierarchical self-standing porous stmctures. For example, multilayers of colloidal zeolite crystals have been coated on polystyrene beads with a size of less than 10 p,m [271,272]. Also, silicalite-1 membranes with a thickness ranging from 20 to several millimeters and controlled mesoporosity [273] have been synthesized by the self-assembly of zeolite nanocrystals followed by high-pressure compression and controlled secondary crystal growth via microwave heating. These structures could be useful for separation and catalysis applications. 

An improved polymerization-induced colloid aggregation (im-PlCA) method was developed to prepare zeolite microspheres with hierarchical porous stractures and a uniform size, which could easily be carried out by adding urea and formaldehyde to an acidic pH precontrolled colloidal solution, as obtained from a hydrothermal crystallization process. After removing the polymeric component, solid and hollow zeolite microspheres can be obtained under different preparation conditions [172]. 

Soft matter science is nowadays an acronym for an increasingly important class of materials, which encompasses polymers, liquid crystals, molecular assembhes building hierarchical structmes, and the whole area of colloidal sciences. Common to all of them is that fluctuations and thus the thermal energy T and the entropy play an important role. Soft then means that these materials are in a state of matter that are neither simple liquids nor hard solids of the type studied in hard condensed matter, hence sometimes soft matter firms also under the name complex fluids. 

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