Solids hierarchical porous

Hierarchical porous materials are solids that are ordered at different length scales. Materials with multiple porosities are of high interest for applications in catalysis and separation, because these applications can take advantages of different pore structures. For example, microporous-mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeolites with high reactant or product mobility due to large uniform mesopores. Several approaches have been reported on the design and synthesis of hierarchical porous materials, as discussed below. 

Regarding the application in supercapacitor, the flexible PANI nanotube arrays with high and stable specific capacitances were prepared via electrochemical polymerization. The specific capacitances increased with decreasing wall thicknesses (Wang et al., 2012). A three-dimensionally hierarchical porous PPy hydrogel with various mechanical and electrochemical properties are fabricated by controlling the ratio of phytic acid to pyrrole monomer in the synthetic process. The solid-state supercapacitor showed a weight specific capacitance of 380 F g and areal specific capacitance of 6.4 F cm at a mass load of 20 mg cm(Shi et al., 2014). 

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]. 

At the highest hierarchical level (1 to 5 mm), there are two types of bone cortical bone, which comes as tightly packed lamellar, Haversian, or woven bone and trabecular bone, which comes as a highly porous cellular solid. In the latter, the lamellae are arranged in less well-organized packets to form a network of rods and plates about 100 to 300 /xm thick interspersed with large marrow spaces. Many common biological materials, such as wood and cork, are cellular solids. ... 

The use of complementary experimental techniques as a numerical and visual basis in the formulation of more realistic and applicable pore structure characterisation models has become widespread. Examples of the use of these techniques include mercury porosimetry [9] in the study of entrapment hysteresis in porous media, and in the characterisation of permeable solids [7], the use of NMR (nuclear magnetic resonance) in the heterogeneous and hierarchical stractural modelling of porous media [8,10], and flie use of SEM imaging techniques [7,11]. 

Yu XI et al (2013) Preparation and electrochemical properties of porous silicon/carbon composite as negative electrode materials. J Inorg Mater. doi 10.3724/SP.J.1077.2013.12672 Yue L et al (2013) Porous silicon coated with S-doped carbon as anode material for lithium ion batteries. J Solid State Electrochem doi 10.1007/s/10008-012-1944-8 Zhang Y, Huang J (2011) Hierarchical nanofibrous silicon as replica of natural cellulose substance. J Mater Chem 21 7161-7165... 

Examples of dense silica, hybrid silica, metal oxides, solid-state metal oxide solutions, or colloidal self-assembly are unlimited. However, the recent developments to accurately control processing conditions (e.g., atmosphere, temperature, and motion) led to films with unique properties (see Figure 9.6) [52,53]. These progresses concern mesoporous coatings with controlled pore size and structure [26], hard template infiltration and/or replication [54-58], nanostructured epitaxial low-quartz thin films [59], ultrathin nanostructured supported networks [60,61], ultrathick porous Ti02 layer prepared from aqueous solutions [51], coatings with hierarchical porosity [62], multilayer porous stacks [63], colloidal MOF layers [64,65], pillar planar nanochannels (PPNs) for nanofluidics [66], and so on. 

---