Hierarchically structured porous materials oxides

Route C mobilizes routes A and B to combine complex precursor systems (aerosol cocktails) in different ratios and yield multifunctional materials with hierarchically structured porosity [18-21]. For instance, latex beads can be combined with a diluted sol-gel surfactant dispersion that contains small inorganic nanoparticles of metallic oxides, metals, and so on. This approach then leack by EISA to micrometer-sized porous spheres with an independent control of dimensions at four levels of size. 

Besides the biomaterials mentioned above, the cuttlebone [49] and chito-san [50] with unique structure have also been used as templates for the formation of hierarchically porous materials, which maintained the biological structure. Starch gel and dextran were also used to produce hierarchically sponge-like micro-, meso/macroporous monoliths of silicalite and meso/macro-porous metal oxides [51,52]. 

Hierarchically porous metal oxide networks can be formed via a spontaneous self-formation phenomenon from metal alkoxides in aqueous solution [113]. Two chemical processes, hydrolysis and condensation, are involved in this spontaneous self-formation procedure to target hierarchically porous structures [114,115]. In fact, the hydrolysis and condensation rates are generally comparable for metal alkoxides [116]. The condensation rate is directly proportional to the rapid hydrolysis rate of reactive metal alkoxides [117,118]. It is well known that the rapid reaction rate of metal alkoxides plays the key role in the formation of hierarchically porous metal oxides [119,120]. The self-formation procedure to form hierarchically porous materials can be achieved by dropping liquid metal alkoxide precursors into an aqueous solution. In this section, the features of self-formation procediu-e and the resulting hierarchically porous materials are summarized. 

Tunable chemical composition Hierarchically porous materials with single or multiple chemical compositions can be prepared via this spontaneous selfformation procedure by using single or multiple alkoxide precursors. Pure oxide materials can be obtained by starting with just the alkoxide or alkylme-tal precursors in aqueous solutions, as opposed to other synthesis strategies where the resulting products are often contaminated by residual species. Furthermore, metal phosphates with hierarchically porous structures can also be prepared by this self-formation process. 

Because of the attractive physicochemical properties and potential applications in catalysis, biotechnology, adsorption, and separation, fabrication of hierarchically porous (macro/mesoporous) materials, especially for the three-dimensional ordered macro/ mesoporous (3DOM) materials, has been a focus in the research on materials science and engineering in recent years [99,199,200], By using close-packed arrays of monodisperse spheres, such as polystyrene (PS), poly(methyl methacrylate) (PMMA), and silica as template, metals [201,202], metal oxides [203-208], metal chalcogenides [209], silica [204,210,211], carbon [212,213], polymers [214,215], and hydroxyapatite [216] with 3DOM structures have been generated. 

---