Hierarchically porous titanias

Titanium oxides with hierarchically meso(micro)-macroporous structure could also be prepared via the spontaneous self-formation phenomenon [137,141,142]. The hierarchically porous titanias can be prepared in acidic or alkaline aqueous solution [137]. Different macroporous structures and macropore sizes were obtained when using different alkoxide precursors and under different pH values. The ethoxide, -propoxide, and n-butoxide alkoxides produced powders with limited and localized macroporosity with irregularity and wide pore size distributions (Figure 32.11e-0 [137]. 

Zhang, Y. X., M. Kuang, X. D. Hao et al. 2014. Rational design of hierarchically porous birnessite-type manganese dioxides nanosheets on different one-dimensional titania-based nanowires for high performance supercapacitors. Journal of Power Sources 270 675-683. 

E.S. Toberer, J.D. Epping, B.F. Chmelka, R. Seshadri, Hierarchically porous rutile titania harnessing spontaneous compositional change in mixed-metal oxides, Chem. Mater. 18 (2006) 6345-6351. 

With the development of the self-formation phenomenon, hierarchically porous materials can be synthesized even without the need for any macro/mesotemplate. In 2004, Davis and coworkers [127] reported the preparation of ordered macroporous titania by dropwise addition of various titanium alkoxides to aqueous ammonia in the absence of siufactants. This work claimed that macroporosity is only dependent on the Ti alkoxides and solutions used. A microphase-separated domain mechanism was therefore proposed. Simultaneously, Su and coworkers [128,129] demonstrated that spontaneous synthesis of macroporous structures can be carried out at different starting pH values in the absence of... 

Cell assemblies. As macroporous templates, these provide a facile bioinspired method for the synthesis of hierarchical macro-mesoporous titania with tunable macroporous morphology and enhanced photocatalytic activity [134]. This is also a simple and facile technique that can be used to prepare many types of metal oxide porous materials with good control over the pore size and morphology. 

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