A New Route for the Synthesis of Mesoporous Materials

-Bishnu Bastakoti, PhD.

The synthesis of mesoporous materials has attracted great interest due to their wide range of applications in drug delivery, catalysis, sensors, photovoltaic cells, and fuel cells.¹ The highly porous structures enable large increases in surface area and access by guest species to the internal pore surface, which can significantly influence the physicochemical properties ofmesoporous materials.There have so far been lots of researches on mesoporous materials prepared using amphiphilic organic molecules like low-molecular-weight surfactants and pluronic-type triblock copolymers after the pioneering works by professor Kuroda² and Mobile group.³

In most cases, co-assembly of soluble inorganic oligomers with amphiphilic molecules has been utilized to synthesize inorganic mesoporous materials. The resultant frameworks are poorly crystallized or even amorphous, which limits their practical usage. Mesoporous metal oxides with highly crystallized walls have been synthesized by using hard-templates (mesoporous silica or carbon).⁴Careful post-treatment is required for improving the crystalline degree in the frameworks. However, scientist have often encountered several issues; incomplete crystallization, destruction of mesoporosity during crystallization, and crack formation due to stress associated during crystallization.

To overcome these issues, several efforts have been made to construct the crystalline frameworks by using pre-formed nanocrystals as building units.⁵Several mesostructured inorganic/organic composites have been prepared through interaction between block copolymer templates and preformed inorganic nanocrystals upon evaporation of solvent.After removal of templates, the robust mesoporous structures can be obtained. Most of the reports have demonstrated the use of sub-10 nm nanocrystals and organic template with neutral blocks. Although the mesoporousstructures have been successfully obtained, the interaction between nanocrystals and polymer unit is not clearly understandable. Judicious selection of solvent compositions, careful understanding of interaction of nanocrystals with block copolymers, strict control on evaporation rate for self-assembly, are always critical factors.

Scheme 1: Schematic Illustration of synthesis of crystalline mesoporous materials from co-assembly of 2-D nanosheet and polymeric micelles. Reprinted from (6)withpermissionfrom the WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Inspired by these situations, a more reliable way was adopted to synthesize mesoporous inorganic materials based on ‘colloidal chemistry’ (Scheme 1).⁶Two-dimensional (2D) crystalline nanosheetswith micrometer size was used as inorganic building blocks for the synthesis of mesoporous architectures, for the first time. The negatively charged 2D nanosheet with thin thickness in atomic scale can be easily mono-dispersed under controlled conditions to show high colloidal stability in aqueous solution. Such charged nanosheets can interact with oppositely charged substances. This noticeable feature motivates to use them as a building block for the synthesis of mesoporous materials. As a model building block, niobate nanosheet was chosen. The negatively charged nanosheet has lateral dimension larger than 200 nm and thickness around 2 nm. Micelles of asymmetric block copolymer,poly (styrene-2-vinylpyridine-ethylene oxide) were used as porogen and structure directing agent.Themselves, the polymeric micelles are positively charged. The strong electrostatic interaction between negatively charged nanosheets and positively charged polymeric micelles enable to form composite micelles with the nanosheets. Even under almost zero charged condition (+3 mV), colloidal hybrid solution is very stable. This is because the PEO corona is still free which stabilizes the colloidal hybrid micelles by steric repulsion between the PEO chains. Very thin nanosheets are flexible, so they can be curled and/or bended on the concave micelle surface as shown in Scheme 1. Removal of micelles by calcination results on robust mesoporous oxides with the original crystalline structure. A large amount of gas (e.g., CO₂) evolved during combustion of polymer template breaks the thin nanosheets, which also induces the formation of open mesopores on the film surface with retention of original crystalinity. This synthesized mesoporous materials shows super performance compared to its counter analogues.This smart synthesis is widely applicable to other inorganic nanosheets for preparation of mesoporous crystalline materials, which could have an enormous potential for new functionality.

(This finding has been highlighted as Hot Paper and Cover Page in AngewandteChemie DOI: 10.1002/anie.201410942)

References

1) Bastakoti et al. Small, DOI: 10.1002/smll.201402573.

2) Yanagisawa et al. Bull. Chem. Soc. Jpn. 1990, 63, 988.

3) Kresgeet al. Nature1992, 359, 710.

4) Tüysüzet al. J. Am. Chem. Soc.2008, 130, 14108.

5) Brezesinskiet al. J. Am. Chem. Soc. 2009, 131, 1802.

6) Bastakoti et al. AngewandteChemie, DOI: 10.1002/anie.201410942