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Highlights

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated in situ


Figure 1.  A) A typical SAXS pattern for the circular hexagonal structure in an AAM host, indexed in the circular hexagonal (p6mm) unit cell (B) with a lattice spacing of 15 nm. The squares show the reflections from the mesophases in the AAM pores while the circles show reflections from a top layer on the membrane. Reprinted with permission from Schuster et al. J. Am. Chem. Soc. 134, 11136 (2012).

A new mechanism for mesostructure formation of ordered mesoporous carbons (OMCs) was investigated at the Austrian SAXS beamline at Elettra with in situ small-angle x ray scattering (SAXS) measurements: thermally induced selfassembly. Unlike the well-established evaporation-induced selfassembly (EISA), the structure formation for organic−organic self-assembly of an oligomeric resol precursor and the blockcopolymer templates Pluronic P123 and F127 does not occur during evaporation but only by following a thermopolymerization step at temperatures above 100 °C.
Ordered mesoporous carbon in bulk or powder form is commonly synthesized either by hard templating, where periodic mesoporous silica is filled with carbon precursors followed by carbonization and removal of the silica, or by soft templating, using the self-assembly of soluble carbon precursors with liquid crystalline phases of surfactants acting as soft templates. The examples for mesoporous carbon thin films or phases still embedded in alumina membrane (AAM) hosts are limited to soft-templating methods. Hardtemplating methods for ordered mesoporous carbon based on porous silica templates have not yet been implemented for these morphologies, which is mainly attributed to weak adhesion of the resulting carbon material to the substrate after etching of the silica template. While the final carbon structure obtained via hard templating is controlled by the solid template, the final structures made by soft templating are much more sensitive to experimental conditions, such as concentrations, temperature or humidity during structure formation. Therefore, the understanding and control of structure formation processes with soft-templating methods concerning mesostructural symmetry, morphology, and orientation of the desired  mesoporous carbon phases are essential, especially for syntheses in confined environments. In situ grazing incidence small-angle x ray scattering (GISAXS) for characterization of thin films and in situ small-angle x ray scattering (SAXS) of AAM/OMC composites are powerful tools to investigate structural changes during all steps of structure formation and processing. The self-assembly mechanisms for OMC materials made by softtemplating have not yet been investigated in detail. For OMC systems, mainly for the popular resol-Pluronic system, the structure formation was mostly described as an EISA process similar to the one found for mesostructured metal oxides (e.g. silica or titania) followed by a thermopolymerization step to cross-link the precursor oligomers. In our study, different OMC phases (2D-hexagonal and orthorhombic as thin films and cubic and circular hexagonal in AAM hosts) were obtained by organic−organic selfassembly of a preformed oligomeric resol precursor and the triblock copolymer templates Pluronic P123 and F127, respectively.

The thermopolymerization step was investigated in detail with in situ grazing incidence small-angle x ray scattering (GISAXS, for films) and in situ SAXS (for AAMs). A typical SAXS pattern and a scheme of the corresponding unit cell for the circular hexagonal structure in an AAM are presented in Fig. 1. The processes in the thermally induced structure formation for this sample are illustrated in Fig. 2. After heating for 15 min at 130 °C, the first reflections related to a circular hexagonal structure start to appear. A diffuse ring attributed to worm-like phases is also visible, thus some parts are oriented randomly, while others already show the final orientation. Upon further heating the intensity of the reflection spots increases, and the structure becomes completely circular hexagonal. This shows that unlike in the case of mesostructured metal oxides, the structure formation in these systems does not occur during evaporation of the solvent but during the thermopolymerization step and should, therefore, rather be called thermally induced self-assembly. As a remarkable consequence, the mesostructure is not fixed but still flexible and can be controlled during this step. Moreover, we find that higher thermopolymerization temperatures result in increased unit cell parameters, caused by swelling of the liquid crystal structures of the block copolymer templates. The new mechanism discovered here offers additional opportunities for mesostructure control. We have demonstrated the influence of different temperatures during this thermally induced self-assembly on the final mesostructure, and we suppose that the change of other synthesis parameters, such as the vapor atmosphere, will also show significant effects and should thus be subject of further studies.

Figure 2.  In situ SAXS of the structure formation during thermopolymerization at 130 °C in AAMs. After heating for 15 min at 130 °C, the first reflections related to a circular hexagonal structure start to appear (indicated with squares), which then increase in intensity until the structure becomes completely circular hexagonal. Reprinted with permission from Schuster et al. J. Am. Chem. Soc. 134, 11136 (2012).

Acknowledgement
The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n° [226716] (ELISA).

Retrieve article

In Situ SAXS Study on a New Mechanism for Mesostructure Formation of Ordered Mesoporous Carbons: Thermally Induced Self-Assembly;
J. Schuster, R. Köhn, M. Döblinger, A. Keilbach, H. Amenitsch and T. Bein;
J. Am. Chem. Soc. 134, 11136 (2012).
10.1021/10.1021/ja208941s
Hot Templating (Editors' Choice); P. Szuromi; Science 337, 270 (2012). 10.1126/science.337.6092.270-d

Ultima modifica il Martedì, 14 Maggio 2019 17:05