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Continuous formation of supported cubic and hexagonal mesoporous films by sol–gel dip-coating

Abstract

Thin films of surfactant-templated mesoporous materials1,2 could find applications in membrane-based separations, selective catalysis and sensors. Above the critical micelle concentration of a bulk silica–surfactant solution, films of mesophases with hexagonally packed one-dimensional channels can be formed at solid–liquid and liquid–vapour interfaces3,4,5. But this process is slow and the supported films3,5 are granular and with the pore channels oriented parallel to the substrate surface, so that transport across the films is not facilitated by the pores. Ogawa6,7 has reported a rapid spin-coating procedure for making transparent mesoporous films, but their formation mechanism, microstructure and pore accessibility have not been elucidated. Here we report a sol–gel-based dip-coating method for the rapid synthesis of continuous mesoporous thin films on a solid substrate. The influence of the substrate generates film mesostructures that have no bulk counterparts, such as composites with incipient liquid-crystalline order of the surfactant–silica phase. We are also able to form mesoporous films of the cubic phase, in which the pores are connected in a three-dimensional network that guarantees their accessibility from the film surface. We demonstrate and quantify this accessibility using a surface-acoustic-wave nitrogen-adsorption technique. We use fluorescence depolarization to monitor the evolution of the mesophase in situ, and see a progression through a sequence of lamellar to cubic to hexagonal structures that has not previously been reported.

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Figure 1: a, Optical interference image of the steady-state film, prepared with initial surfactant concentration co = 0.06 M during dip-coating at 7.6 cm min−1.
Figure 2: a, X-ray diffraction spectra (A–C) recorded on a Siemens D500 diffractometer using Ni-filtered CuKα radiation with λ = 1.5418 Å in θ–2θ scan mode for uncalcined silica/surfactant mesophases prepared with A, co = 0.11 M (5.0 wt% CTAB); B, co = 0.10 M (4.2 wt% CTAB); and C, co = 0.06 M (2.5 wt% CTAB).
Figure 3: Cross-sectional TEM images (recorded with the silicon substrate oriented along the [110] zone axis) of: a, a calcined incipient 1-dH film, the same film used to obtain trace C in Fig. 2b.
Figure 4: a, TEM image of the [100]-zone of a calcined film fragment prepared with co = 0.10 M and detached from the substrate before heating.
Figure 5: N2 sorption isotherms obtained at 77 K for a 400-nm thick incipient 1-dH film prepared with co = 0.06 M (adsorption, white circles; desorption, black circles) and a 250-nm-thick cubic 3-dH film prepared with c0 = 0.10 M (adsorption, white squares; desorption, black squares).

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Acknowledgements

We thank S. Singh, A. Tsai, M. Rodriguez, M. Eatough and R. Tissot for assistance with experiments and M. Aragon for technical illustrations. This work was partially supported by the UNM/NSF Center for Micro-Engineered Materials, by grants from the NSF, by the DOE Basic Energy Sciences, the Electric Power Research Institute and the DOE Federal Energy Technology Center. This work was done under contract from the US Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Department of Energy.

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Correspondence to C. Jeffrey Brinker.

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Lu, Y., Ganguli, R., Drewien, C. et al. Continuous formation of supported cubic and hexagonal mesoporous films by sol–gel dip-coating. Nature 389, 364–368 (1997). https://doi.org/10.1038/38699

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