Nanoscale Control of Homoepitaxial Growth on a Two-Dimensional Zeolite

Zeolites, crystalline microporous solids are of interest for a variety of applications including catalysis, adsorption, ion-exchange, separation membranes etc. Given a pore size, improving the performance of zeolites depends to a great extent on the ability to tune their crystal morphology, size and shape. Advances in the understanding of zeolite synthesis and crystallization have facilitated the development of core-shell catalysts^[1], hierarchical materials^[2], exfoliated two-dimensional nanosheets^[3] and thin films^[4]. These developments reiterate the importance of controlling zeolite growth and crystallization at a nanoscale to tailor their microstructure. However, studies focusing on development of methods to control zeolite crystal growth at a dimension approaching single-unit-cell are still in their infancy.

In this work, we report on solution-based growth conditions that lead to slow and controllable epitaxial growth of two-dimensional zeolite nanosheets^[5]. Growth rates on the order of few nanometers per day were achieved. Anisotropic growth in the absence of misoriented domains was facilitated by use of an organic additive which suppresses the non-classical crystal growth pathway predominant in most zeolites to favor a classical pathway involving growth by addition of molecular silica species. Controlling and monitoring growth at a nanoscale also revealed novel crystal growth phenomenon associated with the lateral size and local surface curvature of 2D zeolites.

The growth method described herein is enabling us to tune the microstructure of zeolite thin films and hierarchical catalysts at a scale approaching single-unit-cell dimensions which offers great potential to improve their performance in a plethora of commercial applications. Recently, using this growth method we were able to obtain zeolite MFI thin films on gold coated Si wafer that demonstrated exceptionally low dielectric constant and great mechanical strength. The method is transferrable and can be used to grow zeolite crystals irrespective of their size and the support material used. This paves way for development of high performing separation membranes.

References

[1] D. Van Vu, M. Miyamoto, N. Nishiyama, S. Ichikawa, Y. Egashira, K. Ueyama, Microporous Mesoporous Mater. 2008, 115, 106–112.

[2] X. Zhang, D. Liu, D. Xu, S. Asahina, K. A. Cychosz, K. V. Agrawal, Y. Al Wahedi, A. Bhan, S. Al Hashimi, O. Terasaki, et al., Science 2012, 336, 1684–1687.

[3] K. Varoon, X. Zhang, B. Elyassi, D. D. Brewer, M. Gettel, S. Kumar, J. A. Lee, S. Maheshwari, A. Mittal, C.-Y. Sung, et al., Science 2011, 334, 72–75.

[4] N. Rangnekar, M. Shete, K. V. Agrawal, B. Topuz, P. Kumar, Q. Guo, I. Ismail, A. Alyoubi, S. Basahel, K. Narasimharao, et al., Angew. Chemie Int. Ed. 2015, 54, 6571–6575.

[5] M. Shete, M. Kumar, D. Kim, N. Rangnekar, D. Xu, B. Topuz, K. V. Agrawal, E. Karapetrova, B. Stottrup, S. Al-Thabaiti, et al., Angew. Chemie Int. Ed. 2017, 56, 535–539.