Sorting molecules with self-assembled nanoscale membranes

December 20, 2017

Schematic illustration of gaseous mixture of acetone (red) and ethyl acetate (green) molecules selectively permeating a self-assembled membrane,

What is the scientific achievement?

We have fabricated highly-porous, highly-uniform silicon nitride membranes by replicating features from self-assembled block copolymer films. With porosities over 30% and thickness <100 nm, the membranes are designed for high throughput. Pore sizes are controllably tuned to molecular scales, for selective gas permeation. Capillary condensation within nanoscale pores enhances selectivity beyond that expected from molecule size differences.

Why does this achievement matter?

Membranes underlie integral separation processes in energy production, water purification, medicine, environmental cleanup, and chemical processing. These highly-uniform, highly-porous inorganic membranes may provide durability for high temperature operation in extreme environments.

What are the details?

We report the fabrication of ultrathin, nanoporous silicon nitride membranes made from templates of regular, nanoscale features in self-assembled block copolymer thin films. The inorganic membranes feature thicknesses less than 50 nm and volume porosities over 30 percent, with straight-through pores that offer high throughout for gas transport and separation applications. As fabricated, the pores are uniformly around 20 nm in diameter, but can be controllably and continuously tuned to single-digit nanometer dimensions by combining with conformal coatings by atomic layer deposition. A deviation from expected Knudsen diffusion is revealed for transport characteristics of saturated vapors of organic solvents across the membrane, which becomes more significant for membranes of smaller pores.  We attribute this to capillary condensation of saturated vapors within membrane pores, which reduces membrane throughput by over one order of magnitude but significantly improves the membrane’s selectivity. Between vapors of acetone and ethyl acetate, we measure selectivities as high as 7:1 at ambient pressure and temperature, four times more than the Knudsen selectivity.

CFN Capabilities

CFN Nanofabrication, Materials Synthesis, and Electron Microscopy facilities were used for membrane synthesis and characterization.

Publication Reference

S. Greil, A.  Rahman, M. Z. Liu, C. T. Black, Gas Transport Selectivity of Ultrathin, Nanoporous Inorganic Membranes Made from Block Copolymer Templates, Chemistry of Materials 29, 9572 (2017).

DOI: 10.1021/acs.chemmater.7b04174

Acknowledgement of Support

This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.

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