Many applications rely on porous membranes to pass specific molecules while excluding others, such as industrial-scale chemical and gas purification. As reported in the October 7, 2012 online issue of Nature Nanotechnology (DOI: 10.1038/NNANO.2012.162), S.P. Koenig and co-workers at the University of Colorado have fabricated molecular sieves by etching pores in bilayer graphene membranes, where the graphene membranes provide atomic thickness, mechanical robustness, chemical inertness and impermeability to standard gases.
The researchers defined an array of 5-μm-diameter microcavities in silicon oxide using standard photolithographic techniques, and then mechanically exfoliated graphene over these wells to form suspended membranes. The as-deposited graphene flakes cling to the oxide substrate through surface forces in a gas-tight manner, although gases are able to enter/exit the microcavity through very slow diffusion through the oxide.
The graphene-sealed microcavities are then loaded with a desired gas species by placing them in a high-pressure (200 kPa above ambient) environment containing the charging gas and allowing the system to equilibrate for 4–12 days. When the samples are brought out to ambient conditions, the membrane bulges outward due to the pressure differential. Slow leak rates were demonstrated through the pristine membranes (on the order of minutes to hours) by using an atomic force microscope to measure the deflection δ of the swollen blister with time.
Finally, pores were generated in the graphene membranes by oxidative etching on exposure to ultraviolet light. The molecular selectivity of these membranes was then demonstrated by the observation that δ decreases two orders of magnitude more rapidly as compared with nonporous graphene for small gas molecules such as H2 and CO2, while the rate remains largely unchanged for Ar and gas molecules with a larger kinetic diameter.
In graphene membranes that have larger pores, faster leak rates were measured using a mechanical resonance method which again demonstrated size-selective leak rates, but this time for molecules above and below the size of SF6.
“Our results are consistent with theoretical models based on effusion through Ångstrom-sized pores,” stated group leader J. Scott Bunch, “and represent an important step toward the realization of macroscopic, size-selective porous graphene barriers.”
