Selective Diffusion of CO2 and H2O through Carbon Nanomembranes in Aqueous Solution as Studied with Radioactive Tracers.

Nanometer-thin carbon nanomembranes (CNMs) are promising candidates for efficient separation processes due to their thinness and intrinsic well-defined pore structure. This work used radioactive tracer molecules to characterize diffusion of [3H]H2O, [14C]NaHCO3, and [32P]H3PO4 through a p-[1,1',4',1″]-terphenyl-4-thiol (TPT) CNM in aqueous solution. The experimental setup consisted of two microcompartments separated by a CNM-covered micropore. Tracers were added to one compartment and their time-dependent increase in the other compartment was monitored. Occurring concentration polarization and outgassing effects were fully considered using a newly developed mathematical model. Our findings are consistent with previous gas/vapor permeation measurements. The high sensitivity toward a small molecule flow rate enables quantification of diffusion through micron-sized CNMs in aqueous solution. Furthermore, the results allow unambiguous distinction between intact and defective membranes. Even for extremely small membrane areas, this method allows detailed insight into the transmembrane transport properties, which is crucial for the design of 2D-separation membranes.

Ultrahigh Ionic Exclusion through Carbon Nanomembranes.

The collective "single-file" motion of water molecules through natural and artificial nanoconduits inspires the development of high-performance membranes for water separation. However, a material that contains a large number of pores combining rapid water flow with superior ion rejection is still highly desirable. Here, a 1.2 nm thick carbon nanomembrane (CNM) made from cross-linking of terphenylthiol (TPT) self-assembled monolayers is reported to possess these properties. Utilizing their extremely high pore density of 1 sub-nm channel nm-2 , TPT CNMs let water molecules rapidly pass, while the translocation of ions, including protons, is efficiently hindered. Their membrane resistance reaches ≈104 Ω cm2 in 1 m Cl- solutions, comparable to lipid bilayers of a cell membrane. Consequently, a single CNM channel yields an ≈108 higher resistance than pores in lipid membrane channels and carbon nanotubes. The ultrahigh ionic exclusion by CNMs is likely dominated by a steric hindrance mechanism, coupled with electrostatic repulsion and entrance effects. The operation of TPT CNM membrane composites in forward osmosis is also demonstrated. These observations highlight the potential of utilizing CNMs for water purification and opens up a simple avenue to creating 2D membranes through molecular self-assembly for highly selective and fast separations.

Rapid Water Permeation Through Carbon Nanomembranes with Sub-Nanometer Channels.

The provision of clean water is a global challenge, and membrane filtration is a key technology to address it. Conventional filtration membranes are constrained by a trade-off between permeance and selectivity. Recently, some nanostructured membranes demonstrated the ability to overcome this limitation by utilizing well-defined carbon nanoconduits that allow a coordinated passage of water molecules. The fabrication of these materials is still very challenging, but their performance inspires research toward nanofabricated membranes. This study reports on molecularly thin membranes with sub-nanometer channels that combine high water selectivity with an exceptionally high permeance. Carbon nanomembranes (CNMs) of ∼1.2 nm thickness are fabricated from terphenylthiol (TPT) monolayers. Scanning probe microscopy and transport measurements reveal that TPT CNMs consist of a dense network of sub-nanometer channels that efficiently block the passage of most gases and liquids. However, water passes through with an extremely high permeance of ∼1.1 × 10-4 mol·m-2·s-1·Pa-1, as does helium, but with a ∼ 2500 times lower flux. Assuming all channels in a TPT CNM are active in mass transport, we find a single-channel permeation of ∼66 water molecules·s-1·Pa-1. This suggests that water molecules translocate fast and cooperatively through the sub-nanometer channels, similar to carbon nanotubes and membrane proteins (aquaporins). CNMs are thus scalable two-dimensional sieves that can be utilized toward energy-efficient water purification.