Symposium NM06—Nanoscale Mass Transport Through 2D and 1D Nanomaterials

TBD

In the last two decades, nanopore technology has revolutionized single-molecule studies of biomolecules by providing researchers with an inexpensive, rapid, and high-throughput tool for sensing, manipulating, and sequencing individual biopolymers (DNA, RNA, and proteins). In this technique, as a biomolecule is electrophoretically pulled through a nanometer-sized pore (either a biological or a synthetic nanopore), occlusion of the biomolecule within the nanopore is detected as a characteristic transient disruption in the ionic current. A hallmark feature of the nanopore technique is the dependence of the sensing resolution on the geometry of the nanopore (diameter and thickness in solid-state nanopores). Therefore, theoretically, two-dimensional (2D) materials afford the highest resolution possible, owing to their single to a few-atomic-layer thickness. We report on a 2D material-based nanopore platform based on MXenes, a new family of 2D transition metal carbides, nitrides, or carbonitrides with a unique combination of electrical, electrochemical, and mechanical properties with facile synthesis and processing in water due to their hydrophilicity¹. MXenes have shown outstanding performance in energy storage, electromagnetic interference shielding, and sensing applications, currently moving toward commercialization and industrial use. Nonetheless, while most 2D materials can be grown on a substrate and then transferred to the device carrier of choice, a major hurdle in MXene implementation into nanoscale actuators and sensors is the lack of a large-scale assembly and fabrication method of MXene films with a controlled number of layers. We have developed and refined a rapid liquid-liquid interfacial self-assembly method to fabricate wafer-scale MXene films with control over nominal film thickness (monolayer to multilayer), film density, and lateral dimension without the need for any specialized instrumentation. In this method, MXene flakes rapidly self-assemble to form a floating mosaic structure film with an arbitrary size at the interface of water and an organic solvent in the presence of co-solvent². Reactive force-field (ReaxFF) molecular dynamics simulations demonstrated the importance of the co-solvent in the ternary mixture, which renders the aqueous/organic interface energetically favorable for MXene flakes to reside while other factors such as surface energy and electrostatic repulsion collectively promote MXene film formation. After transferring the assembled film onto the substrate, we adopted monolayer to multilayer free-standing MXene films for single-molecule biomolecular sensing using nanopores directly fabricated through the membrane. Our results show that ultrathin MXene membranes afford high mechanical robustness, long-time stability, and low-noise ionic current recordings suitable for single-molecule detection³. Moreover, reversible intercalation of cations through the interlayer space of ultrathin free-standing MXene membranes couples a nanopore system to mechanical actuation. This unique coupled nanopore-actuator system could be further enhanced to a new class of nanopore readers with an in-plane cation reservoir to overcome some challenges that other solid-state and biological nanopores face, such as access resistance, which limits the sensing resolution⁴.
Vahid Mohammadi et al., Science 372, 1165 (2021).
Mojtabavi et al., ACS Nano, 15, 625–636 (2021).
Mojtabavi et al., ACS Nano 13, 3042−3053 (2019).
Comer et al., Nanoscale 8, 9600−9613 (2016).

High-performance molecular-selective membranes are expected to play a crucial role in improving the energy-efficiency of separation processes and reducing the industrial carbon emission. Our laboratory is engaged in chemistry and engineering of two-dimensional materials at the angstrom length scale to address challenges in the scalable fabrication of nanoporous membranes separating molecules based on their relative diffusivities through custom-designed nanopores.
In this talk, I will present our work on the synthesis of nanoporous two-dimensional materials focusing on graphene and zeolite precursors using a number of top-down and bottom-up synthetic strategies. In case of graphene, I will discuss defect nucleation and expansion strategies that allow incorporation of vacancy defects (nanopores) at a high density but with a narrow pore-size-distribution with a resolution of 0.3 Å for molecular differentiation, leading to realization of record-high performance in post-combustion carbon capture [1-6]. I will discuss mechanical reinforcement strategies that allows one to scale-up nanoporous single-layer graphene membranes for gas separation [1,5,7] which has led to a pilot plant demonstration project. Finally, I will discuss synthesis and tuning of gas transport pathways from nanoporous two-dimensional nanosheets and their films that allow facile fabrication of membranes for pre-combustion carbon capture [8,9].
References
Nature Communications 2018, 9, 2632
Science Advances 2019, 5, eaav1851
Energy & Environmental Sciences 2019, 12, 3305–3312
Science Advances 2021, 7, eabf0116, In press.
Nature Communications 2018, 9, 2632
PNAS 2021, 118, In Press.
Journal of Membrane Science 2021, 618, 118745
Science Advances 2020, 6, eaay9851
Nature Materials 2021, 20, 362-369.

Single-layer graphene containing molecular-sized in-plane pores is regarded as a promising membrane material for high-performance gas separations due to its atomic thickness and low gas transport resistance. However, typical etching-based pore generation methods cannot decouple pore nucleation and pore growth, resulting in a trade-off between high areal pore density and high selectivity. In contrast, intrinsic pores in graphene formed during chemical vapor deposition (CVD) are not created by etching. Therefore, intrinsically porous graphene can exhibit high pore density while maintaining its gas selectivity. In this work, the density of intrinsic graphene pores is systematically controlled for the first time, while appropriate pore sizes for gas sieving are precisely maintained. As a result, single-layer graphene membranes with the highest H₂/CH₄ separation performances recorded to date (H₂ permeance > 4000 GPU and H₂/CH₄ selectivity > 2000) are fabricated by manipulating growth temperature, precursor concentration, and non-covalent decoration of the graphene surface. Moreover, we identify that nanoscale molecular fouling of the graphene surface during gas separation where graphene pores are partially blocked by hydrocarbon contaminants under experimental conditions, controls both selectivity and temperature dependent permeance. Overall, the direct synthesis of porous single-layer graphene exploits its tremendous potential as high-performance gas-sieving membranes.

Nanoporous atomically thin membranes, wherein nanopores in two-dimensional materials provide pathways for rapid and selective fluidic transport, represent the ideal limit of an ultrathin membrane and have potential for improving the efficiency, selectivity, productivity, versatility, and chemical resistance for a variety of membrane separations. I will discuss our work on understanding mass transport and developing membranes that employ nanoporous single-layer graphene as the selective layer. Through controlled nucleation of defects via ion bombardment and oxidative etching, sub-nanometer pores are created in the otherwise impermeable graphene placed on a porous support. We discuss strategies for the design of defect-tolerant membranes and show that design of the porous support is critical to exploiting the selectivity of nanoporous graphene, which enables molecular sieving of gases across centimeter-scale nanoporous graphene membranes. We investigate the role of defects, illustrate methods to rapidly assess nanoscale defects in graphene, and demonstrate that the impermeability of graphene can be exploited to selectively seal defects and greatly improve its selectivity. Through these developments, we are able to realize centimeter-scale graphene membranes that show high selectivity between ions, molecules, and proteins for nanofiltration and dialysis, and ultrahigh permeance in organic solvent nanofiltration. By comparing pressure-driven flow of various liquids, we observe a breakdown of continuum transport where flow if governed by molecular geometry, allowing for sub-nanometer separations in a wide range of solvents. The membranes retain stability over hundreds of hours of operation in a range of liquids, and exhibit high mechanical strength with ability to withstand applied pressures of 100 bar. These studies illustrate the interplay between material structure and transport in nanoporous graphene and demonstrate its potential for the realization of next-generation membranes for water purification, bio/chemical separations, and other applications to meet the needs for a sustainable future.

Our planet’s health needs an acceleration in the pace of progress towards clean and sustainable technologies that are critically dependent on materials innovation. Materials science and engineering provides the ability to understand and control matter at the atomic scale to realize optimized performance across an exhaustive set of metrics. In this talk, I will discuss our work related to the materials design of resilient, graphene-based nanofiltration membranes for more efficient industrial separations, which are responsible for 15% of global CO2 emissions.

Atomically thin graphene with a high-density of sub-nanometer pores represents the ideal membrane for ionic and molecular separations, offering ultrafast solvent transport and high solute rejection via molecular sieving. However, a single large nanopore can severely compromise membrane performance via non-selective leakage. Forming precise sub-nanometer pores (0.28-0.66 nm) in the graphene lattice over large areas with a high density via scalable processes remains extremely challenging due to differential etching between pre-existing defects/grain boundaries and pristine regions. Here, we show for the first time that size-selective interfacial polymerization after nanopore formation in graphene not only seals larger defects (>0.5 nm) and macroscopic tears effectively, but also successfully preserves sub-nanometer pores (>0.28 nm), thereby enabling fully functional large-area high-performance graphene membranes. Further, low-temperature chemical vapor deposition (CVD) growth followed by mild UV/ozone oxidation allows for facile and scalable introduction of a high density (4-5.5 × 10¹² cm^-2) of useful sub-nanometer pores in the graphene lattice. We demonstrate fully functional centimeter-scale atomically thin membranes with water (~0.28 nm) permeance ~23× higher than commercially available membranes, and excellent rejection to salt ions (~0.66 nm, >97% rejection) and small organic molecules (~0.7-1.5 nm, ~100% rejection) under forward osmosis.

The transport of water through nanoscale capillaries/pores plays a prominent role in biology, ionic/molecular separations, water treatment, breathable and protective applications. However, the transport behavior of water (liquid water and water vapor) through nanoscale apertures remains elusive. Here, we use centimeter-scale atomically thin graphene membrane with angstrom-scale pores as a model to probe the transport of liquid water and water vapor, and report on orders of magnitude differences in transport rates of water vapor and liquid water. This difference is explained by a transport resistance model in which liquid water permeation occurs near the continuum regime while water vapor transport occurs in the free molecular flow regime.

The flow of water in carbon nanochannels has defied understanding thus far, with accumulating experimental evidence for ultra-low friction, exceptionally high water flow rates, and curvature-dependent hydrodynamic slippage. These unique properties have raised considerable interest in carbon-based membranes for desalination, molecular sieving and energy harvesting. However, the mechanism of water-carbon friction remains unknown, with neither current theories, nor classical or ab initio molecular dynamics simulations providing satisfactory rationalisation for its singular behaviour. Here, we develop a non-equilibrium quantum theory of the solid-liquid interface, which reveals a new contribution to friction, due to the coupling of charge fluctuations in the liquid - analogous to phonon modes - to electronic excitations in the solid. We expect that this quantum friction, which is absent in Born-Oppenheimer molecular dynamics, is the dominant friction mechanism for water on carbon-based materials. As a key result, we demonstrate a dramatic difference in quantum friction between the water-graphene and water-graphite interface, due to the coupling of water Debye collective modes with a thermally excited plasmon specific to graphite. This suggests an explanation for the radius-dependent slippage of water in carbon nanotubes, in terms of the nanotubes' electronic excitations. Our findings open the way to quantum engineering of hydrodynamic flows through the confining wall electronic properties.

Reference
N. Kavokine, M.-L. Bocquet and L. Bocquet, arXiv 2105.03413, to appear in Nature (2021)

The growing need for highly efficient water purification and bioseparations necessitates innovations in ultra-filtration membranes. Block copolymer (BCP) membranes have emerged as a promising player for efficient ultra-filtration due to their uniform pore sizes and sharp cut-offs. Most of the work in BCP membranes has focused on using cylindrical pores. In this work, we have developed novel slit-based membranes using lamellar block copolymers. The lamellar BCPs are vertically oriented in one step film casting process by tuning the solvent evaporation rates with additives. The vertically oriented BCPs are converted to slit membranes using a wet etching process. The pore sizes of these slit-based membranes are measured using surface characterization and filtration cut-off experiments. We demonstrate the enhanced separation of 1-D nanomaterials as compared to the 0-D nanomaterials using these slit-based membranes, which is facilitated by the shape similarity of 1-D nanomaterials with the nano-slits. Such membranes possess the potential for shape-selective filtration of biomaterials such as proteins, viruses, and pharmaceutical drugs and open avenues for shape-selective filtration across a spectrum of applications.

Electrospinning, a simple and easily tunable technique that utilizes the force balance between the electrostatic force and the solution surface tension to fabricate membranes out of 1D nanostructured fibers, has garnered much interest especially from the nanotechnology community. Depending on the morphology of the fibers, electrospun membrane has found its way in a wide range of applications, such as oil-water separation, desalination, tissue engineering, and batteries. During an electrospinning process, many factors can affect the morphology of the products, such as viscosity of the solution, voltage supply, discharge distance, feed rate of the solution, and electrospinning time. In this work, we investigated the impacts of the most important parameters on the morphology of electrospun fibrous membrane based on the fractional factorial design. We evaluated the electrospun fibrous membrane morphology using scanning electron microscopy (SEM) and capillary flow porometry. Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was chosen for the current study due to its interesting and versatile functionalities including superior hydrophobicity, high free volume, and piezoelectricity. This work provides valuable insight for the fabrication of electrospun membranes, especially for tailoring membrane properties in accordance with any targeted applications.

Two dimensional (2D) polymers represent crystalline atomically thin films comprised of periodic repeating units connected by covalent bonds. Exfoliation of 2D polymers from bulk materials is a potential synthesis route but typically results in a collage of randomly stacked flakes a few microns in size. Although, surface reactions on single crystalline catalytic substrate in ultra-high vacuum conditions allow for continuous monolayer synthesis these techniques offer limited scalability¹. Here, we leverage interfacial reactions at an immiscible liquid-liquid interface for scalable 2D polymer synthesis². We systematically study and evaluate the effect of individual parameters in allowing scalable 2D polymer synthesis. We report on the underlying mechanisms that control film morphology, thickness, and quality via scanning electron microscopy, Raman spectroscopy, and atomic force microscopy.
References
1. Servalli, M. & Schlüter, A. D. Synthetic Two-Dimensional Polymers. Annu. Rev. Mater. Res. 47, 361–389 (2017).
2. Zhong, Y. et al. Wafer-scale synthesis of monolayer two-dimensional porphyrin polymers for hybrid superlattices. Science 366, 1379–1384 (2019).

To achieve osmotic water transport, nature utilizes membrane proteins (aquaporins) with only 0.3 nm wide channels that efficiently transport water molecules in a “single-file” motion across cell membranes but block all ionic species. This has inspired the creation of artificial membranes with similar “sub-nanometer” channels that combine rapid water flow with superior ion rejection. We show that 1.2 nm thick carbon nanomembranes (CNMs) made from cross-linking of terphenylthiol (TPT) self-assembled monolayers possess an extremely high pore density of ~10¹⁸ m⁻², i.e. one sub-nm channel per square nanometer [1]. It will be demonstrated that TPT CNMs efficiently hinder the translocation of ions, including protons, while they let water molecules rapidly pass through [2]. 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 a ~10⁸ higher resistance than pores in lipid membrane channels and carbon nanotubes. The ultra-high ionic exclusion by CNMs is likely dominated by a steric hindrance mechanism, coupled with electrostatic repulsion, surface functional groups and entrance effects. We demonstrate the operation of TPT CNMs as semipermeable membranes in forward osmosis, and discuss possible applications. Our observations highlight the potential of utilizing CNMs for water treatment and open up avenues to create 2D membranes through molecular self-assembly for highly selective and fast separations.
[1] Y. Yang, P. Dementyev, N. Biere, D. Emmrich, P. Stohmann, R. Korzetz, X. Zhang, A. Beyer, S. Koch, D. Anselmetti, A. Gölzhäuser, ACS Nano 2018, 12, 4695.
[2] Y. Yang, R. Hillmann, Y. Qi, R. Korzetz, N. Biere, D. Emmrich, M. Westphal, B. Büker, A. Hütten, A. Beyer, D. Anselmetti, A. Gölzhäuser, Adv. Mater. 2020, 1907850.

Molybdenum disulfide (MoS₂) is a widely studied transition metal dichalcogenide with a range of potential applications including next-generation electronics, hydrogen evolution, and catalysis. It can also be used as a thermoelectric, exploiting the Seebeck effect to generate an electric voltage in response to a temperature gradient. Additionally, lithium-intercalated MoS₂ is known to undergo a transition from the 2H to 1T phase. Each of these phases has characteristic physical properties such as hydrophobicity and net surface charge that mediate interactions with aqueous salt solutions for selective ion uptake. The combination of these features makes this material suitable as a novel desalination technology that relies on ion movement powered by a thermoelectric potential. Furthermore, this ionic transport can be harnessed to recharge a thermoelectric battery in response to environmental temperature variations.

To assess MoS₂’s capabilities for ion transport, three types of experiments were conducted. The first set of experiments investigated changes in electric potential resulting from dropwise contact of various salt solutions with an MoS₂ membrane. Droplet test data displayed abrupt changes in electric potential followed by an exponential decay representing ion/carrier movement with respect to time. A second set of experiments measured ion concentration changes over time using an MoS2 film in contact with a temperature gradient and separated DI water and salt solutions. Significant changes in solution ion osmolarities were recorded after a duration of one week. Finally, voltages were measured across these films to better understand ionic flow in response to temperature variations. Ongoing research is investigating ionic transport in combined MoS₂/Aquivion structures for future use in batteries. Results are promising for future development of thermoelectric desalination devices as well as new energy storage and medical technologies.

Two dimensional materials such as molybdenum disulfide (MoS₂) are promising candidates for selective liquid-phase separations. Membranes made from restacked MoS₂ nanoflakes are highly scalable and can efficiently separate ions and other molecular species based on size, chemistry and valence. Controlling the interlayer spacing of restacked MoS₂ sheets and mitigating the presence of defects remains a challenge, however. Here, we control the interlayer spacing of MoS₂ membranes via covalent functionalization (Nano Letters, 2020, 20, 11, 7844–7851). The functional groups act as permanent molecular spacers; they prevent local impermeability caused by irreversible restacking and promote the uniform rehydration of the membrane. We use the functionalized membrane as a platform to study the transport of a suite of ions simultaneously and find that specific chemical effects play a strong role in relative uptake and permeance. We employ molecular dynamics simulations to elucidate the interplay of functionalization, equilibrium interlayer spacing, and the structure of water confined in the MoS₂ channels. We also present more recent results in which we control and study the transport of ions through pores in few-layer van der Waals materials. Our work widens the parameter space for studying and utilizing the unique properties of ion and water transport under nanometer-scale confinement.

The ability to precisely control the ion transport and selectivity in sub-nm nanopores would have a transformative impact on a wide range of emerging technologies, from gas separation to water treatment and energy storage. In this talk, we will discuss how multiscale simulations are applied to elucidate the mechanism of ion transport and selectivity in both 1D and 2D nanopores. We will show how first-principles simulations is combined with force-field based approaches to predict ion solvation and transport in narrow carbon nanotubes (CNTs). Ion transport is found to be suppressed under confinement, which is associated with the transition in the Fickian diffusion to a single-file mechanism as the CNT diameter decreases. In addition, we show that ion-pore interactions can significantly influence the transport properties, leading to a strong correlation between ion dehydration, ion pairing, and diffusion. Finally, we will discuss how first-principles simulations can be coupled with implicit solvation models to investigate ion selectivity in 1D and 2D systems. Overall, these simulations highlight a complex interplay between nanopore geometry, ion shape and hydration on the selectivity.
This work was performed under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344. This work was supported as part of the Center for Enhanced Nanofluidic Transport (CENT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0019112.

The surface free energy is one of the most fundamental properties of solids, hence, manipulating the surface energy and thereby the wetting properties of solids, has tremendous potential for various physical, chemical, biological as well as industrial processes. Typically, this is achieved by either chemical modification or by controlling the hierarchical structures of surfaces. Here we report a phenomenon whereby the wetting properties of vermiculite laminates are controlled by the hydrated cations on the surface and in the interlamellar space. We find that by exploiting this mechanism, vermiculite laminates can be tuned from superhydrophillic to hydrophobic simply by exchanging the cations; hydrophilicity decreases with increasing cation hydration free energy, except for lithium. Lithium, which has a higher hydration free energy than potassium, is found to provide a superhydrophilic surface due to its anomalous hydrated structure at the vermiculite surface. Building on these findings, we demonstrate the potential application of superhydrophilic lithium exchanged vermiculite as a thin coating layer on microfiltration membranes to resist fouling, and thus, we address a major challenge for oil-water separation technology.
1. Huang et al. Nature Communication 11, 1097, 2020
2. Nair et al. Acience 335, 442, 2012.
3. J. Abraham et al. Nature Nanotechnology 12, 546-550, 2017

Transport of water and protons through fluctuating and defective materials are areas of great scientific importance. First, we perform comprehensive atomistic simulations to understand coupling between the fluid transport and mechanics of the membrane. For a membrane separating a salt solution and fresh water, we show that considerable deformation and fluctuation in the membrane results in an enhanced water permeability. Calculations on a harmonically vibrating membrane indicate that the coupling between the vibrational density of states between water and the membrane plays an important role. Second, we investigate proton transport across nanometer thick membranes with defects by using ab-initio molecular dynamics simulations. We demonstrate bidirectional translocation of protons which occurs through the interstitial sites in the surface. We show that water dissociation and the subsequent movement of the dissociated proton through the interstitials sites results in bidirectional transport. We also show that this interstitial proton transport results in an induced electric field that is modulated depending on the proton gradient across the membrane.

Nanoporous atomically thin graphene membrane has been considered as an ideal candidate for next-generation membrane-based technologies. To realize this membrane applications, scalable graphene synthesis and facile large-area membrane fabrication are imperative. While catalytic chemical vapor deposition (CVD) has been demonstrated to be an economical and versatile way to enable the commercial production of high-quality graphene, CVD graphene has to be transferred from the growth substrate onto a porous support substrate to provide mechanical stability for membrane applications. However, commonly used polymer scaffold methods typically leave polymer residues detrimental to membrane performance, while transfers without sacrificial polymers suffer from low transfer yield leading to high membrane leakage. Hence, high-yield transfer of clean large-area graphene onto appropriately porous supports (without compromising support porosity/integrity) using scalable processes for membrane applications remains challenging. In this work, we systematically investigate the factors influencing the transfer of CVD graphene from a Cu foil onto a model polycarbonate track etch (PCTE) support for fabricating large-area atomically thin membranes via manual compression, mechanical press and scalable lamination approaches. Particularly, we report on a novel roll-to-roll manufacturing compatible isopropanol-assisted hot lamination that allows for facile, clean and scalable transfer of centimeter-scale graphene onto model PCTE supports with coverage ≥ 99.2% without compromising support porosity, which is one of the best values reported to this date for centimeter-scale atomically thin membranes. The remaining 0.8% of leakage is attributed to defects (>50 nm) primarily along wrinkles formed during CVD growth. Furthermore, water-assisted oxidation effectively detaches CVD graphene from the Cu foil and helps graphene transfer, but significantly damages (~10%) graphene along wrinkles. Finally, fully functional centimeter-scale atomically thin membranes are demonstrated.

Artificial water channels-AWCs and their natural Aquaporin counterparts selectively transport water. They represent a tremendous source of inspiration to devise biomimetic membrane for several applications, including desalination. They contain variable water-channel constructs with adaptative architectures and morphologies. Herein, we critically discuss the structural details that can impact on performances of biomimetic I-quartets obtained via adaptive self-assembly of alkylureido-ethylimidazoles in bilayer or polyamide membranes. We first explore the I-quartet performances in bilayer membranes, identifying that hydrophobicity is an essential key parameter to increase water permeability. We compare various channel types with different hydrophobic tails (from HC4 to HC18) and we reveal that a huge increase in single channel water permeability, from 10⁴ to 10⁷ water molecules/s/channel, is obtained by increasing the size of the alkyl tail. Quantitative assessment of AWC-polyamide-PA membranes shows that water permeability increases roughly from 2.09 to 3.85 L m⁻² h⁻¹ bar⁻¹, for HC4 and HC6 reverse osmosis membranes respectively, while maintaining excellent NaCl rejection (99.3−99.5%). Meanwhile, comparable HC8 loading induces a drop of performance reminiscent with a defective membrane formation. We show that the production of nanoscale sponge-like water channels can be obtained with insoluble low soluble and low dispersed AWCs, explaining observed subpar performance. We conclude that optimal solubility enabling breakthrough performance must be considered to not only maximize the inclusion and the stability in the bilayer membranes but also to achieve an effective homogeneous distribution of percolated particles that minimizes the defects in hybrid polyamide membranes.
[1] M. Di Vincenzo, A. Tiraferri, V.-E. Musteata, S.Chisca, R.Sougrat, L.-B. Huang, S. P. Nunes, M. Barboiu, Nat. Nanotechnol. 2021, 16, 190-196.
[2] M. Di Vincenzo, et al. Proc. Natl. Acad. Sci. USA, 2021, 118(37), e2022200118
[3] L.-B. Huang, M. Di Vincenzo, M. Göktuğ Ahunbay, A. van der Lee, D. Cot, S. Cerneaux, G. Maurin, and M. Barboiu, J. Am. Chem. Soc. 2021, 143, 14386-14393

The mass transfer capacity of a material is an important factor in its application in energy storage and mass conversion systems. Materials capable of vigorous mass transfer are advantageous for a continuous reaction and generation of products. In general, the mass transfer capacity of a material can be related to its surface area. As the surface area of the material increases, the surface energy rises, which can strongly attract and displace other materials towards the material. The surface area of a material is mainly measured through gas adsorption. However, very small amounts of 1D and 2D materials make it difficult to analyze the surface area through gas adsorption. In addition, when measuring the surface area through gas adsorption, only the overall surface area of the material can be measured, which can't reflect the fine structural properties of the material. Meanwhile, another method that is being attempted to determine the mass transfer capacity is various kinds of computer simulations. Computer simulations can effectively show the change in mass transfer capacity of 1D or 2D materials due to microscopic structural differences. However, computer simulations are conducted without many important variables due to oversimplification of the structure of the material, and the results obtained are often not accurately matched to real-world applications. Therefore, more detailed and realistic analysis is required to measure the accurate mass transfer ability of minute amounts of 1D and 2D materials. We confirmed through past experiments that our 1D carbon nitride structure exhibited excellent performance when used as an energy storage electrode and a material conversion catalyst. However, the material had a relatively lower surface area than other studies reported, which didn't match the excellent experimental results. Although, the 1D carbon nitride structure is uniquely a columnar tube with a very thin outer wall, has an independent space with a partition inside, and has micropores passing through the entire structure, which was considered an optimized form for mass transfer. Therefore, we conducted a special analysis to determine the mass transfer ability of the 1D carbon nitride structure and tried to match with the excellent application performance of the material. First, we performed STEM imaging while rotating the 1D carbon nitride structure, and measured 120 times at 1-degree intervals. After that, we succeeded in creating a 3D volume by assembling the measured images, and separated the empty space and the 1D carbon nitride structure from the 3D volume. The separated 3D volume is exactly the same structure as the actual 1D carbon nitride structure down to the sub-nano level. We generated mesh of the separated 3D volume through programming, and analyzed it through FEM software. As a result of the analysis, it was found that the 1D carbon nitride structure has excellent mass transfer ability even with a relatively low surface area, due to the continuous transfer process triggered by a change in surface energy within the continuous micropores and the isolated empty space inside. We expect that these measurement methods and results can be used in a variety of follow-up studies.

Covalent organic frameworks (COFs)offer the ability to construct highly-ordered nanoporous two-dimensional materials with interesting and attractive properties. These materials have been shown to be ideal for nanoscale separations, an area that we have recently focused on.^1-7 In this presentation, we present new highly ordered, and highly-conjugated 2D-COFs synthesized using new organic transformations, which enable variations in pore size from 0.5-2.5 nm. These COFs can be functionalized, either pre or post synthesis, with an extensive range of funtional groups. Membranes fabricated with these materials demonstrate both high solvent throughput and high selectivity to a wide range of mixtures including gases. Pores with functional groups to either increase or reduce pore size or induce cross-linking between layers are also easily constructed. Our self-assembling system of a single monomer will be highlighted.¹
1. Spaulding, V.; Zosel, K. Duong, P. H. H.; Li-Oakey, K. D.; Parkinson, B. A.; Gomez-Gualdron, D. A.; Hoberg, J. O. “A self-assembling, biporous, metal-binding covalent organic framework and its application for gas separation” Mater. Adv. 2021, 2, 3362.
2. Kuehl, V. A.; Duong, P. H. H.; Sadrieva, D.; Amin, S. A.; She, Y.; Li-Oakey, K. D.; Yarger, J. L.; Parkinson, B. A.; Hoberg, J. O.“Synthesis, Postsynthetic Modifications, and Applications of the First Quinoxaline-Based Covalent Organic Framework” ACS Appl. Mater. Interfaces 2021, 13, 37494.
3. Duong, P. H. H.; Shin, Y. K.; Kuehl, V. A.; Afroz, M. M.; Hoberg, J. O.; Parkinson, B. A.; van Duin, A. C. T.; Li-Oakey, K. D. “Molecular Interactions and Layer Stacking Dictate Covalent Organic Framework Effective Pore Size” ACS App. Mater. Interfaces 2021, 13, 42164.
4. Kuehl, V. A.; Wenzel, M. J.; Parkinson, B. A.; Sousa Oliveira, L. de; Hoberg, J. O. “Pitfalls in the Synthesis of Polyimide-Linked Two-Dimensional Covalent Organic Frameworks” J. Mater. Chem. A 2021, 9, 15301.
5. Brophy, J.; Summerfield, K.; Yin, J.; Kephart, J.; Stecher, J.T.; Adams, J.; Yanase, T.; Brant, J.; Li Oakey, K.D.; Hoberg, J.O.; Parkinson, B. A. “The influence of disorder in the synthesis, characterization and applications of a modifiable two dimensional Covalent Organic Framework” Materials 2021, 14, 71.
6. Duong, P. H. H.; Kuehl, V. A.; Mastorovich, B.; Hoberg, J. O.; Parkinson, B. A.; Li-Oakey, K. D. “Carboxyl-functionalized covalent organic framework as a two-dimensional nanofiller for mixed-matrix ultrafiltration membranes” J. Membr. Sci. 2019,574, 338.
7. Kuehl, V. A.; Yin, J.; Duong, P. H. H.; Mastorovich, B.; Newell, B.; Li-Oakey, K. D.; Parkinson, B. A.; Hoberg, J. O. “A highly-ordered nanoporous, two-dimensional covalent organic framework with modifiable pores, and its application in water purification and ion sieving.” J. Am. Chem. Soc. 2018,140, 18200.

3 billion people cannot take access to clean water for granted while over 1 billion do not even have access to clean water. The state-of-the-art method for providing fresh water is from membrane reverse osmosis (RO) which utilizes polyamide membranes that – while having a high salt rejection and water flux – require high operating pressures and are prone to biofouling from chlorine treatments. Other nanostructured materials – such as carbon nanotubes, zeolites, and single-layered graphene – have been studied, but these materials do not show the high salt rejection, ordered pore systems, and stability required to outperform conventional materials A relatively new category of materials, covalent organic frameworks (COFs), can have highly ordered and tunable pores, and a high thermal and chemical stability. Computational studies are an excellent tool for studying the vast space of theoretical COFs and their possible applications at a much faster rate than experimental methods currently allow, which is key to improve desalination technology before access to fresh water is no longer a reversible issue. In this work we present a study of the permeability and salt rejection of water-stable COFs to elucidate trends in performance when evaluated against conventional membrane materials, and the effects of solvation in the solubility and diffusion of ions through highly confined environments. Large-scale ab-initio molecular dynamics simulations were conducted using the tight-binding density functional (DFTB) package DFTB+. DFTB calculations have shown increased speed and minimal loss of accuracy relative to traditional density functional techniques, resulting in efficient methods for geometry optimization and structure calculations. The uncertainty of the DFTB dynamics calculations is estimated based on static modeling of the same COF geometries with standard density functional theory.

Two-dimensional covalent organic frameworks (2D-COFs) have been of increasing interest in the past decade due to their potentially ordered porous structures. One of the most common routes to these polymers relies on Schiff-base chemistry, i.e. the condensation reaction between a carbonyl and an amine. However, the judicious choice of these two building blocks is critical given that many COF forming reactions can lead to an inherent disorder if such a pathway is available. Examples of disorder in 2D-COFs due to both inherent growth mechanisms and reaction pathways will be given and their influence on ion sieving membranes will be discussed. A 2D-COF with negatively charged carboxylated pores, where disorder is minimized, has been shown to be highly charge and size selective for ion conductivity for a series of tetraalkyl ammonium cations. Progress on membranes for desalinization and small ion separations using negatively charged, positively charged and zwitterion pores will also be presented.

Two-dimensional covalent organic frameworks (2D COFs) are a novel class of materials that are ideal for gas storage and separation technologies due to their high porosities and large surface areas. In this work we study the heat transfer mechanisms in 2D COFs with the addition of gas adsorbates, demonstrating the remarkably tunable anisotropic response of the phonon thermal conductivity in 2D COFs during gas adsorption. More specifically, our results from atomistic simulations on COF-5/methane systems show that, as the gas density increases, the cross-plane thermal conductivity along the direction of the laminar pores increases, whereas the in-plane thermal conductivity along the 2D sheets is monotonically decreased. We show that a large portion of heat is conducted along the laminar pore channels by the gas molecules colliding with the solid framework and is directly related to the gas diffusivities.

We report ionic current and double-stranded DNA (dsDNA) translocation measurements through solid-state membranes with two TEM-drilled ~3-nm diameter silicon nitride nanopores in parallel. Nanopores are fabricated with similar diameters but varying in effective thicknesses (from 2.6 nm to 10 nm) ranging from a thickness ratio 1:1 to 1:3.75, producing distinct conductance levels. This was made possible by locally thinning the silicon nitride membrane to shape the desired topography with nanoscale precision using electron beam lithography (EBL). Two nanopores are engineered and subsequently drilled in either the EBL-thinned or the surrounding membrane region. By designing the interpore separation a few orders of magnitude larger than the pore diameter (e.g., ~900 nm vs. 3 nm), we show analytically, numerically and experimentally, that the total conductance of the two pores is the sum of the individual pore conductances. For a two-pore device with similar diameters yet thicknesses in the ratio of 1:3, a ratio of ~1:2.2 in open-pore conductances and translocation current signals is expected, as if they were measured independently. Introducing dsDNA as analytes to both pores simultaneously, we detect more than 12,000 events within 2 minutes and trace them back with a high likelihood to which pore the dsDNA translocated through. Moreover, we monitor translocations through one active pore only, when the other pore is clogged. This work demonstrates how two-pore devices can fundamentally open up a parallel translocation reading system for solid-state nanopores. This approach could be creatively generalized to more pores with desired parameters given a sufficient signal-to-noise ratio.

Nanopore technology has revolutionized single-molecule biomolecular analysis by providing researchers with an inexpensive, rapid, and high-throughput tool for sensing and sequencing biopolymers (DNA, RNA, and proteins). In this technique, as a biomolecule is electrokinetically pulled through a nanometer-sized pore, either within a biological or synthetic nanopore, partial physical obstruction of the nanopore is detected as a characteristic transient disruption in the ionic current. For sequencing, each unique set of building blocks produces a distinct ionic current level detected using a high-bandwidth current reader. The chronological appearance of different signal levels as a molecular strand is passed through the pore reveals the sequence of that strand. A hallmark feature of the nanopore technique is the dependence of the sensing resolution on the geometry of the nanopore (diameter and thickness in solid-state nanopores). Therefore, theoretically, two-dimensional (2D) materials afford the highest resolution possible, owing to their single to a few-atomic-layer thickness. However, practically, a phenomenon known as “access resistance” renders nanopore resolution broader than the physical pore thickness and reduces the sensing resolution¹. To date, no solid-state nanopore has sequenced a biopolymer, because of material properties limitations and difficulties of making small 2D nanopores. I will discuss progress in fabricating nanopores in various materials including van der Waals 2D materials (MoS₂) and the more hydrophilic MXenes, a new family of 2D transition metals carbides, nitrides, or carbonitrides². We have recently implemented MXene nanopores for biomolecule sensing³ and developed a method to produce a large-scale MXene monolayer film⁴. I will discuss how use of ion intercalating MXenes can be useful for nanopore-based biomolecule sequencing, and the requirements of such a device.

1. Comer et al., Nanoscale 8, 9600 - 9613 (2016).
2. Vahid Mohammadi et al., Science 372, 1165 (2021).
3. Mojtabavi et al., ACS Nano 13, 3042 - 3053 (2019).
4. Mojtabavi et al., ACS Nano, 15, 625 - 636 (2021)

We report recent and ongoing work on DNA translocation through 2D nanoslit [1]. 2D nanoslit devices [2], where two crystals with atomically flat surfaces are separated by only a few nanometers, have attracted considerable attention because of their tunable control over the confinement and the discovery of unusual transport behavior of gas, water, and ions. The nanoslits fabricated from exfoliated 2D materials, such as graphene or hexagonal boron nitride, allow for the study of the passage of ds-DNA to be examined in this tight confinement, mimicking the environments found in biological systems. Two types of events are observed in the ionic current: long current blockades that signal DNA translocation and short spikes where DNA enters the slits but withdraws. DNA translocation events exhibit three distinct phases in their current-blockade traces—loading, translation, and exit. Coarse-grained molecular dynamics simulation allows the different polymer configurations of these phases to be identified. DNA molecules, including folds and knots in their polymer structure, are observed to slide through the slits with near-uniform velocity without noticeable frictional interactions of DNA with the confining graphene surfaces. We propose future work through integrated optical and ionic sensing techniques for the study of DNA and other biopolymers. We anticipate that this new class of 2D-nanoslit devices will provide unique ways to study polymer physics and enable lab-on-a-chip biotechnology.
[1] Yang, Wayne, et al. "Translocation of DNA through Ultrathin Nanoslits." Advanced Materials 33.11 (2021): 2007682.
[2] Radha, B. et al. Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222–225 (2016).

Fast and inexpensive DNA sequencing represents a great challenge in personalized medicine and disease diagnosis. Compared to fluorescence-based sequencing, biological nanopores realize real-time and direct sequencing for long DNA chains without the time-consuming amplification/reconstruction process. Yet the fragile lipid bilayer membrane yields short pore lifetime. In this case, solid-state nanopores offer additional advantages including superior mechanical/chemical/thermal robustness, controllable pore sizes and the potential integration with electronics^[1]. Despite the promise, solid-state nanopores fail to perform sequencing due to the poor spatial resolution and fast translocation^[1]. 2D materials turn out to be promising membranes due to the atomic thickness, making them the optimal choice for achieving single-nucleobase resolution^[1]. However, freestanding 2D materials are prone to leakage and mechanically unstable in salt solution, while adding dielectric support/isolation layers adds to the total membrane thickness, offsetting the advantage rising from the atomic thickness^[2,3]. To realize solid-state nanopores competitive with biological ones requires translocation speed control and sensing mechanisms with spatial resolution unlimited by the total membrane thickness.
We propose a novel solid-state nanopore design based on 2D heterostructure with sub-nm spatial resolution, correlated electrical sensing and translocation speed control. We fabricated out-of-plane diode by stacking p-type WSe₂ and n-type MoS₂ and drilled a nanopore in the overlapping region. The diode architecture allows the application and sensing of voltage and current between the two layers separated by only 6.5 angstroms, comparable to the size of one nucleotide unit (3.3 angstroms). The key questions include how interlayer potential tunes the translocation of ions and molecules through the pore, and how the presence of molecules in the pore affects interlayer conductance.
We first characterized the electronic performance of 2D diode nanopore devices in 10 mM KCl solution under ionic bias across the membrane and interlayer bias. The heterostructure displays diode rectification current as a function of interlayer bias. The ionic voltage tunes the magnitude of diode rectification by electrostatically tuning the relative doping of each layer. Meanwhile, we observed tuning of the ion transport through the pore with interlayer bias as the interlayer potential across the vertical p-n diode exerts a local electric field in the nanopore. Finally, we demonstrated simultaneous ionic and electrical sensing of DNA translocation through the 2D heterostructure-based nanopore, a first demonstration of sensing in this architecture. We achieved sensing of pUC19 circular dsDNA with SNR of 2.
Overall, the heterostructure architecture represents a new paradigm for nanopore mass transport control with simultaneous in-situ sensing. This could be applied to applications including gated molecular sieving and slowing translocation for molecular identification with solid-state nanopores.
References:
[1] Y. He, M. Tsutsui, Y. Zhou, X. S. Miao, NPG Asia Mater. 2021, DOI 10.1038/s41427-021-00313-z.
[2] B. M. Venkatesan, D. Estrada, S. Banerjee, X. Jin, V. E. Dorgan, M. H. Bae, N. R. Aluru, E. Pop, R. Bashir, ACS Nano 2012, DOI 10.1021/nn203769e.
[3] S. Banerjee, J. Shim, J. Rivera, X. Jin, D. Estrada, V. Solovyeva, X. You, J. Pak, E. Pop, N. Aluru, R. Bashir, ACS Nano 2013, DOI 10.1021/nn305400n.

The self-organized TiO₂ nanotube layers have attracted considerable scientific and technological interest over the past years motivated by their possible range of applications including photocatalysis, solar cells, hydrogen generation, membrane applications and biomedical uses [1,2]. The synthesis of these 1D TiO₂ nanotube layers is carried out by a conventional electrochemical anodization of valve Ti metal sheet.
Among other TiO₂ nanomaterials, these nanotube layers stand out due to their directionality, tunability of dimensions, ability to absorb significant amount of incident light and the possibility to utilize nanotube interiors and exteriors for catalytic reactions.
One of the most significant applications of TiO₂ nanomaterials is photocatalysis, which is very effective to decompose various pollutants (e.g. dyes) and kill bacteria [3,4]. However, the mass transport within these nanotubes, which influences liquid as well as gas phase reactions, such as photocatalysis, is by far unexplored and not properly understood. Even though the nanotube layers clearly outperform nanoparticulate layers of the same thickness [5, 8], the reasons for enhanced performance remain not very clear.
The presentation will therefore focus on the influence of the thickness of the nanotube layers for photocatalytic removal of various species in the liquid phase [5-7] as well as gas phase [8,9]. We will discuss also the mass transport throughout these excellent 1D nanotubular photocatalysts. Experimental details and some recent photocatalytic results will be presented and discussed.

References:
[1] J. M. Macak et al., Curr. Opin. Solid State Mater. Sci., 2007, 1-2, 3.
[2] K. Lee et al., Chem. Rev., 2014, 114, 9385.
[3] K. Rajeshwar et al., J. Photochem. Photobiol. C: Photochem. Rev. 2008, 9, 171.
[4] Ch. Wei et al., Environ.Sci. Technol. 1994, 28, 934.
[5] J.M. Macak et al., Small, 2007, 3, 300.
[6] H. Sopha et al., Appl. Mater. Today, 2017, 9, 104.
[7] H. Sopha et al, Nano Letters, 2021, 21, 8701
[8] H. Sopha et al., Electrochem. Commun., 2018, 97, 91.
[9] M. Motola et al., Nanoscale, 2019, 11, 23126.

Nanofluidic systems provide an opportunity to control ion and water transport on a molecular scale, which is important for applications ranging from precision separations to industrial water treatment. Living systems, which move ions and small molecules across biological membranes using protein pores, often rely on finely controlled nanoscale confinement effects to achieve efficient and exquisitely selective transport. I will show that carbon nanotube porins—pore channels formed by ultra-short carbon nanotubes assembled in a lipid membrane—can exploit similar physical principles to transport water, protons, and ions with efficiency that rivals and sometimes exceeds that of biological channels. I will discuss how molecular confinement, slip flow, and the nature of the pore walls influence the mechanisms of ion diffusion, ion selectivity, electrophoretic transport and electroosmotic coupling in these nanopores. Overall, carbon nanotube porins represent simple, versatile, and highly controlled biomimetic membrane pores that provide an ideal testbed for development of the next generation of separation technologies.

An important challenge for the membrane community is to mimic natural protein channels that outperform, by orders of magnitude, man-made systems based on pore size and coarse chemical selectivity. To mimic protein channel pumping on a robust engineering membrane platforms, applied bias can be used to actuate charged gatekeepers and induce ionic pumping. Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates [1,2] 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow [3,4]. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices for nicotine addiction [5]. Our most recent work [6] shows highly energy efficient electroosmotic pumping of nicotine with optimal power consumption/flux efficiency of 111(µW/cm2)/µmoles/cm2/h. This allows watch-battery lifetimes of 7-62 days for conventional treatment dosing regimens. On/off nicotine flux ratios of 68 were achieved allowing programmed delivery between therapeutically relevant nicotine patch and nicotine gum levels. By varying degree of chemical functionalization at CNT tips and pH control of ionic/neutral species ratio, the relative contribution of electroosmosis and electrophoresis within atomically smooth CNT conduits was quantified.
1 “Nanoscale hydrodynamics: Enhanced flow in carbon nanotubes” Majumder, M.; Chopra, N.; Andrews, R; Hinds, B.J Nature 2005, 438, 44.
2 ‘Mass Transport through Carbon Nanotube Membranes in three different regimes: ionic diffusion, gas, and liquid flow’ Mainak Majumder, Nitin Chopra, B.J. Hinds ACS Nano 2011 5(5) 3867-3877
3 ‘Highly Efficient Electro-osmotic Flow through Functionalized Carbon Nanotubes Membrane’ Ji Wu, Karen Gerstandt, Mainak Majunder, B.J. Hinds, RCS Nanoscale 2011 3(8) 3321-28
4 “ Programmable transdermal drug delivery of nicotine using carbon nanotube membranes” J. Wu, K.S. Paudel, C.L. Strasinger, D. Hamell, Audra L. Stinchcomb, B. J. Hinds Proc. Nat. Acad. Sci. 2010 107(26) 11698-11702.
5 “Electrophoretically Induced Aqueous Flow through sub-Nanometer Single Walled Carbon Nanotube Membranes” Ji Wu, Karen Gerstandt, Hongbo Zhang, Jie Liu, and B. J. Hinds Nature Nano 2012 7(2) 133-39
6 ‘Electrically controlled nicotine delivery through Carbon nanotube membranes via electrochemical oxidation and nanofluidically enhanced electroosmotic flow’ Gulati G.K., Hinds B.J. Biomedical Microdevices 2021 DOI: 10.1007/s10544-021-00580-1

Enhanced fluid flow in carbon nanotubes (CNT) could enable major advancements in several membrane applications, from efficient water purification to low-cost recovery of high-value components. Challenges in fabricating large-area membranes with a high density of open, small-diameter, CNT pores have hampered the realization of their potential in practical applications. In addition, despite considerable effort in the last 15 years, many fundamental questions on the exact magnitude of the supported transport rates, flow rate dependence on CNT structural features and driving forces, etc. remain open, and this knowledge gap has further complicated the design of CNT-based fluidic systems. In this seminar, I will present work toward 1) addressing pending fundamental questions, such as the precise quantification of mass transport resistances in CNTs and 2) exploiting the enhanced fluid flow through these slippery nanochannels in large-area membranes. As illustrative examples, I will discuss use in hemodialysis and chem/bio protective garments. Demonstrated outstanding performances in these application areas highlight how CNT remarkable fluidic properties allow to overcome the permeance-selectivity trade-off typically encountered in nanoporous membranes and to outperform commercial products.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-827764

The exterior of single-walled carbon nanotubes is shown to facilitate preferential migration of cations over a millimeter length scale. Applying an electric field to droplets of NaCl placed at both ends of the nanotubes causes the transport of a cation-enriched solution along the nanotubes in the direction of the electric field, while the anion-enriched solution countermigrates along the adjacent substrate. This phenomenon is confirmed by Kelvin probe force microscopy and mass spectrometry imaging of individual nanotubes as well as formation of bright and dark lines along the nanotubes in scanning electron microscopy (SEM). Blocking the exterior of the nanotubes prevents formation of both the bright/dark lines in SEM and flow of current through the nanotubes, confirming the insignificance of interior ion transport and electron current. The cation-preferring transport results in the formation of positively charged salt crystals along the nanotubes (with a cation-to-anion ratio of 0.59:0.41 for KCl) followed by the subsequent shrinkage and growth of crystals in the direction of cation flux. Molecular dynamics simulations show that the cation−π interaction is responsible for such cation preference observed during transport. The loss of cation preference upon covalent functionalization of the nanotubes further supports this mechanism. Utilizing the short-range cation−π interaction as a transport mechanism suggests broader applications in areas where charge-specific transport is desired.

The basal plane of graphene is impermeable to all atoms and molecules - even for helium, the smallest - at ambient conditions [1]. Nevertheless, we found that it is permeable to protons at ambient conditions [2]. This opens the possibility of studying ion transport through highly selective, atomically-sharp interfaces. This talk will provide an overview of our investigation of permeation of protons and other small ions through new 2D materials [3-5], including our recent observation of unexpectedly fast ion exchange in atomically thin clays and micas [6].
[1] Bunch, J. S. et al. Impermeable atomic membranes from graphene sheets. Nano Lett. 8, 2458–62 (2008)
[2] Hu, S. et al. Proton transport through one-atom-thick crystals. Nature 516, 227–230 (2014).
[3] Mogg, L. et al. Atomically-thin micas as proton conduting membranes. Nat. Nano 14, 962-966 (2019).
[4] Mogg. L. et al. Perfect proton selectivity in ion transport through two-dimensional crystals. Nat. Commun. 10, 4243 (2019).
[5] Griffin, E. et al. Proton and Li-Ion Permeation through Graphene with Eight-Atom-Ring Defects. ACS Nano 14, 7280-7286 (2020).
[6] Y.-C. Zhou et al, Ion exchange in atomically thin clays and micas. Nat. Materials (2021) https://www.nature.com/articles/s41563-021-01072-6.

Proton selective membranes are essential for the energy devices such as hydrogen and methanol fuel cells. State-of-the-art proton selective membrane, Nafion albeit having a higher proton conductivity (> 1S/cm²), suffers from the crossover of undesired species which limits its long-term operation in these devices.^1,2 Atomically thin graphene membranes can limit the crossover of undesired species due to their impermeable nature but have significantly lower proton conductivity (~ 3×10^-3S/cm²).^3–8 Here, we study the proton conductance of graphene membranes from micrometer to centimeter scale in aqueous phase and discuss the possible implications of unavoidable intrinsic sub-nm defects^9–11 and large defects such as tears.

Angstrom scale proton selective defects introduced into the lattice of atomically thin 2D materials such as graphene via scalable processes present potential for transformative advances as membranes for energy conversion/storage applications. Here, we show for the first time that kinetic control of graphene synthesis during chemical vapor deposition (CVD) can allow for tunable Angstrom scale proton selective pores for facile fabrication of large-area atomically thin proton exchange membranes. Using a resistance-based transport model in conjunction with systematic liquid and gas phase transport measurements on the same membrane, we study the influence of such Angstrom scale intrinsic defects on selective proton transport through centimeter-scale Nafion|Graphene|Nafion sandwich membranes. Additionally, we report on a novel approach that effectively mitigates transport of un-desired species (small ions and gas crossover) without adversely affecting proton transport. Our insights on kinetic control of graphene synthesis for Angstrom scale pore formation over large areas as well as facile approaches for membrane fabrication offer new avenues to enable functional large-area atomically thin proton exchange membranes.

Hydrogen isotope permeation barriers (HIPBs) are a challenging engineering problem for multiple applications such as the hydrogen energy industry, the nuclear power industry, and extreme high vacuum applications.¹ The nuclear fusion industry is especially moving toward using tritium and deuterium as fuel for reactors.¹ Due to its decay half-life of 12.3 years, tritium leakage by permeation is a problem because the permeated container becomes a radiation source.¹ Thus, materials that prevent or have low hydrogen isotope permeabilities are necessary to overcome this problem.¹ HIPB coatings must have good adhesion to the substrate, have high thermal stability, remain chemically inert, and remain mechanically robust.¹ 2D materials, such as graphene, with low permeability, high aspect ratio, high strength, chemical inertness, and atomic thickness may be able to fulfill this need.
Graphene has been shown to be an extremely impermeable barrier, largely due to its unique structure. Graphene is composed of C with sp² bonds in plane with Π orbitals perpendicular to the surface.² The Π orbitals form a delocalized cloud, which repels other molecules, including He.² Graphene is also chemically inert, and a single layer is thermally stable up to 450 °C.² The mechanical strength of graphene is also key to its impermeability, so that it can withstand pressure differences and maintain its structure as a barrier.² When using chemical vapor deposited (CVD) graphene as a separation membrane, the graphene is typically transferred by a wet etch method onto a membrane with a polymer scaffold.³ This transfer method can cause the graphene to easily tear, leaving open holes that allow easy permeation of gas molecules with little to no selectivity.³ Attempts to reduce these issues involve stacking graphene with multiple transfers, filling the transfer-induced holes with atomic layer deposition or polymer, or transferring the graphene to a porous membrane that has comparable pore resistance as the selective pores in graphene.³ Although these methods demonstrate better gas separation selectivity than the as-transferred graphene (with tears), the gas permeation can still be higher than expected.
This work presents the results of gas concentration driven deuterium permeation experiments through CVD-graphene on copper. Since the graphene is synthesized directly onto the copper, the permeation experiments are able to be performed over a large area (16.62 mm²) without the detrimental impact of transfer-induced tears and holes. Thus, permeation through intrinsic defects of the graphene are probed. The graphene-coated copper shows a reduction in permeation by a factor of ∼28 compared to copper alone. The permeation results are modeled with a composite permeation model. The permeation of copper alone is shown to be proportional to the square root of pressure, whereas the permeation through graphene samples is proportional to pressure. The graphene permeance follows an Arrhenius behavior. The effects of graphene grain boundaries on permeation are also studied, and the results suggest that grain boundaries are not the main diffusion pathways, and instead other intrinsic defects in the graphene demonstrate less resistance to permeation. This study advances the fundamental understanding of the intrinsic permeation of CVD-graphene, as well as the use of graphene in HIPB applications.
1. Nemanič, V., Hydrogen permeation barriers: basic requirements, materials selection, deposition methods, and quality evaluation. Nuclear Materials and Energy 2019, 19, 451-457.
2. Berry, V., Impermeability of graphene and its applications. Carbon 2013, 62, 1-10.
3. Boutilier, M. S.; Jang, D.; Idrobo, J.-C.; Kidambi, P. R.; Hadjiconstantinou, N. G.; Karnik, R., Molecular sieving across centimeter-scale single-layer nanoporous graphene membranes. ACS nano 2017, 11 (6), 5726-5736.

The ability of semipermeable membranes to selectively impede the transport of undesirable ions and molecules is key to many applications, from desalination and gas separation, to biological membrane transport. For instance, membranes that are only permeable to water and reject most other ions and molecules are used in water desalination. Designing more selective membranes requires characterizing the kinetics and mechanism of the transport of unwanted entities. The timescales associated with such processes, however, can be too long to be accessible to conventional molecular dynamics (MD) simulations. Moreover, the separation of timescales between the transport of desirable entities (such as water) and undesirable entities (such as salt) will result in considerable changes in concentration throughout an MD trajectory, which can skew the kinetics and mechanism of transport in ways that are difficult to quantify and correct. Recently, we utilized jumpy forward flux sampling (jFFS) [1] and non-equilibrium MD to study pressure-driven ion transport through graphitic nanoporous membranes [2]. This approach not only allows us to accurately estimate arbitrarily long mean passage times, but also to compute fluxes and permeabilities within a pseudo-equilibrium ensemble in which both reservoirs are at different— but almost constant— chemical potentials. In this presentation, we will discuss the technical aspects of this approach in the context of a few more examples, with the goal of addressing interesting questions about ion transport and selectivity. We will also provide a framework for correcting for finite size artifacts in studies of ion transport through ultra-selective membranes [3].
[1] Haji-Akbari, J. Chem. Phys., 149: 072303 (2018).
[2] Malmir, et al., Matter, 2: 735 (2020).
[3] Shoemaker, et al., Under preparation.

The molecular transport phenomena within nanometer-scales constrictions is important for understand basic biological and physical processes, as well as for the rational design of materials for energy and water technologies. 2D materials offer a good model system, with atomically smooth interfaces, sub-nanometer control of geometry, and fine control of surface charges and functionalization.
In this presentation, I will discuss the transport of water and molecules across nanopores in 2D membranes, atomically smooth 2D nanochannels, and laminar graphene-based membranes. Employing a novel micro-pervaporation chamber, we could measure nanoscale transport of liquids and mixtures – through individual nanopores of 10 – 100 nm in diameter, and 1 – 50 nm high nanochannels – at variable temperature, and we could detect differential fluxes of each component. Comparing different 2D materials, and different liquids and binary mixtures, we could discern the effects of geometry vs. materials properties, and deduce the contribution of surface mediated transports. Highly enhanced flux of water is strongly affected by inclusion of different type of alcohol molecules, leading to either alcohol-water correlations, or alcohol-induced anchoring of water molecules. Those nanoscopic insights allowed us to develop the scaled-up membranes with superior molecular separation properties.

In view of process intensification that membrane technology promises to bring in, it is crucial for membrane materials to attain great selectivity, enhanced permeation, and in-operando durability. To this end, understanding of transport phenomena at low dimensions could help renew our insight for membrane pore design. This talk presents selective transport phenomena across 0D, 1D and 2D confined space that atomically thin orifices, nanotubes and 2D material lamellae provide, respectively. As the pore dimension increases, permeation tends to decrease from ultimate permeation to fast transport to unimpeded diffusion, whereas selectivity increases inherently. Proper design of pores and membrane architecture can collaborate with process operation to further tailor the selectivity. Thus obtained knowledge could lay the cornerstone of advancing membrane transport properties toward process intensification.

Understanding ion transport in nano/angstrom scale channels has practical relevance in applications such as membrane desalination, blue energy, supercapacitors and batteries, as well as in understanding ionic flow through biological channels. Synthetic Å-channels are now a reality with the emergence of several cutting-edge bottom-up and top-down fabrication methods. In particular, the use of atomically thin 2D-materials and nanotubes as components to build fluidic conduits has pushed the limits of fabrication to the Å-scale. In this talk, I will discuss about angstrom (Å)-scale capillaries, which can be dubbed as “2D-nothing”. The Å-capillary is an antipode of graphene, created by what is left behind after extracting one-atomic layer out of a crystal [1]. What is intriguing here is, the dimensions of these channels being comparable to the size of a water molecule.
The Å-capillaries have helped probe several intriguing molecular-scale phenomena experimentally, including: water flow under extreme atomic-scale confinement [1] complete steric exclusion of ions [3,5], specular reflection and quantum effects in gas reflections off a surface [2,7], voltage gating of ion flows [4] translocation of DNA [6]. I will present ionic flows induced by stimuli (electric, pressure, concentration gradient) and discuss the importance of ionic parameters that are often overlooked in the selectivity between ions. The mass production and the robustness of the nano/angstrofluidic devices for large-scale applications are still the main challenges. Toward the end of the talk, I will discuss strategies to scale the production of Å-capillaries.
Acknowledgements
B.R. acknowledges the funding from Royal Society university research fellowship and enhancement award RGF\EA\181000, and funding from the European Union’s H2020 Framework Programme/ERC Starting Grant agreement number 852674 - AngstroCAP.
This work was done in collaboration with A.K. Geim, A. Keerthi, K. Gopinadhan, A. Esfandiar, S. Goutham, Y. You, F.C. Wang, S. J Haigh from University of Manchester, and with L. Bocquet, T. Mouterde, A. Poggioli, A. Siria, from Micromégas team, ENS Paris.
References:
[1] B. Radha et al., Molecular transport through capillaries made with atomic-scale precision. Nature 538, 222
(2016).
[2] A. Keerthi et al., Ballistic molecular transport through two-dimensional channels, Nature (2018), 558, 420.
[3] A. Esfandiar et al., Size effect in ion transport through angstrom-scale slits. Science 358, 511 (2017).
[4] T. Mouterde et al., Molecular streaming and voltage gated response in Angstrom scale channels. Nature 567,
87 (2019).
[5] K. Gopinadhan et al., Complete ion exclusion and proton transport through monolayer water. Science 363,
145 (2019).
[6] W. Yang et al., Translocation of dna through ultrathin nanoslits. Advanced Materials 2007682, (2021).
[7] J. Thiruraman et al., Gas flows through atomic-scale apertures, Science Advances 6, eabc7927, (2020)

Permeation through nanometre-pore materials has been attracting unwavering interest due to fundamental differences in governing mechanisms at macroscopic and molecular scales, the importance of water permeation in living systems, and relevance for filtration and separation techniques. Latest advances in the fabrication of artificial channels and membranes using two-dimensional (2D) materials have enabled the prospect of understanding the nanoscale and sub-nm scale permeation behaviour of water and ions extensively. In particular, graphene oxide (GO) membrane containing 2D graphene capillaries shows unique permeation properties such as ultrafast permeation of water and molecular sieving. In my talk, I will discuss our recent results on molecular and ionic permeation properties of GO and other 2D materials-based membranes and their prospect for several applications.
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Q. Yang et al. Nature Materials 16, 1198 (2017)
A Esfandiar et al. Science 358, 511-513 (2017)
K. G. Zhou et al. Nature 559, 236-240 (2018)
Huang et al. Nature Communications 11, 1097 (2020)

In the last decade, atomically thin capillaries made from 2D materials have created a wave of new research especially in hydrodynamics and mass transports properties of fluids.^1-3 Van der Waals assembling of 2D materials such as graphene, molybdenum disulphide and hexagonal boron nitride has already been achieved, but require highly sophisticated environments and nanofabrication techniques such as e-beam lithography, dry etching, and photolithography, and are very time consuming as we make one device at a time.^{1, 2} Herein, we are presenting an unique and novel nano-fabrication process to prepare 2D channels with well-defined geometries in a scalable manner. This technique demonstrates the fabrication of 2D channels from different naturally occurring layered materials such as graphite, metal oxides and sulphide, transition metal dichalcogenides, and phyllosilicates have been prepared in mass production, but in a shorter period. In addition, this method can be applied to prepare 2D channels from different 2D nanosheets such as graphene, molybdenum disulphide and hexagonal boron nitride through Van der Waals assembly. Moreover, this process holds advantages in making 2D channels of different dimensions and shapes with angstrom-scale precisions, alongside of the reduction of time and fabrication cost. The construction of these 2D channels will open doors to the upscale productions of nanofluidics devices, which can greatly impact in studying and understanding the fundamental properties of fluids in confined geometries.

References:
1. Radha et al., Nature 538, 222–225 (2016)
2. Esfandiar et al., Science 358, 511–513 (2017)
3. Keerthi et al., Nature 558, 420–424 (2018)

Biomimetic and bioinspired membranes (BBMs) containing membrane protein (MP) channels and MP-mimic functionalized materials are innovative platforms to develop separation materials with uniform pore size distribution [1]. Relative to commercial polymeric membranes, BBMs, which incorporate well-defined pore-forming structures that are designed to target precise molecular selectivity based on molecular size and shape of separating molecules, could be advantageous [2]. Nevertheless, integrating MP-based membranes into current manufacturing technology for separations remains challenging. These constraints are possibly due to the compatibility of MPs and polymeric matrices, nanoscopic defects on resulting membranes, the usage of large amounts of functionalized porous materials, and time-intensive synthesis techniques.

In this study, we first fabricated MP-block copolymer (BCP) biomimetic membranes with three different types of pore-forming MPs with pore sizes of 0.8 nm, 1.3 nm, and 1.5 nm, respectively, for conducting targeted molecular separations. MPs were integrated into BCP matrices, poly(butadiene)-poly(ethylene oxide) terminated by carboxylic acid group (PB₁₂-PEO₈-COOH), forming two-dimensional (2D) crystals and nanosheets in a 2-hour organic solvent-based self-assembly method [3]. The formation of 2D MP-BCP crystals indicates an optimized packing density of MPs into membrane matrices, which is ~2 orders of magnitude higher than that in proteoliposomes. The resulting three MP-BCP membranes manifested water permeability of ~300-2,000 (l m^-2 h^-1 bar^-1), which is one to three orders of magnitude greater water permeability than commercial nanofiltration membranes, and still maintained a tunable solute selectivity inferred from pore sizes of three MP channels [4]. Following this research, we further applied the modified fabrication process to create BBMs reconstituted with RsAqpZ (an aquaporin isolated from Rhodobacter sphaeroides)-BCP crystals [5] and sealing polymers for water desalination (>99 % rejection of 2,000 ppm NaCl).

To further explore applications of BBMs beyond aqueous separations and sustainable solutions for creating MP-based membranes with fewer defects, we developed a dehydration method and defect-sealing strategies. The tight defect-sealed MP membranes fabricating by the incorporation of nanoscopic defect-filling materials, photo-polymerizable diacetylene fatty acids, 23:2 Diyne PC, achieved >7-log (or >99.99999%) MS2 bacteriophage viral particle removal. Additionally, an efficient dehydration method using a chemical drying agent, Hexamethyldisilazane (HMDS), is investigated to preserve structures, integrity, and functionality of BBMs under dry conditions. The resulting dehydrated MP-based membranes exhibited a high water vapor transport rate (WVTR) of ~245 g m^-2 hr^-1, 1.5 times higher than commercial breathable fabrics [6]. The preservation of MP-based membrane under dry conditions could extend the applications of BBMs such as protective fabrics for medical protections and barriers. The approach of combining MP channels and BCPs could offer the promise of incorporating porous structures into 2D material designs and developing separation materials for efficient transport and specific molecular-based separations in both aqueous and dry conditions.

References
1. Song, W., Tu, Y.M., Oh, H., Samineni, L. and Kumar, M., Langmuir 35, no. 3 (2018): 589-607.
2. Tu, Y.M., Samineni, L., Ren, T., Schantz, A.B., Song, W., Sharma, S. and Kumar, M., Journal of Membrane Science (2021): 118968.
3. Kumar, M., Ren, T., Song, W. and Tu, Y.M., U.S. Patent Application 16/414,517, filed November 21, 2019
4. Tu, Y.M.^†, Song, W.^†, Ren, T.^†, et al., Nature Materials 19, no. 3 (2020): 347-354.
5. Erbakan, M., Shen, Y. X., Grzelakowski, M., Butler, P. J., Kumar, M., & Curtis, W. R. PloS one, 9, no. 1 (2014): e86830.
6. Oh, H.^†, Tu, Y. M.^†, Freeman, B.D. and Kumar, M. (In preparation)

Nanoscale mass transport often occurs through regions that are void of solid material, giving rise to the need to understand the structure of voids and pores. Transmission electron microscopy is one of the leading approaches to directly image structure and developing ways to resolve porosity at atomic resolution without damaging the intrinsic structure is crucial to advancing our basic understanding of nanoscale mass transport. In this talk I will show results related to atomic scale imaging of nanopores in 2D materials, in particular in graphene and transition metal dichalcogenides of MoS2 and WS2. A heating holder is used to study the impact of high temperatures on the structure and dynamics. The stability of nanopores in relation to the dangling bonds at the edges is discussed. Methods to passivate dangling bonds of nanopore edges will be discussed. Finally, the study is extended to 3D porous materials, where tomography is used to reconstruct the 3D structure. Application of the TEM tomography techniques is applied to understanding the nanoscale porosity in water filtration membranes.

Ideal ion-exchange membranes for alternative energy devices would allow high transmission of only charge-balancing ion with a strong repulsion to undesirable species cross-permeation through membrane. These important metrics are far from being satisfactorily achieved with conventional membranes. The interatomic opening in graphene/boron nitride electron density distribution is extremely small for large ions or molecules to penetrate but is well correlated with the effective proton size. Addressing the selectivity issue of polymeric membranes for energy applications has remained a daunting challenge. Thus, the ability to tune 2D materials properties with polymer electrolyte membranes may offer unprecedented selectivity for membranes and consequently improve energy efficiency. In this presentation, research efforts in mitigating unenviable mass transport through ion-exchange membrane by incorporating 2D crystals into the membrane electrode assembly architecture in energy conversion and storage devices will be discussed.

To fulfil the increasing demand for drinking water, researchers are currently exploring nanoporous two-dimensional (2D) materials as potential desalination membranes. Hexagonal boron nitride (hBN), the inorganic analogue of graphene, is one of the promising 2D materials being investigated. A prominent, yet unsolved challenge is to understand how such membranes will perform in the presence of defects in the hBN layer. Indeed, large-area 2D materials are required for membrane separations, which would likely contain grain boundaries (GBs) as inadvertently induced geometrical defects. Additionally, although hBN is a heteropolar 2D material, previous studies have not investigated the role played by electrostatic interactions in its desalination performance. In this work, we study the effect of GBs on and the role played by interfacial interactions in modulating the desalination performance of bicrystalline nanoporous hBN. To this end, we use classical molecular dynamics simulations supported by quantum-mechanical density functional theory (DFT) calculations to investigate three different nanoporous bicrystalline hBN configurations, with symmetric tilt GBs having misorientation angles of 13.2°, 21.8°, and 32.2°. Using lattice dynamics calculations, we find that grain boundaries alter the areas and shapes of nanopores in bicrystalline hBN, as compared to the nanopores in monocrystalline hBN. We observe that bicrystalline nanoporous hBN with the lowest considered misorientation angle of 13.2° shows improved water permeability by ~47% as compared to monocrystalline hBN. We also uncover the role of the nanopore shape in water desalination, finding that more elongated pores with smaller sizes can match the water permeation and ion rejection through less elongated pores of slightly larger sizes. Simulations also predict that the water permeability is significantly affected by interfacial electrostatic interactions. Indeed, the water permeability is the highest when altered partial charges on B and N atoms were determined using DFT calculations, as compared to when no partial charges or bulk partial charges (i.e., charged hBN) were considered. Overall, our work on the role played by GBs and interfacial electrostatic interactions in water/ion transport through nanopores informs the use of large-area, bicrystalline hBN membranes in seawater desalination applications.

Transport in the presence of a fluid-solid interface is affected, and in many cases dominated, by the interaction between the fluid and solid in the interface vicinity. Transport in nanoscale devices and across 2D and 1D nanomaterials in particular, is synonymous with the presence of fluid-solid interfaces. Realizing the considerable potential of the multitude of applications curently being investigated in this context requires improved fundamental understanding of transport processes at the fluid-solid interface.
In this talk we review recent progress in modeling certain canonical transport problems involving mass, momentum and energy transport, as well as the layering behavior of a fluid in the vicinity of a fluid-solid interface. We show that the fluid layering phenomenon can be described using scaling relations derived from the Nernst-Planck equation. Transport across the interface is studied using non-equilibrium statistical mechanics methods and is connected to the layering phenomena where appropriate. All results are validated using molecular dynamics simulations.

Confined (sub)nano fluidic transport exhibits numerous intriguing phenomena which are different from bulk media, as a result of the dominant rules of surface when the dimension approaches molecular level. Owing to the one-atomic-thickness nature and stable physical and chemical properties, graphene is considered a suitable platform to investigate the transport mechanisms and potential applications in a wide variety from biologically aquaporin, DNA sequencing to gas purification, energy conversion, organic solvent nanofiltration, etc. On one hand, revealing the mechanisms of nanofluidic transport is fundamentally significant for understanding of basic physical and chemical phenomena, such as etching dynamics, electrostatic force between graphene and molecules, friction on graphene surface, electron transfer between graphene and substrates, etc. On the other hand, utilizing these mechanisms to tailor the transport process in (sub)nano confinement could guide and promote the design and control the molecule transport under such nano scale for nano fluid devices and specific applications.
Our group follows a thread that runs through basic research and practical applications.
Focusing on the etching dynamics of graphene nanopores, we found out the dominant factors affecting pore size distribution and pore density. Based on this, a method was developed to introduce nanopores with narrow size distribution and high density (exceeds 10¹² cm^-2), exhibiting broad compatibility for different realistic separation processes from sub-nanometer in gas separation, gas–liquid separation and ionic sieving to a few nanometers in dialysis. Moreover, to clarify the transport behavior of ions through graphene nanopores in the above applications, we disclosed the roles of different kinds of defects to tailor ionic transport via defect structure design by either bottom-up method (controllable intrinsic defects introduction during CVD graphene growth step) or top-down method (biomimetic N-doping by post plasma treatment). As a paradigm, proton transport could be tailored with a range of 2 orders of magnitude by grain boundary design during graphene growth process. The enhanced proton transport by biomimetic N doping graphene outperformed the state-of-the-art commercial Nafion membranes in both areal proton conductivity (2−3 orders of magnitude) and selectivity of proton to methanol (1−2 orders of magnitude), which proved great potential in related fields such as proton exchange membranes. Our work highlights the coherence from fundamental to practice. And we provide typical examples starting from basic principles to technologies development, and finally bridging the gap between theory and reality to utilize them in specific applications of nanofluidics.

Mass transport in nanoconfined geometries is different from bulk-phase and exhibits distinctive nanoscale features, including surface effects or quantum effects. In particular, ion-selective effects in nanoconfined electrical double layers have gained significant attention due to their promising benefits in environmental and biomedical applications. Here, we explore two-dimensional interlayer nanochannels with surface charges, formed between randomly stacked neighboring metal carbide and nitride (MXene) sheets, to investigate confined ionic motions under chemical potential gradients. Plenty of surface terminal groups, arising from etching and exfoliation processes during MXene synthesis, endow the planar sheets with negative surface charges and help create the interlayer spacing at the sub-nanometer scale. We exploited the nanoconfined channels to harvest the chemical potential differences by salinity gradients via charge-selective ion transport.[1] The MXene membrane produced an output power density of up to 54 W-m^-2 by thermal regulation of surface charges and ionic mobility. We also report holey MXene-based membranes, exhibiting simultaneous enhancement in permeability and ion selectivity beyond their inherent trade-off.[2] The perforated nanopores could act as a network of cation channels that interconnects interplanar nanocapillaries throughout the lamellar membrane. The constructed internal nanopores lower the energy barrier for cation passage, thereby boosting the preferential ion diffusion across the membrane. Furthermore, inspired by the inherent photothermal conversion performance of MXene materials, we investigated the conversion of light-driven thermochemical potential to active ion transport.[3] The MXene-based ion conductor shows its promising application as an ion channel for photothermal sensory transduction.
References
1. Hong, S. et al., “Two-Dimensional Ti₃C₂T_x MXene Membranes as Nanofluidic Osmotic Power Generators,” ACS Nano 13 (8), 8917-8925 (2019)
2. Hong, S. et al., “Porous MXene Membranes for Highly Efficient Salinity Gradient Energy Harvesting,” Under review (2021)
3. Hong, S. et al., “Photothermoelectric Response of Ti₃C₂T_x MXene Confined Ion Channels,” ACS Nano 14 (7), 9042-9049 (2020)