Decimeter-Scale Atomically Thin Graphene Membranes for Gas-Liquid Separation

Nanofluidic Memristive Transition and Synaptic Emulation in Atomically Thin Pores

Ionic transport across nanochannels is the basis of communications in living organisms, enlightening neuromorphic nanofluidic iontronics. Comparing to the angstrom-scale long biological ionic pathways, it remains a great challenge to achieve nanofluidic memristors at such thinnest limit due to the ambiguous electrical model and interaction process. Here, we report atomically thin memristive nanopores in two-dimensional materials by designing optimized ionic conductance to decouple the memristive, ohmic, and capacitive effects. By conducting different charged iontronics, we realize the reconfigurable memristive transition between nonvolatile-bipolar and volatile-unipolar characteristics, which arises from distinct transport processes governed by energy barriers. Notably, we emulate synaptic functions with ultralow energy consumption of ∼0.546 pJ per spike and reproduce biological learning behaviors. The memristive nanopores are similar to the biosystems in angstrom structure, rich iontronic responses, and millisecond-level operating pulse width, matching the biological potential width. This work provides a new paradigm for boosting brain-inspired nanofluidic devices.

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Anti-Arrhenius passage of gaseous molecules through nanoporous two-dimensional membranes

The passage of molecules through membranes is known to follow an Arrhenius-like kinetics, i.e. the flux is accelerated upon heating and vice versa. There exist though stepwise processes whose rates can decrease with temperature if, for example, adsorbed intermediates are involved. In this study, we perform temperature-variable permeation experiments in the range from -50 to +50 °C and observe anti-Arrhenius behaviour of water and ammonia permeating in two-dimensional freestanding carbon nanomembranes (CNMs). The permeation rate of water vapour is found to decrease many-fold with warming, while the passage of ammonia molecules strongly increases when the membrane is cooled down to the dew point. Liquefaction of isobutylene shows no enhancement for its transmembrane flux which is consistent with the material's pore architecture. The effects are described by the Clausius-Clapeyron relationship and highlight the key role of gas-surface interactions in two-dimensional membranes.

Membranes prepared from graphene-based nanomaterials for water purification: a mini-review

Graphene-based nanomaterials (GBnMs) are currently regarded as a critical building block for the fabrication of membranes for water purification due to their advantageous properties such as easy surface modification of functional groups, adjustable interlayer pore channels for solvent transportation, robust mechanical properties, and superior photothermal capabilities. By combining graphene derivatives with other emerging materials, heteroatom doping and rational design of a three-dimensional network can enhance water transportation and evaporation rates through channels of GBnM laminates and such layered structures have been applied in various water purification technologies. Herein, this mini-review summarizes recent progress in the synthesis of GBnMs and their applications in water treatment technologies, specifically, nanofiltration (NF) and solar desalination (SD). Finally, personal perspectives on the challenges and future directions of this promising nanomaterial are also provided.

Gas Separations using Nanoporous Atomically Thin Membranes: Recent Theoretical, Simulation, and Experimental Advances

Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high-performance gas separations due to their atomic thickness, large-scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas-sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas-separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas-separation applications of nanoporous atomically thin membranes.

Thickness-Varied Carbon Nanomembranes from Polycyclic Aromatic Hydrocarbons

Despite the prospects of intrinsically porous planar nanomaterials in separation applications, their synthesis on a large scale remains challenging. In particular, preparing water-selective carbon nanomembranes (CNMs) from self-assembled monolayers (SAMs) is limited by the cost of epitaxial metal substrates and molecular precursors with specific chemical functionalities. In this work, we present a facile fabrication of CNMs from polycyclic aromatic hydrocarbons (PAHs) that are drop-cast onto arbitrary supports, including foils and metalized films. The electron-induced carbonization is shown to result in continuous membranes of variable thickness, and the material is characterized with a number of spectroscopic and microscopic techniques. Permeation measurements with freestanding membranes reveal a high degree of porosity, but the selectivity is found to strongly depend on the thickness. While the permeance of helium remains almost the same for 6.5 and 3.0 nm thick CNMs, water permeance increases by 2 orders of magnitude. We rationalize the membrane performance with the help of kinetic modeling and vapor adsorption experiments.

Mass Transfer in Boronate Ester 2D COF Single Crystals

Unlike graphene and similar structures, 2D covalent organic frameworks (2D COFs) exhibit intrinsic porosity with a high areal density of well-defined and uniform openings. Given the pore size adjustability, 2D COFs are likely to outperform artificially perforated inorganic layers with respect to their prospects in membrane separation. Yet, exploring the mass transport in 2D COFs is hidden by the lack of laterally extended free-standing membranes. This work reports on direct molecular permeation measurements with single crystals of an interfacially synthesized boronate ester 2D COF. In accordance with the material topography, the atmospheric and noble gases readily pass the suspended nanosheets while their areal porosity is quantified to be almost 40% exceeding that in any 2D membranes known. However, bulkier aromatic hydrocarbons are found to deviate substantially from Graham's law of diffusion. Counterintuitively, the permeation rate is demonstrated to rise from benzene to toluene and further to xylene despite the increase in the molecular mass and dimensions. The results are interpreted in terms of adsorption-mediated flow that appears to be an important transport mechanism for microporous planar nanomaterials.