Nanochannel Size Research Articles

The peculiar size and temperature dependence of water diffusion in carbon nanotubes studied with 2D NMR diffusion-relaxation D -T 2eff spectroscopy.

It is well known that water inside hydrophobic nano-channels diffuses faster than bulk water. Recent theoretical studies have shown that this enhancement depends on the size of the hydrophobic nanochannels. However, experimental evidence of this dependence is lacking. Here, by combining two-dimensional nuclear magnetic resonance diffusion-relaxation ( - ) spectroscopy in the stray field of a superconducting magnet and molecular dynamics simulations, we analyze the size dependence of water dynamics inside Carbon Nanotubes (CNTs) of different diameters ( - nm), in the temperature range of - K. Depending on the CNT diameter, the nanotube water is shown to resolve in two or more tubular components acquiring different self-diffusion coefficients. Most notably, a favorable CNT diameter range ( - nm) is experimentally verified for the first time, in which water molecule dynamics at the center of the CNTs exhibits distinctly non-Arrhenius behavior, characterized by ultrafast diffusion and extraordinary fragility, a result of significant importance in the efforts to understand water behavior in hydrophobic nanochannels.

Regulating the aggregation of anionic nanoparticles for size-tunable nanochannels

Nanochannels fabricated from the aggregation of charged amphiphilic nanoparticles are of growing interest. The nanochannel size can be further affected by electrostatic interactions between these nanoparticles and metal cations. Herein, size-tunable nanochannels (from 0.5 to 1.4 nm in average) are obtained via regulating the aggregation of anionic nanoparticles using sulfonated polysulfone (SPSF) nanoparticles as a model. Transmission electron microscope (TEM) images and dynamic light scattering (DLS) curves show the aggregates of SPSF nanoparticles in a wide range from nanoscale to micrometer-scale via adding different metal cations. Further analysis of the nanochannels shows the quantitative correlation between ionic strength of metal cations (ISMC) and nanochannel size. Preliminary evaluation of the size-tunable nanochannels exhibits a good separation performance with the Na2SO4 rejection of 98.3% and water flux of 5.8 L m-2 h-1 bar-1, indicating a high level among the reported researches.

Finite-Size Charged Species Diffusion and pH Change in Nanochannels.

Molecular transport through nanofluidic structures exhibits properties that are unique at the nanoscale. The high surface-to-volume ratio of nanometer-sized confined spaces renders particle interactions with the surface of central importance. The electrical double layer (EDL) at the solid-liquid interface of charged surfaces generates an enrichment of counterions and the exclusion of co-ions that lead to a change in their diffusivity. In addition, the diffusive transport is altered by steric and hydrodynamic interactions between fluid molecules and the boundaries. An extensive body of literature investigates molecular transport at the nanoscale. However, most studies account for ionic species as point charges, severely limiting the applicability of the results to "large" nanofluidic systems. Moreover, and even more importantly, the change of pH in the nanoconfined region inside nanochannels has been completely overlooked. Corroborated by experimental data, here we present an all-encompassing analysis of molecular diffusion from the micro- to the ultra-nanoscale. While accounting for finite-size ions, we compute self-consistently the pH inside the channels. Surprisingly, we found that the concentration of ions H+ can change by more than 2 orders of magnitude compared to the bulk, hugely affecting molecular transport. Further, we found that counterions exhibit both enrichment and exclusion, depending on the size of nanochannels. Achieving a greater understanding of the effective transport properties of fluids at the nanoscale will fill the gap in knowledge that still limits development of innovative systems for medicine and industrial applications alike.

Features of pulsed photon annealing of graphene oxide membranes for water desalination

Fabrication of reduced graphene oxide/polyvinylidene fluoride (rGO/PVDF) membranes by coating the surface of standard polymeric PVDF membranes with thin graphene oxide (GO) film is demonstrated. rGO/PVDF membranes have a better filtration efficiency than standard polymer PVDF membranes. For stabilization of rGO/PVDF membranes, a pulsed photon annealing has been used. Increase of water flow through the rGO/PVDF membranes with the decrease of the nanochannel sizes has experimentally been shown. Water transportation via planar nanochannels of rGO membranes depends on an rGO reduction time, which is explained by a decrease of the water shear viscosity activation energy in nanochannels.

Polyelectrolyte adsorption in single small nanochannel by layer-by-layer method

Nanochannel size control by using layer-by-layer (LBL) deposition of polyelectrolytes is experimentally studied in single hard Polydimethylsiloxane (PDMS) nanochannel. Electroosmotic flow (EOF) velocity is measured by using the current-slope method in the modified nanochannel with various numbers of the deposition layers. The results reveal that the growth of polyelectrolyte layer in nanochannels is quite different from that on flat substrates. In small nanochannels, the increment of the multilayer thickness will be limited unlike that on flat surfaces. As the number of the deposition layers increases, the nanochannel size will decrease initially and reach a saturation value finally where the nanochannel cannot be filled with the polyelectrolytes. It is observed that both the decrement and the saturation value of the nanochannel size strongly depend on the concentration of the salt addition in polyelectrolyte solutions. At low ionic concentration, increasing the salt addition increases the average layer thickness within the nanochannels and decreases the saturation value of nanochannel size. However, the effects of salt addition can demonstrate opposite behaviors at high ionic concentration. The EOF can be alternatively reversed in the modified nanochannels by repeatedly coating oppositely charged polyelectrolytes, and the EOF velocity will gradually decrease with the increasing deposition cycles.

2D layered double hydroxide membranes with intrinsic breathing effect toward CO2 for efficient carbon capture

2D material membranes with well-defined interlayer nanochannels hold great promise for precise molecular separation, where the size and surface chemical property of the channel determine the separation efficiency. The currently reported 2D material membranes for efficient CO2 separation are primarily built by introducing crosslinkers or intercalators into the interlayer channel. In this study, we demonstrate a novel 2D material membrane with inherent CO2-selective transport channels based on layered double hydroxide (LDH). The intrinsic breathing effect of LDH toward CO2 enables the spontaneous incorporation of CO2 molecules into the interlayer nanochannels and subsequent conversion/transport in the form of CO32− species which are released as CO2 at the downstream side of membrane. Moreover, the intercalated CO32− ions with an ionic radius of 0.136 nm narrow down the nanochannel size from 0.7 nm to 0.3 nm by electrostatic interaction with LDH layers, conferring the membrane a distinct molecular sieving ability for CO2/CH4 separation. The unique breathing effect and size-sieving effect jointly contribute to the high membrane separation performance with CO2 permeance of 150 GPU and CO2/CH4 selectivity of 33. This study is anticipated to extend the materials and strategies for design of the CO2 preferential transport channels in membranes.

Elucidating Ultrafast Molecular Permeation through Well-Defined 2D Nanochannels of Lamellar Membranes.

Lamellar membranes with well-defined 2D nanochannels show fast, selective permeation, but the underlying molecular transport mechanism is unexplored. Now, regular robust MXene Ti3 C2 Tx lamellar membranes are prepared, and the size and wettability of nanochannels are manipulated by chemically grafted hydrophilic (-NH2 ) or hydrophobic (-C6 H5 , -C12 H25 ) groups. These nanochannels have a sharp difference in mass transfer behavior. Hydrophilic nanochannels, in which polar molecules form orderly aligned aggregates along channel walls, impart ultrahigh permeance (>3000 L m-2 h-1 bar-1 ), which is more than three times higher than that in hydrophobic nanochannels with disordered molecular configuration. In contrast, nonpolar molecules with disordered configuration in both hydrophilic and hydrophobic nanochannels have comparable permeance. Two phenomenological transport models correlate the permeance with the mass transport mechanism of molecules that display ordered and disordered configuration.

Elucidating Ultrafast Molecular Permeation through Well‐Defined 2D Nanochannels of Lamellar Membranes

AbstractLamellar membranes with well‐defined 2D nanochannels show fast, selective permeation, but the underlying molecular transport mechanism is unexplored. Now, regular robust MXene Ti3C2Tx lamellar membranes are prepared, and the size and wettability of nanochannels are manipulated by chemically grafted hydrophilic (−NH2) or hydrophobic (−C6H5, −C12H25) groups. These nanochannels have a sharp difference in mass transfer behavior. Hydrophilic nanochannels, in which polar molecules form orderly aligned aggregates along channel walls, impart ultrahigh permeance (>3000 L m−2 h−1 bar−1), which is more than three times higher than that in hydrophobic nanochannels with disordered molecular configuration. In contrast, nonpolar molecules with disordered configuration in both hydrophilic and hydrophobic nanochannels have comparable permeance. Two phenomenological transport models correlate the permeance with the mass transport mechanism of molecules that display ordered and disordered configuration.

Regulating the ionic current rectification behavior of branched nanochannels by filling polyelectrolytes

The overlapping of the electric double layer (EDL) in a nanochannel yields many interesting and significant electrokinetic phenomena such as ionic current rectification (ICR), which occurs only at a relatively low bulk salt concentration (∼1 mM) where the EDL thickness is comparable to the nanochannel size. In an attempt to raise this concentration to higher levels and the ICR performance improved appreciably, a branched nanochannel filled with polyelectrolytes (PEs) is proposed in this study. We show that these objectives can be achieved by choosing appropriate PE. For example, if the stem side of an anodic aluminun oxide nanochannel is filled with polystyrene sulfonate (PSS) an ICR ratio up to 850 can be obtained at 1 mM, which was not reported in previous studies. Taking account of the effect of electroosmotic flow, the underlying mechanisms of the ICR phenomena observed are discussed and the influences of the solution pH, the bulk salt concentration, and how the region(s) of a nanochannel is filled with PE examined. We show that the ICR behavior of a branched nanochannel can be modulated satisfactorily by filling highly charged PE and the solution pH.

Structure-Induced Method for Circular Cross-Sectional Nanochannel Fabrication

Nanochannels are key elements in building nano-systems for many applications, such as ion transportation, single molecule manipulation, and protein analysis. Commercially manufacturing of nanosystems lies in the development of a low-cost and concise method for nanochannels fabrication. In the present work, a novel PDMS nanochannels fabrication method was proposed based on PDMS spin-coating on a structure-induced wafer. All the influencing parameters which can affect the shape and size of PDMS nanochannels were optimized by proposed simulation method. The comparison between experiments and simulation showed good agreement and confirmed the developed simulation method. By the optimized conditions, 64 nm wide and 37 nm deep PDMS nanochannels can be fabricated.

A graphene oxide membrane with self‐regulated nanochannels for the exceptionally stable bio‐oil dehydration

AbstractThe transport behaviors and nanochannel structures of a graphene oxide (GO) membrane were studied for pervaporation dehydration of bio‐oil with a high acidity and a complex composition. The GO membrane showed an unprecedentedly stable water flux of approximately 0.43 kg m−2 hr−1, with a water content of 97 wt% in the permeate throughout 70 hr of pervaporation testing at 30°C. Both the calculated activation energy for water permeation and X‐ray diffraction characterization results confirmed that the nanochannel structures of the GO membrane were temperature‐ and liquid media‐responsive. The molecular intercalation‐induced self‐regulation of the size of laminar nanochannels in the GO membrane was suggested to be primarily responsible for the significantly reduced membrane fouling and the exceptionally stable pervaporation performance for the GO membrane. The mechanistic insights into the nanochannel structures and antifouling properties would provide important inspiration for the design of novel highly fouling‐resistant membrane materials for practical applications.

Hybrid 2D WS2/GO nanofiltration membranes for finely molecular sieving

Hybrid WS2/GO two-dimensional membranes were developed to achieve finely molecular sieving. The size of nanochannels in WS2 membrane became smaller and more regular after the introduction of GO nanosheets into WS2 membrane according to XRD and separation results. The hydrated hybrid WS2/GO membrane with 15 wt% GO (GO15WS2) exhibited high rejection (>90%) for dyes and ions with a hydrated radius above 4.9 Å. A water permeance of 159.6 L/m2·h·bar and a Methylene Blue (MnB) rejection of 96.3% were achieved through hydrated hybrid GO15WS2 membrane, which was excellent compared with reported two-dimensional membranes based on TMDs. Moreover, the nanochannels in GO15WS2 membrane also presented strong rigidity under a pressure loading up to 0.5 MPa. In addition, the half-dried GO15WS2 membrane possessed high tolerance to shear stress, and the separation performance changed slightly during a 120 h-cross-flow filtration, which made the hybrid GO15WS2 membrane promising in water filtration.

Graphene Oxide Membranes for Highly Efficient Rechargeable Li-O2 Batteries: Suppressing the Crossover of Redox Mediator

Lithium-oxygen (Li-O2) battery is a promising energy storage system with high energy density, but several challenges such as its high overpotential and poor cyclability have limited the commercialization of Li-O2 batteries. One of strategies to address such issues is introduction of redox mediators (RMs) as soluble catalysts, which have mobility for decomposition of tethered discharge products on air cathode. Although RMs are expected to efficiently perform the catalytic effects at the initial cycles, undesired crossover of RMs from cathode to Li metal anode could deteriorate the cell performance by accompanying parasitic side reactions. For instance, 5,10-dihydro-5,10-dimethylphenazine (DMPZ) was reported as an oxygen evolution reaction (OER) RM, which showed an exceptionally low charge overpotential at the first cycle, but rapidly lost its high performance due mainly to the transport of DMPZ to the Li metal during repeated operations. Here, we report an excellent suppression of RM crossover for highly efficient Li-O2 batteries using a graphene oxide (GO) membrane prepared by a facile vacuum filtration on a porous support. The size of GO nanochannels is narrow enough to selectively allow the transport of small Li+ ions while rejecting the larger DMPZ molecules. The negative surface charges of GO further repel negative ions via Donnan exclusion, greatly improving the lithium ion transference number. Ultimately, the Li-O2 cells with GO membranes maximize the redox-mediation activity of DMPZ with improved performance, achieving energy efficiency of above 80% for more than 25 cycles, and 90% for 78 cycles when the capacity limits were 0.75 and 0.5 mAh cm-2, respectively. Considering the facile method for GO membrane fabrication, combination of GO membrane and RMs can be cost-effective solution for pragmatic Li-O2 batteries.

Spatiotemporal Control of Enzyme‐Induced Crystallization Under Lyotropic Liquid Crystal Nanoconfinement

AbstractWater nanoconfinement has important effects on the properties of biomolecules and ultimately on their specific functions. By performing experiments and molecular dynamic simulations, we show how intrinsic nanoconfinement controls the crystallization of small organic molecules converted by enzymatic reactions within the water nanochannels of lipid cubic phases (LCPs). By controlling the nanochannel size, enzymatic reactions in LCPs can be engineered to turn the same converted substrate into its soluble, microcrystal, or needle‐like crystal form due to the large variability in water dynamics. Differential scanning calorimetry studies, supported by molecular dynamics simulations, show that most of water within the mesophase nanochannels behaves differently due to interactions with the LCP interface, and that this mechanism has a larger impact for smaller channels. These findings suggest that the amount of free water in the core of the nanochannels is the key factor determining local substrate diffusion and self‐assembly within LCPs.

Spatiotemporal Control of Enzyme-Induced Crystallization Under Lyotropic Liquid Crystal Nanoconfinement.

Water nanoconfinement has important effects on the properties of biomolecules and ultimately on their specific functions. By performing experiments and molecular dynamic simulations, we show how intrinsic nanoconfinement controls the crystallization of small organic molecules converted by enzymatic reactions within the water nanochannels of lipid cubic phases (LCPs). By controlling the nanochannel size, enzymatic reactions in LCPs can be engineered to turn the same converted substrate into its soluble, microcrystal, or needle-like crystal form due to the large variability in water dynamics. Differential scanning calorimetry studies, supported by molecular dynamics simulations, show that most of water within the mesophase nanochannels behaves differently due to interactions with the LCP interface, and that this mechanism has a larger impact for smaller channels. These findings suggest that the amount of free water in the core of the nanochannels is the key factor determining local substrate diffusion and self-assembly within LCPs.

Tuning the nanostructure of nitrogen-doped graphene laminates for forward osmosis desalination.

Studies have concentrated on the physicochemical properties of graphene-based membranes that can replace polymeric membranes for use in forward osmosis (FO) systems. However, recent research studies have focused on mixtures of two or more different materials (e.g., graphene oxide and polymers) due to the need to reinforce underwater stability. Alternatives include reduced forms such as reduced graphene oxide to improve the stability and size-based selectivity, which have resulted in a narrow nanochannel that restricts water permeability. Herein, we propose the use of a novel nitrogen-doped graphene (NG) membrane to solve a trade-off between permeability and selectivity, investigating the nanostructure via N-doping reaction time. In an FO process, NG membranes achieved an outstanding specific salt flux of 0.18 g L-1, compared to commercial membranes (0.55 g L-1). The pyridinic-N bonding structure improved the permeability and selectivity under a similar nanochannel size because of its negatively polarized hole defects with the moderate energy barrier enabling water passage while blocking ions. Our results confirm the possibility of fabricating novel graphene-based FO membranes by tailoring the nitrogen-bonding structure, which will significantly help develop a process for improving the scalability of membrane materials.

Power generation from a pH-regulated nanochannel through reverse electrodialysis: Effects of nanochannel shape and non-uniform H+ distribution

The power generated from the Gibbs free energy of mixing through reverse electrodialysis (RED) is a promising renewable energy of great potential. In this study the performance of a nanochannel having a pH-regulated surface in RED is examined, focusing on the influence of the bulk salt concentration, the distribution of H+, and the nanochannel size and its shape. Taking account of the effect of osmotic flow, we consider three types of nanochannel: bullet-shaped, conical, and trumpet-shaped nanochannels. Because the smaller the nanochannel space the more significant the degree of electric double layer overlapping the trumpet-shaped nanochannel has the best ion selectivity. As the ratio of (nanochannel length/curvature radius) varies the maximum power generated from a bullet-shaped nanochannel has a local maximum. This can be explained by the associated diffusion potential and the electric current passing through it. The results gathered provide desirable and necessary information for designing RED devices. In particular, the maximum power generation can be achieved by choosing appropriately the shape and the size of a nanochannel, and the bulk concentration ratio.

Rightsizing Nanochannels in Reduced Graphene Oxide Membranes by Solvating for Dye Desalination.

Membranes with high water permeance, near-zero rejection to inorganic salts (such as NaCl and Na2SO4), and almost 100% rejection to organic dyes are of great interest for the dye desalination (the separation of dyes and salts) of textile wastewater. Herein, we prepared reduced graphene oxide membranes in a solvation state (S-rGO) with nanochannel sizes rightly between the salt ions and dye molecules. The S-rGO membrane rejects >99.0% of Direct Red 80 (DR 80) and has almost zero rejection for Na2SO4. By contrast, conventional GO or rGO membranes often have channel sizes smaller than divalent ions (such as SO42-) and thus high rejection for Na2SO4. More interestingly, high salinity in typical dye solutions decreases the channel size in the S-rGO membranes and thus increases the dye rejection, while the Na2SO4 rejection decreases because of the negatively charged surface on GO and the salt screening effect. The membranes also show pure water permeance as high as 80 L m-2 h-1 bar-1, which is about 8 times that of commercial NF 90 membrane and 2 times that of a commercial ultrafiltration membrane (with a molecular weight cutoff of 2000 Da), rendering their promise for practical dye desalination.

Water desalination across multilayer graphitic carbon nitride membrane: Insights from non-equilibrium molecular dynamics simulations

Graphitic carbon nitride (g-C3N4) is a novel two-dimensional, nitrogen-doped carbon material with various applications in many fields. Recently, g-C3N4 has exhibited excellent ability in separations. In this work, mechanisms of water and ion permeation across nanoporous g-C3N4 membranes with different pore diameters were studied using non-equilibrium molecular dynamics simulations. The water conduction rates derived from our simulations are in good agreement with previous experimental data. Due to the nanoscale confinement, we observe a stacked water cluster structure within the narrowest nanochannel. Similar structures are obscure as the increase of the nanochannel sizes. For desalination purposes, we find that the nanoporous g-C3N4 membrane with the desired opening may completely reject calcium and chloride ions, whereas sieve out monovalent cations with acceptable rejection rates (over 70%). This work provides molecular insight into the mechanism of water and ion transport through nanoporous g-C3N4, which strongly supports that this novel material is a promising candidate for developing selective membrane for water desalination.

Graphene Oxide Sieving Membrane for Improved Cycle Life in High-Efficiency Redox-Mediated Li-O2 batteries.

As soluble catalysts, redox-mediators (RMs) endow mobility to catalysts for unconstrained access to tethered solid discharge products, lowering the energy barrier for Li2 O2 formation/decomposition; however, this desired mobility is accompanied by the undesirable side effect of RM migration to the Li metal anode. The reaction between RMs and Li metal degrades both the Li metal and the RMs, leading to cell deterioration within a few cycles. To extend the cycle life of redox-mediated Li-O2 batteries, herein graphene oxide (GO) membranes are reported as RM-blocking separators. It is revealed that the size of GO nanochannels is narrow enough to reject 5,10-dihydro-5,10-dimethylphenazine (DMPZ) while selectively allowing the transport of smaller Li+ ions. The negative surface charges of GO further repel negative ions via Donnan exclusion, greatly improving the lithium ion transference number. The Li-O2 cells with GO membranes efficiently harness the redox-mediation activity of DMPZ for improved performance, achieving energy efficiency of above 80% for more than 25 cycles, and 90% for 78 cycles when the capacity limits were 0.75 and 0.5 mAh cm-2 , respectively. Considering the facile preparation of GO membranes, RM-sieving GO membranes can be cost-effective and processable functional separators in Li-O2 batteries.