Graphene Nanochannels Research Articles

Ionic Liquids: Molecular Insights into the Regulatable Interfacial Property and Flow Behavior of Confined Ionic Liquids in Graphene Nanochannels (Small 29/2019)

Ionic Liquids: Molecular Insights into the Regulatable Interfacial Property and Flow Behavior of Confined Ionic Liquids in Graphene Nanochannels (Small 29/2019)

Interfacial properties of water/heavy water layer encapsulate in bilayer graphene nanochannel and nanocapacitor

Water through nanochannels of graphene (G) exposes the capillary pressure, calling fundamental understanding and predictive design of water within G nanochannels. Nanoconfinement induces switching behaviors at an atomic level, altering electronic and geometric structures. Herein, we study the single-layer water (SLW) and double-layer water (DLW) on monolayer G and encap-sulated in G layers and explore their diverse interfacial properties using a number of high level first principles calculations. By correlating the stability of adsorption, and interfacial properties such as intermediate pressure, charge transfer, structural deformation, one can decode various synergies in interaction properties of water on G nanocapillars. The external electric field (Eext) enhances polarization of system. More especially, changing the strength of Eext can effectively modulate the bandgap of monolayer G and bilayer graphene (BLG), and correspondingly causes a semimetal-semiconductor transition, i.e. Eg = 0.8 and 0.9 eV respectively. We also provide a comparison of phonon vibrational modes of water and heavy water (D2O) encapsulated within BLG, capturing their mobility as a key factor of separation mechanism. Moreover, we propose a nanocapacitor array of graphene-water-graphene composite, which electrodes has been separated by a water layer as a dielectric film. Nanometre-scale G capillaries open up a pathway to fabricate atomically channels walls for nanofluidic technology and nanocapacitor as a key device in integrated circuits (ICs).

Molecular Insights into the Regulatable Interfacial Property and Flow Behavior of Confined Ionic Liquids in Graphene Nanochannels.

The understanding of confined structure and flow property of ionic liquid (IL) in a nanochannel are essential for the efficient application of ILs in the green chemical processes. In this work, the ionic structure and various flow behaviors of ILs inside graphene nanochannels via molecular dynamics simulations are shown. The effect of the nanochannel structure on confined flow is explored, showing that the width mainly heightens the viscosity while the oxidation degree primarily enhances the interfacial friction coefficient. Tuning the width and oxidation degree of nanochannel, three different flow behaviors including Poiseuille, partial plunger and full plunger flow can be achieved, where the second one does not occur in water or other organic solvents. To describe the special flow behavior, an effective influence extent of the nanochannel (w EIE ) is defined, whose value can distinguish the above flows effectively. Based on w EIE , the phase diagrams of flow behavior for the nanochannel structure and pressure gradient are obtained, showing that the critical pressure gradient decreases with width and increases with the oxidation degree. Based on the quantitative relations between confined structures, viscosity, friction coefficient, flow behavior, and nanochannel structure, the intrinsic mechanism of regulating the flow behavior and rational design of nanochannel are finally discussed.

Thermal Bubble Nucleation in Graphene Nanochannels

Molecular dynamics simulations are carried out to simulate thermal bubble nucleation processes in homogeneous and heterogeneous systems, respectively. It is found that the nucleation temperature of...

Structure and dynamic properties of stretched water in graphene nanochannels by molecular dynamics simulation: effects of stretching extent.

Water confined in nanochannels can be stretched with variation of the external pressure, leading to unusual properties compared with bulk water. In order to unravel the impacts of stretching extent on the structural and dynamic properties of water confined in hydrophobic graphene nanochannels with various channel widths (L = 1 nm, 2 nm and 3 nm), molecular dynamics (MD) simulations were performed in this work. It was found that an ultrahigh negative pressure was present in the confined space with the increase of stretching extent before cavitation. The interfacial density peak and tetrahedral arrangement were reduced with the increasing hydrogen bond length, indicating the more disordered structural organization, especially in channels with small channel widths. On the other hand, the hydrogen bond lifetime was increased due to the prolonged stability of the hydrogen bond under stretching. The remarkably increased diffusion coefficients of confined water with the increasing stretching coefficient result from the faster diffusion of interfacial water along the channel surface regardless of channel width. At last, the oscillating tangential pressure profile inside the nanochannels demonstrated that the confined water under stretching consists of multiple layers exhibiting alternate positive and negative pressures, which is reduced with the increase of stretching coefficient, corresponding to the enhanced diffusion.

Electro-osmotic flow in hydrophobic nanochannels.

Hydrophobic surfaces with large slip lengths have the potential to enhance electro-osmotic flows. Existing theories of electroosmosis in hydrophobic channels postulate immobile surface charges and/or make a number of simplifying assumptions by considering mostly weakly charged surfaces and thin diffuse layers compared to channel dimension. In this paper, we extend prior models by focusing on planar and cylindrical nanochannels. Our theory accounts for a hydrodynamic slip and a mobility of surface charges, and is valid not only on the scale of the nanochannel with thin diffuse layers, but also on the scale of the overlapping diffuse layer. The model is simple enough to allow us to derive analytical approximations for the electro-osmotic velocities even when the surface potential and charge density are quite large. We also present numerical solutions to validate the analysis and illustrate the variation of electro-osmotic velocities in response to changes in the channel size, potential, surface charge and its mobility, hydrodynamic slip length, and salt concentration. Our results are directly relevant for carbon nanotubes, graphene nanochannels, and conventional nanoporous membranes.

Ionized water confined in graphene nanochannels.

When confined between graphene layers, water behaves differently from the bulk and exhibits unusual properties such as fast water flow and ordering into a crystal. The hydrogen-bonded network is affected by the limited space and by the characteristics of the confining walls. The presence of an extraordinary number of hydronium and hydroxide ions in narrow channels has the following effects: (i) they affect water permeation through the channel, (ii) they may interact with functional groups on the graphene oxide surface and on the edges, and (iii) they change the thermochemistry of water, which are fundamentally important to understand, especially when confined water is subjected to an external electric field. Here we study the physical properties of water when confined between two graphene sheets and containing hydronium and hydroxide. We found that: (i) there is a disruption in the solvation structure of the ions, which is also affected by the layered structure of confined water, (ii) hydronium and hydroxide occupy specific regions inside the nanochannel, with a prevalence of hydronium (hydroxide) ions at the edges (interior), and (iii) ions recombine more slowly in confined systems than in bulk water, with the recombination process depending on the channel height and commensurability between the size of the molecules and the nanochannel height - a decay of 20% (40%) in the number of ions in 8 ps is observed for a channel height of h = 7 Å (bulk water). Our work reveals distinctive properties of water confined in a nanocapillary in the presence of additional hydronium and hydroxide ions.

Confined Structures and Selective Mass Transport of Organic Liquids in Graphene Nanochannels

Selective transport of liquids is an important process in the energy and environment industry. The increased energy consumption and the demands of clean water and fossil fuels have urged the development of high-performance membrane technologies. Nanoscale channels with the critical size for molecular sieving and atomistically smooth walls for significant boundary slippage are highly promising to balance the tradeoff between permeability and selectivity. In this work, we explore the molecular structures and dynamics of organic solvents and water, which are confined within nanoscale two-dimensional galleries between graphene or graphene oxide sheets. Molecular dynamics simulation results show that the layered order and significant interfacial slippage are universal for all molecular liquids, leading to notable flow enhancement for channels with a width of few nanometers, in the order of ethylene glycol > butanol > ethanol > hexane > toluene > water > acetone. The extracted dependence of permeability, selectivity on the channel width, and properties of molecular liquids clarify the underlying mechanisms of selective mass transport in nanofluidics, which help to understand and control the filtration and separation processes of molecular liquids. The performance of graphene oxide membranes for permeation and filtration is finally discussed based on the calculated flow resistance for pressure-driven flow or molecular diffusivity for diffusive flow, as well as the solubility and wettability of membranes.

Renormalization of Ionic Solvation Shells in Nanochannels.

Recently, experimental studies on selective ion transport across nanoporous membranes or through single nanochannels have unveiled interesting behaviors of dissolved ions under nanoconfinement. However, the exploration was limited by the resolution of experimental characterization. In this work, we present an atomistic simulation-based study, showing how the nanoconfinement and surface functionalization of graphene and graphene oxide nanochannels renormalize the solvation of ions (Na+, K+, Mg2+, Ca2+, Cl-). We find that the spatial distribution of dissolved ions demonstrates a layered order in nanochannels. The 1st hydration shell structures of cations are well defined in channels with width beyond ∼1.0 nm, although the rotational degree of freedom is constrained, while the 2nd hydration shells could be destructed. In the graphene oxide nanochannels, oxygen-containing functional groups can participate in the hydration shells of univalent ions but not for the divalent ions, and the valence-dependent reduction in the ionic diffusivity offers good selectivity between the divalent and univalent ions with the interlayer spacing of ∼1.0 nm, which is absent in the graphene nanochannels. With these findings, we conclude that the assessment of permeability and selectivity of ions has to take the renormalized nature of ionic solvation shells into account in the design of nanoporous membranes or nanofluidic devices for energy and environmental applications.

High Performance Humidity Fluctuation Sensor for Wearable Devices via a Bioinspired Atomic-Precise Tunable Graphene-Polymer Heterogeneous Sensing Junction

Measurements of an individual’s water metabolism dynamical information can provide us rich biological information in a noninvasive way. This concept is hindered by the trade-off between the sensitivity and responsive velocity of traditional moisture sensors. Herein, inspired by the molecular detecting system based on weak bond interactions in natural organisms, we designed a new concept of a tunable graphene-polymer heterogeneous nanosensing junction by confining a reasonable thickness sensing material into graphene nanochannels. The fundamentally new sensing mechanism based on dynamical hydrogen bonds endows the sensor with over 4 orders of magnitude sensitivity toward a wide range of relative humidity (RH) (from 0% to 97%) with unprecedented fast response (20 ms) and recovery times (17 ms) with little humidity hysteresis. The promising advantages of the sensor allow us to record humidity fluctuation information in real time during a user’s speech and breath, which can both reveal the speech feature and ...

Electrostrictive behavior of confined water subjected to GPa pressure

Water inside a nanocapillary exhibits unconventional structural and dynamical behavior due to its ordered structure. The confining walls, density, and lateral pressures control profoundly the microscopic structure of trapped water. Here we study the electrostriction of confined water subjected to pressures of the order of GPa for two different setups: (i) a graphene nanochannel containing a constant number of water molecules independent of the height of the channel, (ii) an open nanochannel where water molecules can be exchanged with those in a reservoir. For the former case, a square-rhombic structure of confined water is formed when the height of the channel is $d=6.5$ \AA{} having a density of $\ensuremath{\rho}=1.42\phantom{\rule{4pt}{0ex}}{\mathrm{g}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}3}$. By increasing the height of the channel, a transition from a flat to a buckled state occurs, whereas the density rapidly decreases and reaches the bulk density for $d\ensuremath{\cong}8.5$ \AA{}. When a perpendicular electric field is applied, the water structure and the lateral pressure change. For strong electric fields $(\ensuremath{\sim}1$ V/\AA{}), the square-rhombic structure is destroyed. For an open setup, a solid phase of confined water consisting of an imperfect square-rhombic structure is formed. By applying a perpendicular field, the density and phase of confined water change. However, the density and pressure inside the channel decrease as compared to the first setup. Our study is closely related to recent experiments on confined water, and it reveals the sensitivity of the microscopic structure of confined water to the size of the channel, the external electric field, and the experimental setup.

Competitive adsorption and diffusion of CH4/CO2 binary mixture within shale organic nanochannels

Molecular dynamics simulations were performed to study adsorption and dynamics properties of pure CO2, pure CH4, and their mixtures confined to a graphene nanochannel with width of 3.25 nm at different loadings, temperature and molar ratios. Preferential adsorption CO2 over CH4 on the graphene nanochannel surface is observed by the density profile, the residence time of adsorbed molecules, CO2/CH4 selectivity, and the potential energy surface (PES). The CO2 preferentially adsorbed on the surface reduces the activation energy for CH4 diffusibility, and thus improves CH4 mobility. The increasing temperature leading to the decrease of CO2/CH4 selectivity and the enhancement of CO2/CH4 diffusion. Surface diffusion cannot be neglected on CO2/CH4 diffusion, especially at low loadings. The steric hindrance weakens the diffusion of both species at high loadings. The microscopic insight provided with this study is helpful to estimate and exploit the shale gas and evaluate CO2 sequestration capacity.

Fast water transport in graphene nanofluidic channels.

Superfast water transport discovered in graphitic nanoconduits, including carbon nanotubes and graphene nanochannels, implicates crucial applications in separation processes and energy conversion. Yet lack of complete understanding at the single-conduit level limits development of new carbon nanofluidic structures and devices with desired transport properties for practical applications. Here, we show that the hydraulic resistance and slippage of single graphene nanochannels can be accurately determined using capillary flow and a novel hybrid nanochannel design without estimating the capillary pressure. Our results reveal that the slip length of graphene in the graphene nanochannels is around 16 nm, albeit with a large variation from 0 to 200 nm regardless of the channel height. We corroborate this finding with molecular dynamics simulation results, which indicate thatthis wide distribution of the slip length is due to the surface charge of graphene as well as the interaction between graphene and its silica substrate.

Flow-induced voltage generation by driving imidazolium-based ionic liquids over a graphene nano-channel

In this study, the flow-induced voltage is investigated by driving the pure bulk room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) flowing over a graphene nano-channel consisting of two parallel single-layered graphene sheets using molecular dynamics simulation for the first time.

Surface charge-dependent transport of water in graphene nano-channels

Deionized water flow through positively charged graphene nano-channels is investigated using molecular dynamics simulations as a function of the surface charge density. Due to the net electric charge, Ewald summation algorithm cannot be used for modeling long-range Coulomb interactions. Instead, the cutoff distance used for Coulomb forces is systematically increased until the density distribution and orientation of water atoms converged to a unified profile. Liquid density near the walls increases with increased surface charge density, and the water molecules reorient their dipoles with oxygen atoms facing the positively charged surfaces. This effect weakens away from the charged surfaces. Force-driven water flows in graphene nano-channels exhibit slip lengths over 60 nm, which result in plug-like velocity profiles in sufficiently small nano-channels. With increased surface charge density, the slip length decreases and the apparent viscosity of water increases, leading to parabolic velocity profiles and decreased flow rates. Results of this study are relevant for water desalination applications, where optimization of the surface charge for ion removal with maximum flow rate is desired.

Fullerene-water nanofluid confined in graphene nanochannel

The flow behaviors and boundary slip of the fullerene-water nanofluids (NFs) confined in graphene nanochannels are first investigated by using classical molecular dynamics simulations. The influences of the shear rate in Couette model, the driving force in Poiseuille model, the volume fraction, and the charge magnitude on the motion behaviors and the boundary slip are explored with considering the dynamics and the accumulation of the fullerene within the NFs. The results show that the boundary slip velocity increases almost linearly with the shear rate below a threshold of the shear rate while it increases sharply above the threshold. The relatively large driving force in Poiseuille model and the large shear rate in Couette model can reduce the accumulation of the fullerenes. The increase in the volume fraction of the fullerene in NFs can enhance the shear viscosity, and interestingly, it can increase the boundary slip velocity of the NFs in graphene channels. As the charge magnitude of the graphene channel increases, the boundary slip of fullerene NFs first increases to a threshold and then decreases slightly. The findings may be helpful to the design and fabrication of the low dimensional carbon materials-based nano-apparatus.

Anomalous electrokinetics at hydrophobic surfaces: Effects of ion specificity and interfacial water structure

We study the electrokinetic transport behavior of water molecules and ions in hydrophobic graphene nanochannels with variable surface charge densities as well as the interfacial water structure based on detailed molecular dynamics simulations. The interfacial water structure, described by the water density, hydrogen bonding, diffusion, distribution of the OH bond and dipole orientations, is strikingly influenced by the surface charge sign and density. We find anomalous electrokinetic effects which are related to the distribution of counterions close to the surface, ion-specific effects and the interfacial water structure. On a negatively charged graphene layer, the attraction of Na+ ions towards the surface enhances the interfacial friction. In contrast, if the surface is positively charged, high surface charge density triggers an anomalous enhancement of electroosmotic flow, accompanied by an abrupt change of the interfacial water structure. At high surface charge densities, the mobility of the interfacial water at the positively charged surfaces is suppressed more strongly compared to the negatively charged surface. Our results reveal new electrokinetic phenomena by the comparison of negatively and positively charged surfaces.

Electric field controlled transport of water in graphene nano-channels.

Motivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.

Gated Water Transport through Graphene Nanochannels: From Ionic Coulomb Blockade to Electroosmotic Pump

Understanding and controlling water or ion transport in nanochannels plays an important role in further unravelling the transport mechanism of biological membrane channels and designing functional nanofluidic devices. Molecular dynamics simulations were conducted to investigate water and ion transport in graphene nanochannels. Similar to electron coulomb blockade phenomenon observed in quantum dots, we discovered an ionic coulomb blockade phenomenon in our graphene nanochannels, and another two ion transport modes were also proposed to rationalize the observed phenomena under different electric-field intensities. Furthermore, on the basis of this blockade phenomenon we found that the Open and Closed states of the graphene nanochannels for water transport could be switched according to external electric-field intensities, and electroosmotic flow could further enhance the water transport. These findings might have potential applications in designing and fabricating controllable valves in lab-on-chip nanodev...

Multiscale modeling of electroosmotic flow: Effects of discrete ion, enhanced viscosity, and surface friction

We propose an isothermal, one-dimensional, electroosmotic flow model for slit-shaped nanochannels. Nanoscale confinement effects are embedded into the transport model by incorporating the spatially varying solvent and ion concentration profiles that correspond to the electrochemical potential of mean force. The local viscosity is dependent on the solvent local density and is modeled using the local average density method. Excess contributions to the local viscosity are included using the Onsager-Fuoss expression that is dependent on the local ionic strength. A Dirichlet-type boundary condition is provided in the form of the slip velocity that is dependent on the macroscopic interfacial friction. This solvent-surface specific interfacial friction is estimated using a dynamical generalized Langevin equation based framework. The electroosmotic flow of Na+ and Cl− as single counterions and NaCl salt solvated in Extended Simple Point Charge (SPC/E) water confined between graphene and silicon slit-shaped nanochannels are considered as examples. The proposed model yields a good quantitative agreement with the solvent velocity profiles obtained from the non-equilibrium molecular dynamics simulations.