Nanometer-scale Pores Research Articles

Investigation of the Effectiveness of Graphene for Water Purification

This project proposes another purpose for graphene in tandem with water desalination: water purification. Water consumption surpasses the Earth’s carrying capacity. While steps have been taken, desalination accounts for only 1% of alternative methods in generating clean water. This is mainly due to the large energy footprints and high capital costs. This water that is suitable for human consumption is additionally under new and increasing threats of pollution from industrialization allowing carcinogenic polycyclic aromatic hydrocarbons (PAH) to settle within the ocean long after the initial contamination. The danger of PAHs has been acknowledged by various filtration studies, but there has been a particular focus on an ambient air medium. Through researching the effectiveness of multi-layer freestanding graphene with nanometer scale pores to filter NaCl and PAHs from oil using IR spectroscopy to determine residual species after filtration with graphene and tetrahydrofuran (THF). Understanding the interactions of graphene and THF with NaCl and PAHs will allow great insight into new techniques of water filtration; such insight could be furthered with additional surface chemistry software simulations. This research will determine if graphene holds potential in water purification. Desalination is a promising solution to the restricted quantity of freshwater and the ability to attract and remove dangerous PAHs from ocean water could save aquatic life around the world. Synthesizing a device that reduces toxicity and salinity attributes in seawater could revolutionize the water filtration industry.

High Resolution Physical Characterization of Single Metallic Nanoparticles.

Individual molecules can be detected and characterized by measuring the degree by which they reduce the ionic current flowing through a single nanometer-scale pore. The signal is characteristic of the molecule's physicochemical properties and its interactions with the pore. We demonstrate that the nanopore formed by the bacterial protein exotoxin Staphylococcus aureus alpha hemolysin (αHL) can detect polyoxometalates (POMs, anionic metal oxygen clusters), at the single molecule limit. Moreover, multiple degradation products of 12-phosphotungstic acid POM (PTA, H3PW12O40) in solution are simultaneously measured. The single molecule sensitivity of the nanopore method allows for POMs to be characterized at significantly lower concentrations than required for nuclear magnetic resonance (NMR) spectroscopy. This technique could serve as a new tool for chemists to study the molecular properties of polyoxometalates or other metallic clusters, to better understand POM synthetic processes, and possibly improve their yield. Hypothetically, the location of a given atom, or the rotation of a fragment in the molecule, and the metal oxidation state could be investigated with this method. In addition, this new technique has the advantage of allowing the real-time monitoring of molecules in solution.

A comparison of ion channel current blockades caused by individual poly(ethylene glycol) molecules and polyoxometalate nanoclusters.

Proteinaceous nanometer-scale pores have been used to detect and physically characterize many different types of analytes at the single-molecule limit. The method is based on the ability to measure the transient reduction in the ionic channel conductance caused by molecules that partition into the pore. The distribution of blockade depth amplitudes and residence times of the analytes in the pore are used to physically and chemically characterize them. Here we compare the current blockade events caused by flexible linear polymers of ethylene glycol (PEGs) and structurally well-defined tungsten polyoxymetallate nanoparticles in the nanopores formed by Staphylococcus aureusα-hemolysin and Aeromonas hydrophila aerolysin. Surprisingly, the variance in the ionic current blockade depth values for the relatively rigid metallic nanoparticles is much greater than that for the flexible PEGs, possibly because of multiple charged states of the polyoxymetallate clusters.

Automated, Ultra‐Fast Laser‐Drilling of Nanometer Scale Pores and Nanopore Arrays in Aqueous Solutions

AbstractThe ability to quickly and reliably fabricate nanoscale pore arrays in ultra‐thin membranes such as silicon nitride (SixN) is extremely important for the growing field of nanopore biosensing. Laser‐based etching of thin SixN membranes immersed in aqueous solutions has recently been demonstrated as a method to produce stable functional pores. Herein, the principal mechanism governing material etching and pore formation using light is investigated. It is found that the process is extremely sensitive to the relative content of Si over N atoms in the amorphous membrane, produced by chemical vapor deposition. Commonly, SixN membranes are made to be Si‐rich to increase their mechanical stability, which substantially reduces the material's bandgap and increases the density of Si‐dangling bonds. Hence, even minimal batch‐to‐batch variation may lead to remarkably different etch rates. It is shown that higher Si content results in orders of magnitude faster etching rates. This rate is further accelerated in an alkaline environment allowing on‐demand controlled nanopore formation in about 10 s time even at low laser radiation intensities. These results highlight that photoactivation of the SixN by the incident beam is critical to the chemical etching process and can be used to readily produce nanopore arrays at any specific location.

Detection of metal corrosion characteristics in chlorine solution using solid state nanopore

Nanopore structures were originally proposed for detection of biomolecule translocation through nanometer-scale pores that perforate membranes by transient changes in ionic current. In this study, these changes are utilized to detect corrosion of different metals in aqueous chlorine media. The corrosion behaviors of Cu, Al, Ti, and Cr were analyzed by monitoring the changes in ion current resulting from ion concentration variations in solutions due to corrosion of the metals. We observed that the Cu layer passivated by CuOx was severely corroded when a drastic change of ion current was induced, from 10 to 30 nS to the level of 104 nS, as soon as the bias voltage was applied. In the case of Al passivated by thin AlOx, the conductance increased from 10–30 to 166 ± 52 nS and became saturated. Highly localized pitting corrosion was observed on the periphery of the nanopore, where the electrical field was most concentrated. Finally, we observed that Ti and Cr passivated by oxide showed long-term stability without corrosion in 1 M KCl in the pH range of 4–11.

How Solutes Modify the Thermodynamics and Dynamics of Filling and Emptying in Extreme Ink-Bottle Pores

We investigate the filling and emptying of extreme ink-bottle porous media-micrometer-scale pores connected by nanometer-scale pores-when changing the pressure of the external vapor, in a case where the pore liquid contains solutes. These phenomena are relevant in diverse contexts, such as the weathering of building materials and artwork, aerosol formation in the atmosphere, and the hydration of soils and plants. Using model systems made of vein-shaped microcavities interconnected by a mesoporous matrix, we show experimentally that the presence of a nonvolatile solute shifts the condensation and evaporation transitions and in a way that is consistent with a modified Kelvin-Laplace equation that takes into account the osmotic pressure of the solution. Emptying occurs far below saturation, when the Kelvin stress, mediated by the large curvature of the liquid-vapor interfaces in the nanopores, is negative enough to induce spontaneous bubble nucleation in the microveins. Filling, on the other hand, occurs close to equilibrium (i.e., at saturation, psat for pure water and ps < psat for a solution), driven by the weak capillary pressure of the liquid-vapor interface in the microveins. Interestingly, solutes allow the system to reach situations where the vapor is supersaturated with respect to the solution ( ps < p < psat). We show that in that latter situation, a condensation layer covers the outer surface of the porous system, preventing the generation of Kelvin stresses but inducing osmotic stresses and flows that are vapor pressure-dependent. The timescales and dynamics reflect these different driving forces: emptying proceeds through discrete, stochastic nucleation events with very fast, unsteady bubble growth associated with a poroelastic relaxation process, while filling occurs collectively in all veins of the sample through a slower steady-state process driven by a combination of osmosis and capillarity. The dynamics can however be rendered symmetrical between filling and emptying if bubbles pre-exist during emptying, a case that we explore using cycling of the vapor pressure around equilibrium.

Image-based micro-continuum model for gas flow in organic-rich shale rock

The physical mechanisms that control the flow dynamics in organic-rich shale are not well understood. The challenges include nanometer-scale pores and multiscale heterogeneity in the spatial distribution of the constituents. Recently, digital rock physics (DRP), which uses high-resolution images of rock samples as input for flow simulations, has been used for shale. One important issue with images of shale rock is sub-resolution porosity (nanometer pores below the instrument resolution), which poses serious challenges for instruments and computational models. Here, we present a micro-continuum model based on the Darcy–Brinkman–Stokes framework. The method couples resolved pores and unresolved nano-porous regions using physics-based parameters that can be measured independently. The Stokes equation is used for resolved pores. The unresolved nano-porous regions are treated as a continuum, and a permeability model that accounts for slip-flow and Knudsen diffusion is employed. Adsorption/desorption and surface diffusion in organic matter are also accounted for. We apply our model to simulate gas flow in a high-resolution 3D segmented image of shale. The results indicate that the overall permeability of the sample (at fixed pressure) depends on the time scale. Early-time permeability is controlled by Stokes flow, while the late-time permeability is controlled by non-Darcy effects and surface-diffusion.

Nanoporous PLA/(Chitosan Nanoparticle) Composite Fibrous Membranes with Excellent Air Filtration and Antibacterial Performance.

Particulate matter (PM) pollution, which usually carries viruses and bacteria, has drawn considerable attention as a major threat to public health. In this present study, an environment-friendly antibacterial Poly(lactic acid)(PLA)/chitosan composite air filter was fabricated using the one-step electrospinning technique. The composite PLA/chitosan fibres show a highly porous structure, in which chitosan nanoparticles (NPs) were found to be uniformly distributed throughout the entire fibre. The morphologies, through-pore size and distribution, air filtration and anti-microbial properties of these filter media were studied. The results showed that it was not the chitosan content but instead the concentration of the spinning solutions that had the greatest effect on the morphologies of the porous fibres. The relative humidity influenced the nanometre-scale pores on the surface of PLA/chitosan fibres. The PLA/chitosan fibrous membranes with a chitosan to PLA mass ratio of 2.5:8 exhibited a high filtration efficiency of 98.99% and a relatively low pressure drop (147.60 Pa) when the air flow rate was 14 cm/s, while these also had high antibacterial activity of 99.4% and 99.5% against Escherichia coli and Staphylococcus aureus, respectively. It took 33 min for the PM2.5 concentration to decrease to 0 μg/m3 from 999 μg/m3 using the PLA/chitosan fibrous membranes, which demonstrates obviously effective air purification performance.

Stable, Temperature-Dependent Gas Mixture Permeation and Separation through Suspended Nanoporous Single-Layer Graphene Membranes

Graphene membranes with nanometer-scale pores could exhibit an extremely high permeance and selectivity for the separation of gas mixtures. However, to date, no experimental measurements of gas mixture separation through nanoporous single-layer graphene (SLG) membranes have been reported. Herein, we report the first measurements of the temperature-dependent permeance of gas mixtures in an equimolar mixture feed containing H2, He, CH4, CO2, and SF6 from 22 to 208 °C through SLG membranes containing nanopores formed spontaneously during graphene synthesis. Five membranes were fabricated by transfer of CVD graphene from catalytic Cu film onto channels framed in impermeable Ni. Two membranes exhibited gas permeances on the order of 10-6 to 10-5 mol m-2 s-1 Pa-1 as well as gas mixture selectivities higher than the Knudsen effusion selectivities predicted by the gas effusion mechanism. We show that a new steric selectivity mechanism explains the permeance data and selectivities. This mechanism predicts a mean pore diameter of 2.5 nm and an areal pore density of 7.3 × 1013 m-2, which is validated by experimental observations. A third membrane exhibited selectivities lower than the Knudsen effusion selectivities, suggesting a combination of effusion and viscous flow. A fourth membrane exhibited increasing permeance values as functions of temperature from 27 to 200 °C, and a CO2/SF6 selectivity > 20 at 200 °C, suggestive of activated translocation through molecular-sized nanopores. A fifth membrane exhibited no measurable permeance of any gas above the detection limit of our technique, 2 × 10-7 mol m-2 s-1 Pa-1, indicating essentially a molecularly impermeable barrier. Overall, these data demonstrate that SLG membranes can potentially provide a high mixture separation selectivity for gases, with CVD synthesis alone resulting in nanometer-scale pores useful for gas separation. This work also shows that temperature-dependent permeance measurements on SLG can be used to reveal underlying permeation mechanisms.

DNA capture into the ClyA nanopore: diffusion-limited versus reaction-limited processes

The capture and translocation of biomolecules through nanometer-scale pores are processes with a potentially large number of applications, and hence they have been intensively studied in recent years. The aim of this paper is to review existing models of the capture process by a nanopore, together with some recent experimental data of short single- and double-stranded DNA captured by the Cytolysin A (ClyA) nanopore. ClyA is a transmembrane protein of bacterial origin which has been recently engineered through site-specific mutations, to allow the translocation of double- and single-stranded DNA. A comparison between theoretical estimations and experiments suggests that for both cases the capture is a reaction-limited process. This is corroborated by the observed salt dependence of the capture rate, which we find to be in quantitative agreement with the theoretical predictions.

Dynamics of driven translocation of semiflexible polymers.

We study translocation of semiflexible polymers driven by force f_{d} inside a nanometer-scale pore using our three-dimensional Langevin dynamics model. We show that the translocation time τ increases with increasing bending rigidity κ. Similarly, the exponent β for the scaling of τ with polymer length N,τ∼N^{β}, increases with increasing κ as well as with increasing f_{d}. By comparing waiting times between semiflexible and fully flexible polymers we show that for realistic f_{d} translocation dynamics is to a large extent, but not completely, determined by the polymer's elastic length measured in number of Kuhn segments N_{Kuhn}. Unlike in driven translocation of flexible polymers, friction related to the polymer segment on the trans side has a considerable effect on the resulting dynamics. This friction is intermittently reduced by buckling of the polymer segment in the vicinity of the pore opening on the trans side. We show that in the experimentally relevant regime where viscosity is higher than in computer simulation models, the probability for this buckling increases with increasing f_{d}, giving rise to a larger contribution to the trans side friction at small f_{d}. Similarly to flexible polymers, we find significant center-of-mass diffusion of the cis side polymer segment which speeds up translocation. This effect is larger for smaller f_{d}. However, this speedup is smaller than the slowing down due to the trans side friction. At large enough N_{Kuhn}, the roles can be seen to be reversed, and the dynamics of flexible polymers can be reached. However, for example, polymers used in translocation experiments of DNA are elastically so short that the finite-length dynamics outlined here applies.

An imaging and fractal approach towards understanding reservoir scale changes in coal due to bioconversion

Microbially enhanced coalbed methane (MECBM) aims to replicate the natural process of microbial methane generation in coal under in situ conditions. Considerable work has been reported over the years to optimize the associated microbial geo-chemistry. However, there is very little insight with regards to changes in the physical structure of coal due to bioconversion, and its impact on fluid flow properties. This paper presents the result of an image and fractal-based approach used to evaluate the changes in physical properties of coal. Samples of coal treated over 30, 60 and 120 days respectively were imaged before and after bioconversion. The results revealed that coal bioconversion resulted in swelling of the coal matrix. Cleats narrower than 5 µm reduced in its width post-bioconversion. A 77% drop in the permeability of coal is expected from the obtained results. Bioconversion also resulted in separation of thin layers of coal flakes from the coal surface, serving as a potential source of fines. Fractal analysis of the images revealed a decrease in the fractal dimension post bioconversion, which is in agreement with the previously reported studies, thus corroborating the variations in sorption-trends observed due to bioconversion. Additionally, the methodology to determine the fractal dimension via two-dimensional image processing was modified to improve its accuracy, and remove the dependency of the dimension to the scale of the obtained images. Bioconversion also resulted in formation of new pores/fractures. Shorter treatment duration resulted in nanometer-scale discontinuous pores, which do not contribute to Darcian flow. Longer treatment periods resulted in sub-micron wide continuous pores. Also, few larger fractures (>5 µm wide) saw an increase in its aperture post-treatment. This opens up new avenues, such as, utilizing artificially induced fracturing techniques to enhance biogenic methane production in future.

Extended poromechanics for adsorption-induced swelling prediction in double porosity media: Modeling and experimental validation on activated carbon

Natural and synthesised porous media are generally composed of a double porosity: a microporosity where the fluid is trapped as an adsorbed phase and a meso or a macro porosity required to ensure the transport of fluids to and from the smaller pores. Zeolites, activated carbon, tight rocks, coal rocks, source rocks, cement paste or construction materials are among these materials.In nanometer-scale pores, the molecules of fluid are confined. This effect, denoted as molecular packing, induces that fluid-fluid and fluid-solid interactions sum at the pore scale and have significant consequences at the macroscale, such as instantaneous deformation, which are not predicted by classical poromechanics. If adsorption in nanopores induces instantaneous deformation at a higher scale, the matrix swelling may close the transport porosity, reducing the global permeability of the porous system. This is important for applications in petroleum oil and gas recovery, gas storage, separation, catalysis or drug delivery.This study aims at characterizing the influence of an adsorbed phase on the instantaneous deformation of micro-to-macro porous media presenting distinct and well-separated porosities. A new incremental poromechanical framework with varying porosity is proposed allowing the prediction of the swelling induced by adsorption without any fitting parameters. This model is validated by experimental comparison performed on a high micro and macro porous activated carbon. It is shown also that a single porosity model cannot predict the adsorption-induced strain evolution observed during the experiment. After validation, the double porosity model is used to discuss the evolution of the poromechanical properties under free and constraint swelling.

Synthesis of Catalytic Nanoporous Metallic Thin Films on Polymer Membranes.

Composite membranes were produced with a metallic thin film forming the upper layer of the composite on a porous polymer support. Commercially available membranes were used as supports with both micron and nanometer scale pores. Alloy films of ~110 nm thickness were deposited via magnetron sputtering to produce the top layer of the composite. Dealloying the film with sulfuric acid allowed the creation of a nanoporous film structure with a ligament size of 7.7 ± 2.5 nm. Resulting composite membranes were permeable to water at all stages of production, and a UF PSf membrane with 90 nm of nanoporous Fe/Pd on top showed a flux of 183 LHM/bar. The films were evaluated for dechlorination of toxic polychlorinated biphenyls from water. At a loading of 6.6 mg/L of Pd attached to 13.2 cm2 support in a 2.5 ppm PCB-1 solution with 1.5 ppm dissolved H2, over 90% of PCB-1 was removed from solution in 30 minutes, which produced the expected product biphenyl from the dechlorination reaction.

Structurally Controlled Large-Area 10 nm Pitch Graphene Nanomesh by Focused Helium Ion Beam Milling.

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an energy gap opening. GNMs also have potential for use in phonon control and water filtration. Furthermore, physical phenomena, such as spin qubit, are predicted at pitches below 10 nm fabricated with precise structural control. Current GNM patterning techniques suffer from either large dimensions or a lack of structural control. This work establishes reliable GNM patterning with a sub-10 nm pitch and an < 4 nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Due to the simplicity of the method, no postpatterning processing is required. Electrical transport measurements reveal an effective energy gap opening of up to ∼450 meV. The reported technique combines the highest resolution with structural control and opens a path toward GNM-based, room-temperature semiconducting applications.

Reservoir micro structure of Da'anzhai Member of Jurassic and its petroleum significance in Central Sichuan Basin, SW China

Abstract Based on the qualitative study of microscopic reservoir features using core analysis, cast and fluorescence thin sections inspection, scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) and quantitative examination of pore size and geometry using mercury injection, nano-CT and nitrogen adsorption, reservoir rock of Da'anzhai Member were divided into 9 types, while storage spaces were divided into 4 types and 14 sub-types. The study shows that sparry coquina is the most promising reservoir type. Pores that smaller than 1 μm in diameter contribute 91.27% of storage space volume. Most of them exhibit slot-like geometry with good connectivity. By building up storage space models, it was revealed that micron scale storage spaces mainly composed of fractures and nanometer scale pores and fractures form multi-scale dual porosity system. Low resource abundance, small single well controlled reserve, and low production are related to the nano-scale pore space in Da'anzhai Memer, whereas the dual-porosity system composed of pores and fractures makes for long-term oil yield. Due to the existence of abundant slot-like pore space and fractures, economic tight oil production was achieved without stimulations.

Determining the Physical Properties of Molecules with Nanometer-Scale Pores.

Nanometer-scale pores have been developed for the detection, characterization, and quantification of a wide range of analytes (e.g., ions, polymers, proteins, anthrax toxins, neurotransmitters, and synthetic nanoparticles) and for DNA sequencing. We describe the key requirements that made this method possible and how the technique evolved. Finally, we show that, despite sound theoretical work, which advanced both the conceptual framework and quantitative capability of the method, there are still unresolved questions that need to be addressed to further improve the technique.

Atomistic and continuum scale modeling of functionalized graphyne membranes for water desalination.

Recent theoretical and experimental studies reported ultra-high water permeability and salt rejection in nanoporous single-layer graphene. However, creating and controlling the size and distribution of nanometer-scale pores pose significant challenges to application of these membranes for water desalination. Graphyne and hydrogenated graphyne have tremendous potential as ultra-permeable membranes for desalination and wastewater reclamation due to their uniform pore-distribution, atomic thickness and mechano-chemical stability. Using molecular dynamics (MD) simulations and upscale continuum analysis, the desalination performance of bare and hydrogenated α-graphyne and γ-{2,3,4}-graphyne membranes is evaluated as a function of pore size, pore geometry, chemical functionalization and applied pressure. MD simulations show that pores ranging from 20 to 50 Å2 reject in excess of 90% of the ions for pressures up to 1 GPa. Water permeability is found to range up to 85 L cm-2 day-1 MPa-1, which is up to three orders of magnitude larger than commercial seawater reverse osmosis (RO) membranes and up to ten times that of nanoporous graphene. Pore chemistry, functionalization and geometry are shown to play a critical role in modulating the water flux, and these observations are explained by water velocity, density, and energy barriers in the pores. The atomistic scale investigations are complemented by upscale continuum analysis to examine the performance of these membranes in application to cross-flow RO systems. This upscale analysis, however, shows that the significant increase in permeability, observed from MD simulations, does not fully translate to current RO systems due to transport limitations. Nevertheless, upscale calculations predict that the higher permeability of graphyne membranes would allow up to six times higher permeate recovery or up to 6% less energy consumption as compared to thin-film composite membranes at currently accessible operating conditions. Significantly higher energy savings and permeate recovery can be achieved if higher feed-flow rates can be realized.

Electrochemical synthesis of CuIn(1−x)GaxSe2 nanowires with controlled stoichiometry

We present a solution-based approach for fabrication of CuIn(1−x)GaxSe2 (CIGS) nanowires by electrodeposition method with stoichiometry close to the ideal one for photovoltaic applications. Fabrication of CIGS nanowires is based on a one-step electrodeposition to deposit CIGS nanowires into the nanometer-scale cylindrical pores of the anodized aluminum oxide (AAO) template with molybdenum (Mo) thin-film deposited on the backside of the AAO as the counter electrode. A key to the commercialization of CIGS nanowires as active absorber layer for photovoltaic applications is to improve the composition control and quality of the CIGS nanowires. In this study, CIGS nanowires with close to the ideal stoichiometry and chalcopyrite nanocrystalline structure for photovoltaic applications were obtained by varying the deposition parameters such as deposition potential and electrolyte’s composition during electrodeposition and the annealing process after electrodeposition.

Quantifying the heterogeneity of shale through statistical combination of imaging across scales

Shale is a highly heterogeneous material across multiple scales. A typical shale consists of nanometer-scale pores and minerals mixed with macroscale fractures and particles of varying size. High-resolution imaging is crucial for characterizing the composition and microstructure of this rock. However, it is generally not feasible to image a large sample of shale at a high resolution over a large field of view (FOV), thus limiting a full characterization of the microstructure of this material. We present a stochastic framework based on multiple-point statistics that uses high-resolution training images to enhance low-resolution images obtained over a large FOV. We demonstrate the approach using X-ray micro-tomography images of organic-rich Woodford shale obtained at two different resolutions and FOV. Results show that the proposed technique can generate realistic high-resolution images of the microstructure of shale over a large FOV.