Substantially reducing global carbon emissions requires new processes to curtail greenhouse gas generation during chemical processing and more effectively recycle waste plastics while supplying the necessary hydrogen and advanced materials demanded by emerging clean technologies. Catalytic conversion of light hydrocarbon gas by-products into hydrogen while sequestering the carbon in valuable solid nanotubes and nanofibers is a promising strategy that simultaneously addresses many of these needs. This paper explores the versatility of such a process. Combining measurements of various single-component hydrocarbon feed gases with analytical modeling, we demonstrate a process that can convert a diverse range of hydrocarbon gases into hydrogen and carbon nanotubes and nanofibers. Regardless of the hydrocarbon feed gas composition, at reaction temperatures between 650 degrees C and 1000 degrees C, the product gas consists of 84-99 % hydrogen, with higher hydrogen content at higher temperatures, and the remaining gas being methane. Carbon nanotube and nanofiber production adjusts to accept the balance of the carbon from the feed gas. These findings are further validated by applying the technique to two industrially relevant hydrocarbon gas streams: by-product gases from the pyrolysis of bitumen and recycled waste plastics. The same hydrogen yield is achieved from both complex mixtures as from pure, single-component gases, while producing commercial quality multiwalled carbon nanotubes and carbon nanofibers. The results establish the versatility of a process that simultaneously reduces emissions from industrial by-product gases while generating hydrogen and nanomaterials.

Angstrom‐scale proton‐selective pores in atomically thin 2D materials present fundamentally new opportunities for advancing proton exchange membranes (PEMs). Vanadium Redox Flow Batteries (VRFBs) for grid‐scale energy storage require PEMs with high areal proton conductance (>1 S cm −2 ) and minimal vanadium ion (VO 2+ ) crossover. However, state‐of‐the‐art Nafion 212 membranes (N212 ≈50 µm thick), suffer from persistent VO 2+ crossover reducing performance and efficiency. Here, a layered PEM is demonstrated, comprising monolayer CVD graphene with Angstrom‐scale proton‐selective pores introduced via Ar plasma, integrated with an ultra‐thin ≈300 nm polybenzimidazole (PBI) layer and sandwiched between two Nafion 211 (25 µm thick) layers. The layered architecture facilitates scalable membrane fabrication by mitigating defects while processing and facile stacking of graphene layers allows stochastic non‐selective defect isolation enabling exceptionally low VO 2+ crossover (selectivity (H + areal conductance / VO 2+ permeability) ≈6709 × 10 6 S min cm −4 ), with proton conductance >8 S cm −2 . Systematic transport experiments supported by resistance‐based transport modelling elucidate the role of defect size, defect isolation, and sealing, as well as layering/stacking, to enable orders of magnitude (>671× over N212) improvements in selectivity, along with areal proton conductance >8 S cm −2 . This work highlights the potential of atomic‐scale proton‐selective defect engineering in 2D materials, in conjunction with facile stacking and layering of materials as strategies for scalable, high‐performance advances in PEMs for energy, electrochemical, and separation applications beyond VRFBs.

Liquid transport through membrane nanopores is often modelled as creeping flow through a finite thickness orifice plate. Experiments and molecular simulations have revealed the importance of slip in such pores, where the diameter can be orders of magnitude smaller than the slip length for materials such as carbon nanotubes and graphene. Approximate hydrodynamic resistance models considering access resistance to the pore and fully developed slip flow within the pore are sometimes applied to estimate flow rates. While this approach is very accurate without slip, it can result in large errors for long slip lengths. Even with large slip lengths, flow development in the entry/exit regions contribute significant pressure drops that should be accounted for. In this paper, we extend an infinite series formulation for no-slip creeping flow through a finite thickness orifice plate to slip flow through the same geometry. We develop an algebraic system of equations for the series coefficients that can be efficiently computed to determine the velocity and pressure fields for the selected pore aspect ratio and slip length. Accurate volume flow rates can be quickly calculated, and are tabulated for convenience. We refine the approximate hydrodynamic resistance model for this flow to include losses in the entry region and obtain a fit for the volume flow rate accurate to within 2.5% for all slip lengths and pore aspect ratios.

Permeance-selectivity trade-offs are inherent to polymeric membranes. In fuel cells, thinner proton exchange membranes (PEMs) could enable higher proton conductance and increased power density with lower area-specific resistance (ASR), smaller ohmic losses, and lower ionomer cost. However, reducing thickness is accompanied by an increase in undesired species crossover harming performance and long-term efficiency. Here, we show that incorporating atomically thin monolayer graphene synthesized via scalable chemical vapor deposition (CVD) and tunable defect density into PEMs (Nafion, ∼5-25 μm thick) can allow for reduced H2 crossover (∼34-78% of Nafion of a similar thickness) while maintaining adequate areal proton conductance for applications (>4 S cm-2). In contrast to most prior work using >50 μm symmetric Nafion sandwich structures, we elucidate the interplay of graphene defect density and Nafion proton transport resistance on the performance of Nafion|graphene composite membranes and find high-quality low-defect density CVD graphene (G) supported on Nafion 211 (∼25 μm); i.e., N211|G has a high areal proton conductance (∼6.1 S cm-2) and the lowest H2 crossover (∼0.7 mA cm-2). Fully functional centimeter-scale N211|G fuel-cell membranes demonstrate performance comparable to that of state-of-the-art Nafion N211 at room temperature as well as standard operating conditions (∼80 °C, ∼150-250 kPa-abs) with H2/air (power density ∼0.57-0.63 W cm-2) and H2/O2 feed (power density ∼1.4-1.62 W cm-2) and markedly reduced H2 crossover (∼53-57%).

Interactions between carbon nanotubes (CNTs) and fluid flows are central to the operation of several emerging nanotechnologies. In this paper, we explore the fluid-structure interaction of CNT micropillars in wall-bounded shear flows, relevant to recently developed microscale wall shear stress sensors. We monitor the deformation of CNT micropillars in channel flow as the flow rate and wall shear stress are gradually varied. We quantify how the micropillars bend at low wall shear stress, and then will commonly tilt abruptly from their base above a threshold wall shear stress, which is attributed to the lower density of the micropillars in this region. Some micropillars are observed to flutter rapidly between a vertical and horizontal position around this threshold wall shear stress, before settling to a tilted position as wall shear stress increases further. Tilted micropillars are found to kink sharply near their base, similar to the observed buckling near the base of CNT micropillars in compression. Upon reducing the flow rate, micropillars are found to fully recover from a near horizontal position to a near vertical position, even with repeated on-off cycling. At sufficiently high wall shear stress, the micropillars were found to detach at the catalyst particle-substrate interface. The mechanical response of CNT micropillars in airflow revealed by this study provides a basis for future development efforts and the accurate simulation of CNT micropillar wall shear stress sensors.

Graphene holds potential as a high permeance membrane material for separation applications owing to its single atom thickness. Transport rates through graphene pores ultimately determine membrane performance and are an area of focus of design efforts. In this regard, single pore flow rate measurements are desirable because they are not influenced by material defects present in large-area samples and are unaffected by modeling assumptions used in simulations. However, measuring liquid flow rates through single graphene pores is challenging. In this paper, we establish a micro-particle image velocimetry technique to measure flow rates through single pores or small permeable areas by comparing the velocity decay rate downstream of the pore to analytical predictions for the flow field. The method is validated on silicon nitride micropores by comparison with microfluidic sensor measurements and then applied to measure water permeation rates through single sub-micron graphene pores, below the detection limit of the sensor. A 200 nm diameter pore is measured to have a pore permeation coefficient of 1.5 x 10 - 19 m(3) s(-1) Pa-1, and 500 nm pores are measured to have pore permeation coefficients of 7.0 x 10 - 19 and 14 x 10 - 19 m(3) s(-1) Pa-1. These values are less than half those predicted by continuum theory, but of the same order of magnitude. The results provide measured permeances of experimentally realized flows through single sub-micron graphene pores and a reliable technique for measuring the liquid permeance of micrometer-scale membrane areas.

Increasingly smaller fluid flow devices depend on increasingly smaller sensors to operate. In this work, we develop a miniature wall shear stress sensor consisting of a pair of carbon nanotube pillars that produce a measurable capacitance change when one is deflected by the flow. The sensor, including material producing the capacitance change, has a 50 mu m x 60 mu m footprint and <200 mu m height. It provides 0.05-1 fF/Pa sensitivity with up to +/- 8 Pa range. Most sensors produced were found to have a wider operating range when the thinner pillar was positioned downstream because greater deflection could be achieved without contact between the pillars. Interestingly, a small number of sensors responded differently to flow due to twisting of the sensing element rather than bending or due to a dominant Bernoulli effect. These diverse sensing mechanisms could be exploited to tune sensitivity or operating range in future designs.

Demand is growing for a larger catalogue of experimental techniques to measure flow rates through micro-/nanoscale systems for both fundamental research and device development. Flow emerging from a hole in a plane wall is a common system of interest in such work for its relevance to membrane separation. In this paper, we consider the possibility of measuring volume flow rates through small scale orifice plates from images of dye dispersions downstream. Based on approximate analytical solutions to the advection-diffusion equation, we show that, at low Reynolds numbers, the concentration in the nearly hemispherical plume that forms increases linearly with inverse distance from the pore and that the slope is proportional to volume flow rate. From micrographs of fluorescent dye plumes taken downstream of micropores of three different diameters, we demonstrate that, at Reynolds numbers below 15, the volume flow rate can be determined by extracting this slope from fluorescence intensity images. At higher Reynolds numbers, laminar jets form. In this regime, we derive an approximate similarity solution for the concentration field and show agreement of imaged dye dispersion shapes with both analytical expressions for the streamlines and isoconcentration contours at Reynolds numbers above 25. The results validate a scalable method for flow rate measurements applicable to small micropores of any geometry in plane walls and to small areas of porous materials relevant to membrane systems.

Tactile sensors, or sensors that collect measurements through touch, have versatile applications in a wide range of fields including robotic gripping, intelligent manufacturing, and biomedical technology. Hoping to match the ability of human hands to sense physical changes in objects through touch, engineers have experimented with a variety of materials from soft polymers to hard ceramics, but so far, all have fallen short. A grand challenge for developers of "human-like" bionic tactile sensors is to be able to sense a wide range of strains while maintaining the low profile necessary for compact integration. Here, we developed a low-profile tactile sensor (∼300 μm in height) based on patterned, vertically aligned carbon nanotubes (PVACNT) that can repetitively sense compressive strains of up to 75%. Upon compression, reversible changes occur in the points of contact between CNTs, producing measurable changes in electrical admittance. By patterning VACNT pillars with different aspect ratios and pitch sizes, we engineered the range and resolution of strain sensing, suggesting that CNT-based tactile sensors can be integrated according to device specifications.

We deconstruct the origins of the orders of magnitude difference in proton conductance between micron and centimeter-scale monolayer graphene membranes to help advance atomically thin proton exchange membranes for applications in energy conversion, storage and separation.

Continuum transport equations are commonly applied to nanopores in atomically thin membranes for simple modeling. Although these equations do not apply for nanopores approaching the fluid or solute molecule size, they can be reasonably accurate for larger nanopores. Relatively large graphene nanopores have applications in small particle filtration and appear as unwanted defects in large-area membranes. Solute transport rates through these nanopores determine the rejection performance of the membrane. Atomically thin membranes commonly operate in a regime where advection and diffusion both contribute appreciably to transport. Solute mass transfer rates through larger nanopores have previously been modeled by adding continuum estimates for pure diffusion and pure advection through an infinitesimally thick orifice plate, as if the separate contributions were independent. We show here that estimating the transport rate in this way is accurate to within 30%. We further derive an expression for the net mass transfer rate in advection-diffusion through an infinitesimal thickness orifice plate at low Reynolds numbers that is accurate to within 1% for positive Peclet numbers (where diffusion is in the same direction as advection) and applies for negative Peclet numbers as well. Based on our expression, we devise an equation for the net mass transfer rate in creeping flow through orifice plates of arbitrary thickness that matches finite volume calculations to within 3% for positive Peclet numbers. These simple but accurate analytical equations for mass transfer rates in creeping flow through an orifice plate are useful tools in constructing approximate transport models.

Angstrom-scale pores introduced into atomically thin 2D materials offer transformative advances for proton exchange membranes in several energy applications. Here, we show that facile kinetic control of scalable chemical vapor deposition (CVD) can allow for direct formation of angstrom-scale proton-selective pores in monolayer graphene with significant hindrance to even small, hydrated ions (K+ diameter ∼6.6 Å) and gas molecules (H2 kinetic diameter ∼2.9 Å). We demonstrate centimeter-scale Nafion|Graphene|Nafion membranes with proton conductance ∼3.3-3.8 S cm-2 (graphene ∼12.7-24.6 S cm-2) and H+/K+ selectivity ∼6.2-44.2 with liquid electrolytes. The same membranes show proton conductance ∼4.6-4.8 S cm-2 (graphene ∼39.9-57.5 S cm-2) and extremely low H2 crossover ∼1.7 × 10-1 - 2.2 × 10-1 mA cm-2 (∼0.4 V, ∼25 °C) with H2 gas feed. We rationalize our findings via a resistance-based transport model and introduce a stacking approach that leverages combinatorial effects of interdefect distance and interlayer transport to allow for Nafion|Graphene|Graphene|Nafion membranes with H+/K+ selectivity ∼86.1 (at 1 M) and record low H2 crossover current density ∼2.5 × 10-2 mA cm-2, up to ∼90% lower than state-of-the-art ionomer Nafion membranes ∼2.7 × 10-1 mA cm-2 under identical conditions, while still maintaining proton conductance ∼4.2 S cm-2 (graphene stack ∼20.8 S cm-2) comparable to that for Nafion of ∼5.2 S cm-2. Our experimental insights enable functional atomically thin high flux proton exchange membranes with minimal crossover.

The transport of water through nanoscale capillaries/pores plays a prominent role in biology, ionic/molecular separations, water treatment and protective applications. However, the mechanisms of water and vapor transport through nanoscale confinements remain to be fully understood. Angstrom-scale pores (~2.8–6.6 Å) introduced into the atomically thin graphene lattice represent ideal model systems to probe water transport at the molecular-length scale with short pores (aspect ratio ~1–1.9) i.e., pore diameters approach the pore length (~3.4 Å) at the theoretical limit of material thickness. Here, we report on orders of magnitude differences (~80×) between transport of water vapor (~44.2–52.4 g m −2 day −1 Pa −1 ) and liquid water (0.6–2 g m −2 day −1 Pa −1 ) through nanopores (~2.8–6.6 Å in diameter) in monolayer graphene and rationalize this difference via a flow resistance model in which liquid water permeation occurs near the continuum regime whereas water vapor transport occurs in the free molecular flow regime. We demonstrate centimeter-scale atomically thin graphene membranes with up to an order of magnitude higher water vapor transport rate (~5.4–6.1 × 10 4 g m −2 day −1 ) than most commercially available ultra-breathable protective materials while effectively blocking even sub-nanometer (>0.66 nm) model ions/molecules.

Nanoporous graphene promises extremely high flow rates in membrane separation applications owing to its single atom thickness. While sub-nanometer graphene pores are desirable for rejecting ions and small molecules, transport through larger graphene nanopores is relevant in nanofiltration applications and to leakage flow through membrane defects. The permeance of these pores is commonly estimated using continuum transport relationships in graphene membrane modeling, although errors are expected due to sub-continuum behavior that emerges for smaller nanopores. In this paper we perform molecular dynamics simulations to understand the departure of solute advection–diffusion through graphene nanopores from continuum theory. We find that the same diameter dependent effective pore thickness can be used to adjust continuum models for both the net flow rate and solute mass transfer rate to match simulations. The increased resistance to both modes of transport is attributed to reduced molecule mobility in the dense layers of fluid that form on either side of the graphene and within the pore. The results further indicate the extent of deviations from continuum solute mass transfer that can be expected through graphene nanopores, toward more accurate modeling of nanoporous atomically thin membranes.

Current nanostructured biomaterials-based drug delivery vehicles for bone regeneration applications often show suboptimal cellular uptake and inferior drug loading. To overcome these challenges, we have developed a biomimetic cell-derived nanoparticle (CDN) loaded with the Food and Drug Administration-approved small-molecule therapeutic Dexamethasone, to induce osteogenic differentiation in human adipose-derived stem cells. The drug-loaded CDNs were cytocompatible, maintained hydrodynamic stability with uniform spherical shape and size, and exhibited high-percentage drug loading along with rapid cellular uptake and stem cell differentiation. These results demonstrate for the first time the preparation of Dexamethasone-loaded CDNs capable of directing stem cell fate for advanced bone regeneration applications.KEYWORDS: nanoparticles, drug delivery, bone repair, nanomedicine, regenerative medicine

Surfaces with switchable adhesive properties are employed by robots to quickly grip and release objects and thereby to perform dexterous manipulation and locomotion tasks. Robotic grippers with switchable adhesion have been developed using structured polymers and electrostatic mechanisms. However, manipulating delicate items can be challenging as this requires strong, switchable gripping forces that do not damage the target object. Soft nanocomposite electroadhesives (SNEs) were recently introduced as an option for handling such objects. The technology integrates an electrostatic adhesion mechanism into a mechanically compliant surface formed from dielectric-coated carbon nanotubes (CNTs) to ensure soft contact with target objects. In this study we explore the scaling of the electrostatic adhesion of SNEs, toward their potential application in macroscale grasping and manipulation. We measure electroadhesive pressures on millimeter-scale areas of up to ∼20 kPa with an on/off adhesion ratio of ∼700. Based on the measured forces and simple modeling, we conclude that the maximum achievable SNE adhesion forces are determined by dielectric breakdown in the insulating coating and surrounding air. Consequently, the SNE surface behaves as a parallel capacitor plate placed at an effective distance of 2.9 μm from the target object, despite being in contact with the target and therefore having the contacting CNTs separated from the surface by ∼2 nm dielectric coating. This mechanistic understanding of soft nanocomposite electroadhesives outlines the capabilities of the technology and informs their design for advanced manufacturing applications.

Atomically thin graphene with a high-density of precise subnanometer pores represents the ideal membrane for ionic and molecular separations. However, a single large-nanopore can severely compromise membrane performance and differential etching between pre-existing defects/grain boundaries in graphene and pristine regions presents fundamental limitations. Here, we show for the first time that size-selective interfacial polymerization after high-density nanopore formation in graphene not only seals larger defects (>0.5 nm) and macroscopic tears but also successfully preserves the smaller subnanometer pores. Low-temperature growth followed by mild UV/ozone oxidation allows for facile and scalable formation of high-density (4-5.5 × 1012 cm-2) useful subnanometer pores in the graphene lattice. We demonstrate scalable synthesis of fully functional centimeter-scale nanoporous atomically thin membranes (NATMs) with water (∼0.28 nm) permeance ∼23× higher than commercially available membranes and excellent rejection to salt ions (∼0.66 nm, >97% rejection) as well as small organic molecules (∼0.7-1.5 nm, ∼100% rejection) under forward osmosis.

Printing of ultrathin layers of polymeric and colloidal inks is critical for the manufacturing of electronics on nonconventional substrates such as paper and polymer films. Recently, we found that nanoporous stamps overcome key limitations of traditional polymer stamps in flexographic printing, namely, enabling the printing of ultrathin nanoparticle films with micron-scale lateral precision. Here, we study the dynamics of liquid transfer between nanoporous stamps and solid substrates. The stamps comprise forests of polymer-coated carbon nanotubes, and the surface mechanics and wettability of the stamps are engineered to imbibe colloidal inks and transfer the ink upon contact with the target substrate. By high-speed imaging during printing, we observe the dynamics of liquid spreading, which is mediated by progressing contact between the nanostructured stamp surface and by the substrate and imbibition within the stamp-substrate gap. From the final contact area, the volume of ink transfer is mediated by rupture of a capillary bridge; and, after rupture, liquid spreads to fill the area defined by a precursor film matching the stamp geometry with high precision. Via modeling of the liquid dynamics, and comparison with data, we elucidate the scale- and rate-limiting aspects of the process. Specifically, we find that the printed ink volume and resulting layer thickness are independent of contact pressure; and that printed layer thickness decreases with retraction speed. Under these conditions, nanoparticle films with controlled thickness in the <100 nm regime can be printed using nanoporous stamp flexography, at speeds commensurate with industrial printing equipment.

Automated handling of microscale objects is essential for manufacturing of next-generation electronic systems. Yet, mechanical pick-and-place technologies cannot manipulate smaller objects whose surface forces dominate over gravity, and emerging microtransfer printing methods require multidirectional motion, heating, and/or chemical bonding to switch adhesion. We introduce soft nanocomposite electroadhesives (SNEs), comprising sparse forests of dielectric-coated carbon nanotubes (CNTs), which have electrostatically switchable dry adhesion. SNEs exhibit 40-fold lower nominal dry adhesion than typical solids, yet their adhesion is increased >100-fold by applying 30 V to the CNTs. We characterize the scaling of adhesion with surface morphology, dielectric thickness, and applied voltage and demonstrate digital transfer printing of films of Ag nanowires, polymer and metal microparticles, and unpackaged light-emitting diodes.