High-permeability vacuum membrane distillation utilizing mechanically compressed carbon nanotube membranes.

Membranes for membrane distillation (MD) are mostly made of polymeric and ceramic materials. We demonstrate here that the laterally-compressed, vertically-aligned CNTs (VACNT) obtainable from a CNT forest are an excellent membrane material for vacuum membrane distillation (VMD). The VACNT structure provides interstices between CNTs for extracting vaporized water molecules, while efficiently filtering the impurity salts. The VACNT membrane is shown to deliver excellent performance when tested for the desalination of 3.5 wt% NaCl water solution, as exemplified by the permeability of 68 LMH (liter per square meter per hour) achieved at the salt rejection of over 99.8% at 65 °C. We also demonstrate that the VACNT membrane performance can be maintained with time with the aid of a simple cleaning procedure, which bodes well for a long lifetime of the membrane for VMD application.

Reduced graphene-oxide filter system for removing filterable and condensable particulate matter from source.

Air pollution is one of the most serious problems facing mankind because of its impact on ecosystems and human beings. Although particulate matter (PM) consists of both filterable PM (FPM) and condensable PM (CPM), most research has focused on eliminating only FPM. In this work, we introduce a filter system that removes both FPM and CPM from pollution source with high efficiency. The system consists of two reduced graphene oxide (rGO) filters and a condenser between them that can remove the usual FPM and at the same time CPM-induced FPM that typically leaves the pollution source unabated. The filters, quite effective in removing the PM with their three-dimensional structure, retain the removal capability even at high temperature and in acidic condition that prevail at the pollution source. The proposed rGO system could provide a complete solution for removal of both FPM and CPM from the pollution source.

Synthesis of a Carbonaceous Two-Dimensional Material.

Despite tremendous accomplishments achieved in 2D materials, little progress has been made in carbonaceous 2D materials beyond graphene and graphene oxide. Here, we report a 2D material of carbonaceous nanoplates (CANP). The bottom-up synthesis of CANP is green, separation-free, and massive. The nanoplates are 2 to 3 monolayers thick with an average interlayer spacing of 0.57 nm. The synthesis involves viscosity-aided two-dimensional growth of fragmented glucose derivatives and leads to the complete conversion of glucose to the 2D nanoplates. Application tests demonstrate the usefulness of the affordable 2D material.

High-Performance Field Emission from a Carbonized Cork.

To broaden the range of application of electron beams, low-power field emitters are needed that are miniature and light. Here, we introduce carbonized cork as a material for field emitters. The light natural cork becomes a graphitic honeycomb upon carbonization, with the honeycomb cell walls 100-200 nm thick and the aspect ratio larger than 100, providing an ideal structure for the field electron emission. Compared to nanocarbon field emitters, the cork emitter produces a high current density and long-term stability with a low turn-on field. The nature of the cork material makes it quite simple to fabricate the emitter. Furthermore, any desired shape of the emitter tailored for the final application can easily be prepared for point, line, or planar emission.

High Power Density Electrochemical Thermocells for Inexpensively Harvesting Low-Grade Thermal Energy.

Continuously operating thermo-electrochemical cells (thermocells) are of interest for harvesting low-grade waste thermal energy because of their potentially low cost compared with conventional thermoelectrics. Pt-free thermocells devised here provide an output power of 12 W m-2 for an interelectrode temperature difference (ΔT) of 81 °C, which is sixfold higher power than previously reported for planar thermocells operating at ambient pressure.

Flow-less and shape-conformable CNT sheet nanogenerator for self-powered motion sensor.

A carbon nanotube (CNT) sheet nanogenerator that does not require any liquid or gas flow for power generation is developed on the basis of Coulombic interactions, making the device attractive as a building block for self-powered sensors. The working principle of the CNT nanogenerator is probed in terms of sweeping speed, distance between charged object and nanotube sheet, surface charge, and number of layers of nanotube sheet. The nature of the CNT sheet and its formation process is such that simply winding the CNT sheet stripe n times around a substrate leads to increasing the power n times. For a practical demonstration of the CNT nanogenerator, a self-powered sensor array screen is developed that can read finger movements, just as with a finger command on a smartphone screen.

Membrane of Functionalized Reduced Graphene Oxide Nanoplates with Angstrom-Level Channels.

Membranes with atomic level pores or constrictions are valuable for separation and catalysis. We report a graphene-based membrane with an interlayer spacing of 3.7 angstrom (Å). When graphene oxide nanoplates are functionalized and then reduced, the laminated reduced graphene oxide (rGO) nanoplates or functionalized rGO membrane is little affected by an intercalated fluid, and the interlayer spacing of 3.7 Å increases only to 4.4 Å in wetted state, in contrast to the graphene oxide (GO) membrane whose interlayer spacing increases from 9 Å to 13 Å in wetted state. When applied to ion separation, this membrane reduced the permeation rate of small ions such as K(+) and Na(+) by three orders of magnitude compared to the GO membrane.

Autonomous Graphene Vessel for Suctioning and Storing Liquid Body of Spilled Oil.

Despite remarkable strides in science and technology, the strategy for spilled oil collection has remained almost the same since the 1969 Santa Barbara oil spill. The graphene vessel devised here can bring about an important yet basic change in the strategy for spilled oil collection. When it is placed on the oil-covered seawater, the graphene vessel selectively separates the oil, then collects and stores the collected oil in the vessel all by itself without any external power inputs. Capillarity and gravity work together to fill this proto-type graphene vessel with the spilled oil at a rate that is higher than 20,000 liters per square meter per hour (LMH) with oil purity better than 99.9%, and allow the vessel to withstand a water head of 0.5 m. The vessel also has a superb chemical stability and recyclability. An expanded oil contact area, considerably greater than the thickness of the oil layer, forms at the reduced graphene oxide (rGO) foam interface upon contact with the spilled oil. This expanded contact area does not change much even when the oil layer thins out. As a result, the high oil collection rate is maintained throughout the recovery of spilled oil.

High-efficiency electrochemical thermal energy harvester using carbon nanotube aerogel sheet electrodes.

Conversion of low-grade waste heat into electricity is an important energy harvesting strategy. However, abundant heat from these low-grade thermal streams cannot be harvested readily because of the absence of efficient, inexpensive devices that can convert the waste heat into electricity. Here we fabricate carbon nanotube aerogel-based thermo-electrochemical cells, which are potentially low-cost and relatively high-efficiency materials for this application. When normalized to the cell cross-sectional area, a maximum power output of 6.6 W m(-2) is obtained for a 51 °C inter-electrode temperature difference, with a Carnot-relative efficiency of 3.95%. The importance of electrode purity, engineered porosity and catalytic surfaces in enhancing the thermocell performance is demonstrated.

A carbon nanotube wall membrane for water treatment.

Various forms of carbon nanotubes have been utilized in water treatment applications. The unique characteristics of carbon nanotubes, however, have not been fully exploited for such applications. Here we exploit the characteristics and corresponding attributes of carbon nanotubes to develop a millimetre-thick ultrafiltration membrane that can provide a water permeability that approaches 30,000 l m(-2) h(-1) bar(-1), compared with the best water permeability of 2,400 l m(-2) h(-1) bar(-1) reported for carbon nanotube membranes. The developed membrane consists only of vertically aligned carbon nanotube walls that provide 6-nm-wide inner pores and 7-nm-wide outer pores that form between the walls of the carbon nanotubes when the carbon nanotube forest is densified. The experimental results reveal that the permeance increases as the pore size decreases. The carbon nanotube walls of the membrane are observed to impede bacterial adhesion and resist biofilm formation.

Carbon nanotube-bonded graphene hybrid aerogels and their application to water purification.

We present carbon nanotube (CNT)-bonded graphene hybrid aerogels that are prepared by growing CNTs on a graphene aerogel surface with nickel catalyst. The presence of bonded CNTs in the graphene aerogel results in vastly improved mechanical and electrical properties. A significant increase in specific surface area is also realized. The presence of the CNTs transforms the hybrid aerogels into a mesoporous material. The viscoelasticity of the hybrid aerogels is found to be invariant with respect to temperature over a range of between -150 °C and 450 °C. These characteristics along with the improved properties make the hybrid aerogels an entirely different class of material with applications in the fields of biotechnology and electrochemistry. The mesoporous nature of the material along with its high specific surface area also makes the hybrid aerogel attractive for application in water treatment. Both anionic and cationic dyes can be effectively removed from water by the hybrid aerogel. A number of organics and oils can be selectively separated from water by the hybrid aerogel. The hybrid aerogel is easy to handle and separate from water due to its magnetic nature, and can readily be recycled and reused.

High performance CNT point emitter with graphene interfacial layer.

Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V µm(-1) at 10 µA cm(-2) and a threshold field of 2.00 V µm(-1) at 10 mA cm(-2), as well as the maximum current of 16 mA (current density of 2300 A cm(-2)).

Plasmon enhanced terahertz emission from single layer graphene.

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, 'nonresonant laser-pulse-induced photon drag currents' appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, 'surface-plasmon-enhanced optical rectification', made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices.

Nanotube aerogel sheet flutter for actuation, power generation, and infrasound detection.

Electromagnetic induction (EMI) is a mechanism of classical physics that can be utilized to convert mechanical energy to electrical energy or electrical to mechanical energy. This mechanism has not been exploited fully because of lack of a material with a sufficiently low force constant. We here show that carbon nanotube (CNT) aerogel sheets can exploit EMI to provide mechanical actuation at very low applied voltages, to harvest mechanical energy from small air pressure fluctuations, and to detect infrasound at inaudible frequencies below 20 Hz. Using conformal deposition of 100 nm thick aluminum coatings on the nanotubes in the sheets, mechanical actuation can be obtained by applying millivolts, as compared with the thousand volts needed to achieve giant-stroke electrostatic actuation of carbon nanotube aerogel sheets. Device simplicity and performance suggest possible applications as an energy harvester of low energy air fluctuations and as a sensor for infrasound frequencies.

Self-powered humidity sensor based on graphene oxide composite film intercalated by poly(sodium 4-styrenesulfonate).

This Research Article reports self-powered humidity sensors based on graphene oxide (GO) and poly(sodium 4-styrenesulfonate) (PSS)-intercalated GO composite films used as the humidity-responsive dielectrics. A hydrophilic and electrically-insulating PSS polymer was used as an intercalant between the individual GO platelets to enhance the water permeation characteristics. Capacitive-type humidity sensors fabricated by forming metal electrodes on both sides of the GO and GO-PSS films were installed into the charge pumping system, which can produce a voltage output as a response to humidity sensing. While both sensors based on GO and GO-PSS dielectrics responded stably and reversibly to the changes in RH, the GO-PSS sensor showed enhanced sensing responses compared to the GO sensor, providing ∼5.6 times higher voltage output and 3 times faster responses in humidity sensing.

Superior rechargeability and efficiency of lithium-oxygen batteries: hierarchical air electrode architecture combined with a soluble catalyst.

The lithium-oxygen battery has the potential to deliver extremely high energy densities; however, the practical use of Li-O2 batteries has been restricted because of their poor cyclability and low energy efficiency. In this work, we report a novel Li-O2 battery with high reversibility and good energy efficiency using a soluble catalyst combined with a hierarchical nanoporous air electrode. Through the porous three-dimensional network of the air electrode, not only lithium ions and oxygen but also soluble catalysts can be rapidly transported, enabling ultra-efficient electrode reactions and significantly enhanced catalytic activity. The novel Li-O2 battery, combining an ideal air electrode and a soluble catalyst, can deliver a high reversible capacity (1000 mAh g(-1) ) up to 900 cycles with reduced polarization (about 0.25 V).

Superstrong encapsulated monolayer graphene by the modified anodic bonding.

We report a superstrong adhesive of monolayer graphene by modified anodic bonding. In this bonding, graphene plays the role of a superstrong and ultra-thin adhesive between SiO2 and glass substrates. As a result, monolayer graphene presented a strong adhesion energy of 1.4 J m(-2) about 310% that of van der Waals bonding (0.45 J m(-2)) to SiO2 and glass substrates. This flexible solid state graphene adhesive can tremendously decrease the adhesive thickness from about several tens of µm to 0.34 nm for epoxy or glue at the desired bonding area. As plausible causes of this superstrong adhesion, we suggest conformal contact with the rough surface of substrates and generation of C-O chemical bonding between graphene and the substrate due to the bonding process, and characterized these properties using optical microscopy, atomic force microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy.

Fibers of reduced graphene oxide nanoribbons.

Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7 V µm(-1)) and a giant field emission current density (400 A cm(-2)). Moreover, the fiber maintains a high current level of 300 A cm(-2) corresponding to 1 mA during long-term operation.

Regulation of morphogenesis and neural differentiation of human mesenchymal stem cells using carbon nanotube sheets.

In order to successfully utilize stem cells for therapeutic applications in regenerative medicine, efficient differentiation into a specific cell lineage and guidance of axons in a desired direction is crucial. Here, we used aligned multi-walled carbon nanotube (MWCNT) sheets to differentiate human mesenchymal stem cells (hMSCs) into neural cells. Human MSCs present a preferential adhesion to aligned CNT sheets with longitudinal stretch parallel to the CNT orientation direction. Cell elongation was 2-fold higher than the control and most of the cells were aligned on CNT sheets within 5° from the CNT orientation direction. Furthermore, a significant, synergistic enhancement of neural differentiation was observed in hMSCs cultured on the CNT sheets. Axon outgrowth was also controlled using nanoscale patterning of CNTs. This CNT sheet provides a new cellular scaffold platform that can regulate morphogenesis and differentiation of stem cells, which could open up a new approach for tissue and stem cell regeneration.