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.

Iron (II/III) perchlorate electrolytes for electrochemically harvesting low-grade thermal energy.

Remarkable advances have recently been made in the thermocell array with series or parallel interconnection, however, the output power from the thermocell array is mainly limited by the electrolyte performance of an n-type element. In this work, we investigate iron (II/III) perchlorate electrolytes as a new n-type electrolyte and compared with the ferric/ferrous cyanide electrolyte at its introduction with platinum as the electrodes, which has been the benchmark for thermocells. In comparison, the perchlorate electrolyte (Fe2+/Fe3+) exhibits a high temperature coefficient of redox potential of +1.76 mV/K, which is complementary to the cyanide electrolyte (Fe(CN)63-/Fe(CN)64-) with the temperature coefficient of -1.42 mV/K. The power factor and figure of merit for the electrolyte are higher by 28% and 40%, respectively, than those for the cyanide electrolyte. In terms of device performance, the thermocell using the perchlorate electrolyte provides a power density of 687 mW/m2 that is 45% higher compared to the same device but with the cyanide electrolyte for a small temperature difference of 20 °C. The advent of this high performance n-type electrolyte could open up new ways to achieve substantial advances in p-n thermocells as in p-n thermoelectrics, which has steered the way to the possibility of practical use of thermoelectrics.

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.

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.

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.

Efficiency improvement of organic solar cells by tuning hole transport layer with germanium oxide.

Improving optical property is critical for optimizing the power conversion efficiency of organic solar cells. In the present research, we show that modification of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer with GeO2 leads to 15% improvement of power conversion efficiency in a polymer solar cells through enhancement of short circuit currents. Modified PEDOT:PSS layer with optimized concentration of GeO2 assists active layer absorbing much light by playing a role of optical spacer. Using AFM and grazing incidence X-ray diffraction (GIXD) data, we also present the evidence that an addition of GeO2 does not affect crystallinity of active layer.

Arrays of Lucius microprisms for directional allocation of light and autostereoscopic three-dimensional displays.

Directional and asymmetric properties are attractive features in nature that have proven useful for directional wetting, directional flow of liquids and artificial dry adhesion. Here we demonstrate that an optically asymmetric structure can be exploited to guide light with directionality. The Lucius prism array presented here has two distinct properties: the directional transmission of light and the disproportionation of light intensity. These allow the illumination of objects only in desired directions. Set up as an array, the Lucius prism can function as an autostereoscopic three-dimensional display.

Wettability-controllable super water- and moderately oil-repellent surface fabricated by wet chemical etching.

A facile fabrication method is presented for a super-repellent surface, in which a silicon wafer is etched with a wet chemical method and treated by a fluorinated self-assembled monolayer. This surface is composed of harshly rough nanostructures and highly dense nanoholes. The contact angle of both water and oil with the surface is larger than 150 degrees. The self-cleaning capability of the surface allows for the removal of sticky powders with glycerin droplet. Any desired part(s) of the super-repellent surface can be turned superamphiphilic by simply exposing the desired part(s) to ultraviolet light.

On the mechanism of low-pressure imprint lithography: capillarity vs viscous flow.

Dominant mechanisms in low-pressure imprint lithography processes have been identified for the regimes that are definable in terms of applied pressure, temperature, and mold material characteristics. Capillarity is found to be the dominant mechanism at high temperature and low pressure when stiff, hard molds are used. In the case of flexible thin-film ( approximately 20 microm) molds, both the capillarity and the viscous flow are involved. Both mechanisms are operative in the initial stage of the imprinting, but the capillarity takes over as time progresses.

Direct UV-replica molding of biomimetic hierarchical structure for selective wetting.

describe a one-step UV-replica molding method for fabricating a biomimetic dual-scale hierarchical structure. The use of UV-curable, acrylate-functionalized perfluoropolyethers allows for a high fidelity replication of a low-energy surface with multiscale texture, thereby directly creating a superhydrophobic surface without any complicated processing. The superhydrophobic surface can simply be transformed selectively into a superhydrophilic surface by exposure to deep ultraviolet light. The prepared surface is inert to chemicals and solvents and maintains its wettability over a long period of time.

Complex pattern formation by adhesion-controlled anisotropic wrinkling.

We report complex pattern formation and shape control in the confinement-induced wrinkling that occurs when a poly(dimethylsiloxane) (PDMS) mold is placed on a bilayer of metal and polymer and then heated. Various complex structures that are different from the mold pattern form through the self-organization of wrinkles. These complex structures could be inverted in shape by manipulating the work of adhesion at the interface between the mold and the metal surface. Convex wrinkles result when the work of adhesion is relatively large. However, inverted concave wrinkles emerge when it is relatively small. The ratio of the mold period to the intrinsic wrinkling wavelength is another factor that determines the shape. The ability to tailor the shape of a surface is expected to have a broad range of applications in electro-optics and microfluidics.

A geometry controllable approach for the fabrication of biomimetic hierarchical structure and its superhydrophobicity with near-zero sliding angle.

A simple, geometry controllable method is presented for fabricating multiscale hierarchical polymer structures that exhibit superhydrophobic water-repellent properties with near-zero sliding angle over a large area. A UV-assisted micromolding technique is used to create a microtexture with an ultraviolet (UV)-curable resin containing Al(2)O(3) nanoparticles. A subsequent treatment of ultraviolet ozone (UVO) leads to the formation of nanoscale roughness over the as-formed microstructured surface, resulting in a dual-scale surface texture similar to a lotus leaf, in a reproducible manner. After hydrophobization with a self-assembled monolayer (SAM) in the liquid phase, this hierarchical surface exhibits stable superhydrophobic characteristics, having a water contact angle close to 160° and a contact angle (CA) hysteresis as low as 1°. These characteristics did not change even after exposure to ambient conditions for 6 months.

Anisotropic rupture of polymer strips driven by Rayleigh instability.

We demonstrate that the separated polymer strips of micro- and sub-micro-length-scales rupture anisotropically along the strip direction, resulting in the formation of distinctly observable, regularly spaced polymer drops. The wavelength of the polymer drops and the surface tension dependence of the rupture behavior are found to be well represented by a relationship derived on the basis of Rayleigh instability. The period is proportional to the square root of the cross-sectional area of the strip and the proportionality constant depends on the contact angle. The rupture of polymer strips into polymer blocks instead of drops, which result when annealed with physically confining walls in place, is found to be well described by the same relationship.

Surface modification of poly(dimethylsiloxane) for retarding swelling in organic solvents.

A method of alleviating swelling problems of poly(dimethysiloxane) (PDMS) molds in organic solvents is developed that allows repeated use of the molds without deleterious solvent effects. The method involves surface modification of PDMS surface with poly(urethaneacrylate) that results in a partially modified PDMS surface. This modification leads to a significant reduction in the rate of solvent absorption into PDMS such that the swelling can be controlled.

Formation of regular nanoscale undulations on a thin polymer film imprinted by a soft mold.

We observed the formation of regular nanoscale undulations on a polystyrene film when imprinted by a soft poly(dimethylsiloxane) mold above the polymer's glass transition temperature. The shape of the wave was reminiscent of a buckling wave frequently observed for a metal film supported on an elastomeric substrate. We derived a simple theoretical model based on an anisotropic buckling of the polymer film rigidly bound to a substrate, which agrees well with the experiment.