Janus Graphene Oxide Sponges for High-Purity Fast Separation of Both Water-in-Oil and Oil-in-Water Emulsions.

Membrane separation of oil and water with high purity and high permeability is of great interest in environmental and industrial processes. However, membranes with fixed wettability can separate only one type of surfactant-stabilized emulsion (water-in-oil or oil-in-water). Here, we report on Janus graphene oxide (J-GO) sponges for high purity and high permeability separation of both water-in-oil and oil-in-water emulsions. Millimeter-scale reduced GO sponges with a controlled pore size (11.2 or 94.1 µm) are synthesized by freeze drying, and the wettability is further controlled by fluorine (hydrophobic/oleophilic in air) or oxygen (hydrophilic/oleophilic in air) functionalization. J-GO sponges are prepared by the fluorine functionalization on one side and oxygen functionalization on the other side. Interestingly, the oil wettability of oxygen-functionalized surface turns into an oleophobic surface when immersed in water, which is explained by Young's theory. This effect is further used in the separation of both water-in-oil and oil-in-water emulsions by changing the flow direction. The purity of the separated oil and water is very high (≥99.2%), and the permeability is more than an order of magnitude greater than those of the other Janus membranes reported. J-GO sponges can be reused with an excellent repeatability, demonstrating feasibility in practical applications.

Enhanced water vapor separation by temperature-controlled aligned-multiwalled carbon nanotube membranes.

Here we present a new strategy of selectively rejecting water vapor while allowing fast transport of dry gases using temperature-controlled aligned-multiwalled carbon nanotubes (aligned-MWNTs). The mechanism is based on the water vapor condensation at the entry region of nanotubes followed by removing aggregated water droplets at the tip of the superhydrophobic aligned-MWNTs. The first condensation step could be dramatically enhanced by decreasing the nanotube temperature. The permeate-side relative humidity was as low as ∼17% and the helium-water vapor separation factor was as high as 4.62 when a helium-water vapor mixture with a relative humidity of 100% was supplied to the aligned-MWNTs. The flow through the interstitial space of the aligned-MWNTs allowed the permeability of single dry gases an order of magnitude higher than the Knudsen prediction regardless of membrane temperature. The water vapor separation performance of hydrophobic polytetrafluoroethylene membranes could also be significantly enhanced at low temperatures. This work combines the membrane-based separation technology with temperature control to enhance water vapor separation performance.

Reverse capillary flow of condensed water through aligned multiwalled carbon nanotubes.

Molecular transport through nanopores has recently received considerable attention as a result of advances in nanofabrication and nanomaterial synthesis technologies. Surprisingly, water transport investigations through carbon nanochannels resulted in two contradicting observations: extremely fast transport or rejection of water molecules. In this paper, we elucidate the mechanism of impeded water vapor transport through the interstitial space of aligned multiwalled carbon nanotubes (aligned-MWCNTs)--capillary condensation, agglomeration, reverse capillary flow, and removal by superhydrophobicity at the tip of the nanotubes. The origin of separation comes from the water's phase change from gas to liquid, followed by reverse capillary flow. First, the saturation water vapor pressure is decreased in a confined space, which is favorable for the phase change of incoming water vapor into liquid drops. Once continuous water meniscus is formed between the nanotubes by the adsoprtion and agglomeration of water molecules, a high reverse Laplace pressure is induced in the mushroom-shaped liquid meniscus at the entry region of the aligned-MWCNTs. The reverse Laplace pressure can be significantly enhanced by decreasing the pore size. Finally, the droplets pushed backward by the reverse Laplace pressure can be removed by superhydrophobicity at the tip of the aligned-MWCNTs. The analytical analysis was also supported by experiments carried out using 4 mm-long aligned-MWCNTs with different intertube distances. The water rejection rate and the separation factor increased as the intertube distance decreased, resulting in 90% and 10, respectively, at an intertube distance of 4 nm. This mechanism and nanotube membrane may be useful for energy-efficient water vapor separation and dehumidification.

The effect of different temperature profiles upon the length and crystallinity of vertically-aligned multi-walled carbon nanotubes.

We synthesized vertically-aligned multi-walled carbon nanotubes with an inner diameter of 1.6-7.5 nm and stack height of 80-28600 microm by chemical vapor deposition. The effects of synthesis conditions such as substrate position in the tube furnace, maximum temperature, temperature increasing rate and synthesis duration on the structure of nanotubes were investigated. It was found that slightly faster temperature increase rate resulted in significantly longer length, larger diameter and more defects of nanotubes. Structural parameters such as inner, outer diameters, wall thickness and defects were investigated using transmission electron microscopy and Raman spectroscopy.

Enhanced condensation, agglomeration, and rejection of water vapor by superhydrophobic aligned multiwalled carbon nanotube membranes.

The separation of gas molecules and water vapor has become increasingly important for electronic, energy, and environmental systems. Here we demonstrate a new mechanism of enhanced condensation, agglomeration, and rejection of water vapor by superhydrophobic aligned multiwalled carbon nanotubes with the intertube distance of 73 nm, channel aspect ratio of ~5.5 × 10(4), and tortuosity of 1.157. The array with the characteristic channel dimension some 300 times greater than the target molecule size effectively suppressed water molecular transport at room temperature with the selectivity as high as ~2 × 10(5) (H(2)/H(2)O). The flow through the interstitial space of nanotubes allowed high permeability of other gas molecules (2.1 × 10(-9) to 3.8 × 10(-8) mol · m/m(2) · s · Pa), while retaining high selectivity, which is orders of magnitude greater than the permeate flux of polymeric membranes used for the water-gas mixture separation. This new separation mechanism with high selectivity and permeate flux, enabled by the unique geometry of aligned nanotubes, can provide a low-energy and cost-effective method to control humidity.

Hierarchical construction of self-standing anodized titania nanotube arrays and nanoparticles for efficient and cost-effective front-illuminated dye-sensitized solar cells.

We report on the influence of hierarchical structures, constructed via layer-by-layer assembly of self-standing titania nanotube arrays and nanoparticles, upon charge recombination and photoelectric performance of front-illuminated dye-sensitized solar cells. Both nanotubes and nanoparticles were produced by anodization rather than additionally employing other methods, providing low cost and great simplicity. Electrochemical impedance spectroscopy under AM 1.5 illumination indicates the construction of hybrid morphology has superior recombination characteristics and a longer electron lifetime than nanoparticulate systems. This enhancement with the incorporation of anodized titania nanoparticles with 1D architectures is unprecedented for solar cells. Owing to the better light harvesting efficiency, extended electron lifetime and desirable electron extraction, the short-circuit photocurrent density of solar cell is 18.89 mA cm(-2) with an overall power conversion efficiency of 8.80% and an incident photon-to-current conversion efficiency of 84.6% providing a very promising candidate for sustainable energy production with a high performance/cost ratio.