Recent developments in carbon nanotube membranes for water purification and gas separation

Carbon nanotubes (CNTs) are nanoscale cylinders of graphene with exceptional properties such as high mechanical strength, high aspect ratio and large specific surface area. To exploit these properties for membranes, macroscopic structures need to be designed with controlled porosity and pore size. This manuscript reviews recent progress on two such structures: (i) CNT Bucky-papers, a non-woven, paper like structure of randomly entangled CNTs, and (ii) isoporous CNT membranes, where the hollow CNT interior acts as a membrane pore. The construction of these two types of membranes will be discussed, characterization and permeance results compared, and some promising applications presented.

NMR studies of modified nasicon-like, lithium conducting solid electrolytes*1

²⁷Al, ³¹P and ⁷Li NMR measurements have been performed on lithium conducting ceramics based on the LiTi₂(PO₄)₃ structure with Al, V and Nb metal ions substituted for either Ti or P within the framework NASICON structure. The ²⁷Al magic angle spinning NMR measurements have revealed that, although Al is intended to substitute for octahedral Ti sites, additional substitution into tetrahedral environments (presumably phosphorous sites) occurs with increasing amount of Al addition. This tetrahedral substitution appears to occur more readily in the presence of vanadium, in Li_1+xAl_xTi_2−x(PO₄)_2.9(VO₄)_0.1, whereas similar niobium additions (in place of vanadium) appear to stifle tetrahedral substitution. ⁷Li static NMR spectra reveal quadrupolar structure with C_q approximately 42 kHz, largely independent of substitution. Measurement of the ⁷Li central transition linewidth at room temperature reveals a relatively mobile lithium species (300–900 Hz) with linewidth tending to decrease with Al substitution and increase with increasing V or Nb. This new structural information is discussed in the context of ionic conduction in these ceramics.

Tailoring the surface chemistry of carbon fiber and E-glass composites for improved adhesion

The main challenges in the manufacture of composite materials are low surface energy and the presence of silicon-containing contaminants, both of which greatly reduce surface adhesive strength. In this study, carbon fiber (CF) and E-glass epoxy resin composites were surface treated with the Accelerated Thermo-molecular adhesion Process (ATmaP). ATmaP is a multiaction surface treatment process where tailored nitrogen and oxygen functionalities are generated on the surface of the sample through the vaporization and atomization of n-methylpyrrolidone solution, injected via specially designed flame-treatment equipment. The treated surfaces of the polymer composites were analyzed using XPS, time of flight secondary ion mass spectrometry (ToF-SIMS), contact angle (CA) analysis and direct adhesion measurements. ATmaP treatment increased the surface concentration of polar functional groups while reducing surface contamination, resulting in increased adhesion strength. XPS and ToF-SIMS showed a significant decrease in silicon-containing species on the surface after ATmaP treatment. E-glass composite showed higher adhesion strength than CF composite, correlating with higher surface energy, higher concentrations of nitrogen and CO functional groups (from XPS) and higher concentrations of oxygen and nitrogen-containing functional groups (particularly C₂H₃O⁺ and C₂H₅NO⁺ molecular ions, from ToF-SIMS).

Azide photochemistry for facile modification of graphitic surfaces : preparation of DNA-coated carbon nanotubes for biosensing

A facile, two-step method for chemically attaching single-stranded DNA to graphitic surfaces, represented here by carbon nanotubes, is reported. In the first step, an azide-containing compound, N-5-azido-nitrobenzoyloxy succinimide (ANB-NOS), is used to form photo-adducts on the graphitic surfaces in a solid-state photochemical reaction, resulting in active ester groups being oriented for the subsequent reactions. In the second step, pre-synthesized DNA strands bearing a terminal amine group are coupled in an aqueous solution with the active esters on the photo-adducts. The versatility of the method is demonstrated by attaching pre-synthesized DNA to surfaces of carbon nanotubes in two platforms—as vertically-aligned multi-walled carbon nanotubes on a solid support and as tangled single-walled carbon nanotubes in mats. The reaction products at various stages were characterized by x-ray photoelectron spectroscopy. Two different assays were used to check that the DNA strands attached to the carbon nanotubes were able to bind their partner strands with complementary base sequences. The first assay, using partner DNA strands tethered to gold nanoparticles, enabled the sites of DNA attachment to the carbon nanotubes to be identified in TEM images. The second assay, using radioactively labelled partner DNA strands, quantified the density of functional DNA strands attached to the carbon nanotubes. The diversity of potential applications for these DNA-modified carbon-nanotube platforms is exemplified here by the successful use of a DNA-modified single-walled carbon-nanotube mat as an electrode for the specific detection of metal ions.

Control of porosity and pore size of metal reinforced carbon nanotube membranes

Membranes are crucial in modern industry and both new technologies and materials need to be designed to achieve higher selectivity and performance. Exotic materials such as nanoparticles offer promising perspectives, and combining both their very high specific surface area and the possibility to incorporate them into macrostructures have already shown to substantially increase the membrane performance. In this paper we report on the fabrication and engineering of metal-reinforced carbon nanotube (CNT) Bucky-Paper (BP) composites with tuneable porosity and surface pore size. A BP is an entangled mesh non-woven like structure of nanotubes. Pure CNT BPs present both very high porosity (>90%) and specific surface area (>400 m2/g). Furthermore, their pore size is generally between 20–50 nm making them promising candidates for various membrane and separation applications. Both electro-plating and electroless plating techniques were used to plate different series of BPs and offered various degrees of success. Here we will report mainly on electroless plated gold/CNT composites. The benefit of this method resides in the versatility of the plating and the opportunity to tune both average pore size and porosity of the structure with a high degree of reproducibility. The CNT BPs were first oxidized by short UV/O3 treatment, followed by successive immersion in different plating solutions. The morphology and properties of these samples has been investigated and their performance in air permeation and gas adsorption will be reported.

Poly(triazine imide) with intercalation of Lithium and ChlorideiIons [(C3N3)2(NHxLi1−x)3⋅LiCl]: A Crystalline 2D Carbon Nitride network

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li+Cl−) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li+Cl− resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li+Cl− a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ2) of 5.035 and wRp=0.05937. PTI/Li+Cl− (P63cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li+ and Cl− ions. The presence of salt ions in the nanocrystallites as well as the existence of sp2-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using 15N-labeled PTI/Li+Cl− proved the absence of heptazine building blocks and NH2 groups and corroborated the highly condensed, triazine-based structure model.

Synthesis of single-crystalline LiFePO4 with rhombus-like morphology

In this investigation, carbon-coated LiFePO4 cathode materials were synthesized with a facile hydrothermal method. The structure and electrochemical properties of the materials were investigated by X-ray diffraction (XRD), Roman, transmission electron microscopy-energy dispersive spectroscopy (TEM-EDS), and electrochemical impedance spectroscopy (EIS). By adjusting the mixing concentration of starting materials, a single-crystalline LiFePO4 with an anisotropic rhombus morphology (Space Group: Pmnb No. 62) were successfully synthesized. In addition, the carbon coated on the surface of LiFePO4 material prepared has a lower ID/IG (0.80), which indicates an optimized carbon structure with an increased amount of sp2-type carbon. Electrochemical performance test shows that the carbon-coated LiFePO4 cathode materials have an initial discharge capacity of 146 mAh g−1 at 0.2C.

Temperature-dependent Raman spectra of bamboo-like boron nitride nanotubes

Phonon properties of boron nitride nanotubes (BNNTs) were investigated using Raman spectroscopy at different temperatures and new sp3- bonded BN vibrations were identified. The Raman peak of the E2g mode of BNNTs is found to be downshifted and broadened compared to that of hexagonal BN at the same temperature. By increasing the temperature, the energy of the E2g mode and the sp3-bonding mode are downshifted, with the temperature coefficients being -0.010 and -0.069cm-1/K, respectively. We attribute this downshifting to anharmonic effects as well as the elongation of the B-N bond in BNNT structures with increasing temperature. © 2014 The Japan Society of Applied Physics.

Epoxy nanocomposites containing magnetite-carbon nanofibers aligned using a weak magnetic field

Abstract Novel magnetite-carbon nanofiber hybrids (denoted by Fe3O4@CNFs) have been developed by coating carbon nanofibers (CNFs) with magnetite nanoparticles in order to align CNFs in epoxy using a relatively weak magnetic field. Experimental results have shown that a weak magnetic field (∼mT) can align these newly-developed nanofiber hybrids to form a chain-like structure in the epoxy resin. Upon curing, the epoxy nanocomposites containing the aligned Fe3O4@CNFs show (i) greatly improved electrical conductivity in the alignment direction and (ii) significantly higher fracture toughness when the Fe3O4@CNFs are aligned normal to the crack surface, compared to the nanocomposites containing randomly-oriented Fe3O4@CNFs. The mechanisms underpinning the significant improvements in the fracture toughness have been identified, including interfacial debonding, pull-out, crack bridging and rupture of the Fe3O4@CNFs, and plastic void growth in the polymer matrix.

Improving the toughness and electrical conductivity of epoxy nanocomposites by using aligned carbon nanofibres

There is an increasing demand for high performance composites with enhanced mechanical and electrical properties. Carbon nanofibres offer a promising solution but their effectiveness has been limited by difficulty in achieving directional alignment. Here we report the use of an alternating current (AC) electric field to align carbon nanofibres in an epoxy. During the cure process of an epoxy resin, carbon nanofibres (CNFs) are observed to rotate and align with the applied electric field, forming a chain-like structure. The fracture energies of the resultant epoxy nanocomposites containing different concentrations of CNFs (up to 1.6wt%) are measured using double cantilever beam specimens. The results show that the addition of 1.6wt% of aligned CNFs increases the electrical conductivity of such nanocomposites by about seven orders of magnitudes to 10^-2S/m and increases the fracture energy, G<inf>Ic</inf>, by about 1600% from 134 to 2345J/m². A modelling technique is presented to quantify this major increase in the fracture energy with aligned CNFs. The results of this research open up new opportunities to create multi-scale composites with greatly enhanced multifunctional properties.

Adsorption kinetics and optimum conditions for Cr(VI) removal by activated carbon prepared from luffa sponge

Activated carbon (AC) prepared from luffa sponge was firstly used as an adsorbent to remove Cr(VI) from aqueous solution. The Cr(VI) adsorption behaviors of AC under different conditions, including initial Cr(VI) concentration, quantity of AC, solution pH, and temperature were investigated. The optimal conditions for adsorption of Cr(VI) by AC were pH = 1, initial Cr(VI) concentration = 80 mg/L, T = 303 K, and AC content = 1.6 g/L. The adsorption kinetics could be described by the pseudo-second-order model. Fourier transform infrared spectroscopy was used to investigate the sorption mechanism. Some functional groups such as C–O and O–H were formed on the carbon surface, which could then react with Cr(VI). The surface structure of AC before and after adsorption was analyzed by scanning electronic microscopy. Adsorbed ions choked some of the pores in AC after adsorption. The Brunauer–Emmett–Teller surface area and average pore size of the AC were 834.13 m2/g and 5.17 nm, respectively. The maximum adsorption of Cr(VI) by AC was 149.06 mg/g, which makes AC prepared from luffa sponge promising for removing Cr(VI) from wastewater.

Graphene quantum dots directly generated from graphite via magnetron sputtering and the application in thin-film transistors

This work presents a novel method to prepare graphene quantum dots (GQDs) directly from graphite. A composite film of GQDs and ZnO was first prepared using the composite target of graphite and ZnO via magnetron sputtering, followed with hydrochloric acid treatment and dialysis. Morphology and optical properties of the GQDs were investigated using a number of techniques. The as-prepared GQDs are 4-12 nm in size and 1-2 nm in thickness. They also exhibited typical excitation-dependent properties as expected in carbon-based quantum dots. To demonstrate the potential applications of GQDs in electronic devices, pure ZnO and GQD-ZnO thin-film transistors (TFTs) using ZrOx dielectric were fabricated and examined. The ZnO TFT incorporating the GQDs exhibited enhanced performance: an on/off current ratio of 1.7 × 107, a field-effect mobility of 17.7 cm2/Vs, a subthreshold swing voltage of 90 mV/decade. This paper provides an efficient, reproducible and eco-friendly approach for the preparation of monodisperse GQDs directly from graphite. Our results suggest that GQDs fabricated using magnetron sputtering method may envision promising applications in electronic devices.

Combination of calcium and magnesium ions prevents substrate inhibition and promotes biomass and lipid production in thraustochytrids under higher glycerol concentration

Effect of calcium and magnesium ions was studied in detail in batch mode in shake flask cultures of two fast growing strains of thraustochytrids (Aurantiochytrium sp. DBTIOC-18 and Schizochytrium sp. DBTIOC-1) for biomass and lipid production. These strains were previously isolated from Indian marine biodiversity. Screening of these two strains on different carbon and nitrogen sources revealed the suitability of glycerol over glucose and sodium nitrate over yeast extract for the cultivation of these strains. The presence of higher concentration of glycerol in the medium inhibited the glycerol utilization by the cell thus resulting in lower biomass and lipid production in both the strains. Supplementing media with calcium and magnesium ions promoted glycerol utilization thus resulted in a substantial rise in volumetric production of biomass (55.12 g L-1, 48.12 g L-1), fatty acid for biodiesel (27.14 g L-1, 22.15 g L-1) and docosahexaenoic acid (14.57 g L-1, 10.12 g L-1) with both strains Aurantiochytrium sp. DBTIOC-18 and Schizochytrium sp. DBTIOC-1, respectively. Growth profile study of these two strains showed further improvement in production of biomass, fatty acid for biodiesel and docosahexaenoic acid when cultures were extended up to 7 days. Finding of this work underlines the importance of calcium and magnesium salts in designing new fermentation strategies to prevent substrate inhibition and achieve high cell density culture under high nutrient concentration especially carbon sources.