Study of interactions between single-wall carbon nanotubes and surfactant using molecular simulations

Carbon nanotubes (CNT) exhibit interesting electrical and mechanical properties. However, the insolubility of CNT in either water or organic solvents, poses serious obstacles to their future applications. The main problems are strong van der Waals attractive interactions and CNT tendency to form bundles which are very difficult to disrupt. In this study, molecular dynamics and quantum mechanics simulations were conducted to investigate the interactions between a carbonaceous nanoparticle and surfactants. It was found that a benzoic ring in the surfactant molecule improves its binding to the graphitic surface. It was shown that a structure of two stacked graphene layers causes a significant straightening of the aliphatic tail of the surfactant molecule adsorbed on the outer graphene layer. Binding energy calculations showed the effect of surfactant structure and CNT diameter on their interaction intensity.     

Ferromagnetism/antiferromagnetism transition between semihydrogenated and fully-aminated single-wall carbon nanotubes

We theoretically studied the ferromagnetism/antiferromagnetism (FM/AFM) transition between single-wall carbon nanotubes (SWCNTs) induced by chemical modifications of semihydrogenation (SH-) and full-amination (NH(2)-). We found that armchairs with large diameters of SH-CNTs (n > 3) possess FM functions with intense magnetic moments, while armchair NH(2)-CNTs (n = 4, 6, 8) are antiferromagnetic semiconductors. The FM/AFM transition is mainly dominated by different chemical modifications and sizes of SWCNTs whose distance between carbon atoms of unpaired electrons can regulate the intensity of p-p spin interactions. Moreover, the zigzag SH-CNTs and NH(2)-CNTs are NM semiconductors. Thus, the electronic and magnetic properties of the SH- or NH(2)-CNTs can be precisely modulated by controlling the hydrogenation or amination on the different types and diameters of CNTs, which provides a new and also simple process for magnetism optimization design in SWCNTs.     

The morphology, electrical conductivity and vapour sensing ability of inkjet-printed thin films of single-wall carbon nanotubes

Thin films containing single-wall carbon nanotubes (SWCNTs) have been prepared using the inkjet printing (IJP) technique. Atomic force microscopy (AFM) has been used to investigate the morphology of these layers. The inkjet printed films consisted of small, randomly-oriented islands of nanotubes, the topography of which was dependent on the nature of the substrate surface. The in-plane electrical characteristics of the films were measured at room temperature. The current versus voltage data exhibited non-linear behaviour, which could be fitted to the theoretical model for Poole-Frenkel conductivity. Preliminary measurements are also reported on the use of the thin layers to detect alcohol vapour.     

Enhanced dispersibility and cellular transmembrane capability of single-wall carbon nanotubes by polycyclic organic compounds as chaperon

The common aggregation of single-wall carbon nanotube (SWCNT) in solution is the critical obstacle to elucidate their unique physico-chemical characteristics and biological properties. Therefore, it is very important to overcome this barrier through manipulation of the weak interaction of small molecules with nanotube surface limited interface. A highly dispersed SWCNT system was achieved by binding with polycyclic organic compounds (POCs) including rhodamine 123, ethidium bromide, fluorescein isothiocyanate and 1-pyrene butyric acid as chaperons, in cooperation with sodium dodecyl sulfate. POCs were believed to penetrate through the interstices of aggregated SWCNTs and bind with individual SWCNTs to form highly dispersed and stable SWCNT-POC-surfactant conjugates in both water and phosphate buffer-serum solution, confirmed by gel electrophoresis, transmission electron microscopy and atomic force microscopy. The possible binding interaction includes π-π stacking with side-wall, electrostatic interactions with defect sites and coating surfactants. Compared to pristine SWCNTs, individual SWCNT-POC conjugates had improved transmembrane passage ability through both endocytosis and diffusion pathways, validated by laser scanning confocal microscopy and micro-Raman mapping techniques. For the applications of SWCNTs in drug delivery, in vitro imaging and other research fields, this novel strategy could provide highly dispersed SWCNTs with better efficiency of drug loading and stability.     

Increasing the electrical conductivity of carbon nanotube/polymer composites by using weak nanotube-polymer interactions

A weak interaction between carbon nanotubes (CNTs) and polymers was found to reduce polymer-wrapping on CNT surface, decrease the contact resistance between CNTs, and increase the electrical conductivity of their composites. Thermodynamic properties such as surface energy of components, filler-polymer interactions, and wettability of carbon/polymer systems were analyzed. It was found that the graphitized CNTs filled polyoxymethylene (POM) system exhibits the weakest CNT-polymer interaction among all the investigated systems and a poor wettability. Consequently, the graphitized CNT/POM composites possess a high electrical conductivity and a low percolation threshold of 0.5 wt.% CNT loading, which is associated with the weak CNT-polymer interaction, low contact resistance between CNTs, good connectivity of CNT networks, and high crystallinity of POM in the composites. The results obtained imply that high-performance composites with optimal CNT-network structures can be designed and fabricated by fully considering the surface properties of components and CNT-polymer interactions.     

Enhanced interactions between multi-walled carbon nanotubes and polystyrene induced by melt mixing

The effect of melt mixing on the interaction between multi-walled carbon nanotubes (MWNTs) and polystyrene (PS) matrix has been investigated. The interaction between pristine MWNTs and PS in solution was found to exist but not strong enough to allow MWNTs to be soluble in solvent. In contrast, this interaction between MWNTs and PS was significantly enhanced by melt mixing, which led to increased amount of PS-functionalized MWNT exhibiting good solubility in some solvents. The mechanism of melt mixing on this enhanced interaction was attributed to both chemical bonding and physical interaction during the melt mixing.     

Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning

Single suspended carbon nanofibers on carbon micro-structures were fabricated by directed electrospinning and subsequent pyrolysis at 900 °C of carbon nanotube/polyacrylonitrile (CNT/PAN) composite material. The electrical conductivity of the nanofibers was measured at different weight fractions of CNTs. It was found that the conductivity increased almost two orders of magnitude upon adding 0.5 wt.% CNTs. The correlation between the extent of graphitization and electrical properties of the composite nanofiber was examined by various structural characterization techniques, and the presence of graphitic regions in pyrolyzed CNT/PAN nanofibers was observed that were not present in pure PAN-derived carbon. The influence of fabrication technique on the ordering of carbon sheets in electrospun nanofibers was examined and a templating effect by CNTs that leads to enhanced graphitization is suggested.     

A relationship between electrical conductivity and photodegradation in styrene-butadiene copolymer/multi-wall carbon nanotube composite

An effect of photodegradation on electrical conductivity of a styrene-butadiene copolymer (SBR)/multiwall carbon nanotube (MWNT) composite was studied with a TiO₂/polyethylene oxide/methyl linoleate paint photocatalyst under UV and/or visible light irradiation. An oxidative etching of impurities on the MWNT surface was caused by the UV or visible light irradiation, leading to an increase of quality of MWNT. On the other hand, the photocatalyst addition caused the degradation of MWNT structure. A relationship between the electrical conductivity and MWNT content showed that the MWNT dispersity in a SBR was superior to that in a polystyrene (PS). In addition, the PS addition to SBR matrix caused MWNT aggregation. The electrical conductivity decrease of the MWNT composite was due to electrical percolation structure loss caused by the photocatalyst under the visible light irradiation, and its rate depended on the MWNT dispersity. The PS molecular weight change behavior with the photocatalyst was consistent with the electrical conductivity one of the SBR/MWNT. The photocatalyst ability was estimated from the electrical conductivity of the SBR/MWNT.     

Synthesis of single-wall carbon nanotubes and long nanotube ribbons with Ho/Ni as catalyst by arc discharge

The effect of a new bimetallic catalyst Ho/Ni for synthesis of single-walled carbon nanotubes (SWNTs) by arc discharge has been studied. Long ribbons consisting of roughly-aligned SWNT bundles were obtained by a modified arc discharge apparatus. Ribbon lengths can reach as much as 20 cm. Both elements Ho and Ni play important roles in the synthesis of SWNTs with high yield and purity. Changes in the Ho and Ni concentration in the catalyst hardly affect the diameter distribution of SWNTs, but the yield and purity of SWNTs are very sensitive to the concentration. An optimal range of Ho/Ni compositions for synthesis of SWNTs with relatively high purity and yield is given.     

Efficiency of C60 incorporation in and release from single-wall carbon nanotubes depending on their diameters

We show that the efficiency of incorporating C₆₀ in single-wall carbon nanotubes (SWCNTs) and that of the incorporated C₆₀’s release from the SWCNTs depend on the SWCNT diameter. Through transmission electron microscopy, we found that the C₆₀ incorporation efficiency reached its maximum at diameters of 1-2 nm, while the efficiency of C₆₀ release from SWCNTs in toluene was maximized at 3-5 nm. The difficulty of C₆₀ release from SWCNTs with diameters of 5-6 nm might reflect either the effective packing of C₆₀ inside SWCNTs or a flattened SWCNT structure. We occasionally observed C₆₀ molecules arranged in a line along the sidewall inside SWCNTs with large diameters/width (>7 nm), indicating that large diameter SWCNTs were sometimes flattened.     

Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition

We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.     

Effects of carbon nanotubes aspect ratio on the qualitative and quantitative aspects of frequency response of electrical conductivity and dielectric permittivity in the carbon nanotube/polymer composites

To study the effect of carbon nanotube aspect ratio (AR) on the frequency response of the electrical properties, the alternating current (AC) electrical conductivity and dielectric permittivity of different AR multi-wall carbon nanotubes (MWCNTs)/thermoplastic elastomer (TPE) composites were studied in the AC frequency range of 100 Hz to 10  MHz. Qualitatively, the effect of frequency on the electrical properties of the composites was the same for all AR MWCNTs and shared many typical features of electrically percolative composites. Quantitatively, the frequency responses of electrical properties were found to be independent of nominal AR, concentration, percolation threshold, and the diameter of the MWCNT. Instead, frequency response of electrical properties was dependent on the MWCNT length and initial electrical conductivity of the composites. With the same initial conductivity of the MWNT composites, frequency-conductivity sensitivity varied inversely with the nominal length of the MWCNTs. Composites with MWCNTs of the same nominal length and similar electrical conductivity values, regardless of whether the MWCNT concentration was below or above the percolation threshold, exhibited quantitatively similar frequency-conductivity sensitivity. The frequency-dielectric sensitivity at the percolation threshold was a reflection of frequency-conductivity sensitivity and was also found to be dependent on the initial conductivity of the composites.     

Low electrical conductivity threshold and crystalline morphology of single-walled carbon nanotubes - high density polyethylene nanocomposites characterized by SEM, Raman spectroscopy and AFM

The electrical conductivities (σ) of nanocomposites of single-walled carbon nanotubes (SWCNTs) and high density polyethylene (HDPE) have been studied for a large number of nanocomposites prepared in a SWCNT concentration range between 0.02 and 8 wt%. The values of σ obey a percolation power law with an SWCNT concentration threshold, p_c = 0.13 wt%, the lowest yet obtained for any kind of carbon-polyethylene nanocomposites. Improved electrical conductivities attest to an effective dispersion of SWCNT in the polyethylene matrix, enabled by the fast quenching crystallization process used in the preparation of these nanocomposites. Characterization by scanning electron microscopy (SEM) and Raman spectroscopy consistently points to a uniform dispersion of separate small SWCNT bundles at concentrations near p_c and increased nanotube clustering at higher concentrations. Near p_c, high activation energies and geometries of long isolated rods suggest that electron transport occurs by activated electron hopping between nanotubes that are close to each other but still geometrically separate. The degree of SWCNT clustering given by Raman spectroscopy and the barrier energy for electrical conductivity are highly correlated. The nanotubes act as nucleants in the crystallization of the polyethylene matrix, and change the type of supermolecular aggregates from spherulites to axialitic-like objects. The size of crystal aggregates decreases with SWCNT loading, however, in reference to the unfilled polyethylene, the three-dimensional growth geometry extracted from the Avrami exponents remains unchanged up to 2 wt%. Consistency between SEM, Raman and electrical transport behavior suggests that the electrical conductivity is dominated by dispersion and the geometry of the SWCNT in the nanocomposites and not by changes or lack thereof in the HDPE semicrystalline structure.     

Enhanced electrochemical stability and charge storage of MnO2/carbon nanotubes composite modified by polyaniline coating layer in acidic electrolytes

Manganese dioxide/multiwalled carbon nanotubes (MnO₂/MWCNTs) were synthesized by chemically depositing MnO₂ onto the surface of MWCNTs wrapped with poly(sodium-p-styrenesulfonate). Then, polyaniline (PANI) with good supercapacitive performance was further coated onto the MnO₂/MWCNTs composite to form PANI/MnO₂/MWCNTs organic-inorganic hybrid nanoarchitecture. Electrochemical performance of the hybrid in Na₂SO₄-H₂SO₄ mixed acidic electrolytes was evaluated by cyclic voltammetry (CV) and chronopotentiometry (CP) in detail. Comparative electrochemical tests revealed that the hybrid nanoarchitecture could operate in the acidic medium due to the protective modification of PANI coating layer onto the MnO₂/MWCNTs composite, and that its electrochemical behavior was greatly dependent upon the concentration of protons in the acidic electrolytes. Here, PANI not only served as a physical barrier to restrain the underlying MnO₂/MWCNTs composite from reductive-dissolution process so as to make the novel ternary hybrid material work in acidic medium to enhance the utilization of manganese oxide as much as possible, but also was another electroactive material for energy storage in the acidic mixed electrolytes. It was due to the existence of PNAI layer that an even larger specific capacitance (SC) of 384 F g⁻¹ and a much better SC retention of 79.9% over 1000 continuous charge/discharge cycles than those for the MnO₂/MWCNTs nanocomposite were delivered for the hybrid in the optimum 0.5 M Na₂SO₄-0.5 M H₂SO₄ mixed acidic electrolyte.     

Synthesis of carbon nanotube/epoxy composite films with a high nanotube loading by a mixed-curing-agent assisted layer-by-layer method and their electrical conductivity

A mixed-curing-agent assisted layer-by-layer method is reported to synthesize carbon nanotube (CNT)/epoxy composite films with a high CNT loading from ∼15 to ∼36 wt.%. The mixed-curing-agent consists of two types of agents, one of which is responsible for the partial initial curing at room temperature to avoid agglomeration of the CNTs, and the other for complete curing of epoxy resin at high temperature to synthesize epoxy composite films with good CNT dispersion. The electrical conductivity of the composites shows a value up to ∼12 S/m, which is much higher than that for CNT/epoxy composites with a low CNT loading prepared using conventional methods.     

Advanced water treatment of high turbid source by hybrid module of ceramic microfiltration and activated carbon adsorption: Effect of organic/inorganic materials

We investigated the effect of organic or inorganic materials on membrane fouling in advanced drinking water treatment by hybrid module packed with granular activated carbon (GAC) outside a tubular ceramic microfiltration membrane. Instead of natural organic matters (NOM) and fine inorganic particles in the natural water source, synthetic water was prepared with humic acid and kaolin. Concentrations of kaolin or humic acid were changed to see effects of inorganic or organic matter. And periodic water-back-flushing using permeate water was performed during 10 sec per filtration of 10 min. As a result, both the resistance of membrane fouling (R _f ) and permeate flux (J) were influenced higher by concentration of humic acid rather than kaolin. It was proved that NOM like humic acid could be a more important factor on membrane fouling in drinking water treatment than fine inorganic particles. Treatment efficiencies of turbidity and UV₂₅₄ absorbance were very high above 97.4% and 92.0%, respectively. This article is dedicated to Professor Chang Kyun Choi for celebrating his retirement from the School of Chemical and Biological Engineering, Seoul National University.     

Densification behavior and microstructure of carbon/carbon composites prepared by chemical vapor infiltration from xylene at temperatures between 900 and 1250 °C

Carbon/carbon composites were prepared by film boiling chemical vapor infiltration from xylene pyrolysis. Their densification behaviors such as the mass gain, the deposition rate and the density profile were investigated. The microstructure was studied by polarized light microscopy and characterized quantitatively with average extinction angle (Ae). Results showed that, under the particular experimental equipment and process, the initial deposition rate and the average Ae of pyrocarbon (PyC) increased with the increasing deposition temperature (T_d). The structural transition of PyC from rough laminar (RL) to smooth laminar along both the axial and radial directions of the composites was retarded as T_d increased from 900 to 1100 °C; PyC was deposited by the heterogeneous nucleation and growth. The homogeneous nucleation was generated producing isotropic PyC at the bottom of the composites for 1200–1250 °C deposition. The matrix produced at 1100–1250 °C was dominated by RL PyC, and the composites with high average density and uniform RL matrix were rapidly produced for T_d around 1100 °C.     

A novel nickel/carbon catalyst for CH4 and H2 production from organic compounds dissolved in wastewater by catalytic hydrothermal gasification

A novel Ni/carbon catalyst recently developed by the authors was used to gasify organic compounds dissolved in the wastewater with TOC concentration from 0.2 to 2%. The process removes the organic compounds by gasifying them into high calorific gases like methane and hydrogen. The investigations were focused on the efficiency of the Ni/carbon catalyst in terms of carbon conversion, conversion of big organic molecules, and catalyst deactivation due to sintering. The preliminary results showed that up to 99% carbon conversion can be achieved at 360 °C, and 20 MPa. A conversion mechanism was suggested which consists of: first, decomposition of big molecules to small molecules on the metal surface, steam gasification of small molecules to produce CO and H₂ followed by CO methanation and CO shift reaction to produce CH₄ and CO₂. The catalyst was found to be highly active and stable and no sintering was observed even after 100 h of reaction time.     

Relationship study between crystal structure and thermal/mechanical properties of polyamide 6 reinforced and unreinforced by carbon fiber from macro and local view

Under effect of annealing and reinforcement of carbon fiber (CF), the relationship between crystalline structure of polyamide 6 (PA6) and thermal/mechanical properties was well studied. A local thermal analysis (LTA) method was applied for detecting interface properties between CF and PA6. Significant enhancement of thermal properties of PA6 with annealing and CF reinforcement was confirmed by thermal conductivity, HDT, and tanδ-T DMA curves, which was related with 112% crystallinity improvement by annealing and 105% improvement by CF reinforcement, and crystalline structure variation partly. In LTA results, thermal property near the CF area was improved due to a transcrystalline layer formation, which indicates LTA has great potential to be applied on the interface analysis in composites. Furthermore, flexural strength and modulus got a different degree of improvement by CF, as well as the storage modulus and impact strength, which proved the effective enhancement of strength and toughness of PA6 with strengthening of CF.