Characterization and application of hydrophobin-dispersed multi-walled carbon nanotubes

Hydrophobins are amphiphilic proteins with high surface activity, which can readily adsorb on interfacial surfaces, especially on hydrophobic surfaces. Based on their properties, we used the class I hydrophobin isolated from Grifola frondosa (HGFI) to disperse multi-walled carbon nanotubes (MWCNTs) in water. MWCNTs could be effectively dispersed by 30-min sonication in a 0.1 mg/ml HGFI solution. Optical absorption and transmission electron microscopy provide evidence for individually stable dispersed MWCNTs. X-ray photoelectron, Fourier transform infrared, and Raman spectroscopies and thermogravimetric analysis suggest that HGFI can non-covalently bind to MWCNTs through hydrophobic interaction, rendering them hydrophilic. A quartz crystal microbalance and immunological sandwich assay were used to demonstrate that the HGFI-coated MWCNTs can be used to immobilize human immunoglobulin G in solution.     

Controlling the dispersion of multi-wall carbon nanotubes in aqueous surfactant solution

The sonication-driven dispersion of multi-wall carbon nanotubes (MWCNTs) in aqueous surfactant solution has been monitored by UV-vis spectroscopy and transmission electron microscopy. Time dependent sonication experiments reveal that the maximum achievable dispersion of MWCNTs corresponds to the maximum UV-vis absorbance of the solution. With higher surfactant concentration the dispersion rate of MWCNTs increases and less total sonication energy is required to achieve maximum dispersion. Dispersion of higher MWCNT concentrations requires higher total sonication energy. For effective dispersion the minimum weight ratio of surfactant to MWCNTs is 1.5-1. The surfactant molecules are adsorbed on the surface of the MWCNTs and prevent re-aggregation of MWCNTs so that a colloidal stability of MWCNT dispersions could be maintained for several months. The maximum concentration of MWCNTs that can be homogeneously dispersed in aqueous solution is about 1.4 wt%.     

Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites

A homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in syndiotactic polystyrene (sPS) is obtained by a simple solution dispersion procedure. MWCNTs were dispersed in N-methyl-2-pyrrolidinone (NMP), and sPS/MWCNT composites are prepared by mixing sPS/NMP solution with MWCNT/NMP dispersion. The composite structure is characterized by scanning electron microscopy and transmission electron microscopy. The effect of MWCNTs on sPS crystallization and the composite properties are studied. The presence of MWCNTs increases the sPS crystallization temperature, broadens the crystallite size distribution and favors the formation of the thermodynamically stable β phase, whereas it has little effect on the sPS γ to α phase transition during heating. By adding only 1.0 wt.% pristine MWCNTs, the increase in the onset degradation temperature of the composite can reach 20 °C. The electrical conductivity is increased from 10^{−10∼−16} (neat sPS) to 0.135 S m⁻¹ (sPS/MWCNT composite with 3.0 wt.% MWCNT content). Our findings provide a simple and effective method for carbon nanotube dispersion in polymer matrix with dramatically increased electrical conductivity and thermal stability.     

Characterization of conductive multiwall carbon nanotube/polystyrene composites prepared by latex technology

Conductive multiwall carbon nanotube/polystyrene (MWCNT/PS) composites are prepared based on latex technology. MWCNTs are first dispersed in aqueous solution of sodium dodecyl sulfate (SDS) driven by sonication and then mixed with different amounts of PS latex. From these mixtures MWCNT/PS composites were prepared by freeze-drying and compression molding. The dispersion of MWCNTs in aqueous SDS solution and in the PS matrix is monitored by UV–vis, transmission electron microscopy, electron tomography and scanning electron microscopy. When applying adequate preparation conditions, MWCNTs are well dispersed and homogeneously incorporated in the PS matrix. The percolation threshold for conduction is about 1.5 wt% of MWCNTs in the composites, and a maximum conductivity of about 1 S m⁻¹ can be achieved. The approach presented can be adapted to other MWCNT/polymer latex systems.     

Ultrasonic synthesis of highly dispersed Pt nanoparticles supported on MWCNTs and their electrocatalytic activity towards methanol oxidation

A new method of synthesis of highly dispersed Pt nanoparticles with large catalytic surface area on multi-walled carbon nanotubes (MWCNTs) under high-intensity ultrasonic field was developed. The method, with low processing temperature at 25 °C, took only about 5 min. The surface characterization of MWCNTs was carried out by fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy methods. The electrochemical surface area and pore volume of MWCNTs were also examined. The result showed that functional groups of the MWCNTs which favored the high loading and high dispersion of particles and electrochemical surface area of MWCNTs were reinforced in the case of high-intensity ultrasonic field. The Pt/MWCNT catalysts were characterized by energy dispersion X-ray spectra analysis (EDX), transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements. The prepared Pt nanoparticles were uniformly dispersed on the MWCNT surface. The mean size of Pt particles was 3.4 ± 0.2 nm. The electrocatalytic properties of Pt/MWCNT composites and kinetic characterization for methanol electro-oxidation were investigated by cyclic voltammetry. The Pt/MWCNT catalysts prepared for 5 min in ultrasonic field present excellent electrochemical activities. The schematic of the reaction was also introduced.     

Polyethylene multiwalled carbon nanotube composites

Polyethylene (PE) multiwalled carbon nanotubes (MWCNTs) with weight fractions ranging from 0.1 to 10 wt% were prepared by melt blending using a mini-twin screw extruder. The morphology and degree of dispersion of the MWCNTs in the PE matrix at different length scales was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). Both individual and agglomerations of MWCNTs were evident. An up-shift of 17 cm⁻¹ for the G band and the evolution of a shoulder to this peak were obtained in the Raman spectra of the nanocomposites, probably due to compressive forces exerted on the MWCNTs by PE chains and indicating intercalation of PE into the MWCNT bundles. The electrical conductivity and linear viscoelastic behaviour of these nanocomposites were investigated. A percolation threshold of about 7.5 wt% was obtained and the electrical conductivity of PE was increased significantly, by 16 orders of magnitude, from 10⁻²⁰ to 10⁻⁴ S/cm. The storage modulus (G′) versus frequency curves approached a plateau above the percolation threshold with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behaviour. The ultimate tensile strength and elongation at break of the nanocomposites decreased with addition of MWCNTs. The diminution of mechanical properties of the nanocomposites, though concomitant with a significant increase in electrical conductivity, implies the mechanism for mechanical reinforcement for PE/MWCNT composites is filler-matrix interfacial interactions and not filler percolation. The temperature of crystallisation (T_c) and fraction of PE that was crystalline (F_c) were modified by incorporating MWCNTs. The thermal decomposition temperature of PE was enhanced by 20 K on addition of 10 wt% MWCNT.     

The effect of ionic strength and pH on the stability of tannic acid-facilitated carbon nanotube suspensions

Carbon nanotubes (CNTs) are prone to aggregation and precipitation in water due to their high hydrophobicity and aspect ratio. However, the addition of dissolved organic matter (DOM) has been reported to disperse and stabilize certain CNTs, suggesting the potential transport and bioavailability of CNTs in natural aqueous environments. For a better understanding of the CNT-DOM interaction, five multiwalled CNTs with outer diameters of <10 (MWCNT10), 10-20 (MWCNT20), 20-40 (MWCNT40), 40-60 (MWCNT60), and 60-100 nm (MWCNT100) were used to investigate their sorptive and suspension behavior in tannic acid (TA, as a DOM surrogate) solution; the effects of ionic strength and pH on the TA-CNT interaction were examined. Suspension of MWCNTs sharply improved with increasing TA concentration and leveled off at an initial TA concentration ca. 20 mg/L. Suspension of MWCNTs was in the order of MWCNT40 > MWCNT60 > MWCNT20 > MWCNT100 > MWCNT10. The TA-stabilized CNTs were stable within pH 5-11, while they quickly precipitated at pH < 5. Different ions (Na⁺, Mg²⁺, Ca²⁺, or La³⁺) aggregated the stabilized CNTs, with a critical coagulation concentration exponentially correlated to ionic valence. Changes of steric repulsion and electrostatic interaction with the added TA could account for the variation of CNT stability.     

Preparation and field emission properties of Er-decorated multiwalled carbon nanotubes

The field emission of multiwalled carbon nanotubes (MWCNTs) was improved after decorating their external surface with erbium (Er)-nanoparticles. The decoration was performed by liquid-phase reduction using ethylene glycol as the reducing agent. The oxidation of MWCNTs and the attachment of Er-nanoparticles on the surface of MWCNTs were confirmed by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Raman spectroscopy also revealed the oxidation and functionalization of the nanotubes. Thermogravimetric analysis showed that the decomposition temperature of the MWCNTs decreased gradually as a result of the oxidation process and sequential decoration with uniformly sized Er-nanoparticles (2-3 nm). This means that some of the defects formed by oxidation and decoration with Er-nanoparticles reduced the ignition temperature of the MWCNTs. After decoration with Er-nanoparticles, the MWCNTs showed a significantly better emission current density (3.45 mA/cm² at 3.98 V/μm) and turn-on field (1.8 V/μm) than the pristine MWCNTs.     

Redox polymer covalently modified multiwalled carbon nanotube based sensors for sensitive acetaminophen and ascorbic acid detection

A sensitive electrochemical detection method was developed involving multiwalled carbon nanotubes (MWCNTs) covalently modified with osmium-based redox polymer. The polycationic redox polymer, poly[4-vinylpyridine Os(bipyridine)₂Cl]-co-ethylamine (POs-EA), was first synthesized and covalently attached to MWCNTs. The redox polymer modified MWCNTs were then trapped in a hydrogel formed from polyethyleneglycol diacrylate (PEG-DA) using 1-phenyl-2-hydroxy-2-methyl-1-propanone as a photoinitiator. Upon exposure to aqueous media, the gel swelled to allow movement of analytes in and out of the gel without having any effect on the redox polymer modified nanotube signal. Cyclic voltammetry showed reversible pairs of oxidation-reduction peaks at 0.35 V (vs Ag/AgCl) corresponding to the Os^II/Os^III. This assembly was able to catalytically oxidize both acetaminophen and ascorbic acid (AA). Amperometric data showed a linearity between 0 and 100 μM (R² of 0.999, n = 10) 0.5 mV vs Ag/AgCl (sensitivity 0.003 μA/μM) for ascorbic acid, while for acetaminophen the linearity was between 0 and 1.5 μM (R² of 0.9999, n = 8) with a sensitivity of 65 μA/μM. This sensing system was found to exhibit remarkable stability over several weeks with excellent reproducibility.     

The quantitative characterization of the dispersion state of single-walled carbon nanotubes using Raman spectroscopy and atomic force microscopy

A quantitative method to evaluate the degree of dispersion of single-walled carbon nanotubes (SWCNTs), produced by the high-pressure carbon monoxide process, was developed using Raman spectroscopy and atomic force microscopy (AFM). Nanotubes were dispersed in sodium dodecyl sulfate aqueous solution at seven different dispersion states by controlling ultra-sonication and centrifugation parameters. It is known that the intensity of a Raman peak at 267 cm⁻¹, at the excitation wavelength of 785 nm, is qualitatively proportional to the degree of aggregation. Here, we provide a quantitative calibration technique which involves single-layer spin-deposition of SWCNTs on mica substrates and z-scan analysis of AFM. The trend of the height measurements of AFM precisely matched that of the Raman peak at 267 cm⁻¹. Therefore, this approach can be used to quantitatively characterize the dispersion state of SWCNTs.     

High dispersion and electrochemical capacitive performance of NiO on benzenesulfonic functionalized carbon nanotubes

This work describes an effective method to synthesize structurally uniform composite of nickel oxide/benzenesulfonic functionalized multiwalled carbon nanotubes composite (NiO/f-MWCNTs) using benzenesulfonic MWCNTs as the substrate. Benzenesulfonic group here is bifunctional both for solubilizing MWCNTs into aqueous solution and for tethering Ni²⁺ precursor onto MWCNTs surfaces to facilitate the follow-up chemical deposition of NiO by supplying surface binding and anchoring groups. The composite has a uniform surface dispersion and large coverage of NiO onto f-MWCNTs, which is characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscope, cyclic voltammetry and galvanostatic charge/discharge measurements. The NiO/f-MWCNTs composite improved the utilization of electrochemical capacitive materials and delivered capacity of 384 F/g at the constant current of 0.20 A/g due to f-MWCNTs as substrate.     

The preparation of nano-sulfur/MWCNTs and its electrochemical performance

In this work, a novel nano-sulfur/MWCNTs composite with modified multi-wall carbon nano-tubes (MWCNTs) as sulfur-fixed matrix for Li/S battery is reported. Based on different solubility of sulfur in different solvents, nano-sulfur/MWCNTs composite was prepared by solvents exchange method. The composite was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The modified MWCNTs are considered that not only acts as a conducting material, but also a matrix for sulfur. The electrochemical performance of the nano-sulfur/MWCNTs composite was tested. The results indicated that nano-sulfur/MWCNTs composite had the specific capacity of 1380 mAh g⁻¹, 1326 mAh g⁻¹ and 1210 mAh g⁻¹ in the initial cycle at 100 mA g⁻¹, 200 mA g⁻¹ and 300 mAh g⁻¹ discharge rates respectively, and remained a reversible capacity of 1020 mAh g⁻¹, 870 mAh g⁻¹ and 810 mAh g⁻¹ after 30 cycles. The electrochemical performances confirm that the modified MWCNTs as sulfur-fixed matrix show better ability than any other carbon in cathode of Li/S batteries that had been reported.     

Study of nimesulide and its determination using multiwalled carbon nanotubes modified glassy carbon electrodes

A new method for the determination of nimesulide was established based on the multiwalled carbon nanotubes (MWCNTs) modified glassy carbon electrode (MWCNTs/GCE). In 0.2 M PBS (pH 6.6) buffer solution, the MWCNTs/GCE showed a remarkable catalytic and enhancement effect on reduction of the nimesulide. The reduction peak potential of nimesulide shifted positively from −0.665 V at bare GCE to −0.553 V at MWCNTs/GCE, and the sensitivity increased ca. 7 times. A linear dynamic range of 3.2 × 10⁻⁷-6.5 × 10⁻⁵ M (R = 0.9992) with a detection limit of 1.6 × 10⁻⁷ M was obtained. The electrochemical behaviors of nimesulide were studied and electron-transfer coefficient (α = 0.45), proton number (X = 1) and electron-transfer number (n = 2) have been determined. This method has been used to determine the content of nimesulide in medical tablets. The recovery was determined to be 93.2-106.2% by means of standard addition method. Compared with UV-vis spectrometry, the method was not remarkable difference.     

Surfactant-free dispersion of silver nanoparticles into MWCNT-aqueous nanofluids prepared by one-step technique and their thermal characteristics

This paper reports a significant enhancement in the thermal conductivity of silver-nanoparticle-based aqueous nanofluids with the addition of negligible amounts of multi-walled carbon nanotubes (MWCNTs). The present work was conducted using purified MWCNTs/water nanofluids prepared by a wet grinding method. Silver nanoparticles were dispersed into the MWCNT/water nanofluids via a one-step method using pulse power evaporation, which was observed to improve the dispersibility and thermal conductivity of the nanofluids. A particle sizing system (PSS) and transmission electron microscopy (TEM) were used to confirm the size of silver nanoparticles in base fluids. The PSS measurement results reveal that the size of the silver nanoparticles was approximately 100 nm, which is in good agreement with the results obtained from TEM and SEM. The maximum absorbance (2.506 abs at a wavelength of 264 nm) and highest thermal conductivity enhancement (14.5% at 40 °C) were achieved by a fluid containing ‘0.05 wt% MWCNTs–3 wt% Ag’ composite.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

多壁碳纳米管的改性及其复合材料的制备

用H2SO4/HNO3（体积比3∶1）对碳纳米管进行改性,结果研究表明：与原始碳纳米管相比,改性后的多壁碳纳米管的自身的分散性非常好,表面带有了更多的-OH和-COOH等官能团,碳纳米管在空气中的热稳定性明显下降,而且在碳酸氢铵与氨水和少量SDBS的混合溶液中分散稳定性更好。然后采用原位聚合的方法制备了多壁碳纳米管/碳酸铝铵复合材料,复合粉体的TEM和XRD表明改性后的多壁碳纳米管可以在碳酸铝铵粉体中进行良好的分散。     

Highly dispersed multi-walled carbon nanotubes in ethanol using potassium doping

We demonstrated the production of an effective dispersion of multi-walled carbon nanotubes (MWCNTs) in ethanol using potassium doping (π-stacking interaction). The homogeneous dispersion of individual MWCNTs was achieved without any contamination or severe disruption at the end caps or periphery of the tubes. Potassium as a doping material, phenanthrene as a nonpolar molecule, and 1,2-dimethoxyethane as a dipole solvent were used for our experiment. From UV-visible spectroscopy and visual observation, it was found that the dispersibility of the MWCNTs in ethanol was about 14 mg/dm³. High resolution transmission electron microscopy and Raman spectroscopy showed that disruption of the end caps of the tubes and severance along the tube axis were rarely found. The scanning electron microscopy and corresponding EDX results indicated that the key to the dispersion mechanism was the potassium doping, which is driven by π-stacking complex formation. We suggest that the dispersion of the MWCNTs was influenced by the potassium doping, which caused the enlargement and separation of the entangled-MWCNT networks, and was not affected by defects or modification of the surface morphology.     

Modified carbon nanotube composites with high dielectric constant, low dielectric loss and large energy density

Flexible dielectric polystyrene based composites containing multi-walled carbon nanotubes (MWCNTs) were reported. The MWCNTs were coated with polypyrrole (PPy) by an inverse microemulsion polymerization. Transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy indicated that the MWCNTs were coated with PPy. Our composites presented a stable high dielectric constant (∼44), rather low loss (<0.07), and large energy density (up to 4.95 J cm⁻³). The largely-enhanced dielectric performance originates from the organic shell PPy, which not only ensure good dispersion of MWCNTs in the polymer matrix but also screen charge movement to shut off leakage current. Such MWCNT composites can be used to store charge and electrical energy and play a key role in modern electronics and electric power systems.     

Functionalization of multiwalled carbon nanotubes with polystyrene under atom transfer radical polymerization conditions

Linear Polystyrene (PS) was grafted onto the convex surfaces of multiwalled carbon nanotubes (MWCNTs). Bromine-terminated polystyrene synthesized by atom transfer radical polymerization (ATRP) was directly reacted with MWCNTs under ATRP conditions using CuBr/2,2′-bipyridine as catalyst. The PS-grafted MWCNT samples were characterized by scanning electron microscopy, transmission electron microscopy, FT-IR spectra, Raman spectra, ¹H NMR, UV-vis spectra, thermal gravimetric analyses, and X-ray diffraction. The products can dissolve in organic solvents such as 1,2-dichlorobenzene, tetrahydrofuran and chloroform to form well-dispersed solutions. Optical limiting property measurements in chloroform were carried out at 532 nm using the open-aperture z-scan technique. The results demonstrate that the samples preserve good optical limiting properties when the polymer is covalently attached to the carbon nanotube.     

Surface-enhanced Raman spectra of medicines with large-scale self-assembled silver nanoparticle films based on the modified coffee ring effect

We report here a simple and innovative method to prepare large-scale silver nanoparticle films based on the controlled coffee ring effect. It is demonstrated that the films can be used as surface-enhanced Raman scattering probes to detect low-concentration medicines. Silver nanoparticles with the average size about 70 nm were prepared by reduction of silver nitride. In our experiment, the coffee ring effect was controlled by tilting the substrates during the deposition of silver nanoparticle films. Silver nanoparticle films were spontaneously formed on the surface of silicon substrates at the temperatures about 50°C based on the solvent evaporation and the coffee ring effect. The microstructure of the films was investigated using the scanning electron microscope and atomic force microscope. The surface roughness of the films is found as small as 20 nm. Then, the films were exposed to aqueous solutions of medicine at different concentrations. A comparison with a Raman spectra measured with a conventional Raman spectrometer showed that the Raman signal can be detected in the solution with concentrations as low as 1 × 10⁻⁵ M, and the enhancement factor achieved by the silver nanoparticle film can at least reach to 1.08 × 10⁴. Our experimental results indicate that this technique is promising in the production of large-scale silver nanoparticle films for the surface-enhanced Raman scattering. These may be utilized in biochemical and trace analytical applications.