Acid-oxidized Multiwalled Carbon Nanotubes Research Articles

Improvements in interfacial and heat-resistant properties of carbon fiber/methylphenylsilicone resins composites by incorporating silica-coated multi-walled carbon nanotubes

The multi-scale reinforcement and interfacial strengthening on carbon fiber (CF)-reinforced methylphenylsilicone resin (MPSR) composites by adding silica-coated multi-walled carbon nanotubes (SiO2-CNTs) were investigated. SiO2-CNT has been successfully prepared via the hydrolysis of tetraethoxysilane in the presence of acid-oxidized multi-walled carbon nanotubes. Transmission electron microscopy, X-ray diffraction, and Fourier Transform infrared spectroscopy were carried out to examine the functional groups and structures of CNTs. Then, SiO2-CNT was incorporated into MPSR matrix to prepare CF/MPSR-based composites by the compression molding method. The effects of the introduced SiO2-CNT on the interfacial, impact, and heat-resistant properties of CF/MPSR composites were evaluated by short-beam bend method, impact test, and thermal oxygen aging experiments, respectively. Experimental results revealed that the CF/MPSR composites reinforced with 0.5 wt% SiO2-CNT showed a significant increase 34.53% in the interlaminar shear strength (ILSS) and 20.10% in impact properties. Moreover, the heat-resistant properties of composites were enhanced significantly by adding SiO2-CNT hybrid nanoparticles. These enhancements are mainly attributed to the improved matrix performance resulted from the molecular-level dispersion of SiO2-CNT in MPSR matrix and the strong interfacial adhesion between SiO2-CNT and matrix resin, which are beneficial to improve the mechanical stress transfer from MPSR matrix to CFs reinforcement and alleviate stress concentrations.

Fabrication and characterization of a multi-walled carbon nanotube-based counter electrode for dye-sensitized solar cells

A counter-electrode (CE) for dye-sensitized solar cells (DSSCs) was prepared by coating a slurry containing acid-oxidized multi-wall carbon nanotubes and nano-graphite powder onto a fluorine-doped tin oxide conducting glass substrate. The samples were characterized by SEM, TEM, EDS and IR spectroscopy. The CE performance in the DSSCs with MgO-doped TiO2 as the photoanode was investigated by measurements of current-voltage curves, cyclic voltammetry and electrochemical impedance spectroscopy. Results show that the cell with the CE exhibits the best photoelectric properties of all the carbon-based CEs investigated. The short-circuit current density (Jsc) is 4.67 mA/cm2, the open-circuit voltage (Voc) is 0.53 V and photoelectric conversion efficiency is up to 4.10%, which are comparable with those of the Pt-based CE in DSSCs.

Desirable electrical and mechanical properties of continuous hybrid nano-scale carbon fibers containing highly aligned multi-walled carbon nanotubes

The continuous highly aligned hybrid carbon nanofibers (CNFs) with different content of acid-oxidized multi-walled carbon nanotubes (MWCNTs) were fabricated through electrospinning of polyacrylonitrile (PAN) followed by a series of heat treatments under tensile force. The effects of MWCNTs on the micro-morphology, the degree of orientation and ordered crystalline structure of the resulting nanofibers were analyzed quantitatively by diversified structural characterization techniques. The orientation of PAN molecule chains and the graphitization degree in carbonized nanofibers were distinctly improved through the addition of MWCNTs. The electrical conductivity of the hybrid CNFs with 3wt% MWCNTs reached 26S/cm along the fiber direction due to the ordered alignment of MWCNTs and nanofibers. The reinforcing effect of hybrid CNFs in epoxy composites was also revealed. An enhancement of 46.3% in Young’s modulus of epoxy composites was manifested by adding 5wt% hybrid CNFs mentioned above. At the same time, the storage modulus of hybrid CNF/epoxy composites was significantly higher than that of pristine epoxy and CNF/epoxy composites not containing MWCNTs, and the performance gap became greater under the high temperature regions. It is believed that such a continuous hybrid CNF can be used as effective multifunctional reinforcement in polymer matrix composites.

Exponentially increased nucleation ability for poly(L-lactide) by adding acid-oxidized multiwalled carbon nanotubes with reduced aspect ratios

Acid-oxidized multiwalled carbon nanotubes (A-MWCNTs) with a range of reduced aspect ratios (from about 11 to 5.8) were obtained by acid oxidization of MWCNTs in the mixture of HNO3 and H2SO4 for varying periods of 1, 3, 8 and 12 h, respectively. The aspect ratios and surface functionalization of A-MWCNTs were well characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and thermogravimetric analysis (TGA). Poly(L-lactide)/A-MWCNT composites containing 0.5 wt% A-MWCNTs with a range of reduced aspect ratios were prepared by solution cast. The effects of added A-MWCNTs on the isothermal crystallization kinetics of poly(L-lactide)/A-MWCNT composites were investigated by means of differential scanning calorimetry (DSC), rheology and polarized optical microscopy (POM). It is surprising to find that not only the addition of A-MWCNTs effectively increases the poly(L-lactide) (PLA) crystallization kinetics, but also the nucleation ability of A-MWCNTs for PLA crystallization exponentially increases with the reduced aspect ratio, that is to say, those with lower aspect ratios show much stronger nucleation ability for PLA crystallization than those with higher aspect ratios. The exponentially increased nucleation ability of A-MWCNTs with a range of reduced aspect ratios for PLA crystallization is disclosed.

The role of interfacial chemistry and interactions in the dynamics of thermosetting polyurethane–multiwalled carbon nanotube composites at low filler contents

Acid-oxidized multiwalled carbon nanotubes (MWCNTs) were introduced into a polyurethane (PU) matrix at low filler levels (0.01–0.25 wt%) through either van der Waals or covalent interactions, and their glass transition dynamics using dynamic mechanical analysis and laser-interferometric creep rate spectroscopy was investigated. The nanocomposites reveal substantial impact on the PU glass transition dynamics, which depends on the nanotube content and type of interfacial interactions. The pronounced dynamic heterogeneity within the glass transition covering 200 °C range and the displacement of main PU relaxation maxima from around 0 to 80–140 °C were registered. The results are treated in the framework of chemical inhomogeneity, constrained dynamics effects, and different motional cooperativities. The peculiariaties of the glass transition dynamics in the composites are reflected in their dynamic and static mechanical properties, in particular a two- to threefold increase in modulus and tensile strength for the covalent interfacial interaction of MWCNTs with PU.

Enhancement of Electrical Conductivity by Changing phase Morphology for Composites Consisting of Polylactide and Poly(ε-caprolactone) Filled with Acid-Oxidized Multiwalled Carbon Nanotubes

Composites consisting of polylactide (PLA) and poly(ε-caprolactone) (PCL) filled with acid-oxidized multiwalled carbon nanotubes (A-MWCNTs) were prepared through melt compounding. Phase morphologies of PLA/PCL/A-MWCNT composites with different contents of filled A-MWCNTs and PCL compositions were mainly observed by scanning electron microscope. The results show that A-MWCNTs are selectively dispersed in the PCL phase, regardingless of PCL phase domain sizes. For PLA/PCL/A-MWCNT composites with fixed PLA/PCL ratio of 95/5, the dispersed PCL phase domain sizes in the PLA matrix decrease even though a small content of A-MWCNTs is added, compared with PLA/PCL blend with the same composition, indicating that A-MWCNTs effectively prevent from coalescence of the dispersed PCL phase domains. With filling of 1.0 wt % A-MWCNTs, an interesting change of electrical conductivity for PLA/PCL/A-MWCNT composites is observed, in which the maximum conductivity is observed for PLA/PCL/A-MWCNT composite with PLA/PCL ratio of 60/40. The result is well-explained by the formed cocontinuous phase morphology and effective A-MWCNT content.

Toxicity of Multiwalled Carbon Nanotubes with End Defects Critically Depends on Their Functionalization Density

Carboxylated carbon nanotubes stand as the most promising nanovectors for biomedical and pharmaceutical applications due to their ease of covalent conjugation with eclectic functional molecules including therapeutic drugs, proteins, and oligonucleotides. In the present study, we attempt to investigate how the toxicity of acid-oxidized multiwalled carbon nanotubes (MWCNTs) can be tweaked by altering their degree of functionalization and correlate the toxicity trend with their biodistribution profile. In line with that rationale, mice were exposed to 10 mg/kg of pristine (p) and acid-oxidized (f) MWCNTs with varying degrees of carboxylation through a single dose of intravenous injection. Thereafter, extensive toxicity studies were carried out to comprehend the short-term (7 day) and long-term (28 day) impact of p- and various f-MWCNT preparations on the physiology of healthy mice. Pristine MWCNTs with a high aspect ratio, surface hydrophobicity, and metallic impurities were found to induce significant hepatotoxicity and oxidative damage in mice, albeit the damage was recovered after 28 days of treatment. Conversely, acid-oxidized carboxylated CNTs with shorter lengths, hydrophilic surfaces, and high aqueous dispersibility proved to be less toxic and more biocompatible than their pristine counterparts. A thorough scrutiny of various biochemical parameters, inflammation indexes, and histopathological examination of liver indicated that toxicity of MWCNTs systematically decreased with the increased functionalization density. The degree of shortening and functionalization achieved by refluxing p-MWCNTs with strong mineral acids for 4 h were sufficient to render the CNTs completely hydrophilic and biocompatible, while inducing minimal hepatic accumulation and inflammation. Quantitative biodistribution studies in mice, intravenously injected with Tc-99m labeled MWCNTs, clearly designated that clearance of CNTs from reticuloendothelial system (RES) organs such as liver, spleen, and lungs was critically functionalization density dependent. Well-individualized MWCNTs with shorter lengths (<500 nm) and higher degrees of oxidation (surface carboxyl density >3 μmol/mg) were not retained in any of the RES organs and rapidly cleared out from the systematic circulation through renal excretion route without inducing any obvious nephrotoxicity. As both p- and f-MWCNT-treated groups were devoid of any obvious nephrotoxicity, CNTs with larger dimensions and lower degrees of functionalization, which fail to clear out from the body via renal excretion route, were thought to be excreted via biliary pathway in faeces.

Enhanced Nucleation Rate of Polylactide in Composites Assisted by Surface Acid Oxidized Carbon Nanotubes of Different Aspect Ratios

Biodegradable polylactide (PLA) composites added with acid oxidized multiwalled carbon nanotubes (A-MWCNTs) of two different aspect ratios (length to diameter) were prepared by coagulation. The aspect ratios and surface structures of A-MWCNTs were characterized by TGA, Raman, and SEM measurements. The percolation thresholds for gelation in the PLA composites with A-MWCNTs of large and small aspect ratios are 2.5 and 4.0 wt %, respectively, which were determined by a rheological method, and in turn, the rheological result confirms the aspect ratio differences for the added two types of A-MWCNTs in the composites. Isothermal crystallization kinetics of neat PLA and its composites were further investigated by using polarized optical microscope (POM) and differential scanning calorimetry (DSC) to clarify the effects of A-MWCNTs of different aspect ratios and concentrations. The different aspect ratio A-MWCNTs with the same carboxyl group mass percent show substantial effects on PLA crystallization kinetics. Those with smaller aspect ratios enhance nucleation rate for PLA spherulites much more than those with larger aspect ratios. This phenomenon can be attributed to fewer sidewall carboxyl groups on the surfaces of A-MWCNTs with smaller aspect ratios, which provides more nucleation sites for PLA crystallization than those with larger aspect ratios at the same concentration, resulting in faster PLA nucleation rates for the former one.

Preparation of high performance Pd catalysts supported on untreated multi-walled carbon nanotubes for formic acid oxidation

Due to the inherent inertness of carbon nanotubes (CNTs), one of the most significant challenges in the preparation of CNT-supported catalysts is achieving a uniform deposition of nanoparticles on the surface of the nanotubes. In this paper, we report on the preparation and characterization of Pd nanoparticles supported on untreated multi-walled carbon nanotubes (MWCNTs), synthesized in the presence of glutamate. The results of Raman spectroscopy revealed that this synthetic procedure does not have a detrimental effect on the surface structure of MWCNTs. Transmission electron microscopy (TEM) measurements indicated that the dispersion of Pd nanoparticles on untreated-MWCNTs in the presence of glutamate were uniform, and a narrow particle size was observed. X-ray diffraction (XRD) patterns indicated that the Pd/MWCNT catalyst possessed a face-centered cubic crystal structure. Cyclic voltammetry and chronoamperometry tests demonstrated that the obtained Pd/MWCNT catalyst displayed superior electrocatalytic activity and stability in formic acid oxidation, as compared to both a Pd/MWCNT catalyst synthesized without glutamate and a Pd catalyst on acid-oxidized MWCNTs, under otherwise identical experimental conditions. These results indicate that the catalyst developed in this study is a superior candidate for direct formic acid fuel cells (DFAFCs).

Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors

Stable aqueous dispersions of polymer-modified graphene sheets were prepared via in situ reduction of exfoliated graphite oxides in the presence of cationic poly(ethyleneimine) (PEI). The resultant water-soluble PEI-modified graphene sheets were then used for sequential self-assembly with acid-oxidized multiwalled carbon nanotubes, forming hybrid carbon films. These hybrid films were demonstrated to possess an interconnected network of carbon structures with well-defined nanopores to be promising for supercapacitor electrodes, exhibiting a nearly rectangular cyclic voltammogram even at an exceedingly high scan rate of 1 V/s with an average specific capacitance of 120 F/g.

Surface characterization of acid-oxidized multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWNTs) oxidized with sulfuric acid, nitric acid, and the mixture of sulfuric and nitric acids were characterized by thermogravimetric analysis (TGA), Raman spectroscopy, elemental analysis, X-ray diffraction, and X-ray photoelectron spectroscopy. The TGA data showed that the MWNTs were more resistant to oxidation than C60 or activated carbon fibers. Catalyst particles could be removed by the oxidants containing sulfuric acid, and thus indicative of the tip opening of MWNTs. The sulfuric acid had a propensity to create defect sites and introduce the surface oxides at those defects that already exist or be newly generated on MWNTs. However, the acid mixture could open the caps of MWNTs but preserve the structure homogeneity. The treatment with nitric acid gave rise to the highest bulk oxygen content in MWNTs, while the most abundant surface oxides were provided by sulfuric acid oxidation. In addition, nitric acid exhibited the best ability to transform the phenolic groups into carboxyl groups.

The hepatotoxicity of multi-walled carbon nanotubes in mice

The hepatotoxicity of two types of multi-walled carbon nanotubes (MWCNTs),acid-oxidized MWCNTs (O-MWCNTs) and Tween-80-dispersed MWCNTs(T-MWCNTs), were investigated with Kunming mice exposed to 10 and 60 mg kg−1 by intravenous injection for 15 and 60 d. Compared with the PBS group, thebody-weight gain of the mice decreased and the level of total bilirubin and aspartateaminotransferase increased in the MWCNT-exposed group with a significantdose–effect relationship, while tumor necrosis factor alpha level did not show significantstatistical change within 60 d. Spotty necrosis, inflammatory cell infiltration inportal region, hepatocyte mitochondria swelling and lysis were observed with asignificant dose–effect relationship in the MWCNT groups. Liver damage of theT-MWCNT group was more severe than that of the O-MWCNT group according tothe Roenigk classification system. Furthermore, T-MWCNTs induce slight liveroxidative damage in mice at 15 d, which was recovered at 60 d. Part of the geneexpressions of mouse liver in the MWCNT groups changed compared to the PBSgroup, including GPCRs (G protein-coupled receptors), cholesterol biosynthesis,metabolism by cytochrome P450, natural-killer-cell-mediated cytotoxicity, TNF-α,NF-κB signaling pathway, etc. In the P450 pathway, the gene expressions of Gsta2 (down-regulated),Cyp2B19 (up-regulated) and Cyp2C50 (down-regulated) had significant changes inthe MWCNT groups. These results show that a high dose of T-MWCNTs caninduce hepatic toxicity in mice while O-MWCNTs seem to have less toxicity.

Improved field emission via laser processing of carbon nanotubes on paper substrates

The authors report excellent electron field emission characteristics from acid-oxidized multiwalled carbon nanotubes printed onto paper substrates and subjected to a postdeposition laser treatment. A distinct change in the morphology of the nanotube layer was observed after laser irradiation, and threshold fields improved from 4.8V∕μm before laser irradiation to 2.1V∕μm after laser irradiation. This study shows how field emission cathodes can be fabricated in a straightforward manner on cheap, flexible substrates and that the field emission characteristics can be tailored readily via postdeposition laser treatment.

Kinetics and thermodynamics of sorption of nitroaromatic compounds to as-grown and oxidized multiwalled carbon nanotubes

The sorption kinetics and thermodynamics of 1,3-dinitrobenzene (DNB), m-nitrotoluene (mNT), p-nitrophenol (pNP), and nitrobenzene (NB) on as-grown and nitric acid-oxidized multiwalled carbon nanotubes (MWCNTs) were investigated. The sorption kinetics was well described by a pseudo-second-order rate model, while both Langmuir and Freundlich models described the sorption isotherms well and the sorption thermodynamic parameters of equilibrium constant ( K 0 ), standard free energy (Δ G), standard enthalpy (Δ H), and standard entropy changes (Δ S) were measured. The values of Δ H and Δ G suggested that the sorption of nitroaromatics (NACs) onto MWCNTs was exothermic and spontaneous. The structure, number, and position of nitro groups of NACs were the main factors affecting the sorption rate and capacity. Treatment of the MWCNTs with nitric acid increased both the surface area and the pore volume and introduced oxygen-containing functional groups to the MWCNTs, which depressed the sorption of NACs onto MWCNTs.

Effect of oxidation of activated carbon on its enrichment efficiency of metal ions: Comparison with oxidized and non-oxidized multi-walled carbon nanotubes

The effect of oxidation of activated carbon (AC) with various oxidizing agents (nitric acid, hydrogen peroxide, ammonium persulfate) on preconcentration of metal ions (Cr 3+, Mn 2+, Pb 2+, Cu 2+, Cd 2+ and Zn 2+) from environmental waters prior to their flame atomic absorption spectroscopic analysis was investigated. The highest recoveries and adsorption capacities towards metal ions were achieved when using nitric acid-oxidized AC (sorbent AC-NA) as preconcentrating sorbent at pH 9. A preconcentration procedure was optimized using AC-NA as sorbent, which was then compared with non-oxidized AC in terms of analytical performance of the preconcentration method. Higher sensitivity, lower detection limits and wider linear ranges were achieved when AC-NA was used. The analytical performance of the method using AC-NA as preconcentrating sorbent was also compared with nitric acid-oxidized multi-walled carbon nanotubes (sorbent MWCNT-NA) and non-oxidized multi-walled carbon nanotubes (sorbent MWCNT). The analytical performance of the preconcentration method using AC-NA was close to MWCNT-NA, but AC-NA was better than non-oxidized MWCNT. Application of the optimized preconcentration method (using AC-NA sorbent) to environmental waters (tap water, reservoir water, stream water) gave spike recoveries of the metals in the range 63–104%.

Direct Measurements of Interactions between Polypeptides and Carbon Nanotubes

The interactions of various polypeptides with individual carbon nanotubes (CNTs), both multiwall (MW) and single wall (SW), were investigated by atomic force microscopy (AFM). While adhesion forces arising from electrostatic attraction interactions between the protonated amine groups of polylysine and carboxylic groups on the acid-oxidized multi-wall carbon nanotubes (Ox-MWCNTs) dominate the interaction at a low pH, weaker adhesion forces via the hydrogen bonding between the neutral -NH2 groups of polylysine and -COO- groups of the Ox-MWCNTs were detected at a high pH. The adhesion force was further found to increase with the oxidation time for Ox-MWCNTs and to be negligible for oxidized single-wall carbon nanotubes (Ox-SWCNTs) because carboxylate groups were only attached onto the nanotube tips in the latter whereas onto both the nanotube tips and sidewall in the former. Furthermore, it was demonstrated that proteins containing aromatic moieties, such as polytryptophan, showed a stronger adhesion force with Ox-MWCNTs than that of polylysine because of the additional pi-pi stacking interaction between the polytryptophan chains and CNTs.

Electrochemical impedance study of Li-ion insertion into the raw acid-oxidized carbon nanotubes

Electrochemical impedance spectroscopy (EIS) was applied for the raw acid-oxidized multiwall carbon nanotubes during Li-ion insertion with the electrolyte of 1M LiPF 6 in EC and DMC. Impedance spectroscopy consists of two separated arcs in the high and intermediate range frequency. The high frequency arc is attributed to Li migration within the surface films and the medium frequency arc is associated with the charge transfer. Obtained spectra were analyzed with an equivalent circuit model. Kinetic parameters such as the charge transfer resistance, film resistance, diffusion coefficient and exchange current were evaluated. The contribution of the film to the measured impedance response is discussed.