Physisorption of hydrogen in single-walled carbon nanotubes

The interaction of hydrogen with single-walled carbon nanotubes (SWNTs) was analysed. A SWNT sample was exposed to D₂ or H₂ at a pressure of 2 MPa for 1 h at 298 or 873 K. The desorption spectra were measured by thermal desorption spectroscopy (TDS). A main reversible desorption site was observed throughout the range 77 to 320 K. The activation energy of this peak at about 90 K was calculated assuming first-order desorption. This corresponds to physisorption on the surface of the SWNTs (19.2±1.2 kJ/mol). A desorption peak was also found for multi-walled carbon nanotubes (MWNTs), and also for graphite samples. The hydrogen desorption spectrum showed other small shoulders, but only for the SWNT sample. They are assumed to originate from hydrogen physisorbed at sites on the internal surface of the tubes and on various other forms of carbon in the sample. The nanosized metallic particles (Co:Ni) used for nanotube growth did not play any role in the physisorption of molecular hydrogen on the SWNT sample. Therefore, it is concluded that the desorption of hydrogen from nanotubes is related to the specific surface area of the sample.     

Electrocatalysis and determination of uracil on polythionine/multiwall carbon nanotubes modified electrode

A new type of poly (thionine)/multiwall carbon nanotube/glassy carbon (PTH/MWNTs/GC) electrode was fabricated by electropolymerization thionine onto the surface of MWNTs modified GC electrode. The properties and behaviors of the modified electrode were characterized by scanning electron microscopy (SEM) and cyclic voltammetry (CV). The results show that the high sensitivity and selectivity are mainly caused by the unique carbon surface of the carbon nanotubes and the catalytic activity of thionine. The modified electrode exhibited excellent electrocatalytic behavior to the oxidation of uracil, and was firstly applied to determinate the concentration of uracil for the differential pulse voltammograms. Under the optimum conditions, linear calibration equation was obtained over the uracil concentration range from 1.0 × 10^?5 to 5.5 × 10^?2M with a correlation coefficient of 0.9978 and a detection limit 2.0 × 10^?7M (based on S/N = 3) was also gained. The good electrocatalytic response of uracil at PTH/MWNTs/GC electrode suggests that the PTH/MWNTs are an excellent platform for electrochemical biosensing. The modified electrode displays excellent repeatability, stability, and high sensitivity. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characterization of single-wall carbon nanotubes by N2 adsorption

N₂ adsorption isotherms at 77 K of single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), and mixtures of these carbon nanotubes (CNTs) were analyzed for differences in their pore size distributions (PSDs). The PSDs, calculated in the microporous region by the Horvath–Kawazoe method and in the mesoporous region by the BJH method, are in agreement with the structures of both types of CNTs deduced from high-resolution transmission electron microscopy. A characteristic peak in the microporous region in the PSD of SWNTs is not present in the PSDs of MWNTs and impurities such as amorphous carbon, metal residues of catalysts, etc. The evaluation of this peak is proposed as a convenient tool for the quantitative characterization of SWNT purity in carbon nanotube-containing samples.     

High yield purification of carbon nanotubes with H2S-O2 mixture

Abatract A high yield purification method was developed for multi- and single-walled carbon nanotubes, combining both the gas-phase purification using H₂S and O₂ mixture to remove impurity carbon particles and the acid treatment to remove metal particles. For the purification of MWNTs, the purification yield was about 54%, much higher than the yield previously reported. For the SWNTs, the combined liquid-gas purification process resulted in a high purity of >95% and a high yield of 20–50%, depending on the quality of raw material. Hydrogen sulfide played a role of enhancing the removal of carbon particles as well as suppressing the oxidation of carbon nanotubes. Overall, the purification method developed in this work is simple and quite effective for removing unwanted carbon and metal particles out of MWNTs and SWNTs.     

High growth of SWNTs and MWNTs from C2H2 decomposition over Co-Mo/MgO catalysts

A series of MgO supported catalysts having Co and Mo metals 5-40 wt.% in a ratio of 1:1 was prepared by impregnation method. Carbon nanotubes (CNTs) were grown over the catalysts by decomposition of C₂H₂ at 800 °C for 30 min. It was found that 5 and 10 wt.% Co-Mo/MgO catalysts produced single-wall nanotubes (SWNTs), whereas 20, 30 and 40 wt.% Co-Mo/MgO catalysts produced multi-wall nanotubes (MWNTs). The catalyst Mo/MgO was inactive in growing CNTs. In Co-Mo/MgO catalysts, however Mo generated a favorable environment to grow SWNTs. The growth of SWNTs was strongly dependent on the formation of small clusters of cobalt, which may generate from the decomposition of CoMoO₄ species during the nanotube growth. MWNTs were produced over comparatively larger cobalt clusters generated from Co₃O₄ phase during the nanotube growth stage. The yields of SWNTs were about 6% and 27% over 5 and 10 wt.% Co-Mo/MgO catalysts, respectively. MWNTs yield (576%) was observed over 40 wt.% Co-Mo/MgO catalyst. Carbon yield (%) highly varied with acetylene concentration.     

Functionalization of carbon nanotubes with amphiphilic molecules and their Langmuir-Blodgett films

Crown ether-modified full-length multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) have been prepared by a direct heating method. Subsequently, the Langmuir-Blodgett (LB) method was used to prepare a monolayer of modified carbon nanotubes on an air-water interface. The corresponding Langmuir-Blodgett (LB) films appeared to be very stable and could be transferred onto a hydrophilic silicon substrate easily. Scanning electron microscopy (SEM) images revealed the morphology of the films, in which locally aligned structures could be observed.     

Catalyst effect of metal cations on pyrolysis of hydrocarbon molecules and formation of carbon nanotubes in the channels of AlPO4-5 crystals

Ultra-small single-walled carbon nanotubes (SWNTs) with diameter of 0.4 nm were fabricated in the channels of AlPO₄-5 crystals by pyrolyzing hydrocarbon molecules. In order to improve the structural quality of the SWNTs, we introduced Br?nsted acid sites onto the channel walls by incorporating metal cations (Mn, Mg, Co, and Si) into AlPO₄-5 framework. The Br?nsted acid sites play an important catalytic role in the carbonization of hydrocarbon molecules (tripropylamine) in the AlPO₄-5 channels, and favor the formation of SWNTs, as revealed by the significant decrease in formation energy of the nanotubes. The experimental results agree well with the predictions of first-principles calculations.     

Catalyst effect of metal cations on pyrolysis of hydrocarbon molecules and formation of carbon nanotubes in the channels of AlPO4-5 crystals

Ultra-small single-walled carbon nanotubes (SWNTs) with diameter of 0.4 nm were fabricated in the channels of AlPO₄-5 crystals by pyrolyzing hydrocarbon molecules. In order to improve the structural quality of the SWNTs, we introduced Brønsted acid sites onto the channel walls by incorporating metal cations (Mn, Mg, Co, and Si) into AlPO₄-5 framework. The Brønsted acid sites play an important catalytic role in the carbonization of hydrocarbon molecules (tripropylamine) in the AlPO₄-5 channels, and favor the formation of SWNTs, as revealed by the significant decrease in formation energy of the nanotubes. The experimental results agree well with the predictions of first-principles calculations.     

Buckypaper-based catalytic electrodes for improving platinum utilization and PEMFC's performance

Platinum (Pt) catalytic electrode was developed by using carbon nanotube films (buckypaper) as supporting medium and electrodeposition method to deposit Pt catalyst. Buckypapers are free-standing thin films consisting of single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and/or carbon nanofibers (CNFs) held together by van der Waals forces without any chemical binders. Special mixed buckypapers was developed by layered microstructures with a dense and high-conducting SWNT networks at the surface, as well as large porous structures of CNF networks as back supports. This unique microstructure can lead to improve Pt catalyst accessibility and mass exchange properties. Pt particles of about 6 nm were uniformly deposited in porous buckypapers. A promising electrochemical surface area of ∼40 m²/g was obtained from these electrodes. A Pt utilization as low as 0.28 g_Pt/kW was achieved for the cathode electrode at 80 °C. Pt utilization efficiency can be further improved by optimization of the electrodeposition condition in order to reduce the Pt particle size.     

Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition

High-temperature decomposition of hydrocarbons may lead to the formation of carbon deposits. However in our present studies, we found that the morphology of carbon deposits over MgO supported Fe catalyst during chemical vapor deposition (CVD) process was closely related to the thermodynamic properties and chemical structures of hydrocarbon precursors. Six kinds of hydrocarbons (methane, hexane, cyclohexane, benzene, naphthalene and anthracene) were used as carbon precursors in this study. Methane which has a pretty simple composition and is more chemically stable was favorable for the formation of high-purity single walled carbon nanotubes (SWNTs). For high-molecular weight hydrocarbons, it was found that the chemical structures rather than thermodynamic properties of carbon precursors would play an important role in nanotube formation. Specifically, the CVD processes of aromatic molecules such as benzene, naphthalene and anthracene inclined to the growth of SWNTs. While the cases of aliphatic and cyclic hydrocarbon molecules seemed a little more complicated. Based on different pyrolytic behaviors of carbon precursors and formation mechanism of SWNTs and multi-walled carbon nanotubes (MWNTs), a possible explanation of the difference in CVD products was also proposed.     

A new approach to functionalize multi-walled carbon nanotubes by the use of functional polymers

A new method to graft a large number of long polymer chains or small functional molecules onto multi-walled carbon nanotubes (MWNTs) indirectly is reported. First, MWNTs were slightly functionalized by reversible addition-fragmentation chain transfer (RAFT) copolymerization of styrene and maleic anhydride using the dithioester groups attached to MWNTs as RAFT agents. The highly reactive maleic anhydride groups could further react with a large number of long polymer chains or small functional molecules with hydroxyl or amino group easily. The resulted MWNTs have good solubility in organic solvents and water; the perfect structure of MWNTs is altered very little from the information of Raman spectra.     

A thionine functionalized multiwalled carbon nanotube modified electrode for the determination of hydrogen peroxide

An amperometric sensor for hydrogen peroxide was developed using multiwalled carbon nanotubes (MWCNTs) covalently immobilized with thionine (TH) via a carbodiimide reaction. The thionine functionalized MWCNTs were then abrasively transferred onto a paraffin impregnated graphite electrode followed by a coating of a thin film of nafion (Nf). The immobilization of thionine with MWCNTs was characterized by UV-visible absorption spectroscopy, diffuse reflectance UV-visible spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetry, electrochemical impedance spectroscopy and cyclic voltammetry. The cyclic voltammetric response of thionine grafted onto CNTs indicated that it promotes the low potential, sensitive and stable determination of H₂O₂. The modified electrode showed an inherent stability over a wide pH range, fast response time, high sensitivity, low detection limit and remarkable decrease of over potential for the reduction of hydrogen peroxide that decreases interference in analysis. This TH/MWCNT/Nf modified electrode can be used as an amperometric detector for monitoring oxidase based biosensors in chromatography and in flow injection analysis.     

Grafting polyamide 6 onto multi‐walled carbon nanotubes using microwave irradiation

Chemical reactions under microwave irradiation can be very efficient, with a significant shortening of reaction time. Few studies have reported the use of microwaves to functionalize carbon nanotubes. In the work reported, a new method of formulating functionalized multi‐walled carbon nanotubes (MWNTs) was developed by covalent grafting of polyamide 6 (PA6) chains onto the carbon nanotubes assisted by microwave irradiation. PA6 chains were grafted onto acidified MWNTs through condensation reaction between the carboxylic groups of the MWNTs and the terminal amine groups of PA6 using microwave radiation heating. The functionalized carbon nanotubes (MWNT‐g‐PA6) were characterized systematically using infrared and Raman spectroscopy, transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). TEM showed that the surface of the MWNTs was covered with a layer of PA6. TGA results indicated that the MWNT‐g‐PA6 contained about 47 wt% of polymer. A novel, convenient and efficient functionalization approach is reported, involving covalently grafting PA6 chains onto MWNTs assisted by microwave irradiation. Copyright © 2010 Society of Chemical Industry     

Electrochemical storage of energy in carbon nanotubes and nanostructured carbons

Possibilities of electrochemical energy conversion using carbon nanotubes and related materials in various systems, such as lithium batteries, supercapacitors, hydrogen storage, are considered. It is shown that for these applications the electrochemical properties of multiwalled (MWNTs) and single walled (SWNTs) nanotubes are essentially dominated by their mesoporous character. During lithium insertion into nanotubular materials a high irreversible capacity C_irr (from 460 to 1080 mAh/g) has been observed after the first cycle with a tendency to further decomposition of electrolyte with cycling. Penetration of solvated lithium ions in the accessible mesopores is at the origin of this phenomenon; an almost linear dependence has been found between the mesopore volume and C_irr. Reversible capacity for lithium insertion C_rev ranged between 220 and 780 mAh/g; however, a great divergence (hysteresis) between insertion and extraction characteristics was observed independently on the kind of nanotubes and oxygen content. Amount of lithium stored by electrostatic attraction is negligible in comparison to real redox reactions which for thermodynamic reasons present linear variation of potential, especially during deinsertion (pseudocapacitive effects). During positive polarization, i.e., removal of lithium, resistivity of the electrode also gradually increases. Due to the open network of mesopores formed by the nanotubes entanglement, and consequently an easily accessible electrode-electrolyte interface, nanotubular materials are quite adapted for supercapacitor electrodes in various electrolytic solutions. High values of capacitance (80 F/g) have been obtained in 6 M KOH for materials with a surface area of only ca. 430 m²/g. Capacitance values have been enhanced either by additional oxygenated functionalisation of nanotubes (130 F/g) or by conducting polypyrrole (PPy) electrodeposition where the maximum values reached 170 F/g. The next domain of energy storage in the carbon nanostructures is the accumulation of hydrogen by the electrochemical decomposition of aqueous alkaline medium on a negatively polarized carbon electrode in ambient conditions. For SWNTs only moderate values (below 0.5 wt.% of H₂) have been found, while for activated carbons with highly developed surface area of 1500 m²/g, the amount of reversibly sorbed hydrogen was ca. 2 wt.%, noticeably larger than under dihydrogen pressure (only 0.4 wt.% for the same material at 70 bar and 273 K). The enhancement observed for the activated carbon is interpreted by the formation of nascent hydrogen during water reduction which penetrates easily in the available carbon nanopores. The values obtained by this method are comparable to those of metallic alloys, such as LaNi₅ for example.     

H2S oxidation by multi-wall carbon nanotubes decorated with tungsten sulfide

Tungsten sulfide catalysts decorated on single and multiwall carbon nanotubes (SWNTs & MWNTs) and activated carbon were synthesized, and XRD, ICP, SEM, TEM and ASAP analyses were employed to acquire the characteristics of each catalyst. Afterwards a gas flow containing 5,000 ppm of H₂S was passed over the catalyst in gas hour space velocity (GHSV) of 5,000 h^?1, temperature of 65 °C, steam volume percent of 20 and O₂/H₂S ratio equal to 2. The results revealed that the catalyst supported on MWNTs exhibited higher conversion amongst its counterparts. Then effects of GHSV, steam volume percent in the feed, catalyst loading and temperature were investigated on conversion of hydrogen sulfide to elemental sulfur for tungsten sulfide catalyst decorated on MWNTs.     

Effects of functionalized MWNTs with GMA on the properties of PMMA nanocomposites

The acid modification of multiwall carbon nanotubes (MWNTs) was performed by an HNO₃/H₂SO₄ solution. The glycidyl methacrylate (GMA) undergoing an opening‐ring was grafted onto the surface of acid‐modified MWNTs. The surface properties of MWNTs were investigated by Fourier transform infrared spectrometer (FTIR), Raman spectra, transmission electron microscopy (TEM), X‐ray diffraction, and thermogravimeric analysis. Then the MWNTs/ poly(methyl methacrylate) (PMMA) nanocomposites were prepared by in situ polymerization. The tribological and dielectric properties of nanocomposites were studied. As a result, GMA was grafted on the surface of MWNTs. The tribological and dielectric properties of MWNTs/ PMMA nanocomposites were improved as the content of the surface‐modified MWNT increased. The marked improvement in tribological and dielectric properties were attributed to the good dispersion of MWNTs that were bonded with C?C on the surface that participated in the polymerization of MMA. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Synthesis and characterization of double-walled carbon nanotubes from multi-walled carbon nanotubes by hydrogen-arc discharge

Double-walled carbon nanotubes (DWNTs) were synthesized in a large scale by a hydrogen arc discharge method using graphite powders or multi-walled carbon nanotubes/carbon nanofibers (MWNTs/CNFs) as carbon feedstock. The yield of DWNTs reached about 4 g/h. We found that the DWNT product synthesized from MWNTs/CNFs has higher purity than that from graphite powders. The results from high-resolution transmission electron microscopy observations revealed that more than 80% of the carbon nanotubes were DWNTs and the rest were single-walled carbon nanotubes (SWNTs), and their outer and inner diameters ranged from 1.75 to 4.87 nm and 1.06 to 3.93 nm, respectively. It was observed that the ends of the isolated DWNTs were uncapped and it was also found that cobalt as the dominant composition of the catalyst played a vital role in the growth of DWNTs by this method. In addition, the pore structures of the DWNTs obtained were investigated by cryogenic nitrogen adsorption measurements.     

Effects of oxyfluorination on a multi-walled carbon nanotube electrode for a high-performance glucose sensor

A glucose sensor electrode was prepared with multi-walled carbon nanotubes (MWNTs) because of its effect on surface modification through oxyfluorination. The oxyfluorination of MWNTs was carried out with F₂:O₂ ratios of 7:3, 5:5 and 3:7, which are labeled F7O3-MWNT, F5O5-MWNT, and F3O7-MWNT, based on the oxyfluorination conditions. The hydrophilic functional groups were introduced effectively on the hydrophobic carbon surface. In addition, the amorphous area of the MWNTs was affected by oxyfluorination. The reactivity of the glucose sensor was affected by the oxyfluorination treatment and the existence of amorphous on MWNTs. The optimum O/F percentage was approximately 50%. Therefore, the oxyfluorination conditions are important with amorphous MWNTs. The sensitivity was improved based on the effects of improved interface affinity between the enzyme and the carbon electrode. In addition, the presence of an amorphous area on MWNTs seems to be beneficial for efficient glucose oxidase immobilization, which results in high-performance glucose sensing.     

Preparation of TiO2/carbon nanotubes photocatalysts: The influence of the method of oxidation of the carbon nanotubes on the photocatalytic activity of the nanocomposites

A method for the preparation of efficient TiO₂/multi-wall carbon nanotubes nanocomposite photocatalysts by precipitation of anatase TiO₂ nanoparticles onto differently oxidized carbon nanotubes is presented. The precursor compound titanium(IV) bromide was hydrolyzed producing pure anatase phase TiO₂ nanoparticles decorated on the surface of the oxidized carbon nanotubes. The oxidative treatment of the carbon nanotubes influenced the type, quantity and distribution of oxygen-containing functional groups, which had a significant influence on the electron transfer properties, i.e., the photocatalytic activity of the synthesized nanocomposites. The results of C.I. Reactive Orange 16 photodegradation in the presence of all the synthesized nanocomposites showed their better photocatalytic activity in comparison to the commercial photocatalyst Degussa P-25.     

Evaluation of affinity of molecules for carbon nanotubes

Recent developments of non-covalent functionalization of carbon nanotubes (CNTs) require a systematic understanding of the interaction between molecule and CNTs (CNT-molecular interaction); however, it has been difficult to evaluate the "net" interaction between the CNTs and molecules. We now use silica gel particles coated with the pristine single-walled carbon nanotubes (SWNTs) in a monolayer fashion as the stationary phase of a HPLC column. The newly developed column (SWNT-column) worked as a powerful tool for ranking the interactions between the SWNTs and molecules with a high precision. We describe the binding affinity analysis of polyaromatic hydrocarbons onto the surfaces of SWNTs. The obtained ranking is determined in the order of benzene < naphthalene < biphenyl < fluorene < phenanthrene < anthracene ～ pyrene < triphenylene < p-terphenyl < tetraphene < tetracene.