Complete elimination of metal catalysts from single wall carbon nanotubes

The complete removal of entrapped metallic impurities (i.e. Ni and Co) incorporated within single wall carbon nanotubes (SWNTs) has been a long-standing issue. A sonication-mediated treatment of as-obtained SWNT soot in a 1:1 mixture of aqueous hydrofluoric and nitric acids resulted in the complete elimination of these impurities as shown by energy dispersive X-ray analysis (EDAX). Contact angle measurements indicated that the wetting of SWNTs is enhanced in the presence of HF. The presence of HNO₃ and surfactant was found essential in removing the catalyst due to SWNT etching of end-caps/defects and providing better dispersion, respectively. Moreover, Raman spectroscopy indicated that the structural purity of the SWNTs is not compromised by the HF/HNO₃ purification treatment.     

Purification of single-wall carbon nanotubes produced by arc plasma jet method

In the arc plasma jet method, a large amount of soot including single-wall carbon nanotubes (SWNTs) can be produced in a short time (1–2 g/min). However, a lot of impurities, such as amorphous carbon and catalyst metals, are included in the produced soot besides SWNT. Purification is indispensable to apply SWNTs industrially, but it was difficult until recently. Here, we report that SWNTs can be purified easily in large quantities by reflux in the hydrogen peroxide solution using catalyst of iron particle, which can activate the oxidation reaction of hydrogen peroxide solution. Higher than 90 wt.% purity of SWNTs are obtained by this technique.     

High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell

This research aims to enhance the activity of Pt catalysts, thus to lower the loading of Pt metal in fuel cell. Highly dispersed platinum supported on single-walled carbon nanotubes (SWNTs) as catalyst was prepared by ion exchange method. The homemade Pt/SWNTs underwent a repetition of ion exchange and reduction process in order to achieve an increase of the metal loading. For comparison, the similar loading of Pt catalyst supported on carbon nanotubes was prepared by borohydride reduction method. The catalysts were characterized by using energy dispersive analysis of X-ray (EDAX), transmission electron micrograph (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectrum (XPS). Compared with the Pt/SWNTs catalyst prepared by borohydride method, higher Pt utilization was achieved on the SWNTs by ion exchange method. Furthermore, in comparison to the E-TEK 20 wt.% Pt/C catalyst with the support of carbon black, the results from electrochemical measurement indicated that the Pt/SWNTs prepared by ion exchange method displayed a higher catalytic activity for methanol oxidation and higher Pt utilization, while no significant increasing in the catalytic activity of the Pt/SWNTs catalyst obtained by borohydride method.     

Vertically-aligned carbon nanotubes prepared by water-assisted chemical vapor deposition

Vertically-aligned single walled carbon nanotubes (V-SWNTs) were prepared by water-assisted and ethylene-pyrolyzing chemical vapor deposition. Water vapor was introduced into the growth environment by passing a small fraction of Ar carrier gas through a water bubbler. The effects of processing gas flow rate, growth time, and water vapor content on the growth of SWNTs using Mo/Fe as the catalyst were systematically studied. For the catalyst of SWNTs growth, multi-metal layers consisting of an Al underlayer with 10 nm in thickness and Mo/Fe layers on top were deposited onto the silica film. Evidences of V-SWNTs with high-purity were found by Raman spectroscopy and TEM morphology. The trace amount of water vapor enhanced the activity of metal catalysts to further assist the growth of SWNTs. For the Mo (0.5 nm)/Fe (1 nm) structure, V-SWNTs grew from surface when the growth time lasted 10 min shown in SEM diagram meanwhile the portion of SWNTs increased until 30 min and then a layer of SWNTs developed shown in the RBM peak in the Raman spectra.     

An Attempt at Enhancing the Regioselective Oxidation of Decane Using Catalysis with Reverse Micelles

Abstract  

The regioselective oxidation of linear alkanes to give terminal oxidation products represents a major challenge for catalysis. A number of previous approaches have shown that confinement and encapsulation can offer an experimentally viable way forward. Against this background we have investigated the use of a system comprising gaseous oxygen, a homogenous catalyst (ammonium metavanadate) confined in an aqueous solution within reverse micelles formed in decane using bis(2-ethylhexyl)sulfosuccinate. At low conversion and at short reaction times we show that the approach does lead to a small enhancement in the selectivity to terminal products, but unsurprisingly the surfactant is more readily oxidised than decane and so the small positive effects of the micellar catalyst system are short-lived.     

Purification of Single-Walled Carbon Nanotubes (SWNTs) by Acid Leaching,NaOH Dissolution,and Froth Flotation

The SWNTs with a carbon content of approximately 3% were synthesized via the disproportionation of CO over a CoMo/SiO₂ catalyst. The three sequential steps, including acid treatment, silica dissolution, and froth flotation, were proposed for the purification of the as-synthesized SWNTs. The pretreatment step for the catalyst removal by acid leaching was optimized at an HCl concentration of 6 M, 50°C, and a sonication time of 3 h, corresponding to the Co and Mo removals of 84% and 44%, respectively. For the silica dissolution, the SWNTs sample after the acid leaching step was treated with a 5 M NaOH solution at 50°C and a sonication time of 3 h, leading to a silica removal of 54%. The froth flotation was employed to separate the SWNTs from the remaining silica using SDBS as an anionic frother. The process performance was maximized at an SDBS concentration of 0.1 × CMC, an air flow rate of 120 cm³/min, and a solution pH of 5, yielding a carbon content of the purified SWNTs of 71%. It was also found that the proposed technique successfully purified the as-synthesized SWNTs sample without damaging its nanostructure.     

Properties of a field emission lighting plane employing highly crystalline single-walled carbon nanotubes fabricated by simple processes

We developed a new cathode for a visible ray flat plane-emission device formed from a mixture of highly crystalline single-walled carbon nanotubes (SWNTs) dispersed into an organic In₂O₃–SnO₂ precursor solution and a non-ionic surfactant. A thin film field emission cathode having highly homogeneously dispersed SWNTs was able to be fabricated with low driving voltage, high electron emission homogeneity in the plane, and high brightness efficiency by employing a simple scratching process for the film.The turn-on field of a diode using the optimized cathode was 1.2 V/μm, with the brightness homogeneity in that plane being within 5% against the averaged brightness in the lighting plane. Favorable brightness homogeneity from the lighting device to control the content of SWNTs in the coated film was achieved by employing highly crystalline SWNTs. Furthermore, brightness efficiency and emission life-time, which are important factors when comparing luminance devices, totaled more than 70 lm/W and over 5000 h until reaching the half time against the initial field emission current density.This flat plane-emission device has the potential to provide a new approach to lighting in everyday life as it contributes to energy saving through its low power consumption.     

Low cost growth route for single-walled carbon nanotubes from decomposition of acetylene over magnesia supported Fe-Mo catalyst

A large amount of single wall carbon nanotubes (SWNTs) was successfully produced by thermal decomposition of C₂H, at 800 °C over magnesia supported Fe-Mo bimetallic catalysts in a tubular flow reactor under an atmosphere of hydrogen flow. The growth density of SWNTs increased with increasing the weight percent of the catalyst metals (wt% ratio of two metals: 50 : 50) supported on magnesia (MgO) from 5 to 30 wt%. The yield of SWNTs reached 144.3% over 30 wt% metal-loaded catalyst. Raman measurements showed the growth of bundle type SWNTs with diameters ranging from 0.81 to 1.96 nm. The growth of SWNTs was also identified by thermal gravimetric analysis (TGA) and Raman spectroscopy.     

Diameter control of single-walled carbon nanotubes by plasma rotating electrode process

We investigated plasma rotating electrode process (PREP) as a method for diameter control of single-walled carbon nanotubes (SWNTs) with a Ni–Y catalyst in He ambient. Compared with the diameters of SWNTs produced by the conventional arc discharge (0 rpm), those of SWNTs, which were synthesized by PREP (5000 and 10 000 rpm), were decreased. This result indicated that the centrifugal force by the rotation of anode could control the diameter distributions of SWNTs by controlling kinetic energies of carbon and/or metal species or formation region of SWNTs.     

Covalent functionalization of single-walled carbon nanotubes by aniline electrochemical polymerization

Electrochemical polymerization of aniline in an HCl solution on a single-walled carbon nanotubes (SWNTs) film has been studied by Raman and FTIR spectroscopy. It is shown that this method leads to a covalent functionalization of SWNTs with polyaniline (PANI). A careful study in Raman scattering shows that the increase in the intensity of the band at 178 cm⁻¹ associated with radial breathing modes of SWNTs bundles suggests an additional nanotubes roping with PANI as a binding agent. A post chemical treatment with the NH₄OH solution of polymer-functionalized SWNTs involves an internal redox reaction between PANI and carbon nanotubes. As a result, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one.     

Photooxidation of anionic surfactant (sodium lauryl sulfate) in a three-phase fluidized bed reactor using TiO2/SiO2 photocatalyst

The photooxidation of sodium lauryl sulfate (=sodium dodecyl sulfate) in two different types of three-phase fluidized bed reactors was investigated. A low concentration of sodium lauryl sulfate (0.1–0.6 mM) in aqueous solution was photocatalytically decomposed by a TiO₂ photocatalyst immobilized on a porous SiO₂ support. In order to determine the optimum operating conditions in the fluidized beds, the effects of the air flow rate, amount of catalyst, initial concentration of surfactant, light source power, and pH on the photooxidation rate were investigated. From the experimental results, it was observed that the superficial air velocity was an important parameter in determining the reaction rate for both reactors. The photooxidation reaction rate increased with increasing UV lamp power and the experimentally obtained reaction rates showed good agreement with the Langmuir adsorption model. Also, a higher reaction rate was observed when the aqueous solution was acidic.     

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.     

Controlling the diameter distribution of carbon nanotubes grown from camphor on a zeolite support

Single-wall and multi-wall carbon nanotubes (SWNTs and MWNTs, respectively) of controlled diameter distribution were selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe-Co catalyst. Effects of catalyst concentration, growth temperature and camphor vapor pressure were investigated in wide ranges, and diameter distribution statistics of as-grown nanotubes was analyzed. High yields of metal-free MWNTs of fairly uniform diameter (∼10 nm) were grown at 600-700 °C, whereas significant amounts (∼30%) of SWNTs were formed at 850-900 °C within a narrow diameter range of 0.86-1.23 nm. Transmission electron microscopy and micro-Raman spectroscopy reveal that camphor-grown nanotubes are highly graphitized as compared to those grown from conventional CNT precursors used in chemical vapor deposition.     

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is ∼4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites.     

Determination of critical micelle concentration of surfactant by ultraviolet absorption spectra

A method was investigated for determining the critical micelle concentration (CMC) by the shift of absorption maxima when an organic compound (I) with ultraviolet absorption was added to an aqueous solution of a surfactant. When I was added to the surfactant solution at higher concentrations (above the CMC), λ_max of I approached the value inn-octane, since I was solubilized in the hydrocarbon atmosphere of the inner part of the surfactant micelle. At lower concentrations (below the CMC), however, I was present in the water phase and λ_max approached the value in water. The curve of λ_max vs. surfactant concentration declined from the high concentration values as the CMC was approached and at the CMC, the curve broke upward sharply. Then, it rose for some time and approached the value in water. N,N′-diethylaniline was used because it exhibited larger shifts of λ_max. The standard amount used was 0.002 ml/3–10 ml of aqueous solution of the surfactant. The CMC values obtained agreed with those obtained by the electric conductivity method, dye adsorption method and light scattering method, for surfactants such as tetradecyldimethylbenzylammonium chloride, sodium dodecyl sulfate and polyoxyethylene cetyl ether.     

The stabilization effect of mixed-surfactants in the emulsion polymerization of styrene

Summary The emulsion polymerization of styrene was performed at 50°C with a mixture of anionic and nonionic surfactants using different surfactant concentrations. In the single-surfactant systems, a proportional relationship was observed between the total particle surface area per cm³ of aqueous solution at 90% conversion (TS) and the amount of surfactant used for each polymerization. For mixed-surfactant systems an additivity was established between the TS value and surfactant composition. The study of the particle size data from low to high conversion showed that the particle number changed with conversion.     

Dysprosium-Catalyzed Growth of Single-Walled Carbon Nanotube Arrays on Substrates

In this letter, we report that dysprosium is an effective catalyst for single-walled carbon nanotubes (SWNTs) growth via a chemical vapor deposition (CVD) process for the first time. Horizontally superlong well-oriented SWNT arrays on SiO₂/Si wafer can be fabricated by EtOH-CVD under suitable conditions. The structure and properties are characterized by scanning electron microscopy, transition electron microscopy, Raman spectroscopy and atomic force microscopy. The results show that the SWNTs from dysprosium have better structural uniformity and better conductivity with fewer defects. This rare earth metal provides not only an alternative catalyst for SWNTs growth, but also a possible method to generate high percentage of superlong semiconducting SWNT arrays for various applications of nanoelectronic device.     

Development of Fe-doped carbon electrode for mass-producing high-yield single-wall carbon nanotubes

High crystallinity single-wall carbon nanotubes (SWNTs) can be synthesized by arc discharge evaporation of Fe-doped carbon electrode in hydrogen mixed gas, but the purity of as-grown SWNTs is strongly affected by the kinds of Fe-doped carbon electrode. Various carbon materials (artificial graphite powder, carbon black, calcined coke, etc.) have been tried to prepare Fe-doped carbon electrodes. The calcined coke, a kind of graphitizing carbon, is suitable for preparing high-quality Fe-doped carbon electrode. Moreover, the heat-treatment of Fe-doped carbon electrode in vacuum at the temperature of 1600 °C also plays an important role. At present, the best carbon electrode containing 1 at.% Fe catalyst is capable of continuously generating SWNTs at a production rate of 8 mg/min, which can be easily purified to obtain high purity SWNTs.     

Dissolution mechanism of polyelectrolyte complexes in aqueous surfactant solutions

The dissolution of polyelectrolyte (PE) complexes (PECs) in cetyl trimethyl–ammonium bromide (CTAB) or in sodium dodecyl sulfate (SDS) has been investigated. It was verified that the complex divides first into its two components (anionic PE and cationic PE) in aqueous surfactant solution; then one of the components reacts with oppositely charged surfactant to produce a PE–surfactant complex. The dissolution property of the PEC mainly depends on the solubility of PE–surfactant, ie the PE–surfactant dissolves in its aqueous surfactant solution, the PEC also dissolves or otherwise it is insoluble. A tentative dissolution mode of PEC in aqueous surfactant solution is proposed. © 2000 Society of Chemical Industry     

Aqueous Extended-Surfactant Based Method for Vegetable Oil Extraction: Proof of Concept

The use of hexane to extract vegetable oil from oilseeds is of growing concern due to hexane’s environmental impact and because of worker exposure concerns. The goal of our work is to demonstrate that the aqueous extended-surfactant-based method is a viable alternative for vegetable oil extraction. In our method, ground oilseeds were dispersed in the aqueous surfactant solution, allowing the oil to be liberated from the seeds as a separate phase from the aqueous phase. The impact of pH, shaking intensity, shaking time and seed to liquid ratio on oil yield are presented. Extended-surfactants are a new type of surfactant with propoxylate (PO) and/or ethoxylate (EO) groups inserted between the hydrophilic head and the hydrophobic alkyl chain of the surfactant molecule. This unique structure of extended-surfactants enables them to produce ultralow interfacial tension with vegetable oils. We have found that at low aqueous concentrations (less than 0.3 wt%), extended-surfactant solutions are able to produce ultralow interfacial tension between aqueous extraction and vegetable oil phases. At optimum condition (seed to liquid ratio of 1–5, 30 min extraction at 150 shakes/min and 30 min centrifugation at 2,170×g) we achieved 93–95% extraction efficiency for peanut and canola oils at 25 °C. The oil quality produced from the aqueous extended-surfactant-based method was found to be comparable or even superior to that obtained from hexane-based extraction, further demonstrating the viability of aqueous extended-surfactant based extraction.