Double-walled carbon nanotubes: Quantitative purification assessment,balance between purification and degradation and solution filling as an evidence of opening

We compared the effect of different oxidizing agents on purification, functionalization and opening of double-walled carbon nanotubes. The oxidative treatments were realized in nitric acid solutions at different concentrations (3 M or 15 M), in a mixture of two oxoacids (conc. HNO₃/conc. H₂SO₄) or in sulphuric acid solutions of KMnO₄ or K₂Cr₂O_7. Most of these treatments were very efficient for purification (removal of residual catalytic metal nanoparticles and/or of disorganized carbon) but also caused secondary reactions such as shortening of the nanotubes, creation of functionalized amorphous carbon deposits and covalent functionalization of the outer wall. Secondary treatments were undertaken in order to remove functionalized carbon deposit by washing with sodium hydroxide solutions or by heat treatment in air. A partial filling in solution was obtained with uranyl nitrate, in order to evidence the opening of carbon nanotubes. Effects of purification and filling treatments were characterized both qualitatively by TEM and HRTEM, AFM and Raman spectroscopy, and quantitatively by elemental chemical analysis and chemical titrations.     

Synthesis and characterization of single-walled carbon nanotubes filled with the superionic material SnF2

We report X-ray powder diffraction, high resolution transmission electron microscopy and Raman spectroscopy of the semiconducting single-walled carbon nanotubes filled with the superionic material SnF₂. We first have obtained the Raman spectra of filling SWCNTs using the near-infrared excitation (1064 nm). Our results show that SnF₂ behaves as an electron acceptor with small charge transfer from nanotubes to SnF₂. We interpret the large high-frequency shift of radial breathing modes as due to both charge transfer and the increasing interaction between filling material and nanotubes.     

Influence of Single-Walled Carbon Nanotubes on the Performance of Poly(Azomethine-Ether) Composite Materials

The present work is aimed to fabricate a new set of composite materials containing conducting poly(azomethine-ether) reinforced with single-walled carbon nanotubes in the form of single-walled carbon nanotube/poly(azomethine-ether)_1–5 for excellent enhanced thermal as well as conducting behavior of poly(azomethine-ether). Single-walled carbon nanotubes of variable loading have been embedded into conducting poly(azomethine-ether) using in situ polymerization technique. Before attempting the polymerization, 1,3-thiazole established poly(azomethine-ether) and its conformable monomers have been prepared and their chemical structures have been correlated by spectral analyses. Furthermore, η_inh and M_w values for poly(azomethine-ether) were found 0.89?dL?g^?1 and 39723.6, respectively. The fabricated single-walled carbon nanotube/poly(azomethine-ether)_1–5 composites were specified and characterized by wide-angle X-ray diffraction patterns, Fourier transform infrared spectroscopy, thermal behavior, scanning electron microscopy, and transmission electron microscopy characterization techniques. A perfect indicative response for this composite material was estimated by Fourier transform infrared spectra and X-ray diffraction as well. Both techniques displayed all intensive characteristic peaks regarding single-walled carbon nanotubes and poly(azomethine-ether) in the spectra or diffraction pattern for single-walled carbon nanotube/poly(azomethine-ether)_1–5. The role of single-walled carbon nanotubes on the performance of poly(azomethine-ether) was considerably examined. Single-walled carbon nanotube/poly(azomethine-ether)_1–5 showed relatively higher thermal stability. Single-walled carbon nanotube/poly(azomethine-ether)₁ displayed the lowest final composite degradation temperature value (552°C), whereas single-walled carbon nanotube/poly(azomethine-ether)₅ displayed the highest value (621°C). T₁₀ and T₂₅ values showed a gradual temperature increased while single-walled carbon nanotubes increased. Single-walled carbon nanotube/poly(azomethine-ether)₁ showed the lowest thermal stability and single-walled carbon nanotube/poly(azomethine-ether)₅ showed the highest thermal stability between all fabricated products. Furthermore, transmission electron microscopy images showed a prominent increase in single-walled carbon nanotubes diameters (40–60?nm). The conductivity values were significantly increased while single-walled carbon nanotubes content was increased and reached to the semiconductors. ε′ values were also increased in both single-walled carbon nanotube/poly(azomethine-ether)_4,5 which have higher single-walled carbon nanotubes content.     

The study of thermodynamics and kinetics methyl orange and malachite green by SWCNTs,SWCNT-COOH and SWCNT-NH2 as adsorbents from aqueous solution

The effective removal of dyes from aqueous wastes is among the most important issues for many industrialized countries. Removal of methyl orange (MO) and malachite green (MG) from aqueous solutions were studied using single-walled carbon nanotubes (SWCNTs), carboxylate functionalized single-walled carbon nanotubes (SWCNT-COOH) and amide functionalized single-walled carbon nanotubes (SWCNT-NH₂). The adsorption process was found to be controlled by temperature, ionic strength, initial concentration, adsorbent dosage and contact time. The microstructure of carbon nanotubes was characterized using SEM and FTIR. The adsorbents studied exhibits high efficiency for MO and MG adsorption and the equilibrium states could be achieved in 20, 20, 15 (min) for SWCNTs, SWCNT-COOH, SWCNT-NH₂, respectively. Adsorption capacity of each adsorbent increased with increasing active groups on the surface of carbon nanotube, where SWCNT-NH₂ was the most effectively adsorbent.     

Facile synthesis of amorphous Cr2O3/N‐doped carbon nanosheets and its excellent lithium storage property

The Cr₂O₃ with high‐energy density and relatively low lithium insertion potential is a promising anode candidate for LIBs. However, the intrinsic poor electroconductivity and side effects like volume expansion of Cr₂O₃ severely limit its capacity and cyclability at high charge/discharge rates. To address the problem, the amorphous Cr₂O₃/N‐doped carbon nanosheets (denoted as a‐Cr₂O₃/NC) have been designed and prepared by an easy one‐step solution combustion synthesis method from a uniform solution of chromium nitrate, glucose, and glycine. The as‐synthesized a‐Cr₂O₃/NC consist of amorphous Cr₂O₃ particles and N‐doped carbon sheet, where the amorphous Cr₂O₃ is evenly encapsulated in the carbon sheet support. An anode prepared from the synthesized a‐Cr₂O₃/NC demonstrates much higher specific capacity and better cycling performance than the crystalline Cr₂O₃ anode. Upon extended cycling, the a‐Cr₂O₃/NC anode exhibits good long‐term stability and its reversible capacity retains as high as 782.4 mAh g^?1 after 500 cycles at 1 A g^?1. Such good performance stems from its unique structure. The amorphous structure of Cr₂O₃ can furnish a mass of enterable active sites which can favor the lithium ions insertion/extraction, whereas the sheet‐like N‐doped carbon support can increase the electroconductivity and facilitate the transportation of lithium ions and electrons.     

Synthesizing carbon nanotubes and carbon nanofibers over supported-nickel oxide catalysts via catalytic decomposition of methane

Supported-NiO catalysts were tested in the synthesis of carbon nanotubes and carbon nanofibers by catalytic decomposition of methane at 550 °C and 700 °C. Catalytic activity was characterized by the conversion levels of methane and the amount of carbons accumulated on the catalysts. Selectivity of carbon nanotubes and carbon nanofiber formation were determined using transmission electron microscopy (TEM). The catalytic performance of the supported-NiO catalysts and the types of filamentous carbons produced were discussed based on the X-ray diffraction (XRD) results and the TEM images of the used catalysts. The experimental results show that the catalytic performance of supported-NiO catalysts decreased in the order of NiO/SiO₂ > NiO/HZSM-5 > NiO/CeO₂ > NiO/Al₂O₃ at both reaction temperatures. The structures of the carbons formed by decomposition of methane were dependent on the types of catalyst supports used and the reaction temperatures conducted. It was found that Al₂O₃ was crucial to the dispersion of smaller NiO crystallites, which gave rise to the formation of multi-walled carbon nanotubes at the reaction temperature of 550 °C and a mixture of multi-walled carbon nanotubes and single-walled carbon nanotubes at 700 °C. Other than NiO/Al₂O₃ catalyst, all the tested supported-NiO catalysts formed carbon nanofibers at 550 °C and multi-walled carbon nanotubes at 700 °C except for NiO/HZSM-5 catalyst, which grew carbon nanofibers at both 550 °C and 700 °C.     

Preferential Growth of Semiconducting Single-Walled Carbon Nanotubes on Substrate by Europium Oxide

In this paper, we have demonstrated that europium oxide (Eu₂O₃) is a new type of active catalyst for single-walled carbon nanotubes (SWNTs) growth under suitable conditions. Both random SWNT networks and horizontally aligned SWNT arrays are efficiently grown on silicon wafers. The density of the SWNT arrays can be altered by the CVD conditions. This result further provides the experimental evidence that the efficient catalyst for SWNT growth is more size dependent than the catalysts themselves. Furthermore, the SWNTs from europium sesquioxides have compatibly higher quality than that from Fe/Mo catalyst. More importantly, over 80% of the nanotubes from Eu₂O₃ are semiconducting SWNTs (s-SWNTs), indicating the preferential growth of s-SWNTs from Eu₂O₃. This new finding could open a way for selective growth of s-SWNTs, which can be used as high-current nanoFETs and sensors. Moreover, the successful growth of SWNTs by Eu₂O₃ catalyst provides new experimental information for understanding the preferential growth of s-SWNTs from Eu₂O₃, which may be helpful for their controllable synthesis.     

Carbon nanotubes as a template for mild synthesis of magnetic CoFe2O4 nanowires

Hundred nanometers outer diameter multi-walled carbon nanotubes have been used as suitable host template for synthesizing CoFe₂O₄ nanowires encapsulated inside nanotubes under mild conditions, i.e. 100 °C and atmospheric pressure, with a high filling yield of the nanotubes, using an aqueous nitrate precursor solution and the confinement effect provided by the surrounding walls. The formation of caps near the tube tips at the beginning of the nitrate decomposition led to consider each nanotube as a closed nanoreactor, in which the reaction conditions could be far different from the macroscopic conditions outside the tube. The structure of the CoFe₂O₄ nanowires could be continuously changed from a disordered hair-like dendritic structure at 100 °C to highly crystallized domains when increasing the temperature. A material with high coercivity at room temperature for small particles of about 25 nm in diameter was obtained by submitting the nanowires to an Ar treatment at 550 °C for 2 h.     

Flame synthesis of carbon nanotubes and few-layer graphene on metal-oxide spinel powders

Multi-walled and single-walled carbon nanotubes (CNTs) and few-layer graphene (FLG) are grown directly on spinel powders using flame synthesis. CNT and FLG growth occurs via the decomposition of flame-generated carbon precursors (e.g., CO, C₂H₂, and CH₄) over nanoparticles (i.e., Ni, Co, Fe, and Cu) reduced from the solid oxide. The growth of CNTs is investigated on NiAl₂O₄, CoAl₂O_4, and ZnFe₂O₄, using counterflow diffusion flame and multiple inverse-diffusion flames (m-IDFs), while the growth of FLG is investigated on CuFe₂O₄ using m-IDFs. As shown by analytical electron microscopy techniques, Raman spectroscopy, and X-ray diffraction, substrate temperature and spinel composition play critical roles in the growth of both CNTs and FLG.     

Modelling the surface adsorption of methane on carbon nanostructures

Methane (CH₄) adsorption is investigated on both graphite and in the region between two aligned single-walled carbon nanotubes, which we refer to as the groove site. We exploit the Lennard–Jones potential function and the continuous approximation to determine surface binding energies between a single CH₄ molecule and graphite and between a single CH₄ and two aligned single-walled carbon nanotubes. Our modelling indicates that for a CH₄ molecule interacting with graphite, the binding energy of the system is minimized when the CH₄ carbon is 3.83 Å above the surface of the graphitic carbon, while the binding energy of the CH₄–groove site system is minimized when the CH₄ carbon is 5.17 Å away from the common axis shared by the two aligned single-walled carbon nanotubes. Our results confirm the current view that for larger groove sites, CH₄ molecules in grooves are likely to move towards the outer surfaces of one of the single-walled carbon nanotubes. Our results are computationally efficient and are in good agreement with experiments and molecular dynamics simulations, and show that CH₄ adsorption on graphite and groove surfaces is more favourable at lower temperatures and higher pressures.     

Toughness measurement and toughening mechanisms of arc ion plating Cr2O3 films treated by annealing

Chromium sesquioxide (Cr₂O₃) films were deposited on Ni-based high-temperature alloy substrates by an arc ion plating technique and then annealed at different temperatures and heating rates. The influence of annealing conditions on the toughness of Cr₂O₃ films was calculated according to spherical indentation tests. The increase in grain size and compressive stress, variety of microstructure and surface morphology, and atom diffusion that resulted from annealing caused toughness variations. The increase in grain size closed micro-cracks along the direction of film growth. Compressive stress and a multi-crystal plane led to cracks caused by indentation that required more energy to break through the film. In the process of indentation, turning, bifurcating, and bridging of cracks on film surface was also able to dissipate energy. Atom diffusion in the process of 1000 °C annealing also played a role in grain boundary toughening. The toughness improvement of Cr₂O₃ film significantly improved friction life.     

Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr2 − xFexO3 as catalyst

Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H₂/NG and Ar/NG atmospheres at 950 °C. The Cr_2 − xFe_xO₃ catalyst was capable to produce carbon nanotubes only in H₂/NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO₃.     

Oxidative dehydrogenation of ethane by carbon dioxide over sulfate‐modified Cr2O3/SiO2 catalysts

The oxidative dehydrogenation of ethane into ethylene by carbon dioxide over unsupported Cr₂O₃, Cr₂O₃/SiO₂ and a series of Cr₂O₃/SiO₂ catalysts modified by sulfate was investigated. The results show that Cr₂O₃/SiO₂ is an effective catalyst for dehydrogenation of ethane and CO₂ in the feed promotes the catalytic activity. Sulfation of silica will influence the catalytic behavior of Cr₂O₃/SiO₂ in dehydrogenation of ethane with carbon dioxide depending on the amount of sulfate. Cr₂O₃/6 wt% SO ₄ ^2- –SiO₂ catalysts exhibit an excellent performance for this reaction, giving an ethylene yield of 55% at 67% ethane conversion at 650°C. Characterizations indicate that addition of sulfate changes the bulk and surface properties of Cr₂O₃/SiO₂, promoting the reduction of Cr⁶⁺ to Cr³⁺ and favoring the catalytic conversion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemisorption of gases and reaction of propane dehydration at chromic oxide

Characteristics of Cr₂O₃ as an adsorbent and as a catalyst of propane dehydration by carbon dioxide are studied by adsorption and calorimetric techniques and by kinetic techniques, respectively. Carbon dioxide is usually applied as a dehydrating agent for a number of processes in the coal-chemical industry. For the first time, the molar heats of chemisorption of gases at Cr₂O₃ are studied under elevated temperatures. Cr₂O₃ surface is homogeneous relative to its chemisorption ability. Hydrogen is capable of chemisorbing at the surface (the molar heat of chemisorption is equal to 309.4 kJ/mol) and of sorbing into the solid body (the molar heat of sorption is equal to 170.45 kJ/mol), carbon dioxide is capable of chemisorbing at the surface (the molar heat of chemisorption is equal to 103 kJ/mol). The occurrence of hydrogen in the Cr₂O₃ body induces the Cr₂O₃ capability of sorbing an additional oxygen amount over that characteristic for completely oxidized samples that contain no hydrogen. Chemisorption of excessive oxygen proceeds up to the stoichiometry that corresponds to formation of hydroxyls in the Cr₂O₃ body. Hydrogen sorption by chromic oxide influences its catalytic activity.     

Transmission electron microscopy characterization of nanostructured carbon derived from Cr3C2 and Cr(C5H7O2)3

The preparation, characterization and comparison of nanostructured carbons derived by direct chlorination of Cr₃C₂ and Cr(C₅H₇O₂)₃ are reported in this work. Cr₃C₂ precursor was treated at 400 and 900 °C with a reaction time of 1 h. The nanostructure of the products has been characterized in some detail by means of transmission electron microscopy and associated techniques, such as electron energy-loss and X-ray energy dispersive spectroscopies and high-angle annular dark field imaging. Remains of Cr₃C₂ encapsulated in an amorphous carbon shell were observed at 400 °C, whereas carbon with higher ordering degree was produced at 900 °C. In the latter case, the sample can be described as a continuous variation from poorly-stacked graphene-like carbon to graphitic agglomerates. Remains of the reaction by-product, CrCl₃, are detected in the carbon particles, forming monolayers intercalated inside the graphitic agglomerates and amorphous nanoparticles. As a comparison, carbon samples derived from Cr(C₅H₇O₂)₃ were prepared at 300 and 900 °C. They mainly consist of highly disordered carbon, with local graphite-like stacking in the sample prepared at 900 °C.     

Improved SnO2–Sb2O4 based anode modified with Cr3C2 and CNT for phenol oxidation

Chromium carbide (Cr₃C₂) and carbon nanotubes (CNTs) improved Ti/SnO₂–Sb₂O₄ electrodes were successfully fabricated using pulse electro-co-deposition technique. The morphologies and phase constituents of these electrodes were characterized using scanning electron microscope (SEM) and X-ray diffraction (XRD). The service lifetime of anode was significantly increased by adding Cr₃C₂. The service lifetime of Ti/SnO₂–Sb₂O₄–Cr₃C₂ and Ti/SnO₂–Sb₂O₄–CNT–Cr₃C₂ electrode was 7.4 times and 5.6 times longer than that of the Ti/SnO₂–Sb₂O₄ electrode, respectively. The catalytic activity of phenol oxidation on these electrodes was systematically investigated by the cyclic voltammetry and the chemical oxygen demand (COD) test. The Ti/SnO₂–Sb₂O₄–CNT–Cr₃C₂ electrode shows the highest evolution oxygen potential, COD removal and current efficiency (CE).     

Spray coating as a simple method to prepare catalyst for growth of diameter-tunable single-walled carbon nanotubes

Spray coating is proposed as an optional wet method for preparing nano-sized particles suitable for the growth of single-walled carbon nanotubes (SWCNTs). The obtained SWCNT films are characterized by Raman spectroscopy and electron microscopy, and are confirmed to be comparable to SWCNTs produced by the conventional dip-coating process in terms of crystallinity, tube diameter and carbon yield. The mean diameter of SWCNTs can be effectively reduced from 1.85 to 1.35 nm by prolonging the deposition of Mo. In addition, spray coating allows catalyst preparation on supports other than flat wafers, as demonstrated by the synthesis of high-quality SWCNTs on Al₂O₃ fiber and quartz wool supports.     

Electric field enhancements in In2O3-coated single-walled carbon nanotubes

In₂O₃ nanoparticles are coated on the surfaces of single-walled carbon nanotubes (SWCNTs) by a successive ionic layer adsorption and reaction process. The thickness of the In₂O₃ nanoparticle film is tuned by controlling the number of coating cycles. The electric field around the In₂O₃-coated SWCNTs is compared with that of pristine SWCNTs. Field enhancement of the In₂O₃-coated SWCNTs is confirmed by conductive atomic force microscopy at low electric field (contact mode: 1 V to −1 V) and also field emission (FE) analysis at high electric field (0–4.2 V/μm). The uniformity and emission stability are also measured via FE analysis. Near infrared and X-ray photoemission spectroscopy data are suggested to explain the charge transfer, bandgap change between the In₂O₃ nanoparticles and SWCNTs, and the electric field enhancements in the In₂O₃-coated SWCNTs at both low and high electric field.     

Hot filament enhanced CVD synthesis of carbon nanotubes by using a carbon filament

The low temperature synthesis of single-walled carbon nanotubes (CNTs) on a Si substrate has been reported. Single-walled CNTs were grown from a C₂H₂ and H₂ mixture by a hot-filament enhanced CVD method using a carbon filament. The catalyst was silica-supported iron–cobalt prepared by sol–gel method. We observed the influence of the catalyst material, C₂H₂ concentration, growth pressure and substrate temperature on the formation of the CNTs. By optimizing the catalyst, both single-walled CNTs and multiwalled CNTs could be synthesized, depending on the reaction conditions. The formation of single-walled CNTs occurred when the carbon supply was kept low, i.e. low C₂H₂ concentration and low reaction pressure. The diameter of tubes decreased with increasing the substrate temperature. By optimizing the growth conditions, a small diameter of 0.65 nm single-walled CNTs, estimated from Raman scattering spectrum, was achieved even the low substrate temperature as 660 °C.     

Effects of PTFE activation and carbon sources on combustion synthesis of Cr2AlC/Al2O3 composites

Formation of Cr₂AlC/Al₂O₃ in situ composites was investigated by self-propagating high-temperature synthesis (SHS) involving both PTFE activation and aluminothermic reduction. In addition to Al and Cr₂O₃ as the starting materials, carbon black, graphite, and Al₄C₃ were used as the carbon sources. PTFE was employed not only as a reaction promoter, but also as a carburizing agent. Depending on different sources of carbon, the threshold amounts of PTFE for inducing self-sustaining combustion were 1.5, 4.0, and 3.0 wt% for the samples adopting carbon black, graphite, and Al₄C₃, respectively. The combustion front velocity and temperature increased with increasing PTFE content. Moreover, the sample using carbon black was the most exothermic, while the Al₄C₃-based sample was the least. For the powder compacts adopting carbon black or graphite, Cr₂AlC/Al₂O₃ composites were produced with no impurities. Due to relatively weak reaction exothermicity, however, the synthesized composites containing small amounts of Cr₇C₃ and Al₄C₃ were obtained from the Al₄C₃-based reaction scheme.