Deposition of a thin film of carbon nanotubes onto a glassy carbon electrode by electropolymerization

A novel and easy route for the deposition of a thin film of carbon nanotubes onto an electrode surface by electropolymerization is described. Multi-wall carbon nanotubes (MWNTs) were “dissolved” in aqueous alizarin red S (ARS, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid, sodium salt) solution, and a very stable and well-distributed aqueous MWNTs–ARS solution was obtained. A thin film of MWNTs–ARS was successfully deposited onto the electrode surface by an in situ electropolymerization in aqueous MWNTs–ARS solution. The MWNTs–ARS thin film was characterized by scanning electron microscopy, Raman spectroscopy, UV–Vis spectroscopy and electrochemical techniques.     

Structural properties of carbon materials from the electrochemical reduction of fluorinated naphthalene pitch

Synthesis of carbon materials from fluorinated naphthalene pitch has been carried out by means of electrochemical reduction. Structural and morphological properties of synthesized carbons were investigated using X-ray photoelectron spectrum (XPS), Raman scattering spectrum, X-ray diffraction measurements and scanning and transmission electron microscopy (SEM and TEM). Furthermore, a change in the structure of synthesized carbons with an electron beam irradiation was examined. The synthesized carbon has a disordered structure composed of fragments of the sp² hexagonal network, and also has lots of micro-voids. A carbyne-like structure was found only a little with the reduction of fluorinated naphthalene pitch by Raman spectra. A lot of ribbon-like graphite nanocrystal was formed by irradiation of an electron beam at a high temperature to the defluorinated carbon. Bending of a long ribbon-like crystal of graphite was often observed. Formation mechanism of graphite nanocrystals was discussed.     

Deposition of carbon nitride films from single-source s-triazine precursors

Trisubsituted derivatives of s-triazine (1,3,5-triazine) with N(i-Pr)₂, N(i-Bu)₂, NH(t-Bu), pyridyl, and NHNHMe ligands were used as single-source precursors to produce carbon nitride (CN_x) thin films via hot wall CVD. The precursors are either commercially available or were synthesized in straightforward, one-pot procedures, and the synthesis and characterization of tris-2,4,6-methlyhydrazino-1,3,5-triazine (TMHT) is reported for the first time. All of the precursors studied are thermally stable and volatilize below 250 °C. They thermally decompose between 500 and 1000 °C, resulting in CN_x films with x ranging from 0.95 to 0.03. The film deposition temperature and nitrogen content depend upon the structure and stability of the precursor. The film properties vary from disordered insulating structures with high nitrogen content (CN_0.95) to low nitrogen content turbostratic carbon films. The films on Si and SiO₂ substrates were characterized by Auger surface analysis, FT-IR and Raman spectroscopy, X-ray diffraction, and scanning electron microscopy.     

Synthesis of carbon nanosheets by inductively coupled radio-frequency plasma enhanced chemical vapor deposition

An ultrathin sheet-like carbon nanostructure, carbon nanosheet, has been effectively synthesized with CH₄ diluted in H₂ by an inductively coupled radio-frequency plasma enhanced chemical vapor deposition. Nanosheets were obtained without catalyst over a wide range of deposition conditions and on a variety of substrates, including metals, semiconductors and insulators. Scanning electron microscopy shows that the sheet-like structures stand on edge on the substrate and have corrugated surfaces. The sheets are 1 nm or less in thickness and have a defective graphite structure. Raman spectra show typical carbon features with D and G peaks at 1350 and 1580 cm⁻¹, respectively. The intensity ratio of these two peaks, I(D)/I(G), increases with methane concentration or substrate temperature, indicating that the crystallinity of the nanosheets decreases. Infrared and thermal desorption spectroscopies reveal hydrogen incorporation into the carbon nanosheets.     

High dispersion and electrocatalytic properties of platinum on well-aligned carbon nanotube arrays

Platinum (Pt) electrocatalyst was electrochemically dispersed on well-aligned carbon nanotube (CNT) arrays by a potential-step method. The structure and elemental composition of the resulting Pt/CNT electrode were characterized by scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The electrocatalytic properties of the Pt/CNT electrode for oxygen reduction reaction have been investigated by linear sweep voltammetry. Compared with a Pt/graphite electrode, higher electrocatalytic activity of the Pt/CNT electrode can be observed. This may be attributed to the high dispersion of platinum catalysts and the particular properties of CNT supports. The results imply that the Pt/CNT has good potential applications in proton exchange membrane fuel cells.     

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.     

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.     

Effect of additives on mechanical properties of oxidation-resistant carbon/carbon composite fabricated by rapid CVD method

The optimum composition of additives was obtained by the orthogonal design test method for oxidation-resistant carbon/carbon composites (C/C) fabricated by the rapid CVD method. The effect of additives on mechanical properties was examined. The additives used in this test included silicon carbide, silicon nitride, and metal borides. Additives doped into the matrix of C/C increase not only the initial oxidation temperature from 400 to 657°C (64%), but also its flexural strength from 121 to 254 MPa (110%), and flexural modulus from 25 to 45 GPa (96%). The increase of mechanical properties is considered to be due to the formation of a metallic carbon–boron compound in the microstructure.     

Possible tactics to improve the growth of single-walled carbon nanotubes by chemical vapor deposition

The growth time, growth mode and the method of preparing the supported catalysts play an important role in the growth of single-walled nanotubes (SWNTs). Their effects on the chemical vapor deposition (CVD) growth of SWNTs with MgO-supported catalysts were investigated in this study. It is shown that the growth rate of SWNTs was large during the initial few minutes of growth, however the quality of the tubes was low owing to the formation of many defects. Long term growth may favor the formation of tubes with high quality and high yield, but the introduction of other forms of carbon (impurities) is also unavoidable. There was a balance between the increase in yield and quality and sacrifice of the purity during growth of SWNTs. MgO-supported catalysts prepared by the co-precipitation method were found to be more effective for the synthesis of SWNTs than those prepared by the widely used impregnation method. The size and dispersion state of the catalyst were found to be crucial in enhancing the growth of SWNTs. In addition, growth on the surface of SWNTs over nanosized catalyst films was shown to be more favorable for the synthesis of tube products with higher quality, yield and purity.     

Micromechanical testing of carbon fibers deposited by low-pressure laser-assisted chemical vapor deposition

Carbon fibers were deposited directly from ethylene by laser-assisted chemical vapor deposition. The precursor gas pressures and the incident laser powers were varied. Micro-mechanical testing was carried out using a high-precision micro-manipulator. During three-point bend testing the fibers showed an elastic response, with no residual strain upon unloading, until fracture. The fibers’ strength and Young’s modulus are reported. A model for fiber fracture is proposed based on fiber cross-section analysis. Scanning electron microscopy was used to study the fiber cross-sections and the fiber surface morphology. The mechanical properties are related to the characteristic fiber microstructure investigated by Raman spectroscopy.     

Synthesis and properties of carbon nanospheres grown by CVD using Kaolin supported transition metal catalysts

Carbon spheres with diameters between 400 and 2000 nm were synthesized in large quantities by catalytic chemical vapor deposition (CCVD) method using Kaolin supported transition metal salts (M=Fe, Co, Ni, etc.) as catalysts. The reaction conditions for the synthesis of carbon spheres in different sizes are described. The reactivity of the carbon spheres in various organic solvents is discussed. The as-synthesized carbon spheres are composed of unclosed graphene layers with the interlayer distances 0.33-0.35 nm. These carbon spheres have been characterized by SEM, TEM, HRTEM, XRD, Raman, ESR and SQUID magnetization techniques. From the ESR and SQUID the metallic nature of the carbon spheres is described.     

Synthesis of carbon nanotubes on metallic substrates by a sequential combination of PECVD and thermal CVD

Carbon nanotubes were synthesized from acetylene and hydrogen gas mixture directly on stainless steel plates by sequential combination of rf PECVD and thermal CVD. PECVD was used for nucleation and initial growth of carbon nanotubes while thermal CVD was utilized for further growth of them. In this way decoupling of nucleation and growth of carbon nanotubes was realized, and growth of carbon nanotubes was enhanced compared to growth by PECVD. Synthesized carbon nanotubes were curly in shape, and proper pretreatment of the substrate surface was required for the satisfactory growth of carbon nanotubes. Carbon nanotubes could be fabricated into electrodes for electric double layer capacitors without any further treatment. With an electrolyte composed of lithium hexafluorophosphate, ethylene carbonate and diethyl carbonate, charge/discharge test showed specific capacitance in the range of 33-82 F/g.     

Novel carbon nanofibers of high graphitization as anodic materials for lithium ion secondary batteries

Carbon nanofibers (CNFs) of high graphitization degree were prepared by a CVD process at 550-700 °C. They showed different structures according to catalyst and preparation temperatures. The structure of CNF prepared from CO/H₂ over an iron catalyst was controlled from platelet (P) to tubular (T) by raising the decomposition temperature from 550 to 700 °C. The CNFs prepared over a copper-nickel catalyst from C₂H₄/H₂ showed the typical herringbone (HB) structure regardless of the reaction temperatures. The CNFs prepared over Fe showed d₀₀₂ of 0.3363-0.3381 nm, similar to that of graphite, indicating very high graphitization degree in spite of the low preparation temperature. Such CNFs of high graphitization degree showed high capacity of 297-431 mA h/g, especially in the low potential region. However, low first cycle coulombic efficiency of ≈60% is a problem to be solved. The graphitization of the CNF preserved the platelet texture, however, and formed the loops to connect the edges of the graphene sheets. Higher graphitization temperatures made the loop more definite. The graphitized CNF showed high capacity (367 mA h/g); however, its coulombic efficiency was not so large despite its modified edges by graphitization, indicating that the graphene edges were not so influential for the irreversible reaction of Li ion battery.     

The evaluation of the gross defects of carbon nanotubes in a continuous CVD process

Carbon nanotubes (CNTs) were prepared by ethylene decomposition on an Fe/Al₂O₃ catalyst in a fluidized bed reactor. Their gross defects at different growth periods are evaluated by using a combination of SEM, TGA and Raman spectroscopy. The initially grown CNTs have a much lower thermal stability and more defects as compared to the fully-grown ones. The difference in the defects of CNTs at different reaction times is attributed to the lift up of CNTs with the gradually crashing texture of catalyst and the increasing volume of CNTs in a limited reactor space.     

Open-air carbon coatings on fused quartz by laser-induced chemical vapor deposition

Carbon films are deposited on fused quartz substrates by CO₂ laser-induced chemical vapor deposition (LCVD) using a novel open-air coating system. The hydrocarbon precursor gases are methane, propane and butane. The deposition rates of the three hydrocarbon gases are determined by measuring the mass of the carbon film deposited at constant temperature, and validated by film thickness measurements obtained using an environmental scanning electron microscope. The results indicate that butane and propane have significant deposition at 1375-1500 K, while deposition starts at 1550 K for methane. All three hydrocarbons have an exponential increase in deposition rate. Deposition rates obtained with butane and propane show a much stronger influence of temperature compared to methane. Raman spectra of deposited carbon films indicate that the surface consists of glass-like or nanocrystalline carbon. The ratio of D-peak to G-peak Raman band intensities decreases as the deposition temperature is increased, which is possibly due to self-annealing and additional surface reaction during high temperature deposition. This finding indicates that the carbon film exhibits higher structural order at high deposition temperatures, which can significantly enhance the film’s hermeticity and mechanical properties for use as an optical fiber coating.     

Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm⁻¹, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm⁻¹ was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm⁻¹. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm⁻¹, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.     

Identification of carbon phases and analysis of diamond/substrate interfaces by Raman spectroscopy

Raman spectroscopy is employed to characterize thin diamond and diamond-like carbon films deposited by hot filament chemical vapour deposition (HFCVD). A method is proposed and experimentally verified for a contact-less measurement of the actual substrate temperature by Raman spectroscopy.     

Effect of adding nickel to iron-alumina catalysts on the morphology of as-grown carbon nanotubes

The synthesis of carbon nanotubes (CNTs) from ethylene decomposition by Fe/Al₂O₃ and Fe/Ni/Al₂O₃ catalysts (Fe:Ni=10:1) is studied. A small amount of nickel introduced into the catalyst can significantly increase the yield of CNTs, but the nanotubes change from straight tubes with concentric parallel carbon sheets to helical tubes of the fish-bone type. Raman characterization of CNTs prepared at 823 and 1023 K and CNTs annealed at 2473 K shows that CNTs deposited on the Fe/Ni/Al₂O₃ catalyst have poor crystallinity, as compared with that on the Fe/Al₂O₃ catalyst. These differences are explained by a mechanism of formation of helical tubes of the fish bone type that takes into consideration the differences in the chemical nature of the catalyst with and without nickel.     

Observation of single-walled carbon nanotubes by photoemission microscopy

Single-walled carbon nanotube networks grown on SiO₂ pillars were studied by means of scanning photoemission microscopy. The individual nanotubes or nanotube bundles growing from the pillar tops were observed in C 1s images. Band bending near catalytic Fe/nanotube contacts in an end-bonded configuration was studied by measuring C 1s spectra along the tube axes. Within our experimental resolution, no band bending was observed. This implies that the depletion width is less than the spatial resolution of the scanning photoemission microscope (90 nm) or that the amount of the band bending is less than 0.1 eV.