Methane chemical vapor deposition on transition metal/GaAs samples – a possible route to Haeckelite carbon nanotubes?

We present a systematic study of atmospheric chemical vapor deposition growth of carbon nanotubes (CNTs) on patterned, transition metal/GaAs samples employing methane as the carbon feedstock. Controlled CNT growth was found to occur from the exposed metal‐semiconductor interface, rather than from the metal or semiconductor surfaces themselves. A fast sample loading system allowed for a minimization of the exposure to high temperatures, thereby preventing excessive sample damage. The optimum growth temperature for CrNi/GaAs interfaces is 700 °C (at a methane flow rate of 700 sccm). Possible growth scenarios involving the Ni–As–Ga system and its interaction with C is discussed. Raman spectroscopy of the CNTs revealed the presence of pentagon–heptagon defects. Closer analysis of the spectra points towards a mixture of so‐called Haeckelite CNTs. Copyright © 2011 John Wiley & Sons, Ltd.     

Catalytic synthesis of carbon nanotubes from ethylene in the presence of water vapor

Multiwalled carbon nanotubes were synthesized catalytically from ethylene in the presence of water vapor at transition metals of the iron subgroup. The structure of the obtained nanotubes was studied by transmission electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy. It was shown that the highest yields of carbon nanotubes with diameters between 20 and 40 nm, lengths of more than 1 μm, and average diameter of 0.92 nm for the innermost tube were obtained at a nickel catalyst with a water vapor concentration of 0.32%. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 4, pp. 227–230, July–August, 2006.     

Synthesis of straight and helical carbon nanotubes from catalytic pyrolysis of polyethylene

Straight and helical carbon nanotubes with diameter from 20 to 60 nm have been synthesized through catalytic decomposition of polyethylene in autoclave at 700 °C. The X-ray power diffraction pattern indicates that the products are hexagonal graphite, and transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images reveal the morphologies and structures of carbon nanotubes. The effects of reaction temperature, catalyst and maleated polypropylene on the growth of the carbon nanotubes were also discussed, and the growth mechanism of the CNTs was proposed. Pyrolysis of polyethylene is a promising green chemical method for economically producing carbon nanotubes.     

Combinatorial synthesis of carbon nanotubes by the catalytic chemical vapor deposition method

Summary Bimetallic (Fe-Co) catalyst samples prepared from different precursors over various supports were tested in carbon nanotube (CNT) production. In order to quicken the evaluation of the performance of the catalysts a combinatorial arrangement was used.</o:p>     

Fractal analysis of carbon nanotube agglomerates obtained by chemical vapor decomposition of ethylene over nickel nanoparticles

Fractal geometry was used to describe the morphology of agglomerates of carbon nanotubes (CNT) obtained by the thermal decomposition of ethylene in the presence of water vapor over nickel nanoparticles. Analysis of the structure of the CNT by electron transmission and scanning microscopy showed that CNT agglomerates have fractal structure with fractal dimension D ₀ = 2.65±0.05 and their formation proceeds by the same mechanism regardless of the ethylene partial pressure. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 2, pp. 93-97, March-April, 2009.     

Carbon nanotubes for supercapacitors:Consideration of cost and chemical vapor deposition techniques

In this topic,we first discussed the requirement and performance of supercapacitors using carbon nanotubes(CNTs) as the electrode,including specific surface area,purity and cost.Then we reviewed the preparation technique of single walled CNTs(SWNTs) in relatively large scale by chemical vapor deposition method.Its catalysis on the decomposition of methane and other carbon source,the reactor type and the process control strategies were discussed.Special focus was concentrated on how to increase the yield,selectivity,and purity of SWNTs and how to inhibit the formation of impurities,including amorphous carbon,multiwalled CNTs and the carbon encapsulated metal particles,since these impurities seriously influenced the performance of SWNTs in supercapacitors.Wish it be helpful to further decrease its product cost and for the commercial use in supercapacitors.     

减压化学气相沉积法制备规则螺旋状纳米碳管

以硅胶负载的Co纳米颗粒为催化剂,在低流量的减压体系中通过化学气相沉积法制备了规则螺旋状的纳米碳管.通过TEM和HRTEM研究了螺旋碳管的形貌、尺度分布以及管身、曲面和节点处的晶型;讨论了催化剂制备中pH值对催化剂的尺寸、规则程度和存在形态以致对螺旋碳管产量、管径厚度以及螺旋管的相对比例的影响;此外,还分析了反应压力对碳管生长的影响.     

Chemical vapor deposition of metal borides: 6. The formation of neodymium boride thin film materials from polyhedral boron clusters and metal halides by chemical vapor deposition

The chemical vapor deposition (CVD) of crystalline thin films of neodymium hexaboride (NdB₆) was achieved using either nido ‐pentaborane(9) or nido ‐decaborane(14) with neodymium(III) chloride on different substrates. The highly crystalline NdB₆ films were formed at relatively moderate temperatures (835 °C, ca. 1 µm/h) and were characterized by scanning electron microscopy, X‐ray emission spectroscopy, X‐ray diffraction and glow discharge mass spectrometry. The NdB₆ polycrystalline films were found to be pure and uniform in composition in the bulk material. Depositions using CoCl₂, NdCl₃ and B₅H₉ as the CVD precursors resulted in the formation of a mixture of NdB₆ and CoB phases, rather than the ternary phase. Copyright © 2008 John Wiley & Sons, Ltd.     

Utilization of iron oxide film obtained by CVD process as catalyst to carbon nanotubes growth

Thin films of Fe₂O₃ were obtained on silica glass substrates through the thermal decomposition of ferrocene in air. These films were characterized by Raman spectroscopy and X-ray diffractometry (XRD), and subsequently used as catalyst on the growth of carbon nanotubes, using benzene or a benzene solution of [Fe₃(CO)₁₂] as precursor. A great amount of a black powder was obtained as product, identified as multi-walled carbon nanotubes by XRD, Raman spectroscopy and transmission electron microscopy. The carbon nanotubes formed through the pyrolysis of the [Fe₃(CO)₁₂] solution were identified as structurally better than the one obtained by the pyrolysis of pristine benzene.     

Chemical vapor infiltration method for deposition of gold nanoparticles on porous alumina supports

Knudsen’s effusion method with mass spectral analysis of the composition of the gas phase was used to measure the temperature dependence of the saturated vapor of several (CH₃)₂AuL chelate complexes and to determine the thermodynamic parameters of their sublimation. Based on the results of this study, conditions for chemical vapor deposition of gold using dimethylgold(III) chelates were chosen. Gold nanoparticles were synthesized by chemical vapor deposition (infiltration) of (CH₃)₂Au(acac) on porous granules of γ-Al₂O₃ with subsequent calcination in air at 325°C. Particle size and the chemical state of gold in Au/γ-Al₂O₃ systems were evaluated by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). A vapor infiltration procedure is suggested to prepare metallic gold particles ≤5 nm in diameter from Au/γ-Al₂O₃. It is shown that Au/γ-Al₂O₃ systems obtained by chemical vapor infiltration and containing small gold crystallites possess high catalytic activity in CO oxidation reactions at 40°C.     

The effect of localized lateral growth of multiwalled carbon nanotubes with ammonia plasma post‐treatment

We present horizontally‐oriented multiwalled carbon nanotubes (CNTs) grown by means of thermal chemical vapor deposition. The CNT is across the trenches of the catalytic metals on predefined Ti electrodes. The properties of the lateral multiwalled CNT, following post‐ammonia plasma treatment, are reported. Information about the ammonia plasma treated on the interface structure of CNT is obtained using X‐ray photoelectron spectroscopy. The experimental results show that upon exposure to ammonia, the electrical property of the CNT is found to increase. Copyright © 2012 John Wiley & Sons, Ltd.     

Characterization of the chemical interaction between single-walled carbon nanotubes and titanium dioxide nanoparticles by thermogravimetric analyses and resonance Raman spectroscopy

Raman spectroscopy is a powerful technique that is used to characterize or observe alterations in the structure or properties of carbon nanotubes and its composites. This method can provide information about electronic changes or quantify them. We used Raman spectroscopy to study the chemical and electronic changes in a composite formed by titanium dioxide nanoparticles and single-walled carbon nanotubes. This composite was characterized by scanning electron microscopy to investigate the morphology and by thermogravimetric analyses to assess the thermal stability of the isolated carbon nanotubes as compared with the nanotubes by titanium dioxide nanoparticles. The Raman results showed that the modification of the nanotubes with the TiO₂ nanoparticles generates a new material with different structure of the nanotubes, resulting in a decrease in defects. The charge transfer from the TiO₂ nanoparticles to the nanotubes alters the electronic properties of both moieties in the hybrid material. The interaction between the nanotubes and nanoparticles decreases the CC bound order of the nanotubes and decreases their thermal stability.     

Particle size effects in Fischer-Tropsch synthesis by Co catalyst supported on carbon nanotubes

The effect of Co particle size on the Fischer-Tropsch synthesis (FTS) activity of carbon nanotube (CNT)-supported Co catalysts was investigated. Microemulsion (using water-to-surfactant molar ratios of 2 to12) and impregnation techniques were used to prepare catalysts with different Co particle sizes. Kinetic studies were performed to understand the effect of Co particle size on catalytic activity. Size-dependent kinetic parameters were developed using a thermodynamic method, to evaluate the structural sensitivity of the CNT-supported Co catalysts. The size-independent FTS reaction rate constant and size-independent adsorption parameter increased with increasing reac-tion temperature. The Polani parameter also depended on catalyst particle size, because of changes in the catalyst surface coverage.     

Electrosynthesis of the poly(N-vinyl carbazole)/carbon nanotubes composite for applications in the supercapacitors field

Electrochemical polymerization of N-vinyl carbazole (VK) on carbon nanotube (CN) films was studied by cyclic voltammetry in LiClO₄/acetonitrile solutions. Cyclic voltammograms recorded on a blank Pt electrode were compared with those obtained when single or multi-walled CN films were deposited on the Pt electrode; in the latter case, a down-shift of the VK reduction peak potential was observed. Functionalization of CNs with poly(N-vinyl carbazole) (PVK) was invoked by Raman scattering and UV-VIS-NIR spectroscopic studies. The influence of sweep rate on the electrochemical properties of the PVK/CN nanocomposite and the performance of supercapacitors constructed using PVK-functionalized single-walled carbon nanotube electrodes were also evaluated.     

Preparation of micelle-encapsulated single-wall and multi-wall carbon nanotubes with amphiphilic hyperbranched polymer

The hyperbranched polyester (Boltorn^TM H20) was modified by maleic anhydride and then polystyrene (H20-MAh-PSt) to form amphiphilic micelles in water. The single-wall and multi-wall carbon nanotubes (SWCNTs and MWCNTs, respectively) were encapsulated in the formed micelles through non-covalent interactions. The formed structures were confirmed by FTIR, NMR, GPC, and XPS analysis. The dispersion and aggregation behaviors were observed by TEM and UV-vis and Raman spectroscopic analysis. The results showed that the dispersion performance of the obtained micelle-encapsulated carbon nanotubes in water was greatly improved compared to the pure carbon nanotubes. From the TEM observation, the individual SWCNT structure and the uniform polymer coating around the surface of SWCNT were seen after crosslinking. The Raman spectroscopic measurements also demonstrated that for the crosslinked samples, no effect occurred associated with concentration-dependent carbon nanotube aggregation.     

Effects of temperature on the properties of TiO2 photocatalysts prepared by the chemical vapor deposition (CVD) method

In this study, a TiO₂ film was prepared in an annular reactor by the chemical vapor deposition (CVD) method. Results indicated that anatase crystals were formed, except for at a deposition temperature of 200°C without calcination. At a calcination temperature of 850°C, anatase crystal was the major species formed with a small amount of rutile crystals. After conducting a photocatalytic reaction of toluene, the best activity was found with a preparation temperature of 350°C for the deposition, and 550°C for calcination.     

A catalytic chemical vapor deposition synthesis of double-walled carbon nanotubes over metal catalysts supported on a mesoporous material

Double-walled carbon nanotubes (DWNTs) have been synthesized by catalytic chemical vapor deposition (CCVD) over supported metal catalysts decomposed from Fe(CH₃COO)₂ and Co(CH₃COO)₂ on mesoporous silica. Bundles of tubes with relatively high percentage of DWNTs, in areas where tubular layered structures could be clearly resolved, have been observed by transmission electron microscopy (TEM). In other areas, crystal-like alignment of very uniform DWNTs was observed for the first time, suggesting that mesoporous silica might play a templating role in guiding the initial nanotube growth. In addition, compatible with nano-electronics research, bridging of catalytic islands by DWNTs has also been demonstrated.     

Structural Study of Micro and Nanotubes Synthesized by Rapid Thermal Chemical Vapor Deposition

The structures of micro and nanotubes obtained by pyrolysis of hydrocarbons, hold onto silicon (Si) substrates, are reported in this work. The tubes fabrication experiments were carried out by Rapid Thermal Chemical Vapor Deposition (RTCVD) using propane (C₃H₈) as carbon (C) precursor. Selection of parameters such as temperature of deposition, vacuum conditions or surface cleaning leads to the creation of tubular structures. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), selected area electron diffraction (SAED) and energy dispersive X-ray measurements (EDX) are the microbeam techniques that allow to characterize the tubes found in the studied specimens. Different tube configurations such as isolated nanorods, Y-type junctions or fiber-like layers are evidenced. Metallic catalysis seems to be the mechanism involved in the wires formation since Fe particles are present inside the CNT tubes. Other poly-crystalline inclusions are also evidenced by SAED. The composition of the nanotubes changes from tip to tail in an amorphous matrix. The growth mechanisms leading to tube formation are described.