Multiwall carbon nanotubes grown by thermocatalytic carbonization of polyacrylonitrile

A method is reported for the growth of multiwall carbon nanotubes (MWCNTs) using polyacrylonitrile as a solid carbon source and nanosized SiO₂ particles as catalyst. The nanotubes were grown either on a Si substrate or as a freestanding film at temperatures as low as 800 °C. The smallest measured inner diameter of the resultant MWCNTs is about 0.6 nm and therefore this method provides a new direction to prepare MWCNTs with very small inner diameter from solid carbon source.     

Lithographically defined site-selective growth of Fe filled multi-walled carbon nanotubes using a modified photoresist

Partially Fe filled multi-walled carbon nanotubes (MWCNTs) were grown by chemical vapor deposition with propane at 850 °C using a simple mixture of iron (III) acetylacetonate (Fe(acac)₃) powder and conventional photoresist. Scanning electron microscopy revealed that catalytic nanoparticles with an average diameter of 70 nm are formed on the Si substrate which governs the diameter of the MWCNTs. Transmission electron microscopy shows that the nanotubes have a multi-walled structure with partial Fe filling. A site-selective growth of partially Fe filled MWCNTs is achieved by a simple photolithographic route.     

Efficient growth of MWCNTs from decomposition of liquefied petroleum gas on a NixMg1−xO catalyst

With a solid-solution type of Ni_xMg_1−xO as catalyst, low-cost and easy-handling LPG was used as the carbon-source to efficiently synthesize CNTs of high purity. The prepared CNTs were “Herringbone-type” MWCNTs, with the outer diameters in the range of 10–40 nm and the inner diameters of 3–7 nm. The CNTs were almost the only species in the purified products. The structures of the nanocarbon were predominantly graphite-like, and the content of amorphous carbon was considerably low (6% estimated). The present study provides an alternative route to efficiently synthesize Herringbone-type CNTs of high purity without using CH₄.     

Self-assembling of multi-walled carbon nanotubes on a porous carbon surface by catalyst-free chemical vapor deposition

Synthesis of carbon nanotubes (CNTs) by catalyst-free chemical vapor deposition (CFCVD) is one of the most important challenges in nanotube science. Self-assembling multi-walled CNTs (MWCNTs) were produced on a porous carbon surface using carbon black (CB) as a substrate, at 800 °C by the decomposition of diluted ethylene. MWCNTs with an outer-diameter distribution of 20–80 nm, examined by scanning and transmission electron microscopy, could be self-assembled on pore structures of CB surface by CFCVD.     

Formation and electrochemical properties of composites of the C60–Pd polymer and multi-wall carbon nanotubes

Preparation and electrochemical properties of a novel type of the composite made of multi-wall carbon nanotubes (MWCNTs) and two-component polymer of palladium and C₆₀ (C₆₀–Pd) were investigated using cyclic voltammetry, electrochemical impedance spectroscopy, and piezoelectric microgravimetry. A composite film was prepared by electrochemical deposition of C₆₀–Pd on the layer of MWCNTs immobilized on the electrode surface. The polymer forms islands of shells on the carbon multi-wall core. This composite is electrochemically active in the negative potential range due to the electroreduction of the fullerene moiety. In this potential range, specific pseudo-capacitance of the film of the MWCNT/C₆₀–Pd composite is 425 F g⁻¹ in the acetonitrile solution of tetra(n-butyl)ammonium perchlorate. The presence of MWCNTs makes the composite conductive also at potentials less negative than potentials of the C₆₀ electroreduction. The double-layer specific capacitance of this film is close to 15 F g⁻¹.     

High-Responsivity Heterojunction Photodetector Based on Bi2O3-Decorated MWCNTs Nanostructure Grown on Silicon via Laser Ablation in Liquid

We prepared a high-responsivity bismuth oxide Bi₂O₃ nanoparticles-decorated multiwalled carbon nanotubes MWCNTs/Si heterojunction photodetector by two-step laser ablation in liquid. The structural properties of the hybrid Bi₂O₃NPs-decorated MWCNTs were investigated by X-ray diffraction, which revealed the formation of α-Bi₂O₃ NPs with a monoclinic phase and graphite at the C(002) plane. Scanning electron microscopy results show the formation of spherical Bi₂O₃NPs attached to well-dispersed MWCNTs. The nanotubes' diameters ranged from 30 to 50 nm, their lengths from 1 to 3 m, and the average particle size of Bi₂O₃ NPs is 25 nm. The I–V characteristics for the Bi₂O₃NPs-decorated MWCNTs/Si photodetector were investigated in the dark and under illumination. The Bi₂O₃NPs-decorated MWCNTs/n-Si photodetectors show a responsivity of 1.37 A/W at a wavelength of 560 nm, with a corresponding external quantum efficiency of 3?×?10²%. At equilibrium, band alignment for Bi₂O₃-MWCNTs /Si heterojunction was realized.

     

Field emission properties of Cu/multiwalled carbon nanotube composite films fabricated by an electrodeposition technique

Composite films of Cu and multiwalled carbon nanotubes (MWCNTs) were fabricated by an electrodeposition technique, and their field emission properties were examined. Commercially available MWCNTs with various diameters (60–150 nm) were used. The microstructure of the composite films was analyzed by scanning electron microscopy and the field emission properties were measured using a diode-type system. Cu/MWCNT composite films with homogeneous dispersion of MWCNTs were fabricated using each type of MWCNT. Bare MWCNTs were present on the surface of the composite films and the ends of the protruding tips were fixed by the deposited copper matrix. The composite films produced clear emission currents and the corresponding Fowler–Nordheim (F–N) plots showed that these were field emission currents. The turn-on electric field tended to decrease with decreasing MWCNT diameter. A light-emitting device incorporating the Cu/MWCNT composite film as a field emitter was fabricated, and its light-emitting properties were investigated. Light emission with a brightness of around 100 cd m^?2 was observed for approximately 100 h.     

Size-dependent electrocatalytic reduction of nitrite at nanostructured films of hollow polyaniline spheres and polyaniline–polystyrene core–shells

Nanostructured films of the conducting polymer polyaniline (PANI) were prepared by its potentiostatic polymerization in the presence of thin polystyrene (PS) nanoparticle templates. Two sizes of PS nanoparticles were used (50 and 100 nm) and electron microscopic analysis showed that core–shell composite films of PANI (PANI–PS₅₀ and PANI–PS₁₀₀) and, following removal of the PS, hollow structures (PANI₅₀ and PANI₁₀₀) were formed due to the growth mechanism of PANI around the PS templates. The electrochemical behaviour of the PANI-modified electrodes was studied by cyclic voltammetry, chronocoulometry and amperometry in the presence of nitrite and it was found that nitrite reduction was enhanced by the nanostructuring of the films. The electrocatalysis was dependent on the size of the template, being more pronounced at 50 nm than at 100 nm. The hollow PANI₅₀ film was also a better catalyst than PANI–PS₅₀, whereas for structures based on 100 nm templates, the composite film was better. Such behaviour could be explained in terms of larger surface area and surface concentration of PANI₅₀ and PANI–PS₅₀ films on the electrodes and by higher differential capacitance of those films.     

Synthesis of a highly active carbon-supported Ir–V/C catalyst for the hydrogen oxidation reaction in PEMFC

The active, carbon-supported Ir and Ir–V nanoclusters with well-controlled particle size, dispersity, and composition uniformity, have been synthesized via an ethylene glycol method using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The catalytic activities of these carbon-supported nanoclusters were screened by applying on-line cyclic voltammetry and electrochemical impedance spectroscopy techniques, which were used to characterize the electrochemical properties of fuel cells using several anode Ir/C and Ir–V/C catalysts. It was found that Ir/C and Ir–V/C catalysts affect the performance of electrocatalysts significantly based on the discharge characteristics of the fuel cell. The catalyst Ir–V/C at 40 wt.% displayed the highest catalytic activity to hydrogen oxidation reaction and, therefore, high cell performance is achieved which results in a maximum power density of 563 mW cm⁻² at 0.512 V and 70 °C in a real H₂/air fuel cell. This performance is 20% higher as compared to the commercial available Pt/C catalyst. Fuel cell life test at a constant current density of 1000 mA cm⁻² in a H₂/O₂ condition shows good stability of anode Ir–V/C after 100 h of continuous operation.     

Carbon nanotubes coated with diamond nanocrystals and silicon carbide by hot-filament chemical vapor deposition below 200 °C substrate temperature

Multi-walled carbon nanotubes (MWCNTs) dispersed onto a silicon substrate have been coated with diamond nanocrystals (DNC) and silicon carbide (SiC) from solid carbon and silicon sources exposed to H₂ activated by hot filament chemical vapor deposition (HFCVD) at around 190 °C substrate temperature. MWCNT coating by DNC initiates during filament carburization process at 80 °C substrate temperature under conventional HFCVD conditions. The hybrid nanocarbon material was analyzed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. The structure of the MWCNTs is preserved during coating and the smooth DNC/SiC coating is highly conformal. The average grain size is below 10 nm. The growth mechanism of DNC and SiC onto MWCNT surface is discussed.     

Thermal stability and mechanical properties of SiC particulate-reinforced Si–C–N composites after heating at 2000 °C

A SiC particulate-reinforced Si–C–N ceramic composite was fabricated using the precursor impregnation and pyrolysis method, and its thermal and mechanical properties were analyzed. The weight loss of the composite was 5% after a heating at 2100 °C in Ar. The pores of the composite enlarged at and above 1700 °C in Ar due to the decomposition of the Si–C–N matrix. However, the composite retained mechanical properties such as strength and hardness after heating at 1700 °C. 88% of the original strength was remained after heating at 2000 °C for 10 h although the fabrication temperature was 1350 °C. The weight gain of the composite was 3.2% after an oxidation at 1450 °C for 30 min in air. The inner oxidation of the particulate-reinforced composites (PRC) was suppressed above 1400 °C due to the closure of the open pores by SiO₂. Consequently, the composite possessed excellent creep resistance at 1400 °C in air. The SiC/Si–C–N composite is a challenging candidate for the application at high temperature.     

Characteristics of graphene-layer encapsulated nanoparticles fabricated using laser ablation method

Graphene layer-encapsulated Ni nanoparticles with diameters between 3 and 10 nm were fabricated by laser ablation techniques and deposited directly on the Si substrate at room temperature. It was found from the field-emission type scanning electron microscopy (FE–SEM) and transmission electron microscopy(TEM) analyses that any carbon nanotubes were not fabricated in the deposited nano-materials. High-resolution TEM observation showed the core-shell structure of Ni–C particles with crystalline nickel core surrounded by graphite-like layers. The X-ray diffraction(XRD) pattern also revealed that nanoparticles embedded in graphene capsules are crystalline nickel. With these Ni–C nanoparticles, we demonstrated the growth of vertically aligned carbon nanotubes with low spatial density on a silicon substrate by thermal CVD.     

Growth of multi-walled C nanotubes from pre-heated CH4 using Fe3O4 as a catalyst precursor

The present study aims to investigate influence of pre-heating of CH₄ on the growth of multi-walled C nanotubes (MWCNTs) on a Si (100) substrate by chemical vapor deposition technique using Fe₃O₄ powder as catalyst precursor. Reduction behavior of Fe₃O₄ was also studied in a flowing undiluted CH₄ atmosphere in order to gain better insight into MWCNT synthesis. Mass measurements, XRD and thermodynamic analyses were carried out to determine the extent of reduction of Fe₃O₄ by CH₄. It was found that Fe₃O₄ initially transformed to Fe via FeO within 30 min at 1200 K. Fe₃C and C then formed as reaction time increased to 60 min. It was postulated that reduction of Fe₃O₄ took place by H₂, a product of CH₄ decomposition. The overall reactions leading to the formations of Fe and Fe₃C phases were proposed using equilibrium thermodynamic analysis and the experimental results. Undiluted CH₄ was used to synthesize MWCNTs at temperatures in the range of 1050–1300 K. It was observed that a dense carbon coating was formed at 1300 K owing to self pyrolysis of CH₄, while at 1200 K individual MWCNTs were observed on the Si substrate. Growth of MWCNTs did not take place at the temperature range of 1050–1150 K. The use of CH₄ pre-heated at 1200 K, however, yielded MWCNTs at this temperature range. Experimental results and thermodynamic analysis of the C–H system (excluding graphite) indicated that pre-heating treatment of CH₄ promoted Fe₃O₄ reduction by H₂ and C formations from active intermediate hydrocarbon species of high molecular weights (especially C₆H₆).     

A Quaternary ZnCdSeTe Nanotip Photodetector

The authors report the growth of needle-like high density quaternary Zn_0.87Cd_0.13Se_0.98Te_0.02 nanotips on oxidized Si(100) substrate. It was found that average length and average diameter of the nanotips were 1.3 μm and 91 nm, respectively. It was also found that the as-grown ZnCdSeTe nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. Furthermore, it was found that the operation speeds of the fabricated ZnCdSeTe nanotip photodetector were fast with turn-on and turn-off time constants both less than 2 s.     

Interfacial microstructure characterization and strength evaluation of Si3N4/Si3N4 joints with Si–Mg composite filler for high-temperature applications

Silicon-nitride (Si₃N₄) components were joined under vacuum at 1100 °C for 10 min using Si–Mg composite fillers with Mg contents (X_Mg) that ranged from 0 at.% to 59 at.%. The Si₃N₄/Si₃N₄ joints were fabricated via Si layer formation at the joint interface; the molten Si–Mg liquid was transformed into a solid Si layer after Mg-evaporation-induced isothermal solidification. The joint tensile strength at room temperature increased considerably when X_Mg exceeded the liquidus composition of 37 at.% because of the enhanced densification/thinning of the Si layer. In these cases, some Mg atoms reacted with Si₃N₄ to form a fine-grained MgSiN₂-based layer, whereas relatively large (>0.1 μm) and smaller MgO precipitates (<10 nm) were observed in the Si layer. At a high X_Mg, the MgO precipitates were arranged in a network-like structure, which improved the fracture strength of the Si layer. The joints with a high strength at room temperature were examined using a three-point bending test at 1200 °C in air and endured a maximum fracture stress of ~200 MPa, which confirmed their potential for use in oxidizing atmospheres at least 100 °C above the bonding temperature.     

Microwave plasma chemical vapor deposition of cone-like structure of diamond/SiC/Si on Si (100)

Diamond deposition on 1 × 1 cm² Si (100) substrates with bias was carried out by microwave plasma chemical vapor deposition (MPCVD). Distribution of deposited diamonds has been significantly improved in uniformity over all the Si substrate surface area by using a novel designed dome-shaped Mo anode. The deposits were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman analysis. SEM observations show that there is a high density of cone-like particles uniformly deposited on the surface of the substrate in short bias nucleation period. The average diameter, height and density of cone-like structure were increased with methane concentration in the bias stage. TEM reveals that the cone-like structure is actually composed of Si conic crystal covered with diamond. Between Si and diamond, a thin layer of cubic SiC is found in epitaxy with Si. Furthermore, for 3% CH₄ concentration, the range of diameter of cone-like structure was about 20–90 nm and the size of diamond was about 10–60 nm.     

An easy single step route to synthesize open−ended carbon nanotubes

A simple and easy route is described for the synthesis of multiwalled carbon nanotubes (MWCNTs) by pyrolysis of cetyltrimethyl ammonium decatungstate and cetylpyridinium decatungstate in a specially made LET-LOK union cell. The advantages of using a single component precursor is that the organic moiety present in the precursor acts as the source for carbon and in-situ formed tungsten/tungsten carbide acts as a catalyst for the formation of carbon nanotubes. The method produces large quantities of carbon nanotubes with monodisperse, hollow, open ends. The MWCNTs were characterized systematically using SEM, TEM, HRTEM, Raman and XRD analysis. The MWCNTs have average diameters in the range of 15–35 nm and lengths of several hundreds of nanometers.     

Phase transition behavior and electrical resistance stability of Ge2Sb2Te5/Sb superlattice-like films on a flexible substrate

In order to validate the flexible storage potential, [Ge₂Sb₂Te₅(3 nm)/Sb(7 nm)]₅ superlattice-like phase change films on flexible polyetheretherketone substrates were prepared by magnetron sputtering. The phase transition behavior and the effect of different bending cycles on the properties were investigated. Compared with conventional rigid Si/SiO₂ substrate, [Ge₂Sb₂Te₅(3 nm)/Sb(7 nm)]₅ films based on a flexible substrate possessed better thermal stability and surface adhesion. The superlattice-like structure was confirmed by transmission electron microscopy. PCM devices based on polyetheretherketone substrates were fabricated and the electrical conversion characteristics before and after bending were verified and compared.     

Synthesis of SiC nanowires by a simple chemical vapour deposition route in the presence of ZrB2

The SiC nanowires (NWs) were fabricated by a simple chemical vapour deposition (CVD) method at high temperature using Si, phenolic resin, and ZrB₂ powder. The morphologies of the fabricated SiC NWs included SiC/SiO₂ chain-beads and straight wires with core-shell structures. The fabricated SiC NWs were micrometre-to-millimetre in length, with chains 100–300 nm in diameter and beads with diameters of less than 1 μm. The core-shell-structured SiC NWs consisted of crystalline SiC cores and thin amorphous SiO₂ shells. SiC crystals grew in the [111] direction governed by a vapour-solid (VS) mechanism. The added ZrB₂ promotes the generation of gaseous species at higher gas pressures, which contributes to the formation of SiC NWs by CVD. The fabricated SiC NWs exhibited good photoluminescence properties due to many stacking faults and the presence of amorphous SiO₂.     

Low temperature deposition of titanium oxide containing thin films in trench features from titanium diisopropoxide bis(dipivaloylmethanate) in supercritical CO2

We studied supercritical carbon dioxide fluid deposition of titanium oxide (TiO₂) in trench features on Si substrates using a flow-type deposition apparatus from titanium diisopropoxide bis(dipivaloylmethanate), aiming at fabricating conformal films at a relatively low temperature. We investigated the deposition rate and step coverage under a fluid temperature from 40 to 60 °C, a pressure from 8.0 to 10.0 MPa, and a substrate temperature from 80 to 120 °C. They were dependent on the fluid density, indicating that the solubility difference between the bulk fluid and the neighborhood of the substrate surface plays a decisive role for the deposition. An excellent conformal filling of the trench features was achieved from the fluid of 60 °C under 8 MPa on the substrate kept at 80–100 °C. The XPS spectra of the deposited film suggested partial formation of TiO₂, and the XRD spectra showed the existence of some crystalline TiO₂ (anatase).