Bamboo-shaped carbon nanotubes generated by methane thermal decomposition using Ni nanoparticles synthesized in water-oil emulsions

Ni nanoparticles were synthesized using two water-in-oil emulsions formulated with different surfactants and using n-heptane as the organic phase and aqueous nickel acetate as the catalytic metallic precursor. Characterization by transmission electron microscopy showed that the Ni nanoparticles have diameters ranging from 3 to 12 nm, and that the surface is lightly oxidized. The decomposition of diluted methane catalyzed by the as-prepared Ni nanoparticles was studied in a thermogravimetric analyzer (TGA), operated in the 25–930 °C range. The weight gains measured during the analysis showed that the Ni nanoparticles decomposed methane above 480 °C, producing similar g.C/g.cat ratios (6–7) at the end of the tests. High resolution transmission electron microscopy (HRTEM) confirmed that the carbons collected at 930 °C were bamboo-shaped carbon nanotubes (BSCNTs) with well defined conical compartments. The average outside diameter of the tubes was between 30 and 60 nm.     

In situ growth of CdSe/CdS quantum dots inside and outside of MWCNTs

The CdSe/CdS quantum dots were grown in situ inside and outside of the multiwalled carbon nanotubes in an innocuous solvent and at a lower temperature. The CdSe/CdS-CNTs nanoheterostructures were characterized by TEM, HRTEM, EDS, XPS and PL. The CdSe/CdS nanocrystals with diameters about 5 nm exhibit an improved PL emission.     

Atomic force microscopy of bamboo-shaped multiwalled carbon nanotube structures

Atomic force microscopy (AFM) was employed for the morphology measurements of bamboo-shaped multiwalled carbon nanotubes (BS-MWNTs) grown by thermal chemical vapor deposition on Fe catalyst deposited SiO₂/Ti substrates. Greater diameters and compartment distances of the bamboo structures were observed for the BS-MWNTs grown at 950 °C than for those grown at 850 °C.     

Hot filament CVD of Fe-Cr catalyst for thermal CVD carbon nanotube growth from liquid petroleum gas

A hot filament chemical vapor deposition (HFCVD) method was used to prepare Fe-Cr thin film on Si substrate. The produced layers were used as catalysts for growing carbon nanotubes (CNTs) from liquid petroleum gas (LPG) at 825 °C by thermal CVD (TCVD) method. To characterize the obtained catalysts or CNTs, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Raman spectroscopy were used. CNTs were grown on HFCVD derived Fe-Cr catalyst with the LPG as carbon source successfully. It was found that an annealing process on catalysts enhances the surface concentration of Cr atoms and reduces the sizes of catalyst particles. The grown CNTs on annealed sample were morphologically denser with smaller diameters compared to the as deposited one. In addition, the effect of filament temperature on CNTs was investigated. By increasing the filament temperature from 850 to 1050 °C the surface density and diameters of CNTs were improved.     

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH₄ atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current-voltage (I-V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.     

Influence of pyrolysis temperature on the growth of Y-junction carbon nanotubes

Y-junction carbon nanotubes, grown through thermal pyrolysis of acetylene over nanocrystalline Ni-P deposited SiC whiskers, were investigated by transmission electron microscopy. The pyrolysis temperature, which ranged from 800 °C to 1000 °C, was found to be crucial for the growth of different Y-junction carbon nanotubes. At pyrolysis temperatures below the partial melting point of the Ni-P alloy catalyst, only single-junction nanotubes could be synthesized. Whereas, due to partial melting of the alloy catalyst at higher pyrolysis temperatures, liquid-assisted growth of multiple Y-junction nanotubes could occur. PACS 61.46.+w; 61.48.+c; 68.37.Lp     

Preparation of carbon nanosheets deposited on carbon nanotubes by microwave plasma-enhanced chemical vapor deposition method

Carbon nanosheets were synthesized by microwave plasma-enhanced chemical vapor deposition method on carbon nanotubes substrate which was treated by hydrogen plasma. The results showed that the diameters of carbon nanotubes first got thick and then “petal-like” carbon nanosheets were grown on the outer wall of carbon nanotubes. The diameters of carbon nanotubes without and with carbon nanosheets were 100-150 and 300-500 nm, respectively. Raman spectrum indicated the graphite structure of carbon nanotubes/carbon nanosheets. The hydrogen plasma treatment and reaction time greatly affected the growth and density of carbon nanosheets. Based on above results, carbon nanosheets/carbon nanotubes probably have important applications as cold cathode materials and electrode materials.     

Influence of Hydrothermal Temperature on Structures and Photovoltaic Properties of SnO2 Nanoparticles

Two SnO₂ nanoparticles were synthesized by hydrothermal method at 170°C and 180°C, respectively. Transmission electron microscope observations reveal that the diameters of both the nanoparticles are around 6nm. At the same time, surface photovoltage spectroscopy measurements show that the nanoparticle synthesized at 180°C has more surface electronic states at 0.3eV below the conduction band than the one synthesized at 170°C. This means that the temperatures chosen in hydrothermal synthesis have significant influence on the surface electronic characteristics of resultant SnO² nanoparticles but the effect on their sizes is not obvious. However, after being calcined at 500°C for 2h, the diameter of the nanoparticle synthesized at 180°C increased to 23nm and that of the nanoparticle synthesized at 170°C increased to 32nm as calculated from X-ray diffraction pattern.     

Flame synthesis of carbon nanotubes for panel field emission lamp

Multi-walled carbon nanotubes (CNTs) were synthesized on the surfaces of Ni-alloy plated Fe-wires with the diameter of 2 mm using a conventional laboratory ethanol (C₂H₅OH) flame method at 560 °C. SEM showed that the product had bush-shaped micron-structures with diameters from 100 to 450 nm and lengths of over 1.0 μm. TEM revealed that the micron-structures were composed of multi-walled nanotube bundles with the diameters of about 50 nm. The test on the diode configuration field emission of the Fe-wire arrays was performed. The onset electric field was 2.95 V/μm and the emission current can reach 50 mA/cm² at an electric field of 9 V/μm. The average fluctuation of the emission current density was less than 7%. The result suggests that the field emission was uniform and the present technique was feasible to fabricate Panel Field Emission Lamp (PFEL) with arrays of carbon nanotubes. PFEL has the advantages of high luminescence as well as stability, and thus, it can be used to replace ordinary lights.     

Catalytic activity of Fe, Co and Fe-Co-MCM-41 for the growth of carbon nanotubes by chemical vapour deposition method

Iron, cobalt and a mixture of iron and cobalt incorporated mesoporous MCM-41 molecular sieves were synthesised by hydrothermal method and used to investigate the rules governing their nanotube producing activity. The catalysts were characterised by XRD and N₂ sorption studies. The effect of the catalysts has been investigated for the production of carbon nanotubes at an optimised temperature 750 °C with flow rate of N₂ and C₂H₂ is 140 and 60 ml/min, respectively for a reaction time 10 min. Fe-Co-MCM-41 catalyst was selective for carbon nanotubes with low amount of amorphous carbon with increase in single-walled carbon nanotubes (SWNTs) yield at 750 °C. Formation of nanotubes was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Transmission electron microscope and Raman spectrum was used to follow the quality and nature of carbon nanotubes formed and the graphitic layers and disordered band, which shows the clear evidence for the formation of SWNTs, respectively. The result propose that the diameter of the nanotubes in the range of 0.78-1.35 nm. Using our optimised conditions for this system, Fe-Co-MCM-41 showed the best results for selective SWNTs with high yield when compared with Fe-MCM-41 and Co-MCM-41.     

New aspects on pulsed laser deposition of aligned carbon nanotubes

We have grown carbon nanotubes (CNT) by pulsed laser deposition (PLD) at 1000 °C in Ar atmosphere. A Nd/YAG laser was used for irradiation of a graphite target containing Ni and Co rods. High-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) images showed that “closed” carbon nanotubes were grown between clusters of metallic particles, so that the individual nanotubes were arranged in parallel to each other forming a shape of “Rope-Bridge”. The nanotubes structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and their type was found to be of MWNT, containing about five SWNT. Total diameter was 5-20 nm and their length was about 1 μm. High homogeneous distribution carbon nanotubes were grown and different structures were observed such as well-aligned carbon nanotubes, bamboo-like and Y-junction carbon nanotubes.     

Strong influence of substrate temperature on the growth of nanocrystalline MoO3 thin films

Nanocrystalline MoO₃ thin films are prepared by electron beam evaporation technique. By using the dry MoO₃ pellets, the films are deposited on quartz substrates at substrate temperatures ranging from 100 °C to 400 °C. A phase structure of β-MoO₃ at the substrate temperature 200 °C is characterized by X-ray diffraction, but α-MoO₃ comes forth at 300 °C and 400 °C. The surface morphology, cross section view and plate form nanostructures have been illustrated from the scanning electron microscope images. The absorption spectra of films show that the energy band Eg decreases with the increasing substrate temperature, due to the increase in oxygen vacancy. The resistances of thin films prepared at various substrate temperatures gradually decrease with the increasing environmental temperatures.     

Formation of bamboo-shaped carbon nanotubes on carbon black in a fluidized bed

For the first time, bamboo-shaped multiwalled carbon nanotubes, having diameter of the order of 50 nm, have been grown on carbon black in a fluidized bed in bulk amount. The activation energy for the synthesis of the product was found out to be around 33 kJ/mol in the temperature range of 700−900 °C. The carbon nanotubes were separated from the carbon black by preferential oxidation of the later, the temperature of which was determined by thermogravimetry. The transmission electron microscopy revealed different features of the nanotubes such as “Y” junction, bend, and catalyst filling inside the nanotubes. Small angle neutron scattering was performed on the nanotubes synthesized at different temperatures. The data were fitted into a suitable model in order to find out the average diameter, which decreases with increase in synthesis temperature. The Monte Carlo simulation predicts the same behavior. Based on the above observations, a possible growth mechanism has been predicted. The oscillation in carbon saturation value inside the catalyst in the fluidized bed has been indicated as the responsible factor for the bamboo-shaped structure.     

Observation of ordered phases of fullerenes in carbon nanotubes

We have assembled molecular arrays of C60 inside double-walled carbon nanotubes (DWNTs) with internal diameters of 11-26 A and directly observed the existence of different crystalline phases of C60 previously predicted theoretically. The structure of the encapsulated C60 crystal is defined by the internal diameter of the DWNT, as the molecules adjust their packing arrangement in order to maximize van der Waals interactions. We have also shown that fullerenes in C(60)@DWNT interact with the outer layer of DWNTs, as demonstrated by the efficient filling of DWNTs with internal diameters of less than 12 A.     

Study of multilayer carbon nanotubes subjected to the impact of a nanosecond high-energy ion beam

The impact of a nanosecond high-energy ion beam on the structure and morphology of multilayer carbon nanotubes and their ensembles is studied using transmission electron microscopy and scanning electron microscopy. It is shown that ion-beam irradiation leads to a decrease in the outer diameters of carbon nanotubes, which is related to the destruction of their outer layers. In addition, the secondary growth of carbon nanotubes with smaller diameters and the growth of bulblike formations, inside which structures with interplanar distances that are close to those of nanodiamond are fixed, are observed.     

Hydrogen storage in multi-wall carbon nanotubes using samples up to 85 g

Hydrogen storage in carbon nanotubes (CNTs) is investigated at ambient temperature and pressures of 0–12 MPa, using 35–85 g multi-wall carbon nanotube (MWNT) samples that were synthesized in a nano-agglomerate fluidized bed reactor. The volume of hydrogen gas released by the CNTs was measured by a volumetric flow meter. The capability of H₂ storage in the CNT samples of mass of up to 85 g can be obtained with a precision of 0.01 wt.%. MWNTs with average diameters ranging from 10–30 nm and were pretreated using nitric acid or a sodium hydroxide solution wash and a high temperature treatment. The influence of the hydrogen pressure, hydrogen storage time and treatment method were studied. All data show that the amount of hydrogen released by the MWNTs at room temperature is no more than 0.30 wt.%, while hydrogen released by MWNT at 77 K can reach 2.27 wt.%. PACS 61.46.+w; 68.43.-h; 84.60.Ve     

Standing or lying C70s encapsulated in carbon nanotubes with different diameters

Single-walled carbon nanotubes (SWNTs) encapsulating C₇₀s, so-called C₇₀ peapods, were synthesized in high yield by a vapor-phase doping method. Raman spectra, high resolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) measurement indicate that the tube diameter is one of the important factors to determine the orientation of C₇₀ molecules inside the SWNTs. SWNTs with different diameters give different alignment of C₇₀ molecules. The lying orientation is favorable over the standing orientation in thin nanotube, i.e. 1.36 nm nanotubes, whereas the standing orientation is favorable in thick nanotubes, i.e. 1.49 and 1.61 nm nanotubes.     

Comparative investigation on decorating carbon nanotubes with different transition metals

The diffusion dynamics and structure evolvement of the transition metal (TM=Ni, Cu, Au, and Pt) atoms decorating carbon nanotubes (CNTs) with differences have been systematically studied by Monte Carlo (MC) simulation. The studies show that TM atoms can be encapsulated inside, aggregated and even wrapped on the surface of the CNT, which depend on the interactions among TM–TM and TM–C during the spontaneous diffusion process. The decorating effect is greatly influenced by the diameters of CNTs, TM atoms tend to be encapsulated inside the tube in the relatively large CNTs, while they are inclined to stack on the surface for the small ones. More interestingly, Au and Pt atoms would wrap around the smaller CNT, whereas Ni and Cu atoms are still clustering outside of the CNTs with the increase of the number of TM atoms. Simulation results indicate that Pt and Au possess a better wetting effect with CNT than Ni and Cu.     

A novel continuous process for synthesis of carbon nanotubes using iron floating catalyst and MgO particles for CVD of methane in a fluidized bed reactor

A novel continuous process is used for production of carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CVD) of methane on iron floating catalyst in situ deposited on MgO in a fluidized bed reactor. In the hot zone of the reactor, sublimed ferrocene vapors were contacted with MgO powder fluidized by methane feed to produce Fe/MgO catalyst in situ. An annular tube was used to enhance the ferrocene and MgO contacting efficiency. Multi-wall as well as single-wall CNTs was grown on the Fe/MgO catalyst while falling down the reactor. The CNTs were continuously collected at the bottom of the reactor, only when MgO powder was used. The annular tube enhanced the contacting efficiency and improved both the quality and quantity of CNTs.The SEM and TEM micrographs of the products reveal that the CNTs are mostly entangled bundles with diameters of about 10-20 nm. Raman spectra show that the CNTs have low amount of amorphous/defected carbon with I_G/I_D ratios as high as 10.2 for synthesis at 900 °C. The RBM Raman peaks indicate formation of single-walled carbon nanotubes (SWNTs) of 1.0-1.2 nm diameter.     

Carbon nanotube synthesis on oxidized porous silicon

Carbon nanotubes were grown on thermally oxidized porous silicon by catalytic chemical vapor deposition from the mixture of ferrocene and xylene precursor. The growth rate of carbon nanotubes showed dependence on the oxidation extent of porous silicon. On pristine porous silicon surfaces, only poor nanotube growth was observed, whilst samples oxidized in air at 200, 400, 600 and 800 °C prior to the deposition process proved to be suitable substrates for carbon nanotube synthesis. Networks of carbon tubes with diameter of ∼40 and ∼10 nm observed on the surfaces of samples were investigated by electron microscopy and by energy dispersive X-ray analysis.