Highly ordered carbon nanotubes based on porous aluminum oxide: fabrication and mechanism

Highly ordered carbon nanotubes (CNTs) are wildly pursued due to their unique properties. Anodic aluminum oxide (AAO) exhibits great possibility for this purpose. Here, CNTs based on AAO template were produced using acetylene or ethylene as the hydrocarbon sources with or without the presence of Co catalysts. CNTs grown on the Co-embedded AAO samples were normally confined within the nanopores of the AAO template. It was found that C2H4 normally requires 100 degrees C higher pyrolysis temperature than C2H2 under otherwise identical conditions. The pyrolysis temperature is greatly reduced with the presence of Co catalysts. CNTs can grow out of the nanopores, if Co particles are present at the bottom of the nanopores and if the nanopores are short in length or large in diameter. The graphitization of AAO template grown CNTs was studied by Raman spectroscopy. The CNTs produced from ethylene are generally better in graphitization than those from acetylene, and the CNTs grown with the presence of Co catalysts deposited at the bottom of nanopores are better than those without Co catalysts or with Co catalysts coated on the entire inner wall of nanopores. The growth temperature is found not to play a critical role in graphitization.     

Highly ordered carbon nanotubes based on porous aluminum oxide

Highly ordered carbon nanotubes (CNTs) are widely pursued due to their unique properties. Anodic aluminum oxide (AAO) exhibits great possibility for this purpose. Here, CNTs based on AAO templates were produced using acetylene or ethylene as the hydrocarbon sources with or without the presence of Co catalysts. CNTs grown on the Co-embedded AAO samples were normally confined within the nanopores of the AAO template. It was found that C2H4 normally requires 100 degrees C higher pyrolysis temperature than C2H2 under otherwise identical conditions. The pyrolysis temperature is greatly reduced with the presence of Co catalysts. CNTs can grow out of the nanopores if Co particles are present at the bottom of the nanopores, and if the nanopores are short in length or large in diameter. The graphitization of AAO-template grown CNTs was studied by Raman spectroscopy. CNTs produced from ethylene are generally better in graphitization than those from acetylene, and CNTs grown with the presence of Co catalysts deposited at the bottom of nanopores are better than those without Co catalysts or with Co catalysts coated on the entire inner wall of nanopores. The growth temperature is found not to play a critical role in graphitization.     

Enhanced photocleavage of water using titania nanotube arrays

In this study highly ordered titania nanotube arrays of variable wall thickness are used to photocleave water under ultraviolet irradiation. We demonstrate that the wall thickness and length of the nanotubes can be controlled via anodization bath temperature. We find that the nanotube wall thickness is a key parameter influencing the magnitude of the photoanodic response and the overall efficiency of the water-splitting reaction. For 22 nm inner pore diameter nanotube arrays, those fabricated in a 5 degrees C anodization bath, 224 nm length and 34 nm wall thickness produced a photoanodic response that was thrice that of a nanotube array fabricated in a 50 degrees C anodization bath, 120 nm length and 9 nm wall-thickness. At high anodic polarization, above 1 V, the quantum efficiency under 337 nm illumination was greater than 90%. For the 5 degrees C anodization bath samples (22 nm pore-diameter, 34 nm wall thickness), upon 320-400 nm illumination at an intensity of 100 mW/cm(2), hydrogen gas was generated at the power-time normalized rate of 960 micromol/h W (24 mL/h W) at an overall conversion efficiency of 6.8%. To the best of our knowledge, this hydrogen generation rate is the highest reported for a titania-based photoelectrochemical cell.     

Preparation and modification of carbon nanotubes

Carbon nanotubes (CNTs) were prepared by the catalytic decomposition of methane at 680 °C for 120 min, using nickel oxide–silica binary aerogels as the catalyst. The morphological structure of CNTs was investigated by transmission electron microscopy (TEM), X-ray Diffraction (XRD) and Raman spectroscopy. The results revealed that CNTs with diameter 40–60 nm showed high quality, uniform diameter and high length/diameter ratio, the wall structure of CNTs was similar with that of highly oriented pyrolytic graphite (HOPG), and some metal catalyst particles were encapsulated at the tip of CNTs. Different methods were compared to modify CNTs. Investigated by TEM, XRD, Raman spectroscopy and nitrogen adsorption/desorption for modified CNTs, it was confirmed that after modification treatment by immersion in diluted HNO₃ solution with ultrasonic and then milling by ball at a high velocity, the metal catalyst particles at the tip of CNTs disappeared, the unique cylinder wall structure remained, the CNT length became short, the cap at the tip of nanotube was opened, and thus the internal surface area could be effectively used, leading to the increase of the specific surface area and pore volume. This technique is relatively simple and effective for modifying CNTs which can be scaled up for industrial applications.     

Synthesis of carbon nanotubes by swirled floating catalyst chemical vapour deposition method

A series of developments have been made in synthesizing Carbon Nanotubes (CNTs) by Catalytic Vapour Deposition (CVD) methods since its discovery as a possible route to the large scale and high quality production of CNTs. In this study, CNTs were synthesized continuously in a swirled floating catalytic chemical vapour deposition reactor using acetylene as carbon source, ferrocene as catalyst, with argon and hydrogen as carrier gases within the temperature range of 900-1050 degrees C. The effects of pyrolysis temperature, acetylene flow rate, hydrogen flow rate, and ratio of flow of acetylene to hydrogen on the rate of production of CNTs were investigated. The CNTs produced were purified with dilute nitric acid and the nature and quality of the CNTs were analysed by TEM, Raman spectrometer, EDX, and TGA. Results obtained revealed that a mixture of single and multi wall carbon nanotubes were produced continuously with a maximum yield rate of 0.31 g/min at 1000 degrees C and a flow ratio of acetylene to hydrogen of one to five.     

Germanium nanowires with 3-nm-diameter prepared by low temperature vapour-liquid-solid chemical vapour deposition

We report the growth of germanium nanowires (Ge NWs) with single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300 degrees C, 280 degrees C, and 260 degrees C. The catalyst used in our experiment was Au nanoparticles with equivalent thicknesses of 0.1 nm (average diameter approximately 3 nm), 0.3 nm (average diameter approximately 4 nm), 1 nm (average diameter approximately 6 nm), and 3 nm (average diameter approximately 14 nm). The Gibbs-Thomson effect was used to explain our experimental results. The Ge NWs grown at 300 degrees C tend to have tapered structure while the Ge NWs grown at 280 degrees C and 260 degrees C tend to have straight structure. Tapering was caused by the uncatalysed deposition of Ge atoms via CVD mechanism on the sidewalls of nanowire and significantly minimised at lower temperature. We observed that the growth at lower temperature yielded Ge NWs with smaller diameter and also observed that the diameter and length of Ge NWs increases with the size of Au nanoparticles for all growth temperatures. For the same size of Au nanoparticles, Ge NWs tend to be longer with a decrease in temperature. The Ge NWs grown at 260 degrees C from 0.1-nm-thick Au had diameter as small as approximately 3 nm, offering an opportunity to fabricate high-performance p-type ballistic Ge NW transistor, to realise nanowire solar cell with higher efficiency, and also to observe the quantum confinement effect.     

Ordered fullerene nanocylinders in large-diameter carbon nanotubes

A new ordered fullerene phase encapsulated by large-diameter CNTs is systematically investigated by combining a growth technique by chemical vapour deposition, high-resolution transmission electron microscopy and molecular-dynamics simulations. In contrast to fullerenes in smaller (1-2?nm) diameter CNTs, where fullerenes are packed in linear or helical chains, fullerenes form a nanoscale cylinder in double-walled CNTs with diameters of ～4?nm. The fullerenes were shown to form a nanocylinder with a side wall that resembled the (111) plane of solid C(60). This ordered phase is different from peapods or fullerene solids known so far, and a result of the interaction between the CNT wall and fullerenes. This finding will open up a new field of fullerene science.     

Effect of Co and Ni nanoparticles formation on carbon nanotubes growth via PECVD

The effect of cobalt (Co) and nickel (Ni) nanoparticle catalysts on the growth of carbon nanotubes (CNTs) were studied, where the CNTs were vertically grown by plasma enhanced chemical vapour deposition (PECVD) method. The growth conditions were fixed at a temperature of 700 °C with a pressure of 1000 mTorr for 40 minutes with various thicknesses of sputtered metal catalysts. Only multi-walled carbon nanotubes are present from the growth as large average diameter of outer tube (～10–30 nm) were measured for both of the catalysts used. Experimental results show that high density of CNTs was observed especially towards thicker catalysts layers where larger and thicker nanotubes were formed. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. The average diameter of particle size increases from 4 to 10 nm for Co and Ni catalysts. A linear relationship is shown between the nanoparticle size and the diameter of tubes with catalyst thicknesses for both catalysts. The average growth rate of Co catalyst is about 1.5 times higher than Ni catalyst, which indicates that Co catalyst has a better role in growing CNTs with thinner catalyst layer. It is found that Co yields higher growth rate, bigger diameter of nanotube and thicker wall as compared to Ni catalyst. However, variation in Co and Ni catalysts thicknesses did not influence the quality of CNTs grown, as only minor variation in I_G/I_D ratio from Raman spectra analysis. The study reveals that the catalysts thickness strongly affects not only nanotube diameter and growth rate but also morphology of the nanoparticles formed during the process without influencing the quality of CNTs.     

Double-walled carbon nanotubes in composite powders

Double-walled carbon nanotubes (DWNTs) may be interesting in many applications since the outer wall would provide an interface with the rest of the system, without modifying the inner wall. CNT-Fe/Fe3C-Al2O3 composite powders containing carbon nanotubes (CNTs) (65% of which are DWNTs) are prepared by reduction of an oxide solid solution in a H2-CH4 gas mixture. The powders and CNTs are studied by both local and macroscopical techniques. The influence of the reducing atmosphere composition and of the dwell time at 1050 degrees C is studied. There is a 6-fold increase in CNT content upon the increase in the CH4 content from 3 to 30 mol.%, but the formation of undesirable carbon nanofibers can also be promoted. A CH4 content of 12-18 mol.% is adapted for the particular iron content in these powders. Increasing the dwell time at 1050 degrees C results in the formation of CNTs with more walls.     

Apatite formation on carbon nanotubes

Apatite coating on carbon nanotubes (CNTs) was done with a biomimetic coating method. The multi-walled CNTs (MWNTs) of curled shape with about 30 nm in diameter were immersed for 2 weeks in the simulated body fluid. Observation by scanning electron microscopy (SEM) showed the formation of apatite on the MWNTs surface. The clusters of spherules consisting of needle-shaped apatite crystallites were massively grown on the aggregated MWNTs. The crystallites of 100 nm in width and 200–500 nm in length were grown perpendicularly to the longitudinal direction and radially originating from a common center of a single MWNT. Thus, the architecture of crystalline apatite at nano-scale levels could be produced by simple method and the MWNT may be acting as core for initial crystallization of apatite.     

Synthesis of carbon nanotubes using Ni95Ti5 nanocrystalline film as a catalyst

Carbon nanotubes (CNTs) were synthesized by low-pressure chemical vapour deposition (LPCVD) using N2:C2H2:H2 gas mixtures on nanocrystalline Ni95Ti5 film. This nanocrystalline film was deposited on silicon substrate using vapour condensation method. The growth temperature and growth time was kept at 800 degrees C and 30 mins, respectively and the pressure was maintained at 10 Torr. The growth mechanism of CNTs was investigated using FESEM, TEM, HRTEM, and Raman Spectroscopy. From FESEM image of Ni95Ti5 nanocrystalline film, it is clear that the particle size varies from 5-10 nm. EDX analysis suggests that Ni95Ti5 alloy contains Ni and Ti both. It is clear from TEM images that CNTs are multiwalled with the diameter varying from 10-30 nm and length of several micrometers. HRTEM image shows that the structure of these multi-walled nanotube (MWNTs) is bamboo-shaped and the catalyst exists at the tip of MWNTs. Fourier Transform Raman Spectroscopy confirmed that graphitic structure of as-prepared CNTs. Field emission measurements reveal that the carbon nanotubes grown for 30 mins showed a turn-on field of 7.2 V/microm, when the current density achieves 10 microA/cm2. The field enhancement factor was calculated to be 708.50 for carbon nanotubes grown for 30 mins.     

调变Ni／Mo／MgO催化剂中Ni／Mo比例可控合成薄壁碳纳米管

采用摩尔分数1%Ni及负载少量Mo的Ni/MO/MgO催化剂裂解甲烷合成薄壁碳纳米管.通过SEM、TEM、XRD和Raman光谱表征方法研究了碳纳米管直径和催化剂中Ni/Mo比例关系.实验发现:通过控制Ni/Mo比例可以调变催化剂颗粒大小以及活性相.TEM及XRD表征发现,随着Ni/Mo比例的降低,金属Mo相逐渐从NiMo合金相中析出.NiMo合金相对应的活性组分颗粒很小,容易催化裂解甲烷形成薄壁碳纳米管;而后析出的Mo相则主要形成了大管径厚壁的碳纳米管.当Ni/Mo比例为6时可以高选择性地获得窄分布,内径为1.3nm,外径为3.0nm的溥壁碳纳米管.Raman光谱进一步验证了碳纳米管含有较少的缺陷.薄壁碳纳米管形成的关键因素主要体现为碳在其表而的快速扩散以及小颗粒的碳纳米管催化剂活性相控制.     

Three-dimensional CuO nanobundles consisted of nanorods: hydrothermal synthesis, characterization, and formation mechanism

Novel monoclinic CuO nanobundles, 0.8-1 microm in size, were synthesized at 130 degrees C in the presence of sodium dodecyl benzenesulfonate (SDBS) by a simple hydrothermal method. Each nanobundle was comprised of many nanorods with one ends growing together to form a center and another ends radiating laterally from this center. The length and the diameter of these assembled nanorods are in the range of 200-300 nm and about 20-30 nm, respectively. HRTEM and SAED results indicated that the CuO nanorods grow along the [010] direction. An investigation of the hydrothermal process revealed that the reaction time, temperature and surfactant play important roles in the formation of the resultant CuO nanostructures. Isolated CuO nanorods were obtained when the temperature was increased to 190 degrees C, and CuO microflowers composed of many nanosheets were produced at 130 degrees C when cetyltrimethylammonium bromide (CTAB) was employed instead of SDBS. The possible mechanism for the formation of these CuO nanostructures was discussed simply on the basis of the experimental results.     

Growth and characterization of bamboo-shaped carbon nanotubes using nanocluster-assembled ZnO:Co thin films as catalyst

Bamboo-shaped carbon nanotubes (CNTs) had been successfully fabricated by a plasma enhanced chemical vapor deposition method, in which nanocluster-assembled ZnO:Co thin film was used as catalyst. It was found that bamboo-shaped CNTs were generally grown in a direction perpendicularly to the substrate surface with the tops of CNTs dominated by the droplet-like catalyst covered by the carbon layer. The diameter of CNTs was ranged from 20-50 nm. High resolution of TEM image showed that the typical CNT had a multi-walled structure with an inner core presented. The ordered graphite layers were inclined to an axis of CNT about 18 degrees and the interlayer space of a CNT was about 0.35 nm. Two peaks in Raman spectrum at 1586 cm(-1) and 1372 cm(-1) were identified as G-band and D-band for graphite, respectively. The results showed that catalyst based on ZnO:Co thin films could be used for the growth of CNTs with bamboo-shaped structure.     

High-rate low-temperature growth of vertically aligned carbon nanotubes

We report the low-temperature growth of vertically aligned carbon nanotubes (CNTs) at high growth rates by a photo-thermal chemical vapour deposition (PTCVD) technique using a Ti/Fe bilayer film as the catalyst. The bulk growth temperature of the substrate is as low as 370?°C and the growth rate is up to 1.3 μm min(-1), at least eight times faster than the values reported by traditional thermal CVD methods. Transmission electron microscopy observations reveal that as-grown CNTs are uniformly made of highly crystalline 5-6 graphene shells with an approximately 10 nm outer diameter and a 5-6 nm inner diameter. The low-temperature rapid growth of CNTs is strongly related to the unique top-down heating mode of PTCVD and the use of a Ti/Fe bimetallic solid solution catalyst. The present study will advance the development of CNTs as interconnects in nanoelectronics, through a CMOS-compatible low-temperature deposition method suitable for back-end-of-line processes.     

Growth of carbon nanotubes on cobalt catalyst film using electron cyclotron resonance chemical vapour deposition without thermal heating

This paper demonstrates that carbon nanotubes (CNTs) can be synthesized on a cobalt coated silicon substrate using electron cyclotron chemical vapour deposition and without intentionally heating the substrate. With the mixed gases of C(3)H(8)/N(2), CNTs with a multi-walled structure and a diameter up to 70?nm have been observed. Results show that the diameter of the CNTs increases with the thickness of the cobalt catalyst film and the amount of nitrogen incorporated in the CNT films considerably influences the structures of the CNTs. Vertically aligned CNTs can be fabricated with a microwave power as low as 300?W and the flow rate ratio of C(3)H(8)/N(2) = 20/20?sccm. The CNTs exhibit a turn-on field of 0.2?V?μm(-1) determined at the emission current density of 10?μA?cm(-2).     

Novel strategy for diameter-selective separation and functionalization of single-wall carbon nanotubes

The problem of separating single-wall carbon nanotubes (CNTs) by diameter and/or chirality is one of the greatest impediments toward the widespread application of these promising materials in nanoelectronics. In this paper, we describe a novel physical-chemical method for diameter-selective CNT separation that is both simple and effective and that allows up-scaling to large volumes at modest cost. Separation is based on size-selective noncovalent matching of an appropriate anchor molecule to the wall of the CNT, enabling suspension of the CNTs in solvents in which they would otherwise not be soluble. We demonstrate size-selective separation in the 1-2 nm diameter range using easily synthesized oligo-acene adducts as a diameter-selective molecular anchor. CNT field effect transistors fabricated from diameter-selected CNTs show markedly improved electrical properties as compared to nonselected CNTs.     

Synthesis of multi-walled carbon nanotube/silver nanocomposite powders by chemical reduction in aqueous solution

Carbon nanotube (CNT)/silver nanocomposite powders with different volume fractions of CNTs 2.5, 5 and 10?vol.% were prepared by chemical reduction in solution. Multi-walled CNTs underwent surface modifications for functionalisations by acid treatments. The acid-treated CNTs were investigated by FT-IR and X-ray photoelectron spectroscopy. The spectroscopic investigations of the acid-functionalised CNTs detected that several kinds of functional groups attached with the graphene structure as well as produced short and de-caped CNTs. Acidic stannous chloride solution was used to sensitise the surface of the functionalised CNTs. Silver was deposited on the surface of sensitised CNTs with chemical reduction reaction of alkaline silver nitrate solution by formaldehyde at room temperature and pH?～?9. The morphology of the produced CNT/silver nanocomposite powder was investigated by high-resolution SEM and TEM. It was observed that the produced CNT/silver nanocomposite powders have decorated type of spherical silver particle size 2–5?nm deposited on the surface of CNTs as well as the CNTs were implanted in large spherical silver nanoparticles of particle size ～200?nm. The chemical analysis of the produced powder indicates that some oxygen content is included in the prepared powders which can be reduced by heat treatment at temperatures between 300°C and 400°C under hydrogen atmosphere.     

Synthesis of carbon nanotubes using mesoporous Fe-MCM-41 catalysts

MWNTs (multi-walled carbon nanotubes) were made by catalytic CVD process using iron-containing mesoporous silica, Fe-MCM-41, with 4 mol% Fe loading prepared by direct synthesis route. Uniform 5 nm size Fe2O3 nano-particles impregnated onto a mesoporous silica support, SBA-15 were also prepared for CNTs synthesis. The catalysts were characterized using XRD, SEM/TEM, N2 physisorption, UV-vis diffuse reflectance and FT-IR spectroscopies. Acetylene gas was introduced as a carbon source, and the gas mixture of Ar:H2:C2H2 = 14:5:1 pyrolyzed at 750 degrees C for 60 min was found to be the optimum synthesis condition. Fe-MCM-41 due to higher dispersion of nano-sized Fe-species was efficient as catalyst for MWNTs with more uniform size distribution. Cobalt-impregnated Fe-MCM-41 (Co/Fe = 1) produced a small fraction of SWNTs of ca. 2 nm diameter mixed with MWNTs.     

Synthesis of carbon and carbon–nitrogen nanotubes using green precursor: jatropha-derived biodiesel

The jatropha-derived biodiesel, a green precursor was found to be a new and promising precursor for the synthesis of carbon nanotubes (CNTs) and carbon–nitrogen (C–N) nanotubes. The CNTs and C–N nanotubes have been synthesised by spray pyrolysis of biodiesel with ferrocene and ferrocene–acetonitrile, respectively, at elevated temperature under an argon atmosphere. The typical length and diameter of as-grown CNTs are 20?µm and 20–50?nm, respectively. The C–N nanotubes are found in bundles with effective length of ～30?µm and diameter ranging between 30 and 60?nm with bamboo-shaped morphology. The as-grown CNTs and C–N nanotubes were characterised through scanning and transmission electron microscopes, X-ray photoelectron, Raman and Fourier transform infrared spectroscopic techniques. These investigations revealed that the nanotubes synthesised by jatropha-derived biodiesel are clean from carbonaceous impurities and the bamboo compartment formations in C–N nanotubes are due to nitrogen incorporation. The nitrogen concentration in C–N nanotubes decreases with the increase in synthesis temperature.