Diameter control of carbon nanotubes by changing the concentration of catalytic metal ion solutions

We have developed a simple new method to control the diameter of carbon nanotubes (CNTs) using catalytic nanoparticle arrays fabricated by filling the pores of well-ordered porous anodic aluminum oxide (AAO) templates with a metal ion solution. Fe ion solution was used to fill the pores in which Co had been deposited electrochemically, and then the template was dried naturally on a magnet. After this process, the pores were widened in NaOH solution. Well-graphitized multi-walled CNTs were grown from almost all the pores and were very long in length and homogeneous in diameter. We were able to control the diameter of CNTs, simply, by changing the concentration of iron ion solution. For example, the average outer diameters of the CNTs are 7 ± 1.5, 13 ± 1, and 17 ± 1 nm when the concentrations of Fe ion in their mother solutions were 1.0 × 10⁻³, 3.0 × 10⁻³, and 6.0 × 10⁻³ M, respectively. The inner diameters of these CNTs corresponded to the calculated diameters of Fe nanoparticles by assuming that all Fe ions contained in each pore are reduced to a single nanoparticle. This means that homogeneous nanoparticles are made in each pore. Our new method could be used to fabricate homogeneous nanoparticles from most metal ion solutions.     

Synthesis of vertically aligned, double-walled carbon nanotubes from highly active Fe-V-O nanoparticles

Monodispersed Fe-V-O nanoparticles were prepared by a liquid-phase synthesis to be used as catalysts for carbon nanotube (CNT) growth. Vertically aligned, dense CNTs have been grown from the highly active Fe-V-O nanoparticles by chemical vapor deposition. Diameter distribution of CNTs (3.7 ± 0.6 nm) was consistent with that of the original nanoparticles (3.1 ± 0.5 nm), and the value was smaller than those of other reported vertically aligned CNTs from as-prepared nanoparticles. TEM study showed that the CNTs consisted mainly of double-walled CNTs (single: 14%, double: 74%, and triple: 12%). The CNT diameter increased to 4.4 ± 0.8 nm as the growth temperature was increased from 810 to 870 °C. Energy dispersive X-ray spectroscopy of nanoparticles before and after the CNT growth revealed that the V content decreased from 7.2 to 2.7 at.%, suggesting that the segregation of Fe and V played an important role for the high activity of the Fe-V-O nanoparticles.     

Density measurement of size selected multiwalled carbon nanotubes by mobility-mass characterization

We employ a combination of gas phase particle mobility and mass methods to make the first absolute density measurement of gas phase grown carbon nanotubes (CNTs). The approach combines a tandem differential mobility analyzer and aerosol particle mass analyzer in series to achieve two steps of electrical mobility classifications of the CNTs and one of mass classification. In the first mobility classification step a stream of monodisperse catalytic particles was produced by pulsed laser ablation. These mobility-classified catalysts seeded the aerosol growth of CNTs, where were directly passed to a second electrical mobility classification step which allows classification of the diameter-controlled CNTs in length. These diameter- and length-classified CNTs were finally introduced into the aerosol particle mass analyzer to measure their mass distribution. We found that the condensed phase density of CNTs was 1.74 ± 0.16 g/cm³ for two different groups of CNTs with diameters of ∼15 and ∼22 nm. This value is lower (about 3 sigma) than for graphite, and about 1 sigma lower than the average value for density measurements for carbon black.     

Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

The effect of catalyst calcination temperature on the diameter of carbon nanotubes synthesized by the decomposition of methane

The effect of catalyst calcination temperature on the uniformity of carbon nanotubes (CNTs) diameter synthesized by the decomposition of methane was studied. The catalysts used were CoO-MoO/Al₂O₃ without prior reduction in hydrogen. The results show that the catalyst calcination temperature greatly affects the uniformity of the diameter. The CNTs obtained from CoO-MoO/Al₂O₃ catalysts, calcined at 300 °C, 450 °C, 600 °C, and 700 °C had diameters of 13.4 ± 8.4, 12.6 ± 5.1, 10.7 ± 3.2, and 9.0 ± 1.4 nm, respectively, showing that an increase in catalyst calcination temperature produces a smaller diameter and narrower diameter distribution. The catalyst calcined at 750 °C was inactive in methane decomposition. Transmission electron microscopy (TEM) studies showed that CNTs grown on the catalyst calcined at 700 °C were of uniform diameter and formed a dense interwoven covering. High-resolution TEM shows that these CNTs had walls of highly graphitized parallel graphenes.     

Synthesis of well-aligned carbon nanotubes with open tips

Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10 μm and diameters ranging from 92 to 229 nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.     

Formation of nanocarbons during activation of mesocarbon microbeads with potassium hydroxide

Carbon nanotubes (CNTs) together with carbon nanofibers (CNFs) have been produced on the surface of and inside mesocarbon microbeads containing Co nanoparticles during their activation with potassium hydroxide (KOH). The resulting CNFs consist of a number of platelet-shaped sub-units with width of about 500 nm and thickness of 50 nm. The CNTs, with diameter of about 200 nm, grow from the inside to the surface of the activated carbon beads. The results indicate that, in addition to the Co nanoparticles, the existence of KOH also plays an important role in the nanocarbon growth.     

Fabrication and superhydrophobicity of fluorinated carbon fabrics with micro/nanoscaled two-tier roughness

Superhydrophobic carbon fabric with micro/nanoscaled two-tier roughness was fabricated by decorating carbon nanotubes (CNTs) onto microsized carbon fibers, using a catalytic chemical vapor deposition and subsequent fluorination surface treatment. The superhydrophobic surfaces are based on the regularly ordered carbon fibers (8-10 μm in diameter) that are decorated by CNTs with an average size of 20-40 nm. The contact angle of water significantly increases from 148.2 ± 2.1° to 169.7 ± 2.2° through the introduction of CNTs. This confirms that the wettability of carbon fabric changes from hydrophobicity to superhydrophobicity due to structural transformation. This finding sheds light on how the two-tier roughness surface induces superhydrophobicity of rough surfaces, and how the presence of CNTs reduces the area fraction of a water droplet in contact with the carbon surface with two-tier roughness.     

Fabrication of CNTs with controlled diameters and their applications as electrocatalyst supports for DMFC

A facile synthesis procedure based on chemical vapor deposition (CVD) process has been developed to fabricate carbon nanotubes (CNTs) with controlled diameters and high yields utilizing Fe-containing ordered hexagonal mesoporous silicas (HMSs) such as MCM-41 and SBA-15 having varied pore sizes as the catalysts as well as the templates. It is found that unlike Fe/HMS catalysts prepared by co-precipitation method, samples prepared by the impregnation method gave rise to multi-wall CNTs with uniform diameters, which were largely dictated by the pore size of the Fe/HMS catalysts. Among these uniform MWCNTs, sample with a larger diameter (≥ 8 nm) was found to be more favorable as support for Pt catalyst, leading to a homogeneous dispersion of metal nanoparticles. Consequently, the Pt/CNT electrocatalysts so prepared gave rise to superior methanol oxidation activities as well as tolerances for CO poisoning compared to Pt supported on commercial single-wall CNT (Pt/SWCNT) and XC-72 activated carbon (Pt/XC-72) having a similar metal loading.     

Formation of carbon nanotubes through ethylene decomposition over supported Pt catalysts and silica-coated Pt catalysts

Ethylene decomposition was performed over supported Pt catalysts to fabricate composites of Pt metal nanoparticles and carbon nanotubes (CNTs). All supported Pt catalysts (Pt/carbon black, Pt/CNT, Pt/MgO, Pt/Al₂O₃ and Pt/SiO₂) showed catalytic activity for ethylene decomposition at 973 K to form CNTs. Pt metal particles were found at tips of CNTs. These results indicate that Pt metal particles have catalytic activity for growth of CNTs through hydrocarbon decomposition. A broad range (5-50 nm) of CNT diameters were formed from the use of supported Pt metal catalysts although Pt metal particles in the catalysts before ethylene decomposition were relatively uniform in size (2-5 nm). These results imply that Pt metal particles in the catalysts aggregated during ethylene decomposition at 973 K. Aggregation of Pt metal particles in catalysts during ethylene decomposition could be suppressed by covering catalysts with silica layers that were a few nanometers thick. Silica-coated Pt catalysts showed high activity for ethylene decomposition to form CNTs with uniform diameters (8-10 nm) despite the uniform coverage of Pt metal particles with silica layers.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

Catalytic synthesis of carbon nanotubes using Ni- and Co-doped calcium tartrates

Calcium tartrate doped with Ni and/or Co has been used as a catalyst source in the chemical vapor deposition synthesis of carbon nanotubes (CNTs). Thermolysis of doped calcium tartrate in an inert atmosphere was shown to yield Ni, Co or Ni-Co nanoparticles ∼6 nm in diameter dispersed in a calcium oxide matrix. The CNT synthesis was carried out by ethanol vapor decomposition at 800 °C. The structure of the products was characterized by transmission electron microscopy and Raman spectroscopy. It was found that Ni nanoparticles embedded in CaO provide the narrowest diameter distribution of CNTs, while the bimetallic Ni-Co catalyst allows the formation of the thinnest CNTs with the outer diameter of ∼2 nm. This type of CNT is more likely to be responsible for the lowest value of the turn-on field (∼1.8 V/μm) for the emission current detected for the latter sample.     

Continuous synthesis of metal nanoparticles in supercritical methanol

Metal nanoparticles were synthesized continuously in supercritical methanol (scMeOH) without using reducing agents at a pressure of 30 MPa and at various reaction temperatures ranging 150-400 °C. Wide angle X-ray diffraction (WAXD) analysis revealed that metallic nickel (Ni) nanoparticles were synthesized at a reaction temperature of 400 °C while mixtures of nickel hydroxide (α-Ni(OH)₂) and metallic Ni were produced at lower reaction temperatures of 250-350 °C. In contrast, metallic silver (Ag) nanoparticles were produced at reaction temperatures above 150 °C while metallic cupper (Cu) nanoparticles were produced at reaction temperatures above 300 °C. Mixtures of copper oxide (CuO and Cu₂O) and metallic Cu were produced at lower reaction temperatures of 250 °C. Scanning electron microscopy (SEM) showed that the particles size and morphology changed drastically as the reaction temperature increased. The average diameters of Ni, Cu and Ag particles synthesized at 400 °C were 119 ± 19 nm, 240 ± 44 nm, and 148 ± 32 nm, respectively. The scMeOH acted both as a reaction medium and a reducing agent. A possible reduction mechanism in scMeOH is also presented.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Ionic peapods from carbon nanotubes and phosphotungstic acid

Using a mild wet-chemical route, cage-like phosphotungstic acid (HPW) molecules of 1.2 nm diameter were successfully filled into opened carbon nanotubes (CNTs) with cavity diameter of 2 nm. High resolution transmission electron microscope revealed that HPW molecules arrayed in a chain in the tube cavity, forming a new kind of peapod structure. In aqueous solution these unusual peapods showed higher ionic property than the opened nanotubes, with a ζ-potential of −47.5 ± 1.3 mV, and dissolved finely as a stable solution. Raman spectra confirmed the strong interaction between HPW molecules and CNT graphite wall. It could substantially tune the electronic properties of CNTs and thus promote their applications in novel electronic devices.     

Synthesis of nitrogen-doped horn-shaped carbon nanotubes by reduction of pentachloropyridine with metallic sodium

Nitrogen-doped horn-shaped carbon nanotubes (CNTs) have successfully been prepared by reducing pentachloropyridine with metallic sodium at 350 °C. A typical CNT has an open-end diameter of ∼2 μm, a close-end diameter of ∼0.3 μm, a wall thickness of ∼30 nm, and a length up to 8 μm. TEM observation indicates that the CNTs account for ∼30% of the products, and the rest is solid and hollow carbon nanospheres (CNSs) with a diameter of about 50-290 nm. Elemental analysis shows that the N/C atomic ratio of the carbon nanostructures is about 0.0208. XRD and HRTEM measurements reveal that the CNTs are amorphous. To understand the growth process and refine the growth condition, various control experiments have been finished. At last, a sodium-catalysis-reduction solid-liquid-solid growth mechanism of the CNTs has been suggested on the basis of the experiments.     

A novel hybrid of carbon nanotubes/iron nanoparticles: iron-filled nodule-containing carbon nanotubes

A straightforward, one-step method for the preparation of novel carbon nanotube/iron nanoparticle hybrids with some degree of shape control is reported herein. These carbon nanostructures differ from those reported previously: the nanoparticles were not attached to or coated onto the surface of carbon nanotubes but embedded inside the carbon wall. They were synthesized in good yield by thermolysis of ferrocene and thiophene mixtures in a closed steel vessel. The shapes and compositions of these nanostructures can be simply controlled by adjusting the reaction temperature and relative amounts of the precursors. Iron-filled T-junction carbon nanotubes were also obtained easily by this procedure. These iron-filled nodule-containing carbon nanotubes (INCNTs) are either empty or filled with iron or iron carbide (Fe(C)) nanowires. The outer diameters of these nanotubes range from 70 to 150 nm and the lengths reach up to several micrometers. The average size of the Fe(C) nanoparticles (or empty cores) inside the nodules is about 50 nm in diameter. The carbon in the INCNTs is amorphous. Sulfur was found being responsible for the disordered structure and playing a unique role in promoting the growth of INCNTs as well as the formation of T- or Y-junctions.     

Combined catalyst system for preferential growth of few-walled carbon nanotubes

The effect of a combined catalyst system on the synthesis of carbon nanotubes (CNTs) from methane (CH₄) was investigated using molybdenum trioxide (MoO₃) as a conditioning catalyst and molybdenum-doped iron supported on magnesia as the main catalysts. Without the conditioning catalyst, only single-walled CNTs with diameters smaller than 2 nm were formed on the main catalyst. With the conditioning catalyst, double-walled CNTs and few-walled CNTs with larger hollow diameters than 2 nm were produced with much higher yields. The combination of the two kinds of Mo-containing catalyst would more effectively transform CH₄ into reactive species related to the early stage of nanotube formation on the main catalyst, resulting in the increase of the yield, diameter and layer number of the CNTs.     

Nanodiamond crystallites embedded in carbon films prepared by thermionic vacuum arc method

Nanostructured carbon films with thicknesses of 100 and 200 nm have been deposited from pure vapour carbon plasma using an original thermionic vacuum arc method. Silicon single crystalline wafers, glass and stainless steel held at 400 °C were used for substrates. The films consist of diamond nanoparticles of 5 nm diameter on the average embedded in a disordered graphite matrix as revealed by HRTEM, XPS and visible Raman measurements. The graphitic cluster diameters La range from 1.5 to 2.3 nm. Thicker films (200 nm) on stainless steel exhibit the largest clusters.     

Sonochemical hybridization of carbon nanotubes with gold nanoparticles for the production of flexible transparent conducing films

We have demonstrated the fabrication of flexible, transparent, conducting multiwalled carbon nanotube (MWCNT)/gold nanoparticle hybrid films with improved optoelectronic properties by combining the ionic liquid-assisted sonochemical method (ILASM) for hybrid synthesis with the vacuum filtration (VF) method for thin film preparation. Au nanoparticles (NPs) with diameters of 10.3 ± 1.5 nm were uniformly distributed onto the sidewalls of MWCNTs through ILASM, and flexible, transparent, conducting films of Au/MWCNT hybrids (HBs) were reproducibly fabricated by the VF method. In particular, the sheet resistance of Au-MWCNT-HB films was more than 2-fold lower than the sheet resistance of pristine MWCNT films due to the well-interconnected three-dimensional nanotube network structure and the synergistic effect of hybridization of MWCNTs with Au-NPs.