Oxide-driven carbon nanotube growth in supported catalyst CVD

Detailed HREM studies on carbon nanotubes (CNTs) synthesized via chemical vapor deposition (CVD) using nanoengineered Fe particles on oxide supports show capped tops and open-ended roots. We demonstrate that the pristine catalyst particle dictates the CNT diameter and number of walls at nucleation. The consecutive inward formation of concentric graphene caps during nucleation constricts and elongates the catalyst particle within the tube core. Continued growth stems from the oxide support.     

A carbon nanotube structured biomimetic catalyst for polysaccharide degradation

A unique artificial catalyst that mimics the structure of active sites in real enzymes using functionalized carbon nanotubes is presented. This concept will allow for the potential construction of a library of biomimetic catalysts for enzyme active centers, for which the structure-catalysis relationships are well defined.     

Selective hydrogenation of cinnamaldehyde over carbon nanotube supported pd-ru catalyst

Summary Carbon nanotube supported Pd, Ru and Pd-Ru catalysts have been prepared and tested with the hydrogenation of cinnamaldehyde as a probe reaction. It has been found that the cinnamaldehyde conversion and the selectivity towards the hydrogenation of C=O bond over Pd-Ru/PCNT catalyst could reach 56.6% and 79.1%, respectively, at 120^oC and 5.0 MPa, which is better than Pd/PCNT and Ru/PCNT catalysts under the same reaction conditions. It is assumed that the better performance of Pd-Ru/PCNT catalyst for cinnamaldehyde hydrogenation may be due to the synergic effect of Pd and Ru metals or the promoting effect of Ru metal.     

Layer-by-layer fabrication and characterization of DNA-wrapped single-walled carbon nanotube particles

Carbon nanotubes have been proposed as support materials for numerous applications, including the development of DNA sensors. One of the challenges is the immobilization of DNA or other biological molecules on the sidewall of carbon nanotubes. This paper introduces a new fabrication of DNA-carbon nanotubes particles using the layer-by-layer (LBL) technique on single-walled carbon nanotubes (SWCNTs). Poly(diallyldimethylammonium) (PDDA), a positively charged polyelectrolyte, and DNA as a negatively charged counterpart macromolecule are alternatively deposited on the water-soluble oxidized SWCNTs. Pure DNA/PDDA/SWCNTs particles can be prepared and separated by simple unltracentrifugation. The characterization of DNA/PDDA/SWCNTs particles was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy, Raman spectroscopy, and thermogravimetric analysis (TGA). An electrode modified by the DNA/PDDA/SWCNTs particles shows a dramatic change of the electrochemical signal in solutions of tris(2,2'-bipyridyl)ruthenium(II) ((Ru(bpy)(3)2+) as a reporting redox probe. A preliminary application of the DNA-modified carbon nanotubes in the development of DNA sensors used in the investigation of DNA damage by nitric oxide is presented.     

Clay nanotube encapsulation for functional biocomposites

Natural halloysite clay nanotubes with 50 nm outer- and 15 nm inner- diameters are described as miniature vehicles for sustained release of drugs and proteins. The release time may be adjusted from 10 to 200 h with the tube surface polymeric coating. An explanation of sustained release through locking electrical potential at the nanotube ends is suggested. These biocompatible ceramic tubes may be also used for architectural construction of nanoshells on microbes through alternation with polycations to enhance the intrinsic properties of biological cells. Halloysite nanotubes (pristine or drug-loaded) are well mixable with polar and low-polar polymers allowing for functional biocomposites with enhanced mechanical strength, adhesivity and slow release of drugs or other chemical agents. Halloysite is nontoxic abundantly available from natural deposit material which does not require exfoliation or other complicated energy consuming processing.     

Selective matching of catalyst element and carbon source in single-walled carbon nanotube synthesis on silicon substrates

We have studied the compatibility of various catalysts for ethylene and ethanol chemical vapor deposition (CVD) syntheses of single-walled carbon nanotubes (SWNTs) on Si substrates. A strong selectivity between the catalyst elemental species and carbon source was found; SWNT yield for Fe (Co) catalysts was much higher for ethylene (ethanol) CVD than for ethanol (ethylene) CVD. This strong and completely opposite selectivity implies significantly different SWNT growth mechanisms for ethanol and ethylene CVD on Si substrates.     

Increasing efficiency of photoelectronic conversion by encapsulation of photosynthetic reaction center proteins in arrayed carbon nanotube electrode

The construction of efficient light energy converting (photovoltaic and photoelectronic) devices is a current and great challenge in science and technology and one that will have important economic consequences. Here we show that the efficiency of these devices can be improved by the utilization of a new type of nano-organized material having photosynthetic reaction center proteins encapsulated inside carbon nanotube arrayed electrodes. In this work, a generically engineered bacterial photosynthetic reaction center protein with specifically synthesized organic molecular linkers were encapsulated inside carbon nanotubes and bound to the inner tube walls in unidirectional orientation. The results show that the photosynthetic proteins encapsulated inside carbon nanotubes are photochemically active and exhibit considerable improvement in the rate of electron transfer and the photocurrent density compared to the material constructed from the same components in traditional lamella configuration.     

Molecular dynamics study of the catalyst particle size dependence on carbon nanotube growth

The molecular dynamics method, based on an empirical potential energy surface, was used to study the effect of catalyst particle size on the growth mechanism and structure of single-walled carbon nanotubes (SWNTs). The temperature for nanotube nucleation (800-1100 K), which occurs on the surface of the cluster, is similar to that used in catalyst chemical vapor deposition experiments, and the growth mechanism, which is described within the vapor-liquid-solid model, is the same for all cluster sizes studied here (iron clusters containing between 10 and 200 atoms were simulated). Large catalyst particles, which contain at least 20 iron atoms, nucleate SWNTs that have a far better tubular structure than SWNTs nucleated from smaller clusters. In addition, the SWNTs that grow from the larger clusters have diameters that are similar to the cluster diameter, whereas the smaller clusters, which have diameters less than 0.5 nm, nucleate nanotubes that are approximately 0.6-0.7 nm in diameter. This is in agreement with the experimental observations that SWNT diameters are similar to the catalyst particle diameter, and that the narrowest free-standing SWNT is 0.6-0.7 nm.     

Equilibrium encapsulation of polystyrene latex particles

We have performed a series of seeded styrene emulsion polymerizations in which the second stage of growth was initiated only after the second-stage monomer charge had achieved equilibrium saturation with the seed particles. The final particles were observed in the electron microscope by using two means of distinguishing between the first- and second-generation polymer: (a) butadiene tagging and osmium tetroxide staining coupled with ultramicrotomy and (b) tritiated-styrene tagging coupled with autoradiographic detection. We find that the first- and second-generation polymer chains are not uniformly mixed throughout the final latex product; rather, the second-generation polymer overcoats the seed polymer in a core–shell fashion. In order to explain these results, we present the viewpoint that monomer actually concentrates at the periphery of the swollen particle to form a monomer-encapsulated structure, rather than swelling the particle uniformly as has always been envisioned. We believe the encapsulation phenomena to be governed by the microscopic thermodynamic environment of the latex particles which has, in turn, a profound effect upon the conformational behavior of the long-chain polymer molecules as they interact with the particle–water interface.     

Graphitic mesostructured carbon prepared from aromatic precursors

Mesostructured carbons with graphitic framework walls are conveniently prepared at ambient pressures through the replication of a mesostructured silica template using an aromatic hydrocarbon as the carbon precursor and a catalyst.     

Role of subsurface diffusion and Ostwald ripening in catalyst formation for single-walled carbon nanotube forest growth

Here we show that essentially any Fe compounds spanning Fe salts, nanoparticles, and buckyferrocene could serve as catalysts for single-walled carbon nanotube (SWNT) forest growth when supported on AlO(x) and annealed in hydrogen. This observation was explained by subsurface diffusion of Fe atoms into the AlO(x) support induced by hydrogen annealing where most of the deposited Fe left the surface and the remaining Fe atoms reconfigured into small nanoparticles suitable for SWNT growth. Interestingly, the average diameters of the SWNTs grown from all iron compounds studied were nearly identical (2.8-3.1 nm). We interpret that the offsetting effects of Ostwald ripening and subsurface diffusion resulted in the ability to grow SWNT forests with similar average diameters regardless of the initial Fe catalyst.     

3-D carbon nanotube structures used as high performance catalyst for oxygen reduction reaction

We report a high performance oxygen reduction reaction (ORR) catalyst based on vertically aligned, nitrogen-doped carbon nanotube (VA-NCNT) arrays. Characterization in conditions analogous to the operation of a polymer electrolyte membrane fuel cell show ORR taking place on the catalyst at a favorable reduction potential with a superior current density and greater rate constant.     

Rapid growth and flow-mediated nucleation of millimeter-scale aligned carbon nanotube structures from a thin-film catalyst

We discuss the rapid growth of films and lithographically templated microstructures of vertically aligned small-diameter multiwalled carbon nanotubes (VA-MWNTs), by atmospheric-pressure thermal chemical vapor deposition (CVD) of C2H4/H2/Ar on a Fe/Al2O3 catalyst film deposited by electron beam evaporation. The structures grow to 1 mm height in 15 min and reach close to 2 mm in 60 min. The growth rate and final height of CNT microstructures grown from catalyst patterns depend strongly on the local areal density of catalyst, representing a reverse analogue of loading effects which occur in plasma etching processes. Abrupt transitions between areas of micrometer-thick tangled CNT films and millimeter-scale aligned CNT structures are manipulated by changing the duration of pretreatment by H2/Ar prior to introduction of C2H4 and by changing the configuration of the substrate sample in the furnace tube. This demonstrates that the flow profile over the sample mediates the supply of reactants to the catalyst and that pretreatment using H2 significantly affects the initial activity of the catalyst.     

Carbon nanotube motors driven by carbon nanotube

We propose a new type of carbon nanotube (CNT) motor composed of a single-wall CNT (SWCNT) and a double-wall CNT (DWCNT), that are in mechanical contact. The rotational motion of our CNT motor is controllable by the translational motion of the SWCNT along the axis of the DWCNT. From molecular dynamics simulations, we show how our CNT motor can be driven in a controlled manner.     

Characterization of Pd/TiO2 embedded in multi-walled carbon nanotube catalyst with a high photocatalytic activity

Pd particles loading on TiO₂-embedded multi-walled carbon nanotubes (MWCNTs), MWCNTs, and TiO₂ particles were prepared via an impregnation method with palladium(II) chlorate solution followed by heat treatment at high temperature. To characterize the catalysts, BET surface area, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray, Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy were employed. The prepared catalysts were tested in degradation of methyl orange under visible light. Pd/TiO₂-MWCNTs catalyst demonstrates the highest photocatalytic activity, and the phase transformation from PdO to Pd0 phase takes place at heat treatment of embedded TiO₂. The nanoparticles size of TiO₂ can be decreased by introduction of MWCNTs species. Combining structural characterization with kinetic study results we could conclude that the superior catalytic performance could arise due to the Pd/TiO₂-MWCNTs catalyst’s structure.     

Vertically aligned dense carbon nanotube growth with diameter control by block copolymer micelle catalyst templates

We have grown a dense array of vertically aligned carbon nanotubes (CNTs) with a controlled distribution of diameters by using block copolymer micelles to form and pattern catalyst particles. The block copolymer poly(styrene-block-acrylic acid) (PS16500-PAA4500) was dissolved in toluene to form micelles and then loaded with FeCl3. The metal-loaded micelles were spin-coated on Si and then thermally treated to remove the polymer. Using this process, we produced surfaces patterned with iron oxide catalyst particles with particle densities ranging from 1400 microm(-2) to 3800 microm(-2) and a size distribution of (6.9 +/- 0.8) nm. CNT growth by thermal chemical vapor deposition was then performed on these samples. The low-density catalyst sample produced unaligned, low-density CNTs, whereas the high-density catalyst sample produced vertically aligned, dense CNTs about 10 microm in length. Transmission electron microscopy revealed that the CNTs typically had double and triple graphitic layers with normally distributed diameters of (4.5 +/- 1.1) nm. For comparison, CNTs grown from the standard approach of blanket Fe films had a wide distribution of diameters between 6 and 21 nm. This catalyst preparation approach dramatically sharpens the size distribution of CNTs, compared to standard approaches, and provides a simple means of controlling the areal density of CNTs.     

Preparation of core-shell polymer colloid particles by encapsulation

 By means of heterocoagulation anionic poly-[styrene] particles were coated with smaller electrosterically stabilised cationic particles of poly-[butyl methacrylate]. On heating the heterocoagulated units 45 °C above the glass transition temperature of poly-[butyl methacrylate], as predicted theoretically, the latter polymer spread over the surface of the poly-[styrene] particle to give a composite particle with a core-shell structure. It was found that the extent of packing of the small particles on the larger core particle was a critical feature of the coating process. Received: 12 September 1996 Accepted: 18 September 1996     

Preparation of Co/Pd alloy particles dispersed multiwalled carbon nanotube supported nanocatalysts via gamma irradiation

New multiwalled carbon nanotube/silica supported cobalt-palladium bimetallic nanocatalysts (MWNT@silica/Co–Pd NPs) were prepared by a simple one step gamma irradiation method. The method involves the in-situ surface modification of MWNT with silica (MWNT@silica) and simultaneous formation of Co–Pd bimetallic NPs using gamma irradiation. The bimetallic NPs were stabilized by silica particles formed over the surface of MWNT. Extensive characterization studies have been performed on structural, morphological, and electrochemical, aspects of MWNT@silica/Co–Pd NPs. MWNT@silica/Co–Pd NPs were characterized by field emission scanning microscopy (FESEM), UV–visible spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy. The influence of irradiation dosage levels on the stabilizing effect of silica particles has been studied. The electrolytic activities of the MWNT@silica/Co–Pd NPs were investigated by cyclic voltammetry.     

Large-scale fabrication of aligned single-walled carbon nanotube array and hierarchical single-walled carbon nanotube assembly

Aligned single-walled carbon nanotubes (SWNTs) and hierarchical SWNT assembly were fabricated by electrospinning. The high fiber elongation and high DC electric field applied during the electrospinning process result in the orientation of the SWNTs along the axial direction of the fiber. The alignment of the electropsun composite fiber transfers this local SWNT orientation to macroscopically aligned SWNTs. After removing the polymer component from the aligned composite fiber, we produced large area aligned SWNTs. The results show that the directional control of SWNT alignment and debundling of SWNTs into individual tubes can be simultaneously realized.