Outer Walls Of Carbon Nanotubes Research Articles

Silicon and porous MWCNT composite as high capacity anode for lithium-ion batteries

A silicon/porous multi-walled carbon nanotubes composite was synthesized using a simple method. A mixture comprising silicon nanoparticles and multi-walled carbon nanotubes was prepared by a mini ball milling method followed by annealing at low temperature. The low-temperature annealing treatment allows the aggregation of silicon nanoparticles and propels them to adhere to the outer walls of carbon nanotubes without the formation of a SiOx layer on Si nanoparticles. Mild oxidation occurring on the carbon tube walls provides additional surface defects. The obtained composite, which was studied as an anode for Li-ion batteries, exhibited excellent cyclability and superior rate capability compared with pristine silicon nanoparticles. The improved electrochemical performance of the composite can be attributed to the electrically conductive carbon tubes, easy access of the electrolyte ions into the porous nanotube walls, and mechanical support provided by the carbon matrix. As a result, the proposed composite can sustain high discharge capacities of 1,685 mAh g−1 at 1C rate after 80 cycles and 913 mAh g−1 at 5C rate after 100 cycles.

Effect of carbon nanotubes irradiation by argon ions on the formation of SnO 2-x /MWCNTs composite

Abstract Methods of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES) were used to investigate the influence of irradiation by argon ions of multi-walled carbon nanotubes (MWCNTs) on the composites SnO 2-x /MWCNTs formation. It has been shown that irradiation of the MWCNTs leads to the formation of structural defects and oxygen-containing groups in the walls of carbon nanotubes. The presence of the latest in MWCNTs walls leads to chemical interaction of the tin oxide with the surface of MWCNTs through formation of chemical Sn O C bonds at the interfaces “Tin oxide-MWCNTs” at synthesis of the composite by CVD method. It has been established that the crystalline structure of the tin oxide clusters forming on surface of MWCNTs depends essentially on the defectiveness in the structure of the outer walls of carbon nanotubes.

Low Resistance, Carbon Black-free Magnetite Anode for Li-ion Batteries Obtained by One-step Attachment of Carbon Nanotubes

The paper describes a simple, one-step synthetic route for the fabrication of nanometric iron (III) oxide attached to multiwalled carbon nanotubes (MWCNT). TEM images show that magnetite nanoparticles with primary particle sizes of ca. 10 nm are preferentially located on the outer walls of carbon nanotubes. The obtained nanocomposites have been examined on reversible electrochemical insertion of lithium cations in a Li-ion cell. The presence of MWCNT brings about a substantial increase of the magnetite reversible capacities in the absence of any additional carbonaceous conductivity enhancing agent. With increasing MWCNT content in the material, reversible capacity consistently rises from 207 mAhg-1 for pure iron (III) oxide up to 763 mAhg-1 for the composite with 40 % of MWCNT. Cyclic voltammetry measurements reveal the expected large hysteresis between the reduction and oxidation peaks associated with Li+ insertion/deinsertion into magnetite crystal lattice. The reductive current peak maxima steadily rise with increasing MWCNT content from 0.15 A g-1 for pure iron (III) oxide up to 0.4 A g-1 for the composite with 40 % of MWCNT which confirms faster kinetics of electrochemical processes. EIS measurements directly proved the internal resistance decrease connected with the incorporation of MWCNT.

Tin Dioxide–Carbon Heterostructures Applied to Gas Sensing: Structure-Dependent Properties and General Sensing Mechanism

Carbon materials such as carbon nanotubes (CNTs), graphene, and reduced graphene oxide (RGO) exhibit unique electrical properties, which are also influenced by the surrounding atmosphere. They are therefore promising sensing materials. Despite the existence of studies reporting the gas-sensing properties of metal oxide (MOx) coated nanostructured carbon, an incomplete understanding of their sensing mechanism remains. Here we report a systematic study on the preparation, characterization, and sensing properties of CNT and RGO composites with SnO2 coating. Atomic layer deposition (ALD) was applied to the conformal coating of the inner and outer walls of CNTs with thin films of SnO2 of various thicknesses, while nonaqueous sol–gel chemistry assisted by microwave heating was used to deposit tin dioxide onto RGO in one step. The sensing properties of SnO2/CNTs and SnO2/RGO heterostructures toward NO2 target gas were investigated as a function of the morphology and density of the metal oxide coating. The genera...

Dispersion-corrected density functional theory comparison of hydrogen adsorption on boron-nitride and carbon nanotubes

One of the main challenges for the future hydrogen economy is finding a safe and efficient way to store hydrogen. Materials with large surface areas, like carbon nanotubes and their analogues boron-nitride nanotubes, are being studied as potential candidates for this purpose. We perform density functional theory (DFT) and dispersion-corrected DFT (DFT-D) calculations of the adsorption of molecular hydrogen on graphene and boron-nitride sheets and compare the results against M\o{}ller-Plesset perturbation theory (MP2 and MP2.5). Our results indicate that DFT underestimates the binding energies, while DFT-D gives a very good agreement with the higher-order theory. Within DFT-D, we show that the binding energy of molecular hydrogen to the outer walls of carbon nanotubes is more than 40% larger than that of boron-nitride nanotubes.

Improved field emission performance of carbon nanotube by introducing copper metallic particles

To improve the field emission performance of carbon nanotubes (CNTs), a simple and low-cost method was adopted in this article. We introduced copper particles for decorating the CNTs so as to form copper particle-CNT composites. The composites were fabricated by electrophoretic deposition technique which produced copper metallic particles localized on the outer wall of CNTs and deposited them onto indium tin oxide (ITO) electrode. The results showed that the conductivity increased from 10-5 to 4 × 10-5 S while the turn-on field was reduced from 3.4 to 2.2 V/μm. Moreover, the field emission current tended to be undiminished after continuous emission for 24 h. The reasons were summarized that introducing copper metallic particles to decorate CNTs could increase the surface roughness of the CNTs which was beneficial to field emission, restrain field emission current from saturating when the applied electric field was above the critical field. In addition, it could also improve the electrical contact by increasing the contact area between CNT and ITO electrode that was beneficial to the electron transport and avoided instable electron emission caused by thermal injury of CNTs.

Nickel oxide coated on ultrasonically pretreated carbon nanotubes for supercapacitor

Nickel oxide/carbon nanotubes (NiO/CNTs) composite materials for supercapacitor are prepared by chemically depositing nickel hydroxide onto carbon nanotubes pretreated by ultrasonication and followed by thermal annealing at 300 °C. A series of NiO/CNTs composites with different weight ratios of nickel oxide versus carbon nanotubes are synthesized via the same route. The high-resolution TEM and SEM results show that a lot of nicks, which favored the nucleation of the nickel hydroxide formed on the outer walls of carbon nanotubes due to ultrasonic cavitations, and then nickel oxide coated uniformly on the outer surface of the individual carbon nanotubes. The NiO/CNTs electrode presents a maximum specific capacitance of 523 F/g as well as a good cycle life during 1,000 cycles in 6 M KOH electrolyte. The good electrochemical characteristics of NiO/CNTs composite can be attributed to the three-dimensionally interconnected nanotubular structure with a thin film of electroactive materials.

Effect of Confinement in Carbon Nanotubes on the Activity of Fischer−Tropsch Iron Catalyst

Following our previous findings that confinement within carbon nanotubes (CNTs) can modify the redox properties of encapsulated iron oxides, we demonstrate here how this can affect the catalytic reactivity of iron catalysts in Fischer-Tropsch synthesis (FTS). The investigation, using in situ XRD under conditions close to the reaction conditions, reveals that the distribution of iron carbide and oxide phases is modulated in the CNT-confined system. The iron species encapsulated inside CNTs prefer to exist in a more reduced state, tending to form more iron carbides under the reaction conditions, which have been recognized to be essential to obtain high FTS activity. The relative ratio of the integral XRD peaks of iron carbide (Fe(x)C(y)) to oxide (FeO) is about 4.7 for the encapsulated iron catalyst in comparison to 2.4 for the iron catalyst dispersed on the outer walls of CNTs under the same conditions. This causes a remarkable modification of the catalytic performance. The yield of C5+ hydrocarbons over the encapsulated iron catalyst is twice that over iron catalyst outside CNTs and more than 6 times that over activated-carbon-supported iron catalyst. The catalytic activity enhancement is attributed to the effect of confinement of the iron catalyst within the CNT channels. As demonstrated by temperature-programmed reduction in H2 and in CO atmospheres, the reducibility of the iron species is significantly improved when they are confined. The ability to modify the redox properties via confinement in CNTs is expected to be of significance for many catalytic reactions, which are highly dependent on the redox state of the active components. Furthermore, diffusion and aggregation of the iron species through the reduction and reaction have been observed, but these are retarded inside CNTs due to the spatial restriction of the channels.

Polyoxometallate-stabilized platinum catalysts on multi-walled carbon nanotubes for fuel cell applications

A novel catalyst, Pt-PMo 12-CNT, with well-dispersed Pt nanoparticles and monolayer PMo 12 on multi-walled carbon nanotubes (CNTs) is reported. A polyoxometallate (PMo 12) was chemically adsorbed on the surface of Pt nanoparticles and on outer walls of CNTs. These effectively prevented the agglomeration of Pt nanoparticles and CNTs due to the electrostatic repulsive interactions between negatively charged PMo 12 monolayers. The as-prepared Pt-PMo 12-CNT materials show higher electrocatalytic activity, higher cycle stability, and better tolerance to poisoning species in methanol oxidation than do Pt-CNT catalysts prepared by the same method. The reasons for the improved catalytic performance of the Pt-PMo 12-CNT catalysts are discussed.

Template Synthesis of Aligned Carbon Nanotube Arrays using Glucose as a Carbon Source: Pt Decoration of Inner and Outer Nanotube Surfaces for Fuel‐Cell Catalysts

AbstractA facile method is developed to synthesize aligned arrays of open‐ended carbon nanotubes (CNTs) via in situ glucose polymerization in the inner pores of anodic aluminum oxide templates under hydrothermal conditions, followed by carbonization at high temperature. Pt nanoparticles are decorated on the surfaces of the as‐prepared CNTs using the incipient wet method based on the use of NaBH4 as a reductant. Characterization of the resulting structures by transmission electron microscopy and field‐emission scanning electron microscopy demonstrates that the Pt nanoparticles are anchored on both the inner and outer walls of CNTs, thus giving rise to a shell–core–shell‐like nanotube composite. The electrocatalytic properties of the Pt–CNT–Pt electrodes are investigated for methanol oxidation by cyclic voltammetry and chronoamperometric measurements. It is found that the hybrid electrodes show superior catalytic performance compared to commercial carbon‐black‐supported Pt. The increased catalytic efficiency of Pt might be a result of the unique morphology of these structures.

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.

Shape/Size-Controlled Syntheses of Metal Nanoparticles for Site-Selective Modification of Carbon Nanotubes

Shape- and size-controlled syntheses of metal nanoparticles have been achieved by galvanic displacement reaction between an aqueous solution of metal salt and Cu foil substrate. In particular, cubic and spheric nanoparticles of Pt (Au) with a fairly narrow size distribution were produced by reacting K(2)PtCl(4) (HAuCl(4)) with a Cu foil in an aqueous medium with and without CuCl(2) under different reaction conditions (e.g., different concentrations and reaction times). In conjunction with the substrate-enhanced electroless deposition (SEED) technique (Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806), the shape/size-controlled syntheses have been successfully exploited to site-selectively deposit these metal nanoparticles onto the outerwall, innerwall, or end-tip of carbon nanotubes (CNTs). Asymmetric sidewall modification by attaching the innerwall and outerwall of CNTs with metal nanoparticles of different shapes was also achieved. Furthermore, it was demonstrated that the nanotube-supported Pt nanoparticles could be converted into hollow Au nanoboxes by galvanic displacement of Pt with Au. These CNT-supported metal nanoparticles were shown to possess interesting optical and electrocatalytic properties.

Density Functional Study of H-Induced Defects as Nucleation Sites in Hybrid Carbon Nanomaterials

Recently we have reported on the growth of an exciting new class of hybrid nanostructured carbon materials, coupling nanosized diamond with single-walled carbon nanotubes. The inner structures were shown to be single-walled C nanotubes or bundles of single-walled nanotubes up to 15 {micro}m long, and the outer deposit consisted of faceted diamond crystallites with diameters in the range of 20-100 nm. To aid in understanding the mechanisms responsible for the formation of such materials, the present study uses density functional theory to examine the role of atomic hydrogen in creating localized sp{sup 3} hybridized defects on the outer wall of carbon nanotubes. The results illustrate that certain absorption configurations may produce defects containing dangling carbon bonds, and thus promote the formation of suitable sites for nanodiamond nucleation.

Carbon nanotube/poly(2,4-hexadiyne-1,6-diol) nanocomposites prepared with the aid of supercritical CO2.

Poly(2,4-hexadiyne-1,6-diol)(poly(HDiD)) was coated on the outer walls of carbon nanotubes (CNTs) with the aid of supercritical CO(2), resulting in poly(HDiD)/CNT nanocomposites, which possess optical properties originated from poly(HDiD).