Formation Of Single-walled Carbon Nanotubes Research Articles

Catalytic mechanism of Co-containing layered double hydroxide (Co-LDH) as a precursor in carbon nanotube (CNT) synthesis

Abstract The synthesis of carbon nanotubes (CNTs) by water-assisted chemical vapor deposition using Co-containing layered double hydroxides (Co-LDH) as catalyst precursors and the catalytic mechanism of Co-LDH catalyst precursors during CNT synthesis were investigated. Co-Al and Co-Mg-Al LDH were prepared by the hydrothermal or homogeneous precipitation method using urea. XPS analysis indicated an increase in Co0 content after a reduction process during CNT synthesis, and the TEM images show that metallic Co nanoparticles were formed while maintaining the LDH platelet-like crystal form. Consequently, these metal particles were found to act as the catalyst for the formation of CNTs. Finally, through control of the flow rate of C2H4 gas during CNT synthesis, the formation of single-walled CNTs with diameters of 4 nm or less was demonstrated.

Highly individual single-walled carbon nanotubes synthesized by floating catalyst chemical vapor deposition for transparent conducting films

The bundle formation of single-walled carbon nanotubes (SWCNTs) has dramatically hindered their performances of transparent conducting films (TCFs). Herein, we synthesized less bundled SWCNTs by reducing the concentration of catalyst precursor in floating catalyst chemical vapor deposition. The gas-phase number concentration of SWCNTs could be reduced from 1.6×106 to 4×105 cm−3 by decreasing the catalyst precursor and thus, the bundle formation due to nanotube collisions induced by Brownian diffusion was significantly inhibited. ∼42% of SWCNTs were found to be individual with a mean tube diameter of 1.0 nm, mean length of 2.5 μm and mean bundle diameter of 2.1 nm, the corresponding SWCNT TCF exhibited a sheet resistance of 109 Ω/sq. at 90% optical transmittance. Our work provides a strategy of synthesizing highly individual SWCNTs for enhanced performance of TCFs, implying great potential in electronic applications.

Catalyst geometry dependent single-walled carbon nanotube formation from polyaromatic hydrocarbon molecule: Pt(111) surface versus Pt nanoparticle

The chirality controllable synthesis of single-walled carbon nanotubes (SWCNTs) by the catalytic transformation of designed large polyaromatic hydrocarbon molecules has made significant progress in recent years, but the underlying mechanism, such as the role of the catalyst, has never been revealed at the atomic level. In this study, the energy profiles of the dehydrogenation processes from the C60H30 molecule to a (6,6) SWCNT seed on a Pt (111) surface and a Pt55 particle are calculated using first-principles calculations. Our calculations clearly demonstrate that the SWCNT formation process is catalyst geometry dependent, and that it is substantially easier on a curved catalyst surface, i.e., the Pt55 particle, than on a flat Pt(111) surface. Furthermore, catalytic reactions involving Pt adatom on the catalyst surface can considerably reduce the dehydrogenation barriers. This study reveals that employing catalyst particles to synthesize SWCNT from polyaromatic hydrocarbon molecule is a better approach for chirality-controlled SWCNT development.

Nano-mesh superstructure in single-walled carbon nanotube/polyethylene nanocomposites, and its impact on rheological, thermal and mechanical properties

The mesh formation of single-walled carbon nanotube (SWCNT) superstructure, i.e. nano-mesh, in low- and high-density polyethylene (LDPE and HDPE) matrices was observed by using plasma etching technique and SEM analysis. Nano-mesh was established by two mechanisms: i) alignment of SWCNT bundles (forming network backbone) and ii) shish-kebab induced crystallization (forming network net). The network was formed at low SWCNT content due to the high aspect ratio of nano-filler, which additionally contributed to alignment. Established nano-mesh superstructure caused confinement effect disrupting crystallization and melting kinetics, which directly affected mechanical properties. Although mechanical properties of LDPE and HDPE nanocomposites profoundly improved, i.e. Young’s modulus increased for 230% and 30%, respectively, at only 1 wt% of SWCNTs, HDPE nanocomposites showed visible confinement that reduced effective reinforcement. It seems that HDPE nanocomposites cannot achieve full reinforcing potential since confinement effect restricts the crystal growth, which gradually reduces effective reinforcement in semi-dilute regime.

Controlling anisotropic electrical conductivity in porous graphene-nanotube thin films

We suggest an idea of a new method for controlling the electrical conductivity of graphene-nanotube films using the physical effect that arises from the seamless contact of vertically oriented single-walled carbon nanotubes (SWCNTs) with graphene layers. When increasing the nanotube length by atomic layers in a graphene nanohole, the electrical conductivity of the graphene layer oscillates. This effect is based on the natural feature of armchair nanotubes, whose electronic properties and energy critically depend on the atomic framework length. The results of in silico studies show that the appearance of this effect leads to an increase in the electrical conductivity of graphene layers by 3–10 times. Besides, there is anisotropy: the electrical conductivity is 3–7 times greater in the armchair direction than in the zigzag direction. By adjusting the nanotube length, one can control the magnitude of the electrical conductivity and anisotropy. The amplitude of the oscillations is determined by the nanotube diameter, the symmetry of the transition zone of the seamless contact between graphene and nanotube, and the distance between nanotubes. To conduct in silico studies, an original approach is developed to identify energetically favorable SWCNT-graphene junctions during the formation of SWCNT of various diameters in a graphene nanohole.

A One-Step Chemical Strategy for the Formation of Carbon Nanotube Junctions in Aqueous Solution: Reaction of DNA-Wrapped Carbon Nanotubes with Diazonium Salts.

A single-step chemical strategy allows the formation of single-walled carbon nanotube (SWCNT) molecular junctions in aqueous solution. SWCNTs were first wrapped with DNA to be water soluble and solution processable. Diazonium salts, which have been shown to react spontaneously with carbon nanotubes in water at room temperature, were then employed to covalently link SWCNT segments. The DNA wrapping of the nanotubes acted as a protective layer that limits the functionalization predominantly to the nanotube terminal ends, therefore allowing the assembly of linear SWCNT junctions. Upon increasing the concentration of the linker, we observed first the formation of side-to-end junctions, and eventually the assembly, through side-to-side interactions, of SWCNTs into bundles. This approach demonstrates the possibility of tuning the formation of linear and branched carbon nanotube junctions that in turn is of importance for the sustainable fabrication of solution-processable CNT-based nanoscale systems and devices.

Kinetic study of single-walled carbon nanotube synthesis by thermocatalytic decomposition of methane using floating catalyst chemical vapour deposition

Single walled carbon nanotubes (SWCNTs) have been synthesized from thermocatalytic decomposition of methane by using ferrocene as the catalyst at atmospheric pressure. Floating catalyst chemical vapour deposition using a horizontal two zone reactor was employed to investigate the kinetics. The effects of temperature of synthesis (800–1000 °C), partial pressure (2–40 kPa) of reactant gas and catalyst concentration (0.2–4.6 mol m−3) on the rate of formation of SWCNTs have been studied. The rate is proportional to the partial pressure of methane indicating that the surface reaction of methane on the active site is the rate controlling step. The activation energy was found to be 80.16 kJ mol−1. Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, BET, UV–Vis-NIR and thermogravimetry have been used for the characterization of products. UV–Vis-NIR spectroscopy data reveals presence of 78–82% metallic SWCNTs (m-SWCNTs) in the synthesized samples. The BET surface area of purified SWCNTs were found to be 871 m2 g−1 with pore volume of 0.35 cc g−1 and electrical conductivity of 856–746 S cm−1.

Silica Precursor Effect on the Physical and Chemical Properties of Cobalt Incorportated MCM-41 Catalysts and Their Performance towards Single Wall Carbon Nanotubes

In this work, mesoporous silica (MCM-41) and a cobalt-incorporated catalyst (Co-MCM-41) were prepared using colloidal silica Cab-O-Sil, sodium silicate and tetraethylorthosilicate (TEOS) as the silica sources and cobalt nitrate as the cobalt source. Their physicochemical properties were analyzed, and their catalytic performance for the synthesis of single-wall carbon nanotubes (SWCNT) during chemical vapor deposition (CCVD) of methane was evaluated. When Cab-O-Sil was used, it was possible to incorporate 3% (nominal) cobalt with a good dispersion and without losing mesoporosity, resulting in minimal formation of superficial cobalt oxide. In contrast, the other catalysts product superficial cobalt oxide, according to the temperature programmed reduction (TPR) analysis. Co-MCM-41 prepared using Cab-O-Sil showed the best performance during the formation of SWCNT with a good regularity and selectivity without forming multi-wall carbon nanotubes or amorphous carbon structures.

Effect of gaseous and condensate products of ethanol decomposition on aerosol CVD synthesis of single-walled carbon nanotubes

In this study, the gaseous and condensate products of ethanol decomposition during the synthesis of single-walled carbon nanotubes (SWCNTs) by aerosol chemical vapor deposition (floating catalyst CVD) method were investigated. To determine the conditions for perfect and clean SWCNT formation the products synthesized at different temperatures and collected inside and outside the reactor were studied by transmission electron microscopy, Raman, Fourier transform infrared (FT-IR) and optical absorption spectroscopy. The individual and bundled SWCNTs of a predominantly small diameter (∼1 nm) were revealed. The obtained tubes exhibit a stable distribution of chiral indices in a wide range of the set temperature (750–1000 °C), which is promising for the development of controlled synthesis methods. The FT-IR analysis results show that the main gaseous products of thermal ethanol decomposition in our CVD system are methane and carbon monoxide. Acting as carbon source, the mixture of CO and CH4 makes possible to produce pure SWCNTs, as those obtained in the catalytic CO disproportionation method, with higher yield, taking into account twice the amount of carbon release during CH4 catalytic pyrolysis. The carbon deposit on the reactor wall is found to play important role in maintaining an optimal regime of CNT synthesis.

Evaluation of the enthalpy of formation of carbon nanotubes and their phase diagram

The enthalpy of formation of single-wall carbon nanotubes (SWCNTs) and multiwall carbon nanotubes (MWCNTs) of different geometric sizes is evaluated. The region of phase stability of carbon nanotubes (CNTs) of different geometric shapes (with various lengths, inner radii, and numbers of carbon layers) is determined based on the approximate equation of state of nanocarbon. The estimates do not contradict the experimentally measured parameters of phase transformation of MWCNTs into nanodiamonds. The calculated melting point of nanotubes turned out to be higher than that of graphite. The region of phase stability of CNTs may turn out to be wider than the region of thermodynamic stability of graphite.

Formation of single and multi-walled carbon nanotubes and graphene from Indian bituminous coal

Single-walled carbon nanotubes (SWCNTs) have been synthesized by electric arc discharge method using annealed coal electrode in the presence of Fe as well as Ni–Y as catalysts. Multi-walled carbon nanotubes (MWCNTs) have been synthesized without using any catalysts. The efforts have also been made to synthesized graphene like nanosheets from bituminous coal. The as-synthesized samples have been characterized through scanning and transmission electron microscopy, Raman and Fourier transform infrared spectroscopy. The formation of SWCNTs which holds nearly perfect one dimensional structure is confirmed by the presence of radial breathing mode. A feasible mechanism of CNTs formation from coal described here with the help of Fourier transform infrared spectroscopy.

Selective scission of C-O and C-C bonds in ethanol using bimetal catalysts for the preferential growth of semiconducting SWNT arrays.

For the application of single-walled carbon nanotubes (SWNTs) to electronic and optoelectronic devices, techniques to obtain semiconducting SWNT (s-SWNT) arrays are still in their infancy. We have developed herein a rational approach for the preferential growth of horizontally aligned s-SWNT arrays on a ST-cut quartz surface through the selective scission of C-O and C-C bonds of ethanol using bimetal catalysts, such as Cu/Ru, Cu/Pd, and Au/Pd. For a common carbon source, ethanol, a reforming reaction occurs on Cu or Au upon C-C bond breakage and produces C(ads) and CO, while a deoxygenating reaction occurs on Ru or Pd through C-O bond breaking resulting in the production of O(ads) and C2H4. The produced C2H4 by Ru or Pd can weaken the oxidative environment through decomposition and the neutralization of O(ads). When the bimetal catalysts with an appropriate ratio were used, the produced C(ads) and C2H4 can be used as carbon source for SWNT growth, and O(ads) promotes a suitable and durable oxidative environment to inhibit the formation of metallic SWNTs (m-SWNTs). Finally, we successfully obtained horizontally aligned SWNTs on a ST-cut quartz surface with a density of 4-8 tubes/μm and an s-SWNT ratio of about 93% using an Au/Pd (1:1) catalyst. The synergistic effects in bimetallic catalysts provide a new mechanism to control the growth of s-SWNTs.

Modulating SWCNT–silica interactions for enhanced dispersibility and hybrid cryogel formation

Single walled carbon nanotubes (SWCNTs) form a special class of carbon materials with high electric and thermal conductivity. Ordinary SWCNTs exist in the form of bundles, primarily because of intertubular van der Waals attraction. Here we show that silica nanoparticles (NPs) with tunable surface energy can be used as efficient aqueous dispersant for SWCNTs. A subsequent controlled solvent-evaporation leads to the formation of SWCNT–silica hybrid cryogels. We characterize cryogels using gas/vapor adsorption and electric conductivity measurements and show that their porosity and conductivity can be fine-tuned by altering the silica-to-SWCNT relative concentration. We believe that the SWCNT–silica hybrid cryogels can be used as a precursor for fabricating functional electrodes for advanced energy storage devices.

Ferrocene-ethanol-mist CVD Grown SWCNT Films as Transparent Electrodes

The floating catalyst CVD using ferrocene–ethanol mist was successfully used to deposit single-walled carbon nanotube (SWCNT) films from the gaseous phase onto a Si substrate and a membrane filter. The home-built vertical mist-CVD system was used without the use of H2 or CO gases. The tiny mist was generated using a high-frequency ultrasonic vibration. The effects of various parameters including furnace temperature, ferrocene/ethanol ratio, the deposition position in the reactor, flow rate of carrier gas, and deposition time on the SWCNTs formation were investigated using SEM, TEM and Raman spectroscopy. The furnace temperature and the flow rate of carrier gas were found to determine the diameter and crystallinity of nanotubes. The ferrocene concentration in ethanol influenced the diameter distribution of nanotubes and the amount of impurity particles in the materials. The collected SWCNT films on a filter were directly transferred to a laminate sheet by using a hot press laminator. The properties of SWCNTs were investigated by UV-Vis spectrophotometer and four-point probe measurement. The high sheet resistance was observed for the as-transferred films. However, the initial results show that a sheet resistance of 10 kΩ/cm2 and an 80% transmittance at the wavelength of 550nm after nitric acid treatment.

Theoretical investigation on carbon nucleation on nickel carbides at initial stages of single-walled carbon nanotube formation.

It is a long-standing controversy whether metal carbide clusters do exist during the nucleation and growth process of single-walled carbon nanotubes (SWCNTs). In the current work, we are trying to elucidate the carbon nucleation on nickel carbides during the initial stages based on density functional theory calculated formation energy and chemical potential for a series of Ni55Cn carbides (n is the number of carbon atoms dissolved in the Ni55 cluster). It is found that the formation energies of the Ni55Cn carbides decrease gradually with an increase of dissolved carbon atomic numbers, meaning the Ni55Cn carbides are thermodynamically stable. Meanwhile, the calculated chemical potentials indicate that not only nickel carbides are preferentially formed during the initial stage of the SWCNT nucleation, but also saturated nickel carbides may be able to exist during the nucleation and growth process of SWCNTs. In addition, the nickel carbides have a high selectivity for the formation of the carbon pentagon and carbon structures with pentagon-incorporated end-edge according to the adsorption energies. All of these findings provide opportunities in controlling the growth of the SWCNTs.

Fluctuation theory of single-walled carbon nanotube formation

In the framework of classical fluctuation theory an analytical formula is derived for the reversible work of formation of just detached carbon cap on the surface of catalyst nanoparticle (NP). This cap is considered as single walled carbon nanotube (SWCNT) formation center. The work of cap formation depends on the source carbon chemical potential μC. Using the derived formula for this work an expression for the rate of SWCNT formation is determined. From this expression the SWCNT diameter distributions can be obtained. The obtained distributions have sharp maxima. It is found that the modal SWCNT diameter d(m) increases weakly with μC being in the narrow window of 1.0 < d(m) < 1.8 nm when changing the source carbon chemical potential in a wide range. The determined diameter distributions proved to be in a good agreement with the typical values of the SWCNT diameters as experimentally measured in the chemical vapor deposition process. The increase of d(m) is accompanied by the increase of the distribution width Δd. The selectivity d(m)/Δd is a function of μC, the higher values of μC the worse selectivity is observed. Although the value of the SWCNT formation rate I cannot be calculated precisely the relationship between I and the system parameters, such as the NP radius R(S), can be obtained. This relationship is derived for the solid-liquid-solid system. To determine the function I(R(S)) for nanotubes of a certain diameter d, formulas for catalyst∕amorphous carbon mutual solubilities as functions of NP radius are derived in the framework of the rigorous Gibbs theory of interface. Using the derived formulas an expression giving the dependence I(R(S)) is obtained. The expression predicts an increase of I with the radius R(S). The estimations carried out for the metal/carbon interface surface tension of 1000 mN/m show that the SWCNT formation rate increases by a few orders of magnitude with the radius increase from 1 to 10 nm.

Self-Assembled Microhoneycomb Network of Single-Walled Carbon Nanotubes for Solar Cells

We propose a water vapor treatment to direct the formation of single-walled carbon nanotubes (SWNTs) into a self-assembled microhoneycomb network (μ-HN) for the application to SWNT-Si solar cells. The μ-HN consists of vertical aggregated SWNT walls and a buckypaper bottom. This hierarchical structure exhibits lower sheet resistance and higher optical transmittance compared with buckypaper. The pristine μ-HN SWNT-Si solar cell shows a record-high fill factor of 72% as well as a power conversion efficiency (PCE) of 6% without optimizing the diameter or height of the vertically aligned SWNTs. The PCE remains stable for weeks under ambient condition, and a PCE exceeding 10% is achieved in the dry state after dilute nitric acid treatment.

Efficient cycloaddition of arynes to carbon nanotubes under microwave irradiation

The functionalization of single-wall carbon nanotubes (SWCNT) by cycloaddition reactions with arynes under microwave irradiation has been studied. The efficiency of the functionalization was monitored by XPS, FTIR and Raman spectroscopies, TGA and HR-TEM. All experimental results point to the formation of SWCNT with a high degree of functionalization, with mass increases ranging from 6.4% to 15.1% depending on the aryne substitution.

Pt–Fe catalyst nanoparticles supported on single-wall carbon nanotubes: Direct synthesis and electrochemical performance for methanol oxidation

Single-wall carbon nanotubes (SWCNTs) supported Pt–Fe nanoparticles have been prepared by one-step hydrogen arc discharge evaporation of carbon electrode containing both Pt and Fe metal elements. The formation of SWCNTs and Pt–Fe nanoparticles occur simultaneously during the evaporation process. High-temperature hydrogen treatment and hydrochloric acid soaking have been carried out to purify and activate those materials in order to obtain a new type of Pt–Fe/SWCNTs catalyst for methanol oxidation. The Pt–Fe/SWCNTs catalyst performs much higher electrocatalytic activity for methanol oxidation, better stability and better durability than a commercial Pt/C catalyst according to the electrochemical measurements, indicating that it has a great potential for applications in direct methanol fuel cells.

Preparation and characterization of poly(styrene-co-butyl acrylate)-encapsulated single-walled carbon nanotubes under ultrasonic irradiation

Polymeric composite materials filled with single-walled carbon nanotubes (SWNTs) have attracted much attention, but successful applications of such composites require uniform dispersion of SWNTs in the polymeric matrix and the strong SWNTs-polymer interface interaction. In this paper, chemical modification combined with ultrasonically initiated in situ polymerization was successfully employed to prepare poly(styrene-co-butyl acrylate)/single-walled carbon nanotubes composites [P(St-BA)/SWNTs]. The whole procedure contained two steps: in the first step, 3-(trimethoxy)-propylmethacrylate-silane (silane-coupling agent, KH570), a kind of polymerizable vinyl monomer, was grafted onto the surface of SWNTs, forming KH570-g-SWNTs by reacting KH570 with hydroxyl groups on the surface of SWNTs, which was proved by combination of FTIR and XPS results. Due to the presence of polymerizable KH570 on the surface of SWNTs, this provides a basis for the next stage of polymerization to prepare polymer-encapsulated SWNTs composites. In the second step, an ultrasonically initiated in situ emulsion polymerization of monomer styrene (St) and n-butyl acrylate (BA) proceeded in the presence of KH570-g-SWNTs. Consequently, P(St-BA)/SWNTs composite emulsion was obtained. TEM confirmed that SWNTs were coated with the obtained polymer. FTIR and XPS further showed that even after 72 h of soxhlet extraction with boiling toluene, there were still unextracted polymers in P(St-BA)/SWNTs composite, indicating strong interaction between the polymer and carbon nanotubes. Finally, a mechanism for formation of polymer-encapsulated SWNTs through ultrasonically initiated in situ emulsion polymerization was proposed. This study could provide a new way to resolve the problems of the dispersion, stabilization, and compositing of SWNTs with polymer matrix and prepare polymer/SWNTs composites.