Combinatorial catalyst approach for high-density growth of horizontally aligned single-walled carbon nanotubes on sapphire

We studied effects of metal catalyst and gas composition on the chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWCNTs) on r-plane sapphire substrates. The SWCNTs are sitting on the substrate and aligned along [10] direction of the sapphire surface. A combinatorial metal deposition method was applied for single and binary metal catalysts to systematically investigate the thickness and the composition dependence. The horizontally-aligned SWCNTs grown from stripe-patterned catalysts enable the direct comparison of the catalytic activity based on nanotube density. We found that the SWCNT density strongly depends on the metal catalyst in the order Fe > Co ? Ni ≈ Cu, while no nanotubes were grown over Mo. In addition, the methane concentration during CVD strongly influenced the nanotube density, and the optimal concentration varied depending on the metal species and its thickness. The study on the binary metal catalysts revealed that Fe–Co combination increases the SWCNT density (7–9 tubes/μm) about twice of the original metal film. The Co–Cu binary catalyst also showed the high density (8–10 tubes/μm) under a limited methane concentration. Different catalytic activity of each metal is discussed.     

The growth of single-walled carbon nanotubes on a silica substrate without using a metal catalyst

Single-walled carbon nanotubes (SWCNTs) have been directly grown on a SiO₂ substrate using the chemical vapor deposition (CVD) of ethanol without a catalyst. Care was taken to exclude the possibility that the SWCNT growth was induced by conventional metal catalysts such as Fe, Co and Ni resulting from the contamination. Pretreatment of the SiO₂ at 950 °C or a higher temperature in H₂ before CVD was critical for the synthesis of SWCNTs. After CVD process, nano-scale carbon particles were produced besides SWCNTs. Based on these results, we propose that the annealing of SiO₂ substrates in H₂ at high temperature generates defects on their surfaces, and these defects provide nucleation sites for the formation of carbon nanoparticles and assist the formation of carbon nanocaps, thus leading to the SWCNT growth.     

Multiple “optimum” conditions for Co-Mo catalyzed growth of vertically aligned single-walled carbon nanotube forests

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co-Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 μm in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions.     

Moderating carbon supply and suppressing Ostwald ripening of catalyst particles to produce 4.5-mm-tall single-walled carbon nanotube forests

Millimeter-tall single-walled carbon nanotube (SWCNT) forests were grown by chemical vapor deposition (CVD) from C₂H₂/H₂O/Ar using Fe/Al–Si–O catalysts. Using combinatorial catalyst libraries coupled with real-time monitoring of SWCNT growth, the catalyst and CVD conditions were systematically studied. The keys for this growth are to maintain the C₂H₂ pressure below its upper limit to prevent the killing of the catalysts and to grow the SWCNTs before the catalyst particles lose their activity because of coarsening through Ostwald ripening. Lower temperatures lead to lower limits for the C₂H₂ pressure which result in lower growth rates but also lead to even lower coarsening rates which result in even longer growth lifetimes. Using these principles, we grew 4.5-mm-tall SWCNT forests in 2.5 h at 750 °C.     

Towards high purity graphene/single-walled carbon nanotube hybrids with improved electrochemical capacitive performance

CoMgAl layered double hydroxides were prepared as catalysts for the in situ synchronous growth of graphene and single-walled carbon nanotubes (SWCNTs) from methane by chemical vapor deposition. The as-calcined CoMgAl layered double oxide (LDO) flakes served as the template for the deposition of graphene, and Co nanoparticles (NPs) embedded on the LDOs catalyzed the growth of SWCNTs. After the removal of CoMgAl LDO flakes, graphene (G)/SWCNT/Co₃O₄ hybrids with SWCNTs directly grown on the surface of graphene and 27.3 wt.% Co₃O₄ NPs encapsulated in graphene layers were available. Further removal of the Co₃O₄ NPs by a CO₂-oxidation assistant purification method induced the formation of G/SWCNT hybrids with a high carbon purity of 98.4 wt.% and a high specific surface area of 807.0 m²/g. The G/SWCNT/Co₃O₄ hybrids exhibited good electrochemical performance for pseudo-capacitors due to their high Co₃O₄ concentration and the high electrical conductivity of SWCNTs and graphene. In another aspect, the G/SWCNT hybrids can be used as excellent electrode materials for double-layer capacitors. A high capacity of 98.5 F/g_electrode was obtained at a scan rate of 10 mV/s, 78.2% of which was retained even when the scan rate increased to 500 mV/s.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells

Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter.     

Optimizing substrate surface and catalyst conditions for high yield chemical vapor deposition grown epitaxially aligned single-walled carbon nanotubes

Single-crystal stable-temperature (ST)-cut quartz substrates, which have a (0 1 1 1) crystallographic plane with their surface normal lying close to 38° from the y axis ([0 1 0]), were annealed in air prior to use as a support for aligned carbon nanotube growth by chemical vapor deposition. Very smooth substrate surfaces were obtained with annealing times in the vicinity of 15 h at a temperature of 750 °C. These smooth surfaces are ideal for the growth of horizontally aligned SWCNTs with high spatial density, while less dense SWCNTs were obtained with less smooth surfaces. Under optimized growth conditions, only SWCNT are observed and they can grow to lengths in excess of 100 μm. Our findings suggest structural defects interfere with the growth process. A binary Fe/Co catalyst was employed to grow the nanotubes. No obvious dependence on the Fe:Co ratio is observed.     

Fluidized-bed synthesis of sub-millimeter-long single walled carbon nanotube arrays

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al₂O_x catalysts were deposited on the surface of Al₂O₃ beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C₂H₂ as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2–4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as <0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays.     

Impact of nanometal catalysts on the laser vaporization synthesis of single wall carbon nanotubes

The effects of catalyst particle size on the purity, yield, and purification efficiency of single wall carbon nanotubes (SWCNTs) synthesized via pulsed laser vaporization were investigated. The purity of as-produced SWCNT material synthesized using Ni and Co nanometal (∼13 nm diameter) catalyst particles was compared to material synthesized using conventional micronmetal (2-3 μm diameter) particles. The SWCNT material from nanometal catalysts demonstrated a 50% increase in SWCNT purity as assessed by optical absorption spectroscopy and thermogravimetric analysis (TGA). A change in the thermal oxidation properties was also observed with the nanometal-SWCNTs exhibiting a suppression of the exothermic oxidation of post-synthesis catalyst. Statistical analysis of the TGA residue yielded mean post-synthesis catalyst particle diameters of 18 ± 6 nm and 3 ± 1 nm for the micronmetal and nanometal produced material, respectively. When a thermal oxidation profile was performed, the micronmetal-produced material showed the typical decrease in SWCNT purity with increasing oxidation temperature while the nanometal-produced material showed increasing SWCNT purity with increasing temperature. Overall, the use of nanometal catalysts significantly increases synthesis yield and offers novel thermal oxidation procedures to thermally remove carbonaceous impurities without the aid of acid treatments for the development of potential large-scale purification processing.     

Room-temperature synthesis of single-wall carbon nanotubes by an electrochemical process

Single-wall carbon nanotubes (SWCNTs) were produced by an electrochemical route by applying a small negative potential to a solution of acetic acid over a Au surface supporting Ni nanocatalysts. Ni nanocatalysts were grown electrochemically on Au surface and their particle sizes were controlled by deposition time. Raman spectroscopy and scanning probe microscopy observations of the catalyst and as-deposited samples and revealed that the catalyst structure strongly affects the SWCNT diameter distribution. The deposited carbon structure depended on the catalyst particle size and structure. Raman spectra confirmed the existence of selectively grown semiconducting SWCNTs with very narrow diameter distribution.     

Controlled attachment of metal nanoparticles to single walled carbon nanotubes as a key step in their seeded growth and lengthening

The present study demonstrates the viability of the reductive attachment step of the single walled carbon nanotube (SWCNT) lengthening process in which long SWCNTs are grown from short nanotube seeds. Aryl sulfonate sidewall-functionalized, carboxylate end-functionalized SWCNTs are attached to an inorganic cluster pro-catalyst (FeMoC) via ligand exchange. The SWCNT-FeMoC complex was electrodeposited onto highly ordered pyrolytic graphite (HOPG), heated and exposed to etching conditions. Pre- and post-treatment AFM imaging shows that controlled reductive etching of the SWCNTs is attainable at a variety of pressures and temperatures in hot surface/cold gas and hot surface/hot gas systems.     

Epoxy composite sheets with a large interfacial area from a high surface area-supplying single-walled carbon nanotube scaffold filler

Closed single-walled carbon nanotubes (SWCNTs) with the highest available specific surface area (>1000 m²/g) were used to provide a very large interfacial area with the matrix to fabricate epoxy composite sheets. Through mechanical redirection of the vertically aligned SWCNTs, we created a laterally aligned “scaffold” sheet, into which epoxy resin was impregnated. The SWCNT scaffold was engineered in structure to meet two almost mutually exclusive demands, i.e. to have nanometer mesopores (∼2–50 nm) to facilitate homogeneous impregnation of the epoxy resin and to possess a cohesive mechanical strength to tolerate the compaction forces generated during impregnation. Through this approach, a SWCNT/epoxy composite sheet was realized where long and aligned SWCNTs were loaded at a high weight fraction (33%) with an intertube distance approaching the radius of gyration for polymers. The resultant composite showed a Young’s modulus of 15.0 GPa and a tensile strength of 104 MPa, thus achieving 5.4- and 2.1-fold reinforcement compared to the epoxy resin.     

Layered double hydroxides as catalysts for the efficient growth of high quality single-walled carbon nanotubes in a fluidized bed reactor

A family of layered double hydroxides (LDHs), such as Fe/Mg/Al, Co/Mg/Al, and Ni/Mg/Al LDHs, were used as catalysts for the efficient growth of single-walled carbon nanotubes (SWCNTs) in a fluidized bed reactor. The LDH flakes were agglomerated into clusters with sizes ranging from 50 to 200 μm, and they can be easily fluidized with a gas velocity ranging from 2.3 to 24 cm/s. After calcination and reduction, small metal catalyst particles formed and distributed uniformly on the flakes. At the reaction temperature, the introduction of methane realized the growth of SWCNTs with the diameter of 1-4 nm. The loose structure of LDH agglomerates afforded a yield as high as 0.95 g_CNT/(g_cat h) of SWCNTs with a surface area of 930 m²/g. Compared with Fe/Mg/Al LDH, Ni/Mg/Al and Co/Mg/Al LDHs showed a better selectivity to SWCNTs. The highest selectivity for metallic SWCNTs was obtained using Co/Mg/AI LDHs as the catalyst.     

Deposition of functionalized single wall carbon nanotubes through matrix assisted pulsed laser evaporation

Single-wall carbon nanotubes (SWCNTs) functionalized with oxygen-containing groups were deposited onto glass substrates by matrix assisted pulsed laser evaporation (MAPLE). The experiments were performed by subjecting ultraviolet laser pulses (KrF¹ excimer laser, 248 nm wavelength) to frozen SWCNT-toluene targets placed in a parallel plane a few cm in front of the substrate. The morphology, structure, and chemical composition of the deposited materials were studied through atomic force microscopy, high resolution transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The influence of the laser fluence on the material structure was investigated. The results indicate that the functionalized SWCNTs can be transferred by MAPLE at low laser fluences without the alteration of the structure of the initial material used as targets in MAPLE experiments. An increase of the fluence leads to the decomposition of the functional groups, mainly carboxylic acid groups, without degradation of the SWCNT structure whereas, at the highest fluences, the amorphization and even coalescence of the carbon nanotubes takes place.     

The reasons why metals catalyze the nucleation and growth of carbon nanotubes and other carbon nanomorphologies

Carbon nanotubes (CNTs) and other carbon nanomorphologies are grown using “typical” (Ni, Co, and Fe) and “atypical” (Al, In, Pt, Ti, Mg, Pd, K, Cs, Na, W, Mn, Mo, Ir, and Ni₃C) catalysts by chemical vapor deposition. X-ray diffraction analysis reveals the formation (and decomposition) of metal carbides at different stages of these growth reactions. Based on these results and various pieces of evidence gathered from the literature, we present a model that explains why different metals catalyze the nucleation and growth of CNTs or other carbon nanomorphologies, and in particular, why Ni, Co, and Fe display the highest catalytic activity. The catalytic activity of a metal strongly depends on its electronic structure. This property of the catalyst not only controls the decomposition of the carbon source, but also the formation and stability of metal carbides and, more importantly, the release of carbon atoms. Another property of significance is the enthalpy of formation of the carbon source because it enhances the activation of the catalyst.     

Influence of carbon structure of the anode on the production of graphite in single-walled carbon nanotube soot synthesized by arc discharge using a Fe–Ni–S catalyst

We investigated the production of the graphite contained in the soot of single-walled carbon nanotubes (SWCNTs) synthesized using the arc discharge method with a poorly graphitized carbon (PGC) rod in comparison to a graphite rod. A PGC rod was produced using a mixture of coal tar and carbon black and was heat treated to 1000 °C. The rod was packed with a mixture of iron (Fe), nickel (Ni), sulfur (S), and PGC and used for the production of SWCNT soot using arc discharge. From the results of X-ray diffraction and electron microscopy, the amount of graphite in the SWCNT soot synthesized by PGC rod was lower than that by graphite rod. The production of graphite in the soot was found to be dependent on the carbon structure of the anode and the current density of arc discharge.     

The influence of single-walled carbon nanotube functionalization on the electronic properties of their polyaniline composites

The “in situ” preparation and characterization of composites of polyaniline (PANI) and single-walled carbon nanotubes (SWCNTs) are reported. To improve the dispersion and compatibility with the polymer matrix the raw SWCNTs were modified following different routes. SWCNTs oxidized by chemical or thermal treatments (nitric acid and air oxidation, respectively) were subjected to covalent functionalization with octadecylamine (ODA). SWCNT/PANI composites were prepared either from just oxidized SWCNTs, or from ODA functionalized SWCNTs. Temperature-programmed desorption, elemental analyses, ultraviolet-visible (UV-vis), UV-vis with near infrared and Raman spectroscopy, X-ray diffraction, scanning and transmission electron microscopy and conductivity measurements were used to characterize the functionalized SWCNT materials, dispersions and composites. The PANI composite prepared from air oxidized SWCNTs showed the best electrical conductivity indicating a better interaction with polyaniline than ODA functionalised SWCNTs. The improvement of conductivity is attributed to the doping effect or charge transfer of quinoide rings from PANI to SWCNTs.     

Low resolution Raman spectroelectrochemistry of single walled carbon nanotube electrodes

A new combination of a low-resolution Raman spectrometer with a minipotentiostat is presented in this work to perform in situ Raman measurements during electrochemical experiments with low-cost instrumentation. The instrumental setup has been used to study the electrochemical oxidation of transparent single walled carbon nanotube (SWCNT) films supported on non-conductive substrates. The spectroelectrochemical response provides the dependence of the characteristic signatures of the SWCNT bundles with the applied potential, which is similar to the response observed for SWCNT films deposited on conducting substrates. The evolution of both the electrical current and the Raman features differs considerably for pristine and oxidized films. The spectroscopic data reveal the occurrence of two kinds of irreversible breakdowns, “oxidative burning” and functionalization of the SWCNTs, in addition to reversible p-doping. 2D-correlation has been applied to analyze the evolution of the spectra with potential and has provided more detailed information than expected from a low spectral resolution spectrometer.     

Assembly of untreated single-walled carbon nanotubes at a liquid-liquid interface

Untreated single-walled carbon nanotubes (SWCNTs) were assembled at a liquid-liquid interface to form an ultrathin film. The SWCNTs were dispersed into water using sodium dodecyl sulfate (SDS) as a solubilizing agent. Then, hexane was added to the dispersion to create a liquid-liquid interface. The SWCNTs were assembled at the interface to form a smooth ultrathin film when ethanol was added to the SWCNT water dispersion/hexane solution. The assembly mechanism was considered to be caused by the decreased wettability of SDS-coated SWCNT during the addition of ethanol because of desorption of SDS from the SWCNT surface. The assembly was remarkably robust and easily transferable to substrates. An AFM image of the film transferred onto a silicon substrate shows a closely packed uniform film of 3-8 nm thickness. The SWCNT ultrathin film showed high transparency of ca. 97% with an electrical conductivity of 71.4 S/cm. Fabrication processing was carried out in ambient conditions, thereby making it an attractive application for use in flexible electric devices.