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.     

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.     

Nondestructive purification of single-walled carbon nanotube rope through a battery-induced ignition and chemical solution approach

A two-step method for the purification of single-walled carbon nanotube (SWCNT) rope containing substantial catalyst particles embedded in carbonaceous shells was developed. The first step was the triggering of rope ignition using a 9-V battery, which resulted in pre-oxidization of the carbon shells on the Fe₃C catalyst and oxidation of the exposed Fe₃C to form Fe₂O₃. In addition, SWCNTs with open-end structures due to ignition-induced cutting remained. In the second step, both oxalic acid (H₂C₂O₄) and hydrochloric acid (HCl) were used as the reactants to remove the Fe₂O₃ particles. No damage on the SWCNT walls after H₂C₂O₄ or HCl purification was found. In addition, adsorption of H₂C₂O₄ was also found on the H₂C₂O₄ purified SWCNT rope and it can be effectively removed by heating the rope at 200 °C in vacuum for 40 min. Samples were characterized by SEM, TEM, Raman spectroscopy, TGA, FTIR, XPS, and UV-Vis-NIR.     

Carbon nanotubes based on anodic aluminum oxide nano-template

The effect of reaction gas and catalyst on the growth of carbon nanotubes (CNTs) in the anodic aluminum oxide (AAO) nano-template was investigated. A mechanism of CNT growth was proposed, which involves the competitive catalytic carbon deposition between on the Co catalyst particles electrodeposited at the bottom of the pores and on the AAO template itself. Presence of H₂ in the reacting gas mixture significantly affected the morphology and the wall structure of synthesized CNTs: CNTs of high crystallinity grew out of pores with H₂ while no CNTs overgrew in the absence of H₂. CNT synthesis by CO disproportionation showed a lower growth rate and a higher degree of ordering than those grown by C₂H₂ pyrolysis. The unified mechanism of CNT growth on AAO template is also proposed.     

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.     

Effect of Temperature Gradient Direction in the Catalyst Nanoparticle on CNTs Growth Mode

To improve the understanding on CNT growth modes, the various processes, including thermal CVD, MP-CVD and ECR-CVD, have been used to deposit CNTs on nanoporous SBA-15 and Si wafer substrates with C₂H₂ and H₂ as reaction gases. The experiments to vary process parameter of ΔT, defined as the vector quantities of temperature at catalyst top minus it at catalyst bottom, were carried out to demonstrate its effect on the CNT growth mode. The TEM and TGA analyses were used to characterize their growth modes and carbon yields of the processes. The results show that ΔT can be used to monitor the temperature gradient direction across the catalyst nanoparticle during the growth stage of CNTs. The results also indicate that the tip-growth CNTs, base-growth CNTs and onion-like carbon are generally fabricated under conditions of ΔT > 0, <0 and ~0, respectively. Our proposed growth mechanisms can be successfully adopted to explain why the base- and tip-growth CNTs are common in thermal CVD and plasma-enhanced CVD processes, respectively. Furthermore, our experiments have also successfully demonstrated the possibility to vary ΔT to obtain the desired growth mode of CNTs by thermal or plasma-enhanced CVD systems for different applications.     

Modelling the growth of carbon nanotubes produced by chemical vapor deposition

An improved model of C₂H₂ deposition for the growth of carbon nanotubes (CNT) in a horizontal tube reactor has been developed. This includes detailed gas-phase reactions of acetylene pyrolysis, and surface catalytic reactions for CNT growth. Based on this model, the mechanism of CNT growth has been studied by analyzing the change of the CNT growth rate for different growth conditions such as pressure, temperature, number of catalyst nanoparticles per unit area, and diameters of catalyst nanoparticles. The influence of gas-phase reactions and their products on CNT growth has also been evaluated. It is found that although C₂H₂ is the main contributor to the growth of CNTs, the contribution from the gas-phase products could not be ignored, especially at high temperature.     

Hot filament enhanced CVD synthesis of carbon nanotubes by using a carbon filament

The low temperature synthesis of single-walled carbon nanotubes (CNTs) on a Si substrate has been reported. Single-walled CNTs were grown from a C₂H₂ and H₂ mixture by a hot-filament enhanced CVD method using a carbon filament. The catalyst was silica-supported iron–cobalt prepared by sol–gel method. We observed the influence of the catalyst material, C₂H₂ concentration, growth pressure and substrate temperature on the formation of the CNTs. By optimizing the catalyst, both single-walled CNTs and multiwalled CNTs could be synthesized, depending on the reaction conditions. The formation of single-walled CNTs occurred when the carbon supply was kept low, i.e. low C₂H₂ concentration and low reaction pressure. The diameter of tubes decreased with increasing the substrate temperature. By optimizing the growth conditions, a small diameter of 0.65 nm single-walled CNTs, estimated from Raman scattering spectrum, was achieved even the low substrate temperature as 660 °C.     

Aligned carbon nanotubes fabricated by thermal CVD at atmospheric pressure using Co as catalyst with NH3 as reactive gas

Co is used as a catalyst for chemical vapor deposition (CVD) of vertically aligned multi-walled carbon nanotubes (CNTs) in a tube furnace at atmospheric pressure. C₂H₂ and NH₃ were used for the carbon feedstock and reaction control, respectively. The CVD process parameters determine the chemical properties of the Co particles and subsequently the morphologies and field emission behavior of CNTs as they strongly depend upon the catalyst condition. The flow rate ratio of NH₃ to C₂H₂ is shown to be central to the synthesis of vertically aligned CNTs. Repeatable synthesis of vertically aligned CNTs at atmospheric pressure in a tube furnace is cost effective for large area deposition of such structures which may be used, for example, in vacuum field emission devices.     

Evidence for large hydrogen capacity in single-walled carbon nanotubes encapsulated by thin Pd layers onto a Pd substrate

We report a study of hydrogen storage and its mechanism in a novel material, representing single-walled carbon nanotubes (SWCNTs) encapsulated by thin Pd layers onto a Pd substrate. A synergetic effect resulting in combination of the Pd and the SWCNT properties with regard to hydrogen has been achieved. We showed that adding SWCNTs increases the H₂-capacity of the Pd–SWCNT composite under electrochemical loading only by up to 25% relative to the Pd metal alone. At the same time, with regard to the added SWCNTs, such synergetic approach (providing high H₂ pressure from highly H-loaded massive Pd substrate into a small fraction of deposited SWCNT) allowed us to achieve a net capacity of 8–12 wt.%. H₂, thus, bringing a unique chance to study hydrogen storage mechanism in highly H-loaded SWCNT. Using ESR technique it was established that the Pd–C_x π-complexes forming at the openings of SWCNTs could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT.     

Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C₄₀-Fe₅₅ and C₄₀-Ni₅₅ model complexes at 1500 K. The rate of Fe₅₅-catalyzed SWCNT growth determined in this work was 19% slower than the Fe₃₈-catalyzed growth rate. Conversely, Ni₅₅-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe₅₅-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni₅₅-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe₅₅-catalyzed growth. These polyyne chains usually persisted for 10-30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C_n carbon cluster moieties on the Fe₅₅ and Ni₅₅ surfaces were consistent with the relative strengths of the Fe-C, Ni-C and C-C interactions. The presence of smaller carbon moieties on the Fe₅₅ surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C_n units through the subsurface region of the catalyst particle. Conversely, the Ni₅₅ catalyst particle was observed to be more stable, remaining intact to a greater extent.     

Effect of Fe catalyst thickness and C2H2/H2 flow rate ratio on the vertical alignment of carbon nanotubes grown by chemical vapour deposition

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst thickness and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs by thermal CVD. In order to investigate these aspects, four Fe metal films with thickness of 2.5, 3.5, 7.5 and 16 nm were deposited on Si/SiO₂ substrate and CNTs were grown with different C₂H₂/H₂ flow rate ratios, from 5/95 to 30/70 by thermal CVD at 750 °C. Results showed that CNTs were not vertically aligned with 16 nm catalyst thickness for all flow rate ratios, while CNTs were always vertically aligned for iron thickness less than 3.5 nm and vertically aligned only for a C₂H₂/H₂ flow rate ratio greater than 20/70 for the 7.5 nm catalyst thickness. Morphology and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Our results also indicate that for each flow rate ratio exists a critical thickness of iron catalyst under which vertically aligned CNTs are obtained.     

Preparation of molecular sieving carbon from waste resin by chemical vapor deposition

Molecular sieving carbons (MSCs) were prepared from carbonized phenol-formaldehyde resin wastes by the chemical vapor deposition (CVD) of the pyrolyzed carbon from hydrocarbon species. The pore size of the MSCs could be controlled in the range 0.37-0.42 nm by changing the hydrocarbon species pyrolyzed, the pyrolyzing temperature, and the processing time. It is shown that some of the MSCs have an excellent selectivity for separating CO₂ and CH₄, and others for separating C₃H₈ and C₃H₆. As the mechanism for controlling the pore size during CVD processing, we elucidated that the adsorption of hydrocarbon molecules first takes place on the pore surface and then the adsorbed hydrocarbons pyrolyze into carbon. Therefore, the pore size of the MSC can be adjusted by controlling the amount hydrocarbon adsorbed on the phenol-formaldehyde resin char.     

The effect of processing conditions on carbon nanostructures formed on an iron-based catalyst

The carbon nanostructures formed during chemical vapour deposition (CVD) of ethene and hydrogen over an iron(III) oxide catalyst precursor have been investigated. Graphitic nanofibres and carbon nanotubes were both observed during synthesis, with the structures observed depending on the temperature and gas mix (C₂H₄ and H₂) used. Mixtures ranging from 100% to 5% ethene in hydrogen were investigated at different temperatures. During the synthesis, the iron(III) oxide was transformed into iron carbide (Fe₃C). The samples were characterised using transmission electron microscopy and X-ray diffraction. Filamentous nanostructures were observed with high hydrogen content (?80%) in the gas stream, whereas no filamentous structures were observed with ?20% H₂. The temperature of synthesis was varied between 500 and 800 °C, with higher temperatures giving higher carbon deposition rates, but also affected the nanostructures observed. However, it was found that increasing the temperature during CVD did not change the type of carbon deposited but did increase the deposition rate, providing a route to increase growth yields for a desired type of carbon nanostructure.     

Gas evolution during oil shale pyrolysis. 1. Nonisothermal rate measurements

The rate of evolution of CH₄, CO, CO₂, H₂, C₂ hydrocarbons, and C₃ hydrocarbons during pyrolysis of Colorado oil shale between 25 and 900 °C is reported. All experiments were performed nonisothermally using linear heating rates varying from 0.5 to 4.0 °C min^?1. Hydrogen is the major noncondensable gas produced by kerogen pyrolysis. The amount of H₂ released is influenced, via the shift and Boudouard reactions, by the CO₂ evolved from mineral carbonates. Lesser amounts of C₁, C₂, and C₃ hydrocarbons are produced. On the basis of heat content, however, the combined C₁ to C₃ hydrocarbons contribute twice as much as H₂ to the heating value of the pyrolysis gas. The evolution of H₂ and CH₄ involves processes that are interpreted as a ‘primary’ pyrolysis of the kerogen to generate oil, and a higher temperature ‘secondary’ pyrolysis of the carbonaceous residue. The CO formed is a product of the Boudouard reaction; nearly complete conversion of the carbon residue to CO via this reaction is observed.     

Role of iron catalyst particles density in the growth of forest-like carbon nanotubes

Carbon nanotubes (CNTs) were fabricated by Chemical Vapour Depositon using a C₂H₂/H₂ mixture. They were grown on Si/SiO₂ substrate with Fe film as catalyst, deposited using thermal evaporation technique. The aim of this work is to emphasize the role of the Fe catalyst and the C₂H₂/H₂ flow rate ratio to grow vertically aligned CNTs. Fe metal samples with the deposition times ranging from 1 min to 16 min were deposited and CNTs were grown with different C₂H₂/H₂ flow rate ratio, from 5/95 to 30/70 by thermal CVD at 750 ^oC. Results show that CNTs were not vertically aligned with the longest catalyst deposition time for all flow rate ratios, while CNTs were always vertically aligned for deposition time less than 4 min and vertically aligned only for a C₂H₂ flow rate greater than 20% for the 7 min catalyst deposition time. Morphological and structural information about CNTs and Fe metal clusters were provided by field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). An accurate balance between the Fe metal clusters density and the C₂H₂/H₂ flow rate ratio favours to achieve of a good vertical alignment     

Precursor gas chemistry determines the crystallinity of carbon nanotubes synthesized at low temperature

Despite significant progress in carbon nanotube (CNT) synthesis by thermal chemical vapor deposition (CVD), the factors determining the structure of the resulting carbon filaments and other graphitic nanocarbons are not well understood. Here, we demonstrate that gas chemistry influences the crystal structure of carbon filaments grown at low temperatures (500 °C). Using thermal CVD, we decoupled the thermal treatment of the gaseous precursors (C₂H₄/H₂/Ar) and the substrate-supported catalyst. Varying the preheating temperature of the feedstock gas, we observed a striking transition between amorphous carbon nanofibers (CNFs) and crystalline CNTs. These results were confirmed using both a hot-wall CVD system and a cold-wall CVD reactor. Analysis of the exhaust gases (by ex situ gas chromatography) showed increasing concentrations of specific volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs) that correlated with the structural transition observed (characterized using high-resolution transmission electron microscopy). This suggests that the crystallinity of carbon filaments may be controlled by the presence of specific gas phase precursor molecules (e.g., VOCs and PAHs). Thus, direct delivery of these molecules in the CVD process may enable selective CNF or CNT formation at low substrate temperatures. The inherent scalability of this approach could impact many promising applications, especially in the electronics industry.     

Self-organization of nitrogen-doped carbon nanotubes into double-helix structures

Self-organization of nitrogen-doped carbon nanotube (N-CNT) double helices was achieved by chemical vapor deposition (CVD) with Fe–Mg–Al layered double hydroxides (LDHs) as the catalyst precursor. The as-obtained N-CNT double helix exhibited a closely packed nanostructure with a catalyst flake on the tip, which connected the two CNT strands on both sides of the flake. A mechanism for the self-organization of N-CNTs into double-helix structures with a moving catalyst head is proposed. Effective carbon/nitrogen sources, high-density active catalyst nanoparticles, space confinement, and the precise chiral match between the two CNT strands are found to be crucial for the N-CNT double helix formation. The morphologies of N-CNTs can be well tuned between bamboo-like and cup-stacked structures, and a CNT/N-CNT heterojunction can be constructed by changing the carbon feedstock from C₂H₄ to CH₃CN during CVD growth. N-CNT double helices with a length of 10–36 μm, a screw pitch of 1–2 μm, a CNT diameter of 6–10 nm, and a N-content of 2.59 at.% can be synthesized on the LDH catalysts by the efficient CVD growth.     

Snapshots of the Fragmentation for C70@Single-Walled Carbon Nanotube: Tight-Binding Molecular Dynamics Simulations

In previously reported experimental studies, a yield of double-walled carbon nanotubes (DWCNTs) at C₇₀@Single-walled carbon nanotubes (SWCNTs) is higher than C₆₀@SWCNTs due to the higher sensitivity to photolysis of the former. From the perspective of pyrolysis dynamics, we would like to understand whether C₇₀@SWCNT is more sensitive to thermal decomposition than C₆₀@SWCNT, and the starting point of DWCNT formation, which can be obtained through the decomposition fragmentation of the nanopeapods, which appears in the early stages. We have studied the fragmentation of C₇₀@SWCNT nanopeapods, using molecular dynamics simulations together with the empirical tight-binding total energy calculation method. We got the snapshots of the fragmentation structure of carbon nano-peapods (CNPs) composed of SWCNT and C₇₀ fullerene molecules and the geometric spatial positioning structure of C₇₀ within the SWCNT as a function of dynamics time (for 2 picoseconds) at the temperatures of 4000 K, 5000 K, and 6000 K. In conclusion, the scenario in which C₇₀@SWCNT transforms to a DWCNT would be followed by the fragmentation of C₇₀, after C₇₀, and the SWCNT have been chemically bonding in the early stages. The relative stability of fullerenes in CNPs could be reversed, compared to the ranking of the relative stability of the encapsulated molecules themselves.