Synthesis Of Single-walled Carbon Nanotubes Research Articles

Atomic-Scale Evidence of Catalyst Evolution for the Structure-Controlled Growth of Single-Walled Carbon Nanotubes.

ConspectusKnowing how nanomaterials nucleate and dynamically evolve at the nanoscale is crucial to understanding and in turn controlling the structure and properties of a wide variety of materials, among which single-walled carbon nanotubes (SWCNTs) with chirality-dependent properties is a typical example. Catalyst takes a central role in guiding the SWCNT growth. An in-depth understanding of the growth mechanism of SWCNTs requires knowledge of the catalyst dynamic behavior during the chemical vapor deposition process, where real-time atomic-scale observations are needed. The high spatial, temporal, and energy resolution makes the state-of-the-art aberration-corrected environmental transmission electron microscope (ETEM) a superior tool for tracking the catalyst evolution and the SWCNT growth.Several key factors and processes, including the catalyst stability, carbon diffusion pathway, nucleation site, and growth modes of nanotubes, greatly influence the structure of SWCNTs. This Account summarizes our recent progress in the ETEM investigation of the dynamic catalyst behavior and nucleation of SWCNTs. We first compare the different growth modes of SWCNTs on two types of catalyst-stable solid intermetallic Co7W6 and unstable monometallic catalysts. Then we address the origin of different growth modes and chirality selectivity by revealing the atomic-scale stability and evolution of catalysts under carbon feed conditions and the observation of the in situ growth of SWCNTs on catalysts. We also discuss the catalyst-support interaction and the possible influence on SWCNT growth. In the end, we summarize the present achievements and future challenges.We carefully compare the difference in the ordinary Co catalyst and Co7W6 catalyst which has shown great chirality selectivity in SWCNT growth. Direct imaging by ETEM demonstrated that solid catalysts initiated the growth of SWCNTs with diameters smaller (dNT) than those of the catalyst particles (dNP) (dNT < dNP), whereas molten catalyst nanoparticles produced SWCNTs with similar diameters (dNT ≈ dNP). ETEM combined with in situ synchrotron X-ray absorption spectroscopy demonstrated that the Co7W6 catalyst maintained a solid stable structure under carbon feed conditions at 700-1000 °C, demonstrating the feasibility in acting as a structure template to grow SWCNTs. By contrast, the state and composition of the Co catalyst were changing during SWCNT growth. The near-surface lattice spacings of Co7W6 remained unchanged under carbon feed condition with carbon diffusion on the surface, whereas the solid Co catalyst underwent dynamic expansion and contraction due to carbon penetration into and precipitation out of Co nanoparticles. These two different pathways of carbon diffusion on or in catalysts indicate the distinctly different growth mechanisms of SWCNTs: the epitaxial growth of SWCNTs with specified chirality on the facets of Co7W6 nanocrystals and the nonselective growth of SWCNTs by the Co catalyst with Co/CoC3 as the active species. Besides the SWCNT-catalyst interface, the catalyst-support interface is also of importance in SWCNT growth. The atomic-scale information on catalyst dynamics provides a deep mechanistic understanding of SWCNT growth and will boost the development of the structure-controlled synthesis of SWCNTs and other nanomaterials.

Multi-nucleation of single-walled carbon nanotubes in floating catalyst chemical vapor deposition

Floating-catalyst CVD (FCCVD) is a highly promising technique for the scalable synthesis of single-walled carbon nanotubes (SWCNTs). The number concentration (NC) of catalyst particles is one of the process parameters which mainly influences yield of as-produced SWCNTs. In the conventional FCCVD methods, catalyst particles are usually synthesized by thermal decomposition of organometallic compounds inside the FCCVD reactor. Being in-situ catalyst formation in conventional FCCVD, it is an open question that how many SWCNTs a single catalyst particle can grow? Herein, we employed our novel spark discharged based FCCVD technique which decouples the formation of catalyst particles and SWCNTs synthesis into two subsequent process to investigate effects of catalyst NC on SWCNTs synthesis. The NC of both catalyst particles and SWCNTs in gas-phase were quantitatively compared using differential mobility analyzer (DMA). The DMA measurements corroborated with atomic force microscopy and X-rays photoelectron spectroscopy revealed that under carefully chosen reaction conditions in FCCVD, a single catalyst particle can grow more than one SWCNTs. We propose that after the growth of 1st SWCNT, the catalyst particle detach itself to start nucleating 2nd SWCNT and this process continues as long as the catalyst remains in the growth region of the FCCVD reactor.

Synthesis of high-purity and high-crystallinity single-walled carbon nanotubes homogeneously dispersed by wet process

• The highly crystalline single-walled carbon nanotubes are basic electronic elements. • It is necessary to remove contaminants in hc-SWCNTs to disperse them homogeneously. • Carbon contaminants in hc-SWCNTs disperse the shear force of hc-SWCNT bundles. • The shear force to disperse hc-SWCNTs is required to break up their aggregates. • Highly purified hc-SWCNTs could be dispersed without deteriorated chemical stability. Field emission (FE) electron sources based on high-purity and high-crystallinity single-walled carbon nanotubes (hc-SWCNTs) are promising electronic devices from the viewpoint of industrial application. However, to utilize high-purity hc-SWCNTs as a component of electronic devices, it is necessary to homogeneously disperse hc-SWCNTs using a wet process. Shear force is required to break up aggregates of hc-SWCNTs to produce a film with uniformly dispersed hc-SWCNTs, but contaminants such as fine graphite in the aggregates disperse the shear force of hc-SWCNT bundles. Therefore, it is necessary to remove these contaminants to achieve the uniform dispersion of hc-SWCNTs. In this study, we have succeeded in the highly efficient synthesis of hc-SWCNTs with few contaminants by the arc discharge method using glass-like carbon as a raw material to produce SWCNTs. It was demonstrated that hc-SWCNTs with low foreign matter contamination can be uniformly dispersed without the need for wet dispersion tools such as a homogenizer and a jet mill. Moreover, the long-term stability of the current emitted by FE devices fabricated using the dispersed solution can be obtained.

Machine learning methods for aerosol synthesis of single-walled carbon nanotubes

This work is devoted to the strategy towards the optimal development of multiparametric process of single-walled carbon nanotube (SWCNT) synthesis. Here, we examine the implementation of machine learning techniques and discuss features of the optimal dataset size and density for aerosol chemical vapor deposition method with a complex carbon source. We employ the dataset of 369 points, comprising synthesis parameters (catalyst amount, temperature, feed of carbon sources) and corresponding carbon nanotube characteristics (yield, quality, structure, optoelectrical figure of merit). Assessing the performance of six machine learning methods on the dataset, we demonstrate Artificial Neural Network to be the most suitable approach to predict the outcome of synthesis processes. We show that even a dataset of 250 points with the inhomogeneous distribution of input parameters is enough to reach an acceptable performance of the Artificial Neural Network, wherein the error is most likely to arise from experimental inaccuracy and hidden uncontrolled variables. We believe our work will contribute to the selection of an appropriate regression algorithm for the controlled carbon nanotube synthesis and further development of an autonomous synthesis system for an “on-demand” SWCNT production.

Nucleation of Single-Wall Carbon Nanotubes from Faceted Pt Catalyst Particles Revealed by in Situ Transmission Electron Microscopy.

Revealing the nucleation and growth mechanism of single-wall carbon nanotubes (SWCNTs) from faceted solid catalysts is crucial to the control of their structure and properties. However, due to the small size and complex growth environment, the early stages and dynamic process of SWCNT nucleation have rarely been directly revealed, especially under atmospheric conditions. Here, we report the atomic-resolved nucleation of SWCNTs from the faces of truncated octahedral Pt catalysts under atmospheric pressure using a transmission electron microscope equipped with a gas-cell. It was found that the graphene layers were initially formed preferentially on (111) surfaces, which then joined together to form an annular belt and a hemispherical cap, followed by the elongation of the SWCNT. Based on the observations, an annular belt assembly nucleation model and a possible chirality control mechanism are proposed for SWCNTs grown from well-faceted Pt catalysts, which provides useful guidance for the controlled synthesis of SWCNTs by catalyst design.

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.

Synthesis, Sorting, and Applications of Single-Chirality Single-Walled Carbon Nanotubes.

The synthesis of high-quality chirality-pure single-walled carbon nanotubes (SWCNTs) is vital for their applications. It is of high importance to modernize the synthesis processes to decrease the synthesis temperature and improve the quality and yield of SWCNTs. This review is dedicated to the chirality-selective synthesis, sorting of SWCNTs, and applications of chirality-pure SWCNTs. The review begins with a description of growth mechanisms of carbon nanotubes. Then, we discuss the synthesis methods of semiconducting and metallic conductivity-type and single-chirality SWCNTs, such as the epitaxial growth method of SWCNT (“cloning”) using nanocarbon seeds, the growth method using nanocarbon segments obtained by organic synthesis, and the catalyst-mediated chemical vapor deposition synthesis. Then, we discuss the separation methods of SWCNTs by conductivity type, such as electrophoresis (dielectrophoresis), density gradient ultracentrifugation (DGC), low-speed DGC, ultrahigh DGC, chromatography, two-phase separation, selective solubilization, and selective reaction methods and techniques for single-chirality separation of SWCNTs, including density gradient centrifugation, two-phase separation, and chromatography methods. Finally, the applications of separated SWCNTs, such as field-effect transistors (FETs), sensors, light emitters and photodetectors, transparent electrodes, photovoltaics (solar cells), batteries, bioimaging, and other applications, are presented.

Scalable synthesis of SWCNT via CH4/N2 gas: The effects of purification on photocatalytic properties of CNT/TiO2 nanocomposite

Single-walled carbon nanotubes (SWCNT) were synthesized by a catalytic CVD method over Mo–Fe–MgO and Mo-Fe-Al2O3 catalysts. A systematic investigation was carried out for the purification of SWCNTs by using 20 methods including different acids, acid concentrations, temperatures, and treatment times. The method of consecutive HCl treatment, air oxidation, and second time HCl treatment was successful for the purification of SWCNTs. The purification was effective in the removal of the catalyst support, embedded metal catalysts and non-nanotube carbon materials from the as-synthesized SWCNTs. Effects of purification of SWCNTs on the photocatalytic activity of CNT-based nanocomposites were then studied. The composite SWCNT/TiO2 containing 15 wt% purified SWCNT was synthesized which showed the maximum photocatalytic activity in the degradation of the dye rhodamine B under optimized conditions. The photocatalytic activity of the composite pure-SWCNT/TiO2 was much higher than those of non-purified-SWCNT/TiO2 and neat TiO2. Overall, this work could clarify the scalable synthesis of SWCNTs, an effective purification method for SWCNTs prepared by CCVD and the purity effect on the photocatalytic activity of CNT-based nanocomposites for wastewater treatment.

(Invited) Single Walled Carbon Nanotube (SWNT) Atomic Structure and Thin Film Color during the Floating Catalyst Chemical Vapor Deposition (FC-CVD) Synthesis

We discuss the FC-CVD synthesis of SWNTs, especially the tuning of tube atomic structure i.e. (n,m) distributions and subsequently the thin film color. By introducing various amount of CO2 with CO as the carbon source and in-situ ferrocene decomposition generated Fe catalyst nanoparticles, we directly produced SWNT films with tunable (n,m) i.e. helicity distribution as well as tunable colors. When operating the FC-CVD reactor at the ambient pressure and at 850 oC temperature with 0.25 and 0.37 volume percent of added CO2, the SWNT films display green and brown colors, respectively. We ascribed various colors to suitable diameter and narrow (n,m) distributions, which were determined in detail using the electron diffraction. We calculate the color maps for all SWNT (n,m) structures.N. Wei et al. Adv. Mater. 2021, 33, 2006395.

Constructing ultra-stable photothermal plastics assisted by carbon dots with photocaged reactivity

Constructing ultra-stable photothermal plastics assisted by carbon dots with photocaged reactivity

Subnanometer Single-Walled carbon nanotube growth from Fe-Containing Layered double hydroxides

High-quality single-walled carbon nanotubes (SWNTs) were synthesized on layered double hydroxides (LDHs) containing Fe and FeCo by CO chemical vapor deposition (CVD). Systematic investigations were performed to study the effects of temperature and catalyst composition on the diameter and chirality distributions of SWNTs. It was revealed that both catalysts produced subnanometer SWNTs at low reaction temperatures, correlated with the catalyst performances and reaction parameters. Particularly, SWNTs with enriched (6, 5) species were synthesized at 600 °C from LDHs containing bimetallic FeCo. Besides, the SWNT chirality distribution was further narrowed by a two-phase extraction method and the extracted (6, 5) SWNTs occupy about 57% of all semiconducting species. This work extends the development of robust LDH-based catalysts for chirality-selective synthesis of SWNTs, which paves the way to large scale production of subnanometer SWNTs for potential applications in electronics and optoelectronics.

Multi-step chemical vapor synthesis reactor based on a microplasma for structure-controlled synthesis of single-walled carbon nanotubes

To address the need for the high precision synthesis of nanomaterials with the potential for high productivity, we present a novel design of a multi-step chemical vapor synthesis (CVS) reactor, which centers on the use of an atmospheric, continuous flow microplasma. Based on computational fluid dynamic gas flow simulations, the CVS reactor was designed to provide abrupt interaction steps to provide a well-defined and highly controlled start and stop to the nanomaterial (e.g., nanoparticle) growth. By applying this framework to gas-phase single-walled carbon nanotube (SWCNT) synthesis, the precise start and stop of the catalyst nanoparticle nucleation process can be done using the microplasma reactor (start) and carbon reactant gas to suppress further aggregation/promote SWCNT growth (stop), respectively. In so doing, we demonstrated the generation of controlled size of catalyst nanoparticles and SWCNTs despite high particle densities. We envision that our approach demonstrates the feasibility of fine structural control of nanomaterials through a multi-step CVS process with precisely regulating the aggregation time for high density particle flow, which overcomes the obstacle of both structural control and productivity.

Solid supported ruthenium catalyst for growing single-walled carbon nanotubes with narrow chirality distribution

The past two decades witnessed the thriving of non-conventional nanoparticles in catalytic synthesis of single-walled carbon nanotubes (SWNTs). However, most of the SWNTs are limited to growing on a flat surface which usually suffers low output, thus making the evaluation of SWNT chirality distribution challenging. In this work, we report a solid magnesia (MgO) supported ruthenium (Ru) catalyst for chemical vapor deposition (CVD) growth of SWNTs with sufficient quantity for optical characterizations. Both MgO supported atomically dispersed Ru catalyst and Ru nanoparticles pre-deposited onto porous MgO are active for catalytic synthesis of small diameter SWNTs. Particularly, enriched (6, 5) SWNTs are achieved at temperatures higher than 800 °C by CO CVD, which is partially attributed to the high activity of Ru clusters. Besides, density functional theory (DFT) calculations reveal that the charge transfer from the MgO support to the Ru clusters promotes the carbon precursor absorption and dissociation, accounting for the activation of Ru clusters. This work not only demonstrates the application of atomically dispersed catalyst for CVD growth of SWNTs, but also paves the way for the chirality-selective growth of SWNTs from non-magnetic nanoparticles.

Methods for Estimating Composition of Single Walled Carbon Nanotubes Based on Electronic Type

— Synthesized single-walled carbon nanotubes (SWCNTs) consist of a mixture of chiralities and therefore a post-synthesis separation is essential to separate them based on electronic type i.e., metallic (m-SWCNT) or semiconducting (s-SWCNT) for device applications. A key parameter to measure the effectiveness of separation process is the enrichment composition percentage between m-SWCNT and s-SWCNT, which can be estimated via several methods based on optical characterizations. In this paper, we compare the composition percentage estimations from 3 different methods based on Raman spectroscopy and UV-Vis optical absorption spectroscopy. The estimation methods are radial breathing mode (RBM) peak analysis, optical absorption area under curve (OUA) and first derivative amplitude of the optical absorption curve (FDA). Four different SWCNT sources were used in this study, which were subjected to post-synthesis separation process via agarose gel chromatography. Raman and UV-Vis spectroscopy measurements were carried out on all samples, before and after separation. From the estimations, we observed firstly that there are some variations on the estimated enrichment compositions between different methods, although the values are comparable. Secondly, for some SWCNTs samples, only a certain estimation method showed reliable composition percentage. The results presented in this work may provide viable options for characterizations of SWCNTs as there is no direct method to quantify the absolute composition percentage of SWCNTs based on electronic type. Keywords—single-walledcarbon nanotube, separation, electronic type, optical characterization, purity percentage.

Controlled synthesis of single-walled carbon nanotubes by floating catalyst CVD for transparent conducting films: A critical role of loops

Single-walled carbon nanotubes (SWCNTs) are ideal candidates for transparent conductive films (TCFs) due to their excellent optical transparency and electrical conductivity. The geometry of SWCNTs, including the tube diameter, bundle length and bundle diameter, is vital to high-performance TCFs. Herein, we synthesized SWCNTs by floating catalyst chemical vapor deposition (FCCVD). The SWCNT geometries were tuned by hydrogen (H2), and we found that the tube diameter, bundle length and bundle diameter increase with the H2 concentration. Besides, we observed the formation of SWCNT loop at the tube ends. Both the number and circumference of loops increased with the increment of bundle length. Further, the loops were also found to affect the conductivity of SWCNT thin film. Excessive number of loops with large size could reduce the conductivity of SWCNT thin film. At the optimized H2 concentration, we obtained the SWCNT TCF with sheet resistances of 290 and 95 Ω/sq. for the pristine and AuCl3 doped SWCNT films, respectively, at 90% transmittance. Our work demonstrates the importance of H2 for SWCNT synthesis and the critical role of loops on film conductivity, blazing new ideas for future research to obtain SWCNT TCFs with improved performance.

Selective Growth of Single‐walled Carbon Nanotubes using Cobalt Disilicide

AbstractAfter chirality‐specified growth of single‐walled carbon nanotubes (SWCNTs) was realized by tungsten‐based intermetallic compound catalysts, here we developed a different kind of intermetallic compound, cobalt disilicide (CoSi2), as catalysts to selective grow carbon nanotubes by chemical vapor deposition (CVD). By using methane as carbon source, we obtained (11, 7) tubes with the selectivity of ∼24% and semiconducting SWCNTs (s‐SWCNTs) with the purity of ∼93% simultaneously. Because of the sensitivity of CoSi2 toward the oxygen‐containing environment at high temperatures, ethanol is not a good carbon feedstock. Our results not only expand the chirality library and catalyst database in the chirality‐selective synthesis of SWCNTs, but also further demonstrate the feasibility of intermetallic compound catalysts.

Narrow-chirality distributed single-walled carbon nanotube synthesized from oxide promoted Fe–SiC catalyst

A SiC supported iron (Fe–SiC) catalyst was developed for chemical vapor deposition (CVD) growth of carbon nanotubes. Using CO as the carbon source, sole Fe–SiC catalyst was mostly inactive for synthesizing carbon nanotubes and only trace amount of carbon deposits was produced. In contrast, the powder Fe–SiC catalyst could be activated by pressing them onto a flat SiO2 substrate or by mixing them with porous MgO powders. Particularly, efficient growth of single-walled carbon nanotubes (SWNTs) was achieved on porous MgO promoted Fe–SiC catalyst, and predominant synthesis of (6, 5) SWNTs was realized at a reaction temperature of 650 °C. Systematic characterizations revealed that the reducibility of the Fe–SiC catalyst was enhanced by the physical contact with oxide support, which promotes the formation of small active Fe nanoparticles for the subsequent SWNT nucleation and growth. This work not only deepens our understandings to the catalyst activation mechanisms, but also helps design composite catalyst for synthesizing SWNTs.

Experimental study of thiophene and ferrocene in synthesis of single-walled carbon nanotubes in rich premixed hydrogen/air flames

The effects of varying concentrations of ferrocene and thiophene on the synthesis of carbon nanotubes (CNT) in a floating catalyst, flame-based reaction system were investigated. Pre-vapourised ethanol and a surrounding premixed H2/air flame were used as carbon feedstock and heat source. Samples of the synthesised material were collected and analysed using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffraction to determine their compostitions and properties. Sulphur to iron mass ratios mS/Fe were varied from 0 up to 10.1, by changing the relative amounts of thiophene and ferrocene. The latter was found to marginally improve the yield of CNTs, but rather led to an enhanced yield of iron oxide nanoparticles. In comparison, the addition of sulphur resulted in an improved production of CNTs in terms of number density and lengths. An optimum window of mass ratios mS/Fe for CNT production was identified as 0.1–2. A numerical model for the diffusion of species coupled with an equilibrium chemical model for the resulting species has been constructed to understand the mechanisms of the synthesis process, which demonstrates the range of elemental proportion conditions and residence times favourable to high yield of CNTs.

Joint effect of ethylene and toluene on carbon nanotube growth

This work contributes to the understanding of single-walled carbon nanotube synthesis by an aerosol CVD method using ethylene and toluene as a hybrid carbon source. We evaluated an extensive set of synthesis conditions revealing the role of ferrocene, toluene, and ethylene. We found the fundamental role of ethylene promoting nanotube nucleation and catalyst activation degree at all the concentrations studied, and enhancing nanotube growth at low ethylene content. We observed the interplay effect of toluene and ethylene concentrations on the nanotube growth rate, accompanied by the detrimental effect of toluene on catalyst activation degree. Nevertheless, toluene apparently promotes nanotube crystallinity, increasing the film conductivity while used as an individual carbon source. Adjusting the ethylene and toluene concentrations, we produced carbon nanotube-based transparent and conductive films with an equivalent sheet resistance (at 90% transmittance at 550 nm wavelength) value of 57 Ω/□ at the synthesis yield of 0.24 cm2 L−1, which is at least two times higher than the results reported earlier.

Single-Walled Carbon Nanotube Synthesis Yield Variation in a Horizontal Chemical Vapor Deposition Reactor.

The controlled synthesis of single-walled carbon nanotubes (SWNTs) is essential for their industrial application. This study investigates the synthesis yield of SWNTs, which depends on the positions of the samples on a horizontal chemical vapor deposition (CVD) system. Methane and Fe thin films were used as the feedstock and catalyst for SWNTs synthesis, respectively. A high-resolution scanning electron microscope was used to examine the synthesis yield variation of the SWNTs along the axial distance of the reactor. The morphology and crystallinity of the fabricated SWNTs were evaluated by atomic force microscopy and Raman spectroscopy, respectively. We observed that the highest synthesis yield of the SWNTs was obtained in the rear region of the horizontal reactor, and not the central region. These results can be applied to the synthesis of various low-dimensional nanomaterials, such as semiconducting nanowires and transition metal dichalcogenides, especially when a horizontal CVD chamber is used.