Floating Catalyst Chemical Vapor Deposition Research Articles

Kinetic Modulation of Carbon Nanotube Growth in Direct Spinning for High-Strength Carbon Nanotube Fibers.

With impressive individual properties, carbon nanotubes (CNTs) show great potential in constructing high-performance fibers. However, the tensile strength of as-prepared carbon nanotube fibers (CNTFs) by floating catalyst chemical vapor deposition (FCCVD) is plagued by the weak intertube interaction between the essential CNTs. Here, we developed a chlorine (Cl)/water (H2O)-assisted length furtherance FCCVD (CALF-FCCVD) method to modulate the intertube interaction of CNTs and enhance the mechanical strength of macroscopic fibers. The CNTs acquired by the CALF-FCCVD method show an improvement of 731% in length compared to that by the conventional iron-based FCCVD system. Moreover, CNTFs prepared by CALF-FCCVD spinning exhibit a high tensile strength of 5.27 ± 0.27 GPa (4.62 ± 0.24 N/tex) and reach up to 5.61 GPa (4.92 N/tex), which outperforms most previously reported results. Experimental measurements and density functional theory calculations show that Cl and H2O play a crucial role in the furtherance of CNT growth. Cl released from the decomposition of methylene dichloride greatly accelerates the growth of the CNTs; H2O can remove amorphous carbon on the floating catalysts to extend their lifetime, which further modulates the growth kinetics and improves the purity of the as-prepared fibers. Our design of the CALF-FCCVD platform offers a powerful way to tune CNT growth kinetics in direct spinning toward high-strength CNTFs.

Study of field distribution characteristics in CVD reactors and enhanced growth of SWNCT

The low efficiency of growing single-walled carbon nanotubes (SWCNT) poses a barrier to their application in high-performance electronic devices. However, it is difficult to control the uniform growth of SWCNT in a floating catalytic reactor due to the complex parameter control. Therefore, it is essential to enhance the growth of SWCNT in the floating catalyst chemical vapor deposition (FCCVD) process. In the present work, the influence of the reactive flow field on the growth of SWCNT, which is often neglected, is revealed. To address this issue, this work combines experiments and simulations to obtain the characteristics of field distribution within the reactor and the trend of the products. The results of the flow field analysis indicate that thermal buoyancy is the cause of SWCNT growth limitation in FCCVD. By weakening the thermal buoyancy, a homogeneous reaction field is obtained; vortices in the flow field are reduced or even disappear; the temperature field is more homogeneous, and, importantly, the crystallinity of SWCNT is enhanced (IG/ID up to 20-fold). In addition, the decomposition process of the carbon source is also enhanced, thus suppressing the generation of by-products. Based on the results of the small tube experiments, both the increase in temperature and the decrease in residence time increased IG/ID. Furthermore, the distributions of the maximum and minimum diameters in SWCNT imply variations in the growth modes of SWCNT at different temperatures.

Mapping carbon nanotube aspect ratio, concentration and spinning in FCCVD synthesis controlled by sulphur

Floating catalyst chemical vapor deposition (FCCVD) enables ultrafast synthesis of CNTs and other 1D nanoparticles and their direct assembly as macroscopic solids. The chalcogen growth promotor in FCCVD produces high aspect ratio CNTs that can aggregate in the gas phase and form an aerogel which can be continuously spun as macroscopic fibres or sheets. We study the role of sulphur in controlling CNT morphology and aggregation by synthesising CNTs under a wide range of S/C ratios (0.001 to 5 wt.%) and determining their diameter and length distributions, number concentration and form of aggregation. Increasing S/C ratio in this range increases mean number of CNT walls from 1 to 8, decreases mean length from 34 to 6 µm, but CNT number concentration remains approximately constant at 8 × 108#/cm3. Assuming growth within the first 3 cm of the reactor, longitudinal growth rate spans 1.5- 6.5 µm/s for the different CNT morphologies, but with similar mass throughput of 700 attogram/catalyst. This indicates the amount of carbon reaching the catalyst and solidifying as CNT remains constant regardless of the sulphur available in the catalyst, suggesting the rate limiting process is not at the catalyst/promoter interface but instead in the transport of carbonaceous active precursors to the catalyst, either due to their diffusion in the gas phase or decomposition kinetics. The CNTs produced range from polymer-like, which readily bundle and form aerogels, to rod-like that do not. We include aerogelation “phase diagrams” for different CNT concentrations, aspect ratios and CNT bending stiffness.

Insights into the Nucleation and Structure of Lignin-Based Carbon Nanotubes Synthesized Using Iron via Floating Catalyst Chemical Vapor Deposition

Insights into the Nucleation and Structure of Lignin-Based Carbon Nanotubes Synthesized Using Iron via Floating Catalyst Chemical Vapor Deposition

One-step aerosol synthesis of iron nanoparticles coated single-walled carbon nanotubes (Fe@SWCNT) for multifunctional composite textiles with electromagnetic interference shielding properties

Single-walled carbon nanotubes (SWCNTs) based nanocomposites demonstrate remarkable electromagnetic interference (EMI) shielding potential. However, the efficient and rapid fabrication of SWCNT-based nanocomposite remains a technical challenge. This study successfully one-step synthesizes iron (Fe) nanoparticle-enriched SWCNTs (Fe@SWCNT) using the floating catalyst chemical vapor deposition (FCCVD) method in the aerosol phase. The aerosol-synthesized Fe@SWCNTs were directly integrated with carbon cloth (CC) and polydimethylsiloxane (PDMS) to develop a compelling nanocomposite film for multifunctional EMI shielding. The composite film exhibits exceptional shielding effectiveness of 50.2 dB in the X-band (8.2–12.4 GHz) with absolute shielding effectiveness (SSEt) of 3279.3 dB cm²/g. Notably, the film also displays rapid and uniform electrical heating at low voltages, reaching a temperature of 110.4 °C under a 1.0 V. Overall, this study reveals the potential applications of multifunctional textiles in fields such as EMI shielding, hydrophobicity, and thermal management.

Molecular dynamics simulation of carbon nanotube growth under a tensile strain

We performed molecular dynamics simulations of carbon nanotube (CNT) to elucidate the growth process in the floating catalyst chemical vapor deposition method (FCCVD). FCCVD has two features: a nanometer-sized cementite (Fe3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document}C) particle whose melting point is depressed because of the larger surface-to-volume ratio and tensile strain between the growing CNT and the catalyst. The simulations, including these effects, demonstrated that the number of 6-membered rings of the (6,4) chiral CNT constantly increased at a speed of 1mm/s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${1}\\,{\ extrm{mm}/\ extrm{s}}$$\\end{document} at 1273K\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${1273}\\,{\ extrm{K}}$$\\end{document}, whereas those of the armchair and zigzag CNTs were stopped in the simulations and only reached half of the numbers for chiral CNT. Both the temperature and CNT chirality significantly affected CNT growth under tensile strain.

CNT Sheets Co‐Loaded with Sulfur and Silicon Oxides: Free Standing Anodes for Lithium and Sodium‐Ion Batteries

AbstractSulfur and silicon oxides loaded self‐supporting binder and collector free CNT sheets have been synthesized using floating catalyst chemical vapor deposition technique and investigated for their application as anodes in lithium/sodium ion battery. The CNT sheets have been characterized thoroughly using various microscopic and spectroscopic techniques. The addition of sulfur improves the interplanar spacing in the lattice and also generated abundant defects. The silicon oxides help in enhancing the specific capacity of the CNT sheet via conversion reactions. These features lead to enhanced electrochemical properties and the best performance has been demonstrated by the co‐loaded CNT/S/silicon oxide electrode. As anode in LIB, it delivers 830 mAh g−1 at 1 A g−1 current density after 250 cycles. More importantly, the kinetic analysis confirms that sulfur/silicon oxide co‐loading can improve the Li+ diffusion coefficient in CNT anode and enhance the metal ion storage. The structural modifications also enhance Na+ ion storage and the CNT/S/Silicon oxide anode delivers 368 mAh g−1 after 250 cycles at a current rate of 1 A g−1 with a superior initial coulombic efficiency of 77.5 %.

A new method for evaluation of nanotube growth kinetics in aerosol CVD

Aerosol chemical vapor deposition (CVD) – a particular case of floating catalyst CVD method with an extreme dilution of catalyst – has been demonstrated to be one of the most advanced one-stage and continuous techniques for the production of single-walled carbon nanotubes (SWCNTs) for various applications in electronics, optics, energy storage, etc. Nevertheless, rational scaling of the aerosol CVD reactor is often limited by a fundamental problem of tracking the growth kinetics, in particular, the nanotube growth rate and the catalyst deactivation rate. We propose a robust method for the assessment of nanotube growth kinetics (growth rate and catalyst lifetime) based on the analysis of the SWCNT film equivalent sheet resistance (R90) versus residence time (τ) dependencies. We have demonstrated identical trends of R90-vs-τ and length-vs-τ curves at different concentrations of CO2 used as a promoter in the Boudouard reaction- (CO-) based synthesis. As a result, we have revealed the mechanisms behind the CO2 effect on the catalyst deactivation rate not reported before. In particular, we observed an increase in CO2 concentration to provide a faster growth rate but an earlier catalyst deactivation. We believe this work might be useful for researchers from both academia and industry and facilitate more active development of aerosol CVD methods for the production of new materials.

Gamma-ray irradiation to achieve high tensile performance of unidirectional CNT yarn laminates

Continuous carbon nanotube (CNT) yarn fabricated from a floating catalyst chemical vapor deposition (FCCVD) method is treated under gamma-ray irradiation to enhance the mechanical properties of the CNT yarn and its unidirectional composite laminates. Gammy-ray doses varying from 50 kGy to 1200 kGy are used to irradiate CNT yarns and their microstructures, tensile properties and surface characterizations are studied. The graphitic structure change is not clear from the transmission electron microscopy, however, the specific tensile strength and modulus of yarn vary slightly within 10 % as the dose increased. This modulus trend coincides with mesoscopic distinct element modeling (mDEM) simulation results. Surface characterization shows additional oxygen functional groups and smaller contact angles after irradiation. Interestingly, the specific tensile properties of composite laminates also increase relative to the yarns, and the unidirectional laminate from CNT yarn treated with the optimal dose of 700 kGy achieves specific strength and modulus as high as 1.89 GPa/gcm−3 and 258 GPa/gcm−3, respectively, which are 30.9 % and 37 % increases compared to the control laminate. The results indicate that radiation-induced crosslinking among the CNTs and the formation of surface-active sites leads to enhanced load transfer in the yarns and promote CNT/resin interfacial bonding.

Nitrogen-Doped Single-Walled Carbon Nanotubes by Floating-Catalyst CVD Process

Due to the properties of single-walled carbon nanotube (SWCNT), thin film SWCNTs have been used for many versatile applications. Direct synthesis of the nanotube films with desired electronic structures without prerequisite of post treatment process then becomes one important step. Here, we present the synthesis of nitrogen-doped SWCNTs by floating-catalyst chemical vapor deposition process. With low nitrogen level incorporated into the sp2 carbon network, substitutional and pyridinic nitrogens become predominance. The electronic structure of N-doped SWCNT film could be changed into n-type doping. The localized structures of carbon and nitrogen bonding environments were also revealed by x-ray absorption spectroscopy. The formation of p-n junction was also obtained from the I-V characteristic of N-doped SWCNT heterojunction diode revealing n-type behavior of the sample.

Enhanced water retention in carbon nanotube sheets-sandwiched gas diffusion layer in polymer electrolyte membrane fuel cells operated under low humidity conditions

This study introduces the polymer electrolyte membrane fuel cell (PEMFC) using sandwiched-carbon nanotube (CNT) sheets into the gas diffusion layer (GDL). The CNT sheegts are synthesized via floating catalyst chemical vapor deposition and formed into sheets using the direct spinning technique. The sandwiched-CNT sheets PEMFC indicates high electrochemical performance across a range of relative humidity conditions, remarkably in non-humidified conditions of the cathode. This notable achievement can be attributed to the distinctive physical properties of the CNT sheets. In contrast to the microporous layers, the CNT sheets possess a nanoporous structure that enhances reactant diffusion, and appropriate hydrophobicity enhances the water retention capacity in GDL. These properties are confirmed by polarization curves, electrochemical impedance spectroscopy (EIS), quantitative capillary pressure, and water removal rate test in constant current mode at 0.4 A/cm2. In the non-humidified cathode condition, the peak power density of the sandwiched-CNT sheets PEMFC is 45% higher than conventional PEMFC. Moreover, it demonstrates significantly reduced ohmic resistance in EIS. The water retention capability of the PEMFC is assessed through quantitative capillary pressure and water removal rate test. Based on these assessments, we validate that the sandwiched-CNT sheets PEMFC exhibits high performance even under non-humidified cathode conditions.

Direct synthesis of nitrogen-doped narrow-diameter carbon nanotubes through floating-catalyst chemical vapor deposition with high hydrogen flow rate

The precise control of substitutional nitrogen (N) doping into a carbon nanotube (CNT) lattice is key to tuning their unique one-dimensional electronic properties. Here we report a direct synthesis of high-quality N-doped single-wall CNTs (N-SWCNTs) with ∼1 nm in diameter using a floating-catalyst chemical vapor deposition under a high flow rate of hydrogen as a carrier gas. The high hydrogen flow rate enhances the total N content in the CNT lattice. The N-SWCNTs exhibit an n-type doping behavior induced by enriched graphitic-N as confirmed by Raman analysis. Our finding will be beneficial to tailoring the doping state of N-SWCNTs.

(Invited) Flexible Resistive Random-Access Memories Based on Carbon Nanotubes and Their Application for Reservoir Computing

Reservoir computing (RC) is one of the machine learning methods derived from a recurrent neural network (RNN) and has attracted attention because of the small amount of computation required for learning. In recent years, various forms of physical RC using nonlinear physical systems such as memristors, optical amplifiers, and soft octopus robots have been proposed for low power consumption and high-speed processing. [1] The physical reservoir system needs non-linearity, high dimensionality, and short-term memory. It has also been shown that transient response currents due to electrochemical reactions and ion migration in solution have the ability of reservoirs [2]. In this study, we report carbon nanotube (CNT)-based resistive random-access memories (ReRAMs) and their application for reservoir computing.The network-like CNT film is used as electrodes sandwiching a thin (10 nm) HfO2 oxide film to form CNT/HfO2/CNT ReRAMs. The CNT film was synthesized using the floating-catalyst chemical vapor deposition method. Then, the CNT film was transferred onto a plastic substrate. HfO2 was deposited by the atomic layer deposition method onto CNT film. Finally, another layer of CNT film was transferred to form CNT/HfO2/CNT stack. The ReRAM exhibited clear memory behavior with SET and REST voltages around +6 V and –6 V, respectively. The ReRAM can sustain the current level of each state for more than 1 × 104 s and can be predicted to store them for ten years. The ReRAMs also exhibited stable operation under bending stress with a 0.5 cm bending radius.We used the ReRAMs as a reservoir for reservoir computing. A time-series prediction task called NARMA10, which was often used as a benchmark of reservoir computing, was performed with a normalized mean square error (NMSE) of 0.38. The chaotic motion of a double pendulum was also successfully demonstrated by using the CNT ReRAM-based reservoir.[1] G. Tanaka et al., Neural Netw. 115, 100 (2019), [2] S. Kan et al., Adv. Sci. 9, 2104076 (2022)

Study on the law and mechanism of anisotropic conductivity of carbon nanotubes film prepared by floating catalytic chemical vapor deposition method

Carbon nanotubes (CNTs) film has attracted extensive attention in the field of electronics, sensors, and other potential applications due to their excellent electrical conductivity. The conductivity of CNTs film is one of the most important aspects of engineering applications. This study investigates the conductivity of CNTs film with varying areal density, prepared using the floating catalyst chemical vapor deposition (FCCVD) method and densified by rolling. The square resistance of pre- and post-rolling films was measured to characterize the electrical properties. Experimental results indicate that square resistance decreases with increasing areal density and stabilizes eventually. A mathematical formula was derived to explain the relationship between areal density and square resistance, incorporating volume and areal density formulas. Experimental data curves of pre- and post-rolling films were fitted, yielding mathematical relations consistent with the derived formulas. The electrical conductivity of post-rolling CNTs film was superior to pre-rolling in experiment and calculation. The charge carrier transport mechanism in CNTs film was studied by analyzing its internal structure and electrical properties. Surface conductivity was over 1000 times higher than volume conductivity, attributed to the distribution of CNTs bundles in collection and thickness directions. Charge carrier transport capacity decreased with increasing layers in thickness direction due to contact resistance and large resistance at tube–tube junctions. Layers of CNTs near the current application surface significantly contribute to charge carrier transport at high areal density levels.

Reduction of Metal/Carbon Nano-Tubes Interface Contact Resistance by Floating Catalyst Growing Method and Semimetals

In this work, we investigate the contact resistance between carbon nanotubes (CNTs) and different metal electrodes (In, Cr, Cu, Ni, W, and Bi) grown by two methods: floating catalyst chemical vapor deposition (FCCVD) and thin Fe metal chemical vapor deposition (CVD). The results demonstrate that CNTs grown by FCCVD have lower contact resistance with metal electrodes compared to those grown by thin Fe metal CVD. The study attributes this to the avoidance of metal-silicide formation between the silicon (Si) substrate and the metal catalyst film during the high-temperature growing process in FCCVD. Moreover, the research shows that the FCCVD method results in a higher CNT density grown in the through Si vias (TSVs) and lower contact resistance between metal electrodes and CNTs, moving toward the 3-D integrated circuits (3-DICs). Further, the use of metals with good wettability such as bismuth (Bi) as the metal electrode reduces contact resistance.

Tensile properties of flexible carbon nanotube film/PVA composite at various strain rates

AbstractThe quasi‐static tensile properties of carbon nanotube (CNT) composites have already been extensively studied, while their impact tensile properties under high rates have rarely been investigated. In this work, CNT film/PVA composites were prepared by impregnation and hot press curing, whereas CNT films were prepared by floating catalyst chemical vapor deposition (FCCVD). Instron 3365 and the miniaturized split Hopkinson tensile bar (SHTB) were used for tensile tests at various strain rates. The results show that the tensile properties of PVA‐impregnated CNT film are significantly improved, and the strain rate effect is remarkable. The tensile properties are best when the mass fraction of PVA is 1.5%. At a low strain rate of 0.001 s−1, the maximum stress and energy absorption (EA) of CNT composites are 502.5 MPa and 63.6 MJ/m3, respectively. And at the high strain rate of 3100 s−1, the maximum stress and EA of CNT composites are 969.0 MPa and 166.4 MJ/m3, respectively. Scanning electron microscopy (SEM) was used to observe the failure morphologies at tensile fracture of composites under different strain rates. At low strain rate, the failure mode is the pulling fracture of tube bundles and the crushing of polymers, while at high strain rate is the folding and spring back of tube bundles, as well as the debonding of polymers.

Floating Fe Catalyst Formation and Effects of Hydrogen Environment in the Growth of Carbon Nanotubes

Hydrocarbon conversion to advanced carbon nanomaterials with concurrent hydrogen production holds promise for clean energy technologies. This has been largely enabled by the floating catalyst chemical vapor deposition (FCCVD) growth of carbon nanotubes (CNTs), where commonly catalytic iron nanoparticles are formed from ferrocene decomposition. However, the catalyst formation mechanism and the effect of the chemical environment, especially hydrogen, remain elusive. Here, by employing atomistic simulations, we demonstrate how (i) hydrogen accelerates the ferrocene decomposition and (ii) prevents catalyst encapsulation. A subsequent catalytic dehydrogenation of methane on a liquid Fe nanoparticle showed that carbon dimers tend to be the dominant on-surface species. Such atomistic insights help us better understand the catalyst formation and CNT nucleation in the early stages of the FCCVD growth process and optimize it for potential scaleup.

Floating Catalyst Chemical Vapor Deposition Patterning Nitrogen‐Doped Single‐Walled Carbon Nanotubes for Shape Tailorable and Flexible Micro‐Supercapacitors

AbstractMicro‐supercapacitors (MSCs) as high‐power density energy storage units are designed to meet the booming development of flexible electronics, requiring simple and fast fabrication technology. Herein, a fast and direct solvent‐free patterning method is reported to fabricate shape‐tailorable and flexible MSCs by floating catalyst chemical vapor deposition (FCCVD). The nitrogen‐doped single‐walled carbon nanotubes (N‐SWCNTs) are directly deposited on a patterned filter by FCCVD with designable patterns and facilely dry‐transferred on versatile substrates. The obtained MSCs deliver an excellent areal capacitance of 3.6 mF cm−2 and volumetric capacitance of 98.6 F cm−3 at a scan rate of 5 mV s−1 along with excellent long‐term cycle stability over 125 000 circles. Furthermore, the MSCs show good performance uniformity, which can be readily integrated via connection in parallel or series to deliver a stable high voltage (4 V with five serially connected devices) and large capacitance (5.1 mF with five parallel devices) at a scan rate of 100 mV s−1, enabling powering the light emitting displays. Therefore, this method blazes the trail of directly preparing flexible, shape‐customizable, and high‐performance MSCs.

Highly Conductive Graphenated-Carbon Nanotubes Sheet with Graphene Foliates for Counter Electrode Application in Dye-Sensitized Solar Cells

This work enlightened the synthesis of graphenated-carbon nanotubes sheet (g-CNT) using the floating-catalyst chemical vapor deposition method (FCCVD) for dye-sensitized solar cell (DSSC) application. The carbon injection flow rate in the experiment was varied to 6, 8, and 10 ml/h. The morphological findings revealed that the g-CNT formed a highly conductive network. Excellent conductivity was obtained for the sample g-CNT8 (34.5 S/cm) compared to the sample g-CNT6 (11.2S/cm) and CNT10 (4.76 S/cm). This excellent feature is due to the hybrid structure of the g-CNT8, which creates efficient electron transfer in the materials resulting in higher conductivity. The hybrid structure provides a high surface area that improves conductivity. Therefore, the g-CNT sheet is an excellent candidate to replace the conventional platinum used as a counter electrode (CE) in DSSC.

The improvement of yield and thermal stability of single-walled carbon nanotubes fabricated by Gd-assisted FCCVD

In this work, single-walled carbon nanotubes (SWNTs) were synthesized by a scalable floating catalyst chemical vapor deposition (FCCVD) method, in which Gd was used as growth promotor to improve the yield and quality of SWNTs. Based on the characterization results of electron microscope, thermogravimetric analysis and Raman spectroscopy, the application of Gd leads to a great increase in the production of large-diameter SWNTs with a narrow diameter distribution and an enhanced thermal stability. The newly developed FCCVD technique makes it possible to produce large-diameter SWNTs at a low cost, and the generated structurally homogenous SWNTs are suitable for their applications in the fields of electronic systems and electromagnetic shielding, etc.