Growth Of Carbon Nanotubes Research Articles

(Invited, Digital Presentation) Uniqueness of Cobalt-Tungsten Intermetallic Compounds in Catalyzing Single-Walled Carbon Nanotube Growth

The intermetallic Co7W6 catalysts have shown great success in chirality-specified growth of single-walled carbon nanotubes (SWCNTs). Electron microscopic techniques, particularly the in-situ techniques enable us to study the growth mechanism and the behavior of catalysts at atomic scale. It was found that the structure of Co7W6 nanocrystals were stable at the temperature of 1100 °C under carbon feeding condition. No carbon dispersion within the nanocrystal happened. These observations illustrate why such kind of catalysts can act as the structural template to grow SWCNTs with specified chiralities. Due to the less efficient carbon diffusion and supply on the surface of catalysts in such a vapor-solid-solid process than that in a vapor-liquid-solid process, SWCNTs normally nucleate on larger catalyst nanocrystals. All these behaviors are distinctly different from those of normal metallic catalysts, demonstrating the uniqueness of intermetallic Co7W6 catalysts in chirality-selective growth of SWCNTs. These results can help us to further understand the mechanism of the origination of chirality selectivity in SWCNT growth, benefiting the rational design of catalysts.

(Invited) Kinetic Selectivity of Chemical Vapor Deposition Growth of Carbon Nanotubes

Single-walled carbon nanotubes have been a candidate for outperforming silicon in ultrascaled transistors, but the realization of nanotube-based integrated circuits requires dense arrays of purely semiconducting species. In order to directly growth such nanotube arrays on wafers, control over kinetics and thermodynamics in tube-catalyst systems plays a key role, and further progress requires the comprehensive understanding of seemingly contradictory reports on the growth kinetics. Here, we propose a universal kinetic model that decomposes the growth rates of nanotubes into the adsorption and removal of carbon atoms on the catalysts, and provide its quantitative verification by ethanol-based isotope labeling experiments. While the removal of carbon from catalysts dominates the growth kinetics under a low supply of precursors, our kinetic model and experiments demonstrate that chiral angle-dependent growth rates emerge when sufficient amounts of carbon and etching agents are co-supplied. The kinetic maps, as a product of generalizing the model, include several kinetic selectivities that emerge depending on the balance of gases with opposing effects. Our findings not only resolve discrepancies existing in literature, but also offer rational strategies to control chirality, length, and density of nanotube arrays for practical applications. Part of this work was supported by JSPS (KAKENHI JP20K15137, JP20H00220) and JST (CREST JPMJCR20B5).

Critical Role of the Acetylene Content and Fe/C Ratio on the Thickness and Density of Vertically Aligned Carbon Nanotubes Grown at Low Temperature by a One-Step Catalytic Chemical Vapor Deposition Process.

The present work explores the role of the carbon source content and the Fe/C ratio on the synthesis of vertically aligned carbon nanotubes (VACNTs) by one-step aerosol-assisted CCVD operated at a medium temperature (615 °C) on aluminum substrates. The main objective was to overcome the limitations of VACNT growth, constituting a drawback for applications requiring thick VACNTs. By using acetylene as carbon feedstock and ferrocene as a catalyst precursor, we demonstrate that when acetylene content is reduced to 1.5 vol%, it is possible to grow VACNT carpets up to 700 µm thick while maintaining constant VACNT growth for a long duration (up to 160 min). The carbon conversion yield is significantly improved when the acetylene content reaches 1.5 vol%. The Al surface roughness also influences VACNT growth. An optimum Fe/C ratio of 0.8 wt.% coupled with a low acetylene content gives the highest growth rate (5.4 µm/min) ever reported for a thermal aerosol-assisted CCVD process operated at such a low temperature. The CNT number density can be controlled by varying the Fe/C ratio, enabling high density growth (e.g., 1.3 × 1011 CNT/cm2).

Interface Interaction Dependent Growth of Carbon Nanostructures: An In Situ Study (Adv. Mater. Interfaces 19/2022)

Controllable Growth of Carbon Nanostructures In article number 2200334, Xiaofang Zhang, Rongming Wang, and co-workers demonstrate controllable growth of graphite shells (GSs) and bamboo carbon nanotubes (CNTs) by adjusting the interface interaction between catalyst and substrate via temperature, i.e., high temperature favores tip-growth of bamboo CNTs with larger catalyst nanoparticles, and low temperature favores base-growth of GSs with smaller nanoparticles, which is revealed via in-situ pyrolysis of Co phthalocyanine in an aberration-corrected ETEM.

Carbon nanomaterials for the treatment of water pollutant in the purpose of environmental remediation

A porous graphite carbon/carbon nanotubes material was fabricated by a one-step process using ferrocene, as a catalyzing agent for carbon nanotubes (CNTs) growth, phenolic resin, avicel and graphite powder. SEM images demonstrated that CNTs were grown in different shapes, diameters and lengths. The ferrocene and avicel contents were drastically changed the surface properties of carbon materials. The as-prepared composites were tested as adsorbents of high interest for wastewater treatment for the removal of humic acid (HA) dissolved in aqueous solutions. Results confirmed that porous carbon materials are considered as promising materials acting both as adsorbent and cathode in the adsorption process for wastewater treatment containing natural organic matter.

Twin physically unclonable functions based on aligned carbon nanotube arrays

Physically unclonable functions (PUFs) are a promising technology for generating cryptographic primitives using random imperfections in a physical entity. However, the keys inside PUFs are still vulnerable as they must be written into non-volatile memories and shared with participants that do not hold the PUF before secure communication. Here we show that pairs of identical PUFs (twin PUFs) can be fabricated together on an aligned carbon nanotube array and used for secure communication without key pre-extraction and storage. Two rows of field-effect transistors are fabricated perpendicular to the carbon nanotube growth direction, randomly producing three types of transistor channel—based on metallic nanotubes, semiconducting nanotubes and no nanotubes—that can be used to extract ternary bits for use as a shared key. The twin PUFs exhibit high uniformity, uniqueness, randomness and reliability, as well as a consistency of approximately 95%. We show that separated twin PUFs can provide secure communication with a bit error rate of one bit per trillion via a fault-tolerant design.

Fast microwave-assisted synthesis of iron–palladium catalysts supported on graphite for the direct synthesis of H2O2

While carbon supported noble metal catalysts have been extensively used in the direct synthesis of H2O2 (DSHP), unsatisfactory H2O2 yield remains a difficult problem to solve. In this work, a facile and efficient preparation of the FePd catalysts supported on graphite was realized by fast microwave-irradiated carbon nanotube (CNT) growth and metal reduction. For this purpose, ferrocene was used as a catalyst for microwave-assisted CNT growth as well as a carbon source, and carbon fibers and hexane were only used as carbon sources for CNT growth. The fast and localized microwave heating enabled the fast pyrolysis of ferrocene and the subsequent decomposition of other carbon sources, which efficiently regulated the Pd nanoparticle (NP) size during the microwave-irradiated reduction of Pd. The carbon sources significantly affected the geometric and electronic characteristics of Pd on carbon surface, and therefore, the catalytic activity for the DSHP reaction was affected. The additional use of hexane decreased the Pd NP size and increased the Pd2+/Pd0 ratio under microwave-assisted reduction. In the presence of FePd/GR-H catalyst, the H2O2 productivity and selectivity reached as high as 1492.3 mmol H2O2/g-Pd.h and 68.8%, respectively, in the absence of any corrosive promoters. The result clearly demonstrates that the effect of microwave irradiation for the catalyst preparation highly depended on carbon sources, and therefore, catalytic activity can be finely controlled by microwave irradiation condition.

Effect of electrochemical anodization and growth time on continuous growth of carbon nanotubes on carbon fiber surface

Carbon nanotubes (CNTs)/carbon fiber (CF) reinforcements were prepared by chemical vapor deposition after electrochemical anodization and catalyst impregnation. The results showed that after the electrochemical anodization, the CFs were oxidatively etched and the surface roughness increased, which is helpful to form a uniform catalyst coating on the surface of CF. Under the current of 0.4 A and 0.6 A, CNTs can grow evenly on the surface of CF. Within a certain range, with the increase of growth time, the density and length of CNTs are improved. The CNTs/CF reinforcement prepared at the current intensity of 0.4 A and the growth time of 8 min has the best comprehensive performances compared with other as-fabricated samples. The tensile strength of the sample can reach a high value of 4.56 GPa, and the wettability of resin has an effective improvement.

Parametric investigation of in-situ synthesis of carbon nanotubes on Al2O3 powder by the rotary chemical vapor deposition method

A new kind of rotary chemical vapor deposition (R-CVD) system was built to prepare in-situ uniformly dispersed carbon nanotubes (CNTs) on alumina (Al2O3) powder (matrix) by using Ni as catalyst and acetylene (C2H2) as carbon source. The critical CNT growth factors, such as synthesis temperature, growth time, the carbon-source gas-flow rate ratio, and the rotation speed of the furnace tube were investigated for process optimization. In addition, the quality and quantity of the synthesized CNTs on the surface of Al2O3 were investigated by scanning electron microscope (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), and thermo-gravimetric analysis (TGA). The results showed that the R-CVD allows the use of higher concentrations of a carbon source gas (C2H2:N2 = 1:5) and lower synthesis temperature (500 °C) compared to the traditional CVD process. The synthesized CNTs also exhibit a higher yield rate and more uniform distribution, which has greater application prospects in the field of preparing CNTs reinforced composite materials.

Hydrogen production from catalytic steam reforming of toluene over trace of Fe and Mn doping Ni/Attapulgite

Tar formation has become the bottleneck of biomass gasification due to it serious impacts the gas quality and device safety. Catalytic steam reforming (CSR) is a potential technology that can utilize tar to produce hydrogen. In this work, a series of NixFeyMn/ATP (x, y = 0, 0.1, 0.5, 1 wt%) catalysts were successfully prepared by a sol-gel assisted impregnation method and applied to CSR of toluene, among which toluene was adopted as the biomass tar model. The characterization results of XRD, N2 adsorption-desorption, HRTEM, H2-TPR, NH3/CO2/O2-TPD and XPS demonstrated that the addition of trace of Fe and Mn promoted the uniform dispersion of active Ni metal and increased the reducibility and enhanced CO2 adsorption ability of Ni-based catalysts. Among them, Ni0.5Fe0.5Mn/ATP catalyst exhibited the highest toluene conversion (99.2 %) and H2 yield (80.1 %) and superior stability. This attributes to it has the highest Ni0/Ni2+ proportion and the cooperative effect of electron transfer among the Ni, Fe and Mn components. Additionally, the spent catalysts are characterized by HRTEM, XRD, TG-DTG and Raman. It confirms that trace of Fe and Mn doping improved the catalytic stability is ascribed to the inhibition of amorphous carbon and the growth of carbon nanotubes does not affect the availability of active sites.

Recyclable magnetic carbon foams possessing voltage-controllable electromagnetic shielding and oil/water separation

Lightweight carbon foams with high-performance electromagnetic interference (EMI) shielding have attracted extensive attentions. However, it is challenging for researchers to effectively regulate the EMI shielding of carbon foams independent on shield's thickness. Herein, we proposed a macroscopic magnetic carbon foams through a chemical engineering strategy under the synergies of nickel ion reduction and in-situ growth of carbon nanotubes on carbon frameworks derived from carbonized woods. The macroscopic magnetic carbon foams present high-performance EMI shielding with absorption-dominant mechanism, and the removal of oil pollutants for water purification. Magnetic properties favor the manipulation of oil absorption without direct mechanic contact. After burning, the magnetic carbon foams can be re-used in EMI shielding and oil absorbency. Furthermore, we used a novel electric voltage-driven strategy to upgrade EMI shielding performances of magnetic carbon foams. By the increase in applied voltages, the enhanced absorption-dominant EMI shielding and fast oil absorbency due to the decrease viscosity of crude oil could be simultaneously obtained. The strategy developed here may lead a new avenue toward the design of porous magnetic carbon foams with the enhanced voltage-driven multifunctionalities for practical applications.

1.1‐Millimeter Tall Vertically Aligned‐Carbon Nanotubes Grown by Cold Wall CVD Using a Novel Multiple‐Purge Method

AbstractThis paper reports a multi‐phase deposition for the growth of carbon nanotubes, intercalating phases of effective growth with purges of the reactor, based on the method recently proposed by the author to grow tall vertically aligned carbon nanotubes (VA‐CNT) in a cold‐wall system at low pressure. The test performed consists on running several growths varying the number of cycles during the deposition step and their duration, avoiding a massive study of number of cycles in function of the time and vice versa. The goal here is to verify if the chamber reconditioning and the purge cycles contribute to increase the height and the uniformity of the VA‐CNT forests. The maximum height achieved is 1.1 mm for 11 cycles, in a total time of 110.5 min, with a non‐uniformity of 6.0% under 6.8 Torr. The analysis of the saturation of the deposition is registered around 127 min.

Catalytic Synthesis of Carbon Nanotubes by Ni/ZSM-5 Catalyst from Waste Plastic Syngas

Carbon nanotubes (CNTs) have been proved to be a high-value by-product of hydrogen production which could be obtained through catalytic reforming from waste plastic syngas. Catalyst plays an important role in the growth of carbon nanotubes. The influences of Ni/ZSM-5 catalyst and temperature were performed in a lab-scale tubular reactor. The catalyst and produced carbon were analyzed by different characterization methods. X-ray diffraction, X-ray energy dispersive spectrometer, scanning electron microscopy and transmission electron microscopy. The results showed that in the presence of catalyst, 600 °C is considered the optimal temperature during the operating temperature range of 400 °C~800 °C for carbon yield and hydrogen production rate, the highest carbon yield of 4.83 g/gcatalyst (among which the MWCNTs were the main products) and hydrogen production rate of 0.0199 L/min were obtained. Higher catalytic temperature led to higher average diameter of carbon nanotubes, which increased from 39.5 nm to 55.3 nm. The highest carbon nanotubes proportion of 98.08% to total carbon deposition was obtained under the conditions of 800 °C. It suggested that Ni/ZSM-5 catalyst has the potential for high quality carbon nanotubes and H2-riched gas production from waste plastic syngas.

Effects of Catalytic Promoter Mn on Ni/ZSM-5 Catalyst to Catalyst Waste Plastic Syngas to Prepare Carbon Nanotubes for Co-Production of Hydrogen

Carbon nanotubes (CNTs) produced by catalytic reforming of the waste plastic syngas is a high value-added by-product of hydrogen production. Catalysts play an important role in the growth of carbon nanotubes. The influences of Ni/ZSM-5 catalyst, Ni–Mn/ZSM-5 catalyst and temperature were performed in a lab-scale tubular reactor. The catalysts and product carbon were analyzed by different characterization methods, including temperature-programed reduction/oxidation, X-ray diffractometer, scanning electron microscope, transmission electron microscope, X-ray energy spectrometer. The results showed that in the presence of Ni–Mn/ZSM-5 catalyst with the addition of catalytic promoter Mn, 650 °C was the optimum temperature during the operating temperature range of 600 °C~800 °C for the rate of carbon production and hydrogen production, and the carbon production was the highest, which was 2.95gCNTs/gCatalyst (wherein the Multi-walled carbon nanotubes (MWCNTs) were main product), and compared with the monometallic Ni/ZSM-5 catalyst, the Ni–Mn/ZSM-5 catalyst with Mn addition increased the H2 content in the syngas from 14 Vol.% to 39 Vol.%. The highest carbon nanotubes proportion of 95.81% to total carbon deposition was obtained under the conditions of 750 °C. The introduction of catalytic promoter Mn transforms the growth mode of carbon nanotubes from top growth mode to bottom growth mode, and obtains carbon nanotubes with more regular structure. It suggested that Ni–Mn/ZSM-5 catalyst had the potential for high quality carbon nanotubes and H2-riched gas production from waste plastic syngas.

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.

Optimization of carbon nanotube growth via response surface methodology for Fischer-Tropsch synthesis over Fe/CNT catalyst

Light olefins such as ethylene, propylene, and butylene are key compounds in the chemical industry. Iron catalyst supported on carbon nanotubes (CNTs) is an appropriate candidate for light olefins production using synthesis gas in Fischer-Tropsch synthesis (FTS). First, catalytic chemical vapor deposition (CCVD) method was applied to synthesize CNTs using Fe/CaCO3 and acetylene as catalyst and hydrocarbon source, respectively. Applying response surface methodology, the optimum operating conditions were determined in CVD reactor for maximal yield and purity of CNTs. The effects of reaction time (30–60 min), reaction temperature (700–800 °C), and loading of the catalyst (10–30 wt% Fe) were investigated. The synthesized CNTs was characterized by Raman spectroscopy for its graphitic structure and purity. After acid-treatment of the synthesized CNTs, Fe/CNTs-synthesized catalyst was successfully fabricated by ultrasonic-assisted wet impregnation method. 20Fe/CNTs-synthesized, 20Fe/CNTs-commercial, and 20Fe/Al2O3 were analyzed in terms of physio-chemical properties and FTS catalytic performance. Comparison of transmission electron microscopy (TEM) and CO-chemisorption results showed that 20Fe/CNTs-synthesized reduces iron oxide agglomeration resulting in metal particles well-dispersed among the studied Fe-based catalysts. The catalytic performance of Fe-based catalysts was investigated using a fixed-bed reactor at 280 °C under 290 psi. 20Fe/CNTs-synthesized exhibited a lower rate of water-gas-shift (WGS) reaction compared with 20Fe/CNTs-commercial, with C2-C4 selectivity of 23.6% which is slightly less than that of its commercial counterpart. Analysis of FTS liquid products revealed that 20Fe/CNTs-synthesized provides liquid hydrocarbons in the range of C8-C18, while 20Fe/CNTs-commercial and 20Fe/Al2O3 obtained C9-C52 and C10-C20, respectively. After 120 h time on stream under steady state condition, higher activity had been maintained by the 20Fe/CNTs-synthesized catalyst compared to the 20Fe/CNTs-Commercial and 20Fe/Al2O3 catalysts.

Unraveling the mechanisms of carbon nanotube growth by chemical vapor deposition

The mechanisms of carbon nanotube (CNT) growth by chemical vapor deposition of acetylene on Fe/SiO2:Al2O3 (zeolite Y) catalyst are unraveled through a combined computational and experimental study. CNTs are synthesized in a horizontal reactor under atmospheric pressure within the temperature range 650 °C–850 °C and characterized by SEM, TEM and Raman spectroscopy. A macroscopic computational fluid dynamics (CFD) model accounting for fluid flow, heat transfer and species transport is developed for the process, incorporating also kinetic expressions for acetylene surface decomposition, acetylene gas-phase reactions, carbon diffusion through the bulk of the catalyst, carbon surface accumulation on the catalyst surface and eventually, CNTs growth. The experimental behavior of the CNTs growth can be accurately described by the proposed macroscopic model. Most importantly, theoretical predictions suggest that there are two distinct temperature regimes for CNTs formation: at the low temperature regime, the process is dominated by the competition between carbon diffusion through the catalyst and carbon impurity layer formation on the catalyst surface, while at higher temperature, the gas-phase reactions of acetylene prevails, releasing byproducts that deposit on the catalyst surface in the form of carbon impurities. Finally, using the developed computational approach, the catalyst lifetime, which is directly affected by these mechanisms, is correlated with the CNTs growth process.

Enhancement of catalytic activity by addition of chlorine in chemical vapor deposition growth of carbon nanotube forests

In carbon nanotube (CNT) growth, the role of catalytic nanoparticles involves complex processes of feedstock gas decomposition, carbon feedstock transport, and CNT crystal precipitation. We report on the improvement of the catalytic activity by chlorine addition during the CNT forest growth process. The addition of Cl2 enhanced the diffusion rate of carbon in the catalysts, and thus changed the rate-limiting step from iron diffusion in the catalyst to the feedstock gas decomposition on the catalyst surface, leading to a higher growth rate and longer catalyst lifetime. The addition of Cl2 directly affected numerous CNT structural parameters, including the CNT diameter, number of walls, and crystal quality. Furthermore, the Cl2 addition made the CNT forest super-aligned, and it enhanced the dry spin capability of the CNT forests, allowing CNT webs to be continuously drawn from the forests.

Synergistic Effect of Field Emission Properties on Growth of CNTs by One-Pot Preparation of Various Concentrations Composite Catalyst

Multi-walled carbon nanotubes (MWCNTs) were grown on Silicon (Si) substrate by using low-pressure chemical vapor deposition (LPCVD) technique where iron (Fe) and silver (Ag) colloidal solution act as a catalyst were prepared by chemical route. The as-prepared solutions in varied concentrations were deposited on a Si substrate using a spin coating process at 700[Formula: see text]rpm. The purpose of Fe in composite catalysts is to have high carbon solubility and diffusion rate, and Ag utilization can change catalyst activity temperature and enhance carbon yields. In this work, we demonstrate the growth of carbon nanotubes (CNTs) on different catalyst concentrations ([Formula: see text]Fe/[Formula: see text]Ag), ([Formula: see text]Fe/[Formula: see text]Ag), ([Formula: see text]Fe/[Formula: see text]Ag) and ([Formula: see text]Fe/[Formula: see text]Ag) catalytic films. Field Emission Scanning Electron Microscopy (FESEM) was used to image the morphologies of various catalyst concentrations MWCNTs. The natures of the synthesized CNTs were determined using Raman spectroscopy and it was revealed that as-prepared CNTs are MWCNTs due to the absence of radial breathing mode (RBM). Raman spectroscopy demonstrated the lower ID/IG ratio (Ratio of the intensity of D-Raman peak and G-Raman peak) of ([Formula: see text]Fe/[Formula: see text]Ag) MWCNTs. The ID/IG ratio for ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs was 0.76, while the ID/IG ratio for ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs was 0.89. The ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs were found to be more defective as compared to other. Electron Emission from ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs were much stronger than from other samples, as demonstrated by Field Emission measurement using diode configuration. The turn-on field for ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs was (0.92 V/[Formula: see text]m) which is slightly lower than that of ([Formula: see text]Fe/[Formula: see text]Ag) catalyzed MWCNTs (1.83 V/[Formula: see text]m), indicating superior enhancement.

On the interplay between a novel iron and iron-carbide atomic layer deposition process, the carbon nanotube growth, and the metal–carbon nanotube coating properties on silica substrates

The fabrication of iron and iron carbide nanoparticles (NPs) for catalytic reactions such as the growth of carbon nanotubes (CNTs) compete with the challenge of covering a wide range of substrates with perfect control of the NP reactivity. We present in this work a novel atomic layer deposition (ALD) process to grow Fe/Fe3C thin films over silica flat substrates. The depositions were carried out exposing the surface through various number of ALD cycles, resulting in Fe-based films with thicknesses ranging from 4 nm to almost 40 nm. After a thermal treatment, the film dewetts into nanoparticles, where the efficiency to grow CNTs will depend on the average size distribution of the nanocatalyst. X-ray diffraction and x-ray photoelectron spectroscopy were used to track the elemental, phase, and shape (film to particles) transformation in order to identify the key features of the nanocatalyst, thereby controlling the CNT nucleation and growth. Thin film thickness of around 5 nm promotes the growth of a dense CNT forest. Furthermore, the metal–CNT films reveal optical properties that are totally tailored by the initial number of ALD cycles.