Catalytic Growth Of Carbon Nanotubes Research Articles

In-situ construction of CNTs/laterite composites for enhanced microwave absorption

Carbon nanotubes have the significant disadvantage of insufficient magnetic loss as a microwave absorbing material, while transition metal catalysts for the preparation of carbon nanotubes still limit the cost reduction. In this study, a facile in-situ strategy was developed to synthesize CNTs/laterite composites, smartly employing the catalysts performance of laterite to realize the synthesis of CNTs and simultaneously construct three-dimensional (3D) “caterpillar” structure. As expected, the exposed CNTs with high crystallinity in caterpillar-like CNTs/laterite composites are believed to provide a good conductive 3D network and abundant paths for trapping of incident electromagnetic waves, as well as the iron nanoparticles produced by hydrogen reduction during the in-situ catalytic growth of carbon nanotubes on laterite significantly increase the magnetism of the materials and thus increase the magnetic loss. As a result, the RL curves of the CNTs/laterite-paraffin composites show a maximum reflection loss of −77 dB at 3.2 GHz with 4 mm thickness. Generally, this synthesis strategy presented in this study paves a new way for the fabrication of low-cost CNTs-based composite with high performance for microwave absorbing.

Toward an Intelligent Synthesis: Monitoring and Intervening in the Catalytic Growth of Carbon Nanotubes.

The bottom-up approach to directly synthesizing low-dimensional materials with outstanding performance has extended the material basis for the next generation integrated circuit industry. All the low-dimensional semiconductors, metals, dielectrics, and their heterojunctions are very promising bricks to build faster and more efficient chips because of their atomically smooth surface and interfaces. The greatest challenge in the synthesis of nanomaterials is how to precisely control the structure, crystalline orientation, defects, dimensions, etc. In past decades, both the methodology and the mechanism of synthesis have been systematically investigated to improve the controllability. However, few studies focused on sensing the synthesis processes in situ and responding to the synthesis immediately. Here, we propose the concept of intelligent synthesis in which the final product can be automatically fine-controlled by a closed loop including in situ monitoring and real-time interventions. As a model system, a high-temperature-tolerant circuit is fabricated on the single-walled carbon nanotube (SWCNT) growth substrate for sensing and responding to the synthesis processes. As a result, either highly pure semiconducting (s-) SWCNT arrays or metallic-semiconducting (m-s) junction arrays with different junction positions is simply synthesized by programming the responding signal. The intelligent synthesis shows much higher efficiency and controllability compared to conventional methods and will lead to the next leap in nanotechnology.

Effect of Different Catalyst Supports on the Quality, Yield and Morphology of Carbon Nanotubes Produced from Waste Polypropylene Plastics

The role of the effect of the support on the reactivity of heterogeneous catalysts cannot be over-emphasized. Therefore, the study documented in this article investigated the effect of different metal oxide supports (MgO, CaO and TiO2) and mixed oxide supports (CaTiO3) on the performance of a bimetallic NiMo catalyst prepared via the sol–gel method during the catalytic growth of carbon nanotubes (CNTs) from waste polypropylene (PP). Waste PP was pyrolyzed at 700 °C in a single-stage chemical vapor deposition reactor and off-gas was utilized in-situ as a cheap carbon feedstock for the growth of CNTs under similar conditions for all the prepared NiMo catalysts (supported and unsupported). The structures of the prepared catalysts and deposited carbon were extensively characterized using X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), etc. The catalytic performance of NiMo supported and unsupported catalysts was evaluated in terms of the yield, purity, and morphology of synthesized CNTs. The results revealed that the stabilizing role of supports is fundamental in preventing nanoparticle agglomeration and aggregation, thereby resulting in improved yield and quality of CNTs. Supported NiMo catalysts produced better aligned graphitic and high-quality CNTs. The NiMo/CaTiO3 catalyst produced the highest carbon of 40.0%, while unsupported NiMo produced low-quality CNTs with the lowest carbon yield of 18.4%. Therefore, the type of catalyst support and overall stability of catalytic materials play significant roles in the yield and quality of CNTs produced from waste PP.

SiO2-promoted growth of single-walled carbon nanotubes on an alumina supported catalyst

Developing supported catalyst and elucidating the catalyst activation mechanism are of significant importance for large scale synthesis of single-walled carbon nanotubes (SWNTs). In the work, we designed an alumina-supported iron (Fe–Al2O3) catalyst for catalytic growth of carbon nanotubes by CO chemical vapor deposition (CVD). In the temperature range of 800–1000 °C, the prepared powder catalyst only affords the growth of carbon fibers or multi-walled carbon nanotubes. In contrast, when placing the powder catalyst on flat SiO2 substrates, such as SiO2/Si and quartz, SWNTs were observed to grow at the catalyst-substrate interface, highlighting the contributions of the SiO2 substrate in SWNT synthesis. Systematic characterizations on both the catalysts and carbon nanotubes revealed that the SiO2 substrate promotes the reduction of iron oxide in the adjacent Fe–Al2O3 catalyst, facilitating the generation of small Fe particles at the catalyst-substrate interface, which act as the active phase for the subsequent growth of SWNTs. This work opens a new avenue for growing SWNTs from supported catalyst and sheds light on enhancing SWNT growth efficiency by tuning catalyst reduction behaviors.

Oxygen evolution activity limits the nucleation and catalytic growth of carbon nanotubes from carbon dioxide electrolysis via molten carbonates

Controlled catalytic synthesis of technologically valuable carbon nanomaterials by the electrochemical capture and conversion of CO2 is bottlenecked by limited understanding of combined electrochemical and catalytic mechanisms at play during conversion. Here, our findings provide new insight that a key factor for the electrochemical two-electrode catalytic growth of carbon nanotubes (CNTs) on the cathode surface is the oxygen evolution activity (OEA) occurring at the anode. Poor OEA can inhibit catalytic CNT formation altogether, while increased anode OEA is correlated to changes in diameter and crystallinity of CNTs with otherwise identical cathode catalyst layers and synthesis conditions. Specifically, comparing Cu, Ni/Al2O3, and Pt anodes with Fe/stainless steel cathodes we observe over 6x smaller CNT diameter distribution and over 2x lower D/G ratio when using Pt anodes with the highest OEA. This is attributed to the role of transport between the cathode and anode during synthesis; in the case of poor anode OEA, a build-up of oxide species near the cathode surface influences the surface tension at the catalyst-carbonate interface to modify CNT diameter and properties. This mechanistic insight opens the door to routes to controllably produce high quality carbon nanomaterials from the electrochemical capture and conversion of carbon dioxide.

Transformation of primary siderite during coal catalytic pyrolysis and its effects on the growth of carbon nanotubes

Although the synthesis of carbon nanotubes from coal can efficiently reduce the cost of preparing these materials, the effect of the primary minerals present in coal (e.g., Fe-containing siderite) on the growth of these structures has not been studied in depth. In this study, we used a bituminous coal containing siderite and investigated the changes of the original siderite phase and the growth mechanism of carbon nanotubes during a KOH-catalyzed coal pyrolysis process. The results shown that the primary Fe minerals played an important role in the formation of carbon nanotubes during coal pyrolysis. During the KOH-catalyzed pyrolysis of coal, the Fe present in the raw material migrated from the bulk to the surface of the coal particles, and enriched in some areas of the surface of the coal particle. KOH-catalyzed coal pyrolysis had a significant effect on the formation of micropores and the increase of specific surface area and pore volume in coal. The primary siderite in coal and the catalytic growth of carbon nanotubes, in line with the following mechanism: FeCO3 → α-Fe → Fe3C + Graphite → carbon nanotubes. This study can provide new ideas for the utilization of low-rank coal resources rich in Fe mineral components.

Diffusion effect for the catalytic growth of carbon nanotubes on metal alloys substrate

Multiwall carbon nanotube (MWCNT) films were synthesized on a metal-alloy probe using thermal chemical vapor deposition (CVD) at low temperature (550 °C). The morphology of the Ni catalyst deposited on Co and Ta barrier layers on the metal-alloy substrate was investigated. In addition, we studied the tuning of the thickness of the Ni catalyst (without Co and Ta barrier layers), pretreatment time in a NH3, and the C2H2/NH3 ratio for growing MWCNTs. We showed that buffer layers were unstable during Ni catalyst nucleation on the metal-alloy probe during CNT growth because it diffused into the surface of probe and recrystallized during pretreatment. The results suggest that only using a thicker Ni catalytic layer (50 nm) could it allow for the growth of MWCNTs on the metal-alloy probe. The length of the MWCNTs was ~10 μm. In this paper, we propose a mechanism for thermal CVD of MWCNTs on a metal-alloy probe without barrier layers under a C2H2/NH3 atmosphere.

Synthesis and growth mechanism of carbon nanotubes growing on carbon fiber surfaces with improved tensile strength

An effective approach has been developed for the catalytic decomposition of acetylene (C2H2) by chemical vapor deposition (CVD), to achieve homogeneous growth of carbon nanotubes (CNTs) on the surfaces of carbon fibers. The morphology of CNTs grown on carbon fiber surfaces was observed by a scanning electron microscope and high-resolution transmission electron microscope, which revealed the uniform coverage of CNTs on the carbon fiber surfaces. The single fiber tensile test demonstrated that the tensile strength of carbon fibers could be increased by more than 12% with the catalytic growth of CNTs on their surface. The reparation of the damage caused during the formation of catalyst nanoparticles, and the cross-link of neighboring graphite crystallites induced by CNTs all occurred during the CVD process, which were considered to be the main reasons for improvement. The growth mechanism model of CNTs formation was established based on the thermodynamics principle and the interface microstructure of CNT-grown carbon fiber, illuminating the detailed mechanism for the growth of CNTs and the change of the shape of catalyst particles.

Catalytic effect of montmorillonite nanoparticles on thermal decomposition of coal tar pitch to carbon

The work studies the structure of two types of coal tar pitches (CTPs) modified with montmorillonite (MMT) in the form of nanopowder after annealing to 1000 and 2000 °C. The CTPs differed in polycyclic aromatic hydrocarbons (PAH) content and coking value (CV). The study of scanning electron microscope (SEM), transmission electron microscope (TEM) and x-ray diffraction (XRD) showed that the heat treatment of PAH-containing pitches modified with the ceramic filler to 1000 °C is a way to obtain a new clay-carbon nanocomposite, composed of turbostratic carbon, graphitic carbon, carbon nanotubes (CNT) and MMT. MMT act as a catalyst for carbon nanotubes growth via the gas phase at 1000 °C. Carbon nanotubes growth in pitch-derived carbon material occurred as a result of the presence of hydrocarbon compounds containing PAH evolved during thermal conversion to the carbon phase. Catalytic growth of carbon nanotubes from aromatic hydrocarbons-containing gaseous phase allows to reduce PAH emission during pitch-derived carbon thermal processing.The montmorillonite – modified carbon samples contained a graphitic phase built of preferred orientation crystallites. This phase consisted of well-ordered crystallites with the c-axis spacing, d002 = 0.335 nm. The process occurred due to catalytic graphitization at 1000 °C in the presence of montmorillonite.

A systematic study of the controlled generation of crystalline iron oxide nanoparticles on graphene using a chemical etching process.

Chemical vapor deposition (CVD) of carbon precursors employing a metal catalyst is a well-established method for synthesizing high-quality single-layer graphene. Yet the main challenge of the CVD process is the required transfer of a graphene layer from the substrate surface onto a chosen target substrate. This process is delicate and can severely degrade the quality of the transferred graphene. The protective polymer coatings typically used generate residues and contamination on the ultrathin graphene layer. In this work, we have developed a graphene transfer process which works without a coating and allows the transfer of graphene onto arbitrary substrates without the need for any additional post-processing. During the course of our transfer studies, we found that the etching process that is usually employed can lead to contamination of the graphene layer with the Faradaic etchant component FeCl3, resulting in the deposition of iron oxide FexOy nanoparticles on the graphene surface. We systematically analyzed the removal of the copper substrate layer and verified that crystalline iron oxide nanoparticles could be generated in controllable density on the graphene surface when this process is optimized. It was further confirmed that the FexOy particles on graphene are active in the catalytic growth of carbon nanotubes when employing a water-assisted CVD process.

Theoretical Study of Effective Parameters in Catalytic Growth of Carbon Nanotubes

In this article, an atomic model is used to study the carbon nanotube growth process in the catalytic chemical vapor deposition. The “rate of carbon production” conception is used to explain the feedstock decomposition and carbon diffusion in the catalyst. Some of the parameters, including carbon feedstock, catalyst particle, and growth temperature are the main effective parameters in the decomposition process and carbon diffusion. So the effect of the type and the diameter of the catalyst, the temperature of the growth environment, and the type and flow rate of the carbon feedstock, are assumed in the growth process analysis. On the other hand, these parameters affect the useful life time of the catalyst, the time during which the catalyst could decompose carbon feedstock and products carbon atoms. Theoretical results are compared with experimental results, and a good agreement is obtained. This theory is completed by optimization of the effective parameters in the growth process.

Investigation of the coupled effects of temperature and partial pressure on catalytic growth of carbon nanotubes using a modified growth rate model

An accurate modelling of catalytic growth of carbon nanotubes (CNTs) is needed to model the physics of carbon adsorption and diffusion into the catalyst surface along with the catalyst deactivation. The model should be able to provide a physical response towards the change of temperature and partial pressure. Though the effects of temperature and partial pressure on the growth rate has been studied individually, the coupled effects of the two parameters has yet to be emphasized. A modified growth rate model that unified the terms from previously developed models successfully captured the essential physics during the growth and provided physical response towards the change of temperature and partial pressure. The model validation was done against a chemical vapour deposition (CVD) experiment that employed acetylene and cobalt as the carbon source and the catalyst respectively where the modified model managed to predict the CNT terminal length more accurately compared to the standard model with 5% maximum error. A comprehensive parametric study on the effects of temperature and partial pressure on the growth rate and terminal length successfully reveals the minimum partial pressure of 5 Torr for a given operating condition below which the growth rate is significantly low regardless of any increase of temperature. Three regions of growth in the partial pressure–temperature domain are identified based on the magnitude of terminal length. The model can serve as a guideline for the determination and optimisation of the baseline operating conditions in future experiments on catalytic growth of CNT, with emphasis on the CVD and flame synthesis techniques.

Use of thin film of a Co15Ti40N35 alloy for CVD catalytic growth of carbon nanotubes

It is shown that multiwalled carbon nanotubes can be grown on the catalytic surface of a Co–Ti–N alloy with low (~10 at %) cobalt content by the conventional method of chemical deposition from acetylene. Adding nitrogen to the composition of the Co–Ti contributes the formation of the TiN compound and extrusion of Co onto the surface where it makes a catalytic effect for CNT growth. It was found that the tubes begin growth at a temperature of 400°C. It is shown by studies using Raman spectroscopy that the quality of CNT improves with increasing temperature.

The role of ions in plasma catalytic carbon nanotube growth: A review

While it is well-known that the plasmaenhanced catalytic chemical vapor deposition (PECVD) of carbon nanotubes (CNTs) offers a number of advantages over thermal CVD, the influence of the various individual contributing factors is not well understood. Especially the role of ions is unclear, since ions in plasmas are generally associated with sputtering rather than with growing a material. Even so, various studies have demonstrated the beneficial effects of ion bombardment during the growth of CNTs. This review looks at the role of the ions in plasmaenhanced CNT growth as deduced from both experimental and simulation studies. Specific attention is paid to the beneficial effects of ion bombardment. Based on the available literature, it can be concluded that ions can be either beneficial or detrimental for carbon nanotube growth, depending on the exact conditions and the control over the growth process.

Formation Process of Nickel Nano-Clusters for Catalytic Growth of Carbon Nanotubes for Use in Field-Effect Transistors

Incorporation of carbon nanotubes in conventional silicon technology to exploit their outstanding electrical properties offers opportunities in nanoelectronics. We report on a fabrication technique which utilizes catalytic nickel nano-clusters and chemical vapor deposition to obtain in situ grown carbon nanotubes for field-effect devices. The crucial catalyst formation process is found to be open to influence by an oxygen plasma treatment and the annealing temperature. Here, atomic force microscopy images show the most promising results on plasma treated samples that are annealed at 920°C. Electrical measurements on fabricated devices indicate that by now devices are either well switching (on/off current ratio of 4.5 orders of magnitude) or well current carrying (80 µA).

Promoting effect of group VI metals on Ni/MgO for catalytic growth of carbon nanotubes by ethylene chemical vapour deposition

AbstractThe incorporation of 1 mass % of group VI metals (chromium, molybdenum, and tungsten) into 4 mass % of Ni/MgO catalysts was evaluated for the synthesis of carbon nanotubes (CNTs) by the catalytic chemical vapour deposition of ethylene. All materials were characterised by XRD, surface area, TEM, SEM, Raman spectroscopy, and TGA-DTA. The resulting data demonstrated that the addition of group VI metals improved the surface area and metal dispersion, thereby achieving a remarkable enhancement in catalytic growth activity. Among the metals of group VI, Mo was found to be the most effective promoter for catalysing the CNTs’ growth. From TEM observation, long CNTs with a higher degree of graphitization were obtained on the Ni-Mo/MgO catalyst. TGA and DTA analysis showed that the as-grown CNTs over both Ni-Mo and Ni-W/MgO catalysts exhibited higher thermal stability.

Oxidative heat treatment of 316L stainless steel for effective catalytic growth of carbon nanotubes

Abstract Substrates consisting of stainless steel (SS) type 316L have been typically considered as non-responsive to catalytic formation of multi-walled carbon nanotubes (MW-CNTs) because of the protective chromium oxide layers on their surfaces. In current practices lengthy multi-step pretreatments are needed to break down these protective layers. This study oxidized 316L SS wire mesh in air, at 800 °C, for time periods ranging from 1 to 20 min prior to its use as a substrate catalyst for CNT synthesis from pyrolyzate gases of polyethylene, i.e., a mixture of hydrocarbons and hydrogen. The shortest pretreatment of the substrate (1 min) produced the maximum yield of CNTs (0.0145 g/g of catalyst) in these experiments, whereas longer pretreatments resulted in progressively lower yields. The 10 min pretreatment produced MWCNT forests vertically growing in planes perpendicular to the substrate surface (the periphery of the cylindrical wires of the mesh), 20 μm long and 21 nm in diameter on the average. Shorter (1–9 min) or longer (11–20 min) substrate heat treatments did not produce aligned CNTs at this temperature. Oxidative heat treatments of the substrate lengthier than 20 min did not produced any nanotubes. The results demonstrated an expedient and cost-effective method to activate the 316L SS substrate, and resulted in controllable CNT growth.

Flux and surfactant directed facile thermal conversion synthesis of hierarchical porous MgO for efficient adsorption and catalytic growth of carbon nanotubes

Three-dimensional (3D) hierarchical porous structures have attracted extensive attention owing to their versatile applications. Herein, 3D hierarchical porous MgO superstructures with a well faceted profile and high crystallinity were successfully obtained via a facile flux and surfactant directed decomposition of metal oxalates. As a nonionic surfactant, NP-9 served as the dispersing agent and prevented the porous MgO particles from aggregating with each other. The flux NaCl induced vapor migration through the porous structure, favoring the in situ crystallization of MgO after the phase conversion from the oxalate precursor, enhancing the toughness of the backbone and further realizing the preservation of the polyhedron-like morphology. The as-obtained hierarchical porous MgO superstructures were very efficient and effective as adsorbents for methylene blue (MB) and a catalyst support for single-walled carbon nanotube (SWCNTs) growth. The as-obtained SWCNTs afford an ultrahigh surface area of 1232 m2 g−1. Such superstructures could be employed as great potential candidates in dye-containing water treatment as well as heterogeneous catalysis.

Ni-W-P Catalyzed Growth of Carbon Nanotubes on SiC Whiskers

Ni-W-P nanoislands deposited on the surface of semiconducting SiC whiskers were investigated on its role in a catalytic growth of carbon nanotubes during thermal pyrolysis of acetylene. Experimental results indicated that a large quantity of carbon nanotubes formed although complex chemical reactions among the transition metals (Ni and W), SiC and carbon source had occurred to yield nickel silicides and tungsten carbide (WC). Multi-element catalytic growth of carbon nanostructures was found with the Ni-W-P catalyst. Favorable heterostructures made up of carbon nanotube/ semiconducting SiC whisker are expected.

Fuel Cell Electrodes Based on Carbon Nanotube/Metallic Nanoparticles Hybrids Formed on Porous Stainless Steel Pellets

The preparation of carbon nanotube/metallic particle hybrids using pressed porous stainless steel pellets as a substrate is described. The catalytic growth of carbon nanotubes was carried out by CVD on a nickel catalyst obtained by impregnation of pellets with a highly dispersive colloidal solution of nickel acetate tetrahydrate in ethanol. Granular polyethylene was used as the carbon source. Metallic particles were deposited by thermal evaporation of Pt and Ag using pellets with grown carbon nanotubes as a base. The use of such composites as fuel cell electrodes is discussed.