Growth Of Carbon Nanotube Forests Research Articles

Highly aligned growth of carbon nanotube forests with in-situ catalyst generation: A route to multifunctional basalt fibres

Hierarchical fabrics are gaining ever-growing practical interest, thanks to the possibility of combining different properties into a single textile, particularly useful for applications such as high-performance composites showing in situ structural health monitoring capabilities. In this study, homogeneously aligned carbon nanotube forests are directly grown on basalt fabrics by a fast (∼15 min) chemical vapour deposition process without any external catalyst addition providing, for the first time, a highly dense coverage of the underlying substrate. It is demonstrated by transmission electron microscopy that the direct growth of nanotubes on basalt fibres is catalysed by the microstructural segregation of ferrous iron and its subsequent reduction in hydrogen atmosphere to nanocrystalline metallic iron. It is shown that in situ growth requires a pre-treatment etching which can be conducted in mild basic or acidic conditions, achieving optimum performance with an alkaline attack, without compromising the tensile strength of the fibres. Post-growth Raman analysis shows characteristic features associated with high graphitic ordering which turn an intrinsically insulating substrate into an electrically conductive (∼260 S/m) fabric, thus suggesting the possibility to employ the newly engineered hierarchical fabric in all those applications where the combination of good mechanical properties and electrical conductivity is a key requirement.

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.

Simplification of the CCVD method used in the growth of carbon nanotube forests on titanium substrate

The carbon nanotubes (CNTs) play an important role in nanotechnology research today as they have the potential to be used in several areas. Environmental protection is highly discussed nowadays, which is why it is necessary to be able to produce CNT forests with less energy investment and simply, for these reasons we used the dip-coating layer construction method, which is a simple process. We also would like to simplify the production of CNT forests by changing several parameters used in the syntheses (concentration of ink, catalyst ratios, temperature, the effect of heat-treatment, hydrogen, and water vapor, reaction time, carbon source). For these reasons, this research uses a dip-coating method to form a catalyst layer on the surface of the substrate, as well as to investigate various parameters to produce CNT forests directly on the titanium substrate, as it is important today to use a conductive substrate.

Study of self-organized structure in carbon nanotube forest by fractal dimension and lacunarity analysis

In this work, a fractal analysis of a morphology of carbon nanotube (CNT) forest was conducted. A self-similarity between the formation of catalyst nanoparticles, low and high-density CNT forest growth, and Raman mapping was investigated, for a catalyst thickness of 0.8, 1.0, and 1.2 nm. To explain reasons behind fluctuations of the CNT forest structure, a fractal dimension (Df) and lacunarity (Λ) were calculated, using a box-counting method. During the fractality analysis of catalyst particles, the highest Df was obtained for the 0.8-nm-thick film (Df = 1.91), while the lowest one was noted for the Fe catalyst thickness of 1.2 nm (Df = 1.72). The progressive increase of the catalyst thickness affected the dewetting, Ostwald ripening and diffusion processes, affecting the catalyst size distribution and arrangement. The same thickness variation led to a similar decreasing trend of Df changes, which was observed in the planar and linear analysis of the distribution of CNTs and in the IG/ID ratio distribution in Raman mapping, confirming the complexity and the self-organizing properties of CNT forest. The self-similarity and scaling of properties of self-organized CNT forest structure were proved by the analysis of fractal dimension and lacunarity analysis and revealed different stages of the catalyst annealing.

Decoupling Catalyst Dewetting, Gas Decomposition, and Surface Reactions in Carbon Nanotube Forest Growth Reveals Dependence of Density on Nucleation Temperature

In chemical vapor deposition (CVD) of vertically aligned carbon nanotubes (CNTs), the temperatures for the following three processes are typically coupled: (1) catalyst nanoparticle formation by fi...

High-Density Carbon Nanotube Forest Growth on Copper Foil for Enhanced Thermal and Electrochemical Interfaces

Vertically aligned carbon nanotubes (CNTs)—also called CNT forests—are attractive for use in battery electrodes, capacitive sensors, thermal interface materials, and many other applications. Howeve...

Manual spray coating: A cheap and effective method to build catalyst layers for carbon nanotube forest growth

The main goal of the current work is to investigate the usefulness of manual spray coating method for building Fe-Co layers on titanium substrate, then to grow vertically aligned carbon nanotubes via catalytic chemical vapor deposition (CCVD) technique. The effects of various parameters during both layer formation and CCVD synthesis were investigated to check the tunability of the catalyst layer. The quality of as-prepared samples was examined by scanning and transmission electron microscopies, Raman spectroscopy and X-ray diffraction analysis. The following factors significantly influenced the catalyst layer together with the formation of carbon nanotube forest: (1) application of various synthesis parameters during layer construction, (2) pretreatment of the Ti plate, (3) concentration and composition of catalyst ink. By investigating the effects of various synthesis conditions it was concluded, that the system is not overly sensitive to the CCVD parameters such as reaction time and the volume of water vapor in gas feed.

Rationally designed catalyst layers toward “immortal” growth of carbon nanotube forests: Fe-ion implanted substrates

To increase the length and coherence of carbon nanotube (CNT) forests that are grown by chemical vapor deposition, we implant Fe ions into an Al2O3 supporting layer to weaken the gradient of Fe concentration when Fe catalyst layer is deposited on it. The weakened gradient slows loss of Fe catalyst nanoparticles (NPs) by sub-surface diffusion into the Al2O3 layer. Therefore, Fe catalyst NPs were well maintained on the Fe-implanted substrates after growth for up to 660 min, whereas significant loss of Fe NPs occurred due to sub-surface diffusion into the Al2O3 supporting layer on the non-implanted substrates. The lifetime of the catalyst was extended to 660 min (Fe-implanted substrate) from 540 min (non-implanted substrate), and the lengths of CNTs in the forest were increased to 20.3 ± 1.0 mm (Fe-implanted substrate, maximum 21.5 mm) from 15.8 ± 1.9 mm (non-implanted substrate). This study clearly demonstrates that sub-surface diffusion of Fe catalyst NPs into the supporting layer is the mechanism that limits CNT forest growth and may provide insights into development of methods to design an “immortal” catalyst to grow CNT forests.

Evaluating the forces generated during carbon nanotube forest growth and self-assembly

The time-resolved reaction forces generated by actively growing and interacting carbon nanotube (CNT) forests are investigated using a mechanical finite element model. The CNT–CNT interaction forces are transmitted to the CNT catalyst particle residing at the base of each CNT, which may alter catalyst kinetics and modulate CNT growth rate. The simulation shows that CNTs growing at a rate greater than the population average transmit compressive force to the catalyst particle, while those growing at a slower rate transmitted tensile force. The magnitude of force for CNTs growing at a rate that was +/− 10% of the population average was on the order of 100′s of nanonewtons, corresponding to stress on the order of GPa. When using an Arrhenius-like kinetic model to modulate CNT growth rates, the growth rate of slower CNTs was enhanced by tensile forces, while the growth rate of faster growing CNTs was decreased by compressive forces. The net result of the force-modulation kinetics was a reaction force reduction of approximately an order of magnitude. Understanding how the growth parameters of an individual CNT are related to the mechanical forces it experiences during CNT forest assembly and the overall CNT morphology is expected to improve the control of CNT forest morphology and ensemble forest properties.

Characterization of Vertically Aligned Carbon Nanotube Forests Grown on Stainless Steel Surfaces.

Vertically aligned carbon nanotube (CNT) forests are a particularly interesting class of nanomaterials, because they combine multifunctional properties, such as high energy absorption, compressive strength, recoverability, and super-hydrophobicity with light weight. These characteristics make them suitable for application as coating, protective layers, and antifouling substrates for metallic pipelines and blades. Direct growth of CNT forests on metals offers the possibility of transferring the tunable CNT functionalities directly onto the desired substrates. Here, we focus on characterizing the structure and mechanical properties, as well as wettability and adhesion, of CNT forests grown on different types of stainless steel. We investigate the correlations between composition and morphology of the steel substrates with the micro-structure of the CNTs and reveal how the latter ultimately controls the mechanical and wetting properties of the CNT forest. Additionally, we study the influence of substrate morphology on the adhesion of CNTs to their substrate. We highlight that the same structure-property relationships govern the mechanical performance of CNT forests grown on steels and on Si.

Two step floating catalyst chemical vapor deposition including in situ fabrication of catalyst nanoparticles and carbon nanotube forest growth with low impurity level

Carbon nanotube (CNT) forest with very low concentration of metallic impurity particle was grown by a two-step floating-catalyst chemical vapor deposition (CVD) method. Fe catalyst nanoparticles were uniformly formed on the substrate by flowing ferrocene/ethanol mist generated by an ultrasonic nebulizer. Then, the growth of the CNT forest was followed by a thermal CVD process in an identical furnace with acetylene as a carbon source. The separated process of catalyst formation and CNT growth leads to no formation of impurity particles on the CNT surfaces with a lower impurity level of 0.8 wt%. The gas-phase flow of the catalyst solution enables precise control of catalyst formation including diameter, radius of curvature and areal density of catalyst particle for long and dense CNT forest. The CNT forest grown under optimum conditions had a spin capability because of its high areal density greater than 1 × 1011 cm−2.

Tissue-susceptibility matched carbon nanotube electrodes for magnetic resonance imaging

Test disk electrodes were fabricated from carbon nanotubes (CNT) using the Carbon Nanotube Templated Microfabrication (CNT-M) technique. The CNT-M process uses patterned growth of carbon nanotube forests from surfaces to form complex patterns, enabling electrode sizing and shaping. The additional carbon infiltration process stabilizes these structures for further processing and handling. At a macroscopic scale, the electrochemical, electrical and magnetic properties, and magnetic resonance imaging (MRI) characteristics of the disk electrodes were investigated; their microstructure was also assessed. CNT disk electrodes showed electrical resistivity around 1 Ω·cm, charge storage capacity between 3.4 and 38.4 mC/cm2, low electrochemical impedance and magnetic susceptibility of −5.9 to −8.1 ppm, closely matched to that of tissue (∼−9 ppm). Phantom MR imaging experiments showed almost no distortion caused by these electrodes compared with Cu and Pt-Ir reference electrodes, indicating the potential for significant improvement in accurate tip visualization.

Water-assisted, electron-beam induced activation of carbon nanotube catalyst supports for mask-less, resist-free patterning

The promise of carbon nanotube (CNT)-based technology has driven colossal research efforts for the past three decades. Integration of CNTs onto functional devices often involves the use of time-consuming and laborious patterning techniques to obtain spatially selective growth. In this manuscript, we present a single-step route for CNT forest growth in patterned areas using a novel catalyst support activation process which does not require the use of masks, resists or stencils. The process uses an electron-beam (e-beam) in the presence of water vapor to activate a catalyst support at select locations, on which CNT forests can grow selectively when exposed to the CNT catalyst and precursor. The process, therefore referred to as e-beam chemistry, modifies the support stoichiometry and surface roughness using a radiolysis-driven mechanism to yield catalytically active patterns on an otherwise inactive catalyst support. Because of its simplicity and effectiveness, the presented technique holds great promise in producing high fidelity, chemically-selective patterns.

Carbon nanotube-based black coatings

Coatings comprising carbon nanotubes are very black, that is, characterized by uniformly low reflectance over a broad range of wavelengths from the visible to far infrared. Arguably, there is no other material that is comparable. This is attributable to the intrinsic properties of graphitic material as well as the morphology (density, thickness, disorder, and tube size). We briefly describe a history of other coatings such as nickel phosphorous, gold black, and carbon-based paints and the comparable structural morphology that we associate with very black coatings. The need for black coatings is persistent for a variety of applications ranging from baffles and traps to blackbodies and thermal detectors. Applications for space-based instruments are of interest and we present a review of space qualification and the results of outgassing measurements. Questions of nanoparticle safety depend on the nanotube size and aspect ratio as well as the nature and route of exposure. We describe the growth of carbon nanotube forests along with the catalyst requirements and temperature limitations. We also describe coatings derived from carbon nanotubes and applied like paint. Building the measurement apparatus and determining the optical properties of something having negligible reflectance are challenging and we summarize the methods and means for such measurements. There exists information in the literature for effective media approximations to model the dielectric function of vertically aligned arrays. We summarize this along with the refractive index of graphite from the literature that is necessary for modeling the optical properties. In our experience, the scientific questions can be overshadowed by practical matters, so we provide an appendix of recipes for making as-grown and sprayed coatings along with an example of reflectance measurements.

Patterned growth of carbon nanotube forests using Cu and Cu/Ag thin film reservoirs as growth inhibitors

Lithography and lift-off of the catalyst is the established way to pattern forests of carbon nanotubes (CNTs). We demonstrate an alternative technique where we pattern a thin film reservoir residing below the Al2O3 underlayer to partially or fully deactivate the iron catalyst on the surface, thus leading to short CNTs or no CNTs. High-resolution imaging suggests that mass transfer from the Cu or Cu/Ag alloy reservoir to the surface promotes interalloying with the catalyst, which is the mechanism for deactivation. The lower melting temperature of the Cu/Ag alloy is more effective than Cu in inhibiting CNT growth since it becomes partially liquid at the processing temperature, to faster alloy with the iron catalyst.We show how the lithographic patterning at the level of the reservoir translates into lithographic patterning of the CNT forest. This technique, besides avoiding contamination of the catalytic surface, is an important building block towards fabricating patterned forests with areas of different heights where the reservoir material will modulate the CNT height. Additionally, this research opens the door for testing additional materials as reservoirs and to analyze their effects on CNT growth to achieve fine modulation of the third dimension (height) in patterned CNT forests.

Evolution of implanted Fe ions in SiO2/Si wafer into uniformly sized catalyst particles for carbon nanotube forest growth

We report the synthesis of carbon nanotube (CNT) forests with a narrow diameter distribution based on Fe ion implantation method. By annealing the Fe-implanted SiO2/Si wafer in an Ar atmosphere at 800 °C for 15 min, the Fe particles on the surface of SiO2 layer are successfully formed by the diffusion of Fe atoms from the SiO2 layer. Interestingly, the size distribution of Fe catalyst particles for Fe-implanted SiO2/Si wafers does not change with the prolonged annealing durations of up to 12 h. Using secondary ion mass spectroscopy and transmission electron microscopy (TEM), we confirmed that the implanted Fe atoms diffuse out of the SiO2 layer and form Fe particles on both the SiO2 surface and the interface between SiO2 and Si. The cross-sectional TEM images indicate that the Fe catalyst particles are anchored in the SiO2 layer, which limits the particles' mobility and results in an invariant catalyst size distribution for prolonged annealing durations. Therefore, we anticipate that implantation can be an efficient alternative catalyst preparation method for CNT forest growth which can solve various growth issues that are inherently caused by conventional physical vapor deposition method.

Hard x-ray photoemission study of oxidation states of Ti underlayer in Fe/Ti film system

The Fe/Ti catalyst system that was recently found to be effective for the growth of dense carbon nanotube (CNT) forests was studied using hard x-ray photoemission spectroscopy (HAXPES). It was previously found that the Ti support layer was partially oxidized at room temperature (RT) and absorbed oxygen from the Fe overlayer at higher temperatures, which gave rise to the dense CNT forest growth. The aim of the present study was to elucidate the reason for the initial oxidation of the Ti layer at RT, which remained unclear from the results of the previous study. The control of the initial Ti oxidation is important because it could affect the reduction and activation of the Fe layer at higher temperatures. Depth-dependent HAXPES measurements using different x-ray incidence angles revealed that the degree of oxidation of the 1-nm-thick Ti layer in a sample that had been aged at RT for approximately three months varied depending on the depth. This suggests that oxidation of the Ti layer proceeded after the Fe/Ti film fabrication. Then, the effects of aging on the Ti oxidation states were examined. The Ti 2p3/2 spectrum measured for another sample that had been aged for approximately nine months showed a higher degree of oxidation of the Ti layer than that measured for the sample aged for three months. These results indicate that the Ti layers absorbed oxygen despite the existence of a 2-nm-thick Fe overlayer.

Real-Time Imaging of Self-Organization and Mechanical Competition in Carbon Nanotube Forest Growth.

The properties of carbon nanotube (CNT) networks and analogous materials comprising filamentary nanostructures are governed by the intrinsic filament properties and their hierarchical organization and interconnection. As a result, direct knowledge of the collective dynamics of CNT synthesis and self-organization is essential to engineering improved CNT materials for applications such as membranes and thermal interfaces. Here, we use real-time environmental transmission electron microscopy (E-TEM) to observe nucleation and self-organization of CNTs into vertically aligned forests. Upon introduction of the carbon source, we observe a large scatter in the onset of nucleation of individual CNTs and the ensuing growth rates. Experiments performed at different temperatures and catalyst particle densities show the critical role of CNT density on the dynamics of self-organization; low-density CNT nucleation results in the CNTs becoming pinned to the substrate and forming random networks, whereas higher density CNT nucleation results in self-organization of the CNTs into bundles that are oriented perpendicular to the substrate. We also find that mechanical coupling between growing CNTs alters their growth trajectory and shape, causing significant deformations, buckling, and defects in the CNT walls. Therefore, it appears that CNT-CNT coupling not only is critical for self-organization but also directly influences CNT quality and likely the resulting properties of the forest. Our findings show that control of the time-distributed kinetics of CNT nucleation and bundle formation are critical to manufacturing well-organized CNT assemblies and that E-TEM can be a powerful tool to investigate the mesoscale dynamics of CNT networks.

Multifunctional characterization of carbon nanotube sheets, yarns, and their composites

Carbon nanotube (CNT) based macroscopic objects such as dry-state free-standing sheets and yarns have attracted much attention during more than a decade for their multifunctional features. Thanks to their lightweight, highly conductive, mechanically strong and flexible properties, various applications had been explored so far. However, because of the difficulties in the spinnable CNT forest growth, the sample availability in the academic fields has been quite limited. In this report, various properties of CNT sheets, yarns, and their composites were experimentally presented using the samples prepared from the spinnable CNT forest grown in the newly installed, acetylene-based, chemical-vapor-deposition chamber system. Clear observation of the dimensional effect on the charge transport through CNTs, the enhancement of electro-mechanical actuation owing the volume-expandable infiltration material inside CNTs, and other exemplary evaluations proved the versatility of this macroscopic assembly as well as the good quality of our sample.

Effects of a SiO2 sub-supporting layer on the structure of a Al2O3 supporting layer, formation of Fe catalyst particles, and growth of carbon nanotube forests

We investigate the effects of a SiO2 sub-supporting layer on the growth of carbon nanotube (CNT) forests, especially on the spinnability of the CNT forest into a CNT yarn, which is one of the most promising application areas.