Carbon Nanotube Forests Research Articles

The Effects of Fe Film Thickness and the H₂ Annealing Time on the Spin-Capability of Carbon Nanotube Forest with Chemical Vapor Deposition Method.

The effects of as-deposited iron (Fe) film thickness and the hydrogen (H₂) annealing time on the spin-capability of carbon nanotube (CNT) forest have been studied. Both, the as-deposited Fe film thickness and the H₂ annealing time significantly changed the morphology of Fe nanoparticles (NPs) after annealing process during the synthesis step of spin-capable carbon nanotube (SCNT) forest. The spin capability of CNT forests depended heavily on the different thicknesses of Fe films and the H₂ annealing time. In conclusion, the spin-capability of CNT forest can be achieved by controlling the initial Fe film thickness and/or the H₂ annealing time.

Fabrication of height-modulated carbon nanotube forests: Morphologies and prospects for broadband absorption

Carbon nanotube (CNT) forests can be the darkest manmade surfaces in the visible to near-infrared range of the electromagnetic spectrum, if the dimensions and alignment of the individual tubes and the densities and heights of the forests are in an appropriate range. However, it has been recognized that to push the absorption range beyond mid-infrared, hierarchical structures are needed. Site-selective CNT forest synthesis has been well-developed but not height-modulated CNT forest. Four different approaches and their resulting forest morphologies are analyzed here. Patterning substrates by either reactive ion etching or wet-chemical etching could create height- modulated forests but with regions of imperfect alignment for light absorption. Patterning diffusion barriers with the same catalyst do not always create forest height modulation because lower CNTs were uprooted by the van der Waals forces. Differential growth created by catalyst patterns results in height-modulated forests with vertically aligned CNTs, which is promising for broadband absorption. Novel CNT forest morphologies are revealed by the non-planar substrates and uneven distribution of catalyst. From the forest height ratios, the lifetime of Fe catalyst on silicon dioxide can be estimated to be ~60% of that on aluminum oxide.

Synergistic nanoarchitecture of mesoporous carbon and carbon nanotubes for lithium\u2013oxygen batteries

A rechargeable lithium–oxygen battery (LOB) operates via the electrochemical formation and decomposition of solid-state Li2O2 on the cathode. The rational design of the cathode nanoarchitectures is thus required to realize high-energy-density and long-cycling LOBs. Here, we propose a cathode nanoarchitecture for LOBs, which is composed of mesoporous carbon (MPC) integrated with carbon nanotubes (CNTs). The proposed design has the advantages of the two components. MPC provides sufficient active sites for the electrochemical reactions and free space for Li2O2 storage, while CNT forests serve as conductive pathways for electron and offer additional reaction sites. Results show that the synergistic architecture of MPC and CNTs leads to improvements in the capacity (~ 18,400 mAh g− 1), rate capability, and cyclability (~ 200 cycles) of the CNT-integrated MPC cathode in comparison with MPC.

Estimating carbon nanotube length from isotropic cloud point of carbon nanotube/chlorosulfonic acid solutions

The information about CNT lengths is essential to realize the unique properties of CNTs at the macroscopic scale. Here, we experimentally demonstrate a short-cut method to characterize the average length of carbon nanotubes (CNTs) based on the isotropic-cloud point (ϕICP) of CNT/chlorosulfonic acid (CSA) solutions. We obtained the empirical relationship between the aspect ratio (AR) of CNTs and the ϕICP of CNT/CSA solutions using CNT forests fabricated to have a nearly-uniform length. This empirical relationship follows the Onsager scaling (ϕICP ∼ AR−1) and covers a wide range of the AR. Using this relationship, the average length of CNTs can be estimated from the measurement of ϕICP. This method is applicable to long CNTs (AR>104). The accuracy of the method is supported by comparing the results calculated from this method to those obtained from SEM images. This simple yet accurate method may contribute to the controlled growth of CNTs and fabrication of CNT materials that have desired properties.

The influence of thermal loads on the physical properties of carbon nanotubes forests

Carbon nanotube (CNT) forests have been successfully integrated into wide range of thermal applications due to their high thermal conductance, mechanical compliance, and compatibility with nano-fabrication processes. However, their successful large-scale integration into functional systems requires thorough understanding of their physical behavior. In this work, we studied how the morphological structure of CNT forests is modified due to the application of thermal loads and its interplay with their thermal and mechanical properties. Our investigation has shown that the thermal load enhances the alignment of initially bent CNTs. The CNTs alignment, in turn, influences the thermal transition temperatures and the mechanical stiffness of CNT forests. Therefore, this work sheds light on the coupling between thermal loads and the physics of CNT forest and, thus, will facilitate their successful large-scale use.

Droplet Dynamics and Freezing Delay on Nanoporous Microstructured Surfaces at Condensing Environment

Superhydrophobic surfaces have aroused great interest for being promising candidates in applications such as self-cleaning, anti-icing, and corrosion resistance. However, most of the superhydrophobic surfaces lose their anti-wettability in low surface temperature and high humidity. The loss of superhydrophobicity by condensed liquid is a very common practical incident, yet to be understood properly. Here we report the wettability of the superhydrophobic nanoporous surfaces in condensation and freezing environments. Various structured surfaces fabricated with carbon nanotubes (CNT) and coated by an ultrathin, conformal, and low surface energy layer of poly (1H,1H,2H,2H-perfluorodecylacrylate) (pPFDA) are exploited in humid conditions. Droplet impact dynamics, condensate characteristics, and freezing time delays are investigated on the CNT micropillars with various geometries along with the CNT forest and two commercially available anti-wetting coatings, NeverWet and WX2100. Nanoporous microstructured CNT pillars with the favorable topological configuration demonstrated complete droplet bouncing, significant freezing delays, and considerable durability during several icing/de-icing cycles. This study provides an understanding on the preferable geometry of the highly porous CNT micropillars for retaining hydrophobicity and preventing ice formation, which is of practical importance for the rational development of anti-wetting surfaces and their applications in low temperatures and humid conditions.

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.

Tailoring the surface morphology of carbon nanotube forests by plasma etching: A parametric study

The top surface of carbon nanotube (CNT) “forests” produced by thermal chemical vapor deposition (CVD) often has a tangled morphology, owing to the self-organization of the CNTs at the initial stage of the CVD process. Removal of this top “crust” layer, without damaging the intrinsic microstructure of the CNT forest is often a key step for further applications. This paper studies the tailored use of Ar/O2 plasma etching to modify the surface morphology of multi-walled CNT forests. First, we investigate the effects of process parameters including plasma power, flow rate, and gas composition on the etching of CNT forests. As a result, we identify the plasma conditions that successfully remove the top crust yet maintain the structural shape of a CNT forest. Second, we prepare CNT forests having different packing densities and heights to study the influence of the initial characteristics on the etching result. We experimentally confirm that the etching rate depends strongly on the density and morphology of the crust layer. We also compare the material removal rate in vertical and lateral directions. Finally, we explore the enhancement of alignment and chemical uniformity of the top surface of CNT forests by the Ar/O2 plasma

Liquid electrolyte-free cathode for long-cycle life lithium–oxygen batteries

• An organic liquid electrolyte-free cathode design is proposed for Li–O 2 batteries. • The cathode design involves a duplex-structured Li + -selective solid membrane. • The duplex-structured membrane mitigates the degradation of the metallic Li anode. • The Au-decorated carbon nanotube forests serve as catalytically active sites. • The unique cell architecture results in improved stability during cycling. Ether-based organic liquid electrolytes (OLEs) have been commonly used in lithium–oxygen batteries (LOBs); however, they become unstable and cause rapid performance degradation during LOB operation. To address these problems, in this study we propose an OLE-free cathode architecture based on a Li + -selective solid membrane (LSSM). An LSSM with a seamless duplex (dense/porous) architecture is prepared by a tape casting process combined with co-sintering, and carbon nanotubes (CNTs) decorated with Au nanoparticles (CNT@Au) are directly formed on its porous framework. We show that the duplex-LSSM can effectively protect the metallic Li anode from parasitic reactions with impurity species and improve the cycling stability of Li. Furthermore, an LOB assembled with the duplex-LSSM and CNT@Au components exhibits a discharge capacity as high as 3650 mAh g −1 and improved cycling stability (>140 cycles) compared to a conventional OLE-based LOB; this can be explained in terms of the combined advantages provided by the OLE-free cathode and the LSSM-protected Li anode.

A study on the growth enhancement effects of chlorine on carbon nanotube forest in chloride-mediated chemical vapor deposition

We investigated the growth enhancement effects of chlorine on carbon nanotube (CNT) grown by a chemical vapor deposition method in which FeCl2 is used as a catalyst precursor. Cross-sectional observation of the CNT forest revealed that the FeCl2-originated catalyst showed a liquid-like behavior, and it caused metallic catalyst incorporation into the CNT. When chlorine gas flow was artificially added to the CNT synthesis process, the catalyst was further softened, and it increased the amount of metal filling in CNTs. This softening effect enhanced carbon absorption and diffusion at the catalyst and thus enabled the millimeter-long and high-density forest at a high temperature of 820 °C by suppressing the deactivation of the catalyst.

Different Pathways for Synthesis of WO₃ and Vertically Aligned Carbon Nanotube-Based Nanostructures.

The synthesis and investigation of vertically aligned carbon nanotube (VACNT) based materials are gaining more-and-more interest among scientists due to their specific properties (e.g., electrical, optical, mechanical). Therefore, our interest for the present research has focused on synthesis of WO₃/VACNT based nanostructures (using carbon nanotube forests obtained by catalytic chemical vapor deposition-CCVD method on aluminum substrate) using different synthesis pathways and WO₃ precursors. The obtained composites were investigated by scanning electron microscopy (SEM), Raman spectroscopy, while the obtained crystal structures were characterized by X-ray diffraction (XRD). Results have shown that depending on the synthesis method, and using as template the carbonaceous structure, we can successfully obtain non-stochiometric tungsten oxide (W18O49) or WO₃ composites.

Reducing Variability in Chemical Vapor Deposition of Carbon Nanotubes Based on Gas Purification and Sample Support Redesign

Abstract The synthesis of vertically aligned carbon nanotubes (CNTs), also referred to as CNT forest, by chemical vapor deposition (CVD) is an intricate process that is sensitive to multiple factors other than control of temperature, pressure, and gas flows. In particular, growth is highly sensitive to factors like ambient humidity, as well as small quantities of oxygen-containing species and carbon deposits inside the reactor. These typically uncontrolled factors significantly affect growth reproducibility and hinders the fundamental study of process–structure–property relationship for these emerging materials. Accordingly, universally applicable design modifications and process steps toward improving growth consistency are sought after. In this study, we introduce two new modifications to our custom-designed multizone rapid thermal CVD reactor and demonstrate their impact on growth: (1) reconfiguring the inlet gas plumbing to add a gas purifier to the helium (He) line, and (2) designing a new support wafer for consistent loading of substrates. We use statistical analysis to test the effectiveness of these modifications in improving growth and reducing variability of both CNT forest height and density. Analysis of our experimental results and hypothesis testing show that combining the implementation of He purifier with the redesigned support wafer increases forest height and reduces the variability in height (17-folds), both at statistically significant and practically significant levels.

Thermal properties of single-walled carbon nanotube forests with various volume fractions

Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data.

Horizontally aligned surface segments enhancing the adhesion of carbon nanotube forests

The excellent mechanical properties of carbon nanotube (CNT) and low effective modulus of its forest makes CNT forest (CNTF) promising in dry adhesives. However, the low contact efficiency of buckled nanotube segments on the surface of the CNTF weakened its adhesion strength, which is far below the theoretical value. To improve the orientation, density, and arrangement of these nanotube segments, a layer of patterned aluminum was added to the conventional catalyst. Thus, a CNTF with densely-packed, horizontally-aligned surface nanotube segments was obtained. Nanoindentation results showed that the adhesion force of this CNTF could reach up to 13.69 mN under optimized conditions, nearly 5.5 times the best results of similar works. The results of coarse-grained (CG) simulations revealed the lateral stretching of these nanotube segments induced bond energy variation made a major contribution to the superior adhesion performance. The formation mechanism of these surface nanotube segments was also thoroughly analyzed.

Kinetics of Carbon Nanotubes and Graphene Growth on Iron and Steel: Evidencing the Mechanisms of Carbon Formation.

Carbon formation on steel has recently become an active research area with several important applications, using either carbon nanotubes (CNTs) or graphene structures. The production of vertically aligned CNT (VACNT) forests with combined metals has been explored with important results. Detailed kinetics is the best approach to understand a mechanism. The growth behavior seems complex but can be simplified through the knowledge of the three more common alternative reaction mechanisms/routes. The time required to optimize the production and properties might be reduced. The mechanistic proposal reported in 1971 was better explained recently. The volcano shape Arrhenius plot reported is observed only when Fe, Co, and Ni are used as reaction catalysts. Other metals are catalytically active at higher temperatures, following a different route, which does not require surface catalysis decomposition of the reactive gas. C2H2 and low olefins react well, but CH4 is not reactive via this surface catalysis route. Optimizing production of CNTs, research work is usually based on previous experience, but solid-state science-based studies are available.

Enhanced Thermoelectric Properties of Poly(3-hexylthiophene) through the Incorporation of Aligned Carbon Nanotube Forest and Chemical Treatments.

Carbon nanotube/polymer composites have recently received considerable attention for thermoelectric (TE) applications. The TE power factor can be significantly improved by forming composites with carbon nanotubes. However, the formation of a uniform and well-ordered nanocomposite film is still challenging because of the creation of agglomerates and the uneven distribution of nanotubes. Here, we developed a facile, efficient, and easy-processable route to produce uniform and aligned nanocomposite films of P3HT and carbon nanotube forest (CNTF). The electrical conductivity of a pristine P3HT film was improved from ∼10–7 to 160 S/cm thanks to the presence of CNTF. Also, a further boost in TE performance was achieved using two additives, lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and tert-butylpyridine. By adding the additives to P3HT, the degree of interchain order increased, which facilitated the charge transport through the composite. Under the optimal conditions, the incorporation of CNTF and additives led to values of the Seebeck coefficient, electrical conductivity, and power factor up to rising 92 μV/K, 130 S/cm, and 110 μW/m K2, respectively, at a temperature of 344.15 K. The excellent TE performance of the hybrid films originates from the dramatically increased electrical conductivity and the improved Seebeck coefficient by CNTF and additives, respectively.

Structure-controlled growth of vertically-aligned carbon nanotube forests using iron–nickel bimetallic catalysts

A Fe–Ni catalyst prepared via mechanical alloying using a CCVD system for VACNT growth and its advanced characterization are presented.

Investigation and Optimization of Temperature Dependent Parameters for Growing Millimeter-Long Vertically Aligned Carbon Nanotubes

This study aimed to investigate effects of temperature dependent parameters on vertically aligned carbon nanotube (VACNT) forest height such as catalyst treatment temperature and growth temperature and optimize these parameters to grow long VACNT forests. These growth parameters were examined and optimized on samples including three different thicknesses of Fe catalyst layers which catalyze VACNT growth. Heights of VACNT forests grown on different Fe catalyst layers linearly decreased at various rates with the increment of the catalyst treatment temperature from 500 ℃ to 800 ℃. Moreover, optimum growth temperature to grow long VACNTs was found to be independent from Fe catalyst layer thickness while their height distributions showed variations. As a result of optimization of these parameters, we have found that it is possible to grow millimeter long VACNT forests from all three Fe catalyst layers with substantially low gas flows in 30 min growth. Raman spectroscopy approved that disorder of grown VACNTs is very low. Use of low gas feedstock, achieved in this study, ensures green and economic production of VACNTs.

Towards 3D self-assembled rolled multiwall carbon nanotube structures by spontaneous peel off.

Controlling the 3D assembly of individual nanomaterials can be a challenging task. However, it opens up opportunities for the production of increasingly complex nanostructures. Unusual rolled multiwall carbon nanotube structures are synthesized here by simply inducing a change of precursor composition during the growth of multiwall carbon nanotube forests. The multiwall carbon nanotube structures are comprised of nitrogen-doped and undoped sections, and are obtained via a detailed peel off and roll mechanism. These results open new doors for the development of increasingly complex nanostructures.

Flexible and bio-compatible temperature sensors based on carbon nanotube composites

Flexible temperature sensors are desired for a wide range of applications in engineering, biology, and medicine. In the present paper, we describe the development of novel flexible temperature sensors based on carbon nanotube (CNT) forest composites, and we demonstrate their ability to sense on-skin temperatures. The fabrication process of these sensors is simple and they demonstrate bio-compatibility, high performance, and a simple sensing scheme. When subjected to high temperatures, the CNTs are strained due to the thermal mismatch between the CNT forest and the matrix, which modifies their electrical resistance in a stable and reproducible temperature–resistance relationship. The sensors demonstrate sensitivities of ~0.1Ω/°C or higher, and show stable Young’s modulus of ~0.1MPa for temperatures below 100 °C. These sensors, thus, pave the way toward a new generation of flexible, wearable, and bio-compatible temperature sensors, which can be used as flexible lab-on-a-chip devices or as a sensory artificial skin.