Growth kinetics of vertically aligned carbon nanotube arrays in clean oxygen-free conditions

Dynamics of growing carbon nanotube interfaces probed by machine learning-enabled molecular simulations

Carbon nanotubes (CNTs), hollow cylinders of carbon, hold great promise for advanced technologies, provided their structure remains uniform throughout their length. Their growth takes place at high temperatures across a tube-catalyst interface. Structural defects formed during growth alter CNT properties. These defects are believed to form and heal at the tube-catalyst interface but an understanding of these mechanisms at the atomic-level is lacking. Here we present DeepCNT-22, a machine learning force field (MLFF) to drive molecular dynamics simulations through which we unveil the mechanisms of CNT formation, from nucleation to growth including defect formation and healing. We find the tube-catalyst interface to be highly dynamic, with large fluctuations in the chiral structure of the CNT-edge. This does not support continuous spiral growth as a general mechanism, instead, at these growth conditions, the growing tube edge exhibits significant configurational entropy. We demonstrate that defects form stochastically at the tube-catalyst interface, but under low growth rates and high temperatures, these heal before becoming incorporated in the tube wall, allowing CNTs to grow defect-free to seemingly unlimited lengths. These insights, not readily available through experiments, demonstrate the remarkable power of MLFF-driven simulations and fill long-standing gaps in our understanding of CNT growth mechanisms.

Synergistic enhancement on flexible solid-state supercapacitor based on redox-active Fe3+ions/natural spidroin modified vertically aligned carbon nanotube arrays

The conductive skeleton and aligned carbon nanotube array (CNTA) structure can greatly shorten the ion transfer path and promote the charge transfer speed, which makes the CNTA an ideal electrode material for energy storage application. However, poor mechanical stability and low specific capacitance greatly impede its practical utilization. Here, we introduce a promising flexible electrode material based on the natural spider silk protein (SSP) modified CNTA(SSP/CNTA) with improved hydrophilicity and mechanical flexibility. The redox-active Fe³⁺doped SSP/CNTA flexible solid-state supercapacitor (FSSC) device with superior energy storage performance was assembled in a symmetric 'sandwich-type' structure. The synergetic interaction between Fe³⁺ions and the SSP are proved to greatly enhance the electrochemical performance especially the long-term cyclic stability. The Fe³⁺doped SSP/CNTA FSSCs device achieves an ultra-high volumetric capacitance of 4.92 F cm^-3at a sweep speed of 1 mV s^-1. Meanwhile it exhibited an excellent cycling stability with an increased capacitance by 10% after 10 000 charge-discharge cycles. As a control, a Fe³⁺doped CNTA composite device without SSP will lose over 74% of the capacitance after 10 000 cycles. The energy storage mechanism analysis confirms the dominated capacitive behavior of the device, which explained a considerable power density and rate performance. Our method thus provides a promising strategy to build up highly-efficient redox-enhanced FSSCs for next generation of wearable and implantable electronics.

Conversion of pyrolytic non-condensable gases from polypropylene co-polymer into bamboo-type carbon nanotubes and high-quality oil using biochar as catalyst

The conversion of low-value plastic waste into high-value products such as carbon nanomaterial is of recent interest. In the current study, the non-condensable pyrolysis gases, produced from Polypropylene Copolymer (PPC) feedstock, was converted into bamboo-type carbon nanotubes (BCNTs) through catalytic chemical vapour deposition using biochar. Experiments were conducted in a three-zone furnace fixed bed reactor, where PPC was pyrolysed in the second zone and carbon nanotubes (CNTs) growth was eventuated in the third zone. The effects of different growth temperatures (500, 700, 900 °C) and biochar particle sizes (nanoparticle as well as 0-100 and 100-300 μm) were investigated to optimise the production of hydrogen and the yield of carbon nanotubes on the biochar surface. Biochar samples used in the synthesis of CNTs were obtained from the pyrolysis of saw dust at 700 °C in a muffle furnace. Analyses performed by using Scanning electron microscopy, Transmission electron microscopy, X-ray diffraction, and Raman spectroscopy techniques suggested that the best crystalline structure of CNTs were obtained at 900 °C with nano-sized biochar as a catalyst. The strong gas-solid contact and void fraction of nano-sized particles enhances the diffusion-precipitation mechanism, leading to the growth of CNTs. The nano-sized biochar increased hydrogen production at 900 °C and reduced the polycyclic aromatic hydrocarbons content in oil to only 1%, which is advantageous for further utilisation. Therefore, the production of high-value CNTs from waste plastic using low-cost biochar catalyst can be a sustainable approach in the management of waste plastic while participating in the circular economy.

Metal Cluster Size-Dependent Activation Energies of Growth of Single-Chirality Single-Walled Carbon Nanotubes inside Metallocene-Filled Single-Walled Carbon Nanotubes

By combining in situ annealing and Raman spectroscopy measurements, the growth dynamics of nine individual-chirality inner tubes (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2) with diameters from ~0.8 to 1.1 nm are monitored using a time resolution of several minutes. The growth mechanism of inner tubes implies two successive stages of the growth on the carburized and purely metallic catalytic particles, respectively, which are formed as a result of the thermally induced decomposition of metallocenes inside the outer SWCNTs. The activation energies of the growth on carburized Ni and Co catalytic particles amount to 1.85–2.57 eV and 1.80–2.71 eV, respectively. They decrease monotonically as the tube diameter decreases, independent of the metal type. The activation energies of the growth on purely metallic Ni and Co particles equal 1.49–1.91 eV and 0.77–1.79 eV, respectively. They increase as the tube diameter decreases. The activation energies of the growth of large-diameter tubes (d_t = ~0.95–1.10 nm) on Ni catalyst are significantly larger than on Co catalyst, whereas the values of small-diameter tubes (d_t = ~0.80–0.95 nm) are similar. For both metals, no dependence of the activation energies on the chirality of inner tubes is observed.

Influencing factors and growth kinetics analysis of carbon nanotube growth on the surface of continuous fibers

Carbon nanotubes (CNTs) were continuously grown on the surface of the moving carbon fiber by chemical vapor deposition method using a custom-designed production line to prepare composite reinforcements on a large-scale. The systematic study of different parameters affecting the CNT growth revealed simple growth kinetics, which helps to control the surface morphology and structural quality of CNTs. Since hydrogen maintains the activity of the catalyst, it promotes the growth of CNTs in a continuous process. The increase of acetylene partial pressure promotes the accumulation of amorphous or graphite carbon on the catalyst surface, resulting in the decrease of CNT growth rate when acetylene concentration reaches 40%. The growth temperature significantly affects the CNT diameter and structural quality. As the temperature increases, the crystallinity of the tube wall increases obviously, and the CNT diameter increases due to the aggregate growth of the catalyst particles. According to the Arrhenius formula, the apparent activation energy is observed to be 0.67 eV, which proves that both bulk diffusion and surface diffusion exist when activated carbon passes through the catalyst to form CNTs.

3D silk fibroin/carbon nanotube array composite matrix for flexible solid-state supercapacitors

Silk fibroin decorates CNTA to form 3D microporous N-doped carbon frameworks for high-performance supercapacitors with high flexibility and wettability.

Vertically aligned laser sliced MWCNTs

Applications of multi-walled carbon nanotubes (MWCNTs) benefit from the availability of specific lengths of the material while keeping the outer walls pristine, for example, for applications requiring vertically aligned tubes. To this end, a simple and effective continuous flow 'top down' process to control the length of sliced MWCNTs has been developed using a vortex fluidic device (VFD) coupled with a 1064 nm pulse laser, with the process in the absence of chemicals and any auxiliary substances. Three different length distributions of the sliced MWCNTs, centered at 75 ± 2.1 nm, 300 ± 1.8 nm and 550 ± 1.4 nm, have been generated with the length depending on the VFD operating parameters and laser energy, with the processing resulting in a decrease in side wall defects of the material. We also show the ability to vertically self assemble short MWCNTs on a silicon substrate with control of the surface density coverage using a simple dipping and rinsing method.

Atomic Layer Deposition of Buffer Layers for the Growth of Vertically Aligned Carbon Nanotube Arrays

Vertically aligned carbon nanotube arrays (VACNTs) show a great potential for various applications, such as thermal interface materials (TIMs). Besides the thermally oxidized SiO₂, atomic layer deposition (ALD) was also used to synthesize oxide buffer layers before the deposition of the catalyst, such as Al₂O₃, TiO₂, and ZnO. The growth of VACNTs was found to be largely dependent on different oxide buffer layers, which generally prevented the diffusion of the catalyst into the substrate. Among them, the thickest and densest VACNTs could be achieved on Al₂O₃, and carbon nanotubes were mostly triple-walled. Besides, the deposition temperature was critical to the growth of VACNTs on Al₂O₃, and their growth rate obviously reduced above 650 °C, which might be related to the Ostwald ripening of the catalyst nanoparticles or subsurface diffusion of the catalyst. Furthermore, the VACNTs/graphene composite film was prepared as the thermal interface material. The VACNTs and graphene were proved to be the effective vertical and transverse heat transfer pathways in it, respectively.

Co(ii)Tetraphenyltetraphenanthroporphyrin@MWCNTs: enhanced π–π interaction for robust electrochemical catalysis

A cobalt(ii)tetraphenyltetraphenanthroporphyrin with phenanthrene-fused pyrrole rings was applied for robust HER and ORR.

Effect of Enhanced Thermal Stability of Alumina Support Layer on Growth of Vertically Aligned Single-Walled Carbon Nanotubes and Their Application in Nanofiltration Membranes

We investigate the thermal stability of alumina supporting layers sputtered at different conditions and its effect on the growth of aligned single-walled carbon nanotube arrays. Radio frequency magnetron sputtering of alumina under oxygen-argon atmosphere produces a Si-rich alumina alloy film on a silicon substrate. Atomic force microscopy on the annealed catalysts reveals that Si-rich alumina films are more stable than alumina layers with low Si content at the elevated temperatures at which the growth of single-walled carbon nanotubes is initiated. The enhanced thermal stability of the Si-rich alumina layer results in a narrower (< 2.2 nm) diameter distribution of the single-walled carbon nanotubes. Thanks to the smaller diameters of their nanotube pores, membranes fabricated with vertically aligned nanotubes grown on the stable layers display improved ion selectivity.

Chirality-dependent growth of single-wall carbon nanotubes as revealed inside nano-test tubes

Growth dynamics of single-wall carbon nanotubes (SWCNTs) have been studied with nickelocene as a precursor encapsulated in the interior of template SWCNTs. By means of multi-laser Raman spectroscopy, growth curves of nine different SWCNTs, (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2), have been determined upon in situ annealing at various temperatures. The data reveal that the nanotubes grow through fast and slow reaction pathways with high and low activation energies, respectively. While the activation energy of the slow growth is independent of the nanotube's chiral vector, that of the fast growth exhibits a monotonic increase as the tube diameter reduces from ∼1.1 down to 0.8 nm and no dependency on the chiral angle, which can be attributed to the size-dependent properties of catalyst clusters. The chirality dependent catalytic growth properties exploited in this study provide the basis for a large-scale synthesis of single-chiral vector SWCNTs.

Growth Termination and Multiple Nucleation of Single-Wall Carbon Nanotubes Evidenced by in Situ Transmission Electron Microscopy

In order to controllably grow single-wall carbon nanotubes (SWCNTs), a better understanding of the growth processes and how they are influenced by external parameters such as catalyst and gaseous environment is required. Here, we present direct evidence of growth termination of individual SWCNTs and successive growth of additional SWCNTs on Co catalyst particles supported on MgO by means of environmental transmission electron microscopy. Such in situ observations reveal the plethora of solid carbon formations at the local scale while it is happening and thereby elucidate the multitude of configurations resulting from identical external synthesis conditions, which should be considered in the quest for controlled SWCNT growth. Using CO and a mixture of CO and H₂ as carbon sources, we show that the growth of SWCNTs terminates with a reduced tube-catalyst adhesion strength. Two main reasons for the cessation are proposed: insufficient active carbon species and a certain amount of stress exerted at the tube-catalyst interface. Interestingly, it was observed that catalyst particles stayed active in terms of nucleating additional solid carbon structures after growth termination of the first SWCNT. These observations elucidate the importance of an in-depth understanding of the role of catalysts and carbon sources in the continued growth of SWCNTs. Furthermore, it serves as a guide for further control of carbon nanostructure synthesis via catalyst engineering and synthesis optimization.

Oxygen-promoted catalyst sintering influences number density, alignment, and wall number of vertically aligned carbon nanotubes

A lack of synthetic control and reproducibility during vertically aligned carbon nanotube (CNT) synthesis has stifled many promising applications of organic nanomaterials. Oxygen-containing species are particularly precarious in that they have both beneficial and deleterious effects and are notoriously difficult to control. Here, we demonstrated diatomic oxygen's ability, independent of water, to tune oxide-supported catalyst thin film dewetting and influence nanoscale (diameter and wall number) and macro-scale (alignment and density) properties for as-grown vertically aligned CNTs. In particular, single- or few-walled CNT forests were achieved at very low oxygen loading, with single-to-multi-walled CNT diameters ranging from 4.8 ± 1.3 nm to 6.4 ± 1.1 nm over 0-800 ppm O₂, and an expected variation in alignment, where both were related to the annealed catalyst morphology. Morphological differences were not the result of subsurface diffusion, but instead occurred via Ostwald ripening under several hundred ppm O₂, and this effect was mitigated by high H₂ concentrations and not due to water vapor (as confirmed in O₂-free water addition experiments), supporting the importance of O₂ specifically. Further characterization of the interface between the Fe catalyst and Al₂O₃ support revealed that either oxygen-deficit metal oxide or oxygen-adsorption on metals could be functional mechanisms for the observed catalyst nanoparticle evolution. Taken as a whole, our results suggest that the impacts of O₂ and H₂ on the catalyst evolution have been underappreciated and underleveraged in CNT synthesis, and these could present a route toward facile manipulation of CNT forest morphology through control of the reactive gaseous atmosphere alone.

Investigation of growth dynamics of carbon nanotubes

The synthesis of single-walled carbon nanotubes (SWCNTs) with defined properties is required for both fundamental investigations and practical applications. The revealing and thorough understanding of the growth mechanism of SWCNTs is the key to the synthesis of nanotubes with required properties. This paper reviews the current status of the research on the investigation of growth dynamics of carbon nanotubes. The review starts with the consideration of the peculiarities of the growth mechanism of carbon nanotubes. The physical and chemical states of the catalyst during the nanotube growth are discussed. The chirality selective growth of nanotubes is described. The main part of the review is dedicated to the analysis and systematization of the reported results on the investigation of growth dynamics of nanotubes. The studies on the revealing of the dependence of the growth rate of nanotubes on the synthesis parameters are reviewed. The correlation between the lifetime of catalyst and growth rate of nanotubes is discussed. The reports on the calculation of the activation energy of the nanotube growth are summarized. Finally, the growth properties of inner tubes inside SWCNTs are considered.

The relationship between the growth rate and the lifetime in carbon nanotube synthesis

We report an inverse relationship between the carbon nanotube (CNT) growth rate and the catalyst lifetime by investigating the dependence of growth kinetics for ∼330 CNT forests on the carbon feedstock, carbon concentration, and growth temperature. We found that the increased growth temperature led to increased CNT growth rate and shortened catalyst lifetime for all carbon feedstocks, following an inverse relationship of a fairly constant maximum height. For the increased carbon concentration, the carbon feedstocks fell into two groups where ethylene/butane showed an increased/decreased growth rate and a decreased/increased lifetime indicating different rate-limiting growth processes. In addition, this inverse relationship held true for different types of CNTs synthesized by various chemical vapor deposition techniques and continuously spanned a 1000-times range in both the growth rate and catalyst lifetime, indicating the generality and fundamental nature of this behavior originating from the growth mechanism of CNTs itself. These results suggest that it would be fundamentally difficult to achieve a fast growth with a long lifetime.

Kinetics of laser-assisted carbon nanotube growth

Laser-assisted chemical vapour deposition (CVD) growth is an attractive mask-less process for growing locally aligned carbon nanotubes (CNTs) in selected places on temperature sensitive substrates. The nature of the localized process results in fast carbon nanotube growth with high experimental throughput. Here, we report on the detailed investigation of growth kinetics related to physical and chemical process characteristics. Specifically, the growth kinetics is investigated by monitoring the dynamical changes in reflected laser beam intensity during growth. Benefiting from the fast growth and high experimental throughput, we investigate a wide range of experimental conditions and propose several growth regimes. Rate-limiting steps are determined using rate equations linked to the proposed growth regimes, which are further characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM), therefore directly linking growth regimes to the structural quality of the CNTs. Activation energies for the different regimes are found to be in the range of 0.3-0.8 eV.

Highly flexible, all solid-state micro-supercapacitors from vertically aligned carbon nanotubes

We report a highly flexible planar micro-supercapacitor with interdigitated finger electrodes of vertically aligned carbon nanotubes (VACNTs). The planar electrode structures are patterned on a thin polycarbonate substrate with a facile, maskless laser-assisted dry transfer method. Sputtered Ni is used to reduce the in-plane resistance of the VACNT electrodes. An ionogel, an ionic liquid in a semi-solid matrix, is used as an electrolyte to form a fully solid-state device. We measure a specific capacitance of 430 μF cm(-2) for a scan rate of 0.1 V s(-1) and achieve rectangular cyclic voltammograms at high scan rates of up to 100 V s(-1). Minimal change in capacitance is observed under bending. Mechanical fatigue tests with more than 1000 cycles confirm the high flexibility and durability of the novel material combination chosen for this device. Our results indicate that this scalable and facile fabrication technique shows promise for application in integrated energy storage for all solid-state flexible microdevices.

Statistical analysis of variation in laboratory growth of carbon nanotube forests and recommendations for improved consistency

While many promising applications have been demonstrated for vertically aligned carbon nanotube (CNT) forests, lack of consistency in results (e.g., CNT quality, height, and density) continues to hinder knowledge transfer and commercialization. For example, it is well known that CNT growth can be influenced by small concentrations of water vapor, carbon deposits on the reactor wall, and experiment-to-experiment variations in pressure within the reaction chamber. However, even when these parameters are controlled during synthesis, we found that variations in ambient lab conditions can overwhelm attempts to perform parametric optimization studies. We established a standard growth procedure, including the chemical vapor deposition (CVD) recipe, while we varied other variables related to the furnace configuration and experimental procedure. Statistical analysis of 280 samples showed that ambient humidity, barometric pressure, and sample position in the CVD furnace contribute significantly to experiment-to-experiment variation. We investigated how these factors lead to CNT growth variation and recommend practices to improve process repeatability. Initial results using this approach reduced run-to-run variation in CNT forest height and density by more than 50%.

Laser-assisted simultaneous transfer and patterning of vertically aligned carbon nanotube arrays on polymer substrates for flexible devices

We demonstrate a laser-assisted dry transfer technique for assembling patterns of vertically aligned carbon nanotube arrays on a flexible polymeric substrate. A laser beam is applied to the interface of a nanotube array and a polycarbonate sheet in contact with one another. The absorbed laser heat promotes nanotube adhesion to the polymer in the irradiated regions and enables selective pattern transfer. A combination of the thermal transfer mechanism with rapid direct writing capability of focused laser beam irradiation allows us to achieve simultaneous material transfer and direct micropatterning in a single processing step. Furthermore, we demonstrate that malleability of the nanotube arrays transferred onto a flexible substrate enables post-transfer tailoring of electric conductance by collapsing the aligned nanotubes in different directions. This work suggests that the laser-assisted transfer technique provides an efficient route to using vertically aligned nanotubes as conductive elements in flexible device applications.