Growth Of Nanotubes Research Articles

Floating Fe Catalyst Formation and Effects of Hydrogen Environment in the Growth of Carbon Nanotubes

Hydrocarbon conversion to advanced carbon nanomaterials with concurrent hydrogen production holds promise for clean energy technologies. This has been largely enabled by the floating catalyst chemical vapor deposition (FCCVD) growth of carbon nanotubes (CNTs), where commonly catalytic iron nanoparticles are formed from ferrocene decomposition. However, the catalyst formation mechanism and the effect of the chemical environment, especially hydrogen, remain elusive. Here, by employing atomistic simulations, we demonstrate how (i) hydrogen accelerates the ferrocene decomposition and (ii) prevents catalyst encapsulation. A subsequent catalytic dehydrogenation of methane on a liquid Fe nanoparticle showed that carbon dimers tend to be the dominant on-surface species. Such atomistic insights help us better understand the catalyst formation and CNT nucleation in the early stages of the FCCVD growth process and optimize it for potential scaleup.

Statistical patterns in high-throughput growth of single-wall carbon nanotubes from Co/Pt/Mo ternary catalysts

Designed alloy catalysts have shown promise in structure-controlled growth of single-wall carbon nanotubes (SWCNTs). However, due to the high-dimensional growth parameter space and low efficiency of the conventional trial-and-error optimizing approach, it is still challenging to explore and to establish the relations between catalyst compositions and the structure of SWCNTs. Here, we present a high-throughput strategy to investigate the statistical patterns in catalyst activity and selective growth of SWCNTs using Co/Pt/Mo ternary catalysts. Statistical analysis reveals that in the Co/Pt/Mo ternary diagram, there is an active region in the Co-rich Co–Mo corner to grow SWCNTs with high yield (≥25 tubes/μm2) and high quality (IG/ID ≥ 40). Enriched semiconducting (s-) SWCNTs with a purity higher than 90% were obtained along the boundary of the high yield region, near the Co–Pt axis. The statistical patterns of the yield, quality and selectivity are associated with the alloy composition, revealing a negative correlation between the yield and enrichment of s-SWCNTs. High-resolution transmission electron microscope characterization shows that a Co–Pt alloy catalyst with a uniform size distribution facilitates the s-SWCNT enrichment. And the separate phases of a Co–Mo/Co catalyst stabilize the active Co phase, leading to long catalyst lifetime and high yield of SWCNTs.

Role of Hydrogen in Ethylene-Based Synthesis of Single-Walled Carbon Nanotubes.

We examined the effect of hydrogen on the growth of single-walled carbon nanotubes in the aerosol (a specific case of the floating catalyst) chemical vapor deposition process using ethylene as a carbon source and ferrocene as a precursor for a Fe-based catalyst. With a comprehensive set of physical methods (UV-vis-NIR and Raman spectroscopies, transmission electron microscopy, scanning electron microscopy, differential mobility analysis, and four-probe sheet resistance measurements), we showed hydrogen to inhibit ethylene pyrolysis extending the window of synthesis parameters. Moreover, the detailed study at different temperatures allowed us to distinguish three different regimes for the hydrogen effect: pyrolysis suppression at low concentrations (I) followed by surface cleaning/activation promotion (II), and surface blockage/nanotube etching (III) at the highest concentrations. We believe that such a detailed study will help to reveal the complex role of hydrogen and contribute toward the synthesis of single-walled carbon nanotubes with detailed characteristics.

Enhancing the CO2 capture capacity of natural macroporous carbonized fibers by growing carbon nanotubes on their surface

In this study, a new hybrid material with superior CO2 sorption capacity was prepared by the growth of carbon nanotubes (CNT) into the macroporous structure of acid carbonized fibers (ACF). The growth of CNT was carried out by chemical vapor deposition using acetylene as a carbon source for 3.5 min at 700 °C. Selected materials were characterized by N2 physisorption, FTIR, potentiometric titrations, acid digestions, Raman spectroscopy, scanning and transmission electron microscopy. The CO2 capture was measured in static mode at pressure up to 10 atm in a sorptometer and in dynamic mode in a thermogravimetric analyzer at pressure of 1 atm. CNTs were homogeneously grown on the entire surface of ACF. The surface area of carbonized fibers increased from 2 to 452 m2/g after CNT growth and treatment at 300 °C for 1 min in air. The CO2 capture capacity of the ACF increased from 2.30 to 3.51 mmol/g at 25 °C and 10 atm after CNTs growth and air treatment (ACF3-CNT-AT1). The CO2 capture rate was also higher, from 0.023 to 0.032 mmol/g·min for ACF and ACF3-CNT-AT1 at 25 °C and 1 atm, respectively. These results were mainly attributed to the increase in basic groups and highly reactive edge sites. In addition, regeneration capacities of 99% and 95% were obtained for the first 3 sorption-desorption cycles and an apparent selectivity (QCO2/QN2) of 1.65 and 2.48 for ACF and ACF3-CNT-AT1, respectively. The rapid sorption-desorption kinetics, coupled with the high reversibility of CNT-modified ACFs, makes this hybrid material viable for use in CO2 capture systems.

A feather-like interlayer design with highly exposed active sites for high-performance lithium-sulfur batteries

Improving the conversion efficiency of lithium polysulfides (LiPSs) within the interlayer plays a significant role in inhibiting the shuttle effect of polysulfides for lithium-sulfur (Li-S) batteries, and this can be addressed by constructing highly exposed catalytic active sites. Herein, a feather-like interlayer material featured with highly exposed active sites is prepared by combining electrospinning technology and chemical vapor deposition. In the interlayer design, the growth of carbon nanotubes (CNTs) on the nitrogen-doped carbon fiber (NCF) surface greatly exposes the active Co and N-dopant adsorption-catalytic sites and forms a feather-like Co/CNT@NCF composite. The feather-like structure design endows the interlayer with good blockage effect for LiPSs shuttling. Meanwhile, the abundant active catalytic sites can effectively promote the conversion of polysulfides, and thus further suppress the shuttle of LiPSs. The cell with Co/CNT@NCF interlayer achieves excellent discharge performance (901.5 mAh g−1) with low capacity decay (0.025% per cycle) after 500 cycles at 1 C. Moreover, the cell exhibits a discharge capacity of 809 mAh g−1 at 5 C and an areal capacity of 5.05 mAh cm−2 under a sulfur loading of 6.9 mg cm−2. This work offers a facile interlayer design from the perspective of high active sites exposure in Li-S cells.

Growth of high-density single-wall carbon nanotubes with a uniform structure using a CoRu catalyst

The inefficient production of structurally uniform single-wall carbon nanotubes (SWCNTs) is an obstacle to their practical use in high-performance electronic devices. We have synthesized SWCNTs with a narrow diameter distribution (1.35 ± 0.25 nm) using a CoRu catalyst. Monodispersed nanoparticles with a narrow size distribution (2.4 ± 0.6 nm) and different compositions were prepared and used as catalysts for SWCNT growth. A furnace with an 80 cm-long uniform temperature zone (±10 °C) was designed and used to study the effect of catalyst composition on the growth of SWCNTs under the same conditions. By optimizing the composition of the bimetallic CoRu catalyst, SWCNTs with a uniform structure were efficiently synthesized. In addition, the effect of the growth conditions of temperature and carbon feed rate was investigated, and it was found that with an increase in yield, the structural uniformity of SWCNTs usually became worse. Both catalysts with elements in the suitable proportions and appropriate growth conditions are critical to achieving the high-efficiency structure-controlled growth of SWCNTs.

Multiscale structure and interface engineering of Fe/Fe3C in situ encapsulated in nitrogen-doped carbon for stable and efficient multi-band electromagnetic wave absorption

Most reported electromagnetic wave absorption (EWA) materials show significant effective absorption in a certain frequency range, but their performances deteriorate dramatically as the frequency changes. As the range of working frequencies for electronic devices is gradually widening, it is of great interest to explore frequency-insensitive EWA materials that can achieve efficient absorption in every waveband by simply changing the absorption thickness. To this end, a multi-scale absorber (Fe/Fe3C@NC) is rationally synthesized by chemical foaming and in-situ growth strategy. By controlling the growth of carbon nanotubes, the Fe/Fe3C@NC-2 exhibits a well-constructed 3D multi-scale architecture. Thanks to dipole polarization, interface polarization and magnetic-dielectric energy conversion, the Fe/Fe3C@NC-2 overcomes the frequency dispersion behavior and keeps a stable dielectric attenuation capability across the entire frequency range. Consequently, it delivers a superb full-band absorption of -50.1, -59.83, -55.87 and -51.91 dB in the S, C, X and Ku bands, respectively. The maximum radar cross-sectional reduction reaches 35.44 dB m−2 when the incident θ is 20°, testifying its impressive performance. Surprisingly, this EWA material also shows a remarkable resistance to oxidation and corrosion derived from the tightly coated carbon layers. This work provides new insight into the design of multi-band and stable EWA materials for practical application.

Swinging Crystal Edge of Growing Carbon Nanotubes

Recent direct measurements of the growth kinetics of individual carbon nanotubes revealed abrupt changes in the growth rate of nanotubes maintaining the same crystal structure. These stochastic switches call into question the possibility of chirality selection based on growth kinetics. Here, we show that a similar average ratio between fast and slow rates of around 1.7 is observed largely independent of the catalyst and growth conditions. A simple model, supported by computer simulations, shows that these switches are caused by tilts of the growing nanotube edge between two main orientations, close-armchair or close-zigzag, inducing different growth mechanisms. The rate ratio of around 1.7 then simply results from an averaging of the number of growth sites and edge configurations in each orientation. Beyond providing insights on nanotube growth based on classical crystal growth theory, these results point to ways to control the dynamics of nanotube edges, a key requirement for stabilizing growth kinetics and producing arrays of long, structurally selected nanotubes.

Bithiophene as a Sulfur-Based Promotor for the Synthesis of Carbon Nanotubes and Carbon-Carbon Composites.

We assess bithiophene (C8H6S2) as a novel sulfur-based promotor for the growth of single-walled carbon nanotubes (SWCNTs) in the aerosol (floating catalyst) CVD method. Technologically suitable equilibrium vapor pressure and an excess of hydrocarbon residuals formed under its decomposition make bithiophene an attractive promoter for the production of carbon nanotubes in general and specifically for ferrocene-based SWCNT growth. Indeed, we detect a moderate enhancement in the carbon nanotube yield and a decrease in the equivalent sheet resistance of the films at a low bithiophene content, indicating the improvement of the product properties. Moreover, the relatively high concentrations and low temperature stability of bithiophene result in non-catalytical decomposition, leading to the formation of pyrolytic carbon deposits; the deposits appear as few-layer graphene structures. Thus, bithiophene pyrolysis opens a route for the cheap production of hierarchical composite thin films comprising carbon nanotubes and few-layer graphene, which might be of practical use for hierarchical adsorbents, protective membranes, or electrocatalysis.

Precursor-mediated in situ growth of hierarchical N-doped graphene nanofibers confining nickel single atoms for CO2 electroreduction

Despite the various strategies for achieving metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) with different microenvironments for electrochemical carbon dioxide reduction reaction (CO2RR), the synthesis-structure-performance correlation remains elusive due to the lack of well-controlled synthetic approaches. Here, we employed Ni nanoparticles as starting materials for the direct synthesis of nickel (Ni) SACs in one spot through harvesting the interaction between metallic Ni and N atoms in the precursor during the chemical vapor deposition growth of hierarchical N-doped graphene fibers. By combining with first-principle calculations, we found that the Ni-N configuration is closely correlated to the N contents in the precursor, in which the acetonitrile with a high N/C ratio favors the formation of Ni-N3, while the pyridine with a low N/C ratio is more likely to promote the evolution of Ni-N2. Moreover, we revealed that the presence of N favors the formation of H-terminated edge of sp2 carbon and consequently leads to the formation of graphene fibers consisting of vertically stacked graphene flakes, instead of the traditional growth of carbon nanotubes on Ni nanoparticles. With a high capability in balancing the *COOH formation and *CO desorption, the as-prepared hierarchical N-doped graphene nanofibers with Ni-N3 sites exhibit a superior CO2RR performance compared to that with Ni-N2 and Ni-N4 ones.

Tailoring carbon nanotubes quickly into graphene nanoribbons along axis-direction via dynamic magnetic flux template

So far, the method of top-down preparing graphene nanoribbons by etching of carbon nanotubes (CNTs) mainly relies on the catalytic reaction of Fe, Co, and Ni nanoparticles. However, etching cannot be guaranteed along the CNT axis only. In this study, Ti3AlC2 is decomposed into liquid aluminum in the dynamic magnetic flux template to catalyze the growth of multiwalled carbon nanotubes (MWCNTs). Successively, the obtained MWCNTs are etched in-situ by rutile TiO2 particles to generate multilayered graphene nanoribbons (MLGNRs). Thus-obtained nanoribbons will create zigzag edges full of short-range order segment, demonstrating a spin glass state. This work has paved a new way for a top-down approach to create MLGNRs by tailoring MWCNTs along axis direction only using dynamic magnetic flux template.

Direct Growth of Binder-Free CNTs on a Nickel Foam Substrate for Highly Efficient Symmetric Supercapacitors.

In the modern civilized world, energy scarcity and associated environmental pollution are the center of focus in the search for reliable energy storage and harvesting devices. The need to develop cheaper and more competent binder-free electrodes for high-performance supercapacitors has attracted considerable research attention. In this study, two different procedures are followed to enhance the growth of carbon nanotubes (CNT-E and CNT-NF) directly coated on a Ni-foam substrate by a well-functioning chemical vapor deposition (CVD) method. Thus, directly grown optimized CNT electrodes are used as electrodes for electrochemical devices. Furthermore, solid-state symmetric supercapacitors are fabricated using CNT-NF//CNT-NF, and fruitful results are obtained with maximum specific capacitance (250.51 F/g), energy density (68.19 Wh/kg), and power density (2799.77 W/kg) at 1 A/g current density. The device exhibited good cyclic stability, with 92.42% capacitive retention and 99.68% Coulombic efficiency at 10 000 cycles, indicating the suitability of the electrodes for practical applications. This study emphasizes the importance of studying the direct growth of binder-free CNT electrodes to understand the actual behavior of electrodes and the proper storage mechanism.

Single-walled carbon nanotube growth from Ni catalyst particles under conventional growth conditions by alcohol catalytic chemical vapor deposition: in situ X-ray absorption fine structure study

In situ X-ray absorption fine structure (XAFS) analysis was performed on Ni catalysts during the growth of single-walled carbon nanotubes (SWCNTs) by alcohol catalytic chemical vapor deposition (ACCVD). Ni catalysts were gradually reduced during the heating process and became metallic at the growth temperature (800 °C). With the addition of ethanol during SWCNT growth, Ni catalysts were slightly carbonized. Such behavior was similar to that observed for Ir catalysts in our previous study, although the Ni catalysts had a carbon composition of ∼5%. The results of this study demonstrated the growth of SWCNTs from almost metallic catalysts under the conventional growth conditions of ACCVD using Ni catalysts. Our result obtained by in situ XAFS analysis was consistent with the result of atomistic computer simulations.

On-line intermittent growth of carbon nanotubes in a self-made tapered fluidized bed reactor at different inlet distances

The effect of the gas inlet distance on the quality and yield of carbon nanotubes (CNTs) is studied through the on-line intermittent method via a previous self-made tapered fluidized bed reactor (TFBR). The CNTs are catalytically grown on the Fe-Co-Mo/Al2O3 and Fe/Mo/Vermiculite catalyst at different inlet distances (DIDs), which clarify the effect of inlet distance and explore the optimum location. The whole process of CNTs growth in the TFBR without a distributor is explored. The effects of DIDs on catalyst flow and CNTs growth are discussed as well. Additionally, the morphology, quantity and quality of multi-walled carbon nanotubes (MWCNTs) are systematically characterized by using scanning electron microscopes (SEM), Brunauer Emment Teller (BET) and Raman Spectra. The results show that the CNTs prepared at 50 mm from the bottom of the cone have lower ash content and defect, and higher quality compared to other locations. That is, the airflow from 1/2–3/4 of the cone bottom could improve the catalytic performance of the catalyst, which results in the CNTs prepared have an excellent structure and high-efficiency carbon conversion rate.

(n, m) Distribution of Single-Walled Carbon Nanotubes Grown from a Non-Magnetic Palladium Catalyst

Non-magnetic metal nanoparticles have been previously applied for the growth of single-walled carbon nanotubes (SWNTs). However, the activation mechanisms of non-magnetic metal catalysts and chirality distribution of synthesized SWNTs remain unclear. In this work, the activation mechanisms of non-magnetic metal palladium (Pd) particles supported by the magnesia carrier and thermodynamic stabilities of nucleated SWNTs with different (n, m) are evaluated by theoretical simulations. The electronic metal-support interaction between Pd and magnesia upshifts the d-band center of Pd, which promotes the chemisorption and dissociation of carbon precursor molecules on the Pd surface, making the activation of magnesia-supported non-magnetic Pd catalysts for SWNT growth possible. To verify the theoretical results, a porous magnesia supported Pd catalyst is developed for the bulk synthesis of SWNTs by chemical vapor deposition. The chirality distribution of Pd-grown SWNTs is understood by operating both Pd-SWNT interfacial formation energy and SWNT growth kinetics. This work not only helps to gain new insights into the activation of catalysts for growing SWNTs, but also extends the use of non-magnetic metal catalysts for bulk synthesis of SWNTs.

Heterojunction interlocked catalysis-conduction network in monolithic porous-pipe scaffold for endurable Li-S batteries

The development of high-performance Li-S batteries is impeded by polysulfide (LiPS) shuttling effect and slow conversion kinetics. Building durable catalyst systems with rational adsorption-catalysis-conduction configurations to alleviate these problems still remains a challenge. Herein, a novel heterojunction interlocked catalysis-conduction strategy is proposed to prepare monolithic porous-pipe scaffold for LiPS accommodation. Therein the Co terminal of Co/Mo2C heterostructure catalyzes the in-situ growth of carbon nanotubes (CNTs). The Co-bridging effect reinforces the charge transfer from CNT to the catalytic Mo2C terminal of Co/Mo2C and avoids the coarsening of heterostructure nanodomains with superior anchoring and dispersing abilities. This high tap-density electrocatalytic scaffold (Co/Mo2C@CNTs) without excessive carbon residue as separator modifier enables the excellent rate capability and long cycling stability of Li-S batteries even under the pouch cell configuration, high sulfur loading and decreased electrolyte amount. This work also provides a novel top-down method to prepare mesoporous host with built-in conductive network from simple compact precursor via C3N4 vapor modulation.

Efficient carbon nanotube growth from pyrolysis of citric acid-based small organic molecules

We prepare carbon nanotubes based on the cross-linking reaction between small organic molecules citric acid and amino urea or ethylenediamine. Factors affecting the yield of carbon nanotube are optimized and catalytic effect of hydrated metal nitrate on carbon nanotube growth is investigated. The results demonstrate that carbon nanotube can grow in a wide range of urea content. Even two-dimensional sheet assembled by the prepared carbon nanotubes is observed when ethylenediamine is used. It is believed that such competitive strategy can expand the techniques for efficient, large-scale and low-cost synthesis of carbon nanotubes.

Low-Dimensional Hollow Nanostructures: From Morphology Control to the Release of an Active Pharmaceutical Ingredient

The mechanism guiding the growth of hollow silica nanotubes or nanospheres was unveiled for the first time through in situ liquid-phase transmission electron microscopy, allowing visualization of the morphology of the hydrated species involved in the early stage of material formation. The combined action of a surfactant (F127) and a swelling agent (toluene) was essential for the development of targeted nanostructures, owing to the formation of surfactant-stabilized toluene droplets in the aqueous phase. The quantity of surfactant-stabilized toluene droplets was unambiguously pointed as the key parameter guiding the formation of either tubular or spherical nanostructures. Leveraging on the fundamental understanding of the parameters guiding the formation of hollow silica nanostructures, tubular and spherical carriers were prepared and exploited for the release of an active pharmaceutical ingredient (API). The impregnation of nanotubes and nanospheres with a poorly water-soluble API (curcumin) was investigated, leading to an optimal loading of 20 wt %. The accessibility of nanotubes and nanospheres showed to be highly beneficial to increase the release kinetics of the targeted API in simulated intestinal fluid, opening promising perspectives in the field. Release was more efficient than with other conventional mesoporous silica-based carriers.

An Efficient Way to Model Complex Iron Carbides: A Benchmark Study of DFTB2 against DFT.

Iron carbides have attracted increasing attention in recent years due to their enormous potential in catalytic fields, such as Fischer-Tropsch synthesis and the growth of carbon nanotubes. Theoretical calculations can provide a more thorough understanding of these reactions at the atomic scale. However, due to the extreme complexity of the active phases and surface structures of iron carbides at the operando conditions, calculations based on density functional theory (DFT) are too costly for realistically large models of iron carbide particles. Therefore, a cheap and efficient quantum mechanical simulation method with accuracy comparable to DFT is desired. In this work, we adopt the spin-polarized self-consistent charge density functional tight-binding (DFTB2) method for iron carbides by reparametrization of the repulsive part of the Fe-C interactions. To assess the performance of the improved parameters, the structural and electronic properties of iron carbide bulks and clusters obtained with DFTB2 method are compared with the previous experimental values and the results obtained with DFT approach. Calculated lattice parameters and density of states are close to DFT predictions. The benchmark results show that the proposed parametrization of Fe-C interactions provides transferable and balanced description of iron carbide systems. Therefore, spin-polarized DFTB2 is valued as an efficient and reliable method for the description of iron carbide systems.

Defect-Induced Efficient Heteroepitaxial Growth of Single-Wall Carbon Nanotubes @ Hexagonal Boron Nitride Films.

Carbon nanotube-based derivatives have attracted considerable research interest due to their unique structure and fascinating physicochemical properties. However, the controlled growth mechanism of these derivatives remains unclear, and the synthesis efficiency is low. Herein, we proposed a defect-induced strategy for the efficient heteroepitaxial growth of single-wall carbon nanotubes (SWCNTs)@hexagonal boron nitride (h-BN) films. Air plasma treatment was first performed to generate defects on the wall of SWCNTs. Then, atmospheric pressure chemical vapor deposition was conducted to grow h-BN on the surface of SWCNTs. Controlled experiments combined with first-principles calculations revealed that the induced defects on the wall of SWCNTs function as nucleation sites for the efficient heteroepitaxial growth of h-BN.