Catalytic Carbon Nanotubes Research Articles

Atomistic Mechanisms of Ring Formation during Catalyzed Carbon Nanotube Growth

Atomistic Mechanisms of Ring Formation during Catalyzed Carbon Nanotube Growth

Root characterization of Fe-based catalyzed carbon nanotube forests

For the purpose of understanding the process of forming carbon nanotube forests catalyzed by Fe nanoparticles, a combination of ex-situ transmission electron microscopy observations and molecular dynamics simulation methods were used to study the rooting path of C in Fe and its motion evolution mechanism. The results show that C first reacts with Fe to form Fe3C, and C enters from Fe (110) crystal plane, thus Fe (110) crystal plane transferring to Fe3C (013) crystal plane; then C in Fe3C precipitates and grows into carbon nanotubes, and C precipitates from Fe3C (013) crystal plane, thus creating the relationship of the direction of carbon nanotubes growth parallel to Fe3C (013) crystal plane. The results of the microstructure snapshot over time and radial distribution function calculated by molecular dynamics simulation also show that the motion path of C is the same as the results deduced in the experiment, which confirms the correctness of the results.

Ionic liquid functionalized polylactides: the effect of anions on catalytic activity and carbon nanotube dispersion

A series of novel polylactides bearing an imidazolium group with different anions at one chain extremity were synthesized via ROP of lactide by organocatalysts. The effect of anions was systematically investigated for the first time.

Spark plasma sintered catalytic nickel-copper alloy and carbon nanotube electrodes for the hydrogen evolution reaction.

We report the proof-of-concept of spark plasma sintered (SPS) consolidated mesoporous composite catalytic electrodes based on nickel-copper alloys and carbon nanotubes for the electrocatalytic hydrogen evolution reaction (HER) in alkaline media. The optimized electrode (203 m2 g-1, 5 wt% Ni75Cu25) operated at -0.1 A cm-2 (current of -0.15 A) for 24 h with a stable overpotential of about 0.3 V. This newly described freestanding SPS approach allows the rational control of specific surface area, metal loading, and electrocatalytic performance, thus opening a new route to catalytic electrodes with controllable physical and catalytic properties.

Study on the Effect of Reduction Temperature on the Catalytic Activity of Fe-Mo/Al<sub>2</sub>O<sub>3</sub> Catalyst and the Microstructure of Carbon Nanotubes

It is usually necessary to first perform temperature reduction treatment to enable the catalyst to exert its catalytic activity in the subsequent process of preparing carbon nanotubes by chemical vapor deposition. In this experiment, Fe-Mo/Al2O3 catalyst was prepared based on microreactor, and the effect of reduction temperature on the microstructure of the catalyst and the morphology of carbon nanotubes was investigated. The results show that the reduction temperature has a significant effect on the microstructure of the catalyst, which in turn affects its catalytic activity and the yield and quality of carbon nanotubes. Moderately reducing the reduction temperature during the catalyst reduction process is beneficial to increase the catalytic activity of the catalyst. However, although its sintering degree could be weakened when the catalyst was reduced at an excessively low temperature of 350 °C, its catalytic efficiency was greatly reduced and the degree of defects of the catalyzed carbon nanotubes was increased. When the catalysts calcined at 450 °C and reduced at 600 °C, the catalysts show excellent catalytic activity, and catalytic efficiency can reach 74.76%. In addition, the reduction temperature also has a certain effect on carbon nanotubes. As the reduction temperature increases, the span of carbon nanotubes is relatively concentrated, but the specific gravity of the thicker outer diameter gradually increases. As for the defect degree of carbon nanotubes, the carbon nanotubes M600-600 is better and the defects are fewer when the reduction temperature is reduced from 670 °C to 600 °C.

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

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

Development and research carbon nanotube-based resistive gas sensor

Experimental studies of the processes of formation of catalytic centres (CC) and carbon nanotubes (CNTs) at ITO contacts have been carried out. The regularities of the influence of the annealing temperature on the geometric dimensions of CC have been established. An array of interwoven CNTs with a highly developed surface has been grown. A model of a gas sensor with a sensitive element based on a CNT network has been created. The reaction time and reaction of the sensor, its sensitivity to N2, O2, and Ar have been experimentally investigated. It has been shown that the sensor has a maximum sensitivity of 17.2% to N2, 16.3% to Ar, and 18.7% to O2 in the range of gas concentrations from 30 to 70 ppm. It has been shown that gas detection is possible at room temperature, despite a rather long reaction and reduction time. In this case, an almost complete restoration of the sensitive element of the initial resistance has occurred without additional heating.

Janus Segregation at the Carbon Nanotube-Catalyst Interface.

The contact between a carbon nanotube (CNT) edge and a catalyst is a curvilinear interface of fundamental and practical importance. Here, the first-principles evidence shows that on a rigid/solid catalyst the faceted CNT edge is significantly lower in energy compared to the minimal-length circle, with the interface energy difference decreasing on more compliant surfaces. This universal trend, found for typical monometallic (Ni, Co), bimetallic (Co7W6), and metal carbide (WC) catalysts, results in a peculiar edge segregation into one-dimensional Janus (armchair-zigzag) interface. Its lowered energy greatly enhances the nucleation probability of chiral tubes, dramatically affecting their growth kinetics. This offers a richer basis for understanding, modeling, and control of catalytic CNT synthesis.

Influence of the Sulfur Content Catalyst on the Packing Density of Carbon Nanotube Forests.

For the fabrication of high-performance carbon nanotube (CNT) composites with practical applicability, the development of new methods for the controlled growth of high-aspect-ratio CNTs still constitutes a challenge. With the aim of gaining a deeper understanding of the catalytic CNT growth, in this study, the effect of the catalyst composition is investigated using different mixtures of Fe2(SO4)3 and FeCl2 as catalysts. The relationship between the catalyst chemical state and the growth behavior of CNT forests is demonstrated by evaluating the alignment, diameter, length, and areal density of the CNT forests. When the Fe2(SO4)3 content is increased, the area density, the IG/ID ratio, and the crystallite size of the CNTs increase. Additionally, the obtained CNT forests exhibit good spinnability with increasing the sulfur content.

Structural Modulation of Co Catalyzed Carbon Nanotubes with Cu-Co Bimetal Active Center to Inspire Oxygen Reduction Reaction.

Rational design of highly efficient catalyst for ORR is critical for development of advanced air cathode in Zn-air cells and fuel cells. To optimize the ORR performance of Co based cathode, the structure of carbon nanotube from DCI-Co precursor could be controlled through modulate synthetic parameters. The optimized ORR catalyst Co@NCNT-700 exhibit larger BET area, higher content of Co-N x and graphitic N, which performance could be improved in furtherthrough Cu doping. The experiment data approved that the activity of Co-N x was enhanced by the synergistic effect with introduced Cu. Furthermore, the high-performance zinc-air batteries was fabricated with the bimetal catalyst CuCo@NCNT-700 as an air electrode. The high open-cycle potential (1.54 V) and peak power density (0.275 W.cm-2 at 0.474 A.cm-2) were achieved, which would be potentially used to develop next generation energy conversion devices.

In-situ catalytic growth carbon nanotubes from metal organic frameworks for high performance lithium-sulfur batteries

The highly uniformed hybrid carbon material has been prepared by in-situ growth carbon nanotubes (CNTs) from metal organic frameworks, with the cobalt species as the catalyst for CNTs growth and dicyandiamide (DICY) as a sacrificial agent for the formation of graphitic carbon. The CNTs with extraordinary conductivity supply more electron transport to the cathode and make much more sense in enhancing the rate performance of the sulfur cathode. In the matrix, cobalt nanoparticles and heteroatom nitrogen can help in immobilizing sulfur species, leading to the improvement of polysulfide shutting. Moreover, the increased specific surface area and mesoporous channel enhance the sulfur loading and facilitate electrolyte diffusion. On integrating these fascinating benefits into one electrode material, as a result, the hybrid composite (CNT@CoNC/S) cathode presents a high initial capacity of 1316.1 mAh/g at the current rate of 0.1C. Even at high current rate of 5C, a decent capacity of 620.7 mAh/g can still be achieved. The capacity of hybrids materials can be maintained at 970 mAh/g after 500 cycles at 0.2C, and a capacity retention of 79.8%, revealing it great potential for energy storage application.

The kinetics of chirality assignment in catalytic single-walled carbon nanotube growth and the routes towards selective growth.

Depending on its specific structure, or so-called chirality, a single-walled carbon nanotube (SWCNT) can be either a conductor or a semiconductor. This feature ensures great potential for building ∼1 nm sized electronics if chirality-selected SWCNTs could be achieved. However, due to the limited understanding of the growth mechanism of SWCNTs, reliable methods for chirality-selected SWCNTs are still pending. Here we present a theoretical model on the chirality assignment and control of SWCNTs during the catalytic growth. This study reveals that the chirality of a SWCNT is determined by the kinetic incorporation of pentagons, especially the last (6th) one, during the nucleation stage. Our analysis showed that the chirality of a SWCNT is randomly assigned on a liquid or liquid-like catalyst surface, and two routes of synthesizing chirality-selected SWCNTs, which are verified by recent experimental achievements, are demonstrated. They are (i) by using high melting point crystalline catalysts, such as Ta, W, Re, Os, or their alloys, and (ii) by frequently changing the chirality of SWCNTs during their growth. This study paves the way for achieving chirality-selective SWCNT growth for high performance SWCNT based electronics.

Creation of High-Density Nanostructures for Use in Energy Conversion Applications

Nanostructures have garnered much interest in recent years for their potential in increasing performance of alternative energy devices, such as solar cells, thermoelectrics, batteries, and piezoelectrics. Many methods exist for the fabrication of nanostructures, but one of the most promising methods is the use of nanoporous aluminum oxide templates due to its extremely high density and self-ordering through a hexagonal, close-packed structure. However, the initial aluminum growth substrate is generally not suitable for use in energy conversion devices so a major challenge becomes the practical use of aluminum oxide templates. In this presentation, we demonstrate how to effectively utilize the unique advantages of aluminum oxide templates for a variety of applications. We introduce a method to transfer aluminum oxide templates to a substrate of interest. Using this process, we are able to pattern highly-ordered nanostructured materials with dimensions below 10 nm that may be suitable for use in thermoelectric devices. We also use this pattern transfer process to fabricate porous templates on complex multilayered substrates. These multilayered nanostructures can be used in many applications. Here, we present the growth of highly-ordered, catalyzed carbon nanotubes. These methods highlight an effective approach for creating the highly-dense, highly-ordered nanostructures required for many energy conversion applications.

Reversibility of Noble Metal-Catalyzed Aprotic Li-O₂ Batteries.

The aprotic Li-O2 battery has attracted a great deal of interest because, theoretically, it can store far more energy than today's batteries. Toward unlocking the energy capabilities of this neotype energy storage system, noble metal-catalyzed high surface area carbon materials have been widely used as the O2 cathodes, and some of them exhibit excellent electrochemical performances in terms of round-trip efficiency and cycle life. However, whether these outstanding electrochemical performances are backed by the reversible formation/decomposition of Li2O2, i.e., the desired Li-O2 electrochemistry, remains unclear due to a lack of quantitative assays for the Li-O2 cells. Here, noble metal (Ru and Pd)-catalyzed carbon nanotube (CNT) fabrics, prepared by magnetron sputtering, have been used as the O2 cathode in aprotic Li-O2 batteries. The catalyzed Li-O2 cells exhibited considerably high round-trip efficiency and prolonged cycle life, which could match or even surpass some of the best literature results. However, a combined analysis using differential electrochemical mass spectrometry and Fourier transform infrared spectroscopy, revealed that these catalyzed Li-O2 cells (particularly those based on Pd-CNT cathodes) did not work according to the desired Li-O2 electrochemistry. Instead the presence of noble metal catalysts impaired the cells' reversibility, as evidenced by the decreased O2 recovery efficiency (the ratio of the amount of O2 evolved during recharge/that consumed in the preceding discharge) coupled with increased CO2 evolution during charging. The results reported here provide new insights into the O2 electrochemistry in the aprotic Li-O2 batteries containing noble metal catalysts and exemplified the importance of the quantitative assays for the Li-O2 reactions in the course of pursuing truly rechargeable Li-O2 batteries.

Dye-Sensitized Solar Cell with Electrophoretic Deposited Photocatalytic Carbon Nanotube Counter Electrode: Nanotube Density and Cell Performance

Prospective solar cell technologies such as dye-sensitized solar cells (DSSCs), organic photovoltaics (OPVs) and much recently discovered perovskite solar cells compared to the conventional solid-state pn-junction type silicon solar cells are of high demand for solar energy conversion due to their simple fabrication processes, high energy conversion efficiency, low cost, and potential for flexible solar panels1-4. Among the emerging solar cell technologies, DSSC’s stability and reliability in air is so far most promising compared to OPVs and perovskite cells. While platinum film is used as photocatalyst in the counter electrode of a conventional DSSC, nanostructured alternatives such as carbon nanotubes (CNTs) can further decrease both material and fabrication cost, enhance mechanical flexibility and provide comparable performances5-6. Network of metallic CNTs have demonstrated significant potential for alternate transparent conductors to indium doped tin oxide (ITO). Moreover, its catalytic activity for DSSC counter electrode has also been previously explored to replace platinum7. However, a lower conversion efficiency still persists. Placing the nanotube on counter electrode in a network structure with optimal density is therefore desired for enhances solar cell performance, as the rate of charge transfer reaction on the counter electrode and hence the photo-conversion efficiency is dependent on optimal density of nanotubes. While enormous density of nanotubes in counter electrode can provide large amount of catalytic surface for the I-/I3 -reduction reaction, high nanotube density can be an obstacle for fast charge transport. On the other hand, a sparse nanotube network might provide just enough catalytic surfaces without barricading charge transport. The sparse network also has high light transmittance to build transparent solar cells and solar panels. In this work, we report an electrophoretic deposition of CNT network on to fluorine-doped tin oxide (FTO) glass substrates. We fabricated DSSCs using these CNT-decorated-electrodes as counter electrodes and tested the cells to evaluate their photocatalytic activities using a solar simulator. We have demonstrated optimal CNTs electrophoretic deposited onto the FTO surface (30-45 seconds deposition at a DC bias of 40V) with conversion efficiency comparable to a DSSC with platinum counter electrode. This work demonstrates the efficient use of catalytic CNTs with optimal density for building DSSCs. [1] B. O’Regan and M. Grätzel, Nature 353 (1991) 737 [2] A. Hagfeldt et al., Chem. Rev. 110 (2010) 6595 [3] M. Gratzel, J. Photochemistry and Photobiology C: Photochemistry Reviews 4 (2003) 145 [4] T. Miyazaki, A. Akisawa, and T. Kashiwagi, Renewable Energy 30 (2005) 281 [5] P. Joshi et al., Energy Environ. Sci., 2 (2009), 426-429 [6] E. Ramasamy et al., Appl. Phys. Lett. 90 (2007) 173103 [7] K. Suzuki et al., Chem. Lett. 32 (2003) 28; J. –G. Park et al., Electrochimica Acta 85 (2012) 600 Figure 1

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

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

(Invited) Carbon Nanotubes: From the Origin of Helicity to Application

Despite recent advances in experimental studies on the preferential and helicity selective growths of carbon nanotubes the mechanism of helicity formation still remains elusive, which hinder further progresses in selective synthesis. Here we will present the results of the studies on nucleation of the carbon cap, believed to be responsible for the helicity of the surface catalyzed carbon nanotubes. We found that the cap formation involves graphene growth across various adjacent facets of catalyst particle and an explicit introduction of defects. As a result the graphene embryos incorporate in their hexagonic network various polygons to accommodate the curved 3D geometry that initiates carbon dome formation. Taking into account the census of nanotube cap and the fact that given cap fits only one nanotube wall we suggest that the carbon cap can serve as a precursor that sets the symmetry of growing tube. Our recent progresses in production of carbon nanotubes, nanotube reinforced composites and their potential applications also will be presented.

Carbon nanotube growth: First-principles-based kinetic Monte Carlo model

Growth mechanism of carbon nanotubes (CNT) on nickel nanoparticles has been investigated using a multiscale modeling approach that integrates density functional theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations. The proposed elementary steps that control the catalytic CNT growth were first analyzed using DFT. The energetic results were then used to build an intrinsic kinetic database that was incorporated into a novel KMC framework that has been developed to overcome large disparities in time scales for this system. KMC simulations show that CNT growth is dominated by the surface diffusion of carbon species. Moreover, it was found that the surface diffusion of the small carbon cluster, e.g., trimer, is a critical step in the growth mechanism of the CNT since it prevents further nucleation of the C atoms on the catalyst surface, and therefore inhibits the deactivation of the catalyst. The CNT growth rates predicted by the KMC simulations agree reasonably well with experimental data, verifying the proposed CNT growth mechanism and KMC model.

Origin and Nature of Spontaneous Shape Fluctuations in “Small” Nanoparticles

Normally chemically inert materials such as Au have been found to be catalytically active in the form of particles whose size is about 1 nm. Direct and indirect observations of various types of metal nanoparticles (NPs) in this size range, under catalytically relevant conditions for fuel-cell operation and catalysis, have indicated that such “small” particles can exhibit large spontaneous shape fluctuations and significant changes in shape and chemical activity in response to alterations in environmental conditions. NPs also normally exhibit facile coalescence when in proximity, impacting their stability and reactivity in applications. We perform molecular dynamics simulations on Ni nanoparticles, a commonly used NP in catalytic applications and carbon nanotube growth, in the ≈1 nm size regime where large-scale shape fluctuations have been observed experimentally. An analysis of the large-scale shape fluctuations observed in our simulations of these “small” NPs indicates that they are accompanied by collective motion of Ni atoms through the NP center, and we quantify these dynamic structures and their impact on NP shape. In contrast, stringlike collective atomic motion is confined to the NP interfacial region of NPs having a diameter greater than a few nanometers, and correspondingly, the overall NP shape remains roughly spherical, a case studied in our prior Ni NP simulations. Evidently, the large spontaneous NP shape fluctuations reflect a change in character of the collective atomic dynamics when the NPs become critically small in size.

Multiwalled carbon nanotube thin films prepared by aerosol deposition process for use as highly efficient Pt-free counter electrodes of dye-sensitized solar cells

In this study, transparent and catalytic multiwalled carbon nanotube (MWCNT) thin films with controlled thickness were fabricated using a simple and conventional multiple nozzle-based aerosol deposition process (ADP). MWCNTs were homogeneously dispersed in deionized water via surfactant treatment, and the MWCNT-dispersed aqueous solution was then aerosolized and deposited on fluorine-doped tin oxide glass substrates to form MWCNT thin films. The characteristics of MWCNTs, degree of MWCNT dispersion stability in the aqueous solution, and morphology and light transmittance of the prepared MWCNT thin films were systematically examined. Finally, the prepared MWCNT thin films were used as the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). For comparison purposes, the photovoltaic performance of DSSCs composed of MWCNT thin-film CEs was compared to that of reference DSSCs composed of conventional Pt thin-film CEs. The results showed that the power conversion efficiency of DSSCs composed of critical amount of MWCNTs coated on CEs was almost equal or slightly higher than that of conventional Pt-based DSSCs. Thus, it can be concluded that the ADP-assisted precisely controlled accumulation of transparent and catalytic MWCNT thin films on the CEs of DSSCs is a very promising approach for replacing the expensive Pt metal that is currently used in DSSC industries.