Carbon Nanotubes: An Example of Multiscale Development-A Mechanistic View from the Subnanometer to the Meter Scale

Common Defects Accelerate Charge Separation and Reduce Recombination in CNT/Molecule Composites: Atomistic Quantum Dynamics

Carbon nanotubes (CNTs) are appealing candidates for solar and optoelectronic applications. Traditionally used as electron sinks, CNTs can also perform as electron donors, as exemplified by coupling with perylenediimide (PDI). To achieve high efficiencies, electron transfer (ET) should be fast, while subsequent charge recombination should be slow. Typically, defects are considered detrimental to material performance because they accelerate charge and energy losses. We demonstrate that, surprisingly, common CNT defects improve rather than deteriorate the performance. CNTs and other low dimensional materials accommodate moderate defects without creating deep traps. At the same time, charge redistribution caused by CNT defects creates an additional electrostatic potential that increases the CNT work function and lowers CNT energy levels relative to those of the acceptor species. Hence, the energy gap for the ET is decreased, while the gap for the charge recombination is increased. The effect is particularly important because charge acceptors tend to bind near defects due to enhanced chemical interactions. The time-domain simulation of the excited-state dynamics provides an atomistic picture of the observed phenomenon and characterizes in detail the electronic states, vibrational motions, inelastic and elastic electron-phonon interactions, and time scales of the charge separation and recombination processes. The findings should apply generally to low-dimensional materials, because they dissipate defect strain better than bulk semiconductors. Our calculations reveal that CNT performance is robust to common defects and that moderate defects are essential rather than detrimental for CNT application in energy, electronics, and related fields.

Measurement of Flexural Rigidity of Multi-Walled Carbon Nanotubes by Dynamic Scanning Electron Microscopy

In this work the flexural rigidity of individual large diameter multi-walled carbon nanotubes (MWCNTs) was investigated. The bending modulus were obtained by detecting the resonance frequencies of mechanically excited cantilevered carbon nanotubes using the so-called dynamic scanning electron microscopy technique, and applying the Euler–Bernoulli beam theory. For the nanotubes studied, we determined a modulus of up to 160 GPa. This agrees with values reported by other authors for MWCNTs produced by catalytic chemical vapor deposition, however, it is 6-8 times smaller than values reported for single and multi-walled carbon nanotubes produced by arc-discharge synthesis. Toxicological studies with carbon nanotubes have been showing that inhaled airborne nanofibers that reach the deep airways of the respiratory system may lead to serious, asbestos-like lung diseases. These studies suggested that their toxicity critically depends on the fiber flexural rigidity, with high rigidity causing cell lesions. To complement the correlation between observed toxicological effects and fiber rigidities, reliable and routinely applicable measurement techniques for the flexural rigidity of nanofibers are required.

Spray-Drying-Assisted Layer-by-Layer Assembly of Alginate, 3-Aminopropyltriethoxysilane, and Magnesium Hydroxide Flame Retardant and Its Catalytic Graphitization in Ethylene-Vinyl Acetate Resin

Alginates (nickel alginate, NiA; copper alginate, CuA; zinc alginate, ZnA) and 3-aminopropyltriethoxysilane (APTES) were alternately deposited on a magnesium hydroxide (MH) surface by the spray-drying-assisted layer-by-layer assembly technique, fabricating some efficient and environmentally benign flame retardants (M-FR, including Ni-FR, Cu-FR, and Zn-FR). The morphology, chemical compositions, and structures of M-FR were investigated. With 50 wt % loading, compared with EVA28+MH, the peak heat release rate, smoke production rate, and CO production rate of EVA28+Ni-FR decreased by 50.78%, 61.76%, and 66.67%, respectively. The metals or metal oxide nanoparticles arising from alginates could catalyze the pyrolysis intermediates of EVA into graphene and amorphous carbon, which could bind the inorganic compounds (the decomposition products of MH and APTES) together and form some more protective barriers. For each M-FR, the flame retardant and smoke suppression efficiency were different, which were caused by the diverse carbonization and graphitization behaviors of three alginates. ZnA generated some ZnO aggregations and could not catalyze the graphitization of intermediates. For CuA, the catalytic graphitization was limited by the tightly binding graphene layer. As for NiA, the configuration of the Ni atom could not provide strong binding of Ni substrate and carbon. The liquid-like Ni nanoparticles could restructure and get out from firm graphene shells, so the catalytic graphitization of NiA was efficient and sustainable. This work displayed the catalytic graphitization mechanism of alginates while exploring a simple and novel strategy for fabricating efficient green flame retardants.

Facile synthesis of ultrathin two-dimensional nanosheets-constructed MCo2O4 (M = Ni, Cu, Zn) nanotubes for efficient photocatalytic oxygen evolution

Hierarchical nanosheets-assembled nanotubes are of great interest for their unique physicochemical properties as well as their potential applications in a variety of fields. However, the synthesis of hierarchical mixed transition-metal oxides-based nanosheets-assembled nanotubes for highly efficient photocatalytic oxygen evolution is rarely reported. Herein, a simple and versatile approach was developed to synthesize hierarchical nanosheets-constructed MCo₂O₄ (M = Ni, Cu, Zn) nanotubes. Core-shell polyacrylonitrile (PAN)/M-cobalt hydroxide ultrathin nanosheet composite nanofibers were first synthesized by hydroxylation reaction between PAN/M-cobalt acetate hydroxide precursors and NaBH₄. After calcination, these nanofibers as precursors were easily transformed into the corresponding hierarchical nanosheets-constructed MCo₂O₄ nanotubes. By applying a photocatalyst, the resultant MCo₂O₄ nanotubes, particularly the CuCo₂O₄ nanotubes, exhibited high photocatalytic activity and cycle stability toward water oxidation reaction with O₂ generation rates of 51.1 mmol g^-1 h^-1 under visible light irradiation, which is higher than most reported catalysts. This approach is very versatile and can be applied to synthesize other hierarchical multi-element oxides-based nanosheets-constructed nanotubes for advanced applications.

Discriminating between Metallic and Semiconducting Single-Walled Carbon Nanotubes Using Physisorbed Adsorbates: Role of Wavelike Charge-Density Fluctuations

Discriminating between metallic (M) and semiconducting (S) single-walled carbon nanotubes (SWNTs) remains a fundamental challenge in the field of nanotechnology. We address this issue by studying the adsorption of the isotropic atoms Xe, Kr, and a highly anisotropic molecule n heptane on M- and S-SWNTs with density functional theory that includes many-body dispersion forces. We find that the distinct polarizabilities of M- and S-SWNTs exhibit significantly different physisorption properties, which are also strongly controlled by the SWNT's diameter, adsorption site, adsorbate coverage, and the adsorbate's anisotropy. These findings stem from the wavelike nature of charge-density fluctuations in SWNTs. Particularly, these results allow us to rationalize the unusual sqrt[3]×sqrt[3]R30^{0} phase of Kr atoms on small gap M-SWNTs and the double desorption peak temperatures of n heptane on M-SWNTs in experiments, and also propose the n heptane as an effective sensor for experimentally discriminating M- and S-SWNTs.

Macroscopic Carbon Nanotube-based 3D Monoliths

Carbon nanotubes (CNTs) are one of the most promising carbon allotropes with incredible diverse physicochemical properties, thereby enjoying continuous worldwide attention since their discovery about two decades ago. From the point of view of practical applications, assembling individual CNTs into macroscopic functional and high-performance materials is of paramount importance. For example, multiscaled CNT-based assemblies including 1D fibers, 2D films, and 3D monoliths have been developed. Among all of these, monolithic 3D CNT architectures with porous structures have attracted increasing interest in the last few years. In this form, theoretically all individual CNTs are well connected and fully expose their surfaces. These 3D architectures have huge specific surface areas, hierarchical pores, and interconnected conductive networks, resulting in enhanced mass/electron transport and countless accessible active sites for diverse applications (e.g. catalysis, capacitors, and sorption). More importantly, the monolithic form of 3D CNT assemblies can impart additional application potentials to materials, such as free-standing electrodes, sensors, and recyclable sorbents. However, scaling the properties of individual CNTs to 3D assemblies, improving use of the diverse, structure-dependent properties of CNTs, and increasing the performance-to-cost ratio are great unsolved challenges for their real commercialization. This review aims to provide a comprehensive introduction of this young and energetic field, i.e., CNT-based 3D monoliths, with a focus on the preparation principles, current synthetic methods, and typical applications. Opportunities and challenges in this field are also presented.

Electrocatalytic H2 production from seawater over Co, N-codoped nanocarbons

One of the main barriers blocking sustainable hydrogen production is the use of expensive platinum-based catalysts to produce hydrogen from water. Herein we report the cost-effective synthesis of catalytically active, nitrogen-doped, cobalt-encased carbon nanotubes using inexpensive starting materials-urea and cobalt chloride hexahydrate (CoCl2·6H2O). Moreover, we show that the as-obtained nanocarbon material exhibits a remarkable electrocatalytic activity toward the hydrogen evolution reaction (HER); and thus it can be deemed as a potential alternative to noble metal HER catalysts. In particular, the urea-derived carbon nanotubes synthesized at 900 °C (denoted as U-CNT-900) show a superior catalytic activity for HER with low overpotential and high current density in our study. Notably also, U-CNT-900 has the ability to operate stably at all pH values (pH 0-14), and even in buffered seawater (pH 7). The possible synergistic effects between carbon-coated cobalt nanoparticles and the nitrogen dopants can be proposed to account for the HER catalytic activity of U-CNT-900. Given the high natural abundance, ease of synthesis, and high catalytic activity and durability in seawater, this U-CNT-900 material is promising for hydrogen production from water in industrial applications.

Ni-promoted synthesis of graphitic carbon nanotubes from in situ produced graphitic carbon for dehydrogenation of ethylbenzene

Ni promoted the synthesis of graphitic carbon nanotubes from in situ produced graphitic carbon and exhibited excellent activity in dehydrogenation of ethylbenzene.

A novel strategy to synthesize hierarchical, porous carbohydrate-derived carbon with tunable properties

Hydrothermal carbonization (HTC) of carbohydrate is an interesting candidate for the preparation of carbon materials, as it provides an easy, inexpensive and environmental friendly route. However, it is difficult to prepare porous carbon materials by a straight HTC process. Herein, the solubilising technology of micelles was introduced to direct the HTC of fructose by using an amphiphilic block copolymer, poly-(4-vinylpyridine)-block-poly-(ethylene glycol) (P4VP-PEG), as a structure-directing agent. By this strategy, hierarchical porous carbon materials with tunable properties were prepared. It was found that P4VP-PEG micelles could solubilize fructose and confine the formation of primary carbon domains during a sol-gel process. And the micelle size could be adjusted easily by changing the preparation conditions. Accordingly, the particle size of the obtained carbon materials was effectively tuned from 20 to 100 nm by the direction of the primary micelle size. After calcination, the hierarchical porous carbon materials were evidenced as effective electrode materials for supercapacitor with a capacitance of ∼197 F at 1 A g(-1), which was almost four times higher than the carbon materials prepared by a straight HTC process.

Characterizing the chiral index of a single-walled carbon nanotube

The properties of single-walled carbon nanotubes (SWCNTs) mainly depend on their geometry. However, there are still formidable difficulties to determine the chirality of SWCNTs accurately. In this review, some efficient methods to characterize the chiral indices of SWCNTs are illuminated. These methods are divided into imaging techniques and spectroscopy techniques. With these methods, diameter, helix angle, and energy states can be measured. Generally speaking, imaging techniques have a higher accuracy and universality, but are time-consuming with regard to the sample preparation and characterization. The spectroscopy techniques are very simple and fast in operation, but these techniques can be applied only to the particular structure of the sample. Here, the principles and operations of each method are introduced, and a comprehensive understanding of each technique, including their advantages and disadvantages, is given. Advanced applications of some methods are also discussed. The aim of this review is to help readers to choose methods with the appropriate accuracy and time complexity and, furthermore, to put forward an idea to find new methods for chirality characterization.

Emerging double helical nanostructures

As one of the most important and land-mark structures found in nature, a double helix consists of two congruent single helices with the same axis or a translation along the axis. This double helical structure renders the deoxyribonucleic acid (DNA) the crucial biomolecule in evolution and metabolism. DNA-like double helical nanostructures are probably the most fantastic yet ubiquitous geometry at the nanoscale level, which are expected to exhibit exceptional and even rather different properties due to the unique organization of the two single helices and their synergistic effect. The organization of nanomaterials into double helical structures is an emerging hot topic for nanomaterials science due to their promising exceptional unique properties and applications. This review focuses on the state-of-the-art research progress for the fabrication of double-helical nanostructures based on 'bottom-up' and 'top-down' strategies. The relevant nanoscale, mesoscale, and macroscopic scale fabrication methods, as well as the properties of the double helical nanostructures are included. Critical perspectives are devoted to the synthesis principles and potential applications in this emerging research area. A multidisciplinary approach from the scope of nanoscience, physics, chemistry, materials, engineering, and other application areas is still required to the well-controlled and large-scale synthesis, mechanism, property, and application exploration of double helical nanostructures.

High yield, controlled synthesis of graphitic networks from dense micro emulsions

We propose a new synthesis method to produce hyper-branched carbon nano structures that we call carbon nano networks. These porous, graphitic materials directly grow into a networked structure, do not require the use of an inorganic support, and can be tailored by experimental conditions to better suit their application.