Direct Imaging of Carbon Nanotube Liquid-Crystalline Phase Development in True Solutions

Eco-Friendly Dispersant-Free Purification Method of Boron Nitride Nanotubes through Controlling Surface Tension and Steric Repulsion with Solvents

Boron nitride nanotubes (BNNTs) were purified without the use of a dispersant by controlling the surface tension and steric repulsion of solvent molecules. This method effectively enhanced the difference in solubilities of impurities and BNNTs. The purification process involved optimizing the alkyl-chains of alcohol solvents and adjusting the concentration of alcohol solvent in water to regulate surface tension and steric repulsion. Among the solvents tested, a 70 wt% t-butylalcohol in water mixture exhibited the highest selective isolation of BNNTs from impurities based on differences in solubilities. This favorable outcome was attributed to the surface tension matching with BNNTs, steric repulsion from bulky alkyl chain structures, and differences in interfacial energy between BNNT-liquid and impurity-liquid interfaces. Through this optimized purification process, impurities were removed to an extent of up to 93.3%. Additionally, the purified BNNTs exhibited a distinct liquid crystal phase, which was not observed in the unpurified BNNTs.

Lyotropic Boron Nitride Nanotube Liquid Crystals: Preparation, Characterization, and Wet-Spinning for Fabrication of Composite Fiber

Microfiber fabrication via wet-spinning of lyotropic liquid crystals (LCs) with anisotropic nanomaterials has gained increased attention due to the microfibers' excellent physical/chemical properties originating from the unidirectional alignment of anisotropic nanomaterials along the fiber axis with high packing density. For wet-spinning of the microfibers, however, preparing lyotropic LCs by achieving high colloidal stability of anisotropic nanomaterials, even at high concentrations, has been a critically unmet prerequisite, especially for recently emerging nanomaterials. Here, we propose a cationically charged polymeric stabilizer that can efficiently be adsorbed on the surface of boron nitride nanotubes (BNNTs), which provide steric hindrance in combination with Coulombic repulsion leading to high colloidal stability of BNNTs up to 22 wt %. The BNNT LCs prepared from the dispersions with various stabilizers were systematically compared using optical and rheological analysis to optimize the phase behavior and rheological properties for wet-spinning of the BNNT LCs. Systematic optical and mechanical characterizations of the BNNT microfibers with aligned BNNTs along the fiber axis revealed that properties of the microfibers, such as their tensile strength, packing density, and degree of BNNT alignment, were highly dependent on the quality of BNNT LCs directly related to the types of stabilizers.

Versatile acid solvents for pristine carbon nanotube assembly

Chlorosulfonic acid and oleum are ideal solvents for enabling the transformation of disordered carbon nanotubes (CNTs) into precise and highly functional morphologies. Currently, processing these solvents using extrusion techniques presents complications due to chemical compatibility, which constrain equipment and substrate material options. Here, we present a novel acid solvent system based on methanesulfonic or p-toluenesulfonic acids with low corrosivity, which form true solutions of CNTs at concentrations as high as 10 g/liter (≈0.7 volume %). The versatility of this solvent system is demonstrated by drop-in application to conventional manufacturing processes such as slot die coating, solution spinning continuous fibers, and 3D printing aerogels. Through continuous slot coating, we achieve state-of-the-art optoelectronic performance (83.6 %T and 14 ohm/sq) at industrially relevant production speeds. This work establishes practical and efficient means for scalable processing of CNT into advanced materials with properties suitable for a wide range of applications.

Aligned 2D carbon nanotube liquid crystals for wafer-scale electronics

Semiconducting carbon nanotubes promise faster performance and lower power consumption than Si in field-effect transistors (FETs) if they can be aligned in dense arrays. Here, we demonstrate that nanotubes collected at a liquid/liquid interface self-organize to form two-dimensional (2D) nematic liquid crystals that globally align with flow. The 2D liquid crystals are transferred onto substrates in a continuous process generating dense arrays of nanotubes aligned within ±6°, ideal for electronics. Nanotube ordering improves with increasing concentration and decreasing temperature due to the underlying liquid crystal phenomena. The excellent alignment and uniformity of the transferred assemblies enable FETs with exceptional on-state current density averaging 520 μA μm⁻¹at only −0.6 V, and variation of only 19%. FETs with ion gel top gates demonstrate subthreshold swing as low as 60 mV decade⁻¹. Deposition across a 10-cm substrate is achieved, evidencing the promise of 2D nanotube liquid crystals for commercial semiconductor electronics.

Enhanced ordering in length-polydisperse carbon nanotube solutions at high concentrations as revealed by small angle X-ray scattering

Carbon nanotubes (CNTs) are stiff, all-carbon macromolecules with diameters as small as one nanometer and few microns long. Solutions of CNTs in chlorosulfonic acid (CSA) follow the phase behavior of rigid rod polymers interacting via a repulsive potential and display a liquid crystalline phase at sufficiently high concentration. Here, we show that small-angle X-ray scattering and polarized light microscopy data can be combined to characterize quantitatively the morphology of liquid crystalline phases formed in CNT solutions at concentrations from 3 to 6.5% by volume. We find that upon increasing their concentration, CNTs self-assemble into a liquid crystalline phase with a pleated texture and with a large inter-particle spacing that could be indicative of a transition to higher-order liquid crystalline phases. We explain how thermal undulations of CNTs can enhance their electrostatic repulsion and increase their effective diameter by an order of magnitude. By calculating the critical concentration, where the mean amplitude of undulation of an unconstrained rod becomes comparable to the rod spacing, we find that thermal undulations start to affect steric forces at concentrations as low as the isotropic cloud point in CNT solutions.

Continuous Binder-Free Fibers of Pure Imogolite Nanotubes

Imogolite nanotubes (INTs) display a range of useful properties and provide an ideal material system to study the assembly of nanomaterials into macroscopic fibers. A method of wet spinning pure, binder-free imogolite fibers has been developed using double-walled germanium imogolite nanotubes. The nanotube aspect ratio can be controlled during the initial synthesis and is critical to the spinning process. Fibers made from short nanotubes (<100 nm) have very low gel strengths, while dopes with longer nanotubes (500-1000 nm) are readily spinnable. The tensile behavior of the resulting imogolite nanotube fibers is strongly influenced by relative humidity (RH), with a modulus of 30 GPa at 10% RH compared to 2.8 GPa at 85% RH, as well as a change in failure mode. This result highlights the importance of inter-nanotube interactions in such assemblies and provides a useful strategy for further exploration. Interestingly, in the absence of a matrix phase, a degree of misorientation appears to improve load transfer between the individual INTs within the porous fiber, likely due to an increase in the number of interparticle contacts. Imogolite nanotubes are an appealing analogue to other nanotube fiber systems, and it is hoped that learnings from this system can also be used to improve carbon nanotube fibers.

Electrical Properties Enhancement of Carbon Nanotube Yarns by Cyclic Loading

Carbon nanotube yarns (CNTYs) possess low density, high conductivity, high strength, and moderate flexibility. These intrinsic properties allow them to be a preferred choice for use as conductive elements in high-performance composites. To fully exploit their potential as conductive reinforcing elements, further improvement in their electrical conductivity is needed. This study demonstrates that tensile cyclic loading under ambient conditions improves the electrical conductivity of two types of CNTYs. The results showed that the electrical resistance of untreated CNTYs was reduced by 80% using cyclic loading, reaching the resistance value of the drawn acid-treated CNTYs. Scanning electron microscopy showed that cyclic loading caused orientation and compaction of the CNT bundles that make up the CNTYs, resulting in significantly improved electrical conductivity of the CNTYs. Furthermore, the elastic modulus was increased by 20% while preserving the tensile strength. This approach has the potential to replace the environmentally unfriendly acid treatment currently used to enhance the conductivity of CNTYs.

Thermodynamic stability condition can judge whether a nanoparticle dispersion can be considered a solution in a single phase

Establishing that a nanoparticle dispersion can, in fact, be treated as a solution has an important practical ramification, namely the application of solubility theories for solvent selection. However, what distinguishes a solution and dispersion has remained ambiguously understood. Based on the recent progress in statistical thermodynamics on multiple-component solutions, here we establish the condition upon which a nanoparticle dispersion can be considered a single-phased solution. We shall provide experimental evidence already found in the literature showing the solution nature of nanoparticle dispersions.

Effect of Carbon Nanotube Diameter and Stiffness on Their Phase Behavior in Crowded Solutions

The unique carbon nanotube (CNT) properties are mainly determined by their geometry, e.g., their aspect ratio, diameter, and number of walls. So far, chlorosulfonic acid is the only practical true solvent for carbon nanotubes, forming thermodynamically stable molecular solutions. Above a critical concentration the system forms an ordered, nematic liquid-crystalline phase. That phase behavior is the basis for liquid-phase processing and the optimal translation of the carbon nanotube molecular properties to the macroscopic scale. The final material properties depend on the phase behavior of the "dope" from which it is prepared, which depends on the CNT parameters themselves. Earlier work determined that CNT aspect ratio controls the phase behavior, in accordance with classical rigid-rod theories. Here we use cryogenic transmission electron microscopy and Raman spectroscopy to understand the relation between the geometry of the CNTs, the chemical interaction with chlorosulfonic acid, and the phase behavior of crowded solutions. We show that the CNT diameter and number of walls also play an independent role in the phase transition and phase morphology of the system because of their effect on the CNT bending stiffness.

The Synthesis and Properties of Liquid Crystalline Polyurethanes, Chemically Modified by Polyhedral Oligomericsilsesquioxanes

In this work, we report for the first time on the influence of polyhedral oligomericsilsesquioxanes (POSS) on the structure and properties of liquid crystalline polyurethane (LCPU). LCPU/POSS hybrids were synthesized via a two-step method. In the first step, 4,4′-methylenephenyl diisocyanate (MDI) and polytetramethylene ether glycol (PTMG) reacted with functionalized trisilanolphenyl POSS (TSP-POSS) bearing three hydroxyl groups. In the second step, the growing chain was extended with 4,4′-bis(hydroxyhexoxy)biphenyl (BHHBP). FTIR measurements confirmed the chemical bonding between the POSS and LCPU matrix and showed the influence of the silsesquioxane modification on the intensity of hydrogen bonds. The DSC and POM techniques confirmed the formation of liquid crystalline phases. The incorporation of silsesquixanes into the LC matrix leads to higher melting and isotropization temperatures along with the broadening phase transition effect. Scanning electron microscopy showed a good distribution of POSS moieties, both in the bulk and on the surface of the liquid crystalline PU matrix, whereby wide-angle X-ray diffraction (WAXD) patterns revealed halos from both the liquid crystalline and unmodified polyurethane matrix. The stress at the breaking points for LCPU/POSS hybrids containing 50% and 60% of elastic segments is greater than the stress at the breaking point of the reference material (LCPU), what is due to good dispersion of POSS in less elastic matrix. Thermal properties of the LCPU/POSS materials obtained, determined by TGA, revealed that the char residue increased with the amount of POSS for 40% of elastic segments materials.

Next-Generation Multifunctional Carbon-Metal Nanohybrids for Energy and Environmental Applications

Nanotechnology has unprecedentedly revolutionized human societies over the past decades and will continue to advance our broad societal goals in the coming decades. The research, development, and particularly the application of engineered nanomaterials have shifted the focus from "less efficient" single-component nanomaterials toward "superior-performance", next-generation multifunctional nanohybrids. Carbon nanomaterials (e.g., carbon nanotubes, graphene family nanomaterials, carbon dots, and graphitic carbon nitride) and metal/metal oxide nanoparticles (e.g., Ag, Au, CdS, Cu₂O, MoS₂, TiO₂, and ZnO) combinations are the most commonly pursued nanohybrids (carbon-metal nanohybrids; CMNHs), which exhibit appealing properties and promising multifunctionalities for addressing multiple complex challenges faced by humanity at the critical energy-water-environment (EWE) nexus. In this frontier review, we first highlight the altered and newly emerging properties (e.g., electronic and optical attributes, particle size, shape, morphology, crystallinity, dimensionality, carbon/metal ratio, and hybridization mode) of CMNHs that are distinct from those of their parent component materials. We then illustrate how these important newly emerging properties and functions of CMNHs direct their performances at the EWE nexus including energy harvesting (e.g., H₂O splitting and CO₂ conversion), water treatment (e.g., contaminant removal and membrane technology), and environmental sensing and in situ nanoremediation. This review concludes with identifications of critical knowledge gaps and future research directions for maximizing the benefits of next-generation multifunctional CMNHs at the EWE nexus and beyond.

Macroscopic Freestanding Nanosheets with Exceptionally High Modulus

Macroscopic single-wall carbon nanotube (SWCNT) films of nanoscale thickness have significant potential for an array of applications that demand thin, transparent, conductive coatings. Using macroscopic micrometer thick polystyrene sheets as a reference, we characterize the elastic response of freestanding multifunctional SWCNT nanosheets possessing both exceptionally high Young's modulus and good durability. Thin SWCNT films (20-200 nm thick) asymmetrically "doped" with dilute concentrations of superparamagnetic colloids were suspended in ethanol as freestanding nanosheets. Through repeated and controlled deformation in an external magnetic field, we measure the temporal relaxation of nanosheet curvature back to equilibrium. From the relaxation time and its dependence on nanosheet thickness and length, we extract the SWCNT nanosheet modulus through a simple viscoelastic model. Our results are consistent with nearly ideal SWCNT rigidity percolation with moduli approaching 200 GPa and limited plasticity for sufficiently thick sheets, which we attribute to the screening of van der Waals interactions by the surrounding solvent and the macroscopic nature of the deformation.

Liquid crystallinity of carbon nanotubes

In this review, we first briefly recapitulate the orientation characteristics of liquid crystalline carbon nanotubes (CNTs), emphasizing their inherent properties. Both the high Young's modulus and the strong attractive interaction between them make the liquid crystallinity apt to show splay deformations (splay defects). It is these defects that often produce apparent low-order structures for long and deformed nanotubes. However, the application of doping, shearing, magnetic or electric fields will be efficient routes toward highly ordered CNT assemblies from such defects. Then, we describe the electrical behavior of CNTs in the electric field, which combines desirable features of the CNTS with those of classical liquid crystals (LCs). An electric field will generate an induced dipole moment on CNTs and align them in the field direction, minimizing the dipolar energy. Finally, we review the potential application of CNTs in the area of liquid crystal displays (LCD). In the LC cell unit, CNTs as dopants in LC layers can have compatible stability with LCs, with the orientation consistent and with surprising complementary advantages. And also CNT films as nanostructured electrodes can substitute ITO electrodes in the LC cell unit, exhibiting a strong electrical anisotropy due to their excellent axial conductivity. Furthermore, CNT films as an alignment layer have the potential to replace the traditional PI film, aligning LC molecules effectively along the direction of the nanotubes. Besides, CNTs acting as polarizers can absorb or transmit incident light when the electric vector propagates parallel or perpendicular to the nanotube axis. All of these applications demonstrate that CNTs in LC ordering will effectively improve the performance of materials and their related devices. Thus, we should improve the ordering of CNT assemblies as far as possible, which is critical to make full use of their exceptional axial properties and further to develop novel materials and applications successfully.

Quantification of Carbon Nanotube Liquid Crystal Morphology via Neutron Scattering

Liquid phase assembly is among the most industrially attractive routes for scalable carbon nanotube (CNT) processing. Chlorosulfonic acid (CSA) is known to be an ideal solvent for CNTs, spontaneously dissolving them without compromising their properties. At typical processing concentrations, CNTs form liquid crystals in CSA; however, the morphology of these phases and their concentration dependence are only qualitatively understood. Here, we use small-angle neutron scattering (SANS), combined with polarized light microscopy and cryogenic transmission electron microscopy to study solution morphology over a range of concentrations and two different CNT lengths. Our results show that at the highest concentration studied the long CNTs form a highly ordered fully nematic phase, while short CNTs remain in a biphasic regime. Upon dilution, long CNTs undergo a 2D lattice expansion, whereas short CNTs seem to have an intermediate expansion between 2D and 3D probably due to the biphasic nature of the system. The average spacing between the CNTs scaled by the CNT diameter is the same in both systems, as expected for infinitely long aligned rods.

Dissolution and Characterization of Boron Nitride Nanotubes in Superacid

Boron nitride nanotubes (BNNTs) are of interest for their unique combination of high tensile strength, high electrical resistivity, high neutron cross section, and low reactivity. The fastest route to employing these properties in composites and macroscopic articles is through solution processing. However, dispersing BNNTs without functionalization or use of a surfactant is challenging. We show here by cryogenic transmission electron microscopy that BNNTs spontaneously dissolve in chlorosulfonic acid as disentangled individual molecules. Electron energy loss spectroscopy of BNNTs dried from the solution confirms preservation of the sp² hybridization for boron and nitrogen, eliminating the possibility of BNNT functionalization or damage. The length and diameter of the BNNTs was statistically calculated to be ∼4.5 μm and ∼4 nm, respectively. Interestingly, bent or otherwise damaged BNNTs are filled by chlorosulfonic acid. Additionally, nanometer-sized synthesis byproducts, including boron nitride clusters, isolated single and multilayer hexagonal boron nitride, and boron particles, were identified. Dissolution in superacid provides a route for solution processing BNNTs without altering their chemical structure.

Influence of Carbon Nanotube Characteristics on Macroscopic Fiber Properties

We study how intrinsic parameters of carbon nanotube (CNT) samples affect the properties of macroscopic CNT fibers with optimized structure. We measure CNT diameter, number of walls, aspect ratio, graphitic character, and purity (residual catalyst and non-CNT carbon) in samples from 19 suppliers; we process the highest quality CNT samples into aligned, densely packed fibers, by using an established wet-spinning solution process. We find that fiber properties are mainly controlled by CNT aspect ratio and that sample purity is important for effective spinning. Properties appear largely unaffected by CNT diameter, number of walls, and graphitic character (determined by Raman G/D ratio) as long as the fibers comprise thin few-walled CNTs with high G/D ratio (above ∼20). We show that both strength and conductivity can be improved simultaneously by assembling high aspect ratio CNTs, producing continuous CNT fibers with an average tensile strength of 2.4 GPa and a room temperature electrical conductivity of 8.5 MS/m, ∼2 times higher than the highest reported literature value (∼15% of copper's value), obtained without postspinning doping. This understanding of the relationship of intrinsic CNT parameters to macroscopic fiber properties is key to guiding CNT synthesis and continued improvement of fiber properties, paving the way for CNT fiber introduction in large-scale aerospace, consumer electronics, and textile applications.

Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals

Thin bilayers of contrasting nanomaterials are ubiquitous in solution-processed electronic devices and have potential relevance to a number of applications in flexible electronics. Motivated by recent mesoscopic simulations demonstrating synergistic mechanical interactions between thin films of single-wall carbon nanotubes (SWCNTs) and spherical nanocrystal (NC) inclusions, we use a thin-film wrinkling approach to query the compressive mechanics of hybrid nanotube/nanocrystal coatings adhered to soft polymer substrates. Our results show an almost 2-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of a thin interpenetrating overlayer of semiconductor NCs. Mesoscopic distinct-element method simulations further support the experimental findings by showing that the additional noncovalent interfaces introduced by nanocrystals enhance the modulus of the SWCNT network and hinder network wrinkling.