Nanotechnology: 'buckypaper' from coaxial nanotubes

A Double-walled Noncovalent Carbon Nanotube by Columnar Packing of Nanotube Fragments

Herein, we report the synthesis of double-walled noncovalent carbon nanotubes (CNTs) through host-guest complexation of nanotube fragments and tube-forming crystal engineering. As the smallest fragment of double-walled CNTs, a host-guest complex of perfluorocycloparaphenylene (PFCPP) and carbon nanobelt (CNB) was synthesized by mixing them in solvents. The immediate complexation of the PF[12]CPP⸧(6,6)CNB complex with a remarkably high association constant (Ka) of 2×105 L/mol was observed. Time-dependent 1H NMR and thermogravimetry measurements revealed that the stability of (6,6)CNB was significantly improved by encapsulation. X-ray crystallography confirmed the robust belt-in-ring structure of this complex. As indicated by the short distance between PF[12]CPP and (6,6)CNB (2.8 Å), intermolecular orbital interactions exist between the belt and the ring, which were further supported by theoretical calculation and phosphorescence quenching experiments. While the PF[12]CPP⸧(6,6)CNB complex adopts various crystal packing structures, chloroform was discovered to be a magic "glue" solvent inducing one-dimensional alignment of the PF[12]CPP⸧(6,6)CNB complex to build an unprecedented double-walled noncovalent CNT structure.

A single carbon nanotube-entangled high-performance buckypaper with tunable fracture mode

It is well known that the traditional buckypaper (BP) is composed of a certain number of short carbon nanotubes (CNTs) intertwined with each other and sliding always happens when the BP is under tensile and impact loading, which results in inferior mechanical properties compared to single CNTs. In this work, a highly-entangled single-wire BP (SWBP) structure is constructed by a modified self-avoiding random walk approach. The in-plane mechanical properties and impacting behaviors of the SWBPs with different entanglement degrees and interface frictions are systematically investigated via newly developed coarse-grained molecular dynamics (CGMD) simulation. A coarse-grained method can effectively reflect the inter-tube van der Waals (vdW) interactions and the mechanical behaviors of CNTs, including tension, bending and adhesion. In this work, from the tensile simulations of the SWBP, the results showed that the self-locking mechanism between entangled CNTs could significantly enhance the tensile resistance of the film. Besides, the mechanical properties of the SWBP are highly dependent on the entanglement degree and the interface friction between CNTs. Furthermore, two distinct fracture modes, ductile fracture and brittle fracture, are revealed, which can be efficiently controlled by changing the related friction between CNTs. From the impacting simulations, it is found that the impacting performance can be effectively tuned by adjusting the entanglement degree of the film. In addition, the kinetic energy of the projectile could be rapidly dissipated through the stretching and bending of CNTs in the SWBP. This work provides an in-depth understanding of the effect of interface friction and entanglement degree on the mechanical properties of the buckypaper and provides a reference for the preparation of strong CNT-based micromaterials.

Submillimeter-Long WS2 Nanotubes: The Pathway to Inorganic Buckypaper

WS2 nanotubes present many new technologies under development, including reinforced biocompatible polymers, membranes, photovoltaic-based memories, ferroelectric devices, etc. These technologies depend on the aspect ratio (length/diameter) of the nanotubes, which was limited to 100 or so. A new synthetic technique is presented, resulting in WS2 nanotubes a few hundred micrometers long and diameters below 50 nm (aspect ratios of 2000-5000) in high yields. Preliminary investigation into the mechanistic aspects of the two-step synthesis reveals that W5O14 nanowhisker intermediates are formed in the first step of the reaction instead of the ubiquitous W18O49 nanowhiskers used in the previous syntheses. The electrical and photoluminescence properties of the long nanotubes were studied. WS2 nanotube-based paper-like material was prepared via a wet-laying process, which could not be realized with the 10 μm long WS2 nanotubes. Ultrafiltration of gold nanoparticles using the nanotube-paper membrane was demonstrated.

Synthesis and Characterization of Carbon Microbeads

We report a microfluidic-based droplet generation platform for synthesizing micron-sized porous carbon microspheres. The setup employs carbon materials such as graphite, carbon nanotubes, graphene, fullerenes, and carbon black as starting materials. Custom composition, structure, and function are achieved through combinations of carbon materials, cross-linkers, and additives along with variations in process parameters. Carbon materials can be assembled into spheres with a mean diameter of units to hundreds of μm with relatively tight size distribution (<25% RSD). Pore structure and size (tens to hundreds of angstrom) can be modulated by incorporating porogen/coporogen dilutants during synthesis. The microbeads have excellent mechanical stability with an elastic modulus of hundreds of MPa. They can sustain high dynamic fluid flow pressures of up to 9000 psi. This work lays the foundation for synthesizing novel tailorable and customizable carbon microbeads. It opens avenues for applying these novel materials for composite and additive manufacturing, energy, life science, and biomedical applications.

Thermal Conductivity of Carbon/Boron Nitride Heteronanotube and Boron Nitride Nanotube Buckypapers: Implications for Thermal Management Composites

To date, there has been limited reporting on the fabrication and properties of macroscopic sheet assemblies (specifically buckypapers) composed of carbon/boron nitride core-shell heteronanotubes (MWCNT@BNNT) or boron nitride nanotubes (BNNTs). Herein we report the synthesis of MWCNT@BNNTs via a facile method involving Atmospheric Pressure Chemical Vapor Deposition (APCVD) and the safe h-BN precursor ammonia borane. These MWCNT@BNNTs were used as sacrificial templates for BNNT synthesis by thermal oxidation of the core carbon. Buckypaper fabrication was facilitated by facile sonication and filtration steps. To test the thermal conductivity properties of these new buckypapers, in the interest of thermal management applications, we have developed a novel technique of advanced scanning thermal microscopy (SThM) that we call piercing SThM (pSThM). Our measurements show a 14% increase in thermal conductivity of the MWCNT@BNNT buckypaper relative to a control multiwalled carbon nanotube (MWCNT) buckypaper. Meanwhile, our BNNT buckypaper exhibited approximately half the thermal conductivity of the MWCNT control, which we attribute to the turbostratic quality of our BNNTs. To the best of our knowledge, this work achieves the first thermal conductivity measurement of a MWCNT@BNNT buckypaper and of a BNNT buckypaper composed of BNNTs not synthesized by high energy techniques.

Development and Optimization of Water-Soluble Double-Walled Carbon Nanotubes by Effective Surface Treatment of Inner Walls

Carbon nanotubes are a significant class of nanomaterials with distinctive properties that have led to their application in a variety of fields, such as polymer composites, medicine, electronics, and material science. However, their nonpolar nature and insolubility in polar solvents limit their applications. To address this issue, highly functionalized and water-soluble double-walled carbon nanotubes (DWNTs) were developed by selectively oxidizing the inner walls of the DWNTs using oleum and nitric acid. The impact of reaction time on the chemical functionalization of DWNTs was investigated under two different reaction durations of 2 and 24 h. The presence of highly oxygenated functional groups resulted in high water solubility, which was confirmed by high- and low-frequency Raman spectroscopy, high-resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) method, and optical spectroscopy. The conductivity of highly water-soluble W-DWNTs (24 h) was 122.65 × 10² S cm^-1. After annealing for 12 h at 140 °C, the W-DWNTs retained 72% of their conductivity (88.79 × 10² S cm^-1).

Ultrafast growth of carbon nanotubes using microwave irradiation: characterization and its potential applications

Carbon nanotubes (CNTs) have been studied for more than twenty-five years due to their distinguishing features such as high tensile strength, high elastic module, high surface area, high thermal and electrical conductivity, making them ideal for a variety of applications. Nanotechnology and nanoscience researchers are working to develop CNTs with appropriate properties for possible future applications. New methodologies for their synthesis are clearly needed to be developed and refined. In this research, the authors look at the history and the recent developments of carbon nanotubes synthesis methods for CNTs, such as arc discharge, laser ablation, chemical vapour deposition and microwave irradiation. New immerging methods like microwave irradiation for the growth of CNTs and their composite was extensively reviewed. Low temperature and ultrafast growth of CNT through microwave irradiation technique were examined and discussed. In addition, all the techniques used for the CNTs characterization were also briefly discussed. Special attention was dedicated to the application of CNTs. This review has extensively explored future applications in the biomedical sector, industrial water purifications, CNTs composites, energy and storage devices.

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Synthesis of carbon nanotubes using different methods.

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Microwave irradiation techniques are used for the growth of CNTs.

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Current challenge and future aspects of CNTs growth.

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Detailed characterization and application of CNTs.

Electrical and Electrochemical Sensors Based on Carbon Nanotubes for the Monitoring of Chemicals in Water-A Review

Carbon nanotubes (CNTs) combine high electrical conductivity with high surface area and chemical stability, which makes them very promising for chemical sensing. While water quality monitoring has particularly strong societal and environmental impacts, a lot of critical sensing needs remain unmet by commercial technologies. In the present review, we show across 20 water monitoring analytes and 90 references that carbon nanotube-based electrochemical sensors, chemistors and field-effect transistors (chemFET) can meet these needs. A set of 126 additional references provide context and supporting information. After introducing water quality monitoring challenges, the general operation and fabrication principles of CNT water quality sensors are summarized. They are sorted by target analytes (pH, micronutrients and metal ions, nitrogen, hardness, dissolved oxygen, disinfectants, sulfur and miscellaneous) and compared in terms of performances (limit of detection, sensitivity and detection range) and functionalization strategies. For each analyte, the references with best performances are discussed. Overall, the most frequently investigated analytes are H+ (pH) and lead (with 18% of references each), then cadmium (14%) and nitrite (11%). Micronutrients and toxic metals cover 40% of all references. Electrochemical sensors (73%) have been more investigated than chemistors (14%) or FETs (12%). Limits of detection in the ppt range have been reached, for instance Cu(II) detection with a liquid-gated chemFET using SWCNT functionalized with peptide-enhanced polyaniline or Pb(II) detection with stripping voltammetry using MWCNT functionalized with ionic liquid-dithizone based bucky-gel. The large majority of reports address functionalized CNTs (82%) instead of pristine or carboxyl-functionalized CNTs. For analytes where comparison is possible, FET-based and electrochemical transduction yield better performances than chemistors (Cu(II), Hg(II), Ca(II), H2O2); non-functionalized CNTs may yield better performances than functionalized ones (Zn(II), pH and chlorine).

Treatment technologies to mitigate the harmful effects of recalcitrant fluoroquinolone antibiotics on the environ- ment and human health

Antibiotic proliferation in the environment and their persistent nature is an issue of global concern as they induce antibiotic resistance threatening both human health and the ecosystem. Antibiotics have therefore been categorized as emerging pollutants. Fluoroquinolone (FQs) antibiotics are an emerging class of contaminants that are used extensively in human and veterinary medicine. The recalcitrant nature of fluoroquinolones has led to their presence in wastewater, effluents and water bodies. Even at a low concentration, FQs can stimulate antibacterial resistance. The main sources of FQ contamination include waste from pharmaceutical manufacturing industries, hospitals and households that ultimately reaches the wastewater treatment plants (WWTPs). The conventional WWTPs are unable to completely remove FQs due to their chemical stability. Therefore, the development and implementation of more efficient, economical, convenient treatment and removal technologies are needed to adequately address the issue. This review provides an overview of the technologies available for the removal of fluoroquinolone antibiotics from wastewater including adsorptive removal, advanced oxidation processes, removal using non-carbon based nanomaterials, microbial degradation and enzymatic degradation. Each treatment technology is discussed on its merits and limitations and a comparative view is presented on the choice of an advanced treatment process for future studies and implementation. A discussion on the commercialization potential and eco-friendliness of each technology is also included in the review. The importance of metabolite identification and their residual toxicity determination has been emphasized. The last section of the review provides an overview of the policy interventions and regulatory frameworks that aid in retrofitting antibiotics as a central key focus contaminant and thereby defining the discharge limits for antibiotics and establishing safe manufacturing practices.

Nanotube-Based 1D Heterostructures Coupled by van der Waals Forces

1D van der Waals heterostructures based on carbon nanotube templates are raising a lot of excitement due to the possibility of creating new optical and electronic properties, by either confining molecules inside their hollow core or by adding layers on the outside of the nanotube. In contrast to their 2D analogs, where the number of layers, atomic type and relative orientation of the constituting layers are the main parameters defining physical properties, 1D heterostructures provide an additional degree of freedom, i.e., their specific diameter and chiral structure, for engineering their characteristics. The current state-of-the-art in synthesizing 1D heterostructures are discussed here, in particular focusing on their resulting optical properties, and details the vast parameter space that can be used to design heterostructures with custom-built properties that can be integrated into a large variety of applications. First, the effects of van der Waals coupling on the properties of the simplest and best-studied 1D heterostructure, namely a double-walled carbon nanotube, are described, and then heterostructures built from the inside and the outside are considered, which all use a nanotube as a template, and, finally, an outlook is provided for the future of this research field.

A comprehensive review on magnetic carbon nanotubes and carbon nanotube-based buckypaper for removal of heavy metals and dyes

Industrial effluents contain several organic and inorganic contaminants. Among others, dyes and heavy metals introduce a serious threat to drinking waterbodies. These pollutants can be noxious or carcinogenic in nature, and harmful to humans and different aquatic species. Therefore, it is of high importance to remove heavy metals and dyes to reduce their environmental toxicity. This has led to an extensive research for the development of novel materials and techniques for the removal of heavy metals and dyes. One route to the removal of these pollutants is the utilization of magnetic carbon nanotubes (CNT) as adsorbents. Magnetic carbon nanotubes hold remarkable properties such as surface-volume ratio, higher surface area, convenient separation methods, etc. The suitable characteristics of magnetic carbon nanotubes have led them to an extensive search for their utilization in water purification. Along with magnetic carbon nanotubes, the buckypaper (BP) membranes are also favorable due to their unique strength, high porosity, and adsorption capability. However, BP membranes are mostly used for salt removal from the aqueous phase and limited literature shows their applications for removal of heavy metals and dyes. This study focuses on the existence of heavy metal ions and dyes in the aquatic environment, and methods for their removal. Various fabrication approaches for the development of magnetic-CNTs and CNT-based BP membranes are also discussed. With the remarkable separation performance and ultra-high-water flux, magnetic-CNTs, and CNT-based BP membranes have a great potential to be the leading technologies for water treatment in future.

Controlled Removal of Surfactants from Double-Walled Carbon Nanotubes for Stronger p-Doping Effect and Its Demonstration in Perovskite Solar Cells

Double-walled carbon nanotubes (DWNTs) have shown potential as promising alternatives to conventional transparent electrodes owing to their solution processability as well as high conductivity and transparency. However, their DC to optical conductivity ratio is limited by the surrounding surfactants that prevent the p-doping of the DWNTs. To maximize the doping effectiveness, the surfactants are removed from the DWNTs, with negligible damage to the nanotubes, by calcination in an Ar atmosphere. The effective removal of the surfactants is characterized by various analyses, and the results show that the optimal calcination temperature is 400 °C. The conductivity of the DWNTs films improves when doped by triflic acid. While the conductivity increase of the surfactants-wrapped DWNT films is 31.9%, the conductivity increase of the surfactants-removed DWNT is found to be 59.7%. Using the surfactants-removed, p-doped, solution-processed transparent electrodes, inverted-type perovskite solar cells are fabricated, resulting in a power conversion efficiency of 17.7% without hysteresis. This work advances the application of DWNTs in transparent conductors, as the efficiency obtained is the highest value achieved to date for carbon nanotube electrode-based perovskite solar cells and solution-processable transparent electrode-based solar cells.

Carbon nanotubes for flexible batteries: recent progress and future perspective

Flexible batteries, which maintain their functions potently under various mechanical deformations, attract increasing interest due to potential applications in emerging portable and wearable electronics. Significant efforts have been devoted to material synthesis and structural designs to realize the mechanical flexibility of various batteries. Carbon nanotubes (CNTs) have a unique one-dimensional (1D) nanostructure and are convenient to further assemble into diverse macroscopic structures, such as 1D fibers, 2D films and 3D sponges/aerogels. Due to their outstanding mechanical and electrical properties, CNTs and CNT-based hybrid materials are superior building blocks for different components in flexible batteries. This review summarizes recent progress on the application of CNTs in developing flexible batteries, from closed-system to open-system batteries, with a focus on different structural designs of CNT-based material systems and their roles in various batteries. We also provide perspectives on the challenges and future research directions for realizing practical applications of CNT-based flexible batteries.

High Capacity and Energy Density Organic Lithium-Ion Battery Based on Buckypaper with Stable π-Radical

Owing to an increasing demand on high performance and rare-metal free energy storage systems, organic rechargeable battery has attracted much attention. To increase the capacity of the whole battery, we have fabricated coin-type buckypaper cells composed of a trioxotriangulene neutral radical derivative (H₃ TOT) and single-walled carbon nanotubes as a cathode and lithium metal plate as an anode without current collector. The cells exhibited a stable charge-discharge behavior even at a 90 wt % H₃ TOT content with a high-rate performance of 10 C originating from high electrical conductivity of H₃ TOT. Furthermore, based on the four-stage redox ability of H₃ TOT, the H₃ TOT 90 wt % cathode showed a high capacity of approximately 260 mAh g^-1 and a high energy density of 546 Wh g^-1 . In view of the simple fabrication of the cathode and excellent performance, TOT-based buckypaper will open a new strategy for the flexible cells for next-generation energy storages.

Toward Confined Carbyne with Tailored Properties

Confining carbyne to a space that allows for stability and controlled reactivity is a very appealing approach to have access to materials with tunable optical and electronic properties without rival. Here, we show how controlling the diameter of single-walled carbon nanotubes opens the possibility to grow a confined carbyne with a defined and tunable band gap. The metallicity of the tubes has a minimal influence on the formation of the carbyne, whereas the diameter plays a major role in the growth. It has been found that the properties of confined carbyne can be tailored independently from its length and how these are mostly determined by its interaction with the carbon nanotube. Molecular dynamics simulations have been performed to interpret these findings. Furthermore, the choice of a single-walled carbon nanotube host has been proven crucial even to synthesize an enriched carbyne with the smallest energy gap currently reported and with remarkable homogeneity.

Multi-Functional Properties of MWCNT/PVA Buckypapers Fabricated by Vacuum Filtration Combined with Hot Press: Thermal, Electrical and Electromagnetic Shielding

The applications of pure multi-walled carbon nanotubes (MWCNTs) buckypapers are still limited due to their unavoidable micro/nano-sized pores structures. In this work, polyvinyl alcohol (PVA) was added to a uniform MWCNTs suspension to form MWCNT/PVA buckypapers by vacuum infiltration combined with a hot press method. The results showed an improvement in the thermal, electrical, and electromagnetic interference (EMI) shielding properties due to the formation of dense MWCNTs networks. The thermal and electrical properties rose from 1.394 W/m·k to 2.473 W/m·k and 463.5 S/m to 714.3 S/m, respectively. The EMI performance reached 27.08 dB. On the other hand, ABAQUS finite element software was used to simulate the coupled temperature-displacement performance. The electronic component module with buckypapers revealed a homogeneous temperature and thermal stress distribution. In sum, the proposed method looks promising for the easy preparation of multi-functional nanocomposites at low-cost.

WO3/Buckypaper Membranes for Advanced Oxidation Processes

Photocatalytic materials, such as WO₃, TiO₂, and ZnO nanoparticles, are commonly linked onto porous polymer membranes for wastewater treatment, fouling mitigation and permeation enhancement. Buckypapers (BPs) are entanglements of carbon nanotubes, which have been recently proposed as innovative filtration systems thanks to their mechanical, electronic, and thermal properties. In this work, flexible membranes of single wall carbon nanotubes are prepared and characterized as efficient substrates to deposit by chemical vapor deposition thin layers of WO₃ and obtain, in such a way, WO₃/BP composite membranes for application in advanced oxidation processes. The photocatalytic efficiency of WO₃/BP composite membranes is tested against model pollutants in a small continuous flow reactor and compared with the performance of an equivalent homogeneous WO₃-based reactor.

Steroid hormone micropollutant removal from water with activated carbon fiber-ultrafiltration composite membranes

Short activated carbon fibers (ACF) with high surface area were fabricated via carbonization in N₂ and activation in CO₂ at high temperatures, with cellulose fibers as the raw materials. The obtained ACF were subsequently deposited into the support layer of a polyethersulfone (PES) ultrafiltration membrane by a facile filtration process to obtain the sandwich structured ACF-PES composite membrane. The hormone (17β-estradiol, E2) adsorption kinetics and isotherm of ACF in static conditions, as well as E2 removal by filtration with the ACF-PES composite membrane were investigated. In static conditions, ACF rapidly and efficiently adsorbs E2 evidenced by a high removal of >97 %. The fitting of second order kinetics and linear (Henry) adsorption isotherm models indicated the availability of easily accessible adsorption sites. Besides, such efficient E2 adsorption was contributed by many interactions between E2 and ACF, namely hydrophobic interactions, hydrogen bonding and π-π stacking. The incorporation of ACF in a PES membrane resulted in a minor loss of filtration flux compared with the control membrane, but significantly improved E2 removal through adsorption pathway. With only 1.0 mg ACF incorporated (loading 2.0 g/m²), the composite membrane could reject 76 % of E2 from a 100 ng/L solution at a flux of 450 L/m²∙h, demonstrating that ACF-PES can overcome the permeability-selectivity trade-off of traditional UF membranes.

A Hybrid Nanomaterial Based on Single Walled Carbon Nanotubes Cross-Linked via Axially Substituted Silicon (IV) Phthalocyanine for Chemiresistive Sensors

In this work, the novel hybrid nanomaterial SWCNT/SiPc made of single walled carbon nanotubes (SWCNT) cross-linked via axially substituted silicon (IV) phthalocyanine (SiPc) was studied as the active layer of chemiresistive layers for the detection of ammonia and hydrogen. SWCNT/SiPc is the first example of a carbon-based nanomaterial in which an axially substituted phthalocyanine derivative is used as a linker. The prepared hybrid material was characterized by spectroscopic methods, thermogravimetry, scanning and transmission electron microscopies. The layers of the prepared hybrid were tested as sensors toward ammonia and hydrogen by a chemiresistive method at different temperatures and relative humidity as well as in the presence of interfering gases like carbon dioxide, hydrogen sulfide and volatile organic vapors. The hybrid layers exhibited the completely reversible sensor response to both gases at room temperature; the recovery time was 100-200 s for NH3 and 50-120 s in the case of H2 depending on the gas concentrations. At the relative humidity (RH) of 20%, the sensor response was almost the same as that measured at RH 5%, whereas the further increase of RH led to its 2-3 fold decrease. It was demonstrated that the SWCNT/SiPc layers can be successfully used for the detection of both NH3 and H2 in the presence of CO2. On the contrary, H2S was found to be an interfering gas for the NH3 detection.

Effective Heat Transfer Pathways of Thermally Conductive Networks Formed by One-Dimensional Carbon Materials with Different Sizes

We investigated the heat transfer behavior of thermally conductive networks with one-dimensional carbon materials to design effective heat transfer pathways for hybrid filler systems of polymer matrix composites. Nano-sized few-walled carbon nanotubes (FWCNTs) and micro-sized mesophase pitch-based carbon fibers (MPCFs) were used as the thermally conductive materials. The bulk density and thermal conductivity of the FWCNT films increased proportionally with the ultrasonication time due to the enhanced dispersibility of the FWCNTs in an ethanol solvent. The ultrasonication-induced densification of the FWCNT films led to the effective formation of filler-to-filler connections, resulting in improved thermal conductivity. The thermal conductivity of the FWCNT-MPCF hybrid films was proportional to the MPCF content (maximum thermal conductivity at an MPCF content of 60 wt %), indicating the synergistic effect on the thermal conductivity enhancement. Moreover, the MPCF-to-MPCF heat transfer pathways in the FWCNT-MPCF hybrid films were the most effective in achieving high thermal conductivity due to the smaller interfacial area and shorter heat transfer pathway of the MPCFs. The FWCNTs could act as thermal bridges between neighboring MPCFs for effective heat transfer. Furthermore, the incorporation of Ag nanoparticles of approximately 300 nm into the FWCNT-MPCF hybrid film dramatically enhanced the thermal conductivity, which was closely related to a decreased thermal interfacial resistance at the intersection points between the materials. Epoxy-based composites loaded with the FWCNTs, MPCFs, FWCNT-MPCF hybrids, and FWCNT-MPCF-Ag hybrid fillers were also fabricated. A similar trend in thermal conductivity was observed in the polymer matrix composite with carbon-based hybrid films.

Mechanical behavior of nonwoven non-crosslinked fibrous mats with adhesion and friction

We present a study of the mechanical behavior of planar fibrous mats stabilized by inter-fiber adhesion. Fibers of various degrees of tortuosity and of infinite and finite length are considered in separate models. Fibers are randomly distributed, are not cross-linked, and interact through adhesion and friction. The variation of structural parameters such as the mat thickness and the mean segment length between contacts along given fibers with the strength of adhesion is determined. These systems are largely dissipative in that most of the work performed during deformation is dissipated frictionally and only a small fraction is stored as strain energy. The response of the mats to tensile loading has three regimes: a short elastic regime in which no sliding at contacts is observed, a well-defined sliding regime characterized by strain hardening, and a rapid stiffening regime at larger strains. The third regime is due to the formation of stress paths after the fiber tortuosity is pulled out and is absent in mats of finite length fibers. Networks of finite length fibers lose stability during the second regime of deformation. The scaling of the yield stress, which characterizes the transition between the first and the second regimes, and of the second regime's strain hardening modulus, with system parameters such as the strength of adhesion and friction and the degree of fiber tortuosity are determined. The strength of mats of finite length fibers is also determined as a function of network parameters. These results are expected to become useful in the design of electrospun mats and other planar fibrous non-cross-linked networks.

Ultrahigh-Areal-Capacitance Flexible Supercapacitor Electrodes Enabled by Conformal P3MT on Horizontally Aligned Carbon-Nanotube Arrays

Nanocarbon electronic conductors combined with pseudocapacitive materials, such as conducting polymers, display outstanding electrochemical properties and mechanical flexibility. These characteristics enable the fabrication of flexible electrodes for energy-storage devices; that is, supercapacitors that are wearable or can be formed into shapes that are easily integrated into vehicle parts. To date, most nanocarbon materials such as nanofibers are randomly dispersed as a network in a flexible matrix. This morphology inhibits ion transport, particularly under the high current density necessary for devices requiring high power density. Novel flexible densified horizontally aligned carbon nanotube arrays (HACNTs) with controlled nanomorphology for improved ion transport are introduced and combined with conformally coated poly(3-methylthiophene) (P3MT) conducting polymer to impart pseudocapacitance. The resulting P3MT/HACNT nanocomposite electrodes exhibit high areal capacitance of 3.1 F cm^-2 at 5 mA cm^-2 , with areal capacitance remaining at 1.8 F cm^-2 even at a current density of 200 mA cm^-2 . The asymmetric supercapacitor cell also delivers more than 1-2 orders of magnitude improvement in both areal energy and power density over state-of-the-art cells. Furthermore, little change in cell performance is observed under high strain, demonstrating the mechanical and electrochemical stability of the electrodes.

Carbon-Nanomaterial-Based Flexible Batteries for Wearable Electronics

Wearable electronics have received considerable attention in recent years. These devices have penetrated every aspect of our daily lives and stimulated interest in futuristic electronics. Thus, flexible batteries that can be bent or folded are desperately needed, and their electrochemical functions should be maintained stably under the deformation states, given the increasing demands for wearable electronics. Carbon nanomaterials, such as carbon nanotubes, graphene, and/or their composites, as flexible materials exhibit excellent properties that make them suitable for use in flexible batteries. Herein, the most recent progress on flexible batteries using carbon nanomaterials is discussed from the viewpoint of materials fabrication, structure design, and property optimization. Based on the current progress, the existing advantages, challenges, and prospects are outlined and highlighted.

N-Doped carbon nanotubes enriched with graphitic nitrogen in a buckypaper configuration as efficient 3D electrodes for oxygen reduction to H2O2

Herein, a series of N-doped carbon nanotube (CNx) samples were obtained by modifying the synthesis temperature. Consequently, the proportion of graphitic nitrogen (Ngraph) in the samples was systematically increased as a function of temperature. This allowed evaluation of the role of the CNx graphitic nitrogen in the oxygen reduction reaction (ORR). A correlation between the Ngraph content and the ORR onset potential was observed, which shifted to more positive potentials with an increase in kinetic current density (jk); this showed that Ngraph played a significant catalytic role in the ORR. The samples with high Ngraph content favored the two-electron pathway for the ORR not only in basic media (pH = 13) but also in neutral media (pH = 7), representing an attractive alternative for wastewater remediation through the on-site generation of H2O2. The energetic calculations showed that the formation of H2O2 must be favorable in the presence of graphitic nitrogen sites. Finally, the performance of the buckypaper arrangement was evaluated, and the CNx buckypaper showed a higher cathodic current peak as compared to CNx traditional ink dispersions. Overall, this study not only sheds light on the role of Ngraph in the ORR, but also demonstrates that CNx buckypaper is an efficient 3D electrode for electrocatalytic applications.

Templated direct growth of ultra-thin double-walled carbon nanotubes

Double-walled carbon nanotubes (DWCNTs) combined the advantages of multi-walled (MW-) and single-walled (SW-) CNTs can be obtained by transforming the precursors (e.g. fullerene, ferrocene) into thin inner CNTs inside SWCNTs as templates. However, this method is limited since the DWCNT yield is strongly influenced by the filling efficiency (depending on the type of the filled molecules), opening and cutting the SWCNTs, and the diameter of the host SWCNTs. Therefore, it cannot be applied to all types of SWCNT templates. Here we show a universal route to synthesize ultra-thin DWCNTs via making SWCNTs stable at high temperature in vacuum. This method applies to different types of SWCNTs including metallicity-sorted ones without using any precursors since the carbon sources were from the reconstructed SWCNTs and the residue carbons. The resulting DWCNTs are with high quality and the yield of inner tubes is comparable to/higher than that of the DWCNTs made from the transformation of ferrocene/fullerene peapods.

Synthesis of outer tube-selectively nitrogen-doped double-walled carbon nanotubes by nitrogen plasma treatment

We demonstrate selective nitrogen doping on the outer tubes of double walled carbon nanotubes by nitrogen plasma treatment. Such selectivity improves the chemical activity of the outer tubes, including their wettability and oxygen reduction reaction properties, while retaining the structural, optical, and electrical properties of the inner tubes.

Crystalline multiwall carbon nanotubes and their application as a field emission electron source

Using super-aligned carbon nanotube (CNT) film, we have fabricated van der Waals crystalline multiwall CNTs (MWCNT) by adopting high pressure and high temperature processing. The CNTs keep parallel to each other and are distributed uniformly. X-ray diffraction characterization shows peaks at the small angle range, which can be assigned to the spacing of the MWCNT crystals. The mechanical, electrical and thermal properties are all greatly improved compared with the original CNT film. The field emission properties of van der Waals crystalline MWCNTs are tested and they show a better surface morphology stability for the large emission current. We have further fabricated a field emission x-ray tube and demonstrated a precise resolution imaging ability.

A wet-filtration-zipping approach for fabricating highly electroconductive and auxetic graphene/carbon nanotube hybrid buckypaper

A combination of carbon nanotubes (CNT) and graphene in the form of macroscopic hybrid buckypaper (HBP), exhibits a unique set of properties that can be exploited for many emerging applications. Here, we present a simple, inexpensive and scalable approach for the synthesis of highly conductive auxetic graphene/CNT HBP via wet-filtration-zipping and demonstrate the electrical, electrochemical and mechanical performance (tensile, mode I and mode III fracture) of synthesized HBP. An overall increase in electrical conductivity of 247% is observed for HBP (50 wt.% graphene and 50 wt.% CNT) as compared to BP (100 wt.% CNT) due to effective electronic percolation through the graphene and CNT. As a negative electrode for lithium-ion batteries, HBP shows 50% higher gravimetric specific capacity and 89% lower charge transfer resistance relative to BP. The graphene content in the HBP influences the mechanical performance providing an auxetic structure to HBP with large negative Poisson's ratio. The facile green-chemistry approach reported here can be readily applied to any other 1D and 2D materials and solves key challenges associated with existing buckypaper manufacturing methods. The potential of the synthesis method to integrate with current cellulose paper manufacturing technology and its scalability demonstrate the novelty of the work for industrial scale production.

A Review of Carbon Nanomaterials' Synthesis via the Chemical Vapor Deposition (CVD) Method

Carbon nanomaterials have been extensively used in many applications owing to their unique thermal, electrical and mechanical properties. One of the prime challenges is the production of these nanomaterials on a large scale. This review paper summarizes the synthesis of various carbon nanomaterials via the chemical vapor deposition (CVD) method. These carbon nanomaterials include fullerenes, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, carbide-derived carbon (CDC), carbon nano-onion (CNO) and MXenes. Furthermore, current challenges in the synthesis and application of these nanomaterials are highlighted with suggested areas for future research.

Carbon nanotube conditioning part 1-effect of interwall interaction on the electronic band gap of double-walled carbon nanotubes

Ab initio density functional theory simulations were used to calculate the electronic structure and the total energy of double-walled carbon nanotubes (DWCNTs). The relaxed configurations studied were uncapped, infinitely-long zigzag@zigzag double-walled carbon nanotubes. The lowest energy configuration was found to correspond to an interwall distance of 0.35 nm, except for the configurations with inner tube chiral indices (5,0), (6,0) and (7,0). The largest binding energies were found to correspond to a 0.35 nm interwall distance for all the DWCNT configurations studied, and increasing with DWCNT average diameter. In terms of the effect of the interwall interaction on the electronic band gap of DWCNTs, four regions of band gap were obtained which were termed: zero band gap, narrow band gap, small band gap, and medium band gap regions. These regions offer the possibility to first tune the electronic band gap to a region with a desired range, and further tune that choice within the region itself by varying the interwall distance. It was also found that zigzag@zigzag DWCNTs with outer tube leading chiral index n = 3k + 1 or n = 3k + 2 (k being an integer) follow, as a general trend, an inversely proportional relation of the electronic band gap with respect to the average diameter.

Real time monitoring of the curing degree and the manufacturing process of fiber reinforced composites with a carbon nanotube buckypaper sensor

This paper proposes a flexible and highly sensitive carbon nanotube buckypaper as a sensing layer embedded within a composite for cure monitoring applications. The buckypaper was fabricated with mono-dispersion of multi-wall carbon nanotubes by a spray-vacuum filtration method. Six different curing conditions (with maximum heating temperatures of 120 °C, 108 °C, 95 °C, 90 °C, 85 °C and 75 °C) were designed to characterize and analyze the electromechanical response of the BP sensor to the composite structure, and the results indicated that the temperature coefficient of resistance of buckypaper is associated to the resin curing behavior. The critical value (−7.18 × 10⁻⁴ °C⁻¹) of the temperature coefficient of resistance was determined. Experimental results also show that a stable three-dimensional network of resin molecular chains is formed and that the polymer presents a glassy state when the value of the temperature coefficient of resistance is greater than the critical value. Based on this relationship, a hypothesis was raised that for the complete curing of the resin, the temperature coefficient of resistance of the buckypaper sensor should meet the critical value condition, which was also consistent with the differential scanning calorimetry testing of the curing degree. The buckypaper sensor was found to be sensitive to the curing degree of the resin, and has a promising future in applications in composite manufacturing processes. Moreover, the properties of composite components are indeed able to be improved via the monitoring and optimization of the curing parameters.

A flexible and highly sensitive carbon nanotube buckypaper (BP) as a sensing layer was embedded within composite for cure monitoring applications. BP sensor can monitor the resin phase transition and resin cure degree during composite manufacturing.

Electrical properties of O-self-doped boron-nitride nanotubes and the piezoelectric effects of their freestanding network film

Boron-nitride nanotubes (BNNTs) are a common one-dimensional (1D) nanostructure that possess piezoelectric potential due to ion-covalent boron-nitride (BN) bonding. Harnessing the advantages offered by high-stability BN structures, these materials have been used for various new applications such as nanogenerators, nanotransistors, and nano-artificial eardrums. In this paper, we used nano-iron oxide red as a catalyst and boron powder in an aqueous dispersion as the boron source to synthesize high-purity O-self-doped BNNTs and film. We investigated the electrical properties of O-self-doped BNNTs and the piezoelectricity of freestanding BNNT film and demonstrated that the electrical properties of O-self-doped BNNTs improved dramatically compared to those of non-doped BNNTs. We also analyzed the band gaps and density of states (DOS) of the O-self-doped BNNTs with the Spanish Initiative for Electronic Simulation with Thousands of Atoms (SIESTA) code to explain the improvement. In addition, we revealed the piezoelectric voltage coefficient g₃₁ of O-self-doped BNNTs (0.28 V m N⁻¹) network films, which can guide future applications for vibration nanosensors and transducers under extreme conditions.

Boron-nitride nanotube (BNNT) freestanding network films have been applied in various new fields, such as nanogenerators, nanotransistors, and nano-artificial eardrums.

Effective NaCl and dye rejection of hybrid graphene oxide/graphene layered membranes

Carbon nanomaterials are robust and possess fascinating properties useful for separation technology applications, but their scalability and high salt rejection when in a strong cross flow for long periods of time remain challenging. Here, we present a graphene-based membrane that is prepared using a simple and environmentally friendly method by spray coating an aqueous dispersion of graphene oxide/few-layered graphene/deoxycholate. The membranes were robust enough to withstand strong cross-flow shear for a prolonged period (120 h) while maintaining NaCl rejection near 85% and 96% for an anionic dye. Experimental results and molecular dynamic simulations revealed that the presence of deoxycholate enhances NaCl rejection in these graphene-based membranes. In addition, these novel hybrid-layered membranes exhibit better chlorine resistance than pure graphene oxide membranes. The desalination performance and aggressive shear and chlorine resistance of these scalable graphene-based membranes are promising for use in practical water separation applications.

Paper-Based Electrodes for Flexible Energy Storage Devices

Paper-based materials are emerging as a new category of advanced electrodes for flexible energy storage devices, including supercapacitors, Li-ion batteries, Li-S batteries, Li-oxygen batteries. This review summarizes recent advances in the synthesis of paper-based electrodes, including paper-supported electrodes and paper-like electrodes. Their structural features, electrochemical performances and implementation as electrodes for flexible energy storage devices including supercapacitors and batteries are highlighted and compared. Finally, we also discuss the challenges and opportunity of paper-based electrodes and energy storage devices.

Self-Expansion Construction of Ultralight Carbon Nanotube Aerogels with a 3D and Hierarchical Cellular Structure

A novel and simple strategy is developed to construct ultralight and 3D pure carbon nanotube (CNT) aerogels by the spontaneous expansion of superaligned CNT films soaked in a piranha (mixed H₂ SO₄ and H₂ O₂ ) solution, followed by cryodesiccation. The macroscopic CNT aerogels have an extremely low apparent density (0.12 mg cm^-3 ), ultrahigh porosity (99.95%), high specific surface area (298 m² g^-1 ), and a hierarchical cellular structure with giant and ultrathin CNT sheets as cell walls. The pure CNT aerogels show high adsorption abilities for various kinds of solvents, and have great potential in widespread applications such as energy storage, catalysis, and bioengineering.

Carbon Nanotube Membranes: Synthesis, Properties, and Future Filtration Applications

Over the course of the past decade, there has been growing interest in the development of different types of membranes composed of carbon nanotubes (CNTs), including buckypapers and composite materials, for an ever-widening range of filtration applications. This article provides an overview of how different types of CNT membranes are prepared and the results obtained from investigations into their suitability for different applications. The latter involve the removal of small particles from air samples, the filtration of aqueous solutions containing organic compounds and/or bacteria, and the separation of individual liquids present in mixtures. A growing number of reports have demonstrated that the incorporation of CNTs into composite membranes confers an improved resistance to fouling caused by biomacromolecules and bacteria. These results are discussed, along with evidence that demonstrates it is possible to further reduce fouling by taking advantage of the inherent conductivity of composite membranes containing CNTs, as well as by using different types of electrochemical stimuli.

Soft Actuators for Small-Scale Robotics

This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Cm-size free-standing self-organized buckypaper of bucky-onions filled with ferromagnetic Fe3C

Novel cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe₃C single crystals were serendipitously discovered.

Fullerene and nanotube growth: new insights using first principles and molecular dynamics

Shortly after the discovery of fullerenes, many researchers pointed out that carbon nanotubes could be considered as elongated fullerenes. However, the detailed formation mechanism for both structures has been a topic of debate for several years, and consequently it has been difficult to draw a clear connection between the two systems. While the synthesis conditions appear to be different for both fullerenes and nanotubes, here, we demonstrate that it is highly likely that, at an initial growth stage, single-walled carbon nanotubes begin to grow from a hemisphere-like fullerene cap. More importantly, by analysing the minimum-energy path, it is shown that the insertion of C2 fragments drives the transformation of this fullerene cap into an elongated structure that leads to the formation of very short carbon nanotubes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Growth of single-layer boron nitride dome-shaped nanostructures catalysed by iron clusters

We report on the growth and formation of single-layer boron nitride dome-shaped nanostructures mediated by small iron clusters located on flakes of hexagonal boron nitride. The nanostructures were synthesized in situ at high temperature inside a transmission electron microscope while the e-beam was blanked. The formation process, typically originating at defective step-edges on the boron nitride support, was investigated using a combination of transmission electron microscopy, electron energy loss spectroscopy and computational modelling. Computational modelling showed that the domes exhibit a nanotube-like structure with flat circular caps and that their stability was comparable to that of a single boron nitride layer.

Confined linear carbon chains as a route to bulk carbyne

Strong chemical activity and extreme instability in ambient conditions characterize carbyne, an infinite sp(1) hybridized carbon chain. As a result, much less has been explored about carbyne as compared to other carbon allotropes such as fullerenes, nanotubes and graphene. Although end-capping groups can be used to stabilize carbon chains, length limitations are still a barrier for production, and even more so for application. We report a method for the bulk production of long acetylenic linear carbon chains protected by thin double-walled carbon nanotubes. The synthesis of very long arrangements is confirmed by a combination of transmission electron microscopy, X-ray diffraction and (near-field) resonance Raman spectroscopy. Our results establish a route for the bulk production of exceptionally long and stable chains composed of more than 6,000 carbon atoms, representing an elegant forerunner towards the final goal of carbyne's bulk production.

Adsorptive removal of antibiotics from aqueous solution using carbon materials

Antibiotics, an important type of environmental contamination, have attracted many researchers to the study of their removal from aqueous solutions. Adsorption technology is a fast, efficient, and economical physicochemical method that is extensively used in wastewater treatment. From original activated carbon and carbon nanotubes to the latest graphene-based materials, carbon-based materials have been widely used as highly effective adsorbents for contaminant removal from aqueous solution because of their large specific surface area, high porosity, and high reaction activity. In this article, adsorption removal methods for four major types of antibiotic (tetracyclines, sulfonamides, macrolides, and quinolones) are reviewed. We also provide an overview of the application development of carbon materials as adsorbents for antibiotic removal from aqueous solution. The most promising works are discussed, and the main challenges in preparing high-performance adsorbents and the development tendency of adsorbents are also analyzed. This work provides theoretical guidance for subsequent research in the design and modification of carbon materials for applications in the adsorption removal of antibiotics from aqueous solution.