Free-standing Single-walled Carbon Nanotube Research Articles

Janus Electrochemistry: Asymmetric Functionalization in One Step.

Janus structures represent an overwhelming member of materials with adaptable chemical and physical properties. Development of new synthesis routes has allowed the fabrication of Janus architectures with specific characteristics depending on the final applications. In the case of the membranes, the improvement of wet routes has been limited to the capillary effect, in which the solution can gradually penetrate through the membrane, avoiding a double modification different at each face of the membrane. In this work, we propose a new electrochemical methodology to circumvent the capillary limitation and obtain a double electrochemical functionalization in only one step in a controlled way. This innovative methodology has been validated using a tridirectional spectroelectrochemistry setup. Moreover, the information provided by this optical arrangement should be especially useful for the study of the different processes (ion transfer, assisted ion transfer, and electron transfer) that can take place at liquid/liquid interfaces. Janus electrochemistry allows us to modify the two faces of a free-standing single-walled carbon nanotube electrode in a single experiment. As proof of concept, the free-standing films have been functionalized with two different conducting polymers, polyaniline and poly(3-hexylthiophene), in one electrochemical experiment. According to the obtained results, this new electrochemical methodology will open new gates for the design and functionalization of Janus materials.

From mesoscale to nanoscale mechanics in single-wall carbon nanotubes

The analysis of the radial collapse of individualized and isolated single-wall carbon nanotubes under high pressure as function of their diameter, d, distinguishes their mesoscale and their nanoscale mechanics. The evolution with pressure of the Raman spectra for nine tube chiralities and the theoretical modelling reveal a deviation from the continuum mechanics prediction of a collapse pressure PC∝d−3. Nanotubes show a normalized collapse pressure PN=PCd3=24αD(1−β2/d2) both in experiment and in very different theoretical models. In this expression β=0.44±0.04nm represents the smallest diameter for a stable freestanding single-wall carbon nanotube and D is the bending stiffness of graphene. From the experimental data D=1.7±0.2eV. Deviations from the continuum mechanics predictions start to be of significance for diameters smaller than ∼1nm. The associated reduction of their collapse pressure is attributed to the discretization of the elastic compliances around the circumference of the tubes.

Silver nanoparticles/free-standing carbon nanotube Janus membranes.

Janus materials are attracting intense interest due to their exceptional possibilities in the design of structures with specific properties. In this work, Janus structures were constructed by the functionalization of free-standing single-walled carbon nanotube (FS-SWCNT) membranes with silver nanoparticles (AgNPs) electrodeposited on one face of the free-standing structure. The modification of the FS-SWCNT films with AgNPs was carried out from an ionic liquid solution, a green and environmentally friendly procedure. Before the functionalization of the free-standing membranes, the optimal electrodeposition conditions of AgNPs from an ionic liquid in different carbon substrates were studied. We demonstrate by UV–vis spectroelectrochemistry that the functionalization of the FS-SWCNT membranes on only one face is achieved, remaining unmodified the other face, as SEM images confirm. Moreover, the morphology of AgNPs, and, consequently, the properties of the Janus structures strongly depend on the specific electrodeposition conditions selected, determining their future applicability.

Unsaturated-sulfur-rich MoS2 nanosheets decorated on free-standing SWNT film: Synthesis, characterization and electrocatalytic application

Herein, we report a bottom-up solvothermal route to synthesize a flexible, highly efficient MoS2@SWNT electrocatalyst for hydrogen evolution reactions (HER). Characterization revealed that branch-like MoS2 nanosheets containing sulfurrich sites were in situ uniformly dispersed on free-standing single-walled carbon nanotube (SWNT) film, which could expose more unsaturated sulfur atoms, allowing excellent electrical contact with active sites. The flexible catalyst exhibited excellent HER performance with a low overpotential (~150 mV at 10 mA/cm2) and small Tafel slope (41 mV/dec). To further explain the improved performance, the local electronic structure was investigated by X-ray absorption near-edge structure (XANES) analysis, proving the presence of unsaturated sulfur atoms and strong electronic coupling between MoS2 and SWNT. This study provides an in-situ synthetic route to create new multifunctional flexible hybridized catalysts and useful insights into the relationships among the catalyst microstructure, electronic structure, and properties.

(Invited) Influence of Purity and Processing on Freestanding SWCNT Nanosheets and Nanotube-Silicon Heterojunctions

Single-wall carbon nanotubes (SWCNTs) of varied polydispersity (length, electronic type and chirality) are used to study the impact of SWCNT purity and processing on both the durability of freestanding SWCNT nanosheets and the performance of SWCNT-silicon heterojunctions. Solution-processed SWCNT films are deposited on n-doped silicon substrates and p-doped to yield photovoltaic devices with competitive photo-conversion efficiency, fill factor and open-circuit voltage. The temperature dependence of the dark-current density and its transient recovery after an abrupt change in bias are used to clarify the nature of the junctions over a broad range of SWCNT polydispersity and processing schemes, where the latter varies from the vacuum filtration of aqueous colloidal SWCNTs to the flow-deposition of aligned nanotube super-acid suspensions. As an intermediate step in all of these processes, the durability of freestanding SWCNT nanosheets in a variety of solvents is considered in the context of large-deformation elasticity and plasticity.

Temperature induced modification of the mid-infrared response of single-walled carbon nanotubes

The temperature dependences of the absorbance spectra of thin free-standing single-walled carbon nanotube (SWCNT) films were studied in the infrared range (700–6200 cm−1) while heating the air from 300 to 575 K. The observed temperature variation in the infrared absorbance spectra has been explained by two different physical factors. The first one is the strong temperature dependence of the conductivity of p-type doped semiconducting SWCNTs. The second one is the temperature dependence of electron relaxation time of intraband electron transitions in metallic SWCNTs. The possibility of the separation of contributions from the interband and intraband transitions to the infrared spectra of SWCNT films has been demonstrated.

Laser images recording on aerosol-synthesized single-walled carbon nanotube films

It is shown that images on semitransparent conducting films of aerosol-synthesized single-walled carbon nanotubes (SWCNTs) can be recorded using focused low-power radiation of a He–Ne laser operating at a wavelength of 632.8 nm. Both free-standing films and those deposited on glass or polymer substrates can be used. Laser recording of images on the polymer-supported films is possible due to their transparency increased as a result of chemical reactions between iron nanoparticles encapsulated in SWCNTs and the products of local thermal decomposition of the polymer. Recording on the free-standing SWCNT films and those supported on glass substrates was performed upon acid treatment of the film surface.

Texture and light-induced anisotropic terahertz properties of free-standing single-walled carbon nanotube films with random networks

We present an anisotropic property investigation of the free-standing single-walled carbon nanotube (SWCNT) random networks by terahertz time-domain spectroscopy (THz-TDS). Strongly anisotropic transmission in the spectral region 0.3–2.5 THz is observed. The experimental observation implies that our pristine SWCNT networks show preferential order in micrometer scale, which are attributed to the formation of the texture pattern. Modulation of this exceptional anisotropy by mechanical stretching and light illuminating indicates that the anisotropic response can be enhanced. The light-induced anisotropic property shows a hyperbolic tangent dependence with the pump power. The results suggest the potential applications and manipulations of SWCNT networks for THz electro-optical and polarized devices.

A Facile Synthesis of High-Surface-Area Sulfur-Carbon Composites for Li/S Batteries.

Small-grained elemental sulfur is precipitated from sodium thiosulfate (Na2 S2 O3 ) in a carbon-containing oxalic acid (HOOC-COOH) solution through a novel spray precipitation method. Surface area analysis, elemental mapping, and transmission electron micrographs revealed that the spray-precipitated sulfur particles feature 11 times higher surface area compared to conventional precipitated sulfur, with homogeneous distribution in the carbon. Moreover, the scanning electron micrographs show that these high-surface-area sulfur particles are firmly adhered to and covered by carbon. This precipitated S-C composite exhibits high discharge capacity with about 75 % capacity retention. The initial discharge capacity was further improved to 1444 mA h g(-1) by inserting a free-standing single-walled carbon nanotube layer in between the cathode and the separator. Moreover, with the help of the fixed capacity charging technique, 91.6 % capacity retention was achieved.

A Facile Route to Metal Oxides/Single-Walled Carbon Nanotube Macrofilm Nanocomposites for Energy Storage

Nanocomposites consisting of transition-metal oxides and carbon nanomaterials with a desired size and structure are highly demanded for high performance energy storage devices. Here, a facile two-step and cost-efficient approach relying on directly thermal treatment of chemical-vapor-deposition products is developed as a general synthetic method to prepare a family of metal oxides (MxOy (M=Fe, Co, Ni))/single-walled carbon nanotube (SWNT) macrofilm nanocomposites. The MxOy nanoparticles obtained are of 3-17 nm in diameter and homogeneously anchor on the free-standing SWNT macrofilms. NiO/SWNT also exhibits a high specific capacitance of 400 F g-1 and fast charge-transfer Faradaic redox reactions to achieve asymmetric supercapacitors with a high power and energy density. All MxOy/SWNT nanocomposites could deliver a high capacity beyond 1000 mAh g-1 and show excellent cycling stability for lithium-ion batteries. The impressive results demonstrate the promise for energy storage devices and the general approach may pave the way to synthesize other functional nanocomposites.

Carbon nanotube network varactor

Microelectromechanical system (MEMS) varactors based on a freestanding layer of single-walled carbon nanotube (SWCNT) films were designed, fabricated and tested. The freestanding SWCNT film was employed as a movable upper patch in the parallel plate capacitor of the MEMS. The measurements of the SWCNT varactors show very high tunability, nearly 100%, of the capacitance with a low actuation voltage of 10 V. The functionality of the varactor is improved by implementing a flexible nanocellulose aerogel filling.

Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries

We demonstrate a surfactant-free, solution processing route to form three-dimensional freestanding foams of pristine single-walled carbon nanotubes (SWCNTs) and explore the diverse electrochemical energy storage applications of these materials. This route utilizes SWCNT dispersions in organic n-methylpyrrolidone solvents and subsequent electrophoretic assembly onto a metal foam sacrificial template which can be dissolved to yield surfactant-free, binder-free freestanding SWCNT foams. We further provide a comparison between surfactant-free foams and conventional surfactant-based solvent processing routes and assess performance of these foams in supercapacitors, lithium-ion batteries, and lithium–air batteries. For pristine SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors, specific capacity up to 2210 mAh/g for lithium-ion batteries with up to 50% energy efficiency, and specific discharge capacity up to 8275 mAh/g in lithium–air batteries. For lithium–air batteries, this corresponds to a total energy density of 21.2 and 3.3 kWh/kg for the active mass and total battery device, respectively, approaching the 12.7 kWh/kg target energy density of gasoline. In comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric behavior in all devices and compromised Faradaic storage that leads to depreciated amounts of usable, stored energy. This work demonstrates the broad promise of SWCNTs as lightweight and highly efficient energy storage materials but also emphasizes the importance of clean nanomanufacturing routes which are critical to achieve optimized performance with nanostructures.

Electrochemical behaviour of different redox probes on single wall carbon nanotube buckypaper-modified electrodes

In the present work, the electrochemical properties of single-walled carbon nanotube buckypapers (BPs) were examined in terms of carbon nanotubes nature and preparation conditions. The performance of the different free-standing single wall carbon nanotube sheets was evaluated via cyclic voltammetry of several redox probes in aqueous electrolyte. Significant differences are observed in the electron transfer kinetics of the buckypaper-modified electrodes for both the outer- and inner-sphere redox systems. These differences can be ascribed to the nature of the carbon nanotubes (nanotube diameter, chirality and aspect ratio), surface oxidation degree and type of functionalities. In the case of dopamine, ferrocene/ferrocenium, and quinone/hydroquinone redox systems the voltammetric response should be thought as a complex contribution of different tips and sidewall domains which act as mediators for the electron transfer between the adsorbate species and the molecules in solution. In the other redox systems only nanotube ends are active sites for the electron transfer. It is also interesting to point out that a higher electroactive surface area not always lead to an improvement in the electron transfer rate of various redox systems.In addition, the current densities produced by the redox reactions studied here are high enough to ensure a proper electrochemical signal, which enables the use of BPs in sensing devices.

Mechanics and morphology of single-walled carbon nanotubes: from graphene to the elastica

The elastica is referred to the shape of the curve into which the centreline of a flexible lamina is bent. Hence, single-walled carbon nanotubes (SWCNTs) are treated as the elastica obtained from bending of graphene. The corresponding large deformation accompanies both the material and geometrical non-linearities. The morphology of the free-standing SWCNTs such as the natural angle of twist, bond lengths, tube radius and wall thickness are determined. Moreover, it is shown that the induced self-equilibriated strain field has a remarkable impact on the mechanical behaviour of the nanotube. Utilization of an appropriate non-linear continuum constitutive relation for graphene leads to exact formulation of the governing equations of SWCNTs. Subsequently, through perturbation analysis, the asymptotic solutions of the initial elastic fields for the SWCNTs are presented. By performing ab initio calculations, the components of the fourth and sixth-order elastic moduli tensors in the constitutive model of graphene needed in this study are computed.

Ultrafast Separation of Emulsified Oil/Water Mixtures by Ultrathin Free‐Standing Single‐Walled Carbon Nanotube Network Films

As an alternative to polymer membranes, ultrathin free-standing single-walled carbon-nanotube network films are used to realize oil/water separation with ultrahigh flux. The films with tunable thickness of the tens of nanometer scale can effectively separate both micrometer and nanometer-sized surfactant-free and surfactant-stabilized water-in-oil emulsions with a flux 2-3 orders of magnitude higher than commercial filtration membranes with similar separation performance.

Free-standing single-walled carbon nanotube–CdSe quantum dots hybrid ultrathin films for flexible optoelectronic conversion devices

In this work, we described a facile route for the fabrication of free-standing single-walled carbon nanotubes (SWCNT)-CdSe quantum dots (QDs) hybrid ultrathin films and investigated their optoelectronic conversion properties. A free-standing SWCNT film with thickness of ∼36 nm was firstly prepared via vacuum filtration. The film was then immersed into the pre-synthesized oleic acid-capped CdSe QDs (average diameter of 3.5 nm) solution, where CdSe QDs anchored spontaneously onto the surface of SWCNT film to produce SWCNT-CdSe QDs hybrid film. By using pure SWCNT films in different thicknesses as bottom and top electrodes, a flexible all-carbon electrode optoelectronic conversion device with sandwich structure of SWCNT film (thickness of ∼200 nm)/SWCNT-CdSe QDs hybrid film (thickness of ∼36 nm)/SWCNT film (thickness of ∼36 nm) was constructed to generate optoelectronic conversion under illumination of solar-simulated light. Our results demonstrated that the all-carbon electrode structure was effective for charge separation and a sensitive and stable photocurrent signal could be produced in such a device. In addition, our SWCNT-CdSe QDs hybrid film exhibited high flexibility and durability. No clear change in the resistance of the film was detected under bending in various bending angles.

Freestanding single-walled carbon nanotube bundle networks: Fabrication, properties and composites

As a type of thin film, two dimensional (2D) reticulate architectures built of freestanding single-walled carbon nanotube (SWCNT) bundles are suitable for scalable integration into devices and nanocomposites for many applications. The superior properties of these films, such as optical transparency, unique electrical properties and mechanical flexibility, result not only from the outstanding properties of individual SWCNTs but also from the collective behavior of the individual tubes, with additional properties arising from the tube-tube interactions. In this review, the synthesis, structure and fundamental properties, such as conductivity, transparency, optical nonlinearity and mechanical performance, of “freestanding SWCNT bundle network” thin films and nanocomposites, as well as their application as supercapacitors are highlighted. Some long-standing problems and topics warranting further investigation in the near future are addressed.

Hybrid Germanium Nanoparticle–Single-Wall Carbon Nanotube Free-Standing Anodes for Lithium Ion Batteries

Germanium nanoparticles (Ge-NPs) were synthesized through a one-step chemical vapor deposition process and were included in a hybrid free-standing single-wall carbon nanotube (SWCNT) electrode. The Ge-NPs were characterized through scanning electron microscopy and Raman spectroscopy to confirm the presence of crystalline nanoparticles with average diameters of 60 nm. Electrochemical testing of the Ge-NPs shows high reversible lithium ion capacity up to 900 mAh g–1 and a Coulombic efficiency of 96% on the first cycle, with capacities realizing 1000 mAh g–1 and a Coulombic efficiency of 98% on the second cycle. The use of SWCNTs to provide a stable nanoscale electrical network to support Ge-NPs resulted in a hybrid three-dimensional free-standing electrode, which is an attractive alternative to the conventional composite-current collector approach. The Ge-NP:SWCNT hybrid electrode with thin film titanium contacts produced electrode capacities of 983 mAh g–1 versus Li/Li+ up to 3 V. The higher anode capacity for the hybrid is maintained at modest cycling rates up to 1C. The pairing of the hybrid electrode with a commerical LiFePO4 cathode showed excellent performance with anode capacities of 800 mAh g–1 over a 1 V discharge range. Even at higher discharge rates, up to 1C, the anode energy density changes by only 8.5%. Thus, this demonstrates the first full battery comprising a free-standing Ge-based anode with a high power cathode exhibiting improved energy and power density.

Fabrication of Superstrong Ultrathin Free-Standing Single-Walled Carbon Nanotube Films via a Wet Process

A one-pot and readily practical approach is described for the preparation of superstrong, ultrathin, free-standing single-walled carbon nanotube (SWNT) films. The SWNT films, with controlled thicknesses of tens to hundreds of nanometers, are prepared from commonly commercialized SWNTs via a wet process. The SWNTs could be easily transferred onto any substrates after self-releasing from filter membranes without further treatment. The obtained films exhibit excellent performances with sheet resistance of 223 O sq-1 and a transparency of 90% at 550 nm was obtained. Most important is that the as-prepared free-standing SWNT ultrathin films showed extremely high tensile strength up to 850 MPa for only about a 20-nm thick film, which has great significance for practical applications, for example, as flexible electrode materials. The SWNT film is used to construct a capacitive touch-screen prototype, which has a highly sensitive and quick signal touch response.

Multifunctional Free-Standing Single-Walled Carbon Nanotube Films

We report a simple and rapid method to prepare multifunctional free-standing single-walled carbon nanotube (SWCNT) films with variable thicknesses ranging from a submonolayer to a few micrometers having outstanding properties for a broad range of exceptionally performing devices. We have fabricated state-of-the-art key components from the same single component multifunctional SWCNT material for several high-impact application areas: high efficiency nanoparticle filters with a figure of merit of 147 Pa(-1), transparent and conductive electrodes with a sheet resistance of 84 Ω/◻ and a transmittance of 90%, electrochemical sensors with extremely low detection limits below 100 nM, and polymer-free saturable absorbers for ultrafast femtosecond lasers. Furthermore, the films are demonstrated as the main components in gas flowmeters, gas heaters, and transparent thermoacoustic loudspeakers.