Multi-walled Structures Research Articles

Nanostructured carbon electrocatalyst supports for intermediate-temperature fuel cells: Single-walled versus multi-walled structures

Abstract It is unknown if nanostructured carbons possess the requisite electrochemical stability to be used as catalyst supports in the cathode of intermediate-temperature solid acid fuel cells (SAFCs) based on the CsH2PO4 electrolyte. To investigate this application, single-walled carbon nanohorns (SWNHs) and multi-walled carbon nanotubes (MWNTs) were used as supports for Pt catalysts in SAFCs operating at 250 °C. SWNH-based cathodes display greater maximum activity than their MWNT-based counterparts at a cell voltage of 0.8 V, but are unstable in the SAFC cathode as a consequence of electrochemical carbon corrosion. MWNT-based cells are resistant to this effect and capable of operation for at least 160 h at 0.6 V and 250 °C. Cells fabricated with nanostructured carbon supports are more active (52 mA cm−1 vs. 28 mA cm−1 at 0.8 V) than state-of-the-art carbon-free formulations while simultaneously displaying enhanced Pt utilization (40 mA mgPt−1 vs. 16 mA mgPt−1 at 0.8 V). These results suggest that MWNTs are a viable support material for developing stable, high-performance, low-cost air electrodes for solid-state electrochemical devices operating above 230 °C.

Synthesis of palladium nanoparticle modified reduced graphene oxide and multi-walled carbon nanotube hybrid structures for electrochemical applications

In this work, palladium (Pd) nanoparticles functionalized reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs) hybrid structures (Pd/rGO-MWCNTs) were successfully prepared by a combination of electrochemical reduction with electrodeposition method. The morphology, structure, and composition of the Pd/rGO-MWCNTs hybrid were characterized by scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy. The as-synthesized hybrid structures were modified on the glassy carbon electrode (GCE) and further utilized for hydrazine sensing. Electrochemical impedance spectroscopic, cyclic voltammetry and single-potential amperometry experiments were carried out on Pd/rGO-MWCNTs hybrid structures to investigate the interface properties and sensing performance. The measured results demonstrate that the fabricated Pd/rGO-MWCNTs/GCE sensor show a high sensitivity of 7.09μAμM−1cm−2 in a large concentration range of 1.0 to 1100μM and a low detection limit of 0.15μM. Moreover, the as-prepared sensor exhibits good selectivity and stability for the determination of hydrazine under interference conditions.

High Current Emission from Patterned Aligned Carbon Nanotubes Fabricated by Plasma-Enhanced Chemical Vapor Deposition.

Vertically, carbon nanotube (CNT) arrays were successfully fabricated on hexagon patterned Si substrates through radio frequency plasma-enhanced chemical vapor deposition using gas mixtures of acetylene (C2H2) and hydrogen (H2) with Fe/Al2O3 catalysts. The CNTs were found to be graphitized with multi-walled structures. Different H2/C2H2 gas flow rate ratio was used to investigate the effect on CNT growth, and the field emission properties were optimized. The CNT emitters exhibited excellent field emission performance (the turn-on and threshold fields were 2.1 and 2.4 V/μm, respectively). The largest emission current could reach 70 mA/cm2. The emission current was stable, and no obvious deterioration was observed during the long-term stability test of 50 h. The results were relevant for practical applications based on CNTs.

A study of thermal properties of sodium titanate nanotubes synthesized by microwave-assisted hydrothermal method

Sodium titanate nanotubes (NaTiNTs) were synthesized by microwave-assisted hydrothermal treatment of commercial TiO2, at constant temperature (135°C) and different irradiation times (15min, 1, 4, 8 and 16h). The products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, differential scanning calorimetry and specific surface area measurements. The irradiation time turned out to be the key parameter for morphological control of the material. Nanotubes were observed already after 15min of microwave irradiation. The analyses of the products irradiated for 8 and 16h confirm the complete transformation of the starting TiO2 powder to NaTiNTs. The nanotubes are open ended with multi-wall structures, with the average outer diameter of 8nm and specific surface area up to 210m2/g. The morphology, surface area and crystal structure of the sodium titanate nanotubes synthesized by microwave-assisted hydrothermal method were similar to those obtained by conventional hydrothermal method.

Transformation of carbon black into carbon nano-beads and nanotubes: the effect of catalysts

Structural transformation of carbon black (CB) into carbon nano-beads and nanotubes was achieved at 1000 °C using ferrocene and nickelocene as catalyst precursors using a simple and single step chemical vapor deposition method. The samples were characterized by XRD, SEM, TEM, HR-TEM and Raman spectroscopy. Results indicate that different morphological and high quality nano carbon structures were obtained using different weight ratios of catalyst to precursor. The use of bimetallic catalysts provides many different morphologies and a higher degree of crystal order of the carbon nanostructures than the use of mono-metallic catalysts. The nanotubes were mostly filled with metal nanoparticles and the degree of metal-filling is dependent on the weight ratio of catalyst precursor to CB. Metal–filled multi-walled carbon nano-bead structures with a high degree of crystalline order are also obtained at weight ratios of CB: ferrocene: nickelocene of 1:2:2.

Effects of multi-walled carbon nanotube structures on the electrical and mechanical properties of silicone rubber filled with multi-walled carbon nanotubes

The specific surface area is a key factor that determines both the electrical and mechanical properties of silicone rubber/MWCNTs.

Uniform shell modeling of vertically aligned multi-walled carbon nanotube arrays

Vertically aligned multi-walled carbon nanotube arrays (VA-MWCNTs) are novel carbon nanomaterials that have broader applications than individual carbon nanotubes (CNTs). This paper presents the uniform shell modeling of VA-MWCNTs which allow effectively design and characterize these complex materials. Multi-walled nanotubes in the arrays are approximated as equivalent single-walled CNTs and modeled with shell elements. The stiffness and Young's modulus obtained from the equivalent thickness method are comparable to those obtained from the modeling of actual multi-wall structures, but at much efficient computational efforts. VA-MWCNTs have also been designed at various architectures (square, FCC) and areal densities, and the resultant mechanical properties are analyzed.

Cellular Ti6Al4V with carbon nanotube-like structures fabricated by selective electron beam melting

Purpose – The purpose of this paper is to fabricate cellular Ti6Al4V with carbon nanotube (CNT)-like structures by selective electron beam melting and study the resultant mechanical properties based on each respective geometry to provide fundamental information for optimizing molecular architectures and predicting the mechanical properties of cellular solids. Design/methodology/approach – Cellular Ti6Al4V with CNT-like zigzag and armchair structures are fabricated by selected electron beam melting. The microstructures and mechanical properties of these samples are evaluated utilizing scanning electron microscopy, synchrotron radiation X-ray and compressive tests. Findings – The mechanical properties of the cellular solids depend on the geometry of strut architectures. The armchair-structured Ti6Al4V samples exhibit Young’s modulus from 501.10 to 707.60 MPa and compressive strength from 8.73 to 13.45 MPa. The zigzag structured samples demonstrate Young’s modulus from 548.19 to 829.58 MPa and compressive strength from 9.32 to 16.21 MPa. The results suggest that the zigzag structure of the Ti6Al4V cellular solids can achieve improved mechanical properties and the mechanism for the enhanced mechanical properties in the zigzag structures was revealed. Originality/value – The results provide an innovative example for modulating the mechanical properties of cellular titanium by adjusting the unit cell geometry. The Ti6Al4V cellular solids with single-walled CNT-like structures could be used as light-weight construction components or filters in industries. The Ti6Al4V with multiwalled CNT-like structures could be used as new scaffolds for biomedical applications.

Impurity magneto-optical absorption with the participation of resonance states of D 2 − centers in quantum wells

The dependence of the average binding energy of the resonance g-state of a D2− center on the induction of an external magnetic field in a quantum well with a parabolic confining potential is studied using the zero-range potential method. It has been shown that with an increasing exchange interaction, the character of the dependence of the average binding energy of the resonance g-state of the D2− center on the induction of the external magnetic field changes. It has been assumed that in GaAs/AlGaAs quantum wells alloyed with small Si donors, resonance D2− states can exist under conditions of exchange interaction. It has been found that in spectra of impurity magneto-optical absorption in multiwall quantum structures, exchange interaction manifests itself as oscillations of interference origin.

Structure in nascent carbon nanotubes revealed by spatially resolved Raman spectroscopy

The understanding of carbon nanotube (CNT) growth is crucial for the control of their production. In particular, the identification of structural changes of carbon possibly occurring near the catalyst particle in the very early stages of their formation is of high interest. In this study, samples of nascent CNT obtained during nucleation step and samples of vertically aligned CNT obtained during growth step are analysed by combined spatially resolved Raman spectroscopy and X-ray diffraction measurements. Spatially resolved Raman spectroscopy reveals that iron-based phases and carbon phases are co-localized at the same position, and indicates that sp2 carbon nucleates preferentially on iron-based particles during this nucleation step. Depth scan Raman spectroscopy analysis, performed on nascent CNT, highlights that carbon structural organisation is significantly changing from defective graphene layers surrounding the iron-based particles at their base up to multi-walled nanotube structures in the upper part of iron-based particles.

A Non‐Enzymatic Carbohydrate Sensor Based on Multiwalled Carbon Nanotubes Modified with Adsorbed Active Gold Particles

AbstractThe voltammetric behavior of a glassy carbon electrode modified with multiwalled carbon nanotubes (MWCNTs) and Au particles was studied in alkaline medium towards the electrooxidation of some carbohydrates used as model compounds. The influence of carbohydrate concentration and scan rate on peak potentials, peak currents, etc., observed at the modified electrode was evaluated and critically discussed. The Au particles dispersed into multiwalled carbon nanotubes structures showed favorable electrocatalytic and analytical properties towards the electrooxidation of xylose and glucose molecules. Atomic force microscopy performed on the resulting modified electrode showed a well‐efficient 3D distribution of Au active particles having sharp‐edged and elongated grains along bundles of the MWCNTs. The three‐dimensional MWCNT‐Au composite structure of the catalyst act as a promoter to enhance the diffusive character of recorded currents and probably also increases the rate of the heterogeneous electron transfer of the electrooxidation process considered.

Influence of Methane Gas on Growth of Multi-Walled Carbon Nanotubes by Arc Discharge Process

The influence of methane (CH4) gas as feedstock and ambient pressures on growth of multi-walled carbon nanotube (MWCNT) and carbon nano structures is studied by arc discharge process. The discharge is produced between graphite rods at different 100, 300 and 500 torr ambient pressures. Plasma kinematics such as density and temperature is determined by optical emission spectroscopy to investigate the contribution of ambient environment and pressure towards the growth of MWCNT and nano structures. The samples of synthesized MWCNT are analyzed transmission electron microscope, scanning electron microscope, Raman spectroscope and X-ray diffraction analysis. An increase in the growth of the carbon nanotubes with increase in the ambient pressure of the CH4 gas is observed. The CH4 molecules serve as feed stock and enhance the growth of carbon nanotubes. An increase in the growth of other carbon nanostructures such as graphene are also observed with increase in pressure.

J012013 電子デバイス用カーボンナノチューブの高密度電流下における強度特性([J012-03]電子情報機器、電子デバイスの強度・信頼性評価と熱制御(3))

Carbon nanotube (CNT) is expected to use as the materials of nano-scale components of electronic devices. In this study, we investigated the damage mechanism of Multi-walled (MW) CNT structures used as nano-component of electronic devices. An acceleration testing system of direct current loading was designed and the test structures collected MWCNT at the gap of Pt thin-film electrodes were treated in the system. The damage mechanisms of MWCNT are regarded as the effects of oxidation by Joule heating and/or the electromigration (EM) by high-density electron flows. Therefore the tests were conducted under the two kinds of current density and temperature conditions, and furthermore in both of air and low oxygen conditions. Lifetime of the specimen was determined by voltage measurements during the acceleration tests under the constant current condition. Their fracture phenomena were evaluated by means of microscopic observations. As the results, the amounts of lifetime of MWCNT were longer in the lower oxygen concentrations than in the air condition. In the microscopic studies, it was confirmed that the disconnections of MWCNT were occurred at the cathode side of the MWCNT structures under low current density, and the center area of MWCNT under high current density. Both types of damage morphologies induced by oxidation and EM were observed at the damaged MWCNT.

Electrospinning of multilevel structured functional micro-/nanofibers and their applications

Electrospinning is a straightforward and versatile way to fabricate multilevel structured ultrafine fibers down to the micro-/nanometer scale. In this review, recent achievements in electrospun fibers with multilevel surfaces and inner structures are presented. The multilevel surface structures include branched structures, porous structures, necklace structures and non-cylinder deformed structures. The multilevel inner structures are classified as peapod structures, multiwalled tube structures, wire-in-tube structures and multichannel structures. Furthermore, the outstanding properties of multilevel structured fibrous materials are discussed and highlighted. These electrospun multilevel structured materials have wide applications in sensing, filtration and adsorption, catalysis, energy storage, bio-field, and many other fields.

On Finite Element Modeling of Single- and Multi-Walled Carbon Nanotubes

In this study, Single-Walled and Multi-Walled Carbon Nanotubes in their perfect forms were investigated by the Finite Element Method. Details on the modeling of the structure are provided in this paper, including the appropriate elements, the element properties that should be defined based on the atomic structure of Carbon Nanotubes and the corresponding chemical bonds. Non-covalent van der Waals interactions between two neighbor atoms as well as the required approximations for the modeling of the structures with this kind of interaction are also presented. Specific attention was dedicated to the necessity of using some time- and energy-consuming steps in the simulation process. First, the effect of simulating only a single ring of the whole structure is studied to find out if it would represent the same mechanical behavior as the long structure. Results show that by applying an appropriate set of boundary conditions, the stiffness of the shortened structure is practically equal to the long perfect structure. Furthermore, Multi-Walled Carbon Nanotube structures with and without defining the van der Waals force are studied. Based on the observations, applying the van der Waals force does not significantly influence the obtained Young's modulus of the structure in the case of a uniaxial tensile test.

Control over the Diameter, Length, and Structure of Carbon Nanotube Carpets Using Aluminum Ferrite and Iron Oxide Nanocrystals as Catalyst Precursors

Both colloidal iron oxide and aluminum ferrite nanocrystals can initiate the growth of vertically aligned carbon nanotube (CNT) carpets through water-assisted chemical vapor deposition (CVD). The outer diameters of the CNTs ranged from 3 to 18 nm, and this dimension correlated well with the diameters of the starting nanocrystals. The smallest particles (4 nm aluminum ferrite nanocrystals) yielded CNT carpets with high percentages (60%) of single-walled CNT species, whereas larger particles (iron oxide nanocrystals over 25 nm) formed carpets with more double, triple, and larger multiwalled structures. CNTs grown by aluminum ferrite nanocrystals were uniformly of higher quality (IG/ID = 11.4) than comparable materials formed from pure iron oxides (IG/ID = 9.8). We speculate that the presence of aluminum in the nanocatalyst may slow the acetylene decomposition at the catalyst surface, leading to less production of amorphous carbon material.

Electron field emission properties of vertically aligned carbon nanotube point emitters

We fabricated vertically aligned carbon nanotube (CNT) point emitters directly on Cu wires with flat or sharp tips. The nanostructures and field emission properties of the two kinds of emitters were investigated. Both emitters showed vertically aligned, multi-walled bamboo-liked CNT structures and demonstrated good field emission properties. The emitter on the Cu wire with a flat tip (F-tip) attained an emission current of 70μA at 4200V whereas the emitter on the Cu wire with a sharp tip (S-tip) reached an emission current of 65μA at 3700V. Both of them demonstrated excellent field emission stability for 40h at emission currents of about 40μA and 10μA for the F-tip and S-tip emitters, respectively.

Growth of Metal-Free Carbon Nanotubes on Glass Substrate with an Amorphous Carbon Catalyst Layer

We have investigated the direct growth of metal-free carbon nanotubes (CNTs) on glass substrates with microwave-plasma enhanced chemical vapor deposition (MPECVD). Amorphous carbon (a-C) films were used as a catalyst layer to grow metal-free CNTs. The a-C films were deposited on Corning glass substrates using RF magnetron sputtering with the use of a carbon target (99.99%) at room temperature. They were pretreated with hydrogen plasma using a microwave PECVD at 600 degrees C. Then, CNTs were prepared using microwave PECVD with a mixture of methane (CH4) and hydrogen (H2) gases. The CNTs were grown at different substrate temperatures (400 degrees C, 500 degrees C, and 600 degrees C) for 30 minutes. Other conditions were fixed. The growth trends of CNTs against substrate temperature were observed by field emission scanning electron microscopy (FE-SEM). The structure of a-C catalyst layer and grown CNTs were measured by Raman spectroscopy. High-resolution transmission electron microscopy (HR-TEM) images showed that the CNTs had bamboo-like multi-walled structures. Energy dispersive spectroscopy (EDS) measurements confirmed that the CNTs consisted of only carbon.

Enhanced Photoelectrochemical Properties of CdS/ZnO Supported on Carbon Nanotube Films

We report here a facile vacuum filtration method to fabricate CdS and CdS-ZnO particles wrapped with carbon nanotubes on conductive glass substrates. The net-like nanotube structures facilitate carrier transport and greatly improve photon-response characteristics of CdS and CdS-ZnO particles. The enhancement is related to the structural characteristics of the nanotubes and the configuration of CdS and ZnO particles. For instance, single-walled carbon nanotube-based ZnO-CdS structures have higher photo-response currents than CdS-ZnO structures, whereas, multi-walled carbon nanotube-based CdS-ZnO structures have better photo-response characteristics than ZnO-CdS structures.

Shape-controlled synthesis of metal nanocrystals–multiwalled carbon nanotubes hybrid structures via electrodeposition

We have exploited a method for the synthesis of metal nanocrystals (NCs) with distinct geometries on multiwalled carbon nanotubes (MWCNTs) by employing the electrodeposition technique at room-temperature. The method has been demonstrated for the Cu NCs–MWCNTs hybrid structures. The shape of the Cu NCs on MWCNTs has been effectively manipulated by simply tuning the applied potential. The present work provides an alternative approach for the large scale synthesis of shape-controlled metal or/and alloy NCs–MWCNTs hybrid structures for a wide range of applications.