Surface graphitization and mechanical properties of hot-pressed bulk carbon nanotubes compacted by spark plasma sintering

The carbon nanotube bulk material was prepared by the spark plasma sintering of multi-walled carbon nanotubes. When the surface of the bulk material was polished unexpected graphitization happened and the surface became covered by interlocked graphene layers. A similar phenomenon was observed in polishing raw multi-walled carbon nanotubes, indicating that a sheer stress caused graphene layers to peel away from the multi-walled carbon nanotubes. Nanoindentation and scratch techniques were used to evaluate the mechanical properties of this material. It was suggested that the easy sliding between graphene layers in a multi-walled carbon nanotube played a major role in determining its mechanical behavior.     

The effect of sulfur on the number of layers in a carbon nanotube

This study shows that S concentration in the Fe catalyst plays an important role in controlling the number of graphene layers during carbon nanotube (CNT) growth via chemical vapor deposition. Using different S concentrations, we have grown thin carbon nanotubes with from 1 to 5 layers. Single-walled carbon nanotubes growing from the active surface of an FeS-Fe eutectic were observed. A growth model basing on the HRTEM characterization and thermodynamic analysis is suggested in which the CNT nucleates and grows on the active surface area of the catalyst where S accumulates in the form of an FeS-Fe eutectic, and the S distribution on the catalyst particle surface determines the number of CNT layers.     

Semiconducting properties of cup-stacked carbon nanotubes

The current-voltage characteristics of individual cup-stacked carbon nanotubes (CSCNTs) were investigated in situ inside the transmission electron microscope. Different from other quasi-1D carbon structures such as multi-walled carbon nanotubes, carbon nanofibers or graphitic fibers that normally behave as a metallic conductor of electrons, individual CSCNTs were found to exhibit unexpectedly semiconducting behaviors due to the special stacking microstructure of graphene layers. The band gap of the CSCNTs was obtained with the value of about 0.44 eV, in contrast to the zero-gap semiconducting quasi-2D graphene. These findings provide new information about the effect of the stacking graphene layers on their electronic properties, and will widen the usefulness of such stacking structure for the application in nanoelectronics.     

Synthesis of large area carbon nanosheets for energy storage applications

We report the large area growth of highly conductive carbon nanosheets (CNS) composed of few layer graphene on 200 mm diameter Si substrates using conventional radio frequency plasma-enhanced chemical vapour deposition. Raman spectroscopy is used to characterise the evolution of the CNS nucleation and growth with time in conjunction with TEM revealing the nano-sized graphene-like nature of these films and the intimate contact to the substrate. An individual sheet can have edges as thin as 3 graphene layers. The influence of the growth support layer is also discussed as film growth is compared on titanium nitride (TiN) and directly on Si. Electrochemical cyclic voltammogram (CV) measurements reveal these layers to form an excellent electrical contact to the underlying substrate with excellent stability towards oxidation whilst having a large electrochemical surface area. The resistance of a 150 nm film was measured to be as low as 20 μohm cm. The high percentage of narrow few layer graphene edge sites exposed allows for faster electrochemical reaction rates compared to carbon nanotubes (CNTs) and other electrode materials (glassy carbon and Pt).     

MgO-catalyzed growth of N-doped wrinkled carbon nanotubes

We report a MgO-catalyzed growth of N-doped carbon nanotubes (N-CNTs) that are constructed by graphene layers with wrinkles. Introducing NH₃ to a chemical vapor deposition process and keeping a high reaction temperature (∼900 °C) are key factors to the N-CNT growth from MgO. Without N-doping or at a lower temperature, only graphene sheets deposited on MgO surfaces were obtained. Compared to the Fe-catalyzed N-CNTs, the MgO-catalyzed N-CNTs have larger diameters, thinner walls, more surface wrinkles and open ends with polygonal graphene edges. Because carbon dissolves into MgO in a very limited amount, the structural defects and broken sites formed in the graphene layers due to N-doping might have contributed to the formation of new graphene layers and thus lead to the growth of the wrinkled tubes.     

The surface acidity of acid oxidised multi-walled carbon nanotubes and the influence of in-situ generated fulvic acids on their stability in aqueous dispersions

The oxidation of multi-walled carbon nanotubes (MWCNTs) with nitric acid was studied. In addition to the formation of oxygen-containing surface functional groups, the oxidative digestion of graphene caps and layers generated polycyclic aromatic substances, classified as fulvic acids (FAs). The latter remained immobilised on the MWCNT surface in acidic and neutral solutions but were released in basic pH solutions, which were subsequently separated, purified and characterised by high-performance liquid chromatography and mass spectrometry. Using acid-base titrations, the number of surface acidic functional groups was determined, which was shown to significantly decrease after FAs were removed. Depending on the length of oxidation, FAs account for up to 43% of the surface acidity of MWCNTs. The protonated solubilised fulvic acids can be readsorbed on the surface of oxidised or unfunctionalised MWCNTs, which assists the stability of carbon nanotube suspensions in the aqueous phase.     

The effects of confinement inside carbon nanotubes on catalysis

The unique tubular morphology of carbon nanotubes (CNTs) has triggered wide research interest. These structures can be used as nanoreactors and to create novel composites through the encapsulation of guest materials in their well-defined channels. The rigid nanotubes restrict the size of the encapsulated materials down to the nanometer and even the sub-nanometer scale. In addition, interactions may develop between the encapsulated molecules and nanomaterials and the CNT surfaces. The curvature of CNT walls causes the π electron density of the graphene layers to shift from the concave inner to the convex outer surface, which results in an electric potential difference. As a result, the molecules and nanomaterials on the exterior walls of CNTs likely display different properties and chemical reactivities from those confined within CNTs. Catalysis that utilizes the interior surface of CNTs was only explored recently. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside CNTs often leads to a different catalytic activity with respect to the same metals deposited on the CNT exterior surface. Furthermore, this inside and outside activity difference varies based on the metals used and the reactions catalyzed. In this Account, we describe the efforts toward understanding the fundamental effects of confining metal nanoparticles inside the CNT channels. This research may provide a novel approach to modulate their catalytic performance and promote rational design of catalysts. To achieve this, we have developed strategies for homogeneous dispersion of nanoparticles inside nanotubes. Because researchers have previously demonstrated the insertion of nanoparticles within larger nanotubes, we focused specifically on multiwalled carbon nanotubes (MWCNTs) with an inner diameter (i.d.) smaller than 10 nm and double-walled carbon nanotubes (DWCNTs) with 1.0-1.5 nm i.d. The results show that CNTs with well-defined morphology and unique electronic structure of CNTs provide an intriguing confinement environment for catalysis.     

Melt Compounding of Thermoplastic Polyurethanes Incorporating 1D and 2D Carbon Nanofillers

Thermoplastic polyurethane nanocomposites incorporating carbon nanotubes and graphene nanoplatelets were prepared through melt blending and compression molding, and the compounding process was optimized taking into account the different physical properties of one-dimensional carbon nanotubes and two-dimensional graphene nanoplatelets. Filler dispersion was further improved in the case of carbon nanotubes by noncovalent surface modification using a specific surfactant. The well-dispersed nanofillers favored enhanced phase separation in the thermoplastic polyurethane, leading to a better microstructure, which is able to improve the load transfer and maximize the tensile and viscoelastic properties.     

Catalyst faceting during graphene layer crystallization in the course of carbon nanofiber growth

The low temperature catalytic growth of multiwall carbon nanotubes (MWCNTs) rests on the continuous nucleation and growth of graphene layers at the surface of crystalline catalyst particles. Here, we study the atomic mechanisms at work in this phenomenon, by observing the growth of such layers in situ in the transmission electron microscope, in the case of iron-based catalysts. Graphene layers, parallel to the catalyst surface, appear by a mechanism of step flow, where the atomic layers of catalyst are “replaced” by graphene planes. Quite remarkably, catalyst facets systematically develop while this mechanism is at work. We discuss the origin of faceting in terms of equilibrium particle shape and graphene layer nucleation. Step bunching due to impeded step migration, in certain growth conditions, yields characteristic catalyst nail-head shapes. Mastering the mechanisms of faceting and step bunching could open up the way to tailoring the structure of low temperature-grown MWCNTs, e.g. with highly parallel carbon walls and, ultimately, with controlled structure and chirality.     

In situ transmission electron microscopy of electrochemical lithiation,delithiation and deformation of individual graphene nanoribbons

We report an in situ transmission electron microscopy study of the electrochemical behavior of few-layer graphene nanoribbons (GNRs) synthesized by longitudinal splitting the multi-walled carbon nanotubes (MWCNTs). Upon lithiation, the GNRs were covered by a nanocrystalline lithium oxide layer attached to the surfaces and edges of the GNRs, most of which were removed upon delithiation, indicating that the lithiation/delithiation processes occurred predominantly at the surfaces of GNRs. The lithiated GNRs were mechanically robust during the tension and compression tests, in sharp contrast to the easy and brittle fracture of the lithiated MWCNTs. This difference is attributed to the unconfined stacking of planar carbon layers in GNRs leading to a weak coupling between the intralayer and interlayer deformations, as opposed to the cylindrically confined carbon nanotubes where the interlayer lithium produces large tensile hoop stresses within the circumferentially-closed carbon layers, causing the ease of brittle fracture. These results suggest substantial promise of graphene for building durable batteries.     

Comparative study of methylene blue dye adsorption onto activated carbon,graphene oxide,and carbon nanotubes

Three different carbonaceous materials, activated carbon, graphene oxide, and multi-walled carbon nanotubes, were modified by nitric acid and used as adsorbents for the removal of methylene blue dye from aqueous solution. The adsorbents were characterized by N₂ adsorption/desorption isotherms, infrared spectroscopy, particle size, and zeta potential measurements. Batch adsorption experiments were carried out to study the effect of solution pH and contact time on dye adsorption properties. The kinetic studies showed that the adsorption data followed a pseudo second-order kinetic model. The isotherm analysis indicated that the adsorption data can be represented by Langmuir isotherm model. The remarkably strong adsorption capacity normalized by the BET surface area of graphene oxide and carbon nanotubes can be attributed to π–π electron donor acceptor interaction and electrostatic attraction.     

Multilayered silicon embedded porous carbon/graphene hybrid film as a high performance anode

Silicon (Si) has been regarded as one of the most attractive anode materials for the next generation lithium-ion batteries because of its large theoretical capacity, high safety, low cost and environmental benignity. However, the architecture of Si-based anode material still needs to be well designed to overcome the structure degradation and instability of the solid-electrolyte interphase caused by a large volume change during cycling. Here we report the electrochemical performances of a novel binder-free Si/carbon composite film consisting of alternatively stacked Si-porous carbon layers and graphene layers, which is synthesized by electrostatic spray deposition followed by heat treatment. For this composite film, Si nanoparticles are embedded in the porous carbon layer composed of nitrogen-doped carbon framework, carbon black and carbon nanotubes. And the combined Si-porous carbon layer is further sandwiched by flexible and conductive graphene sheets. The multilayered Si-porous carbon/graphene electrode shows a maximum reversible capacity of 1020 mAh g⁻¹ with 75% capacity retention after 100 cycles and a good rate capability on the basis of the total electrode weight. The excellent electrochemical performances are attributed to the fact that the layer-by-layer porous carbon matrix can accommodate the volume change of Si particles and maintain the structural and electrical integrities.     

A flexible nanostructured paper of MnO NPs@MWCNTs/r-GO multilayer sandwich composite for high-performance lithium-ion batteries

Designing a high capacity and long cycle life MnO-based composite material for lithium ion batteries (LIBs) is still a great challenge because of the intrinsically low electrical conductivity and dramatic volume variations during lithiation/delithiation. In this paper, the MnO nanoparticles (MnO NPs) are recombined with multi-walled carbon nanotubes (MWCNTs) and reduced graphene oxide (r-GO) to rationally construct a novel MnO NPs@MWCNTs/r-GO multilayer sandwich structure via electrostatic interaction self-assembly and vacuum filtration processes. As a result, the MnO NPs are closely attracted in the conductive MWCNTs network, and the MWCNTs adsorbed on the surface of MnO NPs can be served as a soft and flexible carbon coating layer to self-adapt the huge volume expansion. For another, the r-GO between two MnO NPs@MWCNTs layers is the cause to form a free-standing paper, enhancing the transverse conductivity of the whole electrode simultaneously. These features will contribute to achieve excellent cycling stability and improved rate capability.     

Chemical-free synthesis of graphene–carbon nanotube hybrid materials for reversible lithium storage in lithium-ion batteries

Graphene–carbon nanotube hybrid materials were successfully prepared through the π–π interaction without using any chemical reagent. We found that the ratio between carbon nanotube and graphene had critical influences on the state in aqueous solution and morphology of hybrid materials. Field emission scanning electron microscope and transmission electron microscope analysis confirmed that graphene nanosheets wrap around individual carbon nanotubes and form a homogeneous three-dimensional hybrid nanostructure. When applied as an anode material in lithium ion batteries, graphene–carbon nanotube hybrid materials demonstrated a high reversible lithium storage capacity, a high Coulombic efficiency and an excellent cyclability.     

Chirality dependent surface adhesion of single-walled carbon nanotubes on graphene surfaces

The adhesion of single-walled carbon nanotubes to the surface of graphene has been studied, mixing shortened tubes of different chiralities and few-graphene crystal membranes. The spontaneous atomic match of the two lattices was directly imaged using high-resolution transmission electron microscopy and carefully analyzed by means of electron diffraction, and we found evidences that surfaces of few-graphene crystal act as tangential nano-sieves, preferentially retaining zig-zag tubes to their surface.     

Graphene unrolled from ‘cup-stacked’ carbon nanotubes

High quality graphene with a large area and smooth edges has been obtained by unrolling the so-called ‘cup-stacked’ carbon nanotubes (CSCNTs) by the solution-phase oxidation and reduction. Atomic force microscopy and transmission electron microscopy observations reveal that the obtained graphene layers can even have a size of 20 μm in width and 100 μm in length, much larger than that of graphene unzipped from multi-walled carbon nanotubes. The low ratio of the D to G band intensities (within the 0.15–0.20 range) in Raman spectra indicates high quality of the obtained graphene, when compared to other graphene produced by the solution-phase oxidation. A formation mechanism is suggested for the graphene unrolled from the CSCNTs, providing an insight into the real microstructure of the CSCNTs, which are essentially continuous graphene layers rolled along the tube axis, yielding a pseudo cup-stacked like structure.     

A new molecular model for water adsorption on graphitized carbon black

Adsorption of water on graphitized carbon black at various temperatures has been studied with a new molecular model of graphitized carbon black using Monte Carlo simulation. The model is a collection of graphene layers, modelled by the Steele potential, and a number of phenol groups forming clusters of various sizes which are placed randomly at the graphene edge sites to give an O/C ratio of 0.006. The results are compared with experimental data reported by Kiselev et al. [1] in 1968 for a range of temperatures, and for the first time a reconciliation between the experimental data and simulation has been successfully achieved. The simulation results show that water adsorbs preferentially around the functional groups to form clusters, which then grow and merge at the edges of the graphene layers, rather than adsorbing onto the basal planes of the graphene because the electrostatic interactions (hydrogen bonding) between water molecules are stronger than the basal plane–water dispersion interactions.     

Alignment of graphene nanoribbons by an electric field

In this paper, we develop an analytical approach to predict the field-induced alignment of cantilevered graphene nanoribbons. This approach is validated through molecular simulations using a constitutive atomic electrostatic model. Our results reveal that graphene’s field-oriented bending angle is roughly proportional to the square of field strength or to the graphene length for small deformations, while is roughly independent of graphene width. The effective bending stiffness and the longitudinal polarizability are found to be approximately proportional to the square of graphene length. Compared with carbon nanotubes, graphene nanoribbons are found to be more mechanically sensitive to an external electric field.     

Carbon nanotubes coated by carbon nanoparticles of turbostratic stacked graphenes

Carbon nanotubes (CNTs) decorated by a high density of carbon nanoparticles of turbostratic graphene stacks have been fabricated by low energy hydrocarbon ion deposition at 700 °C. Transmission and scanning electron microscopy show that the carbon particles of turbostratic graphene stacks extend from the nanotube surface. The diameter of CNTs decreases with the increasing percentage of hydrogen in the gas phase. Raman spectroscopy indicates that the formation of carbon nanoparticles of turbostratic graphene stacks results from the high temperature used in the experiment. Meanwhile, Raman spectroscopy and high resolution transmission electron microscopy indicate that the carbon nanoparticle degree of crystallinity is lower with increasing hydrogen content in the gas phase due to the etching effect of hydrogen ions.     

Layer-dependent nanoscale electrical properties of graphene studied by conductive scanning probe microscopy

The nanoscale electrical properties of single-layer graphene (SLG), bilayer graphene (BLG) and multilayer graphene (MLG) are studied by scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM). The quantum capacitance of graphene deduced from SCM results is found to increase with the layer number (n) at the sample bias of 0 V but decreases with n at -3 V. Furthermore, the quantum capacitance increases very rapidly with the gate voltage for SLG, but this increase is much slowed down when n becomes greater. On the other hand, the magnitude of the EFM phase shift with respect to the SiO₂ substrate increases with n at the sample bias of +2 V but decreases with n at -2 V. The difference in both quantum capacitance and EFM phase shift is significant between SLG and BLG but becomes much weaker between MLGs with a different n. The layer-dependent quantum capacitance behaviors of graphene could be attributed to their layer-dependent electronic structure as well as the layer-varied dependence on gate voltage, while the layer-dependent EFM phase shift is caused by not only the layer-dependent surface potential but also the layer-dependent capacitance derivation.