Ballistic conduction in multiwalled carbon nanotubes

The electrical transport in multiwalled carbon nanotubes is shown to be ballistic at room temperature with mean free paths on the order of tens of microns. The measurements are performed both in air and in the transmission electron microscope by contacting the free end of a nanotube pointing out of a fiber to a liquid metal and measuring the dependence of the nanotube resistance between the contacts. For a specific representative nanotube the resistance per unit length is found to be Rt = 31 +/- 61 omega/micron and the contact resistance with the liquid metal, Rc = 165 +/- 55 omega microns, corresponding to a mean free path l = 200 microns. Current-to-voltage characteristics are in accord with the electronic structure. The nanotubes survive high currents (up to 1 mA, i.e., current density on the order of 10(9) A/cm2). In situ electron microscopy shows that a relatively large fraction of the nanotubes do not conduct (even at high bias), consistent with the existence of semiconducting nanotubes. Discrepancies with other measurements are most likely due to damage caused to the outer layer(s) of the nanotubes during processing. The measured mean free path of clean, undamaged arc-produced multiwalled carbon nanotubes is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon nanotubes.     

Enhanced electrical conduction in aluminum wires coated with carbon nanotubes

Electrophoretic deposition was used to create SWCNT nanosized coatings on commercial aluminum wires. Electrical measurements showed an increase in the conductivity of about 160%. Due to the semiconductor nature of a fraction of the deposited SWCNTs, the high conductivity of the coated wires did not substantially decrease with increasing temperature (up to 600 K). Instead, increasing temperature and current were observed to affect the arrangement of the CNTs on the substrate, such that a web-like structure of SWCNTs was induced and resulted in further resistivity decrease.     

Vibrating carbon nanotubes as water pumps

Nanopumps conducting fluids directionally through nanopores and nanochannels have attracted considerable interest for their potential applications in nanofiltration, water purification, and hydroelectric power generation. Here, we demonstrate by molecular dynamics simulations that an excited vibrating carbon nanotube (CNT) cantilever can act as an efficient and simple nanopump. Water molecules inside the vibrating cantilever are driven by centrifugal forces and can undergo a continuous flow from the fixed to free ends of the CNT. Further extensive simulations show that the pumping function holds good not only for a single-file water chain in a narrow (6,6) CNT, but also for bulk-like water columns inside wider CNTs, and that the water flux increases monotonically with increasing diameter of the nanotube.     

Self-assembly of single-walled carbon nanotubes into multiwalled carbon nanotubes in water: molecular dynamics simulations

We report discoveries from a series of molecular dynamics simulations that single-walled carbon nanotubes, with different diameters, lengths, and chiralities, can coaxially self-assemble into multiwalled carbon nanotubes in water via spontaneous insertion of smaller tubes into larger ones. The assembly process is tube-size-dependent, and the driving force is primarily the intertube van der Waals interactions. The simulations also suggest that a multiwalled carbon nanotube may be separated into single-walled carbon nanotubes under appropriate solvent conditions. This study suggests possible bottom-up self-assembly routes for the fabrication of novel nanodevices and systems.     

Curvature effects on pressure-induced buckling of empty or filled double-walled carbon nanotubes

Summary The curvature effects of interlayer van der Waals (vdW) forces on pressure-induced buckling of empty or filled double-walled carbon nanotubes (DWNTs) are studied for various radii, length-to-radius ratios, end conditions and internal-to-external pressure ratios. The analysis is based on a double-elastic shell model and assumes that the interlayer vdW pressure at a point between the inner and outer tubes depends not only on the change of the interlayer spacing, but also on the change of the curvatures of the inner and outer tubes at that point. Here the role of filling substances inside DWNTs is modeled by a uniformly distributed internal pressure. The present work aims to study the curvature effects on critical radial pressure. An explicit formula is obtained for the external buckling pressure of empty or filled DWNTs. The critical value of external pressure is estimated with various internal-to-external pressure ratios. It is shown that the curvature effects play a more significant role in buckling problems under radial pressure for small radii DWNTs than under pure axial stress. Our results show that loading transfer through vdW forces prior to buckling is important for the pressure-induced buckling of DWNTs rather than axially compressed buckling.     

Evaporation of water droplets containing carbon nanotubes

The evaporation of water droplets containing carbon nanotubes has been experimentally studied. The droplets were evaporated in a flow of dry air at temperatures in a range of T ₀ = 20−200°C and Reynolds numbers designed on the initial diameter were Re = 500−2000. The results of measurements of the droplet surface temperature and evaporation rate show that the addition of ∼0.1 wt % nanoparticles to the base liquid (water) virtually does not change the laws of heat and mass transfer.     

Functionalization of carbon nanotubes by water plasma

Multiwall carbon nanotubes grown by plasma enhanced chemical vapour deposition were functionalized by H(2)O plasma treatment. Through a controlled functionalization process of the carbon nanotubes (CNTs) we were able to modify and tune their chemical reactivity, expanding the range of potential applications in the field of energy and environment. In particular, different oxygen groups were attached to the surfaces of the nanotubes (e.g. carboxyl, hydroxyl and carbonyl), which changed their physicochemical properties. In order to optimize the main operational parameters of the H(2)O plasma treatment, pressure and power, a Box-Wilson experimental design was adopted. Analysis of the morphology, electrochemical properties and functional groups attached to the surfaces of the CNTs allowed us to determine which treatment conditions were suitable for different applications. After water plasma treatment the specific capacitance of the nanotubes increased from 23?up to 68?F?g(-1) at a scan rate of 10?mV?s(-1).     

Reassessing fast water transport through carbon nanotubes

Pressure-driven water flow through carbon nanotubes (CNTs) with diameters ranging from 1.66 to 4.99 nm is examined using molecular dynamics simulation. The flow rate enhancement, defined as the ratio of the observed flow rate to that predicted from the no-slip Hagen-Poiseuille relation, is calculated for each CNT. The enhancement decreases with increasing CNT diameter and ranges from 433 to 47. By calculating the variation of water viscosity and slip length as a function of CNT diameter, it is found that the results can be fully explained in the context of continuum fluid mechanics. The enhancements are lower than previously reported experimental results, which range from 560 to 100 000, suggesting a miscalculation of the available flow area and/or the presence of an uncontrolled external driving force (such as an electric field) in the experiments.     

Proteins and carbon nanotubes: close encounter in water

The interactions between biological substances and carbon nanotubes (CNTs) and their effect on the nanotubes are of significant importance in this emerging era of nanobiotechnology. Consequently, highly stable dispersions of debundled CNTs in aqueous solution are an important prerequisite for their applications and for the development of nanotube-based molecular electronic and nanobiomedical devices. Here, we report that proteins can work as tools to this end if their primary structure and the pH value of the system are chosen appropriately. Proteins containing a large number of basic residues, for example, histone, are found to be the most promising protein tools for the dispersion of nanotubes. Apart from other interactions, the polarity of the protein seems to play a vital role in obtaining high yields of debundled nanotubes. In addition, an enrichment of metallic nanotubes in the products is observed, which offers a facile approach for separating nanotubes according to their electronic properties in the bulk.     

Ultrasonication study for suspending single-walled carbon nanotubes in water

A systematic calorimetry-based technique was developed to standardize single-walled carbon nanotube (SWNT) dispersion protocol. Simple calorimetric experiments were performed to benchmark the performance of the ultra-dismembrator. Temperature profiles for the sonication period were utilized to estimate energy input to the system. Energy loss profile was generated for the ultradismembrator in use and a calibration relationship was formulated that could standardize the sonication process. The standardized protocol was used to prepare aqueous SWNT suspensions-sonicating SWNTs in a varied range of input energy (18-100 kJ) in water. SWNT mass fractions suspended for each energy input was accurately measured and the suspended SWNT samples were characterized for morphology, surface potential, cluster size and structure, and chemical functionality using high resolution transmission electron microscopy (HRTEM), electrophoresis, dynamic and static light scattering (DLS/SLS), and Raman spectroscopy. The study demonstrated that suspended mass of SWNTs increased up to 18 kJ of energy input with no further increase upon continued energy input. The physicochemical properties showed similar trend for energy input. The aggregate cluster size, surface potential behavior, as well as the Raman defect properties plateaued after the initial energy input. The significant changes observed were limited to morphological properties, i.e., shorter length, debundled, and sharp edged SWNTs and fractal cluster formation (lower D(f)) with increased input energy.     

Dispersion of multiwalled carbon nanotubes in water by lignin

Multiwalled carbon nanotubes (MWCNT) can be stably dispersed in water with small amount of lignin. One-step dispersion in the 20.0 g/L concentration range is achieved at room temperature with excellent electrical properties of MWCNT. Lignin is depicted to act as an anti-static agent.     

The mechanism of water diffusion in narrow carbon nanotubes

Carbon nanotubes show exceptional physical properties that render them promising candidates as building blocks for nanostructured materials. Many ambitious applications, ranging from gene therapy to membrane separations, require the delivery of fluids, in particular aqueous solutions, through the interior of carbon nanotubes. To foster these and other applications, it is necessary to understand the thermodynamic and transport properties of water confined within long narrow carbon nanotubes. Previous theoretical work considered either short carbon nanotubes or short periods of time. By conducting molecular dynamics simulations in the microcanonical ensemble for water confined in infinitely long carbon nanotubes of diameter 1.08 nm, we show here that confined water molecules diffuse through a fast ballistic motion mechanism for up to 500 ps at room temperature. By comparing the results obtained for the diffusion of water to those obtained for the diffusion of a reference Lennard-Jones fluid, we prove here that long-lasting hydrogen bonds are responsible for the ballistic diffusion of water clusters in narrow carbon nanotubes, as opposed to spatial mismatches between pore-fluid and fluid-fluid attractive interactions which, as shown previously by others, are responsible for the concerted motion of simple fluids in molecular sieves. Additionally we prove here for the first time that, despite the narrow diameter of the carbon nanotubes considered which may suggest the existence of single-file diffusion, when the trajectories of confined water are studied at time scales in excess of 500 ps, a Fickian-type diffusion mechanism prevails. Our results are important for designing nano fluidic apparatuses to develop, for example, novel drug-delivery devices.     

Self-adjusted sustaining oscillation of confined water chain in carbon nanotubes

We show by molecular dynamics and first principle calculations that a water chain confined in carbon nanotubes can self-adjust into regular oscillation with remarkably lower entropy from random thermal motion with higher entropy at room temperature. The turning between the two phases is triggered by the water orientation fluttering into or from the energy optimum configuration of the chain. The findings are expected to be helpful in creation of self-sustaining nanoelectromechanical systems driven by ambient energy.     

水辅助化学气相沉积制备定向碳纳米管

为提高定向碳纳米管(Aligned carbon nanotubes,ANTs)的长度与面积,以二茂铁为催化剂前驱体、二甲苯为碳源,生长温度为850℃,采用水辅助化学气相沉积法在石英基底上制备ANTs。借助透射电镜(TEM)、扫描电子显微镜(SEM)分析了ANTs的形貌。结果表明,在优化的生长条件下,可获得面积为60×140 mm2、长为2.05 mm的ANTs;在微波辐射下ANTs会产生灼热的白光,是潜在的吸波材料。     

Adsorption of fluoride from water by aligned carbon nanotubes

Aligned carbon nanotubes (ACNTs), a new type of carbon material, were prepared by catalytic decomposition of xylene using ferrocene as catalyst. The kinetics experiment of ACNTs shows that fluoride adsorption rate is fast in the first 60 min and the adsorption capacity reaches 3.0 mg/g rapidly, then it decreases and adsorption achieves equilibrium gradually in about 180 min. The fluoride adsorption of ACNTs depends slightly on the solution pH value. The highest adsorption capacity of ACNTs occurs at pH 7 and reaches 4.5 mg/g at equilibrium fluoride concentration of 15 mg/l. The experimental results indicate that ACNTs are promising candidate materials for fluoride removal.     

Metal--semiconductor transition in single-walled carbon nanotubes induced by low-energy electron irradiation

We report the effect of low-energy (1 keV) electron beam irradiation on gated, three-terminal devices constructed from metallic single-walled carbon nanotubes. Pristine devices, which exhibited negligible gate voltage response at room temperature and metallic single-electron transistor characteristics at low temperatures, when exposed to an electron beam, exhibited ambipolar field effect transistor (room temperature) and single-electron transistor (low temperature) characteristics. This metal-semiconductor transition is attributed to inhomogeneous electric fields arising from charging during electron irradiation.     

PEGylation of double-walled carbon nanotubes for increasing their solubility in water

Polyethylene glycol (PEG) functionalized double-walled carbon nanotubes (DWNTs) have been synthesized by a [2 + 1] cycloaddition reaction and characterized by transmission electron microscopy, atomic force microscopy, Raman spectroscopy, thermal gravimetric analysis, and UV-visible spectroscopy. Functionalization affords a large increase in the aqueous solubility of DWNTs. The saturated concentrations of DWNTs functionalized with diazido-terminated PEG₈₀₀ (with a molecular weight of 800) and azido-terminated PEG₇₅₀ monomethylether (with a molecular weight of 750) are very similar—0.36 and 0.37 mg/mL (DWNTs equivalent concentration), respectively.     

Dispersion study of long and aligned multi-walled carbon nanotubes in water

Dispersion of nanotubes is a crucial step for many applications. The properties of the final nanotube-based material are strongly dependent on the quality of nanotube suspensions. In this study, long and aligned multi-walled carbon nanotubes obtained by catalytic chemical vapour deposition were dispersed in water with different dispersing agents using high intensity ultrasounds. Among different additives, we selected sodium dodecyl sulfate (SDS) as dispersing agent to prepare suspensions of nanotubes. UV-Visible spectrometry method was used to measure the influence of dispersion parameters (power and duration of sonication) on dispersion state and suspension stability. Therefore, we demonstrated that, even if high intensity ultra-sounds are breaking nanotubes, it is possible to obtain stable water-based suspensions containing MWNTs which exhibit length up to 20 microm.     

碳纳米管对水体有机污染物的吸附研究进展

独特的物理化学性质使新型碳纳米材料-碳纳米管( CNTs)具有优异的吸附性能,被广泛的应用在污染水体治理中.详细阐述了CNTs对水体有机污染物的吸附研究进展,并总结了影响因素和吸附机理,最后展望了碳纳米管吸附应用的前景.     

Chemically functionalized water soluble single-walled carbon nanotubes modulate neurite outgrowth

We report the use of chemically-functionalized water soluble single-walled carbon nanotube (SWNT) graft copolymers for modulation of outgrowth of neuronal processes. The graft copolymers were prepared by the functionalization of SWNTs with poly-m-aminobenzene sulphonic acid and polyethylene glycol. When added to the culturing medium, these functionalized water soluble SWNTs were able to increase the length of various neuronal processes.