Showing papers by "Pulickel M. Ajayan published in 1999"

Improved Charge Transfer at Carbon Nanotube Electrodes

Abstract: The closed topology and tubular structure of carbon nanotubes make them unique among different carbon forms and provide pathways for chemical studies. A number of investigations have been carried out to find applications of nanotubes in catalysis, hydrogen storage, intercalation, etc. Since carbon-electrode-based fuel cells have been experimented with for decades, it is of importance to learn the electrodic performance of these new carbon structures. We report here results of the electrocatalytic reduction of dissolved oxygen (important H2±O2 fuel cell reaction), using microelectrodes constructed from multiwalled nanotubes. In parallel, ab initio calculations were performed for oxygen deposited on the lattice and defect sites of nanotube surfaces to determine the charge transfer during oxygen reduction and compared with similar reactions on planar graphite. The microelectrodes were constructed in the following way (see Fig. 1). Multiwalled nanotubes (10 mg) prepared by the electric arc discharge process and liquid paraffin (4 mL) were intimately mixed, placed in the narrow cylindrical slot of a Perspex holder and then packed by smooth vibration. The assembly was cured at 50 C for 30 min. From the inner side of the Perspex, contact to a copper lead was made through conducting paint. Carbon paste electrodes (based on commercially available graphite powder) were prepared similarly. Carbon nanotube electrodes were prepared earlier by similar techniques to probe bioelectrochemical reactions. The need for oxygen reduction at catalytic surfaces has been recognized in fuel cells, batteries, and many other electrodic applications. Hence, oxygen reduction at nanotube surfaces is of great interest. Electrochemical reduction of dissolved oxygen is carried out in aqueous acidic (H2SO4) and neutral media (1 M KNO3). The solution is first degassed by bubbling nitrogen gas for about 15± 30 min in order to record the background current±voltage curves. Under these conditions, no cyclic voltammetric peak in the potential range 0 to ±0.8 V were observed. The same solutions were then saturated with oxygen by bubbling oxygen gas for 15 min. The cyclic voltammetric curve showed a well-defined peak at Epc = ±0.31 V vs. SCE (saturated calomel electrode) in H2SO4 solution (pH 2) at the carbon nanotube electrodes. At the carbon paste electrodes only an ill-defined peak is seen at Epc = ±0.48 V. In the KNO3 medium (pH 6.2), the reduction of dissolved oxygen is observed at Epc = ±0.51 V at the carbon nanotube electrode. This peak is shifted at the carbon paste electrode by about 30 mV. The shift of the peaks, corresponding to the reaction on the nanotube electrodes, is a strong indication of the electrocatalysis on this electrode (see discussion below). The shift may be considered as an overpotential, which indicates a more facile reaction occurring at the nanotubes compared to other carbons. The electrochemical reduction of oxygen is a function of pH of the medium as proton participation occurs as described by Equation 1.

Boron-mediated growth of long helicity-selected carbon nanotubes

Abstract: We investigate the growth of B-doped carbon nanotubes combining experimental and theoretical techniques. Electron microscopy observations and electron diffraction patterns reveal that B doping considerably increases the length of carbon tubes and leads to a remarkable preferred zigzag chirality. These findings are corroborated by first-principles static add dynamical simulations which indicate that, in the zigzag geometry, B atoms act as a surfactant during growth, preventing tube closure. This mechanism does not extend to armchair tubes, suggesting a helicity selection during growth.

Carbon nanotubes : from macromolecules to nanotechnology

Abstract: The discovery of fullerenes (1) provided exciting insights into how architectures built from pure carbon units can result in new symmetries and structures with remarkable physical properties. Carbon nanotubes represent the most striking example (2). A carbon nanotube can be considered as the ultimate fiber, reflecting highly organized, near ideal sp2 bonded carbon structure. The organization of the hexagonal honeycomb carbon lattice into cylinders with helical arrangement of hexagonal arrays has created a very unusual macromolecular structure that is by far the most superior carbon fiber ever made.

Fractionation of Multiwalled Carbon Nanotubes by Cascade Membrane Microfiltration

Abstract: Multiwalled carbon nanotubes were purified and size-separated by a multistep microfiltration process through a sequence of track-etched polycarbonate membranes of various pore sizes in both dead-ended and cross-flow mode. For this cascade microfiltration, the electric arc derived raw multiwalled samples were suspended in an aqueous solution of sodium dodecyl sulfate in deionized water. By examining the deposits on the membrane surfaces and in the permeate suspensions with scanning electron microscopy and atomic force microscopy, the nanotube fractionation was confirmed and analyzed. These scanning techniques showed that the components of the crude sample, which included carbon nanotubes, polyhedral nanoparticles, and large aggregates, were separated from each other during the filtration. In addition, fractionation of the multiwalled carbon nanotubes according to length was possible.

High spatial resolution imaging and spectroscopy in nanostructures

Abstract: The challenge during the characterization of nanostructures is in extracting consistent local and spatially varying information from the structure and correlating it to the new physics that appears at the nanoscale. Recent years have seen exciting advances in imaging and spectroscopy techniques that can achieve this goal. The techniques offer the possibility to directly investigate local properties of materials with sub-nanometre spatial resolution. In this paper we review, from the perspective of our own contributions to the field of carbon nanostructures, recent advances in the application of nanoanalysis. Complementary techniques of spatially resolved electron, and scanning tunneling microscopy and spectroscopy, can be used to characterize and correlate the structure, topology, chemistry and electronic properties of nanostructures. Often the interpretation of the data needs to come from comparison with adequate theoretical models.