Julio A. Alonso

Improved Charge Transfer at Carbon Nanotube Electrodes

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.

Enhancement of hydrogen physisorption on graphene and carbon nanotubes by Li doping

Density-functional calculations of the adsorption of molecular hydrogen on a planar graphene layer and on the external surface of a (4,4) carbon nanotube, undoped and doped with lithium, have been carried out. Hydrogen molecules are physisorbed on pure graphene and on the nanotube with binding energies about 80-90 meV/molecule. However, the binding energies increase to 160-180 meV/molecule for many adsorption configurations of the molecule near a Li atom in the doped systems. A charge-density analysis shows that the origin of the increase in binding energy is the electronic charge transfer from the Li atom to graphene and the nanotube. The results support and explain qualitatively the enhancement of the hydrogen storage capacity observed in some experiments of hydrogen adsorption on carbon nanotubes doped with alkali atoms.

Hydrogen storage in pure and Li-doped carbon nanopores: Combined effects of concavity and doping

Density functional calculations are reported for the adsorption of molecular hydrogen on carbon nanopores. Two models for the pores have been considered: (i) The inner walls of (7,7) carbon nanotubes and (ii) the highly curved inner surface of nanotubes capped on one end. The effect of Li doping is investigated in all cases. The hydrogen physisorption energies increase due to the concavity effect inside the clean nanotubes and on the bottom of the capped nanotubes. Li doping also enhances the physisorption energies. The sum of those two effects leads to an increase by a factor of almost 3 with respect to the physisorption in the outer wall of undoped nanotubes and in flat graphene. Application of a quantum-thermodynamical model to clean cylindrical pores of diameter 9.5 A, the diameter of the (7,7) tube, indicates that cylindrical pores of this size can store enough hydrogen to reach the volumetric and gravimetric goals of the Department of Energy at 77 K and low pressures, although not at 300 K. The results are useful to explain the experiments on porous carbons. Optimizations of the pore size, concavity, and doping appear as promising alternatives for achieving the goals at room temperature.

Optical to ultraviolet spectra of sandwiches of benzene and transition metal atoms: Time dependent density functional theory and many-body calculations

The optical spectra of sandwich clusters formed by transition metal atoms (titanium, vanadium, and chromium) intercalated between parallel benzene molecules have been studied by time-dependent density functional theory (TDDFT) and many-body perturbation theory. Sandwiches with different number of layers, including infinite chains, are considered. The lowest excitation energy peaks in the spectra are characteristic of the robust bonding in these complexes. The excitation energies vary in a systematic way with the metal atoms and with the cluster size, and so these materials could be used to tune the optical properties according to specific functionality targets. The differences in the spectra could be used to identify relative abundances of isomers with different spins in experimental studies. As a salient feature, this theoretical spectroscopic analysis predicts the metallization of the infinite (TiBz)(infinity) chain, which is not the case of (CrBz)(infinity).

Deformations and thermal stability of carbon nanotube ropes

Structural and thermal characteristics of crystalline ropes of single-wall carbon nanotubes (SWCNTs) are investigated. Novel crystalline ropes of polygonized SWCNTs produced by laser irradiation exhibit rounded-hexagonal cross sections in contrast to earlier observations of circular tubes. Extensive molecular dynamics (MD) simulations lead to several metastable structures of the lattice characterized by different tube cross sections, hexagonal, rounded-hexagonal and circular, and increasing cell volume. The competition between different tube shapes is analyzed and compared to experiments. On the other hand, bundles of SWCNTs coalesce, forming multiwall carbon nanotubes under thermal treatment at high temperatures. Extensive MD simulations confirm the single-wall-to-multiwall transformation and suggest the physical patching-and-tearing mechanism underlying the concerted coalescence of the tubes.

Photoabsorption spectra of Ti8C12 metallocarbohedrynes: Theoretical spectroscopy within time-dependent density functional theory.

The photoabsorption spectra of several of the most stable isomers of the Ti8C12 metallocarbohedryne are calculated using time-dependent density functional theory. Several ground-state magnitudes have been also calculated, such as cohesive energies, electronic gaps between the highest occupied and lowest unoccupied molecular orbitals, and static polarizabilities. Since significant differences are found among the photoabsorption spectra of the different isomers in the low energy region (0-5 eV), we propose the comparison of experimental and the calculated absorption spectra as a tool to elucidate the isomers that appear to form in the experiments. Between 10 and 13 eV all the spectra show a region of high absorption that we identify as due to collective electronic excitations. The existence of this prominent feature explains the occurrence of delayed ionization and delayed ion emission phenomena observed in previous experiments.

Theoretical study of the reactivity of cesium with benzene and graphitic CxHy clusters

The adsorption of a Cs atom on planar (C6H6 and C24H12) and nonplanar (C20H10 and C21H9) carbon clusters has been studied using the density-functional theory, with the local-density approximation and atomic pseudopotentials. Binding energies as a function of separation have been calculated for several configurations of the Cs atom on the different substrates. The adsorption on sites above the center of carbon rings is more stable than adsorption on top of carbon atoms and C-C bonds. In the case of the curved clusters, adsorption on the concave side is preferred compared to the convex side. The Cs bonding is stronger on the nonplanar clusters. The strength of the binding energy depends on two effects: the magnitude of the highest occupied molecular orbital-lowest unoccupied molecular orbital (LUMO) energy gap of the substrate, and the energy of the valence state of Cs relative to the LUMO of the substrate. Due to a favorable relative position of those two energy levels, charge transfer occurs from Cs to the two nonplanar clusters, and this provides an ionic contribution to the bonding. The analysis of the electronic density redistribution and of the local Fukui functions helps in the interpretation of the charge transfer and the reactivity.

Interaction of Hydrogen with Palladium Clusters Deposited on Graphene

Hydrogen adsorption on nanoporous carbon materials is a promising technology for hydrogen storage. However, pure carbon materials do not meet the technological requirements due to the week binding of hydrogen to the pore walls. Experimental work has shown that doping with Pd atoms and clusters enhances the storage capacity of porous carbons. Therefore, we have investigated the role played by the Pd dopant on the enhancement mechanisms. By performing density functional calculations, we have found that hydrogen adsorbs on Pd clusters deposited on graphene following two channels, molecular adsorption and dissociative chemisorption. However, desorption of Pd-H complexes competes with desorption of hydrogen, and consequently desorption of Pd-H complexes would spoil the beneficial effect of the dopant. As a way to overcome this difficulty, Pd atoms and clusters can be anchored to defects of the graphene layer, like graphene vacancies. The competition between molecular adsorption and dissociative chemisorption o...