Session K26 - Focus Session: Carbon Nanotube Gas Absorption.
FOCUS session, Tuesday afternoon, March 04
Room 17B, Austin Convention Center

[K26.001] Gas-surface interactions and charge-carrier dynamics in single-wall carbon nanotubes

The perfect surface-to-volume ratio of single-wall carbon nanotubes (SWNTs) whose atoms may all be exposed to the environment has raised prospects for the fabrication of SWNT based nanoscaled gas sensors with unique properties. Here, we report on a systematic study of the interaction of various gases and solvents including water with graphite, C_60- and SWNT surfaces. The experiments help to clarify questions related to the sensitivity of the electronic transport properties of SWNTs to different adsorbates as well as the wetting properties and solubility of these materials. We have furthermore performed a time-domain study of \ite-ph interactions in SWNT ropes using femtosecond time-resolved photoemission. We find that the \ite-ph mass enhancement parameter \lambda is extraordinarily small which explains why electronic transport in SWNTs can be ballistic over several microns, even at room temperature.

[K26.002] Carbon nanotubes as gas sensors: a first-principles study

Carbon nanotubes have recently been proposed as chemical sensors(J.Kong et al.), Science 287, 622 (2000)., due to their fast response and high sensitivity towards environmental gaseous molecules. However, the chemical and physical interactions between molecules and sensing nanotubes are not yet completely understood. Within this framework, first principles calculations within the density functional theory have been performed for simple molecules (such as NO_2, CO and O_3) adsorbed on (10,0) carbon nanotubes (CNT), using the Dmol^3 code(B. Delley, J. Chem. Phys. 113, 7756 (2000)). The effects of the different gases on the CNTs are discussed in terms of binding energies, charge transfer and density of states. Our findings indicate that NO_2 and O_3 molecules are charge acceptors, whereas CO is a charge donor. The CNT density of states is sensitive to the adsorption of NO_2 and O_3, with a high peak close to the CNT valence band maximum, leading to an enhanced p-type conductivity. On the other hand, the CO adsorption does not alter the CNT electronic properties. Our theoretical results are in excellent consistency with experimental changes of the tube conductivity upon different gas exposure.

[K26.003] CARBON NANOTUBES BASED RESONANT CIRCUIT SENSORS FOR GAS DETECTION

A circular disc resonator was used to study the gas sensing properties of carbon nanotubes. Single walled nanotube (SWNT) bundles were prepared using the electric arc method. These air-exposed SWNT bundles were then coated on the conducting circular disc. Significant shift in the resonant frequency (f_0) was detected when exposed to polar gases while no shift was observed when exposed to non-polar gases. Next, the SWNT coated resonator was degassed and noticeable shift in f_0 was observed upon exposure to both polar and non-polar gases. It was concluded that the already adsorbed oxygen on the air-exposed SWNT bundles masked the shifts induced by the non-polar gases. Effective medium approximation model was to explain shifts in f_0 when nanotubes were exposed to gases of different polarity. Using this configuration, a carbon nanotube based resonant circuit sensor is demonstrated which can detect the presence of a number of gases (He, O_2, CO, CO_2, NH_3).

[K26.004] Quantum Dents by Gas Atoms

We formulate a theory which gives a quantum mechanical description of the effects of collisions by gas atoms on the electrical transport properties of nanowires and nanotubes. The collisions of gas atoms transfer energy to the structure, resulting in phonon excitation. From the scattering amplitude we estimate the average additional number of phonons due to these collisions. We also calculate the electron-phonon matrix element for various structures; this allows the computation of the life time of electrons via the self-energy. Finally we discuss the implication of the results to the electrical resistivity of the carbon nanotubes caused by collisions by gas atoms.

[K26.005] Gas Interactions with Single-walled Carbon Nanotubes

We present reults of extensive investigations of the interaction of gases and molecular vapors with carbon nanotubes. The experiments were carried out on thin films of purified bundles of carbon nanotubes and include four-probe resistance, thermoelectric power and Raman scattering measurements. We find that the transport properties are most sensitive to gases that chemisorb and undergo weak charge transfer reactions. It is perhaps surprising that these transport parameters are even sensitive to physisorbed molecular gases, for example, six-membered ring molecules with the strength of the effect correlating with the number of pi electrons on the molecule. Furthermore, even contact with gases such as He and N2, where the collisions must be driving the perturbation on the transport properties, can be detected. Finally, we present our data on the effects of the contact of molecular oxygen (below 200 C) with carbon nanotubes and attempt to reconcile what we see as a charge transfer phenomenon with experimental and theoretical results of other groups.

[K26.006] Adsorption isotherm study of neon on the outer surface of SWNT bundles

We have investigated the adsorption of neon on unpurified samples of SWNTs prepared by arc-discharge. We have measured isotherms at temperatures between 19.5 and 30 K. The coverages studied in these measurements are in the region above the completion of the 1st layer on the outside surface of the bundles. We will present values for the isosteric heat in the coverage region studied. Our results will be compared with those from computer simulations for this system. Preliminary results, obtained on unpurified HiPCO nanotube samples using the same adsorbate, will also be discussed. This work is supported by the NSF through grant DMR-0089713.

[K26.007] H2 and N2 Adsorption on Activated-Nanofiber-Doped Carbon Liquid Crystals

Activated-nanofiber-doped carbon liquid crystals were made by doping mesogenic carbon materials with nanofibers, and activating them using water vapor. The adsorption of N2 and H2 was investigated on the resulting composite substrates. Isotherms were performed at 77.3 K. The BET equation was used to determine the surface area of the substrate. From the N2 measurements we found a specific area of 1660.6 m2/gram. We also investigated H2 adsorption for pressures between 0 and 7 atmospheres. At 77.3 K we obtained 3 percent, by weight, H2 adsorption at the highest pressures studied.

[K26.008] High temperature phase transition of hydrogen in nanotube bundles

In recent work [1,2] we have shown that the ground state energy of small molecules adsorbed within interstitial channels of nanotube (NT) bundles is reduced when dilation of the NT lattice is included. Here, we consider the local deformation (bending) of the tubes produced by a single molecule in the interstitial channel and we evaluate the molecule's energy as it gets dressed by the NT deformation "field". The effective interaction between two such dressed particles is computed, and we consider the possibility of a high temperature condensation transition originated by that effective attraction.

[1] M.M. Calbi, F. Toigo, M.W. Cole, Phys. Rev. Lett. 86, 5062 (2001). [2] M.M. Calbi, F. Toigo, M.W. Cole, J. Low Temp. Phys. 126, 179 (2002)

[K26.009] Diffusion of H2 on Single Walled Carbon Nanotubes

>From their discovery, carbon nanotubes have drawn interest for a variety of reasons. The attention has been focused on practical applications, such as hydrogen storage and isotope and spin selectivity, to novel effects resulting from the manifestation of reduced dimensionality due to the geometry of the tubes in the bundles. Of particular interest is the adsorption and storage of molecular hydrogen in the carbon nanotube bundle due to the possibility of using the nanotubes as a fuel cell for molecular hydrogen. This interest has arisen from the need for a clean fuel source and a safe and effective method to transport such a fuel. An investigation of the microscopic properties of the adsorbed hydrogen is essential in understanding the suitability of such a system in storing and transporting molecular hydrogen.

We report quasielastic neutron scattering (QENS) measurements performed on molecular hydrogen adsorbed on single walled carbon nanotubes (SWNT’s). These measurements inidicate that no quasielastic component to the scattering is present below 30K. A quasielastic component is present at 30K, indicating the onset of mobility of the adsorbed hydrogen molecules. The diffuse component to the scattering is well described by a liquid-like diffusion model, with a diffusion coefficient an order of magnitude greater than that of bulk liquid molecular hydrogen. The observed diffusion is consistent with 2-dimensional diffusion on Grafoil and indicates that the adsorbed hydrogen is leaving the groove sites of the nanotubes bundles before diffusing on the outer surface of the bundles.

[K26.010] Specific heat and thermodynamic behavior of gases adsorbed on the outside of nanotube bundles

>From grand canonical Monte Carlo simulations we derive the specific heat of several phases of methane as a function of the temperature. At certain temperature there appears a peak, due to thermal excitation of the particles to regions of higher potential energy (outside the groove and/or monolayer zone). For very low coverage, we compare these results with those from a simple model which retains the main features of the system and allows us to investigate some quantum corrections. Results of phonon calculations provide an explicit basis for computing quantum corrections at higher density. We discuss a possible experimental evidence of this behavior [1] as well as of other thermal properties [2,3].

[1] D.G. Narehood et al, submitted to Phys. Rev. Lett.

[2] A.D. Migone and S. Talapatra, to appear in the Encyclopedia of Nanoscience and Nanotechnology (American Scientific Publishers).

[3] T. Wilson et al, J. Low Temp. Phys. 126, 403 (2002); M. Muris et al, Surf. Sci. 492, 67 (2001); M. Muris et al, Langmuir 16, 7019 (2000).

[K26.011] Ab initio simulations of H_2 in Li-doped graphite and carbon nanotube systems

The issue of hydrogen storage in nanotube systems is great technological interest and importance. Recent experiments seem to indicate that the uptake of molecular hydrogen is greatly enhanced in Li-doped graphite and carbon nanotube systems \citeChen. Hence, we have examined this problem with standard density functional methods. Our results, however,are not in agreement with the experiments, and indicate that the uptake in Li-doped graphite is not thermodynamically favored unless the distance between the graphene sheets is substantially increased. Similar results are obtained for carbon nanotube systems. In addition, we have examined the exchange of H_2 molecules with nanotube interiors via defects in the nanotube sidewalls. We find that there are substantial barriers for H_2 molecules to enter, in all but the largest defects, indicating that the processing of nanotubes by mechanical means, such as ball milling, is not likely to lead to a substantial increase in the H_2 uptake.

[K26.012] Li and H2 Diffusion in Carbon Nanotubes for Li-Batteries and Hydrogen Storage: A Predictive Model

We present here theory of diffusion of atoms like Li and H and diatomic molecules like Li2 and H2 in the armchair nanotubes. We derived the Arrhenius type diffusion coefficient in terms of the quantum Boltzmann equation. The diffusion coefficient was derived with the consideration both the motion of the particle in the cylindrical periodic potential of the nanotube and the interaction of the diffuse particle with lattice vibrations such as longitudinal, torsion and deformation vibrations of the tubule. We also evaluated the energies of activation of the diffusion in terms of the pair interaction potential. The pair interaction potential is derived for arbitrary position of two interacting charges with respect to the tubule. We argue that there is a minimal diameter of the tubule below which the diffusion is significantly reduced.

[K26.013] ADSORPTION OF ACETONE ON CARBON NANOTUBES

Strong polar solvents are known to interact chemically with carbon nanotubes. The experiment indicate that the acetone molecule chemisorbs on nanotube surfaces and alters their surface properties. We have studied interaction of acetone using hybrid quantum mechanical and semi-empirical methods based on ONIOM scheme. We find that presence of topological defects could explain the strong chemical interaction between acetone and nanotube surface. The computed binding energies are in good agreement with the experimental thermal desorption data.

Part K of program listing