Bhabendra K. Pradhan

Experimental probes of the molecular hydrogen–carbon nanotube interaction

Electrical transport (resistance R and thermoelectric power S), Raman scattering, and hydrogen adsorption are used to study the interaction of hydrogen molecules with ropes of single-walled carbon nanotubes. The data are consistent with H2 physisorption under the experimental conditions investigated (4 KoTo500 K; 0.1 atmoPo20 atm). The response of S; R to 1 atm hydrogen at 500 Kis consistent with the introduction of a new scattering channel for electrons/holes in the metallic tubes. Raman scattering from the Q-branch of hydrogen molecules adsorbed on the surface is found shifted only by 1–2 cm � 1 from their frequencies in the free molecule and indicates that two different adsorption sites can be detected. Finally, H2 wt% storage in heavily processed ropes of SWNTs are found to exceed 6% at B1 atm and T ¼ 77 Kand the isosteric heat of adsorption is found to be 120 meV. r 2002 Elsevier Science B.V. All rights reserved.

Thermoelectric chemical sensor based on single wall carbon nanotubes

Thermoelectric properties of single wall carbon nanotubes (SWNT) are quite sensitive to gases in contact with the tube walls. This effect makes possible a thermoelectric chemical sensor. Large, reversible swings in thermoelectric power (S), sometimes even involving sign changes in S, have been observed. Even contact of the SWNTs with He and N 2 and H 2 result in easily detectable and reversible changes in S. Smaller, polar alcohol molecules stimulate a large thermoelectric response, although H 2 O has no effect. For adsorption of six membered ring molecules C 6 H n in SWNTs, the large thermoelectric response observed for Benzene (n=6) is seen to decrease as the π electrons in the molecule are removed, and the coupling between the molecules and the SWNT is thereby reduced. These effects are discussed in terms of the diffusion thermopower for a rope, and a new scattering channel associated with adsorbed molecules.

Raman spectroscopic investigation of H2, HD, and D2 physisorption on ropes of single-walled, carbon nanotubes.

We have observed the S- and Q-branch Raman spectra of H2, HD, and D2 adsorbed at 85 K and pressures up to 8 atm on single-walled, carbon nanotubes (SWNT). Comparative data for H2 on graphite and C60 were also collected. Frequency-downshifted and upshifted features were observed in the Q-branch spectra of H2 on C60 and SWNT. These shifts are small and are therefore inconsistent with charge transfer. An H2-surface potential with van der Waals and electrostatic terms was developed and used to estimate the shifts in the frequency of the Q(0) transition of H2 adsorbed in two types of sites. These calculations corroborate the experimental findings and indicate physisorption in multiple sites of the SWNT ropes.

Thermoelectric study of hydrogen storage in carbon nanotubes

In situ resistivity (p) and thermoelectric power (S) have been used to study the nature of the adsorption of hydrogen in bundles of single-walled carbon nanotubes for H 2 pressure P ≤ 1 atm and temperatures 77 K <T<500 K.Isothermal plots of S vs Δρ/ρ 0 are found to exhibit linear behavior as a function of gas coverage, consistent with a physisorption process. Studies of S, p at T = 500 K as a function of pressure exhibit a plateau at a pressure P∼40 Torr, the same pressure where the H wt. % measurements suggest the highest binding energy sites are being saturated. The effects of H 2 exposure at 500 K on the thermoelectric transport properties are fully reversible.