Single-walled nanotubes (SWNTs) produced by plasma laser vaporization (PLV) and containing oxidized surface functional groups have been studied for the first time with NEXAFS. Comparisons are made to SWNTs made by catalytic synthesis over Fe particles in high-pressure CO, called HiPco material. The results indicate that the acid purification and cutting of single-walled nanotubes with either HNO3/H2SO4 or H2O2/H2SO4 mixtures produces the oxidized groups (O/C = 5.5-6.7%), which exhibit both pi*(CO) and sigma*(CO) C K-edge NEXAFS resonances. This indicates that both carbonyl (C=O) and ether C-O-C functionalities are present. Upon heating in a vacuum to 500-600 K, the pi*(CO) resonances are observed to decrease in intensity; on heating to 1073 K, the sigma*(CO) resonances disappear as the C-O-C functional groups are decomposed. Raman spectral measurements indicate that the basic tubular structure of the SWNTs is not perturbed by heating to 1073 K, based on the invariance of the ring breathing modes upon heating. The NEXAFS studies agree well with infrared studies which show that carboxylic acid groups are thermally destroyed first, followed by the more difficult destruction of ether and quinone groups. Single-walled nanotubes produced by the HiPco process, and not treated with oxidizing acids, exhibit an O/C ratio of 1.9% and do not exhibit either pi*(CO) or sigma*(CO) resonances at the detection limit of NEXAFS. It is shown that heating (to 1073 K) of the PLV-SWNTs containing the functional groups produces C K-edge NEXAFS spectra very similar to those seen for the HiPco material. The NEXAFS spectra are calibrated against spectra measured for a number of fused-ring aromatic hydrocarbon molecules containing various types of oxidized functional groups present on the oxidized SWNTs.

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The adsorption of Xe into carbon single walled nanotubes with both closed and open ends has been investigated using temperature programmed desorption and other surface analytical tools. It has been found that opening the ends of the nanotube by chemical cutting increases both the kinetic rate and the saturation capacity of the nanotubes for Xe at 95 K. Further enhancement in Xe adsorption kinetics and capacity are achieved by treating the nanotubes in vacuum at 1073 K where CO, CO 2 , CH 4 , and H 2 are evolved. On this basis it is postulated that surface functionalities such as ϪCOOH block entry ports for adsorption at the nanotube ends and at the defect sites on the walls. The thermal destruction of these functionalities leads to enhanced adsorption. The denser phase of Xe inside the saturated nanotubes desorbs by zero-order kinetics (E d ϭ26.8Ϯ0.6 kJ/mol͒. It is postulated that a quasi-one-dimensional Xe confined phase in equilibrium, with a two-dimensional Xe gas phase on the exterior, provides a phase transition governing the zero-order kinetics desorption process.

Closed end (10, 10) single walled carbon nanotubes (SWNTs) have been opened by oxidation at their ends and at wall defect sites, using ozone. Oxidation with ozone, followed by heating to 973 K to liberate CO and CO2, causes etching of the nanotube surface at carbon atom vacancy defect sites. The rate of adsorption of Xe has been carefully measured as a function of the degree of nanotube etching by ozone. It is found that a level of etching corresponding to wall openings of about 5–7 Å radius is optimal for maximizing the rate of Xe adsorption. Beyond this level of etching, the rate of Xe adsorption decreases as the surface area of the SWNTs decreases due to further carbon atom removal. Both experiment and modeling show that the presence of polar oxidized groups, such as –COOH or –COR groups, with dipole moments in the range 1.5–3.0 D at the perimeter of the defect sites, causes a retardation of the rate of Xe adsorption due to dipole-induced dipole interactions. This effect is larger for smaller radius defect sites and decreases as the defect sites increase in size beyond about 7 Å radius. At large defect radii, the energetic profile of the adsorption pathway controls the physisorption rate. Modeling shows that after Xe adsorption has been completed inside the nanotubes, then Xe clusters begin to form on the outer surface of the nanotubes at the defect sites where polar groups are present. The Xe clustering effect also occurs to a smaller degree when the defect sites are not decorated by polar groups. The experiments and modeling demonstrate how one may optimize the rate of adsorption of a gas into nanotubes by the adjustment of the size and polar character of the vacancy-site entry ports in the walls of the nanotubes.

Adsorption of xenon on single-walled (10,10) carbon nanotubes at a temperature of 95 K has been studied by molecular simulation and the results have been compared with recent experiments [A. Kuznetsova, J. T. Yates, Jr., J. Liu, and R. E. Smalley, J. Chem. Phys. 112, 9590 (2000)]. Simulations indicate that adsorption takes place primarily on the inside of the nanotubes at the experimental conditions. Interstitial and external adsorption were found to be negligible in comparison with adsorption inside the nanotubes. The coverage computed from simulation of 0.06 Xe–C is in good agreement with the experimentally measured value of 0.042 Xe–C. The isosteric heat of adsorption from simulation ranges from about 3000 to 4500 K as a function of coverage, which is consistent with the experimental desorption activation energy of 3220 K. Adsorption on the external surfaces of the nanotubes is observed to take place at Xe pressures that are larger than those probed in the experiments. The good agreement between simulations and experiments for the coverage and heat of adsorption indicate that the curvature of the nanotube does not substantially perturb the adsorption potential from that of a graphene sheet.

A recently developed technique for the artificial production of an Al2O3 film on ultrahigh-purity polycrystalline Al samples was employed. Electrochemical impedance spectroscopy (EIS), Auger electron spectroscopy (AES), and grazing angle X-ray diffraction (GAXRD) were used to investigate the artificial oxide film. The growth of aluminum oxide in water vapor (5 × 10 -7 Torr) enhanced by 100 eV electron bombardment resulted in an artificial oxide film which exhibited a 10-30-fold improvement in electrical resistance in 3.5% NaCl solution compared to oxide films of the same thickness grown at 300 K in the absence of electron bombardment (thermal excitation only). These measurements suggest that the artificial aluminum oxide film may provide superior corrosion passivation qualities compared to thermally grown oxide films. The amorphous nature of the artificial oxide film was confirmed by GAXRD.

The adsorption and thermal stability of vinyltriethoxysilane (VTES) was studied on a γ-Al2O3 surface. Adsorption occurs near room temperature with the production of ethanol from the reaction with isolated surface AlOH groups, producing the surface species AlOSi(OC2H5)2CHCH2. As the surface is heated, decomposition of AlOSi(OC2H5)2CHCH2 occurs first via loss of the OC2H5 groups (starting at ∼520 K) and then with loss of the CHCH2 groups (starting at ∼820 K). Below 375 K, ethanol is the sole gas phase product produced, and near 520 K, both ethanol gas and ethylene gas are produced. At higher temperatures, ethanol decomposes on the surface and ethylene is the only gas phase species present. The thermal stability of ethoxy groups on the VTES-treated Al2O3 surface decreases considerably under strong hydrolysis conditions in water vapor. There is also a decrease in the stability of the vinyl groups under strong hydrolysis conditions. Above ∼525 K, SiOH infrared bands are observed to develop as AlOH groups disappear. Above ∼1100 K, organic ligands are no longer detected on the surface by FTIR spectroscopy and the surface remains covered with a silicate overlayer.