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J Karl Johnson

Publications and source records attributed to J Karl Johnson.

At least 19 recordsLinked to original sources
PubMed2006

Unusual hydrogen bonding in water-filled carbon nanotubes.

We present the first experimental vibrational spectroscopy study providing direct evidence of a water phase inside single-walled carbon nanotubes that exhibits an unusual form of hydrogen-bonding due to confinement. Water adopts a stacked-ring structure inside nanotubes, forming intra- and inter-ring hydrogen bonds. The intra-ring hydrogen bonds are bulk-like while the inter-ring hydrogen bonds are relatively weak, having a distorted geometry that gives rise to a distinct OH stretching mode. The experimentally observed infrared mode at 3507 cm(-1) is assigned to vibrations of the inter-ring OH-groups based on detailed atomic-level modeling. The direct observation of unusual hydrogen bonding in nanotubes has potential implications for water in other highly confined systems, such as biological channels and nanoporous media.

Journal Article↗
PubMed2006

Spectroscopic measurement of diffusion kinetics through subnanometer and larger Al2O3 particles by a new method: the interaction of 2-chloroethylethyl sulfide with gamma-Al2O3.

A new method to study the diffusion properties of molecules into porous materials using transmission IR spectroscopy is employed. A measurement of the diffusion of the 2-chloroethylethyl sulfide (2-CEES) molecule into two types of gamma-Al2O3 powder is performed, showing that the diffusion rate into subnanometer crystallite particle size gamma-Al2O3 powders (subnano-Al2O3) is higher than that into the larger crystallite particle size powder. It is shown that a surface diffusion mechanism can be used to model the diffusion process giving good agreement with the experimental results, where Dsubnano-Al2O3 is approximately 5 times larger than Dmultinano-Al2O3 at 170 K for the 2-CEES molecule.

Journal Article↗
PubMed2006

Identification of destabilized metal hydrides for hydrogen storage using first principles calculations.

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through alloying with other elements. A very large number of possible destabilized metal hydride reaction schemes exist. The thermodynamic data required to assess the enthalpies of these reactions, however, are not available in many cases. We have used first principles density functional theory calculations to predict the reaction enthalpies for more than 100 destabilization reactions that have not previously been reported. Many of these reactions are predicted not be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low. More importantly, our calculations identify five promising reaction schemes that merit experimental study: 3LiNH(2) + 2LiH + Si --> Li(5)N(3)Si + 4H(2), 4LiBH(4) + MgH(2) --> 4LiH + MgB(4) + 7H(2), 7LiBH(4) + MgH(2) --> 7LiH + MgB(7) + 11.5H(2), CaH(2) + 6LiBH(4) --> CaB(6) + 6LiH + 10H(2), and LiNH(2) + MgH(2) --> LiMgN + 2H(2).

Journal Article↗
PubMed2006

Transport diffusion of gases is rapid in flexible carbon nanotubes.

Molecular dynamics simulations of rigid, defect-free single-walled carbon nanotubes have previously suggested that the transport diffusivity of gases adsorbed in these materials can be orders of magnitude higher than any other nanoporous material (A. I. Skoulidas et al., Phys. Rev. Lett. 2002, 89, 185901). These simulations must overestimate the molecular diffusion coefficients because they neglect energy exchange between the diffusing molecules and the nanotube. Recently, Jakobtorweihen et al. have reported careful simulations of molecular self-diffusion that allow nanotube flexibility (Phys. Rev. Lett. 2005, 95, 044501). We have used the efficient thermostat developed by Jakobtorweihen et al. to examine the influence of nanotube flexibility on the transport diffusion of CH4 in (20,0) and (15,0) nanotubes. The inclusion of nanotube flexibility reduces the transport diffusion relative to the rigid nanotube by roughly an order of magnitude close to zero pressure, but at pressures above about 1 bar the transport diffusivities for flexible and rigid nanotubes are very similar, differing by less than a factor or two on average. Hence, the transport diffusivities are still extremely large compared to other known materials when flexibility is taken into account.

Journal Article↗
PubMed2006

Adsorption and diffusion of carbon dioxide and nitrogen through single-walled carbon nanotube membranes.

We have used atomically detailed simulations to examine the adsorption and transport diffusion of CO2 and N2 in single-walled carbon nanotubes at room temperature as a function of nanotube diameter. Linear and spherical models for CO2 are compared, showing that representing this species as spherical has only a slight impact in the computed diffusion coefficients. Our results support previous predictions that transport diffusivities of molecules inside carbon nanotubes are extremely rapid when compared with other porous materials. By examining carbon nanotubes as large as the (40,40) nanotube, we are able to compare the transport rates predicted by our calculations with recent experimental measurements. The predicted transport rates are in reasonable agreement with experimental observations.

Journal Article↗
PubMed2006

Quantum sieving in single-walled carbon nanotubes: effect of interaction potential and rotational-translational coupling.

The selective adsorption of heavy isotopes in narrow nanotubes, known as quantum sieving, is studied using a simple approximate theory for several different potential models. We address the reasons for wide disagreement among previously published results for quantum sieving. We analyze the sensitivity of quantum sieving to perturbations in the potential parameters used in the calculations. The selectivities are very sensitive to changes in the atomic diameter parameter and less sensitive to changes in the potential well depth. We present an approximate method for accounting for rotational-translational coupling that is computationally efficient and accurate for the narrowest nanotubes. For wide nanotubes, the estimation of rotational-translational coupling becomes inaccurate because of neglect of the effect of rotational states on the translational degrees of freedom.

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PubMed2006

First principles study of adsorption and dissociation of CO on W(111).

The adsorption and dissociation of carbon monoxide on the W(111) surface is studied with density functional theory. The CO molecule is found to adsorb in end-on configurations (alpha states) and inclined configurations (beta states). The dissociation of the most strongly bound beta state CO is found to have an activation energy of about 0.8 eV, which is lower than the energy required to desorb CO molecularly from the surface. The diffusion of CO and O on W(111) is predicted to be facile at room temperature, whereas C atoms are virtually immobile up to approximately 600 K, according to our calculations. Preadsorbed carbon atoms are shown to prevent the dissociation of CO by blocking the most strongly bound beta state adsorption site and by blocking the dissociation pathway. We predict that dissociation of CO on W(111) is a self-poisoning process.

Journal Article↗
PubMed2005

Observation of a one-dimensional adsorption site on carbon nanotubes: adsorption of alkanes of different molecular lengths.

Three well-defined adsorption sites have been found on opened single-wall carbon nanotubes by temperature-programmed desorption measurements for several alkanes. A series of linear chain alkanes from pentane to nonane, as well as a branched alkane molecule, 2,2,4-trimethylpentane, were used to elucidate the effect of molecular length on the capacity of the adsorption sites. The two highest-energy adsorption sites were assigned as the nanotube interior sites and groove sites on the outside of the nanotube bundles. Hybrid Monte Carlo simulations were performed to probe the molecular-level details of adsorption. Both in experiments and in the simulation, the groove sites were seen to behave as one-dimensional adsorption space, demonstrating an inverse dependence of capacity on the length of the adsorbed molecule. In contrast, the capacity of the internal sites was found to depend inversely on the volume occupied by the molecule.

Journal Article↗
PubMed2005

Oxygenated hydrocarbon ionic surfactants exhibit CO2 solubility.

Several oxygenated hydrocarbons, including acetylated sugars, poly(propylene glycol), and oligo(vinyl acetate), have been used to generate CO2-soluble ionic surfactants. Surfactants with vinyl acetate tails yielded the most promising results, exhibiting levels of CO2 solubility comparable to those associated with fluorinated ionic surfactants. For example, a sodium sulfate with single, oligomeric vinyl acetate (VAc) tails consisting of 10 VAc repeat units was 7 wt % soluble in CO2 at 25 degrees C and 48 MPa. Upon introduction of water to these systems, only surfactants with the oligomeric vinyl acetate tails exhibited spectroscopic evidence of a polar environment that was capable of solubilizing the methyl orange into the CO2-rich phase. For example, a single-phase solution of CO2, 0.15 wt % sodium bis(vinyl acetate)8 sulfosuccinate, and water, at water loading (W) values ranging from 10 to 40 at 25 degrees C and 34.5 MPa, exhibited a methyl orange peak at 423 nm. This result indicated that the core of a reverse micelle provided a microenvironment with a polarity similar to that of methanol. Quantum chemical calculations indicate that the acetylated sugars may be too hydrophilic to readily form reverse micelles, whereas the VAc-based surfactants appear to have the correct balance of hydrophilic and hydrophobic forces necessary to form reverse micelles.

Carbon Dioxide↗
PubMed2005

Adsorption of gases in metal organic materials: comparison of simulations and experiments.

Molecular simulations using standard force fields have been carried out to model the adsorption of various light gases on a number of different metal organic framework-type materials. The results have been compared with the available experimental data to test the validity of the model potentials. We observe good agreement between simulations and experiments for a number of different cases and very poor agreement in other cases. Possible reasons for the discrepancy in simulated and measured isotherms are discussed. We predict hydrogen adsorption isotherms at 77 and 298 K in a number of different metal organic framework materials. The importance of quantum diffraction effects and framework charges on the adsorption of hydrogen at 77 K is discussed. Our calculations indicate that at room temperature none of the materials that we have tested is able to meet the requirements for on-board hydrogen storage for fuel cell vehicles. We have calculated the volume available in a given sorbent at a specified adsorption energy (density of states). We discuss how this density of states can be used to assess the effectiveness of a sorbent material for hydrogen storage.

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PubMed2005

Formation of odd-numbered clusters of CO2 adsorbed on nanotube bundles.

Simulations show that CO2 adsorbed in the groove sites of carbon nanotubes displays unique quasi-one-dimensional behavior. Clusters containing only odd numbers of molecules are formed at finite CO2 coverages and low temperatures. The molecules are orientationally ordered with respect to the nanotube groove axis and azimuthally ordered in the plane perpendicular to the groove axis. This ordering is a result of a delicate balance between solid-fluid and fluid-fluid forces; the CO2 quadrupole plays a critical role in the cluster formation and orientational ordering.

Journal Article↗
PubMed2005

Dimensional effects on the LO-TO splitting in CF4: first-principles and infrared absorption studies.

The development of longitudinal optical-transverse optical (LO-TO) modes in CF(4) has been studied experimentally and theoretically as a function of dimensionality. Infrared absorption experiments for CF(4) adsorbed on single-walled carbon nanotubes indicate a lack of LO-TO splitting at low coverage and a gradual appearance of LO-TO modes as the coverage of CF(4) on the nanotubes is increased. We have performed density functional perturbation theory calculations for the vibrational frequencies, IR absorption spectra, and phonon density of states for CF(4) in one, two, and three dimensions. The calculations demonstrate that LO-TO splitting in 1D is qualitatively different from that computed for 2D or the bulk. The magnitude of the splitting in 1D is about one-half that computed for the bulk, and the LO mode is very weakly blue-shifted in 1D. We predict that the phonon density of states changes dramatically as the dimensionality of the crystal is changed. This prediction can be tested experimentally via inelastic neutron scattering. We conclude that LO-TO splitting can be used as a probe to identify 1D states of matter.

Journal Article↗
PubMed2004

Surface tension of quantum fluids from molecular simulations.

We present the first molecular simulations of the vapor-liquid surface tension of quantum liquids. The path integral formalism of Feynman was used to account for the quantum mechanical behavior of both the liquid and the vapor. A replica-data parallel algorithm was implemented to achieve good parallel performance of the simulation code on at least 32 processors. We have computed the surface tension and the vapor-liquid phase diagram of pure hydrogen over the temperature range 18-30 K and pure deuterium from 19 to 34 K. The simulation results for surface tension and vapor-liquid orthobaric densities are in very good agreement with experimental data. We have computed the interfacial properties of hydrogen-deuterium mixtures over the entire concentration range at 20.4 and 24 K. The calculated equilibrium compositions of the mixtures are in excellent agreement with experimental data. The computed mixture surface tension shows negative deviations from ideal solution behavior, in agreement with experimental data and predictions from Prigogine's theory. The magnitude of the deviations at 20.4 K are substantially larger from simulations and from theory than from experiments. We conclude that the experimentally measured mixture surface tension values are systematically too high. Analysis of the concentration profiles in the interfacial region shows that the nonideal behavior can be described entirely by segregation of H(2) to the interface, indicating that H(2) acts as a surfactant in H(2)-D(2) mixtures.

Journal Article↗
PubMed2004

Vibrational behavior of adsorbed CO2 on single-walled carbon nanotubes.

We present theoretical and experimental evidence for CO(2) adsorption on different sites of single walled carbon nanotube (SWNT) bundles. We use local density approximation density functional theory (LDA-DFT) calculations to compute the adsorption energies and vibrational frequencies for CO(2) adsorbed on SWNT bundles. The LDA-DFT calculations give a range of shifts for the asymmetric stretching mode from about -6 to -20 cm(-1) for internally bound CO(2), and a range from -4 to -16 cm(-1) for externally bound CO(2) at low densities. The magnitude of the shift is larger for CO(2) adsorbed parallel to the SWNT surface; various perpendicular configurations yield much smaller theoretical shifts. The asymmetric stretching mode for CO(2) adsorbed in groove sites and interstitial sites exhibits calculated shifts of -22.2 and -23.8 cm(-1), respectively. The calculations show that vibrational mode softening is due to three effects: (1) dynamic image charges in the nanotube; (2) the confining effect of the adsorption potential; (3) dynamic dipole coupling with other adsorbate molecules. Infrared measurements indicate that two families of CO(2) adsorption sites are present. One family, exhibiting a shift of about -20 cm(-1) is assigned to internally bound CO(2) molecules in a parallel configuration. This type of CO(2) is readily displaced by Xe, a test for densely populated adsorbed species, which are expected to be present on the highest adsorption energy sites in the interior of the nanotubes. The second family exhibits a shift of about -7 cm(-1) and the site location and configuration for these species is ambiguous, based on comparison with the theoretical shifts. The population of the internally bound CO(2) may be enhanced by established etching procedures that open the entry ports for adsorption, namely, ozone oxidation followed by annealing in vacuum at 873 K. Xenon displacement experiments indicate that internally bound CO(2) is preferentially displaced relative to the -7 cm(-1) shifted species. The -7 cm(-1) shifted species is assigned to CO(2) adsorbed on the external surface based on results from etching and Xe displacement experiments.

Journal Article↗
PubMed2004

Microporous metal organic materials: promising candidates as sorbents for hydrogen storage.

Advancement in hydrogen storage techniques represents one of the most important areas of today's materials research. While extensive efforts have been made to the existing techniques, there is no viable storage technology capable of meeting the DOE cost and performance targets at the present time. New materials with significantly improved hydrogen adsorption capability are needed. Microporous metal coordination materials (MMOM) are promising candidates for use as sorbents in hydrogen adsorption. These materials possess physical characteristics similar to those of single-walled carbon nanotubes (SWNTs) but also exhibit a number of improved features. Here, we report a novel MMOM structure and its room-temperature hydrogen adsorption properties.

Journal Article↗
PubMed2003

Gas adsorption on heterogeneous single-walled carbon nanotube bundles.

Optimization of carbon nanotube bundles containing a distribution of nanotube diameters always gives structures with packing defects that form relatively large interstitial channels. Experimental data for CH4, Ar, and Xe adsorption are compared with simulations. Low coverage experimental isosteric heats are in excellent agreement with simulations of gases adsorbing into interstitial channels of defective nanotube bundles, whereas adsorption onto perfect bundles does not agree with experiments. Thus, an accurate description of adsorption on nanotube bundles must account for interstitial adsorption.

Journal Article↗
PubMed2003

Adsorption of CF4 on the internal and external surfaces of opened single-walled carbon nanotubes: a vibrational spectroscopy study.

Infrared spectroscopy has been used to make the first experimental discrimination between molecules bound by physisorption on the exterior surface of carbon single-walled nanotubes (SWNTs) and molecules bound in the interior. In addition, the selective displacement of the internally bound molecules has been observed as a second adsorbate is added. SWNTs were opened by oxidative treatment with O(3) at room temperature, followed by heating in a vacuum to 873 K. It was found that, at 133 K and 0.033 Torr, CF(4) adsorbs on closed SWNTs, exhibiting its nu(3) asymmetric stretching mode at 1267 cm(-1) (red shift relative to the gas phase, 15 cm(-1)). Adsorption on the nanotube exterior is accompanied by adsorption in the interior in the case of opened SWNTs. Internally bound CF(4) exhibits its nu(3) mode at 1247 cm(-1) (red shift relative to the gas phase, 35 cm(-1)). It was shown that, at 133 K, Xe preferentially displaces internally bound CF(4) species, and this counterintuitive observation was confirmed by molecular simulations. The confinement of CF(4) inside (10,10) single-walled carbon nanotubes does not result in the production of lattice modes that are observed in large 3D ensembles of CF(4).

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