Role of vacancies in the adsorption of quantum noble gases inside a bundle of carbon nanotubes

Diffusion Monte Carlo calculations on the He4 and Ne20 adsorption in the interstices of a bundle of (10,10) carbon nanotubes are reported. The results indicate that the presence of a carbon vacancy is enough, at least in some cases, to impede the adsorption of quantum gases in those systems. This could explain some discrepancies between the experimental data and the theoretical calculations about the possibility of adsorption of gases inside a bundle of carbon nanotubes.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

H2 in the interstitial channels of nanotube bundles

The equation of state of H2 adsorbed in the interstitial channels of a carbon nanotube bundle has been calculated using the diffusion Monte Carlo method. The possibility of a lattice dilation, induced by H2 adsorption, has been analyzed by modeling the cohesion energy of the bundle. The influence of factors like the interatomic potentials, the nanotube radius and the geometry of the channel on the bundle swelling is systematically analyzed. The most critical input is proved to be the C-H2 potential. Using the same model than in planar graphite, which is expected to be also accurate in nanotubes, the dilation is observed to be smaller than in previous estimations or even inexistent. H2 is highly unidimensional near the equilibrium density, the radial degree of freedom appearing progressively at higher densities.Comment: Accepted for publication in PR

Conditions for H2 adsorption in the interstitials of a bundle of carbon nanotubes

The influence of corrugation, impurities, heterogeneity of tube diameters, shape of the interchannels, and carbon vacancies in the H2 uptake by a bundle of carbon nanotubes was studied. It was found that both the existence of tubes of different chiralities and the presence of vacancies in narrow interstitials could prevent the hydrogen adsorption, making the system less useful as a reservoir in fuel cells than previously thought.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

4He/H2 binary clusters : a path-integral Monte Carlo study

The structure and thermodynamic properties of the 4He/H2 binary clusters are studied by path-integral Monte Carlo simulations. It has been found that, despite their lower mass, the H2 molecules form a subcluster of their own at the center of the system. When the number of helium atoms increases for a fixed number of hydrogen molecules, those inner clusters get compressed but retain their original form. The fraction of H2 in the superfluid state decreases with the subcluster compression, but remains finite for small H2 concentrations.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Temperature effects on the static and dynamic properties of liquid water inside nanotubes

We report a molecular dynamics simulation study of the behavior of liquid water adsorbed in carbon nanotubes under different thermodynamic conditions. A flexible simple point charged potential has been employed to model internal and intermolecular water interactions. Water-carbon forces are modeled with a Lennard-Jones-type potential. We have studied three types of tubes with effective radii ranging from 4.1 to 6.8 Å and three temperatures, from 298 to 500 K for a fixed density of 1 g/cm3. Structure of each thermodynamic state is analyzed through the characterization of the hydrogen-bond network. Time-dependent properties such as the diffusive behavior and molecular vibrational spectra are also considered. We observe the gradual destruction of the hydrogen-bond network together with faster diffusive regimes as temperature increases. A vibrational mode absent in bulk unconstrained water appears in the power spectra obtained from hydrogen velocity autocorrelation functions for all thermodynamic states. That frequency mode should be attributed to confinement effects.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Structure of water adsorbed on a single graphene sheet

We present the result of molecular-dynamics simulations of water adsorbed on top of a single graphene layer at temperatures between 25 and 50¿°C. The analysis of the energy per particle and the density profiles indicate that the behavior of the adsorbed liquid is similar to the case of multiple graphene layers (graphite) with the only difference being the values of configurational energy. Other structural properties, such as stability ranges, hydrogen bond distributions, and molecular orientations are also presented.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

High temperature behavior of water inside flat graphite nanochannels

By means of molecular dynamics calculations performed at high temperatures, we were able to calculate the phase diagram of liquid water confined inside narrow graphite channels. We found stable liquid phases all the way to 398K, failing to reach the critical temperature for all of the systems under consideration. We also analyzed how the temperature variations in the range of 298 to 398K affected the hydrogen bond network and found that the main variation in the infrared spectra of confined water at high temperatures was the forward shifting of the bending peak.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Effects of confinement on the vibrational spectra of liquid water adsorbed in carbon nanotubes

We report the results of a calculation of the vibrational and rotational spectra of water confined in carbon nanotubes. Water has been modeled with a flexible version of the simple point charge model of Berendsen et al., while the carbon nanotubes are taken to be soft potential walls interacting with water through a Lennard-Jones-type potential. The comparison of the computed absorption line shapes with the corresponding ones of liquid bulk water shows significant shifts in the positions of the spectral bands directly related to the tube radii.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Liquid-gas transition of neon in quasi-one-dimensional environments

We characterize the behavior of a system of Ne20 atoms in a pure one-dimensional environment between 12 and 30 K by means of path integral Monte Carlo calculations. This is a reasonable model to describe neon absorbed inside a narrow carbon nanotube. When embedded in a bundle of those tubes, this quasi-one-dimensional system can undergo a liquid-gas phase transition if nanotubes are narrow and close enough.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito

Water on the outside of carbon nanotube bundles

Molecular-dynamics simulations of water on the outer surface of a bundle of carbon nanotubes are reported. We observed a first-order phase transition from a low-density regime in which the water molecules are confined in the grooves between tubes and a high-density one, characterized by adsorption of water on all the surface exposed. Both phases could be experimentally detected by the differences in the locations of the main peaks of their infrared spectra.Universidad Pablo de Olavide. Departamento de Sistemas Físicos, Químicos y NaturalesVersión del edito