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In this work, graphene layers on SiO(2)/Si substrate have been chemically decorated by radio frequency hydrogen plasma. Hydrogen coverage investigation by Raman spectroscopy and micro-X-ray photoelectron spectroscopy characterization demonstrates that the hydrogenation of single layer graphene on SiO(2)/Si substrate is much less feasible than that of bilayer and multilayer graphene. Both the hydrogenation and dehydrogenation process of the graphene layers are controlled by the corresponding energy barriers, which show significant dependence on the number of layers. The extent of decorated carbon atoms in graphene layers can be manipulated reversibly up to the saturation coverage, which facilitates engineering of chemically decorated graphene with various functional groups via plasma techniques.
ACS Nano 2009 Jul 28
PMID:Thickness-dependent reversible hydrogenation of graphene layers. 1949 23

The spillover phenomenon, which essentially involves transfer of H from a metal catalyst to a graphitic receptor, has been considered promising for efficient hydrogen storage. An open question about the spillover mechanism is how a H atom binds to graphene instead of forming the thermodynamically preferred H(2). Using ab initio calculations, we show that the catalyst saturation provides a way to the adsorption of hydrogen on the receptor by increasing the H chemical potential to a spillover favorable range. Although it is energetically unfavorable for the spillover to occur on a pristine graphene surface, presence of a phase of hydrogenated graphene facilitates the spillover by significantly improving the C-H binding. We show that thermodynamic spillover can occur, both from the free-standing and from the receptor-supported clusters. Further, the computed energy barrier of the motion of a H from the catalyst to the hydrogenated graphene is small (0.7 eV) and can be overcome at operational temperatures.
ACS Nano 2009 Jul 28
PMID:H-Spillover through the Catalyst Saturation: An Ab Initio Thermodynamics Study. 1953 42

Understanding transport phenomena of fluids through nanotubes (NTs) is of great interest in order to enable potential application of NTs as separation devices, encapsulation media for molecule storage and delivery, and sensors. Single-walled metal oxide NTs are interesting materials because they present a well-defined solid-state structure, precisely tunable diameter and length, as well as a hydrophilic and functionalizable interior for tuning transport and adsorption selectivity. Here, we study the transport properties of hydrogen-bonding liquids (water, methanol, and ethanol) through a single-walled aluminosilicate NT to investigate the influence of liquid-surface and liquid-liquid interactions and the effects of competitive transport of different chemical species using molecular dynamics (MD) simulations. The self-diffusivities (D(s)) for all the three species decrease with increasing loading and are comparable to bulk liquid diffusivities at low molecular loadings. We show that the hydrogen-bond network associated with water makes its diffusion behavior different from methanol and ethanol. Mixtures of water and methanol show segregation in the NT, with water located closer to the tube wall and the alcohol molecules localized near the center of the NT. D(s) values of water in an analogous aluminogermanate NT are larger than those in the aluminosilicate NT due to a larger pore diameter.
ACS Nano 2009 Jun 23
PMID:Self-diffusion of water and simple alcohols in single-walled aluminosilicate nanotubes. 1954 68

We investigate the electronic and magnetic properties of hydrogenated carbon nanotubes using ab initio spin-polarized calculations within both the local density approximation (LDA) and the generalized gradient approximation (GGA). We find that the combination of charge transfer and carbon network distortion makes the spin-polarized flat-band appear in the tube's energy gap. Various spin-dependent ground state properties are predicted with the changes of the radii, the chiralities of the tubes, and the concentration of hydrogen. It is found that strain or external electric field can effectively modulate the flat-band spin-splitting and even induce an insulator-metal transition.
ACS Nano 2009 Jul 28
PMID:Itinerant flat-band magnetism in hydrogenated carbon nanotubes. 1954 40

Prosthesis of non-critical parts of a polypeptide backbone is an attractive strategy to simplify bioactive peptides. This approach was applied to an opioid neuropeptide, Met-enkephalin, in which two adjacent Gly2-Gly3 residues were replaced with a series of non-peptidic backbone spacers varying in length and/or physicochemical properties. The backbone spacers did not affect the overall structural properties of the analogues, but they did dramatically reduce their affinities and agonist activities toward delta- and mu-opioid receptors. Molecular modeling suggested that the decrease of the affinity of Met-enkephalin to delta-opioid receptor could be accounted for by the loss of a single hydrogen bond. Remarkably, the analogues containing the most isostere spacers retained potent antinociceptive and anticonvulsant properties that were comparable to that of the endogenous peptide. This unexpected high in vivo potency could not be accounted for by an increase in metabolic stability. Moreover, the antiepileptic activity could not be reversed by opioid receptor antagonists. In summary, the results obtained with the analogues containing backbone spacers suggest a novel mechanism for seizure control in the brain that involves alternative non-opioid signaling.
ACS Chem Biol 2009 Aug 21
PMID:Anticonvulsant Met-enkephalin analogues containing backbone spacers reveal alternative non-opioid signaling in the brain. 1963 61

We have measured the dispersibility of single-walled carbon nanotubes in a range of solvents, observing values as high as 3.5 mg/mL. By plotting the nanotube dispersibility as a function of the Hansen solubility parameters of the solvents, we have confirmed that successful solvents occupy a well-defined range of Hansen parameter space. The level of dispersibility is more sensitive to the dispersive Hansen parameter than the polar or H-bonding Hansen parameter. We estimate the dispersion, polar, and hydrogen bonding Hansen parameter for the nanotubes to be <delta(D)> = 17.8 MPa(1/2), <delta(P)> = 7.5 MPa(1/2), and <delta(H)> = 7.6 MPa(1/2). We find that the nanotube dispersibility in good solvents decays smoothly with the distance in Hansen space from solvent to nanotube solubility parameters. Finally, we propose that neither Hildebrand nor Hansen solubility parameters are fundamental quantities when it comes to nanotube-solvent interactions. We show that the previously calculated dependence of nanotube Hildebrand parameter on nanotube diameter can be reproduced by deriving a simple expression based on the nanotube surface energy. We show that solubility parameters based on surface energy give equivalent results to Hansen solubility parameters. However, we note that, contrary to solubility theory, a number of nonsolvents for nanotubes have both Hansen and surface energy solubility parameters similar to those calculated for nanotubes. The nature of the distinction between solvents and nonsolvents remains to be fully understood.
ACS Nano 2009 Aug 25
PMID:Multicomponent solubility parameters for single-walled carbon nanotube-solvent mixtures. 1965 24

Using density-functional tight-binding (DFTB)-based quantum chemical molecular dynamics at 2500 and 3000 K, we have performed simulations of benzene combustion by gradually reducing the hydrogen to carbon (H/C) ratio. The accuracy of DFTB for these simulations was found to be on the order of 7-9 kcal/mol when compared to higher-level B3LYP and G3-like quantum chemical methods in extensive benchmark calculations. Ninety direct-dynamics trajectories were run for up to 225 ps simulation time, during which hydrocarbon cluster size, curvature, and C(x)H(y) composition, carbon hybridization type, and ring count statistics were recorded. Giant fullerene cage formation was observed only after hydrogen was completely eliminated from the reaction mixture, with yields of around 50% at 2500 K and 42% at 3000 K. Cage sizes are mostly in the range from 152 to 202 carbon atoms, with the distribution shifting toward larger cages at lower temperature. In contrast to previous simulations of dynamics fullerene assembly from ensembles of C(2) molecules, we find that the resulting cages show smaller number of attached carbon chains (antenna) surviving until cage closure. Again, no direct formation pathway for C(60) from smaller fragments was observed. Our results challenge the idealized picture of "ordered" growth of PAHs along a route involving only maximally condensed and fully hydrogenated graphene platelets, and favor instead fleeting open-chains with ring structures attached, featuring a large number of hydrogen defects, pentagons, and other nonhexagon ring species.
ACS Nano 2009 Aug 25
PMID:Quantum chemical molecular dynamics simulations of dynamic fullerene self-assembly in benzene combustion. 1970 22

Nanostructures based on multiwalled carbon nanotubes (MWNTs) are fabricated using plasma of the mixture of hydrogen and nitrogen gases. The plasma-sharpened tips of nanotubes contain only a few tubes at the apex of the structure and lead to the dramatic enhancement in the emission current density by a factor >10(6) with the onset field as low as 0.16 V/microm. We propose that the nature of the tunneling barrier changes significantly for a nanosize tip at very high local electric field and may lead to the saturation in the emission current density.
ACS Nano 2009 Sep 22
PMID:Dramatic enhancement of the emission current density from carbon nanotube based nanosize tips with extremely low onset fields. 1971 50

Local probe oxidation experiments by conductive AFM have been performed on a hexadecyl monolayer and a N-hydroxysuccinimide (NHS)-ester-functionalized undecyl (NHS-UA) monolayer assembled on hydrogen-terminated (i.e., unoxidized) silicon. The oxidation conditions for the mild oxidation of the top terminal groups of monolayers and the deep oxidation of the underlying silicon into silicon oxide were investigated. The results show that the bias threshold for the AFM tip-induced oxidation of the top groups of monolayers on oxide-free silicon can be reduced by 2 V for the methyl-terminated hexadecyl monolayer and even by 3.5 V for the active NHS-ester-terminated undecyl monolayer, in comparison to a methyl-terminated octadecyl trichlorosilane (OTS) monolayer on oxidized silicon. Upon such local mild oxidation, the active NHS ester group of the NHS-UA monolayer is selectively cleaved off to generate carboxyl-containing monolayer nanopatterns, opening further possibilities for subsequent patterned multifunctionalization.
ACS Nano 2009 Oct 27
PMID:Local probe oxidation of self-assembled monolayers on hydrogen-terminated silicon. 1975 33

By using the first-principles DFT calculations, we design a novel hydrogen storage material, Li(12)Si(60)H(60) composite, and validate its geometric stability. It is found that the adsorbed Li atoms do not cluster on the Si(60)H(60) fullerene unlike other metals such as Ti, owing to the relatively low Li-Li binding energy and the inhibition of Si-H bonds. Our results show that the Li-doping enhances the hydrogen adsorption ability of Si(60)H(60) significantly, owing to the charge transfer from the doped Li atoms to the host material and the polarization of the adsorbed H(2) molecules. By combining the first-principles calculation and grand canonical Monte Carlo simulation, we further investigate the hydrogen storage capacity of the simulation-synthesized exohedral Li(12)Si(60)H(60) composite at T = 77 K. As the vdW gap (i.e., the separation between the surfaces of two Li(12)Si(60)H(60) fullerenes) is equal to 8.2 A, the total hydrogen uptake of the square-arranged Li(12)Si(60)H(60) array reaches 12.83 wt % at p = 10 MPa, while the excess hydrogen uptake shows a maximum of 7.46 wt % at p = 6 MPa. Impressively, at T = 298 K and p = 10 MPa, the Li(12)Si(60)H(60) array still exhibits a total hydrogen uptake of 3.88 wt % at the vdW gap of 8.2 A. These results clearly indicate that the composite, Li(12)Si(60)H(60) fullerene, is a promising candidate for hydrogen storage.
ACS Nano 2009 Oct 27
PMID:Li(12)Si(60)H(60) fullerene composite: a promising hydrogen storage medium. 1976 Nov 95


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