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PubMed2006

Crystalline graphite from an organometallic solution-phase reaction.

We report a chemical method by which graphitic carbon is prepared at reaction temperatures as low as 110 degrees C from readily available molecular reagents. This process requires no forcing conditions such as high pressures, intense light, or electrical discharge but is a simple, catalytic organometallic reaction. The carbon forms in a variety of morphologies including graphene sheets and nanotubes. The mild reaction conditions are similar to those routinely employed in homogeneous catalysis; therefore, they will allow the subtle manipulation of reaction variables to give desired morphologies selectively.

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PubMed2005

Electrodeposition of platinum on highly oriented pyrolytic graphite. Part I: electrochemical characterization.

The electrochemical deposition of Pt on highly oriented pyrolytic graphite (HOPG) from H2PtCl6 solutions was investigated by cyclic voltammetry and chronoamperometry. The effects of deposition overpotential, H2PtCl6 concentration, supporting electrolyte, and anion additions on the deposition process were evaluated. Addition of chloride inhibits Pt deposition due to adsorption on the substrate and blocking of reduction sites, while SO4(2-) and ClO4- slightly promote Pt reduction. By comparing potentiostatic current-time transients with the Scharifker-Hills model, a transition from progressive to instantaneous nucleation was observed when increasing the deposition overpotential. Following addition of chloride anions the fit of experimental transients with the instantaneous nucleation mode improves, while the addition of SO4(2-) induces only small changes. Chloride anions strongly inhibit the reduction process, which is shifted in the cathodic direction. The above results indicate that the most appropriate conditions for growing Pt nanoparticles on HOPG with narrow size distribution are to use an H2PtCl6 solution with HCl as supporting electrolyte and to apply a high cathodic overpotential.

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PubMed2005

Ab initio studies on nanoscale friction between graphite layers: effect of model size and level of theory.

Ab initio methods were used to investigate the nanoscale friction between two graphite layers placed in contact. The interaction energies were calculated for four two-layer models in series, C(6(n+1))2H(6n+1))-C(6)(n)2H(6)(n) with n = 1, 2, 3, and 4, and additionally for C(54)H(18)-C(6)H(6) and C(150)H(30)-C(6)H(6). The study was done with the Hartree-Fock method using basis sets 3-21G and 6-31G and with the second-order Møller-Plesset theory using basis set 6-31G. A density functional method (B3PW91) was also tested for reference purposes. The main interest was how the model size and level of theory affect the nanoscale friction coefficient. Most of the calculated friction coefficients fell within the range of values of 0.07-0.14.

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PubMed2005

Dynamic and spectroscopic studies of single molecules physisorbed on graphite substrates. 1. Theoretical approach.

The homogeneous line shapes of the infrared spectrum of single small molecules adsorbed on a graphite substrate are determined by applying a renormalization technique to the Hamiltonian of the system. The absorption profile can be determined by using a cumulant series expansion theory to treat the dynamic couplings between the optical system (formed by the vibration-orientation modes of the molecule) and the bath modes (characterized by molecular translations and substrate phonons). The various causes of homogeneous broadening of the lines are studied, with emphasis on phase relaxations through orientation-phonon transfers.

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PubMed2005

Spatially anisotropic etching of graphite by hyperthermal atomic oxygen.

The spatially anisotropic kinetics involved in the chemical reaction between highly ordered pyrolytic graphite (HOPG) and a beam containing hyperthermal (approximately 8 km s(-1)) O((3)P) atomic oxygen and molecular oxygen yields unique surface morphologies. Upon exposure at moderate sample temperatures (298-423 K), numerous multilayer circular pits embedded in the reacted areas have been observed with the use of atomic force microscopy and scanning tunneling microscopy. These pits have diameters spanning nanometers to micrometers and depths from a few to tens of nanometers. The most striking characteristic of these pits is the convex curvature of the pit bottoms, where the highest point on the pit bottom is at the center and the lowest point occurs around the peripheral edge. Such structure arises by the interplay between kinetics of pit nucleation, the spatially anisotropic kinetics involved in the lateral and downward reactivity of HOPG, and the fluence of atomic oxygen. These kinetics, which are also influenced by the high reactivity of the translationally hot impinging oxygen atoms, govern the overall morphological evolution of the surface.

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PubMed2005

Solid-state NMR (19F and 13C) study of graphite monofluoride (CF)n: 19F spin-lattice magnetic relaxation and 19F/13C distance determination by Hartmann-Hahn cross polarization.

Graphite monofluoride (CF)(n) was studied by solid-state NMR. (19)F spin-lattice relaxation time T(1) and second moment measurements of the (19)F line are presented. A "chair" conformation structure is found to be compatible with the experimental data. Relaxation is shown to be mainly due to paramagnetic oxygen. The presence of a molecular motion with an activation energy of 1.685 kJ.mol(-1) (202.7 K) is also evidenced. (19)F magic angle spinning (MAS) NMR and (13)C MAS NMR with (19)F to (13)C cross-polarization allows the determination of CF and CF(2) groups. Reintroduction of dipolar coupling by cross-polarization is used for C-F bond length determination (0.138 +/- 0.001 nm).

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PubMed2005

Reflection absorption infrared spectroscopy and temperature-programmed desorption studies of the adsorption and desorption of amorphous and crystalline water on a graphite surface.

Reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) have been used to perform a detailed investigation of the adsorption of water on highly oriented pyrolytic graphite (HOPG) at 90 K. RAIRS shows that water is physisorbed on HOPG at all coverages, as expected. Experiments at higher surface temperatures show marked changes in the O-H stretching region of the spectrum which can be assigned to the observation of the amorphous to crystalline ice phase transition. The infrared signature of both phases of solid water has been determined on HOPG and can be used to identify the phase of the ice. TPD spectra show the desorption of multilayers of crystalline ice. At high exposures a small bump appears in the TPD spectrum, on the low temperature side of the main peak, which is attributed to the amorphous to crystalline phase transition. At very low exposures of water, it is possible to distinguish the desorption of water from two- and three-dimensional islands and hence to determine the growth mode of water on the HOPG surface. Isothermal TPD studies have also been performed and show that the desorption of water does not obey perfect zero-order kinetics. Desorption orders, derived directly from the TPD spectra, confirm this observation. Desorption energies and preexponential factors have also been determined for this adsorption system.

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PubMed2005

Interaction of evaporated nickel nanoparticles with highly oriented pyrolytic graphite: Back-bonding to surface defects, as studied by X-ray photoelectron spectroscopy.

Ni nanoparticle evaporation onto highly oriented pyrolytic graphite (HOPG), with low and high surface defect densities, has been studied in situ by X-ray photoelectron spectroscopic (XPS) techniques, including binding energy shifts, Auger parameters, and Wagner plots. Ni nucleates at defect sites, whether initially present or those introduced by Ar+ bombardment, with the formation of spherical nanoparticles, which adhere strongly through Ni/HOPG back-bonding. The variation of the C 1s peak intensity with Ni coverage suggests that the photoelectron emission yield may be enhanced at lower Ni coverages, due to Ni nanoparticle-induced electron localization and work function reduction at the HOPG surface, which is evidence of such back-bonding.

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PubMed2005

Weak hydrogen bonds as a structural motif for two-dimensional assemblies of oligopyridines on highly oriented pyrolytic graphite: an STM investigation.

We present the STM investigation of four different oligopyridines at the liquid/highly oriented pyrolytic graphite interface. The heteroaromatic compounds are constitutional isomers showing the same overall shape regardless of their actual conformation. On the basis of weak intermolecular C-H...N hydrogen-bonding interactions, different nanopatterns are formed following a simple general concept for the two dimensional self-assembly. The molecules arrange either in linear or in cyclic structures. Though the oligopyridines are achiral, the formation of prochiral trimeric superstructures leads to chiral phases due to the immobilization on the surface. Some of the molecules show polymorphic structures depending on the solvent. The large variety of the presented structures formed by self-assembly of the different oligopyridines which retain the same functional heteroaromatic backbone shall open the possibility of exploiting these patterns as templates for the nanostructuring of surfaces with guests such as small molecules or metal ions for intriguing applications in, for example, catalysis.

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PubMed2006

Self-assembly and odd-even effects of cis-unsaturated carboxylic acids on highly oriented pyrolytic graphite.

The self-assembly of several cis-unsaturated carboxylic acids of the structure cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH on highly oriented pyrolytic graphite (HOPG) was studied. The impact of the interior cis-CH=CH group and the molecular chain length on their self-assembled structures was considered. Due to the cis conformation of the -HC=CH- group in the interior of these molecules, they display self-assembled structures significantly different from saturated acids with all-trans configurations. As an example of the class of molecules cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) (p=8, n=7), cis-CH3(CH2)7CH=CH(CH2)13COOH self-assembles into two kinds of enantiomer domains with opposite 2-D chirality. Due to the steric restriction of the interior cis-HC=CH group, all chains with acid groups are packed at the same side of a lamella, a head-to-head arrangement which is different from the head-to-tail packing of saturated all-trans acids. However, cis-CH3(CH2)7CH=CH(CH2)8COOH, considered as one example of the group cis-CH3(CH2)p-1CH=CH(CH2)2n-2COOH (p not equal 2n-1) (p=8, n=5), does not form any stable self-assembled domain, consistent with the molecular arrangement model. This difference in self-assembly behavior between cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) and cis-CH3(CH2)p-1CH=CHC2n-2COOH (p not equal 2n-1) shows an odd-even chain-length effect of cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH (p not equal m, m=2n or 2n-1). For another category of molecules, cis-unsaturated acids with equal numbers of all-trans carbon atoms on both sides of the cis-CH=CH group, cis-CH3(CH2)m-1CH=CH(CH2)m-1COOH (m=2n or 2n-1), display another odd-even effect. cis-CH3(CH2)7CH=CH(CH2)7COOH, one example of cis-CH3(CH2)2n-1-CH=CH(CH2)2n-1COOH (n=4), is predicted to form both an enantiomer and a nonchiral racemic structure, which is in accordance with the experimental observation of its self-assembled monolayer. However, cis-CH3(CH2)2n-2CH=CH(CH2)2n-2COOH does not form a stable self-assembled domain due to the same steric repulsion as that seen in the cis-CH3(CH2)7CH=CH(CH2)8COOH structure. These odd-even effects demonstrate that molecular self-assembly can be significantly tailored by slightly changing the molecular chain length.

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PubMed2006

The crystalline structures of carboxylic acid monolayers adsorbed on graphite.

X-ray and neutron diffraction have been used to investigate the formation of solid crystalline monolayers of all of the linear carboxylic acids from C(6) to C(14) at submonolayer coverage and from C(8) to C(14) at multilayer coverages, and to characterize their structures. X-rays and neutrons highlight different aspects of the monolayer structures, and their combination is therefore important in structural determination. For all of the acids with an odd number of carbon atoms, the unit cell is rectangular of plane group pgg containing four molecules. The members of the homologous series with an even number of carbon atoms have an oblique unit cell with two molecules per unit cell and plane group p2. This odd-even variation in crystal structure provides an explanation for the odd-even variation observed in monolayer melting points and mixing behavior. In all cases, the molecules are arranged in strongly hydrogen-bonded dimers with their extended axes parallel to the surface and the plane of the carbon skeleton essentially parallel to the graphite surface. The monolayer crystal structures have unit cell dimensions similar to certain close-packed planes of the bulk crystals, but the molecular arrangements are different. There is a 1-3% compression on increasing the coverage over a monolayer.

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PubMed2006

Solvent effect on self-assembled structures of 3,8-bis-hexadecyloxy-benzo[c]cinnoline on highly oriented pyrolytic graphite.

3,8-Bis-hexadecyloxy-benzo[c]cinnoline (BBC16) self-assembled into two structures at highly oriented pyrolytic graphite (HOPG) surface: one was formed by molecules with a V-like configuration (C2v symmetry) and the other by molecules with a Z-like configuration (C(s) symmetry). The self-assembled structures could be tweaked by the solvents used. In the self-assembled monolayers (SAMs) on HOPG, the BBC16 molecule adopted the V-like configuration in polar solvents and the Z-like configuration in nonpolar solvents. Moreover, the solvent viscosity, solvent dissolvability of BBC16, and substrate temperature also played some roles in tuning the two-dimensional self-assembled structures.

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PubMed2006

Classical studies of H atom trapping on a graphite surface.

The trapping and sticking of H and D atoms on the graphite (0001) surface is examined over the energy range 0.1-0.9 eV. Total electronic energy calculations based on density functional theory are used to develop a potential energy surface that allows for the full three-dimensional motion of the incident atom and the reconstruction of the bonding carbon atom, which must pucker out of the surface to form a stable bond. Classical methods are used to compute trapping cross sections as a function of incident energy. The C-H bond, once formed, rapidly dissociates without a mechanism to dissipate its excess energy. However, a number of long-lived trapping resonances exist, and for impact parameters below 1 A or so, several percent of the incident H atoms can remain trapped for 1 ps or more. This long-time trapping probability increases significantly when additional lattice degrees of freedom are added to carry energy away from the C-H stretch. Trapping can also increase with an increasing collision impact parameter, as H vibrations parallel to the surface become excited, leaving less energy in the C-H stretch. The trapping cross section at 1 ps reaches a maximum of 0.2 A2 for an H atom energy of 0.3 eV. Assuming that any atoms remaining trapped after 1 ps fully relax and stick, we estimate a lower bound for the sticking probability of H and D to be 0.024 and 0.050, respectively, about an order of magnitude below the experimental values.

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PubMed2006

Evidence of the interaction of evaporated Pt nanoparticles with variously treated surfaces of highly oriented pyrolytic graphite.

The interactions of Pt nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite surfaces modified by kiloelectronvolt Ar+ beam treatment, have been studied by X-ray photoelectron spectroscopy core-level line shape analysis. The C1s and Pt4f7/2 peaks were each considered to be composed of one asymmetric peak, and changes in their asymmetry parameters were used to study their interfacial interactions. In addition to these changes, strong signal intensity changes with time were found for both the C1s and Pt4f peaks, indicating an initial crystalline orientational instability of the Pt nanoparticles, which is supported by time-dependent high-resolution electron microscopy studies at elevated temperatures.

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PubMed2006

Tubular carbon nano-/microstructures synthesized from graphite powders by an in situ template process.

Through the use of commercial graphite powders as the carbon sources, a variety of interesting tubular carbon nano- and microstructures, such as networked carbon nanotubes, aligned carbon microtubes with hexagonal cross-sections, aligned tapered carbon tubes, and hollow carbon microhorns, have been successfully synthesized. As-grown tubular carbon structures were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray energy-dispersive spectroscopy. An in situ template mechanism was proposed to explain the possible growth process. The vibrational properties of the synthesized tubular carbon structures were also studied by Raman spectroscopy.

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PubMed2006

A comparison between interfacial electron-transfer rate constants at metallic and graphite electrodes.

The Fermi golden rule formalism has been used to derive the rate constant for interfacial electron transfer from a semimetallic electrode, such as highly ordered pyrolytic graphite (HOPG), to a redox couple in solution. A simple expression is presented that semiquantitatively relates the electron-transfer rate constant at a semimetallic electrode to that at a metallic electrode. The approach allows for the estimation of the value of the rate constant for interfacial charge transfer to nonadsorbing outer-sphere redox species at semimetallic electrodes. Rate constants for interfacial electron transfer for a variety of one-electron redox couples at semimetallic electrodes have been calculated relative to the rate constant of the ferrocenium/ferrocene redox couple at a gold electrode. Good agreement is found, in general, between the calculated and observed rate constants.

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PubMed2006

Molecular architecture of oligothiophene on a highly oriented pyrolytic graphite surface by employing hydrogen bondings.

To achieve a controllable and predictable molecular architecture on a two-dimensional (2D) surface, a series of oligothiophenes with carboxylic groups and alkane chains were synthesized. The alkane chains and carboxylic groups, which can form hydrogen bonding, were intentionally designed in different positions of the oligothiophenes. The resulted molecular architectures by using the so-prepared oligothiophenes on a highly oriented pyrolytic graphite (HOPG) surface were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT). It is found that the hydrogen bonding plays an essential role in the formation of the ordered assemblies. A controlled 2D molecular assembly could be fabricated by using hydrogen bondings.

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PubMed2006

Molecular electrostatic potential devices on graphite and silicon surfaces.

We demonstrate that molecular gates using molecular electrostatic potentials (MEP) can be used on hydrogen-passivated silicon substrates without any disturbance of their behavior in vacuum; however, the use of graphite as a substrate strongly affects such behavior. As expected, the substrate may become one more design variable. The ability to have several substrate alternatives is very important for the practical implementation of this new scenario based on molecular potentials. In general, the effect of the substrate can be predetermined by calculating the MEP of the surface as this indicates how strongly its intrinsic potential is.

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