Basal Graphite Research Articles

Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes.

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag-S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverageand less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not onlystrictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

Microscopic Analysis of the Different Perchlorate Anions Intercalation Stages of Graphite

Driven by the perspective of large-scale, high-quality graphene production via chemical routes, the investigation of electrochemical anion intercalation between the basal graphite planes has seen a renewed interest among the scientific community. At relatively high electrochemical potentials, when oxidation occurs, graphite electrodes undergo significant anion intercalation processes. The latter swell the uppermost graphite layers (i.e., graphene sheets), reduce the interplane interaction and favor the graphite delamination in liquid. Different intercalation stages are observed in a perchloric acid electrolyte, which are usually interpreted in terms of different perchlorate penetration depths. Nonetheless, the understanding of the morphological changes occurring at the electrode surface during the different intercalation stages is still not completely clear. We combine different microscopy techniques including optical, scanning electron and electrochemical atomic force microscopies to analyze the morpholo...

Polymer lead pencil graphite as electrode material: Voltammetric, XPS and Raman study

Mechanical pencil leads were studied as disposable, low-cost electrodes. Lateral surfaces of mechanical pencil leads branded as “polymer” show high electron transfer rates for hexaamineruthenium chloride, potassium ferricyanide, ascorbate, ferric chloride and dopamine electrochemical probes, and are significantly better electrode materials than either classic woodcase clay–graphite pencil compositions or non-polymer mechanical pencil leads. Best polymer leads outperform glassy carbon, basal and edge graphite and boron-doped diamond electrodes. In addition to electrochemical experiments, the studied pencil leads were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and Raman spectroscopy. High content of sp3 hybridized carbon (up to 80%) with a low degree of surface oxidation and occurrence of silicon monoxide (SiO) on the surfaces of the polymer pencil leads were found using X-ray photoelectron spectroscopy. Low double layer capacitance values of similar magnitude as that found for boron-doped diamond electrodes are at the origin of favourably low background currents on the polymer pencil lead electrodes. SiO containing polymer pencil lead electrodes allow electrochemical analysis that is more sensitive than common carbon electrodes, as demonstrated by voltammetry of adenine and xanthine.

State of the Active Component and Catalytic Properties of Pd/C Cata- lysts on the Selective Hydrogenation of Butadiene-1,3 into Butylenes

<p>Palladium (0.25-5.8 wt.%) was supported on three different types of filamentous carbon in which the basal graphite planes are arranged along, crosswise and at a 45° angle to the axis of a nanofiber. HREM, XPS and X-ray techniques were used to study the state of high dispersed Pd particles supported on the<br /> filamentous carbon. It was established that the type of the carbon support affects the properties of the active component. The high disperse Pd particles most strongly interact with the prismatic planes of graphite to provide electron transfer from palladium to the support, which stabilizes palladium in a more dispersed state. Because of this strong interaction, Pd<sup>2+</sup> is stabilized in the ionic state. A change in the properties of the palladium particles initiates changes in the catalytic properties of Pd/C catalysts in the reaction of selective hydrogenation of 1,3-butadiene into butylenes. An increase in the fraction of Pd<sup>2+</sup> in the catalyst composition results in the decrease of both total activity and selectivity of Pd/C catalysts in the considered reaction.</p>

A molecular dynamics simulation study of the electric double layer and capacitance of [BMIM][PF6] and [BMIM][BF4] room temperature ionic liquids near charged surfaces

A molecular dynamics simulation study of electric double layer (EDL) structure and differential capacitance (DC) of two 1-butyl-3-methylimidazolium (BMIM)-based room temperature ionic liquids, i.e. [BMIM][BF4] and [BMIM][PF6], has been conducted on basal and prismatic graphite as well as (001) and (011) gold electrode surfaces. The influence of the electrode surface and electrolyte structure on electrode capacitance and EDL structure are discussed. For a given electrode surface both the [BMIM][BF4] and [BMIM][PF6] electrolytes generate very similar DC and EDL structures. The DC for these ionic liquids in contact with atomically flat surfaces (i.e. basal graphite and (001)Au) shows very small variations within the electrolyte chemical stability potential window and fluctuates around an average value of ∼5 μF cm(-2). On atomically more corrugated surfaces (i.e., Au(011) and prismatic graphite) the DC shows more variation with electrode potential and depends on the correspondence between dimensions of the surface roughness and electrolyte ion sizes. The trends and dependencies obtained for DC are used to discriminate between mutually contradictory experimental data reported in the literature for related systems.

Molecular Dynamics Simulation Study of the Interfacial Structure and Differential Capacitance of Alkylimidazolium Bis(trifluoromethanesulfonyl)imide [Cnmim][TFSI] Ionic Liquids at Graphite Electrodes

The dependence on electrode potential of the interfacial structure and differential capacitance (DC) for 1-alkyl-3-methyimidazolium bis(trifluoromethanesulfonyl)imide ([Cnmim][TFSI], n = 2, 4, 6, and 8) ionic liquids (IL) near basal (flat) and prismatic edge face (rough) graphite electrodes was investigated here with atomistic simulations. Overall camel-shaped DCs were observed for both surfaces. The prismatic graphite generated systematically larger capacitances than the atomically flat basal face. Although on the flat electrodes the DC is almost constant at electrode potential bellow saturation (i.e., roughly within ±2 V), on the prismatic edge face the DC showed large amplitude changes between minima and maxima. This trend in DC was explained from the dependence versus potential of the structure and composition of the interfacial electrolyte layer; specifically, faster counterions accumulation and ion segregation in the interfacial layer are observed for atomically corrugated electrode surfaces as compared to the flat ones. Surprisingly, the increase of the charge-neutral alkyl tail length of the cation resulted only in a small reduction in DC, indicating ions ability to rearrange/reorient charge-caring groups such that it maximizes the counterions charge near the surface. This finding shows a promising route for optimization of ions structure to achieve the desired/optimal properties of electrolyte (e.g., low melting point and viscosity) without significant reduction of energy density storage capabilities.

Hydrogen adsorption on functionalized graphene

The hydrogen adsorption on basal graphite planes functionalized by hydrogen atoms is studied by molecular modeling and numerical simulation at temperatures of 77 K and 293 K up to high pressure. At 77 K and pressure of 1 MPa, on such an adsorbing surface, the excess hydrogen physisorption is estimated equal to ∼ 7.0 wt % . At 293 K and 30 MPa, the excess physisorption reaches ∼ 1.5 wt % . A comparison between the hydrogen adsorption properties of functionalized graphite basal planes and plain graphite basal planes is presented for materials exhibiting similar porosities.

An experimental and molecular-statistical study of the adsorption of the iodobenzene, 2-iodothiophene, and isomeric iodoadamantane molecules on the graphite basal face surface

The thermodynamic characteristics of adsorption of iodobenzene, 2-iodothiophene, and 1- and 2-iodoadamantanes on the surface of graphitized thermal carbon black were determined experimentally. The influence of the special features of the molecular structure of the adsorbates on the thermodynamic characteristics of adsorption was studied. The atom-atom approximation of the semiempirical molecular-statistical theory of adsorption was used to calculate the thermodynamic characteristics of adsorption of the adsorbates using the newly determined potential function parameters of pair intermolecular interaction (φ(r)) of I with C atoms of the basal graphite face. For the example of isostructural monohalogenated benzenes, thiophenes, and adamantanes, a comparative analysis of the contributions of the F, Cl, Br, and I atoms to the thermodynamic characteristics of adsorption was performed for the nonspecific adsorption of these compounds on a plane graphitized carbon black surface.

DFT and kinetics study of O/O2 mixtures reacting over a graphite (0001) basal surface

Spin-polarized density functional calculations were used to investigate the interaction of atomic and molecular oxygen on the basal graphite surface at several atomic coverages. Two carbon layers were enough to represent the graphite surface. Oxygen atoms bind mainly over C–C bridge sites forming an epoxide-like structure with a two carbon puckering and with adsorption energies in the 0.95–1.28 eV range, depending on the atomic coverage. Molecular oxygen only shows a very weak physisorption. Atomic adsorption and diffusion along with atomic recombination via Eley–Rideal and Langmuir–Hinshelwood mechanisms were studied. All surfaces processes were activated with energy barriers that decreased for lower atomic coverages. Relaxation effects were non-negligible. A microkinetic model with six elementary surface processes was proposed to see the overall behaviour of several initial O/O2 mixtures flowing over a graphite surface at 300–1,000 K. Thermal rate constants were derived from Density Functional Theory data and standard Transition State Theory. A very low steady-state atomic coverage (θΟ < 0.5%) was predicted, and also very low atomic recombination coefficients were observed (γO < 5 × 10−4). The Eley–Rideal together with the adsorption and desorption processes was much more important than the Langmuir–Hinshelwood reaction.

Dissociative adsorption of atmospheric molecules at vacancies on the graphite (0 0 0 1) surface of samples exposed to plasma

The quantum Density Functional Theory (DFT) model of a single atom vacancy on the basal graphite surface is first validated through comparison of the theoretical vibrational spectra of the fully hydrogenated defect with the corresponding High Resolution Energy Loss Spectroscopy (HREELS) spectra. We then proceed to investigate atmospheric molecules adsorption on vacancy defects on the (0001) graphite surface. Carbon dioxide and nitrogen have no interaction with the defect whereas carbon monoxide is incorporated into the vacancy with an activation energy of 1.5eV. Water dissociates with a barrier of 1.6eV, forming an adsorbed hydroxyl radical. Molecular oxygen dissociates with a very low barrier (0.2eV).

Quantum Chemical Prediction of Reaction Pathways and Rate Constants for Dissociative Adsorption of COx and NOx on the Graphite (0001) Surface

We present predictions of reaction rate constants for dissociative adsorption reactions of CO(x) (x = 1, 2) and NO(x) (x = 1, 2) molecules on the basal graphite (0001) surface based on potential energy surfaces (PES) obtained by the integrated ONIOM(B3LYP:DFTB-D) quantum chemical hybrid approach with dispersion-augmented density functional tight binding (DFTB-D) as low level method. Following an a priori methodology developed in a previous investigation of water dissociative adsorption reactions on graphite, we used a C(94)H(24) dicircumcoronene graphene slab as model system for the graphite surface in finite-size molecular structure investigations, and single adsorbate molecules reacting with the pristine graphene sheet. By employing the ONIOM PES information in RRKM theory we predict reaction rate constants in the temperature range between 1,000 and 5,000 K. We find that among CO(x) and NO(x) adsorbate species, the dissociative adsorption reactions of CO(2) and both radical species NO and NO(2) are likely candidates as a cause for high temperature oxidation and erosion of graphite (0001) surfaces, whereas reaction with CO is not likely to lead to long-lived surface defects. High temperature quantum chemical molecular dynamics simulations (QM/MD) at T = 5,000 K using on-the-fly DFTB-D energies and gradients confirm the results of our PES study.

Exploring the electrocatalytic sites of carbon nanotubes for NADH detection: an edge plane pyrolytic graphite electrode study

The electrocatalytic properties of multi-walled carbon nanotube modified electrodes toward the oxidation of NADH are critically evaluated. Carbon nanotube modified electrodes are examined and compared with boron-doped diamond and glassy carbon electrodes, and most importantly, edge plane and basal pyrolytic graphite electrodes. It is found that CNT modified electrodes are no more reactive than edge plane pyrolytic graphite electrodes with the comparison with edge plane and basal plane pyrolytic graphite electrodes allowing the electroactive sites for the electrochemical oxidation of NADH to be unambiguously determined as due to edge plane sites. Using these highly reactive edge plane sites, edge plane pyrolytic graphite electrodes are examined with cyclic voltammetry and amperometry for the electroanalytical determination of NADH. It is demonstrated that a detection limit of 5 microM is possible with cyclic voltammetry or 0.3 microM using amperometry suggesting that edge plane pyrolytic graphite electrodes can conveniently replace carbon nanotube modified glassy carbon electrodes for biosensing applications with the relative advantages of reactivity, cost and simplicity of preparation. We advocate the routine use of edge plane and basal plane pyrolytic graphite electrodes in studies utilising carbon nanotubes particularly if 'electrocatalytic' properties are claimed for the latter.

A comparison of different types of gold?carbon composite electrode for detection of arsenic(III)

A study has been conducted using abrasively modified basal and edge-plane graphite, carbon-paste, and carbon-epoxy electrodes to create gold-carbon composite electrodes. Using either nano or micro-sized gold particles their suitability for use in detecting arsenic(III) is assessed. It was found that gold arrays prepared from micron-sized particles gave the best performance for arsenic detection. In particular micron arrays produced in carbon-paste electrodes with an easily renewable surface work well for detection of arsenic, producing a detection limit of 5(+/-2)x10(-9) mol L(-1), with a high sensitivity of 10(+/-0.1) A mol(-1) L.

Detailed atomistic Monte Carlo simulation of grafted polymer melts. I. Thermodynamic and conformational properties

The thermodynamic and conformational properties of polymer melts grafted on a solid substrate as obtained from detailed, atomistic Monte Carlo simulations with the end-bridging algorithm are presented. The interface between a basal graphite plane (as well as a non-interacting hard surface) and a bulk polyethylene (PE) melt, a few or all chains of which are grafted on the plane, has been studied. Three different PE melts, of mean molecular length C78, C156, and C250, have been investigated, at grafting densities ranging from 0.54 to 2.62 nm−2. For melts composed of grafted and free chains, it is observed that, at moderate to high surface densities (σ⩾1 nm−2), the region close to the substrate is fully occupied by segments belonging to grafted chains, which are forced by their chemical grafting to have their first segment on the interface. As the grafting density increases, free chains are progressively expelled from the surface region, in agreement with scaling arguments and the predictions of lattice-based self-consistent mean-field (SCF) theory. For melts grafted on a graphite plane, it is also seen that the local melt density in the region closest to the interface is systematically higher than in the bulk, exhibiting distinct local maxima due to polymer adsorption. Results for the chain conformation tensor demonstrate that chains are significantly stretched in the direction perpendicular to the surface, even for moderate surface densities. For the C250 (PE) melt at a grafting density σ=1.31 nm−2, for example, the average chain dimension perpendicular to the interface is 1.9 times larger than its equilibrium value in the bulk. The profile of the chain end density is seen to exhibit universal behavior in agreement with the predictions of the SCF theory. Additional results concerning the mean chain conformational path and the structure of the interfacial area for both systems studied (fully grafted and mixtures of grafted and free chain systems) are also presented.

Electrocatalysis of the hydrogen production by [Fe] hydrogenase from Desulfovibrio vulgaris Hildenborough

Hydrogenases are enzymes which catalyze hydrogen production/consumption reactions. In this paper, the [Fe] hydrogenase from Desulfovibrio vulgaris Hildenborough is shown to catalyze the direct hydrogen production from protons, in the absence of any promoter, at a basal pyrolytic graphite electrode using cyclic voltammetry techniques. The effect of several parameters upon catalytic current is investigated: pH, hydrogenase concentration, ionic strength, competition with another protein (bovine serum albumin), cycling repetition, mode of electrode polishing. The extent and efficiency of the hydrogenase electroactivity are examined in the presence of either an artificial electron carrier (methylviologen) or a physiological partner cytochrome c 3 isolated from the same bacterial strain. Results suggest that the electron-transfer process is essentially controlled by a complex between both hydrogenase and cytochrome c 3. Electrocatalysis appears to be largely governed by the adsorption of hydrogenase on the electrode surface very likely involving hydrophobic interactions.

First-principles study of the atomic oxygen adsorption on the (0 0 0 1) graphite surface and dissolution

In the framework of the density-functional theory we studied the atomic oxygen adsorption on the basal graphite surface at three imposed oxygen coverages 50, 25, 5.5% for three different adsorption sites, and also the dissolution of the atomic oxygen in graphite at one fixed coverage. Independently of the concentration, the most stable position of oxygen atoms is directly above a carbon–carbon bond (bridge), followed by the directly above a carbon atom position (top), and the directly above a hexagonal hollow (hex) site. Intercalating oxygen just below the surface layer, shows that dissoluted oxygen is more stable than adsorbed one.

Carbon Nanofibers: Catalytic Synthesis and Applications

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (˜200 m2/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.

Planar clusters of Rydberg matter K N ( N=7, 14, 19, 37, 61) detected by multiphoton fragmentation time-of-flight mass spectrometry

We report a direct observation of large planar clusters characteristic for Rydberg matter. This state of matter has low density with interatomic distances of the order of tens of nanometers. We observe the laser multiphoton-induced fragmentation and ionization of potassium clusters K N + with N (the magic numbers) =7, 14, 19, 37 and 61 in processes due to 5–12 photons of 2.2 eV (564 nm) in 5 ns pulses, using time-of-flight (TOF) mass spectrometry. The clusters are formed by desorption of K * Rydberg atoms from a basal graphite surface, which is intercalated by a weak molecular beam of K. The fragmentation is strongly influenced by the electric field used, below 4 V/cm. The efficient multiphoton fragmentation is probably due to the low rate of E–V transfer in the cluster.

Micromechanisms of metal dusting on Fe-base and Ni-base alloys

Phenomena of metal dusting on iron and nickel (and their alloys) were studied by characterizing both microstructure and nanochemistry of the reaction zones using TEM and AEM techniques. While in case of iron a metastable carbide (cementite) is formed nickel directly disintegrates. In the chromium-rich steel HK40 and the chromium-rich Ni-base alloy Inconel 600 some microstructural chromium dependent features were observed with protective effects against metal dusting. Independent of these differences, in all groups of materials a fundamental common starting mechanism on the atomic scale could be deduced, which first of all comprises the arrangement of basal graphite lattice planes perpendicularly oriented to the carbide (or metal) surface acting as active sites in the disintegration process. Mikromechanismen beim Metal Dusting von Eisen- und Nickelbasislegierungen Unter Verwendung von TEM- und AEM-Techniken wurden sowohl die Mikrostruktur als auch die Nanochemie der beim Metal Dusting von Eisen und Nickel (und deren Legierungen) auftretenden Reaktionszonen charakterisiert. Während im Falle von Eisen ein metastabiles Carbid (Zementit) gebildet wird, zerfällt Nickel direkt. Im chromreichen Stahl HK40 und in der ebenfalls Chrom enthaltenden Nickelbasislegierung Inconel 600 wurden mikrostrukturelle Prozesse mit einer Schutzwirkung gegen Metal Dusting beobachtet. Unabhängig von diesen Unterschieden wirkt in allen untersuchten Materialgruppen auf atomarem Niveau ein gemeinsamer, grundlegender Schädigungsmechanismus: Basisebenen des Graphitgitters wachsen senkrecht auf die Carbid- (bzw. Nickel-) Oberfläche auf, wobei ihre reaktiven, frei endenden Kanten als aktive Zentren beim Zerfallsprozeß der Materialien fungieren.

Carbon nanostructures — Diamond nucleation centers formed during the bias pretreatment

Abstract Current models of diamond nucleation during the so-called bias pretreatment in chemical vapour deposition (CVD) consider the possibility that nuclei form in or on top of a carbonaceous layer that is deposited initially on the substrate. In this context we have investigated the stability of different carbon modifications under plasma conditions. Microcrystalline graphite, hydrogenated amorphous carbon, and turbostratic carbon films were etched within a few minutes under both bias and growth conditions without enhancing the nucleation density appreciably. In contrast, fibrous carbon is observed in scanning electron micrographs to grow during the bias pretreatment. The growth of these fibers appears to be initiated by defects and metal contamination on the silicon substrate. Even on already existing diamonds, prolonged biasing yields fibrous structures. On the fibrous material a high nucleation density was achieved without further pretreatment. A detailed TEM characterization reveals that the fibers are some tens of nanometers thick, and that they are hollow carbon tubes with a layered wall structure. The layer distance was determined to be 0.335 nm which is the distance of the basal graphite layers observed also in carbon nanotubes. We thus conclude that the formation of carbon nanostructures is one possible mechanism of diamond nucleation during the bias pretreatment. This conclusion is supported by other authors, who observed the formation of small diamond particles in fullerene onions during electron bombardment.