Surface Structure Of Graphite Research Articles

Effect of Surface Structure of Graphite on the Passivation Ability of Solid Electrolyte Interphases

Effect of Surface Structure of Graphite on the Passivation Ability of Solid Electrolyte Interphases

Effect of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans on preg-robbing of gold by graphite from thiourea leaching solution

Carbonaceous matter (CM) is one of the main reasons for the decrease of gold recovery in refractory gold ores. Bio-pretreatment of carbonaceous gold ores makes the reduction the preg-robbing of gold, and has attracted significant attention recently due to its economic feasibility and environmental friendliness. In this study, the graphite was used as surrogate material to investigate the effect of Acidithiobacillus ferrooxidans (A. ferrooxidans) and Leptospirillum ferrooxidans (L. ferrooxidans) on the gold adsorption by graphite in thiourea system. The results showed that graphite had a significant inhibitory effect on the growth of A. ferrooxidans and L. ferrooxidans. After bio-oxidation, the lamellar structure of graphite surface was blurred, and there was the formation of jarosite. In addition, some oxygen-containing groups such as O–H, SO and Fe–O appeared on the surface, but the CC bond cleaved. The adsorption capacity of graphite to gold decreased from 35.8% to 16.6% (A. ferrooxidans) and 18.4% (L. ferrooxidans) respectively. It can be concluded that the bio-oxidation by A. ferrooxidans and L. ferrooxidans can passivate and erode graphite, and contribute to decrease preg-robbing of gold from thiourea leaching solution.

Diamond Deposition on Graphite in Hydrogen Microwave Plasma

Hydrogen plasma etching of graphite generates radicals that can be used for diamond synthesis by chemical vapor deposition (CVD). We studied the etching of polycrystalline graphite by a hydrogen microwave plasma, growth of diamond particles of the non-seeded graphite substrates, and characterized the diamond morphology, grain size distribution, growth rate, and phase purity. The graphite substrates served simultaneously as a carbon source, this being the specific feature of the process. A disorder of the graphite surface structure reduces as the result of the etching as revealed with Raman spectroscopy. The diamond growth rate of 3 – 5 µm/h was achieved, the quality of the produced diamond grains improving with growth time due to inherently nonstationary graphite etching process

Effect of Surface and Defect Chemistry on the Photocatalytic Properties of Intentionally Defect-Rich ZnO Nanorod Arrays.

Due to the abundance of intrinsic defects in zinc oxide (ZnO), the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photocatalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photocatalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore, strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g., shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photocatalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which, it is suggested, should be employed when assessing the photocatalytic properties of defect-rich ZnO.

Overpotentials and solid electrolyte interphase formation at porous graphite electrodes in mixed ethylene carbonate–propylene carbonate electrolyte systems

The first electrochemical lithium insertion was characterized for several graphite materials with high degree of crystallinity, different particle size distributions and surface morphologies in an ethylene carbonate (EC)/propylene carbonate (PC) electrolyte. For coarser graphite materials and graphites with a low superficial defect concentration, an irreversible process was observed which correlated with the electrochemical exfoliation of graphite. Different natural and synthetic graphites with similar particle size distribution and active surface area showed differences in the passivation behavior during the first electrochemical reduction. The fraction of graphite particles exfoliating during the first galvanostatic lithium insertion linearly increased with length of the irreversible plateau, which concomitantly moved to more positive potentials. This behavior can be rationalized when considering, besides the surface structure, local overpotentials for the solid electrolyte interphase formation process, and especially the overpotential distribution through the graphite electrode. These overpotentials cause a distribution of the local current density attributed to the passivation process. Optimizing the particle contacts in the electrode by applying mechanical pressure or by selecting the proper binder decreased the overpotentials and suppressed the graphite exfoliation in the EC/PC electrolyte. Therefore, both graphite surface structure and electrode engineering aspects have to be considered for successful passivation against exfoliation.

A comparison between physically and chemically driven etching in the oxidation of graphite surfaces

The etching of graphite surfaces by two different types of oxidative treatments, namely dielectric barrier discharge (DBD) air plasma and ultraviolet-generated ozone (UVO), has been investigated and compared by means of scanning tunneling microscopy (STM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Although the attack is initiated in both cases with the formation of individual, isolated atomic-scale defects (in particular, atomic vacancies), its subsequent evolution indicated that different mechanisms drive the surface modification in the two types of treatment, which greatly differ in etching selectivity. Thus, physical processes (i.e., ion bombardment) dominate the attack by DBD air plasma, which are not present in the case of UVO oxidation. The effects of the different etching mechanisms on the graphite surface structure, as visualized by STM down to the atomic scale, are discussed and found to be consistent with the Raman spectroscopy and XPS data. This type of information can be relevant when selecting the most appropriate type of surface modification of carbon materials for specific purposes.

Study of the effect of nitric acid and metal-based chemical modifiers on graphite platform surfaces by Raman spectrometry

Graphite platforms containing metal-based chemical modifiers and nitric acid solutions, individually and in combination, were heated using a typical temperature program for ETAAS; for some measurements, aluminium nitrate was also present as a matrix component. Changes in the graphite surface structure that occurred after heating the platform to various temperatures were evaluated using Raman spectrometry. The presence of thorium, zirconium, palladium or nitric acid resulted in an increase in the content of sp3-bonded carbon clusters on the platform surface. In addition, nitric acid affected the chemical modifier phases present, particularly for palladium. The surface alterations observed help explain the effect of the chemical modifiers on platform lifetime and the stabilization mechanism of the modifiers on analyte elements.

Influences of surface fluorination and carbon coating with furan resin in natural graphite as anode in lithium-ion batteries

Graphite and carbonaceous materials are promising as the anode for lithium-ion batteries. Although natural graphite exhibits a high specific capacity, violent irreversible capacity occurs after a few charge/discharge cycles. Carbon coating from furan resin pyrolysis on natural graphite surface can enhance graphite surface structure and promote the electrochemical properties by increasing C–C bonding. In this research, electrochemical characteristics of surface fluorination natural graphite and fluorination carbon-coated graphite are investigated. The C–F bonds formed at a temperature of around 500 °C heat treatment under 5 at% fluorine gas can improve lithium-ion intercalation and de-intercalation during charge/discharge. Both methods of carbon coating and surface fluorination can increase the first charge capacity and decrease the irreversibility of natural graphite. The analysis on surface properties is investigated by Raman spectroscopy and the bulk is analyzed by X-ray diffraction measurements.

Performance enhancement of spherical natural graphite by phenol resin in lithium ion batteries

The capacity of natural graphite in the lithium ion battery anode decays seriously. The phenol resin is used as a reaction material to modify the electrochemical performance of spherical graphite as the anode material in lithium ion batteries. Measuring the reversible capacity indicates change in the surface structure of spherical graphite. A dense layer of methyl groups was thus formed. Some structural imperfections are removed and the stability of the graphite structure is increased. Clearly, reducing the irreversible capacity is beneficial in controlling the uniformity of the spherical graphite surface structure.

Generation of circular and hexagonal microholes in a graphite surface

Circular and hexagonal microholes, ranging in size from several nanometers to micrometers, were generated in a graphite surface by electron beam irradiation. The depth of the holes was always 0.34 nm, indicating that only the uppermost layer was modified. The shape of the holes was switched from circular to hexagonal when the size reached about 500 nm. Although the hexagons appeared at random on the surface, they were aligned in one direction, implying that the formation and growth of the holes were determined by the atomic structure of graphite. The results described in this study will be used to modify the surface structure of graphite for many applications.

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.

Effect of Graphite Surface Structure on Initial Irreversible Reaction in Graphite Anodes

The initial irreversible reaction that occurs in graphite anodes during the first lithium intercalation in lithium rechargeable batteries was studied in view of graphite surface structure. Graphitized mesophase spheres and pitch‐based carbon fibers, which show low irreversible capacity, were shown to have turbostratic surface regions and highly graphitized cores using Ar‐ion laser Raman spectroscopy. Burning off these surface regions resulted in remarkable increases of initial irreversible capacity. Those results can be explained by a proposed model that a turbostratic structure of the graphite surface region resists drastic swelling of interlayer spaces arising from cointercalation of solvated ions and depresses the side reaction. © 1999 The Electrochemical Society. All rights reserved.

In Situ Roman Studies of Graphite Surface Structures during Lithium Electrochemical Intercalation

Surface structural changes were investigated using in situ Raman microspectroscopy during lithium intercalation into Lonza KS‐44 and KS‐6 graphite materials in solutions of ethylene carbonate (EC)/dimethyl carbonate (DMC) or EC/1,2‐dimethoxyethane (DME). In the 1 M EC/DMC solution, the Raman spectral changes indicate that graphite undergoes a series of transitions during the electrochemical lithium intercalation process from initial formation of the dilute stage 1 GIC (graphite intercalation compound) to a stage 4 GIC, then through a stage 3 to stage 2 and finally to a stage 1 GIC. The reversibility of the Raman spectral changes indicates that the top layers of the graphite are not damaged during the first charge and discharge cycle. Intercalation‐induced strain causes the E2g2 (i) mode of the unbound graphite layers to shift toward lower frequency in the stage 4 and stage 3 GICs. In the 1 M EC/DME solution, the Raman spectra changed dramatically in the high‐potential range (0.9–0.5 V) during the discharge and charge processes, indicating that the graphite surface structure is altered even at the high potentials where major lithium intercalation is not expected to occur. Irreversible spectral changes in the E2g2 band shoulders suggest that the surface structural changes are also irreversible. Raman spectral changes of the E2g2 band in the potential range 0.9–0.5 V are attributed to extensive surface graphite exfoliation caused by solvent cointercalation with lithium ions. This is in accordance with the large irreversible capacity consumption in the potential range 0.9–0.5 V observed during the first discharge process. In the low‐potential range, the Raman spectral changes of the graphite in EC/DME solutions are similar to those in EC/DMC solution.

Kinetic models of horseradish peroxidase action on a graphite electrode

The direct and mediated mechanisms of the electroreduction of hydrogen peroxide at a graphite electrode modified with horseradish peroxidase (HRP) were studied. The turnover number of the heterogeneous electron transfer between adsorbed HRP and the electrode was found to be equal to 0.66 ± 0.28 s −1 . The rate of the reaction of H 2 O 2 with HRP on the graphite surface was found to be 385 times slower than that in solution. p -Cresol, phenol and p -chlorophenol behaved as efficient mediators in the process of bioelectrochemical H 2 O 2 reduction. From the comparison of kinetically limited currents observed during direct and mediated reduction of H 2 O 2 it was concluded that the population of adsorbed HRP molecules and/or the graphite surface structure cannot be treated as homogeneous. It was found that 42% of the total amount of HRP molecules adsorbed on the electrode were accessible for direct unmediated electron transfer from the graphite electrode.

Bundles of carbon nanotubes generated by vapor-phase growth

Bundles of nanotubes were produced by vapor-phase condensation of carbon on highly oriented pyrolytic graphite and analyzed by scanning tunneling microscopy. The bundles are found to have widths between 15 and 50 nm, and up to 200 nm long. They are composed of concentric tubules with 2–4 nm in diameter. Atomic resolution images reveal their graphitic surface structure.

Vapor-Phase Grown Bundles of Carbon Nanotubes

We have produced bundles of nanotubes by vapor-phase condensation of carbon on highly-oriented pyrolytic graphite (HOPG). The bundles are found to have widths between 15 nm and 50 nm, and up to 200 nm long. They are composed of concentric tubules with 2-4 nm in diameter. Atomic resolution STM images reveal their graphitic surface structure.

原子間力顕微鏡によるフッ素-黒鉛層間化合物の界面構造の解析

Atomic force microscopy (AFM) has been used to study the interface structure of stage-1 and stage-2 fluorine-graphite intercalation compounds (CXF) synthesized under the gaseous HF. The results obtained are summarized as follows;(1) It is difficult to prepare the atomically-flat surfaces of the stage-1 CXF by cleaving.(2) We have recognized a well ordered 2×\sqrt3 and 1×\sqrt3 superlattice structures of fluorine atoms with twofold symmetry on the cleaved surfaces of the stage-1 CXF synthesized from natural graphite. This suggests that the 1×1 ordered structure and the new 1×\sqrt3 superlattice structure of fluorine atoms coexist in the interface of the stage-1 CXF.(3) Only the graphitic surface structure has been observed on the cleaved surfaces of the stage-2 CXF. This suggests that the cleavage occurs at the graphite layer having the weak bonds preferentially.(4) We have found that the force between the tip of AFM and a sample surface for the intercalant layer of CXF is larger than that for the graphite layer.

An ultrahigh vacuum scanning tunneling microscope for the investigation of clean surfaces

An ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been built into a VG surface analysis system consisting of a VG Escalab and three additional UHV chambers. Samples and tips can be prepared in UHV. Surface analysis can be performed by low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), UV-photoelectron spectroscopy (UPS), and scanning electron/Auger microscopy (SEM/SAM). The STM design is based on an electromagnetically driven approach system. Easy exchange of samples is possible since the scanner is incorporated in a scan head which can be flipped backwards. The performance of the STM was tested by resolving the surface structure of graphite and Si(111)7×7. However, the main application will be the investigation of well characterized metal surfaces. We have studied the Au(111) surface where series of monoatomic steps could be imaged. In addition, we resolved the atomic surface structure. The conditions for atomic resolution imaging on close-packed metal surfaces are discussed. For comparison, we have also studied polycrystalline Au films vacuum deposited onto various substrates. For the first time, atomic resolution could also be obtained on these polycrystalline films. Finally, we present preliminary results of STM studies on a gallium single crystal.

Surface structure of graphite intercalation compounds resolved in real space by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to study the freshly cleaved surfaces of stage 1 donor graphite intercalation compounds at room temperature. STM images on a submicrometer scale reveal surface defects such as steps with unprecedented spatial resolution. On atomically flat terraces, ordered superlattices commensurate as well as incommensurate with the graphite lattice have been observed on the surface of C 6Li. In contrast, STM measurements performed so far on the surface of C 8K have revealed a graphitic surface structure. We suggest that the different superlattices observed locally on the surface of C 6Li are due to a lithium layer being on top of the first graphitic layer after cleaving. The lithium forms either superlattices commensurate with the graphite lattice or an incommensurate, nearly close-packed two dimensional overlayer.

Donor graphite intercalation compounds studied with a high stability STM

SUMMARYWe have designed a small, rigid and easy operable scanning tunnelling microscope (STM) which shows low thermal drift rates due to its compact construction and the chosen materials. Using this instrument we have studied stage 1 donor graphite intercalation compounds (C6Li and C8K) with atomic resolution either in the current imaging, constant current or local tunnelling barrier height mode of operation. Ordered superlattices commensurate as well as incommensurate with the graphite lattice have been observed on the C6Li surface. STM images on C8K revealed a graphitic surface structure.