Intercalant Concentration Research Articles

Raman-scattering and transmission-electron-microscopy studies of fluorine-intercalated graphite fibers CxF (7.8 >= x >= 2.9).

Raman-scattering and transmission-electron-microscopy (TEM) experiments are reported which provide information on the microstructure of fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F) for stoichiometries in the range 7.8\ensuremath{\ge}x\ensuremath{\ge}2.9. Lorentzian fits to our Raman spectra indicate the presence of a doublet near 1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a broad line near 1360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. An anomalous frequency downshift of the doublet was observed in ${\mathrm{C}}_{\mathit{x}}$F fibers upon increase in the intercalate concentration, unlike the behavior observed in other acceptor graphite intercalation compounds. This downshift in frequency of the doublet can be understood in terms of a decrease in the hole concentration in the graphene layers with increasing fluorine concentration, resulting in an increase in the in-plane lattice constant and a decrease in the in-plane mode frequency. These Raman results are consistent with high-resolution TEM experiments on the same fibers. Lattice fringe images of ${\mathrm{C}}_{\mathit{x}}$F fibers show wavy fringes, and the waviness is identified with semi-ionic (covalent) bonding. Evidence for unintercalated graphite regions is also presented. As the fluorine concentration of the ${\mathrm{C}}_{\mathit{x}}$F fibers increases to high values, the amount of waviness increases.

The effects of the preparation conditions on the electrochemical properties of graphiteCrO 3 intercalation compounds in acid electrolyte—II. The influence of the graphite flake size and the intercalation time

The slow potentiodynamic method and galvanostatic measurements in 0.25 M H 2SO 4 have appeared to be very useful tools to determine the quantity of the lower chromium oxides built into graphiteCrO 3 compounds (GICCrO 3's). From the charge and discharge curves it has been possible to calculate the oxidation state of chromium in these oxides. The results obtained showed that the amount of lower oxides and the oxidation state of chromium depend both on the graphite flake size and the intercalation time. The concentration of intercalant in GIC-CrO 3's decreases on prolonging the intercalation process as a result of the increase of the amount of lower oxides and the decrease of the oxidation state of chromium. The relations between the parameters determined chemically and electrochemically are considered to explain the mechanism of intercalation.

Resistivity studies and phase transitions in HOPG/IrF 6

The in-plane resistivity ▪ a, and the c-axis resistivity, ▪-1 c, of HOPG/IrF 6 were measured as a function of temperature and intercalant concentration. ▪ a could be fitted to the equation ▪ a = A+BT+CT 2 but exhibits a shallow minimum for some of the stage I GIC. ▪ c is metallic and d ▪ c/dT exhibits nonmonotonic behaviour suggesting a first order phase transition at T c = 240K for C 10IrF 6 (st. I). Neutron scattering measurements suggest a liquid to incommensurate three dimensional solid phase transition.

Graphite compound with BF 3/F 2: Conductivity and its relation to charge transfer

In-plane and c-axis resistivities of HOPG/BF 3/F 2 were studied as a function of temperature and intercalant concentration. The charge transfer was estimated from reflectivity measurements. The in-plane residual resistivity, the charge transfer and the repeat distances allow estimation of the mean free path (domain size). Various models for the c-axis resistivity are discussed.

Angle resolved photoemission study of first and second stages alkali metal graphite intercalation compounds

We present Angle Resolved Photoemission measurements for Cs and K compounds peeled at 150K under ultrahigh vacuum. The structure and the composition of (at least) the uppermost intercalated layer in alkali metals GIC, differs strongly from the bulk structure and composition. The observed bands are in excellent agreement with σ and π bands expected from recent theoretical calculations, except for (i) the zone folding and (ii) the shape of the bands within 2 eV from the Fermi level. Near the Fermi level, a high intensity band is observed with a low dispersion (≈0:5 eV) throughout the whole Brillouin zone. We show that a first intercalated layer with a high intercalant concentration, exhibiting an incommensurate structure, accounts very well for all observed features. This structure gives rise to the band observed near the Fermi level, which is most probably a charge density wave.

Surface segregation in donor and acceptor graphite intercalation compounds

Among the different spectroscopies applied to the study of graphite intercalation compounds (GIC), Auger electron spectroscopy (AES), and angle resolved photoemission spectroscopy show that the composition of the uppermost unit cell is different from the bulk composition. According to these measurements, donor compounds exhibit an increased intercalant concentration near the surface, while acceptor compounds at equilibrium exhibit two or three nearly pure graphite layers at the surface. We present here AES measurements for donor GIC (cesium compounds), and x-ray photoelectron spectroscopy and AES measurements for acceptor GIC (manganese chloride compounds). A simple model accounts for the observed surface concentration in both donor and acceptor compounds. The screening of a surface state involves an electrostatic energy, which depends on the Fermi level density of states in the most superficial unit cell or sandwich. The minimum of the free energy at equilibrium implies enrichment in cations and a depletion of anions near the surface of the compound.

Solid-state properties of some polycrystalline alkali-metal tungsten bronzes

The structures, electrical conductivities, oxygen deficiencies and surface activities of polycrystalline Li, Na and K bronzes of tungsten with varying alkali-metal contents have been considered in detail for the first time. There is clear evidence that as the temperature is raised so ionic mobility makes a significant contribution to the total electrical conduc­tivity. The processes involved in ionic mobility may be diffusion of O 2− lattice anions, the intercalate alkali-metal ion M + , and a lower oxidation-state lattice cation W y + ( y < 6). All bronzes studied are shown to be oxygen deficient (to an extent that increases with the concentration of the alkali-metal intercalate) and, because their ionic conductivity is similar to that of lower oxides, O 2− migration may be more significant than previously recognized.

Surface composition of graphite intercalation compounds

We present Auger spectroscopy measurements on graphite intercalation compounds, for donor (cesium) and for acceptor (MnCl 2) intercalants. The surface composition of both compounds is steeply different from their bulk composition. In the case of high stage cesium compounds, a segregation of the intercalant towards the surface layers is observed, leading to an increased concentration between the first two graphite layers (CsC 8 or CsC 6). On the opposite, an intercalant depletion between these first layers is observed for the first stage (MnCl 2)C 6 compounds. Both effects are explained by the screening of surface states, which involves an electrostatic energy of the same order of magnitude as the intercalant chemical potential. This is thus sufficient to allow the local intercalant concentration to change.

A new approach utilizing high‐resolution proton NMR in structural analysis of intercalation complexes of natural DNA

AbstractProtons on the antimalarial intercalator quinacrine and a substituted phenanthrene intercalator were monitored on titration with sonicated DNA. The titrations were carried out below the DNA Tm but at a temperature, ionic strength, and intercalator concentration sufficient to give fast exchange conditions and a minimum of dimerization of the planar aromatic intercalators. Line widths were kept within reasonable limits by using sonicated DNA, high temperature, fast exchange conditions and an excess of intercalator over DNA binding sites.Proton assignments were made using analysis of chemical shifts, coupling patterns, NOE and 2D methods. 2D COSY experiments on DNA‐intercalator complexes were performed to assist in assigning protons for the bound intercalator. Line broadening with quinacrine is more severe than with the phenanthrene, and the acridine essentially represents the limit of the NMR titration method. The phenanthrene intercalator, on the other hand, illustrates the power of the method, particularly when analyzing a series of compounds. Using chemical shift changes of the phenanthrene on titration with DNA and space filling molecular modeling studies, we have been able to propose a general structure for the phenanthrene intercalation complex.

Preparation and Electrical Resistivity of Intercalation Compounds of Graphitized Cokes with Antimony Halides

AbstractPreparation of intercalation compounds of a graphitized electrode coke and a synthetic graphite with SbCl5, SbCl4F, SbCl2F3 and SbF6 have been described and stages have been characterized by X‐ray diffraction technique. On applying pressure a de‐intercalation of antimon pentahalides have been found. The stability of intercalation compounds of graphitized cokes to pressure, temperature, organic solvents and aqueous HCI is lower than that of natural graphite compounds. Electrical resistivity of compacted intercalation compounds of different stages parallel and perpendicular to pressure direction decreases with increasing pressure, intercalant concentration and particle size. Intercalation increases anisotropy of electrical conductivity in graphitized coke and decreases it in synthetic graphite.

New theory for magnetic graphite intercalation compounds: Superferromagnetism in two dimensions

We calculate the high-field and low-field in-plane magnetic susceptibilities of the transition metal chloride graphite intercalation compounds (GIC) whose spins are confined to the plane. We show that the two observed low temperature susceptibility peaks can be due to a single phase transition which occurs near the higher temperature peak. The model assumes superferromagnetism, intra-plane and inter-plane super-moment interactions, and plane to plane variations of the intercalant concentration. The parameters which we introduce are estimated for several compounds. The resultant susceptibility curves have the observed temperature and field dependences and many other formerly peculiar aspects of the magnetism are explained. Notibly, the exceptionally large magnitude of the measured susceptibility, the non-zero sample magnitization between the two susceptibility peaks, and the stage dependence of the susceptibility curves.

On the fidelity of DNA replication. Specificity of nucleotide substitution by intercalating agents.

The effects of intercalating agents on the fidelity of DNA synthesis in vitro have been investigated. The accuracy of DNA synthesis with Escherichia coli DNA polymerase I with both the poly[d(A-T)] and poly[d(G-C)] templates is decreased in the presence of the intercalating agents proflavin, ethidium bromide, acridine orange, ICR-170, and ICR-191. Nearest neighbor analyses of the product of the reaction indicate that two different types of misincorporations occur in the presence of intercalating agents, frameshifts, and single-base substitutions. With alternating polynucleotide templates, frameshifts involving pyrimidines are the most frequent change in sequence observed. Overall, frameshift misincorporations occur with frequencies of one complementary pyrimidine for each intercalated site and one noncomplementary pyrimidine for each 150 sites. From analysis of nearest neighbor frequencies in the product, it is inferred that the intercalating agents interact specifically with pyrimidine (3' leads to 5') purine sequences. An analysis of ratios of correct nucleotide incorporations as a function of intercalator concentration indicates that frameshifts are predominantly additions; however, one cannot rule out infrequent deletions. Base substitutions in the presence of intercalators occur less frequently than frameshifts. From the results of reaction kinetics and nearest neighbor frequencies, it is concluded that the noncomplementary nucleotides are incorporated in phosphodiester linkage and are present as single-base substitutions. Taken together, the results of these studies suggest at least two different modes of action for intercalating agents on the accuracy of DNA synthesis: one leading to frameshift misincorporations and the other leading to single-base substitutions.

Structure and electrical properties of potassium-graphite-dimethylsulfoxide ternary intercalation compound

The chemical composition, the stage number, and the identity period, I c, are determined for ternary intercalation compounds prepared by the cathodic reduction of graphite in solution of KI in dimethylsulfoxide (DMSO) and their electrical properties are investigated. Gravimetry reveals that the chemical composition of the compounds is expressed as KC n (DMSO) 3 with n ≥ 24. Stepwise changes in the potential of graphite electrode on reduction are found to correspond to the formation of stage 2 compound KC 24(DMSO) 3 with I c of 18.4 Å and stage 4 compound KC 48(DMSO) 3 with I c of 25.1 Å. The spacing between adjacent carbon planes containing intercalate layer is 15.0 Å. The electrical conductivity along the layer plane exhibits a significant increase followed by saturation with intercalate concentration. Compared with the corresponding binary compound KC n , the role of the organic molecule in the ternary compound is found to enhance the conductivity at the initial intercalation.

Thermal gravimetric analysis of graphite-bromine compounds

Thermal gravimetric analysis was applied for the first time to the study of graphite intercalation compounds. The gravimetric measurements were carried out on graphite-bromine compounds in the temperature range from room temperature to 140 °C. Intercalate desorption was found to be thermally activated. Its rate is anomalously high near the intralayer intercalate position order-disorder transformation temperature (100 °C) and the exfoliation temperature. This rate was also found to increase with increasing intercalate concentration. The mechanism of desorption depends on the intercalate concentration. At high intercalate concentrations, desorption appears to be dominated by a diffusion process and has an activation energy of 14 kcal (mol Br 2) −1, in good agreement with the activation energy for the diffusion of bromine in graphite. As desorption ends, the intercalate concentration approaches a minimum value which depends on the desorption temperature. Much of the retained intercalate is ordered. Below the intralayer order-disorder transformation temperature an initially saturated graphite-bromine sample has a minimum concentration of 18 – 19 wt.% Br 2. Above the order-disorder transformation temperature the minimum concentration is smaller. The energy of the desorption process is utilized mainly for the vaporization of the intercalate, as suggested by our differential scanning calorimetry results.

Graphite intercalation compounds: A simple model of Fermi surface and transport properties

A parametrized model of the $\ensuremath{\pi}$ bands of $n$-layer films of graphite is developed for the purpose of studying the Fermi-level properties of graphite intercalation compounds. Specifically, the charge transfer, the density of states, and the conductivity tensor as a function of magnetic field are considered. The first two quantities are proportional to the de Haas-van Alphen frequency and cyclotron mass, respectively. A numerical study is carried out for a single layer of graphite as a function of excess charge. It is shown that the anisotropic dispersion of the energy bands causes unusual galvanomagnetic behavior at small magnetic fields, such as a nonzero magnetoresistance mobility for moderate intercalant concentrations and also a change in the sign of the low-field Hall coefficient at high intercalant concentrations.

Infrared active lattice modes in graphite alkali metal compounds

The first observation of IR-active optic modes in donor graphite intercalation compounds is reported. For stage 1 and 2 compounds with K, Rb and Cs no graphitic optic mode in the 1600 cm -1 region is observed, while more dilute compounds exhibit strong IR-active modes with a stage-dependent mode frequency that is downshifted with increasing donor intercalate concentration. This mode frequency shift is opposite in sign and much larger in magnitude than for acceptor compounds. Analysis of the lattice mode effective charge indicates a significant amount of dynamic charge residing in the interior graphitic layers.

Electronic properties of graphite intercalation compounds

New preparation techniques for Li- and the high stages of the heavy Alkali Graphite Intercalation Compounds (AGIC's) were developed. A new electrochromic effect based on electrochemical intercalation of graphite electrodes was discovered. Electron spectroscopy studies such as XPS, UPS and AES on AGIC's revealed a drastic increase of the density of states and alkali-like s-type conduction bands at the Fermi energy. The shift at EF appears to be almost independent of the intercalant concentration. From optical reflectivity experiments we observe interband transitions to disappear with the ordering of the intercalant layer. The resistivity and magnetic susceptibility measurements indicate no significant difference between a dilute and a concentrated regime.

Calorimetric study of the order-disorder transformations in graphite-halogens

The first determination of the enthalpy and entrophy of transformation for the order-disorder transformations in graphite intercalation compounds is reported here. By using differential scanning calorimetry, the order-disorder transformation associated with the intralayer intercalate position ordering has been observed at 375 ± 2 K in graphite-Br 2 and at 314 ± 2 K in graphite-ICl. The transformation temperatures show a strong dependence on the intercalate species but relatively little dependence on the intercalate concentration. The enthalpies of transformation are 108 ± 15 cal mol −1 bromine and 460 ± 50 cal mol −1 ICl for graphite-Br 2 and graphite-ICl respectively. Moreover, for graphite-Br 2 an endothermic peak at 277 ± 2 K has tentatively been identified with the order-disorder transformation associated with the intralayer intercalate molecular orientation ordering. The observed order-disorder transformations have all been found to be first-order transformations.

Elektrochemische Intercalation und elektrische Leitfähigkeit von Graphitfasern / Electrochemical Intercalation and Electrical Conductivity of Graphite Fibers

Abstract Lamellar compounds of graphite fibers have been prepared by electrochemical intercalation. The dependence of the electrical resistance on the intercalate concentration was determined by a quasi simultaneous method. A factor 30 decrease of the relative fiber resistance was obtained with fluorosulfuric acid.

The electronic properties of lamellar compounds of graphite — An introduction

A brief review is given of the electronic properties of the lamellar compounds of graphite, with particular emphasis on the dilute limit. Experimental methods of determining these properties are discussed and the applicability of a rigid band model with variable electrochemical potential considered. The role of the micro- and macro-scopic structure in determining properties is stressed. In this context, the change of conductivity with concentration of intercalant is used as an example of a property whose analysis depends sensitively on structure. Finally, a brief discussion is given of charge density waves, and a tentative mode for the intercalation process is outlined.