Surface Properties and Gas Adsorption

It is well known that alkali metal binary GICs adsorb gaseous species (H2, N2, Ar, CH4, etc.) physisorptively at low temperatures, where physisorbed gaseous molecules are accommodated in the interstitials of the alkali metal lattice within the graphitic galleries (Lagrange and Hérold, 1975; Lagrange et al., 1972, 1976; Watanabe et al., 1971, 1972, 1973). The capacity for hydrogen adsorption, which is estimated at 144 cm3/g in KC24, for example, is large and comparable to the capacity in other adsorbers such as zeolite or activated charcoal. Interestingly, the physisorption phenomenon in alkali metal GICs has different features from that in conventional adsorbents such as zeolite or activated charcoal; that is, guest molecules in alkali metal GICs are not simply bonded to the adsorbents through weak van der Waals forces without any change in the electronic structures. Here we discuss the gas physisorption phenomenon in alkali metal GICs from general aspects, in relation to their specific features. Then in subsequent sections, we will give details of actual cases. Hydrogen is a typical gaseous molecule adsorbed in alkali metal GICs. Hydrogen physisorption takes place at low temperatures below about 200 K, where hydrogen molecules are accommodated in the graphitic galleries and are not dissociated into atomic hydrogen species. When the temperature is increased to over 200 K, the alkali metal GICs work as catalysts to hydrogen, resulting in the occurrence of hydrogen chemisorption. Hydrogen physisorption will be discussed in Section 8.1.2, hydrogen chemisorption and related issues have been discussed partly in Sections 2.2.1 and 5.4.1 from the viewpoints of structure and electronic properties, and will be discussed again in Section 8.1.2. Figure 8.1 represents the composition dependence of the amount of physisorption of hydrogen molecules in KCm at 77 K (Lagrange and Hérold, 1975). The composition of 1/m = 1/8 corresponds to the stage-1 compound and the composition 1/m = 1/24 to the stage-2 compound; intermediate compositions between 1/8 and 1/24 are considered to have a mixed structure of stage-1 and stage-2 compounds. The stage-1 compound does not adsorb hydrogen at all.

Magnetic Properties

Two-dimensional (2D) magnetic phase transition has been one of the major topics of condensed matter physics. There are many materials in which the magnetic ions are arranged in planes so that there is strong coupling between the spins within a plane, but only a weak coupling between spins in different planes. In materials such as transition metal chlorides (FeCl2, CoCl2, and NiCl2) there is a strong ferromagnetic coupling within the planes and a coupling between planes which is weaker by a factor of 10 or so and antiferromagnetic. In more complicated ferromagnetic layer compounds such as (CH3 NH3)2 CuCl4, the interlayer coupling is down by a factor of more than a thousand. There are also many materials, such as Rb2MnF4, Rb2CoF4, and K2CoF4, in which the coupling within the plane is antiferromagnetic. This can result in a very much reduced coupling between the planes, since one spin may have four spins in the next plane which are at an equal distance, and their effects cancel out. As a result, the coupling between layers in this material is down by a factor of 106. In these layered compounds the magnetic behavior is only 2D-like at a certain distance away from the critical temperature. Close to the critical temperature, there is a long-range correlation of the spins within the layer and such a correlated region will interact between one layer and the next even if the coupling between individual spins is weak, since many spins can contribute coherently. Thus the spin order near the critical temperature is essentially three-dimensional (3D). What condition should be required for the occurrence of a real 2D spin order in layered magnetic systems? We consider a system consisting of only two magnetic layers separated by a distance. The effective interplanar exchange interaction J'eff is given by J'(ξa)2, but not by J', where ξa is the in-plane spin correlation length and J' is the interplanar exchange interaction. If ξa diverges on approaching a critical temperature, the effective interplanar exchange interaction J'eff becomes comparable with the intraplanar exchange interaction J.

Introduction

There are two important features in the structure and electronic properties of graphite: a two-dimensional (2D) layered structure and an amphoteric feature (Kelly, 1981). The basic unit of graphite, called graphene is an extreme state of condensed aromatic hydrocarbons with an infinite in-plane dimension, in which an infinite number of benzene hexagon rings are condensed to form a rigid planar sheet, as shown in Figure 1.1. In a graphene sheet, π-electrons form a 2D extended electronic structure. The top of the HOMO (highest occupied molecular orbital) level featured by the bonding π-band touches the bottom of the LUMO (lowest unoccupied molecular orbital) level featured by the π*-antibonding band at the Fermi energy EF, the zero-gap semiconductor state being stabilized as shown in Figure 1.2a. The AB stacking of graphene sheets gives graphite, as shown in Figure 1.3, in which the weak inter-sheet interaction modifies the electronic structure into a semimetallic one having a quasi-2D nature, as shown in Figure 1.2b. Graphite thus features a 2D system from both structural and electronic aspects. The amphoteric feature is characterized by the fact that graphite works not only as an oxidizer but also as a reducer in chemical reactions. This characteristic stems from the zero-gap-semiconductor-type or semimetallic electronic structure, in which the ionization potential and the electron affinity have the same value of 4.6 eV (Kelly, 1981). Here, the ionization potential is defined as the energy required when we take one electron from the top of the bonding π-band to the vacuum level, while the electron affinity is defined as the energy produced by taking an electron from the vacuum level to the bottom of the anti-bonding π*-band. The amphoteric character gives graphite (or graphene) a unique property in the charge transfer reaction with a variety of materials: namely, not only an electron donor but also an electron acceptor gives charge transfer complexes with graphite, as shown in the following reactions: . . .xC + D → D+ C+x. . . . . .(1.1). . . . . .xC + A → C+x A−. . . . . .(1.2). . . where C, D, and A are graphite, donor, and acceptor, respectively.

Lattice Dynamics

Pristine graphite crystallizes according to the D46h space group. There are twelve modes of vibration associated with the three degrees of freedom of the four atoms in the primitive cell. The hexagonal Brillouin zone and the phonon dispersion curves of pristine graphite, calculated by Maeda et al. (1979), are shown in Figure 4.1. The zone-center (Γ point) modes are labeled as three acoustic modes (A2u + Elu), three infrared active modes (A2u + Elu), four Raman active modes (2E2g), and two silent modes (2Blg). The first calculation of phonon dispersion for the stage-1 compounds KC8 and RbC8 was presented by Horie et al. (1980) on the basis of the model of Maeda et al. (1979) for the lattice dynamics of pristine graphite. Although the calculated phonon energies do not agree well with the experimental data, the model has most of the ingredients for describing the lattice dynamics of stage-1 GICs. A simple review of their work is presented as follows. The primitive cell of KC8, having a p(2 × 2)R0° superlattice, contains 16 carbon atoms and two K atoms. Note that only an αβ stacking sequence is assumed here (see Section 3.6.1). The primitive translation vectors are given by t1 (0, a, 0), t2 = (−√3a/2, a/2, 0), and t3 = (−√3a/4, −a/4, c), where a = 2aG = 4.91 Å and c = 5.35 × 2 = 10.70 Å. The corresponding Brillouin zone is shown in Figure 4.2b. The phonon dispersion for KC8 has been calculated by Horie et al. (1980) on the basis of the Born-von Karman force constant model. This dispersion curve is compared with that of pristine graphite by folding the dispersion curves of graphite into the first Brillouin zone of KC8. Since the side of the Brillouin zone in KC8 is not flat in two directions, as shown in Figure 4.2b, it is a little difficult to transfer the information on the dispersion curves in the first Brillouin zone of graphite into the Brillouin zone of KC8. For simplicity, nevertheless, we assume that the side of the Brillouin zone in KC8 is flat like that of graphite.

Highly Conductive Graphite Fibers

From the viewpoint of applications of GICs, a very interesting development is the enhancement in conductivity of the host graphite up to the range of metals, especially for pristine materials with fibrous forms. Much attention has been paid to the exploitation of the order of magnitude intercalation-induced enhancement of the electrical conductivity of graphite fibers for the fabrication of practical high-conductivity, lightweight conductors (Vogel et al., 1977; Goldberg and Kalnim, 1981; Manini et al., 1983, 1985; Murday et al., 1984; Meschi et al., 1986; Natarajan and Woollam, 1983; Natarajan et al., 1983a). The fiber geometry (large aspect length/diameter) ratio offers advantages relative to highly oriented pyrolytic graphite (HOPG) or bulk graphite for the measurement of several of the transport properties of GICs and for increasing the compositional stability of GICs both under ambient conditions and at elevated temperatures (Endo et al., 1981, 1983a). For bulk GICs, intercalation increases the density of carriers by the injection of electrons into the graphite planes in the case of donor guest species, and by injection of holes in the case of acceptor type (see Chapters 5 and 6). The intercalation-induced decrease in carrier mobility that results from the increased scattering by defects and the increased effective mass is outweighed by the larger increase in carrier density, resulting in a large conductivity enhancement as discussed in Section 6.1. The carriers are localized in the graphene planes, and for high-stage compounds (n ≥ 2) the carrier density falls off rapidly with distance from the graphite bounding layer owing to the screening of the charged intercalate layer by the surrounding graphite bounding layers. From an application standpoint, many of the applications of intercalated carbon fibers exploit the high specific conductivity of GICs, which can be expressed as a figure of merit in terms of the conductivity σ divided by the mass density ρm; for a good conductor like copper this is ~ 6 x 10−2 cm /gμΩ. For example, intercalated carbon fibers can provide a lightweight conductor for huge aircraft or motor vehicles, in which, respectively, about 1.5 tonne or 30 kg conventional metallic conductor is used.

Electron Transport Properties

In GICs, charge transfer between graphite and intercalate produces a large concentration of charge carriers, featuring an electron or hole nature in donor or acceptor GICs, respectively, as discussed in Chapter 5. GICs are therefore metallic, in contrast with the semi-metallic properties of host graphite. The typical inplane conductivity values for GICs are in the range of ~ 105 Ω−1 cm−1, which is one order of magnitude larger than the in-plane conductivity of pristine graphite (Delhaes, 1977). It is well known that the conductivity of some GICs, such as AsF5, exceeds that of copper, suggesting the properties of synthetic metals (Vogel et al., 1977). As discussed in Chapter 5, GICs have two-dimensional (2D) features in the electronic properties inherent to their stacking structure, so that electron transport is considerably anisotropic between in-plane and interplane electron conduction processes. In the in-plane process, conduction electrons, whose concentration is estimated from eq (5.9), contribute to the coherent electron conduction, and the electrical conductivity σa or resistivity ρa is described as follows (Drude formula): . . .σa =1/ρa = Neμ= Ne2τ/(m*). . . . . .(6.1). . . where N, μ, τ, and m* are the density, mobility, relaxation time, and effective mass of the conduction carriers (electrons or holes), respectively.

Electronic Structures

In the structure of graphite, graphene hexagon sheets are stacked in an AB stacking mode as shown in Figure 1.3. Such as mode is characterized by strong in-plane C-C bonds and weak interlayer interaction. This structural feature imparts two-dimensionality to its electronic structure. In the in-plane hexagon carbon network, the combination of sp2 σ- and π-bonds is the origin of the strong intralayer interaction, while the overlap of π-bonds between adjacent graphene sheets contributes to the weak interlayer interaction. In discussing the electronic properties around the Fermi energy, which is particularly important for the electronic structure of GICs, the π-electron orbitals play an essential role, giving graphite its unique properties. On the other hand, σ-bands, which have larger energy than π-bands, are located far from the Fermi energy. Thus they do not contribute to any serious change in the electronic properties when intercalates are introduced in the graphitic galleries. Here, we start with the electronic structure of graphite on the basis of the tight binding model. The discussion is first devoted to a graphene sheet, which is considered to be an infinite 2D hexagonal conjugated π-electron system (Wallace, 1947). Figure 5.la presents a unit cell of graphene sheet comprising two kinds of carbon atoms, α and β, where τ2 is the vector connecting the two atoms. The corresponding Brillouin zone in the reciprocal lattice is shown in Figure 5.1b.

Structures and Phase Transitions

Graphite intercalation compounds (GICs) have unique layered structures where intercalate layers are arranged periodically between graphite layers. This phenomenon is known as staging, and the number of graphite layers between adjacent intercalate layers is known as the stage number n. As the stage number n increases, the separation between adjacent intercalate layers becomes larger. So the interlayer interactions between intercalate layers become weak, leading to a crossover of dimensionality from three-dimensional (3D) to two-dimensional (2D). Because of their intrinsic anisotropy, GICs exhibit a great variety of structural orderings such as staging, in-plane ordering of intercalate layers, and stacking ordering of both graphite and intercalate layers. The stable stage and in-plane ordering of the intercalate layers depend on the relative strength of the intercalate-graphite interaction to the intercalate-intercalate interaction. At low temperatures the intercalate layers form a variety of 2D superlattices resulting from the competing interactions. At elevated temperatures the superlattice undergoes a transition to a 2D liquid. The migration of intercalate atoms is restricted to the 2D gallery between graphite layers. The critical temperature below which the stacking order appears is equal to or lower than the critical temperature below which the in-plane order appears. In this chapter, we review the subject of structures, phase transitions, and kinetics for donor and acceptor GICs. The subject matter of this chapter is organized as follows. Section 3.1 deals with the general structural characteristics of GICs. Sections 3.2 and 3.3 are respectively devoted to descriptions of liquid state and phase transitions of stage-2 alkali metal GICs. Section 3.4 deals with the discommensuration domain model for high-stage alkali metal GICs. Sections 3.5 and 3.6 are devoted to descriptions of liquid-solid transitions in stage-1 K and Rb GIC. In Sections 3.7-3.9, we describe the stage transition, Kirczenow’s model, Hendricks-Teller-type stage disorder, and fractional stage. In Sections 3.10 and 3.11 we describe the phase transitions of acceptor-type GICs (Br2 GIC and SbCl5 GIC). Section 3.12 treats the ordering kinetics in K GIC and SbCl5 GIC.

Intercalated Fullerenes and Carbon Nanotubes

Since the discovery of soccer-ball-shaped C60 (Kroto et al., 1985), fullerenes have been added to the family of allotropes of carbon element. During the fullerene formation by arc-discharge of graphite electrodes, carbon nanotubes were simultaneously grown as a deposit on the electrode (Iijima, 1991). The carbon nanotubes consist of single or multiple graphene sheets rolled in the form of a seamless cylinder, with the diameter of the hollow core being almost 10 Å (similar to that of fullerenes) or even as small as 4 Å. For these new forms of nanometer-sized carbon, so-called nanocarbons, basically similar concepts as GICs have been applied from the aspects of structures, electronic properties, and functionalities that can be controlled by doping or intercalation process. That is, the bonding force between nanoballs or nanotubes is governed by weak van der Waals forces, so that foreign species such as atoms or molecules can be intercalated (or doped) in the van der Waals gaps, similar to graphite. So, from applications and the basic science of these new carbon families, intercalation as well as doping to these hosts has been studied intensively in the last ten years. There are three kinds of doping reactions of guest species into these host materials, which are reflected in their specific structure. Guest atoms can be introduced by substituting the carbon atoms of the hosts. This process is generally called “doping,” as there is a similarity with the doping process in semiconductors, where, in general, there is no long-range periodicity in the guest arrangement against the host crystal. Guests can locate in the hollow cores of fullerenes or carbon nanotubes as well as on their outer surfaces. GICs establish the super-lattice structure between the host of the graphite lattice and the inserted guest species, where the long-range periodicity along the c-axis as well as on the a-b plane is formed. According to the original meaning of “intercalation,” periodic doping to the host materials is defined as intercalation. So, the three kinds of doping are: (1) endohedral doping into the hollow cage; (2) substitutional doping by replacing the carbon atoms on the cages; and (3) exohedral doping where the dopants are sited in the gaps between the cage molecules of a fullerene crystal or between carbon nanotubes in the array of the bundle form.

Exfoliated Graphite Formed by Intercalation

When a graphite intercalation compound (GIC) is heated, a thermal expansion, large (~3.5 × 10−5/K) compared with most materials, occurs along the c-axis, while the in-plane lattice constant remains almost unchanged. This anisotropic thermal expansion behavior is reversible and can be modeled in terms of a superposition of the c-axis thermal expansion coefficients of the constituent layers (Salamanca-Riba and Dresselhaus, 1986). However, when a GIC is heated above a specific critical temperature, a gigantic c-axis expansion can occur, with sample elongations (ΔLS/LS) of a factor of 300 (Inagaki et al., 1983). This extremely large elongation, also called “exfoliation,” is generally irreversible, and leads to a spongy, foamy, low-density, high-surface-area carbon material of about 85 m2/g (Thomy and Duval, 1969). This exfoliation effect alters the structural integrity of the GIC material and therefore is undesirable for structural applications of GICs. However, this spongy, foam-like material is advantageous for some applications, such as gas adsorption. The commercial version of the exfoliated wormy-like material is called vermicular graphite from its appearance, and is used for high-surface-area applications. Furthermore, when pressed into sheets, it is called grafoil and is widely used as a high-temperature gasket or packing material because of its flexibility, chemical inertness, low transverse thermal conductivity, and ability to withstand high temperatures. Grafoil-type products can also be used to contain molten corrosive liquid metals at high temperatures and to extinguish metal fires (Anderson and Chung, 1984). These products have been expected to be useful for oil adsorption (Toyoda et al., 1998a, 1998b, 1999). In this chapter, the preparation and conventional as well as advanced applications of exfoliated graphite with unique properties, obtained from QIC-based materials, is demonstrated.