Graphite and Related Materials

Graphite may be the thermodynamically most stable modification of carbon. Still it is chemically attacked more easily than diamond due to its layered structure and the comparatively weak interaction between the graphene sheets. Altogether the graphite's reactivity toward many chemicals is rather low nevertheless. With chlorine, for example, it does not react at all under usual conditions, and even with fluorine reaction occurs only at more than ~400 °C. Suitable performance yields the transparent, colorless carbon monofluoride CF (up to CF112 due to additional fluorine atoms at marginal positions and defects), a chemically very resistant

Figure 1.21 (a) Structure of the compound (CF)X resembles an undulated graphene sheet or the structure observed for Lonsdaleite, (b) mellitic acid.

insulator that is employed as dry lubricant. The individual layers of the graphitic lattice are conserved in (CF)* (Figure 1.21a), with the fluorine atoms alternatingly pointing upward and downward. This causes an interlayer distance of 6 A, compared to 3.35A in graphite. On nonstoichiometrical fluorination (CF08-09), the obtained graphite fluoride retains its electrical conductivity. Fluorination at temperatures >700 °C converts graphite into CF4, whereas concentrated nitric acid turns it into mellitic acid C6(CO2H)6 that contains planar C12-units (Figure 1.21b). In the mixtures of HNO3 /H2SO4, graphite reacts with potassium perchlorate to give the so - called graphitic acid, named such for the existence of weakly acidic OH-groups. This compound is obtained as greenish-yellow, lamellar crystals that explode on heating. Aqueous bases do not perceptibly attack graphite, and the reaction with hydrogen succeeds only at high temperatures, for example, in an arc, mainly to yield acetylene. Graphite is not soluble in any of the common solvents; only molten iron does solve significant amounts.

A particularity within the chemical behavior of graphite is the formation of the so-called intercalation compounds. These result from embedding atoms or molecules in between single graphene sheets ("sandwich" structure). Their weak interaction enables the formation of this kind of compounds. There is a rearrangement of charges upon intercalation so the carbon layers are polarized (or even ionized) either negatively or positively, depending on the kind of intercalate (embedded atom or molecule). The exchange of electrons between the intercalate and the graphite's n-bands increases the number of charge carriers in the conduction or the valence band, respectively. Electron-withdrawing species cause holes in the valence band of the graphite, whereas electron donors partly fill the conduction band with electrons. Therefore graphite intercalation compounds exhibit an increased electrical conductivity.

The longest known among these compounds are the CnK phases (n = 8, 24, 36, 48), with the first of them, C8K, having been made in 1926 already. Depending on the stoichiometry, the potassium atoms are separated by one to four graphene layers (Figure 1.22) in these compounds, which are referred to as steps, The first of these steps deviates from the C- 2nK-rule (n = 2, 3, 4) that holds for higher homologs - it represents the compound with the highest possible number of potassium atoms incorporated. It is noteworthy that the distance between graph-ene layers increases only by 2.05 A, which after all is much less than could be

Potassium Graphite Structure Distance

Figure 1.22 Intercalation compounds of potassium with different contents of metal are highly ordered structures. (a) Stacking of graphene layers and metal atoms (side view), (b) top view of the metal atoms arranged in the graphene lattice. The dark circles indicate positions occupied in all cases, while the position marked by the light circle is only occupied in C8K.

Figure 1.22 Intercalation compounds of potassium with different contents of metal are highly ordered structures. (a) Stacking of graphene layers and metal atoms (side view), (b) top view of the metal atoms arranged in the graphene lattice. The dark circles indicate positions occupied in all cases, while the position marked by the light circle is only occupied in C8K.

expected upon the intercalation of metal atoms measuring 3.04 A. This indicates an interaction of the potassium atoms with the clouds of n-electrons of the surrounding graphene sheets.

Actually, K+ ions are formed-the released electron is transferred into the graphite ' s conduction band, causing the high electrical conductivity. C8K is a versatile reducing agent in organic synthesis. Besides potassium, other alkaline or alkaline earth metals may form ionic intercalation compounds as well. Their composition is C„M (e.g., M = Rb, Cs, Ca with n = 8, 24, 36, 48orM = Li, Ca, Sr, Ba with n = 6) with the graphite being reduced. Sodium-graphite compounds, on the other hand, are hard to obtain. The golden-colored C6Li is employed in efficient batteries.

As the valence and the conduction band exhibit similar energies, the graphite in intercalation compounds may also be oxidized when electron acceptors are embedded. The intercalation compounds with bromine or certain interhalides are important examples here: iodine and chlorine do not react with graphite, whereas with fluorine, the aforementioned carbon fluorides are obtained. In C8Br, then, electrons from the valence band of graphite are transferred to the bromine. However, it is not present as isolated anions, but in the shape of polybromide ions. The Br-Br distance of 254 pm correlates quite well with that between the centers of the hexagons in the graphite lattice (256 pm). For other polyhaloge-nide anions, on the other hand, the interatomic distance (Cl-Cl: 224 pm and I-I: 292 pm) is too low or too high, respectively, to perfectly fit in these voids, and consequently no interstitial compounds are formed. Iodine monochloride (expected I-Cl distance: 255 pm) then again is incorporated. Halogen intercalation com pounds are also good electric conductors. However, the charge transport occurs via holes (defect electrons) instead of electrons, similar to the mechanism in p-doped semiconductors.

Apart from graphite halogenides, there are also intercalation compounds with sulfuric, nitric, phosphoric, perchloric, and trifluoroacetic acid. They all exhibit a considerably larger distance between their graphene layers, and the graphite donates electrons to the respective intercalate. With sulfuric acid, for example, graphite hydrogensulfate is obtained, and the reaction with nitric acid yields graphite nitrate. Also metal salts and oxides like FeCl3, AlCl3, SbF5, CrO3, CuS, etc. may be embedded between the graphene sheets. The chromium oxide compound CrO3-graphite is a selective oxidant to convert secondary and tertiary alcohols into the related keto-compounds (Lalancette reagent), whereas AlCl3-graphite is employed as a selective Friedel-Crafts catalyst.

0 0

Post a comment