Physical chemistry and mechanism of intercalation in graphite

Abstract

If one exposes a 10 mm-thick piece of pyrolytic graphite to Br 2 and follows the thickness changes of five marked sections in the 10 mm, it is apparent that intercalation starts in the interlayer regions next to the two basal planes and not throughout the 10 mm. In fact, if one caps the ends to prevent this end reaction, then no expansion occurs in any region. Furthermore, the threshold pressure required to initiate the end reaction increases with the total thickness of the slab of graphite. A similar behavior has been observed with four other reactants in flakes of natural graphite whose dimensions are about 2 mm in a direction parallel to the carbon layers. They are (1) CrO 2Cl 2, which will intercalate thin flakes but will cause no weight increase or expansion in flakes thicker than about 100 μm. (2) CrO 3 at 220 °C which gives a higher content of intercalated material in thin- than in thick flakes. (3) MoCl 5 vapor at 300 °C which gives a higher content and gives it more rapidly in thin- than in thick flakes. (4) FeCl 3 in nitromethane which, as a 3 M solution, with a 20 μm thick flake gives an expansion of 150%. With a 90 μm flake the expansion is 60% on initial exposure, but after exposure to pure solvent, with some accompanying shrinkage, the expansion is a total of 80% during a second exposure to FeCl 3 solution. Repeated cycles finally raise this to the 150% of the thin flake. A theory to account for the above behavior tentatively suggests that the charge transfer between the reactant and the two basal planes of a flake modifies the band structure so that the nth space is intercalated, where n is the stage number (number of carbon layers between reactant layers). The effect is transferred through the layer system as intercalation proceeds. It is, however, countered by the energy required initially to distort two layer planes when intercalation starts at the periphery of those two planes. This energy requirement increases with the flake thickness, thereby accounting for the above effects.