The effect of surface passivation on the preparation and stability of the graphite intercalation compounds containing tetra-n-alkylammonium cations

The kinetic stability of graphite intercalation compounds (GICs) is markedly increased by a surface passivation reaction that occurs under strong reducing conditions in the presence of long-chain tetra-n-alkylammonium cations. A simple alkylation model is proposed. Surface alkylation allows the formation of a stable, isolable, graphite intercalation compound of tetra-n-ethylammonium, (C2H5)4N+ for the first time, by chemical surface passivation of [Na(en)1.0]C15 (en = ethylenediamine) with R4N+, R = C6H13, C7H15 or C8H17, followed by an ion exchange reaction to displace the Na(en)+ complex with (C2H5)4N+. One GIC thus obtained using dimethylsulfoxide (DMSO) as solvent has composition [(C2H5)4N]C57 0.5DMSO, and is a stage-1 compound with a gallery expansion of 0.47 nm. This relatively small expansion indicates a monolayer of intercalate and additionally requires an unusually flattened cation conformation. Electrophoretic analyses indicate that the ion exchange within the graphene galleries goes to completion. Additionally, the passivated GIC surfaces afford a dramatic increase in the stability of GICs, in protic solvents, aqueous media, and the ambient environment.