12 - Hydrogen behaviour in graphite–nitric acid intercalation compounds

Summary

Graphite intercalation compounds

Graphite crystallizes in a structure which consists of stacked carbon atom layers in which each carbon atom is bonded to three neighbours in a hexagonal lattice. Foreign atoms or molecules can slip between graphite layers without completely disrupting the structure of the host material. During the past ten years, these GICs have been the subject of numerous studies as they present interesting application possibilities: electrical conductors, catalysts, high energy density batteries, lubricants and membranes. A good survey is given in the review Intercalated Materials where the synthesis, structure and physical properties of GICs with metals, halogens, halogenides, oxides and acids are described.

In the last century, Brodie and Schafhautl reported the intercalation, between graphite layers, of a mixture of sulphuric and nitric acids. More recently, Rüdorff determined the X-ray diffractograms of (GNCs) Graphite-Nitric acid Compounds. GNCs can be prepared by using dilute nitric acid or pure HNO₃ as well as dinitrogen pentoxide mixed with, or without, nitric acid. The best way to avoid swelling and distortion of pyrolytic graphite samples and to prepare high quality GNCs, is to intercalate, step by step, small amounts of pure nitric acid vapour.

The GNCs behave as good p-type conductors, which is the reason why Ubbelhode described them as ‘Synthetic Metals,’ in agreement with a structural model in which the carbon hexagonal network acts as a macrocation with intercalation of nitrate ions and molecules of nitric acid.