Stage Transitions in Graphite Intercalation Compounds:\nRole of the Graphite Structure

Abstract

Despite\nintensive and long-lasting research on graphite intercalation\ncompounds (GIC), the mechanism of the stage transitions remains unclear.\nUsing optical and Raman microscopy, we perform direct real-time monitoring\nof stage transitions in H₂SO₄-GICs made from\nhighly oriented pyrolitic graphite (HOPG). We observe that stage transitions\nin HOPG-based GICs occur very differently from those in GICs made\nfrom the natural flake graphite. During the stage-2 to stage-1 transition,\nformation of the stage-2 phase begins nearly simultaneously over the\nentire graphite surface that is exposed to the media. We attribute\nthis concerted transition to the movement of the small intercalant\nportions toward the points of attraction, thus growing continuous\nislands. During the reverse process, the stage-1 to stage-2 transition\nbegins strictly from the edges of the graphite sample and propagates\ntoward the center of the graphite sample. The deintercalation front\nis discontinuous; the selected micrometer-sized domains of the graphite\nsurface deintercalate preferentially to release the strain that had\nbeen induced by the intercalation. The intercalant dynamics in the\ntwo-dimensional (2D) graphite galleries, occurring at the speed >\n240 μm/s, has fast kinetics. The initial intercalation process\nis different from the rest of the reintercalation cycles. The difference\nin the mechanisms of the stage transitions in natural flake graphite-based\nGICs and in the HOPG-based GICs exemplifies the role of the graphite\nstructure for the intercalant dynamics in 2D graphite galleries.