Stage Transitions in Graphite Intercalation Compounds: Role of the Graphite Structure

Abstract

© 2019 American Chemical Society. Despite intensive and long-lasting research on graphite intercalation compounds (GIC), the mechanism of the stage transitions remains unclear. Using optical and Raman microscopy, we perform direct real-time monitoring of stage transitions in H2SO4-GICs made from highly oriented pyrolitic graphite (HOPG). We observe that stage transitions in HOPG-based GICs occur very differently from those in GICs made from the natural flake graphite. During the stage-2 to stage-1 transition, formation of the stage-2 phase begins nearly simultaneously over the entire graphite surface that is exposed to the media. We attribute this concerted transition to the movement of the small intercalant portions toward the points of attraction, thus growing continuous islands. During the reverse process, the stage-1 to stage-2 transition begins strictly from the edges of the graphite sample and propagates toward the center of the graphite sample. The deintercalation front is discontinuous; the selected micrometer-sized domains of the graphite surface deintercalate preferentially to release the strain that had been induced by the intercalation. The intercalant dynamics in the two-dimensional (2D) graphite galleries, occurring at the speed > 240 μm/s, has fast kinetics. The initial intercalation process is different from the rest of the reintercalation cycles. The difference in the mechanisms of the stage transitions in natural flake graphite-based GICs and in the HOPG-based GICs exemplifies the role of the graphite structure for the intercalant dynamics in 2D graphite galleries.