Disclosing the Early Stages of Electrochemical Anion Intercalation in Graphite by a Combined Atomic Force Microscopy/Scanning Tunneling Microscopy Approach

In view of large-scale applications, electrochemical exfoliation of graphite for the production of graphene sheets must follow chemical processes that ensure high quality of the products -wide-size graphene foils, single- or few-layer thickness, and low level of defectivity -in order to guarantee high electrical transport and good mechanical properties. Understanding the exfoliation process of graphite at the atomic scale, that is, the intercalation of graphene layers in the electrolyte solution, is fundamental to really be able to control and optimize such processes. This can be obtained, for instance, by investigation of the exfoliated graphite -the surface of the original crystal left behind in the chemical solution- and by real-time monitoring of graphite surface morphological and structural modifications during the exfoliation process. Here, we monitor graphite surface changes as a function of the electrochemical potential by both electrochemical (EC) atomic force microscopy and EC scanning tunneling microscopy coupled with cyclic voltammetry. Following this strategy, we disclose the surface modifications encountered during the early stages of anion intercalation, for different electrolytes: surface faceting, step erosion, terrace damages, and nanoprotrusions, all affecting the graphite surface and therefore the exfoliation process. Our results represent a key step toward a full investigation of the intercalation process in graphite. Within the current debate on the exfoliation of layered crystals, these data potentially represent important information for investigation of the intercalation process in graphite and, on the other hand, for further optimization of the electrochemical protocol for graphene production.