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

Abstract

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

Evolution of the graphite surface in phosphoric acid: an AFM and Raman study

“…At the nanometer scale, protrusions have been observed together with an increase of the surface roughness caused by graphite oxidation [7–8]. HClO 4 solutions show a similar phenomenology during intercalation of anions [7]. In particular, the evolution of blisters as a function of time has been analyzed in the past [9–10], supporting the theoretical model proposed by Murray [11].…”

“…After that single- or multi-layer graphene sheets with a size of about 1 μm can be retrieved from the electrochemical bath. The electronic and mechanical properties of the graphene sheets [1–6] and the main characteristics of the graphite crystals subjected to EC delamination [6–7] have been studied extensively. This allows one to shed light on the correlation between the modifications induced on the graphite crystal and the structure of the exfoliated graphene sheets.…”

“…This allows one to shed light on the correlation between the modifications induced on the graphite crystal and the structure of the exfoliated graphene sheets. H 2 SO 4 produces blisters at the micrometer scale, i.e., local swellings of the surface caused by the production of gases (O 2 , CO and/or CO 2 ) due to the oxidation processes occurring at high anodic potentials [7]. At the nanometer scale, protrusions have been observed together with an increase of the surface roughness caused by graphite oxidation [7–8].…”

“…H 2 SO 4 produces blisters at the micrometer scale, i.e., local swellings of the surface caused by the production of gases (O 2 , CO and/or CO 2 ) due to the oxidation processes occurring at high anodic potentials [7]. At the nanometer scale, protrusions have been observed together with an increase of the surface roughness caused by graphite oxidation [7–8]. HClO 4 solutions show a similar phenomenology during intercalation of anions [7].…”

“…However, a detailed analysis of the surface modification of a graphite crystal subjected to EC processes in phosphoric acid solution is still missing. In a recent work [7], we have shown that the EC characterization of the system, i.e., the cyclic-voltammetry (CV) curve, presents a single feature during the first EC potential sweep, but it disappears during the second scan. On the other hand, EC atomic force microscopy (EC-AFM) measurements, performed in situ in the EC cell, reveal a significant increase of the surface roughness.…”