In-Depth Investigations of Graphene Oxide Reduction via in situ TEM Measurements

Graphene oxide (GO) strongly attracts research interest due to its use as precursor material for graphene-based material and its high flexibility in terms of functional modification offering promising applications in numerous fields [1–5]. It is worth to mention that graphene oxide as well as reduced graphene oxide (rGO) have become two materials of abundant importance within the realm of 2D materials in general and chemically modified graphene in particular. Despite the efforts for investigating its atomic structure, which is critical for knowing its properties, several aspects remain unknown, in particular the behavior of the physi/chemisorbed and of the oxygen functional groups (OFGs) during GO reduction. Indeed, the processes taking place in the thermal reduction of GO into rGO are still not completely clear. In situ TEM studies involving GO heating, including EELS analyses, seem like an ideal method for achieving this goal. Here we present two different approaches for studying, in depth, the GO reduction via in situ TEM investigations: thermal reduction and Joule heating, see Figs.1 and 2, respectively. For achieving these comprehensive studies [7], we have performed high-resolution TEM (HRTEM) imaging and electron energy-loss spectroscopy (EELS) measurements and coupled (for the second of these studies) with electrical conductivity measurements. In both studies, we have identified the transformations of different oxygen functional groups, the desorption of physisorbed and chemisorbed water and the graphitization of the GO flakes. These in-depth analyses provide a detailed roadmap of the behavior of GO during its reduction and graphitization. All these findings improve the knowledge of this complex and heterogeneous material, which is crucial for the study of their physical and chemical properties and its future applications [8]. General schematic on the main results of this study showing the reduction of GO via detailed in-situ heating TEM (HRTEM & EELS) [6]. This concern water/OFG removal. Left: Schematic of the structure of GO. Center: Atomic structure and desorption temperature of water and OFGs. Right: ideal structure of rGO. In the works showed in this figure, the reduction of GO has been followed via detailed in situ Joule heating TEM (HRTEM, electron diffraction & EELS) studies and coupled with conductivity investigation by electrical measurements [7]. This in-depth analysis demonstrates that this approach allows reducing the GO in a highly homogeneous, uniform, and localized manner. During this process, the conductivity of the GO film increases more than four orders of magnitude reaching a value of 3x103 Sm-1 with a final O content of less than 1%.