Revisiting Graphene Oxide Structure via Spatially‐Resolved Electron Energy Loss Spectroscopy

Abstract

Graphene oxide (GO) is obtained by chemical oxidation and exfoliation of natural graphite. In the last decade, it has attracted a widespread interest for its mechanical strength, tunable optoelectrical properties, simple processability and its potential as precursor for a low‐cost and large‐scale production of graphene. Indeed, chemical and thermal treatments allow to almost completely remove the oxygen, yielding reduced graphene oxide (RGO). Nevertheless, after about 150 years, the atomic structure of GO and RGO is still greatly debated. At present, the most acknowledged model for GO considers a random functionalization of the carbon basal plane with epoxide and hydroxyl groups, forming graphitic and partially oxidized domains. However, no definitive evidence of this model has been reported due to the lack of chemical analysis at the proper scale. For these reasons, nanometrically spatially‐resolved spectroscopy of GO and RGO is highly suitable. In this work we provide the first chemical characterization of GO and RGO thin flakes at the scale of few nanometres, thanks to core Electron Energy Loss Spectroscopy (EELS) in a STEM microscope. A major issue is represented by the extreme sensitivity of these materials to illumination and the use of this technique on GO and RGO has been so far very restricted. A new experimental set up combining a liquid nitrogen cooling system at the sample stage, a low accelerated electron beam (60 keV) and a liquid nitrogen cooled CCD camera with a low read‐out noise of three counts r.m.s. and a negligible dark count noise has allowed us to overcome this limitation. Optimal illumination conditions have been defined by monitoring the evolution of the sample under continuous illumination, defining a maximal electron dose before substantial chemical modification of the order of 10 3 e ‐ Å ‐2 and hence a 3 nm lower limit on the hyperspectral spatial resolution. Chemical maps of the atomic oxygen content of few layers GO and RGO show well separated domains on the scale of tens of nanometres. Overall, the oxygen amount has been observed to vary within 10‐50 at.% in GO and 5‐20 at.% in RGO. Energy‐Loss Near‐Edge Structures (ELNES) at the carbon K‐edge exhibit well‐defined features related to C‐O bonding, previously not reported. Moreover different oxidation levels in GO and RGO are characterized by specific ELNES profiles. The highly oxidized regions in GO (~50 oxygen at.%, i.e. 1:1 C/O ratio) correspond to a full functionalization of the carbon network. With the support of complementary DFT numerical calculations, we suggest a model for the highly oxidized regions consisting in a full functionalization with hydroxyls, forming a 2D‐sp 3 system.