Revisiting Graphene Oxide Chemistry via Spatially-Resolved Electron Energy Loss Spectroscopy

TL;DR

This study uses advanced electron energy loss spectroscopy with a specialized setup to map and analyze the distribution of oxygen functional groups in graphene oxide at nanometric resolution, revealing detailed chemical structures and proposing a new model.

Contribution

It introduces a novel experimental approach to reduce electron damage in spectroscopy, enabling detailed nanometric chemical mapping of GO and RGO, and proposes a new structural model for highly oxidized domains.

Findings

Oxygen distribution varies from 4:1 to 1:1 C/O ratio.

Residual oxygen in RGO concentrates in nanometer-sized regions.

Identified specific energy-loss structures for different oxidation levels.

Abstract

The type and distribution of oxygen functional groups in graphene oxide (GO) and reduced graphene oxide (RGO) remain still a subject of great debate. Local analytic techniques are required to access the chemistry of these materials at a nanometric scale. Electron energy loss spectroscopy in a scanning transmission electron microscope can provide the suitable resolution, but GO and RGO are extremely sensitive to electron irradiation. In this work we employ a dedicated experimental setup to reduce electron illumination below damage limit. GO oxygen maps obtained at a few nanometers scale show separated domains with different oxidation levels. The C/O ratio varies from about 4:1 to 1:1, the latter corresponding to a complete functionalization of the graphene flakes. In RGO the residual oxygen concentrates mostly in regions few tens nanometers wide. Specific energy-loss near-edge structures are observed for different oxidation levels. By combining these findings with first-principles simulations we propose a model for the highly oxidized domains where graphene is fully functionalized by hydroxyl groups forming a 2D-sp$^3$ carbon network analogous to that of graphane.