Detailed thermal reduction analyses of graphene oxide via in-situ TEM/EELS studies

M Pelaez-Fernandez,A Bermejo,A.M Benito,W.K Maser,R Arenal

doi:10.1016/j.carbon.2021.03.018

Abstract

We report an in-depth study of the reduction of graphene oxide (GO) by in-situ thermal transmission electron microscopy (TEM) analysis. In-situ heating high-resolution TEM (HRTEM) imaging and electron energy-loss spectroscopy (EELS) measurements have been combined to identify the transformations of different oxygen functional groups, the desorption of physisorbed and chemisorbed water and the graphitisation as a function of the temperature in the range from 70 up to 1200 °C. A model for the general removal of water and OFGs is proposed based on different chemical and physical parameters that have been monitored. All this unique information provides a detailed roadmap of the thermal behaviour of GO at an extended range of temperature. This is not only of interest to understand the thermal reduction process of GO but also of critical relevance to the response of GO in applications when exposed to thermal effects.

Highlights

Since the first production of isolated graphene in 2004 [1, 2], 2D nanomaterials have experienced unprecedented interest in research, due to their promising mechanical [3], thermal [4], and electrical properties [5, 6, 7]
The obtained graphite oxide material was mildly sonicated and centrifuged yielding the corresponding graphene oxide (GO) suspension in water. This suspension was drop cast on our custom-modified DENS chips for in-situ transmission electron microscopy (TEM) studies, resulting in samples consisting of GO films, with thicknesses ranging from ~5 to ~50 nm, composed of several stacked GO flakes
The analysis of the Energy loss near-edge structure (ELNES) features has been performed for the low and high temperature regimes, and the different peaks of the C-K and O-K edges in the different films have been assigned taking into account both energy-loss spectroscopy (EELS) edges, previous EELS and X-Ray photoelectron spectroscopy (XPS) studies [17, 35, 37, 41, 52, 68, 69, 70], as well as studies focusing on the desorption temperature for different oxygen-containing functional groups (OFGs) [33, 35, 37, 71]

Summary

Introduction

Since the first production of isolated graphene in 2004 [1, 2], 2D nanomaterials have experienced unprecedented interest in research, due to their promising mechanical [3], thermal [4], and electrical properties [5, 6, 7]. We have developed in-situ TEM studies ranging from 70 to 1200°C (a temperature well above the known limit for the reduction of GO [35]) to analyse the structural modifications of the GO material upon heating This includes an in-depth characterisation of the water desorption and losses of OFGs from the GO material as well as of the concomitant structural defects created through their removal. We present our results combining TEM imaging and EELS during heating These thermal in-situ analyses have provided simultaneous and detailed measurements of the oxidation rate, thickness, mass density and sp bond fraction in the C atoms of the GO, all while performing an ELNES analysis on the presence of different OFGs in the sample. To the best of our knowledge, this represents the most comprehensive approach for an in-depth study on the processes of GO thermal reduction to date

Experimental

Data Analysis

Results and discussion

Conclusion