Long-Term Aging of Concentrated Aqueous Graphene Oxide Suspensions Seen by Rheology and Raman Spectroscopy.

Benjámin Gyarmati,Loredana Mirela Voelker-Pop,Attila Farkas,Shereen Farah,Krisztina László,György Sáfrán

doi:10.3390/nano12060916

Abstract

Today, graphene oxide (GO) has gained well-deserved recognition, with its applications continuing to increase. Much of the processing of GO-based devices occurs in a dispersed form, which explains the commercialization of GO suspensions. Aging of these suspensions can, however, affect the shelf life and thus their application potential. Aging of GO preparations is often acknowledged, but no longer-term systematic study has been reported on the alteration of GO suspensions. This paper investigates high-concentration (10 mg/mL) aqueous GO suspensions over a 2-year time scale. In addition to steady shear tests, the dynamic behavior of the suspensions was studied in more detail by transient shear and frequency sweep measurements. Both the viscosity and the dynamic moduli increased with age, particularly within the first year. The results of the complementary Raman spectroscopic studies indicate that the change in the rheological behavior with aging results from a slow oxidation process occurring in the highly acidic aqueous medium during the relatively long-term storage. The (over)oxidized layers peel off spontaneously or are removed by high shear stress, resulting in increased viscosity, as it was corroborated by XRD and XPS.

Highlights

For many years, graphene oxide (GO) has been mainly considered as an intermediate of wet chemical graphene production
Direct ink writing (DIW), 3D printing, film casting, wet spinning, etc., are techniques that require GO dispersed in liquid medium [3–5]
Scanning electron microscopic (SEM) images of the gold-coated freeze-dried samples were taken by a JEOL

Summary

Introduction

Graphene oxide (GO) has been mainly considered as an intermediate of wet chemical graphene production. Direct ink writing (DIW), 3D printing, film casting, wet spinning, etc., are techniques that require GO dispersed in liquid medium [3–5]. For this reason, GO is commercialized in the form of aqueous dispersions. The performance of the nonstoichiometric nanoplates is influenced by multiple, occasionally interrelated factors, in the dry state but even more so in the suspended form. The list starts with the precursor graphite material, its origin (natural or synthetic), particle size, crystallinity or inorganic impurities [7,8]. It continues with the disintegration method of the graphite.

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