Short CommunicationDispersion behaviour of graphene oxide and reduced graphene oxide
Graphical abstract
Introduction
Graphene is an atomically thin layer of sp2-bonded carbon atoms, stacked in a two-dimensional (2D) honeycomb lattice, forming the basic building block for carbon allotropes of any dimensionality [1]. Since its isolation as a monolayer, graphene has attracted an extraordinary amount of interest due to its potential application in the fastest growing scientific fields, such as supercapacitors [2], biosensors [3], photovoltaics [4] and touch panels [5].
Chemical vapour deposition (CVD) [6] and micromechanical exfoliation of graphite are the most widely used fabrication methods of less defective graphene films. However, the CVD deposition of uniform large area graphene films on arbitrary substrates at low temperatures is not possible and furthermore this method is incompatible with roll to roll mass production processes. At the same time, the exfoliated graphene exhibits very low solubility in common organic solvents [7], due to the essential addition of a stabilizer as the exfoliation liquid medium [8].
On the other hand, exfoliated graphene oxide (GO) is the ideal alternative for the production of solution processable graphene, as it can be synthesized in large quantities from inexpensive graphite powder and can readily yield stable dispersions in various solvents [9]. GO is an oxidized graphene sheet having its basal planes decorated mostly with epoxide and hydroxyl groups, in addition to carbonyl and carboxyl groups located at the edges [10].
The covalent character of C–O bonds disrupts the sp2 conjugation of the hexagonal graphene lattice, making GO an insulator. Nevertheless, GO can be partially reduced to conductive graphene-like sheets by removing the oxygen-containing groups [11], [12], [13]. In this way the conjugated structure of graphene can be recovered, resulting in reduced graphene oxide (rGO) with important electrical properties partially restored [14].
However, the preparation of dispersed form of graphene for applications in printed flexible electronics is not a straightforward process, since its stability in various solvents is a critical point. In this context, the solubility of GO in various solvents has been recently examined by several groups [9], [15], [16]. However, there is a gap in the literature on the direct comparison on solubility values on GO and rGO, which in principle they are different. Therefore, the knowledge on how conductive rGO stable solution can be obtained in common organic solvents is vital.
In this work, the dispersion behaviour of GO and chemically rGO is compared, aiming to get an insight into how the removal of oxygen containing groups during the reduction process affects its dispersion quality. The solubility/dispersibility of rGO is investigated in eighteen different solvents and directly compared with the pristine GO. In this way, critical solubility values are recorded aiming at the application of conductive rGO inks on printed flexible electronic devices [17].
Section snippets
Results and discussion
For the preparation of GO and rGO dispersions, the products prepared as described in the Experimental Section (SI), were first grounded with a mortar and pestle. In order to compare the dispersion behaviour in the different solvents, the same quantity of GO and rGO powder (∼1 mg) was added to a given volume of solvent (∼2 mL), with an initial concentration of 0.5 mg/mL. GO and rGO dispersions were tested in the following organic solvents: (DI) water, acetone, methanol, ethanol, 2-propanol,
Conclusions
The dispersion behaviour of GO and rGO in eighteen solvents was compared. The Hansen and Hildebrand parameters of GO and rGO were estimated verifying that the reduction process has a strong effect on the solubility and stability. Solutions of GO in NMP, ethylene glycol and water presented significant long-term stability with solubility values reaching ∼8.7 μg/mL for NMP. While, the dispersion behaviour of GO changed after its reduction, presenting better interaction with solvents like o-DCB (∼9
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