Water-assisted stable dispersal of graphene oxide in non-dispersible solvents and skin formation on the GO dispersion Rana Tariq Mehmood Ahmad, Seung-Ho Hong, Tian-Zi Shen, Jang-Kun Song * School of Electronic & Electrical Engineering, Sungkyunkwan University, Suwon, Gyeonggi-do, 440-746, Republic of Korea article info Article history: Received 23 June 2015 Received in revised form 2 November 2015 Accepted 3 November 2015 Available online 10 November 2015 abstract Graphene oxide (GO) particles disperse well in a few polar solvents including water but do not disperse in non-polar and low dielectric constant solvents. The limitation of available solvents can restrict the application of GO compositional materials. Herein, we report that by adding small amount of water in a water-miscible solvent, the dispersity of GO can be significantly improved. The scheme works even in solvent with low dielectric constant; the water-added dioxane exhibited even better GO dispersity than pure water that is known to be the best solvent for GO dispersion to date. The method increases the spectrum of available solvents for dispersing GO particles. We also report a formation of skin layer with random birefringent wrinkles on the surface of water-dioxane binary GO dispersion. The skin layer in gel phase was partially stretchable and less rigid as compared to a fully dried GO film. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Graphene oxide (GO) is a water-dispersible, single layered, carbon-based material [1]. Aqueous GO dispersions exhibit unique properties including spontaneous nematic assembly [2e4], sensi- tive electro-optical switching [5,6], and easy reducibility to conductive reduced GO [7]. In particular, the solution processability of aqueous GO dispersions has attracted significant attention for use in various applications including flexible electronics, batteries, conductive paper, and wire fabrications. Compositional materials of GO and polymers were also widely used in these studies [8,9]. The widespread use of various solution-based applications of GO stems from their ability to form stable dispersions, and water is, in fact, considered the best solvent for this purpose [10]. Although few polar organic solvents such as dimethylformamide and tetrahy- drofuran can disperse GO particles, GO dispersions in non-polar solvents or low dielectric constant solvents have not been ach- ieved [11,12]. To date, the dispersion of GO in low-polar organic solvents or polymer has been accomplished through covalent functionalization of GO particles with different molecules such as long alkyl chains [13,14]. However, the presence of such stabilizers on GO surface is not desirable for most applications [12]. Increase in the spectrum of available solvents for stable GO dispersions can lead to new possibilities in the area of composite materials, but remains quite challenging. The GO basal plane is decorated by surface functional groups including carboxyls (eCOOH) and hydroxyls (eOH) [1,15]. When GO particles are dispersed in water, these functional groups become negatively ionized, thereby generating protons [10,16,17]. The repulsive electrostatic forces between the GO flakes, and in turn the stable dispersity of GO particles in water, arise from the negative charges on their surface. However, the ionization of the functional groups accompanying the protonation of the solvent occurs only weakly in non-polar solvents. Therefore, GO particles exhibit low levels of dispersity in these solvents [12]. In this study, we introduce a new approach to enhance the GO dispersity in GO-immiscible solvents. We demonstrate that a small amount of water can lead to a significant increase in the dispersity of GO in water-miscible solvents; this indicates that water can act as a surfactant in these dispersions. The GO dispersity in these water- added solvents was even better than that in pure water, which was typically considered the best solvent for GO dispersion. We also report a skin formation on water-added dioxane GO dispersion. 2. Experimental work An aqueous GO dispersion was prepared by using the Hummers method [18]. 60 ml of 98% H 2 SO 4 was added to a mixture of 2 g of graphite flake (7e10 micron 99% Alfa Aesar) and 1.5 g of NaNO 3 within a bath at 0  C. Then, 7.0 g of KMnO 4 was added slowly into * Corresponding author. E-mail address: jk.song@skku.edu (J.-K. Song). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon http://dx.doi.org/10.1016/j.carbon.2015.11.007 0008-6223/© 2015 Elsevier Ltd. All rights reserved. Carbon 98 (2016) 188e194