Aqueous Colloidal Stability of Graphene Oxide and Chemically Converted Graphene

Graphene oxide (GO) was prepared by modified Hummer’s method, and chemically converted graphene (CCG) was prepared by further reduction of the aqueous GO colloid. The effect of pH on particle size, particle charge, and light absorption of the aqueous colloids of GO and CCG was studied with titration against HCl or NaOH, to find the ideal characteristics for a stable dispersion. The GO colloid was stable in the pH range of 4–11, whereas the CCG colloid gained stability at a relatively narrower pH range of 7–10. Poor stability of the colloids was observed for both GO and CCG colloids at both extremes of the pH scale. Both of the colloids exhibited average size of ~1 micron in the low pH range, whereas for higher pH the size ranged between 300 and 500？nm. The UV-Vis spectra showed absorption peak at 230？nm for GO colloids that shifted to 260？nm for the CCG colloid. Such shift can be ascribed to restoring of electronic conjugation of the C=C bonds in CCG. 1. Introduction Graphene is a flat 2D layer of carbon atoms packed in a honeycomb lattice and is the basal building block in all graphitic materials [1]. Its different synthesis routes and the possession of unique mechanical, thermal, and electrical properties make it a material of great interest [2–7]. In spite of the viable structure-property relationship, large volume production of graphene is still a challenge. Graphene oxide syntheses via chemical methods using acids and oxidants are the most widely followed procedures, which can be further reduced to produce graphene [8–10]. Graphene oxide (GO) can be best described as a single layer planar hexagonal array of carbon atoms to which functional groups, including carboxylic acid, hydroxyl, epoxy, and carbonyl groups, are attached. According to the Lerf-Klinowski model, the carbonyl and carboxylic groups remain attached to the edges of the sheet, while the hydroxyl and epoxy groups are found on the basal plane [11]. These groups render the GO sheets hydrophilic due to the H-bonding present between the carboxylic and hydroxyl groups. This property opens up many opportunities for chemical reactions and schemes, including self-assembly and thin films processing, to exploit the beneficial properties of graphene oxide. On the contrary, graphene sheets that are devoid of any O-containing functional groups are insoluble in water, thus posing a challenge for the exploitation of graphene in aqueous processing methods. Several methods, including functionalization [12–14], in situ reduction [15, 16], or polymeric reduction [17] of GO sheets in aqueous medium, can be