Electrolyte-induced precipitation of graphene oxide in its aqueous solution.

Graphene oxide (GO) can easily dissolve in water to form a stable homogeneous solution due to its hydrophilic property and ionization of functional groups. However, in this paper, it is reported that a strong electrolyte (HCl, LiOH, LiCl, LiBr, KCl, or KBr) can destabilize the GO solution, causing GO precipitation. This indicates that the electrostatic repulsion plays a critical role in stabilizing aqueous GO solution. The electrolyte-induced precipitates were characterized by transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). The oxygen-containing functional groups of GO sheets, which are carboxyl, epoxy, and hydroxyl groups, remained unchanged during acid (HCl)- and salt (LiCl)-induced precipitations. In contrast, during the GO precipitation induced by a base (LiOH), the carboxyl group of GO sheets disappeared with a remarkable increase in hydroxyl group and aromatic C=C bonds. This indicates that the LiOH-induced GO precipitation resulted in the partial reduction of GO sheets. Furthermore, it was demonstrated that the HCl-induced GO precipitation is a feasible approach to deposit GO on a substrate as a Pt-free counter electrode for a dye-sensitized solar cell (DSSC), which exhibited 1.65% power conversion efficiency.