Zinc Oxide/Reduced Graphene Oxide Composites and Electrochemical Capacitance Enhanced by Homogeneous Incorporation of Reduced Graphene Oxide Sheets in Zinc Oxide Matrix

ZnO/reduced graphite oxide composites were synthesized using a two-step method in which KOH reacts with Zn(NO3)2 in the aqueous dispersions of graphite oxide (GO) to form a Zn(OH)2/graphite oxide precursor, followed by thermal treatment in air. It was found that the dispersion of reduced graphene oxide (rGO) sheets within composites was key for achieving an excellent capacitive performance of the samples. However, the mass ratio of ZnO to rGO determined whether rGO sheets within composites were dispersed or agglomerated. The composite achieved homogeneous incorporation of rGO sheets within the ZnO matrix when the mass ratio of ZnO to rGO was equal to 93.3:6.7. This composite, in which the weight percent of rGO was only 6.7%, appeared in the SEM images to be almost entirely filled with rGO sheets coated by ZnO and exhibited high specific capacitance and excellent cycling ability. Furthermore, the sheets overlapped to form a three-dimensional network structure, through which electrolyte ions easily access the surface of the rGO or electrochemical active sites. The homogeneously incorporated rGO sheets were shown to provide 128% enhancement in specific capacitance compared with 135 F g−1 for pure zinc oxide samples. Also, the unexpected phenomena involved in the experimental processes are discussed in detail.