ZnO nanoparticles-graphene oxide-reduced graphene oxide thin films assembled layer-by-layer through non-electrostatic interactions

Ultrathin films of zinc oxide nanoparticles (ZnONP), graphene oxide (GO) and reduced graphene oxide (RGO) are assembled layer-by-layer (LbL) onto quartz and ITO substrates. UV-vis absorption, micro Raman, and x-ray photoelectron spectroscopies show a linear dependence between the mass of adsorbed nanomaterials and the number of ZnONP/GO and ZnONP/RGO deposited bilayers and suggest the establishment of covalent bonds rather than electrostatic interactions between ZnONP and GO/RGO sheets. In addition, impedance spectroscopy and scanning electron microscopy reveal that the films are formed via a nucleation-coalescence mechanism, in which a continuous surface is established after the deposition of twenty bilayers. Cyclic voltammetry and galvanostatic charge-discharge measurements confirm their double-layer capacitor behavior and that the ZnONP/RGO exhibits a capacitor performance better than ZnONP/GO. The charge stored in ZnONP/RGO films remains unaltered during 1,000 charge-discharge cycles without apparent film degradation. The proposed LbL method is advantageous since ZnONP are assembled as-produced without surface functionalization or employment of surfactants.