Electrochemically deposited zinc oxide morphologies on graphene on insulators

Abstract

Hybrid structure of semiconductor nanostructures on graphene has attracted much attention due to their potentials in optoelectronic and electronic. A synthesis of zinc oxide (ZnO) nanostructures on graphene/insulator substrates was carried out using electrochemical deposition. Several growth parameters such as current density, temperature and supporting electrolyte have been investigated. ZnO nanostructures were directly grown onto graphene layer whereby zinc nitrate hexahydrate (Zn(NO3)2.6H2O) solution was used as an electrolyte. The growth temperature was varied from 75°C - 90°C at current densities of -0.1 to -3.0 mA/cm2 for 45 minutes. Vertically aligned nanorods were obtained at low growth temperatures. The diameters of grown nanorods were increased with the current densities. Film-like structure produced by the coalescences between the neighboring nanorods with large diameters was observed on the samples grown at high current densities and high temperatures. Low temperature and low current density tended to exhibit the highest density of nanorod. The grown ZnO structures were highly oriented along the c-axis and at low current densities seem to show fewer structural defects. The addition of supporting electrolyte of hexamethylenetetramine (HMTA) shows improvement of hexagonal shape and smooth surface of ZnO nanorods at equimolar ratio. HMTA plays its role as mineralizer to supply additional OH- ions in the formation of ZnO structures. Meanwhile for addition of potassium chloride (KCl) solution, instead of nanorods, vertically ZnO nanowalls were also formed. It proves that Cl- ions as a capping agent on (0001) plane and control the formation of two dimensional (2D) ZnO nanowalls structures and one dimensional (1D) nanostructures with large diameter. The diffraction peak intensity of HMTA and KCl solution shows the crystallinity of the grown structure. This study offers significant benefit for the ZnO morphology in order to realize high crystalline and uniform ZnO nanostructures.