Effect of Zn content on the structural, optical, electrical and supercapacitive properties of sol–gel derived ZnCo2O4 nanostructured thin films

Abstract

Pure Co3O4 and ZnCo2O4 (Co/Zn: 9/1, 8/2 and 7/3) nanostructured thin films were successfully deposited on the glass and indium tin oxide substrates from acetate based sols, by spin coating technique. The effect of Zn amount on the structural, optical, electrical and supercapacitive properties of ZnCo2O4 nanostructured films were characterized. X-ray diffraction, infrared spectroscopy and field emission scanning electron microscopy results revealed that the increase of Zn content does not lead to a significant change in the structure and morphology, nevertheless it caused drastic effects on the optical, electrical and electrochemical properties of the films. Mono-phase ZnCo2O4 structure even in high amounts of Zn exhibited good solubility of Zn in the Co3O4 structure in this preparation method. The quantity of allowed direct, forbidden direct, allowed indirect and forbidden indirect transitions were calculated from the UV–Visible spectroscopy results, and the nature of allowed direct band gap of Co3O4 in 1.52 eV was discussed as a controversial topic. Resistance measurements as a function of temperature suggested variable range hopping as predominant mechanism for transport in room temperature, which switches to thermionic conduction in higher temperatures. Cyclic voltammetry results revealed that, addition of Zn to the Co3O4 structure results interfacial capacitance of the films to be increased more than 12 times.