Nanostructured CuO: Facile synthesis,optical absorption and defect dependent electrical conductivity

Nanostructured CuO with different average crystallite sizes in the range ~11–48 nm is synthesized by thermolysis of carbonate precursor at different temperatures. Structural characterization of the samples is done using X-ray powder Diffraction and Transmission Electron Microscopy techniques. Analysis of UV–Visible absorption spectra of the samples reveal direct band gaps in the range 1.49–1.79 eV which is blue shifted in comparison with that corresponding to single crystalline CuO. DC electrical conductivity of the samples is found to increase with increase in crystallite size/decomposition temperature. The conduction mechanism is found to be defect dependent with holes associated with uncompensated Cu²⁺ vacancies being the charge carriers. A comparison of the DC electrical conductivities in vacuum and air ambience reveal that there is a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature. X-ray photoelectron spectral studies confirm an increase in the concentration of Cu³⁺ ions in samples with higher electrical conductivity. Analysis of Raman spectra indicates a decrease in the concentration of O^2- vacancies with increase in the decomposition temperature confirming the proposed role of Cu²⁺ and O^2- vacancies in determining the electrical conductivity.