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 O2- 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 O2- vacancies with increase in the decomposition temperature confirming the proposed role of Cu2+ and O2- vacancies in determining the electrical conductivity.