Enhanced dielectric constant of Co-doped ZnO nanoparticulate powders

In this work we studied the optical band-gap and dielectric properties of nanoparticulate powders of Co-doped ZnO with x=0.0, 0.5, 1.0, 3.0, 5, 7 and 9.0 at %Co synthesized by the combustion reaction method. X-ray diffraction patterns (XRD) of each sample showed broad peaks consistent with the ZnO wurtzite structure. The absence of extra reflections in the diffraction patterns ensures the phase purity. The average crystallite size determined from the most prominent (101) peak of the diffraction using Scherrer's equation was in good agreement with those determined by transmission electron microscopy (TEM), being ∼16 nm for undoped-ZnO sample and ∼21 nm for all Co-doped samples. Diffuse reflectance spectrum of each sample was obtained by using a UV/VIS/Near spectrometer and the optical band-gap Eg decreased with Co doping amount. The dielectric constant ε was determined using the Brus model (L.E. Brus, J. Chem. Phys. 80 (1984) 4403 [38]) increased with Co doping amount, reaching the maximum value of ∼10 for x=1.0 at %Co. These results were discussed in terms of the defects such as oxygen vacancies VO¨ or/and interstitial oxygen Oι¨″ present in Co-doped ZnO nanoparticles, which may be introduced during sample preparation.