Defect induced ferromagnetism in undoped ZnO nanoparticles

Undoped ZnO nanoparticles (NPs) with size similar to 12 nm were produced using forced hydrolysis methods using diethylene glycol (DEG) [called ZnO-I] or denatured ethanol [called ZnO-II] as the reaction solvent; both using Zn acetate dehydrate as precursor. Both samples showed weak ferromagnetic behavior at 300 K with saturation magnetization M-s = 0.077 +/- 0.002 memu/g and 0.088 +/- 0.013 memu/g for ZnO-I and ZnO-II samples, respectively. Fourier transform infrared (FTIR) spectra showed that ZnO-I nanocrystals had DEG fragments linked to their surface. Photoluminescence (PL) data showed a broad emission near 500 nm for ZnO-II which is absent in the ZnO-I samples, presumably due to the blocking of surface traps by the capping molecules. Intentional oxygen vacancies created in the ZnO-I NPs by annealing at 450 degrees C in flowing Ar gas gradually increased M-s up to 90 min and x-ray photoelectron spectra (XPS) suggested that oxygen vacancies may have a key role in the observed changes in M-s. Finally, PL spectra of ZnO showed the appearance of a blue/violet emission, attributed to Zn interstitials, whose intensity changes with annealing time, similar to the trend seen for M-s. The observed variation in the magnetization of ZnO NP with increasing Ar annealing time seems to depend on the changes in the number of Zn interstitials and oxygen vacancies. (C) 2014 AIP Publishing LLC.