Correlation of defects and crystallite size of ZnO nanoparticles synthesized by mechanical milling

For practical applications of zinc oxide (ZnO) nanoparticles (NPs) in the field of optoelectronics, a control over particle size with proper understanding of defects in the NPs has become crucial nowadays. In the present study, an effort has been given to both controlling the size and understanding the evolution of defects in ZnO NPs synthesized using mechanical (ball) milling. Ball milling of ZnO nanopowder (of average size ~ 46.8 nm) in air ambient was found to reduce the size of NPs very fast reaching a value as small as ~ 16.3 nm for a ball milling time of 10 h. A size saturation of the NPs was seen beyond the time of milling of 10 hours. X-ray diffraction and optical absorption spectroscopy studies confirmed that the stress and the band gap of the NPs were found to increase with decrease in particle size due to increased surface to volume ratio of the NPs and the quantum confinement effects, respectively. Raman and photoluminescence spectroscopy studies revealed the increase in zinc interstitials (Zn i ) and decrease in oxygen vacancies ( V o ) with increase in size of ZnO NPs. A correlation between NP size and the defects ( V o and Zn i ) in it is established. These correlations can be utilized for relative quantification of the defects in ZnO NP of a particular size. It is concluded that the defects in the small ZnO NPs are inherently induced because of the reduced size but not due to ball milling. So it is very unlikely to synthesize defect-free ZnO NPs.