Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200°C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1to22±0.5nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425°C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (>700°C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.