Comparative study on the effects of different annealing conditions on the surface morphology, crystallinity, and optical properties of ZnO micro/nanoparticle-based discs

► High-density zinc oxide discs were prepared using ZnO micro- and nanoparticles. ► Unique secondary growth in ZnO nanoparticles and multilayer grains were observed. ► Annealing can be employed as a new technique for controlling the ZnO properties.