FORMATION MECHANISM AND SYNTHESIS OF HIGHLY CRYSTALLINE POROUS ZnO SUPERSPHERES VIA SOL-GEL PROCESS: EVOLUTION FROM INTACT HOLLOW STRUCTURES TO POLLEN-LIKE MORPHOLOGIES

Abstract

The synthesis of hollow and porous (HP) ZnO superspheres with intact hollow structures is crucial for various applications, yet it poses significant challenges. This article explores the fundamental aspects of preparing highly crystalline HP ZnO superspheres via a sol-gel process, elucidating the synthetic strategy, formation mechanism, and subsequent utilization for generating pollen-like ZnO superstructures. Under solvothermal conditions at 200°C, employing zinc acetate (0.065 M) in diethylene glycol with varying molar ratios of H2O/Zn, a range of ZnO particles and superspheres were synthesized. Within a molar ratio range of 2 - 4, initial nanoparticles self-assembled into solid and porous (SP) superspheres, evolving dominantly towards or along the c-axis, resulting in HP superspheres with intact hollow structures. Conversely, molar ratios of 6 - 20 yielded only separate nanocrystals instead of superspheres. The critical role of the H2O/Zn molar ratio in forming HP superstructures with intact hollows was highlighted, controlling Ostwald ripening and outward diffusion rates, with a molar ratio of 2 identified as a prerequisite for intact HP superspheres. The resultant intact HP superspheres exhibited intense and sharp band gap emission at 389 nm, indicating potential for optoelectronic applications. Furthermore, the study introduces pollen-like ZnO colloids derived from these intact HP ZnO superspheres, offering stable dispersion. Crystal growth predominantly occurred along the outward-oriented c-axis. The intact HP ZnO superspheres serve as a promising template system for biomimetic pollen-like ZnO superstructures.