Fabricating BaZrO3 hollow microspheres by a simple reflux method

Abstract

A simple reflux method was developed to fabricate hollow microparticles of complex oxides. Single crystalline perovskite-type BaZrO₃ hollow microspheres were synthesized in a concentrated KOH solution via Ostwald ripening. The high KOH concentration resulted in the increased supersaturation of the reactants and grain growth of primary particulates that readily agglomerated into aggregated particles. The high KOH concentration also decreased the size of the aggregated particles due to enhanced interparticle attractive forces. Heat treatment promoted the hollowing process. Refluxing a mixture in 20 mol L⁻¹ KOH solutions in a 200 °C oil bath produced perovskite BaZrO₃ hollow microspheres with an average size of 92 nm. XRD patterns demonstrated the cubic perovskite structure of the BaZrO₃ hollow microspheres. HRTEM and SAED measurements disclosed the single crystalline nature of a single BaZrO₃ hollow microsphere. The products showed variations in morphology, from solid to core–shell and hollow microspheres, at various reaction durations. The highly concentrated KOH solution not only led to smaller aggregated particle sizes but also afforded a high boiling temperature to accelerate the diffusion rate and promote the ripening process. Similarly, perovskite SrZrO₃ and SrHfO₃ hollow microparticles were also fabricated by this method.