Morphology and magnetic properties of hollow Co$_{3}$O$_{4}$ spheres

Nanostructured hollow Co$_{3}$O$_{4}$ spheres are synthesized by ultrasonic spray pyrolysis. Nanocrystals forming the sphere structure have different sizes and packing density, depending on the synthesis conditions. The samples are studied by X-ray diffraction, scanning electron microscopy, low-temperature nitrogen adsorption, vacuum pycnometry, and Raman and X-ray photoelectron spectroscopies. The field and temperature dependences of magnetization show that the magnetic properties and the Néel temperature of Co$_{3}$O$_{4}$ hollow spheres are determined by the size and packing density of nanocrystals.