Cobalt oxide is often obtained by hydrothermal synthesis in an autoclave.
One-pot hydrothermal synthesis of metal oxide hollow spheres starts with carbohydrates and metal salts dissolved in water at 100-200 °C.  The reaction produces carbon spheres, with metal ions integrated into the hydrophobic shell.  The carbon cores are removed by calcination, leaving hollow metal oxide spheres.  Surface area and thickness of the shell can be manipulated by varying the carbohydrate to metal salt concentration, as well as the temperature, pressure, and pH of the reaction medium, and the cations of the starting salts.  The completion time for the procedure varies from hours to days.
A drawback of this approach is its smaller yield compared to other methods.