Synthesis of hausmannite and kempite nanoparticles at low temperature via an epoxide precipitate route

A simple synthesis procedure, using epoxide as the precipitation agent, was applied at low temperature to prepare well size-controlled hausmannite (Mn₃O₄) and kempite (Mn₂(OH)₃Cl) nanoparticles. During the preparation, the hydrolysis of aquo complexes of metal ions was promoted by the ring opening reaction of epoxide through the protonation of its oxygen. Owing to the low acidity of aquo Mn²⁺ complexes in solution, the slow protonation of epoxide could also lead to the subsequent rise in the pH of the solution, which allows alternative side reactions to occur to a significant degree and cause precipitation to take place instead of sol or gel. Owing to the unique chemistry of propylene oxide, the structure of the as-prepared nanoparticles can be well controlled through the controlling of the reaction temperature and Mn²⁺ concentration. The hydrolysis and further condensation of aquo Mn²⁺ complexes resulted in the formation of Mn₃O₄ nanoparticles. On the other hand, their partial hydrolysis occurred without further condensation of the generated hydroxide intermediate under high Mn²⁺ concentration. This induced the interposition of Cl^- ions into intermediate to form the structure of Mn₂(OH)₃C1. As confirmed by characterisation results, the size of nanoparticles with the corresponding pure phases increased with the increase in Mn²⁺ concentration and reaction temperature.