Synthesis of nanocrystalline manganese oxide powders: Influence of hydrogen peroxide on particle characteristics

Nocrystalline manganese oxide powders have been prepared at 25 °C by precipitation from Mn(NO₃)₂ aqueous solution. The presence and addition sequence of H₂O₂ significantly influence particle characteristics of the resulting manganese oxides, including crystal structure, particle size and morphology, and surface area, depending upon molar ratio of H₂O₂ with respect to Mn. The precipitation from preoxidized manganese solution by H₂O₂ results in flakelike-shaped amorphous hydrous manganese oxide (MnO₂·xH₂O). In the absence of H₂O₂, on the other hand, amorphous Mn(OH)₂ is obtained, and a part of Mn(OH)₂ subsequently transforms into crystalline Mn₃O₄ by oxidation in air. Relative population of amorphous Mn(OH)₂ decreases by dissolution when post-treated with H₂O₂. At Mn:H₂O₂ = 1:4, the well-defined 16-nm-sized nanocrystalline Mn₃O₄ with homogenous particle morphology is prepared. The treatment with excess H₂O₂, however, destroys crystalline Mn₃O₄ and leads to further oxidation of the aqueous manganese species. Under these conditions, a mixture of needlelike Mn₂O₃ and cubelike Mn₃O₄, including amorphous MnO₂·xH₂O, is obtained.