Low-Temperature Hydrothermal Synthesis of Mn3O4 and MnOOH Single Crystals: Determinant Influence of Oxidants

The determinant influences of oxidants on the single-crystalline nature of manganese oxides (i.e., Mn3O4 and MnOOH single crystals) through a low-temperature hydrothermal synthesis route from a simple aqueous solution containing 20 mM Mn(CH3COO)2·4H2O at 120 °C are demonstrated in this work. The absence of oxygen molecules in the precursor solution limits formation of Mn3+, while saturation of oxygen in the precursor solution causes partial oxidation of Mn2+, favoring direct synthesis of Mn3O4 single crystals (hausmannite). Addition of K2S2O8 causes complete oxidation of Mn2+ to Mn3+, favoring formation of MnOOH single crystals. The shape of as-prepared Mn3O4 examined by HR-TEM is polyhedral, i.e., cubic and rhombohedral, while MnOOH prefers to form nanowires. X-ray diffraction, HRTEM, electron diffraction, and Raman spectroscopic analyses confirm the single-crystalline nature of the as-synthesized Mn3O4 and MnOOH. With potentiodynamic (CV) activation for 200 cycles between 0 and 1.0 V in 1 M Na2SO4 at 25 mV s−1, the activated Mn3O4 shows relatively high capacitance (∼170 F g−1 obtained at 500 mV s−1), high-power nature, and excellent stability for the supercapacitor application. The ideal capacitive responses of activated Mn3O4 are definitely different from those of the potentiodynamically activated MnOOH.