Low-temperature hydrothermal synthesis of Mn3O4 and MnOOH single crystals:: Determinant influence of oxidants

The determinant influences of oxidants on the single-crystal line 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)center dot 4H(2)O at 120 degrees C are demonstrated in this work. The absence of oxygen molecules in the precursor solution limits formation of Mn, while saturation of oxygen in the precursor solution causes partial oxidation of Mn, favoring direct synthesis of Mn3O4 single crystals (hausmannite). Addition of K2S2O8 causes complete oxidation of Mn to Mn, 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. I With potentiodynarnic (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 (similar to 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 potentio-dynamically activated MnOOH.