Abstract
The determinant influences of oxidants on the single-crystalline nature of manganese oxides (i.e., Mn 3 O 4 and MnOOH single crystals) through a low-temperature hydrothermal synthesis route from a simple aqueous solution containing 20 mM Mn(CH 3 COO) 2 ·H 2 O at 120°C are demonstrated in this work. The absence of oxygen molecules in the precursor solution limits formation of Mn 3+ , while saturation of oxygen in the precursor solution causes partial oxidation of Mn 2+ , favoring direct synthesis of Mn 3 O 4 single crystals (hausmannite). Addition of K 2 S 2 O 8 causes complete oxidation of Mn 2+ to Mn 3+ , favoring formation of MnOOH single crystals. The shape of as-prepared Mn 3 O 4 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 Mn 3 O 4 and MnOOH. With potentiodynamic (CV) activation for 200 cycles between O and 1.0 V in 1 M Na 2 SO 4 at 25 mV s -1 , the activated Mn 3 O 4 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 Mn 3 O 4 are definitely different from those of the potentiodynamically activated MnOOH. © 2008 American Chemical Society.