Room Temperature Synthesis Routes to the 2D Nanoplates and 1D Nanowires/Nanorods of Manganese Oxides with Highly Stable Pseudocapacitance Behaviors

Abstract

The 2D nanoplates of δ-MnO2 and the 1D nanowires/nanorods of α-MnO2 can be synthesized at room temperature via one-pot oxidation reaction of commercially available divalent manganese compounds. Treating the MnO or MnCO3 precursor with persulfate ions for 1–2 days yields layered δ-MnO2 2D nanoplates, whereas the same oxidation reaction for the MnSO4 precursor produces γ-MnO2-structured 3D urchins. As the reaction time is extended for ∼14–21 days, not only δ-MnO2 nanoplates but also γ-MnO2 urchins are changed to well-separated 1D nanostructured α-MnO2 materials with controllable diameters. According to N2 adsorption–desorption isotherm measurements and Mn K-edge X-ray absorption spectroscopy, all the obtained manganate nanostructures show expanded surface areas of ∼50–120 m2 g–1 and the mixed oxidation state of Mn3+/Mn4+, respectively. All the present nanostructured manganese oxides exhibit pseudocapacitance behaviors with large specific capacitance and excellent capacitance retention, highlighting their promising functionality as a supercapacitor electrode. Among the materials under investigation, the δ-MnO2 2D nanoplates show the largest specific capacitance (∼180–210 F g–1). The present finding clearly demonstrates that the room-temperature oxidation reaction of the MnO or MnCO3 precursor can provide a facile soft-chemical route to 2D δ-MnO2 nanoplates and 1D α-MnO2 nanowires/nanorods with highly stable pseudocapacitance behaviors.