Synthesis of amorphous manganese oxide nanoparticles – to – crystalline nanorods through a simple wet-chemical technique using K+ ions as a ‘growth director’ and their morphology-controlled high performance supercapacitor applications

Abstract

Highly crystalline manganese-oxide nanostructures are fabricated by acidic reduction of KMnO₄ solution followed by air-annealing. During annealing, the nanostructures are converted from nanoparticles (diameters ∼ 100 nm) to nanorods (width ∼ 20 nm), which depends on the K⁺ ion content within the samples. K⁺ ions are considered to act as a ‘growth-director’ for the nanoparticle-to-nanorod conversion process. By controlling the K⁺ content through a simple rinsing step, the nanostructures are effectively controlled to be either only nanorod structures, or of pure nanoparticle structure or a mixture of both. Electrochemical characterization of these three types of nanostructures revealed that nanorod–nanoparticle mixture samples have superior electrochemical performance compared to others, which is attributed to their unique morphology, with a combination of highly crystalline 1D-nanorods and the porous structure of 3D-nanoparticles. This provides a high active surface area in the pores of nanoparticles and high surface-to-volume ratio in the nanorods for considerably higher utilization of the active materials during electrochemical performance.