Shape-selective synthesis, magnetic properties, and catalytic activity of single crystalline β-MnO2 nanoparticles

The shape-controlled synthesis of exclusively single crystalline beta-MnO2, nanospheres on one hand and nanorods on the other, has been achieved through air oxidation of MnCl2 in variable SDBS micellar templates under alkaline condition at room temperature. Raman measurement and TEM unequivocally confirm the changes in the structural aspects of the particles from sphere to rod. Field-dependent magnetization measurement indicates superparamagnetic nature of MnO2 rods and spheres having a blocking temperature of 4 and 40 K, respectively. These observations together with other physical parameters explicitly confirm the changes in the structural aspects of the particles with the changes in surfactant concentration. Thus, nanorods of high aspect ratio (similar to 25) were obtained exclusively leaving aside the embryonic spherical particles (5 +/- 2 nm) at lower surfactant concentration. This Article also focuses for the first time on the change in magnetic properties and cyclic voltammograms when we enroute nanospheres to nanorods. The catalytic activity of spherical and rod shaped particles has also been demonstrated.