Thermally activated structural transformations in manganese oxide nanoparticles under air and argon atmospheres

Abstract

The properties of manganese oxide nanomaterials are dictated by the structure and morphology of the particular phases they can adopt. The effects of post-synthesis heat treatments on amorphous monodisperse manganese oxide nanoparticles at ≤1300° under lab. air and argon atmospheres were investigated using X-ray diffraction, thermal analysis and electron microscopy techniques. During heat treatments in air, the nanoparticles undergo three transformations, resulting in: crystallization to cubic Mn 2 O 3 at ~500°, followed by the transformation to tetragonal Mn 3 O 4 at ~1010°, and to cubic Mn 3 O 4 at ~1190°. The first two transformations are irreversible, are associated with oxygen loss, and involve reductions. of the Mn ions. The latter transformation is polymorphic and spontaneously reversible, and so, tetragonal Mn 3 O 4 is observed at ambient temperature in samples heat-treated at >1000°. The samples heat treated in argon firstly crystallize to a mixt. of monoclinic Mn 2 O 3 and tetragonal Mn 3 O 4 at ~475°, followed by complete transformation to tetragonal Mn 3 O 4 at ~820°, and then to mostly cubic MnO at ~1145°; here again, the residual tetragonal Mn 3 O 4 undergoes a reversible polymorphic transformation at ~1180°, whereas the other transformations are irreversible. In both atmospheres, the amorphous material exhibits short-range cryptomelane-type order with a mixture of Mn 3+ and Mn 4+ prior to crystallization. Furthermore, these data indicate that most of the stable manganese oxide phases can be obtained from initially amorphous nanoparticles by heat treatment under appropriate conditions.