Synthesis of Manganese Oxide Nanoparticles in Premixed Flames

Abstract

Synthesis of manganese oxide nanoparticles was carried out in premixed stagnation flames. The deposition surface was stationary to enable rigorous comparison to flame structure computations using pseudo one-dimensional and full two-dimensional calculations. The pseudo one-dimensional assumption taken by OPPDIF to calculate the flame structure performs reasonably well for the narrow aspect ratio stagnation flow currently studied. Agreement between the measured flame position and both computational methods was within 0.25 cm. The variation in manganese oxidation state in the products observed here is shown to be caused by competing oxidation and particle nucleation processes. Complimentary experimental and modeling studies of flame synthesis were carried out under well-defined boundary conditions to examine this competition. Manganese oxide products having II, II,III, III and IV valence were observed depending on the flame conditions. Unlike iron oxide and titanium oxide systems, complete oxidation to MnO2 is only observed for the most oxidizing growth conditions. The first observation of phase pure MnO by flame synthesis is reported here. Manipulation of the balance between manganese oxidation and particle nucleation through control of the time-temperature-oxygen history may enable selective synthesis of manganese oxide nanoparticles with tailored morphology and oxidation state.