Size-dependent Lattice Contraction and Oxidation State Ratio in Nano-MnO: Potential Tunable Biomedical Applications

Abstract

<img src=” https://s3.amazonaws.com/production.scholastica/article/56898/large/prnano_852022_ga.jpg?1670345556”> Manganosite (MnO) nanocrystals ranging from 22 to 36 nm have been prepared by reducing hausmannite (Mn<sub>3</sub>O<sub>4</sub>) nanocrystals with hexamethylenetetramine (C<sub>6</sub>H<sub>12</sub>N<sub>4</sub>) in a heated, H<sub>2</sub>/N<sub>2</sub> gaseous environment. X-ray Diffraction analysis indicates that the lattice parameter decreases by up to 0.18% (4.4379 Å) as the crystalline diameter decreases to 23 nm. X-ray Absorption Near Edge Spectroscopy demonstrates increasing Mn<sup>3+</sup> fraction from 8.9% for 36 nm diameter crystallites to 14.5% for 23 nm diameter crystallites. Thus, the lattice contraction could be due in part to the decrease of the relative cation radii from Mn<sup>2+</sup> to Mn<sup>3+</sup>. Yet, straight calculations of lattice parameter from the Mn<sup>3+</sup> concentrations yield a 10x larger lattice contraction. We suspect the magnetic properties of the nano-MnO plays a role since most oxides show a lattice expansion due to surface adsorbents. Though it is expected that the band gap should increase with decreasing size, the increasing concentration of Mn<sup>3+</sup> would result in states inside the bandgap, potentially giving the appearance of a smaller bandgap. Further, the increased concentration of Mn<sup>3+</sup> has potential antioxidant properties due to the dual oxidation state as observed in nanocrystals of Mn<sub>3</sub>O<sub>4</sub> and CeO<sub>2</sub> in other studies.