Particle size control and self-assembly processes in novel colloids of nanocrystalline manganese oxide

The synthesis of semiconducting nanocrystals of manganese oxide of controlled sizes and their manipulation to form ordered arrays is described. Nanocrystalline mixed-valent manganese oxides have been prepared as colloidal solutions via reduction of tetraalkylammonium (methyl, ethyl, propyl, and butyl) permanganate salts in aqueous solutions with 2-butanol and ethanol. Reduction with the poorly water miscible 2-butanol produces aqueous colloids for the methyl, ethyl, and propyl systems, whereas 2-butanol colloids are produced for the butyl system. The colloids are reddish-brown, have an average manganese oxidation state of 3.70-3.79, and have been prepared in manganese concentrations up to 0.57 M. The sols will gel upon aging, and the gel time depends on the cation, the amount of alcohol, the temperature, and the concentration of manganese. Small angle neutron scattering (SANS) data indicate that the particles are disklike in shape with radii in the range 20-80 A and are largely unassociated in solution. Thin films produced from evaporation of the colloid or spreading of the gel onto glass slides demonstrate long-range order, yielding an X-ray diffraction (XRD) pattern consistent with a structure of Cdl₂-type layers of manganese oxide with tetraalkylammonium cations and water molecules interspersed between the layers. The SANS and XRD evidence are consistent with a mechanism of self-assembly of unassociated layers upon concentration and evaporation. Consistent with a mechanism of quantum confinement, UV/visible spectroscopy of the colloids reveals two absorbances, one near 220 nm and the other in the range 290-310 nm, blue-shifted from the maxima observed for bulk manganese oxide (400 nm). When the samples are aged, these bands shift to the red. The growth of particles in solution with aging, indicated by the shifts in absorbance, has been confirmed by SANS experiments.