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

The synthesis of semiconducting nanocrystals of manganese oxide of controll ed sizes and their manipulation to form ordered arrays is described. Nanocr ystalline mixed-valent manganese oxides have been prepared as colloidal sol utions via reduction of tetraalkylammonium (methyl, ethyl, propyl, and buty l) permanganate salts in aqueous solutions with 2-butanol and ethanol. Redu ction with the poorly water miscible 2-butanol produces aqueous colloids fo r the methyl, ethyl, and propyl systems, whereas 2-butanol colloids are pro duced 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 manganes e concentrations up to 0.57 M. The sols will gel upon aging, and the gel ti me depends on the cation, the amount of alcohol. the temperature, and the c oncentration of manganese. Small angle neutron scattering (SANS) data indic ate that the particles are disklike in shape with radii in the range 20-80 Angstrom and are largely unassociated in solution. Thin films produced from evaporation of the colloid or spreading of the gel onto glass slides demon strate long-range order, yielding an X-ray diffraction (XRD) pattern consis tent with a structure of CdI2-type layers of manganese oxide with tetraalky lammonium cations and water molecules interspersed between the layers. The SANS and XRD evidence are consistent with a mechanism of self-assembly of u nassociated layers upon concentration and evaporation. Consistent with a me chanism of quantum confinement, UV/visible spectroscopy of the colloids rev eals 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 part icles in solution with a,aging, indicated by the shifts in absorbance, has been confirmed by SANS experiments.