Structural analysis of colloidal MnO x composites

Abstract

We report on the detailed structure of MnOx nanoparticles (MnOxNP) which are either stabilized by cationic spherical polyelectrolyte brushes or by star-shaped cationic polyelectrolyte chains. In both cases, the polycation is composed of 2-(trimethylammonium)ethyl methacrylate chloride (TMAEMC). The analysis by transmission electron microscopy (TEM), cryogenic transmission electron microscopy (cryoTEM), and powder X-ray diffraction leads to the conclusion that the MnOx nanoparticles in aqueous dispersed state are composed of only a few or even single lamellae of c-disordered potassium birnessite (birnessite). Using star-shaped pTMAEMC homopolymer for the synthesis of composite particles, we obtain MnOxNP with an average diameter of about 5 nm. MnOxNP immobilized on cationic spherical polyelectrolyte brush have a length of about 20 nm and a width of 1.6 nm. Comparison of the extended X-ray absorption fine structure (EXAFS) spectra of the MnOx composites with reference spectra leads to the conclusion that all materials include c-disordered birnessite-type nanoparticles. A comparison of the energy shift of the Mn K-edge absorption peak of the X-ray absorption near-edge structure spectra of different manganese oxide reference materials with the different MnOxNP revealed an average oxidation state of about 3.5–3.7 for synthesized compounds. No distinct structural difference is found when comparing the dried samples to samples dispersed in water. A comparison of the EXAFS data of the birnessite nanoparticles with the crystal structure of macroscopic systems showed a compression along the c direction accompanied by a slight elongation within the ab plane of the layered material.