Prof. Dr. Axel Müller

We report on the detailed structure of MnO_x nanoparticles (MnO_xNP) 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 (PXRD) leads to the conclusion that the MnO_x 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 MnO_xNP with an average diameter of about 5 nm. MnO_xNP immobilized on cationic spherical polyelectrolyte brush (SPB) 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 MnO_x 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 (XANES) spectra of different manganese oxide reference materials with the different MnO_xNP 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.