Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes

A new route of in situ formation and stabilization of ultrathin, needle-like manganese dioxide nanoparticles (MnO₂NP) in aqueous solution by using spherical polyelectrolyte brush particles (SPB) is presented. The SPB that act as carrier particles consist of a solid polystyrene core of about 50 nm radius onto which long chains of the positively charged polyelectrolyte poly(trimethyl ammonium ethyl methacrylate chloride) (pTMAEMC) are grafted to yield an overall radius of about 85 nm. Potassium permanganate (KMnO₄) is directly reduced within the brush layer of these particles because of the basic environment within this layer. This mechanism appears to limit the size of the MnO₂NP to the dimensions of the brush layer. Powder X-ray diffraction, (PXRD), transmission electron microscopy (TEM), and cryogenic transmission electron microscopy (cryoTEM) prove that birnessite-type MnO₂NP with a c*-disorder are generated on the SPB without adding any reducing agent. The birnessite nanoparticles have an average length of 20 nm and a breadth of about 1.6 nm. They are composed of single lamellae or of ultrathin stacks of very few lamellae. Energy-dispersive X-ray spectroscopy (EDX) demonstrates that most of the charges of the thin birnessite platelets are balanced by potassium ions. The excellent stabilization by the SPB carrier particles in aqueous solution can be traced back to a strong interaction of the birnessite particles with the positively charged pTMAEMC chains of the SPB.