Synthesis and characterization of birnessite and cryptomelane nanostructures in presence of hoffmeister anions

Abstract

The effect of Hoffmeister anions Cl , SO 4 2-, and ClO 4 - on the structure and morphology of birnessite and cryptomelane-type manganese dioxide nanostructures, produced by the reduction reaction of KMnO 4 and MnSO 4 in aqueous acidic media, was studied. The syntheses were based on the decomposition of aqueous KMnO 4 in presence of HCl for birnessite-type and acidified MnSO 4 for cryptomelane-type manganese dioxide under soft hydrothermal conditions. They were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) techniques. XRD patterns show the formation of birnessite for the first synthesis and a mixture of cryptomelane and birnessite-types MnO 2 for the second synthesis. XRD data revealed that the Hoffmeister anions have a significant effect on the nanostructures of birnessite. The sulphate ion-treated birnessite has the smallest crystals, whereas the chloride ion-treated birnessite has the largest crystals. Their TEM and HRTEM studies revealed a transformation from nanoplatelet morphology for chloride-treated samples to nanofibrous morphology for sulphate-treated birnessite. For the cryptomelane nanostructures, Hoffmeister anions also show a profound effect on their crystalline structures as determined by XRD analyses revealing a transformation of the cryptomelane phase to birnessite phase of MnO 2. This transformation is also supported by TEM and HRTEM studies. Copyright © 2009 Marcos A. Cheney et al.