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.