Coupled Morphological and Structural Evolution of δ-MnO2 to α-MnO2 through Multistage Oriented Assembly Processes: the Role of Mn(III)

Abstract

α-MnO2 is a typical tunneled Mn oxide (TMOs) which frequently associated with δ-MnO2 in the environment exhibits strong adsorption and oxidation activity. The morphology of α-MnO2 which controlled by oriented attachment process (OA) is one of the key factors affecting its reactivity. However, the detailed crystal growth process and coupling between morphology and structure of α-MnO2 during OA processes remain poorly understood. We propose that the transformation of the layer-based δ-MnO2 to tunnel-based α-MnO2 occurs via a multistage OA process. In the initial stage, the produced δ-MnO2 nanoflakes are found to spontaneously self-assemble into a nanoribbon with a large number of lattice defects via edge-to-edge OA. The presence of lattice defects promotes the generation of oxygen vacancies, and the Mn(IV) ions in the [MnO6] octahedral layers of δ-MnO2 is reduced to Mn(III)/Mn(II). The reduced ions subsequently migrate from the [MnO6] octahedral layers to the interlayers during this process. Driven by hydrogen bonding between hydroxide which acts in coordination with the interlayer Mn(III)/Mn(II) and oxygen atoms coordination with adjacent nanoribbons attach to each other and form primary nanorods through a face-to-face OA mechanism along the c-axis. Concomitantly, the bonding of [Mn(III)O6] octahedra in the interlayer of the nanoribbons with adjacent [MnO6] octahedral layers lead to the fabrication of a new 2 * 2 α-MnO2 tunnel structure from the original δ-MnO2. These findings provide insights into both the transformation mechanisms of the layer-based to the tunnel-based nanoparticles and methods for efficient and controlled synthesis of nanomaterials.