Mechanism of coupled phase/morphology transformation of 2D manganese oxides through Fe galvanic exchange reaction

Abstract

Nanostructured manganese oxides have a rich variety of morphologies and crystal phases which can undergo transformations during synthesis and application. Although these structural features are crucial for their performance, the mechanisms behind such transitions are not well understood. Herein, we describe the mechanism of transformation from layered 2D δ-MnO₂ nanosheets to the scarcely reported γ-MnO₂ nanocone morphology. Despite the common purpose of introducing Fe dopants to enhance the conductivity of layered manganese oxides, the Fe galvanic exchange reaction was found responsible for such coupled phase/morphology transition. Electrochemical characterization confirmed a distinct electrochemical behaviour of the nanocones, emphasizing the need to unravel the mechanism of 2D MnO₂ transformation. Such mechanistic insights were gained by systematic and rigorous electron microscopy studies. The effect of the local chemical composition was determined by energy dispersive X-ray spectroscopy while electron energy loss spectroscopy unravelled the key influence of the oxidation state of Mn ions within nanosheets and nanocones. We propose and demonstrate a Mn²⁺-mediated oxidative mechanism of coupled morphology/phase transformation subjected to the equilibrium of Fe and Mn ions during galvanic exchange reaction. These findings contribute to the understanding of the growth and morphology/phase transformations of manganese oxide nanostructures, providing insights for the rational design of nanomaterials.