Unconventional structural and morphological transitions of nanosheets, nanoflakes and nanorods of AuNP@MnO2

Abstract

Two-dimensional (2-D) layered inorganic materials with ultra-high surface area and mechanical strength have shown impressive photo-/electro-catalytic activities. We herein report a facile synthetic strategy to grow monodispersed 2-D MnO₂ nanosheet on an individual gold nanoparticle (AuNP@MnO₂ nanosheet), and demonstrate that the strongly interacted AuNP and MnO₂ nanosheet could greatly improve the electrocatalytic activity of the MnO_x family for electrocatalytic oxygen reduction reactions (ORRs). AuNP@MnO₂ nanosheets were prepared using a hydrothermal reduction of KMnO₄ by citrate ligands capped on AuNPs. Because of the metastability of the layered MnO₂ nanosheets, we observed unconventional structural and morphological transitions of amorphous MnO₂ nanosheets to δ-MnO₂ nanoflakes, and eventually to α-MnO₂ nanorods under hydrothermal conditions. The layered MnO₂ nanosheets underwent a structural expansion to nanoflakes before the curling and re-folding of layered MnO₂ nanosheets occurred. The intermediate states and structural transitions via a “layer compression”, for the first time, were experimentally recorded at a single-NP scale using electron microscopy. Moreover, we found the electrocatalytic activity of AuNP@MnO₂ nanosheets was enhanced roughly 30–40 times, compared to that of pure MnO₂ nanosheets and AuNPs. The strong interaction of metal–oxide interfaces (MnO₂ nanosheets and AuNPs) was likely responsible for the improved electrocatalytic activity. This interaction of MnO₂ and AuNPs was weakened in the course of hydrothermal treatment where partially positively charged Au⁺ was reduced at elevated temperatures, accompanying with the decrease of ORR activity. This insight into the effect of topological nanostructures and metal–oxide interactions on the electrocatalytic performance of the MnO_x family is believed to illustrate an alternative pathway to develop new efficient electrocatalysts.