Phase and Morphology Transformation of MnO2 Induced by Ionic Liquids toward Efficient Water Oxidation

Water oxidation through the Mn4Ca-oxo complex in photosystem II has fascinated many researchers because of its high efficiency and low energy input; therefore, it has triggered great interest in various polymorphs of manganese oxides for electrocatalysis. Herein we report a facial ionic liquid (IL)-assisted [IL: 1-butyl-3-methylimidazohum tetrafluoroborate (BMIM-BF4)] hydrothermal approach for tuning both crystallographic phase and nanostructure morphology of MnO2, enabling the excellent oxygen evolution reaction (OER) activity with an overpotential of 394 mV at 10 mA cm(-2) and a Tafel slope of 49 mV dec(-1). The roles of IL in the crystallographic and morphological transformation from beta-MnO2 nanorods to alpha-MnO2 nanowires and in the OER are carefully scrutinized. TEM, EDX, FTIR, XPS characterizations all reveal the capping of IL cations on the surface of alpha-MnO2, where the amphiphilic nature, the electrostatic interaction, the steric hindrance, and the pi-pi stacking of IL cations collectively serve as entropic drivers for the templated growth of 2 X 2 tunnel structure incorporating K+ ions. This structure has been particularly beneficial for OER, owing to a concerted synergy from the nanostructured morphology, suitable tunnel structure with rich di-mu-oxo bridges, alkali-metal incorporation, as well as higher content of trivalent Mn3+. What's more, our investigation indicates the surface-immobilized IL plays a crucial role toward efficient OER by facilitating the formation and stabilization of oxygen vacancies on the surface of alpha-MnO2 nanowires.