δ-MnO₂-Mn₃O₄ Nanocomposite for Photochemical Water Oxidation: Active Structure Stabilized in the Interface

Pure phase manganese oxides have been widely studied as water oxidation catalysts, but further improvement of their activities is much challenging. Herein, we report an effective method to improve the water oxidation activity by fabricating a nanocomposite of Mn₃O₄ and δ-MnO₂ with an active interface. The nanocomposite was achieved by a partial reduction approach which induced an in situ growth of Mn₃O₄ nanoparticles from the surface of δ-MnO₂ nanosheets. The optimum composition was determined to be 38% Mn₃O₄ and 62% δ-MnO₂ as confirmed by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The δ-MnO₂-Mn₃O₄ nanocomposite is a highly active water oxidation catalyst with a turnover frequency (TOF) of 0.93 s^-1, which is much higher than the individual components of δ-MnO₂ and Mn₃O₄. We consider that the enhanced water oxidation activity could be explained by the active interface between two components. At the phase interface, weak Mn-O bonds are introduced by lattice disorder in the transition of hausmannite phase to birnessite phase, which provides active sites for water oxidation catalysis. Our study illustrates a new view to improve water oxidation activity of manganese oxides.