Energy-efficient fabrication of a novel multivalence Mn3O4-MnO2 heterojunction for dye degradation under visible light irradiation

• A novel Mn3O4-MnO2 valence state heterojunction was first fabricated.
• The visible light driven Mn3O4-MnO2 exhibited high photocatalytic capacity.
• The type II heterojunction accelerated the separation of photo-carriers.
• The superoxide radicals played the major role in dye degradation.
• The surface oxygen took part in the formation of superoxide radicals.

A novel Mn3O4-MnO2 valence state heterojunction was fabricated for the first time by a low-temperature hydrothermal method and further applied for methylene blue (MB) degradation under visible light irradiation at room temperature. The Mn3O4 core with tetragonal hausmannite structure was coated by a thin amorphous MnO2 shell. The specific surface area and light-response range of Mn3O4-MnO2 photocatalyst were significantly enhanced by doping MnO2. Noticeably, the Mn3O4-MnO2 exhibited excellent photocatalytic activity with the degradation efficiency of MB up to 93.5% within 1 h. Moreover, the mechanism for MB photocatalytic degradation by Mn3O4-MnO2 was also explored. The superoxide radicals made a major contribution to the photo-degradation process, followed by the photo-induced holes and hydroxyl radicals. The surface oxygen of Mn3O4-MnO2 also took part in the formation of superoxide radicals, which could be replenished by atmospheric oxygen. More importantly, the formation of type II heterojunction in Mn3O4-MnO2 composite could accelerate the spatial separation of photo-induced carriers. This novel multivalence Mn3O4-MnO2 core-shell composite photocatalyst proved to have great potentials for organic contaminants degradation in water.

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