Oxygen Vacancy Formation Energy Determines the Phase-Activity Relationship of MnO2 Laccase Nanozymes

Although manganese dioxide (MnO2) has been explored as a powerful laccase nanozyme for pollutant oxidation in wastewater treatment, the phase-activity relationship of multiphase MnO2 remains ambiguous and controversial. Herein, the experimental studies show that the laccase-like activities and aerobic catalytic oxidation toward tetracycline antibiotics of the six types of MnO2 are in the following order: beta- > lambda- > gamma- > alpha- > epsilon- > delta-MnO2. Density functional theory (DFT) calculations revealed that the catalytic activities are inversely proportional to the oxygen vacancy formation energies of the different MnO2 materials. Further investigation of surface oxygen species with reactivity demonstrated that rich oxygen vacancies boost the oxygen mobility and catalytic efficiency of MnO2 nanozymes, which is in good agreement with both experimental and DFT results. Hence, this study reveals the decisive role of the crystal phase in the oxygen vacancy generation, which elucidates the laccase-like catalytic mechanism of MnO2 nanozymes and is valuable for the future design and synthesis of MnO2 nanocatalysts.