Regulating of MnO₂ photocatalytic activity in degradation of organic dyes by polymorphic engineering

Different phase nanostructured MnO2, including alpha-, beta-, gamma-, and delta-MnO2, were prepared by a hydrothermal route. Their photocatalytic activities in the degradation of cationic (methylene blue) and anionic (methyl orange) dyes were compared for the first time. The as-prepared oxides were characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption-desorption techniques. It was shown that the highest efficiency was achieved in the degradation of cationic methylene in the presence of the delta-MnO2 photocatalyst (conversion of methylene blue was 90.4% after 180 min). These results are explained by unique physicochemical properties of the delta-MnO2 polymorph. It was observed that there is a synergetic effect between various physicochemical properties of delta-MnO2 which allows to obtain the high photodegradation efficiency: high adsorption ability towards methylene blue, unique 2D morphology, lower Mn-O bond strength, and high surface Mn4+/Mn3+ molar ratio. The reaction mechanism as well as the main products of the methylene blue photooxidation process over delta-MnO2 were also investigated.