Mn3O4/graphene nanocomposites: outstanding performances as highly efficient photocatalysts and microwave absorbers

Mn3O4 (M) incorporated graphenes (G) synthesized by a deposition-solvothermal process, formed at various nominal weight percentages (G1M1, G3M1 and G1M3), were efficiently used for the photodegradation of methylene blue dye (MB) under visible light illumination (lambda > 420 nm, 88 W, 20 ppm, 298 K) and under microwave irradiation (800 W, 2.45 GHz, 373 K). These materials were characterized using XRD, TEM-SAED, UV-Vis diffuse reflectance, N-2 sorptiometry, FTIR and Raman techniques. Amongst the nanocomposites, G3M1 of polyhedral structure and an average domain equal to 10-12 nm has presented unique photo-degradation performance (100% degradation, 60 min, 0.0791 min(-1) and TOC of 60%) exceeding the rest of the materials. This was mainly due to the extraordinary optical properties and to the strong interaction between Mn3O4 and graphene through which charge recombination is hampered. Based on the conduction and valence band edges together with the studied reactive species, it has been shown that (OH)-O-center dot was the dominant species responsible for the MB degradation. Interestingly, the G3M1 nanocomposite has shown fascinating microwave absorption properties and is capable of degrading MB at a faster rate (0.287 min(-1)) than the one conducted via photocatalysis. Scavenger studies have shown that (OH)-O-center dot and electrons were responsible for the excellent performance of the MB microwave degradation. The microwave results were discussed in view of the marked increase in dielectric constant (epsilon(-)) and dielectric loss (epsilon '') in the studied frequency range of 1.0 Hz to 100 kHz, in addition to the electronic conductivity measurements. This work offers an exceptional approach for exploring high-performance microwave absorption as well as distinctive visible light photocatalytic reaction for organics degradation.