Synthesis of a hydrogen producing nanocrystalline ZnFe2O4 visible light photocatalyst using a rapid microwave irradiation method

A rapid microwave solid-state synthesis method is systematically investigated to achieve a H-2 producing visible light active spinel photocatalyst. ZnFe2O4 nanocrystallites were obtained by microwave irradiation of precursor compacts under optimized conditions. This investigation led to a uniform sized nanocrystalline photocatalyst that yielded a quantum-yield of H-2 evolution similar to 3.8 times higher than that of conventionally synthesized ZnFe2O4. The synthesis parameters - microwave power, synthesis temperature, and time, were found to control the physico-chemical properties viz phase formation kinetics, phase purity, crystallinity, specific surface area and photochemical efficiency, of the synthesized photocatalyst. The study reveals that the threshold microwave power of >= 3 kW was necessary to obtain a spinel phase structure, while lower power (<3 kW) could not induce the crystallization even after prolonged low-power irradiation of 180 min. At the threshold power, a minimum of 10 min. synthesis time was enough to obtain uniform sized nanocrystallites, indicating that the synthesis method is similar to 24 times faster than the solid state reaction method, which needs nearly 4 h. The particle morphology evolution with irradiation time from 10-150 min. exhibited de-crystallization phenomena. Longer irradiation displayed a morphological crystallization probably induced due to the simultaneous area and volumetric heating effect. The possible "formation mechanism'' of these uniform nanocrystallites has been presented here for qualitative understanding. Thus synthesized photocatalysts generated hydrogen from a water-methanol mixture even without the co-catalyst loading. The ferrite photocatalyst was found to decolorize methylene blue dye with a maximum decay constant of 0.232 h(-1), thereby demonstrating its capability in the pollutant decomposition applications, all under visible light photons.