Preparation of h-MoO3 and α-MoO3nanocrystals: comparative study on photocatalytic degradation of methylene blue under visible light irradiation

Abstract

A detailed study on visible light photocatalytic degradation of methylene blue (MB) has been investigated in aqueous heterogeneous media containing hexagonal phase molybdenum oxide (h-MoO₃) nanocrystals (NCs) which was identified as a new material for visible light driven photocatalysis. A simple and template-free solution based chemical precipitation method was employed to synthesize h-MoO₃ NCs by reacting ammonium heptamolybdate tetrahydrate (AHM) with nitric acid. The formation and growth mechanism of h-MoO₃ microstructures was explained. In addition, by annealing the h-MoO₃ sample, the phase stability of hexagonal was retained up to 410 °C and showed an irreversible phase transition from hexagonal (h-MoO₃) to highly stable orthorhombic phase (α-MoO₃). Finally, the photocatalytic activities of h-MoO₃ and α-MoO₃ samples were evaluated using the degradation of MB, representing an organic pollutant of dye wastewater. The effects of various experimental parameters such as catalyst loading, initial dye concentration, light intensity, and operating temperature were analyzed for the degradation of MB. The results demonstrated that the efficiency of visible light assisted MB degradation using h-MoO₃ NCs can be effectively enhanced by catalyst loading, light intensity, and operating temperature. However, the efficiency declined with the increase in initial dye concentration. Optimum conditions for higher photocatalytic performance were recognized as a catalyst loading of 100 mg L⁻¹, a dye concentration of 12 mg L⁻¹, a light intensity of 350 mW cm⁻², and an operating temperature of 45 °C.