Temperature induced modifications in shapes and crystal phases of MoO3 for enhanced photocatalytic degradation of dye waste water pollutants under UV irradiation

Rod shaped structures of molybdenum trioxide (MoO₃) were synthesized at low temperature using simple and low cost method. The synthesized materials were characterized by different characterization techniques. The effect of annealing temperature on crystal structure and optical properties of MoO₃ was investigated. The hexagonal phase (h-MoO₃) was observed up to 300 °C and beyond this temperature, the phase of the MoO₃ is changed to orthorhombic (α-MoO₃). The transition of crystal phase (h-MoO₃ to α-MoO₃) was investigated by analyzing the X-ray diffraction (XRD), Raman and DSC measurements. The shift in the wavenumber position of 971 cm⁻¹ (Mo–O stretching) peak with temperature also substantiates the phase transition in MoO₃ structure. The shape of nanostructured MoO₃ was modified with annealing temperature. The change in crystal phases and shapes with temperature may result defects in the structure, which essentially modify the structural and optical properties of the material. The photocatalytic properties of the synthesized materials were studied on methylene blue (MB) dye under UV-irradiation. The sample annealed at 400 °C (M4) show excellent photodegradation compared to other samples. It could be due to the combined effects of shape (helical) and defects (oxygen vacancies) present in M4, as supported by the electron paramagnetic resonance (EPR) measurements. Among the various radicals, OH plays an important role in photodegardation of the MB dye.