Photocatalytic properties of polyoxometalate–thionine composite films immobilized onto microspheres under sunlight irradiation

Multilayer films (PW12–TH)n (PW12 = PW12O403−, TH = thionine) were prepared on quartz slides and MnCO3 microspheres by an electrostatic layer-by-layer (LbL) self-assembly method. The composite films on quartz slides were characterized by UV-vis spectra and atomic force microscopy (AFM) images. MnCO3 microspheres coated with (PW12–TH)n composite films were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. The microspheres exhibited photocatalytic activity for methyl orange (MO) under sunlight irradiation. Compared with the (PW12–TH)10 composite films on quartz slides, the microspheres showed a higher photoactivity due to the increase of the specific surface area. The photocatalytic properties of the microspheres toward a rhodamine B (RhB) solution were also investigated. The results indicated that the decrease of the absorbance was due to the destruction of the dye chromophore. The kinetics of the photodecomposition follows the first-order reaction. The dye molecule TH of the composite films is excited by sunlight irradiation to the appropriate excited states. Then, the electron injection into the conduction band of polyoxometalates (POMs) follows and results in the photodegradation of the target dye molecules.