Visible-light sensitive cobalt-doped BiVO4 (Co-BiVO4) photocatalytic composites for the degradation of methylene blue dye in dilute aqueous solutions

A series of visible-light sensitive Co-BiVO4 photocatalysts were synthesized by heteronuclear complexing method using diethylenetriamine pentaacetic acid (DTPA) as the chelating agent. The photocatalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), UV–vis diffuse reflectance spectroscopy (DRS), and Raman spectroscopy. Results indicated that all Co-BiVO4 photocatalysts had a crystal structure of monoclinic scheelite. Loading BiVO4 with cobalt did not alter the crystal structure of the composites. Cobalt was present as oxides and was deposited on the surface of larger BiVO4 particles. The oxidation state of cobalt varied with its content in Co-BiVO4. The photocatalytic activity of Co-BiVO4 was studied by the decolorization of methylene blue (MB). BiVO4 containing 5% (molar wt) of cobalt exhibited the greatest photocatalytic activity with a 85% of MB removal versus 65% by pure BiVO4 in 5 h. Factors such as pH, initial MB concentration, electrolytes, and irradiation conditions that may affect the photodegradation of methylene blue were studied. High pH and low initial MB concentration resulted in fast photocatalytic reaction. Electrolytes, especially those capable of scavenging hydroxyls, can inhibit MB degradation. The stability of the photocatalysts was confirmed using reclaimed Co-BiVO4 in three successive runs. There was no loss of photocatalytic ability in three successive runs each of which lasted for 6 h with BM removal remained high at >90%. Results demonstrated clearly that Co-BiVO4 was stable and resistant to photocorrosion during the photocatalytic oxidation of organic compounds such as methylene blue.