Tungsten-based Lindqvist and Keggin type polyoxometalates as efficient photocatalysts for degradation of toxic chemical dyes

Abstract

Photocatalytic dye degradation employing polyoxometalates (POMs) has been a research focus for several years. We report the facile synthesis of tungsten-based Lindqvist and Keggin-type POMs that degrade toxic chemical dyes, methyl orange (MO) and methylene blue (MB), respectively. The Lindqvist POM, sodium hexatungstate, Na2W6O19, degrades MO under 100�W UV light irradiation within 15�min, whereas the Keggin POM, Ag4PW11VO40, degrades MB under 20�W visible light source within 180�min. The effect of various operating parameters, such as photocatalyst concentration, pH, time, and initial dye concentration, were assessed in the degradation of both dyes. The photoelectrochemical performance of the as-synthesized polyoxometalates shows that the Ag4PW11VO40 shows 2.4 times higher photocurrent density than Na2W6O19 at a potential of 0.9�V vs. Ag/AgCl. Electrochemical impedance analysis reveals that Ag4PW11VO40 exhibits much lower charge transfer resistance as compared to Na2W6O19, which indicates facile charge transfer at the electrode-electrolyte interface. Further Mott-Schottky measurements reveal that both the catalysts possess n-type semiconductivity and the charge carrier concentration of Ag4PW11VO40 (5.89�נ1019�cm?3) is 1.4 times higher as compared to Na2W6O19 (4.25�נ1019�cm?3). This work offers a new paradigm for designing polyoxometalates suitable for efficient photocatalytic degradation of organic dyes.