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 × 10¹⁹ cm^-3) is 1.4 times higher as compared to Na₂W₆O₁₉ (4.25 × 10¹⁹ cm^-3). This work offers a new paradigm for designing polyoxometalates suitable for efficient photocatalytic degradation of organic dyes.

Keywords: Dye degradation; Methyl orange; Methylene blue; Photocatalysis; Photoelectrochemical response; Polyoxometalate.