Tailoring photocatalytic performance through EDTA-2Na coordination: Competition of electron transfer to conduction band and valence band in bismuth-based compounds

Abstract

The effect of EDTA-2Na on the photocatalytic performance of bismuth-based compounds varied with their chemical compositions and EDTA dosage. For Bi₂O₃ and Bi₅O₇I with high Bi/anions ratios, coordinating with EDTA-2Na at molar ratios of 1.5:1 for Bi₂O₃ and 5:1 for Bi₅O₇I significantly improved photocatalytic activity of methyl violet (MV, 50 mg·L^-1) degradation under visible irradiation, with apparent rate constants (k) of 0.0715 min⁻¹ and 0.0261 min⁻¹, which were 102 and 44 times of those for pure Bi₂O₃ and Bi₅O₇I, respectively. Besides, these samples also exhibited enhanced adsorption performance to cation dye MV due to decreased zero zeta potential. However, If the molar ratio of EDTA to Bi₂O₃ exceeded 4:1 and the molar ratio of EDTA to Bi₅O₇I exceeded 5:1, the samples possessed the poor photocatalytic performance for the degradation of MV, even inferior to pure Bi₂O₃ and Bi₅O₇I. Conversely, BiOX (X = Br, I) and Bi₂O₂CO₃ with low Bi/anion ratios didn’t show significant improvement in photocatalytic activity, and BiVO₄ and Bi₂MoO₆ with lower Bi/anion ratios exhibited even worse photocatalytic performance after coordinating with EDTA-2Na. Photocatalytic mechanism suggests that EDTA-2Na’s electron transfer capabilities to the conduction band of high Bi/anion ratio compounds promote reactive oxygen species generation, enhancing their photocatalytic performance. For the low Bi/anion ratio compounds, the electrons of EDTA would transit to VB and trap the holes, hindering the hole-dominant photo catalysis process.