Interfacial charge transfer mechanism in bismuth oxide/tungsten oxide p-n heterojunction photocatalyst for methylene blue degradation

Abstract

In this study, tungsten trioxide (WO₃) photocatalysts were synthesized through a hydrothermal process, utilizing sodium tungstate (Na₂WO₄) as the starting material in the presence of polyvinylpyrrolidone (PVP) at different heat-treatment temperatures (180 °C, 190 °C, and 200 °C). The 190 °C sample exhibited a hybrid rod-sheet morphology, achieving the highest methylene blue (MB) degradation (47.6 % in 60 min under light). In order to improve the activity, the molar ratio of Bi₂O₃/WO₃ was adjusted, and the Bi₂O₃/WO₃ p-n heterojunction was constructed. The ratio critically governed both morphology and charge dynamics: increasing Bi₂O₃ content promoted the growth of spherical Bi₂O₃ particles on WO₃ rods/sheets, optimizing interfacial contact and light harvesting. When the molar ratio of Bi₂O₃/WO₃ was 60 %, the composite photocatalyst displayed the greatest photocatalytic property, the first-order reaction rate constant of 0.0100 min⁻¹, which was 3.6 times WO₃. The Mott-Schottky curve indicated that p-n heterojunctions were formed between Bi₂O₃ and WO₃, and the internal electric field (IEF) at the interface effectively promoted the separation of charge carriers. During illumination, photogenerated electrons in the conduction band (CB) of Bi₂O₃ migrate to the CB of WO₃, and holes in the valence band (VB) of WO₃ migrate to the VB of Bi₂O₃, enhancing the utilization of charge carriers.