Efficient visible light photodegradation of BiVO4:Yb3+/Tm3+ with high content of tetragonal phase

In photocatalytic studies, tetragonal bismuth vanadate (t-BiVO₄) is not promising due to its wide band gap. However, according to previous studies, the tetragonal phase is inevitable when the monoclinic bismuth vanadate (m-BiVO₄) is modified. Therefore, it is necessary to find ways to improve the photoresponse and photocatalytic ability of t-BiVO₄ under visible light. In this study, Yb³⁺ and Tm³⁺ co-doped BiVO₄ was synthesized by a simple hydrothermal method, and its microstructure, morphology and optical properties were characterized and analyzed by scanning electron microscope, transmission electron microscopy, Brunauer-Emmett-Teller, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, diffuse reflectance spectra, photoluminescence, upconversion luminescence and other means. The results show that BiVO₄:Yb³⁺/Tm³⁺ (BVYT) has a high content of tetragonal phase (about 80%), showing the characteristics of t-BiVO₄. BVYT shows a higher separation efficiency of photogenerated carriers, its transient photocurrent response intensity increased by about 3 times, and the photocatalytic efficiency is significantly improved compare with the undoped m-BiVO₄. Under simulated sunlight, BVYT completely degraded methylene blue (MB) solution and rhodamine B solution in 45 and 90 min, respectively, and the reaction rate was significantly improved. BVYT also shows excellent photocatalytic ability under visible light, about 35% of MB solution was degraded within 45 min under visible light irradiation (> 420 nm), this is because Yb³⁺ effectively promotes the upconversion luminescence of Tm³⁺ in response to visible light, and the energy cycle mechanism of Yb-Tm-Tm is proposed. Consequently, BiVO₄ with high content of tetragonal phase has excellent photoactivity, even exceeding m-BiVO₄. This is a novel discovery in the field of photocatalysis, which provides a broader application prospect for BiVO₄ in photocatalysis.