Mesoporous TiO2 encapsulating a visible-light responsive upconversion agent for enhanced sonocatalytic degradation of bisphenol-A

Abstract

Herein, we report the integration of a visible-light active upconversion agent (Er:Y₂O₃) with mesoporous TiO₂ via a modified two-step sol–gel coating method. The resultant material possesses a well-defined core–shell structure with a good upconversion property and exhibits uniform worm-like mesopores (∼3.8 nm), a high BET surface area (∼151.5 m² g⁻¹) and a large pore volume (∼0.23 cm³ g⁻¹). The composite was demonstrated as an advanced sonocatalyst, showing a superior degradation performance for bisphenol-A (BPA). The effect of the erbium dopant content on the upconversion property and sonocatalytic performance was evaluated for the production of the best sonocatalyst. We found that the maximum pseudo first-order reaction rate constant in the presence of the composite with an Er³⁺ content of 3 wt% in Y₂O₃ nanocrystals is calculated to be 0.155 min⁻¹, which is 2.9 and 2 times larger than that of US alone (0.054 min⁻¹) and hollow structured mesoporous TiO₂ (0.077 min⁻¹), respectively. More importantly, the degradation rate is much higher than that of the sonocatalysts reported previously for treatment of BPA (0.09–0.14 min⁻¹). The superior catalytic activity can be attributed to an intensified cavitation reaction zone and an enhanced amount of photo-generated charges. Moreover, the recycling test shows that a constant catalytic activity is retained even after 4 cycles. This study paves a promising way for the development of a multi-functional catalyst for sonochemical processes.