Enhanced photo-induced charge generation of floating sulfur-doped TiO2 double-walled nanotubes for efficient sunlight-driven photocatalytic degradation of bisphenol A

Floating photocatalysts are an innovative class of immobilized semiconductors that can address the limitations of traditional photocatalysis by directly harvesting light, enhancing oxygen transfer from the air, and enabling easy photocatalyst recovery. In the present study, we developed a novel ultrasound-assisted solvothermal method to synthesize sulfur-doped TiO2 nanotubes (S-TNT). The floating photocatalysts were prepared by immobilizing S-TNT onto buoyant polyurethane foam (PUF) with an open-cell structure using a wet chemical deposition method. The synthesis yielded double-walled TNT doped with 3 nm sulfur nanodots. The ultrasound-assisted solvothermal synthesis method using thiourea as a sulfur precursor resulted in the incorporation of anionic sulfur (S2−) and cationic sulfur (S4+, S6+) into TNT. Among different sulfur dopants (0.5–2 wt%), 1 wt% (S-TNT-1) was optimal to achieve superior optical properties, enhanced photo-induced charge generation, and effective electron-hole pair separation. Compared to bare TNT, the specific surface area of S-TNT-1 increased from 110 to 267 m2/g, while the bandgap energy decreased from 3 to 2.48 eV. The photocurrent density of S-TNT-1 (0.6 mA/cm2) was threefold higher than that of TNT. Evaluating operating parameters indicated that >96 % of 10 mg/L Bisphenol A (BPA) was degraded with 0.2 g S-TNT-1@PUF at pH = 8 after 75 min of simulated sunlight irradiation. This degradation followed first-order kinetics, with an apparent rate constant of 0.05 min−1, in which •OH was the reactive species that contributed most (31 %).