Black TiO2 Nanotube Array/BiVO4 Heterojunction Photocatalysts for Tetracycline Removal with High Solution Detoxification Efficiency

Pulsed laser deposition was employed to accurately vary the BiVO4 content on the outermost surface of black TiO2 (B-TiO2) nanotube arrays. The optimal B-TiO2(225) led to >80% tetracycline (TC) antibiotic removal, while only 27% TC removal was obtained by bare TiO2 nanotubes after 300 min of photocatalytic reaction under visible-light irradiation. Moreover, the removal rate constant using the optimal heterojunction sample was 0.005 min–1, which was more than 5 times higher than that of bare TiO2 nanotubes. Such an improvement in the performance was attributed to the one-dimensional structure, enhanced light absorption of B-TiO2, formation of a thermodynamically favorable type II heterojunction, and deposition of BiVO4 on the top surface of B-TiO2, which increases the light utilization in front illumination. On the basis of the identification of intermediates and byproducts via high-resolution tandem mass spectrometry analysis and the Fukui index, it was revealed that demethylation and oxidation were the main degradation pathways. The toxicity change after treatment was assessed by using the optical density method and the Ecological Structure Activity Relationship (ECOSAR) program. The results indicate that the mass ratio between B-TiO2 and BiVO4 is a critical parameter for determining the photocatalytic activity, in terms of not only the TC degradation efficiency and kinetics but also the type and quantity of byproducts and, hence, the toxicity of the treated solution.