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.