N-doped TiO2 nanotubes synthesized by atomic layer deposition for acetaminophen degradation

Titanium dioxide (TiO2) is widely used in photocatalysis applications for wastewater treatment. Investigations demonstrated that titanium dioxide structure strongly contributes to increase micropollutant degradation efficiency. Indeed, pollutant degradation rates are enhanced when using photocatalysts with highly ordered structures and high specific surface area, such as TiO2 nanotubes (NTs) synthesized by atomic layer deposition (ALD). Here, TiO2 NTs, fabricated by ALD followed by nitrogen doping via thermal treatment, were compared with TiO2 nanofibers (NFs), prepared by electrospinning. Their morphology and structure were investigated by scanning and transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Photoluminescence measurements showed that TiO2 NT photoluminescence intensity was lower than that of TiO2 NFs, due to lower electron-hole pair recombination, and consequently their degradation efficiency was higher. As the surface to volume ratio was higher in TiO2 NTs than NFs, the rate of non-radiative surface recombination also was higher in TiO2 NTs. Comparison of their performance for photocatalytic degradation of acetaminophen showed higher degradation activity with TiO2 NTs than TiO2 NFs. TiO2 NT doping with nitrogen (N-TiO2 NTs) further enhanced their photocatalytic activity that was 5 times higher than that of TiO2 NFs (degradation rates: 0.05 and 0.01 mg.L-1.min(-1), respectively). The N-TiO2 NT photocatalyst was stable after four cycles of acetaminophen degradation. Acute toxicity assays confirmed the release of harmful by-products during the first hours of acetaminophen degradation, but toxicity strongly decreased after 5 h.