titanium dioxide nanotubes (TiO 2 Nts) are able to produce oxidizing agents that can
degrade pharmaceuticals in the absence of light. However, its electrochemical
hydroxyl radical production is scarce but can be improved by annealing the
nanotubes in a reducing atmosphere that will create defects that allow the material
to generate oxidizing species by applying a voltage. Notably Carlson et al. were able
to degrade ibuprofen up to 50% in 15 min by using TiO 2 NTs annealed under a
reductive atmosphere [81].
Titanium dioxide nanotubes can also be used as support materials to hold
electroactive [82] or photoactive species. They have been employed to support
TiO 2 nanoparticles (TiO 2 NPs), N,S decorated TiO 2 nanocrystallites (N, S-TiO 2
NCs), reduced graphene oxide (RGO), and Pd nanoparticles (Pd NPs). Among these,
the photoelectrode that had shown the best photoelectrochemical performance, the
N,S co-doped TiO 2 nanocrystallites decorated TiO 2 nanotube arrays (N, S-TiO 2
NCs/TiO 2 NTAs) was used to carry on the photoelectrochemical degradation of
diclofenac. With this electrode, about 70% diclofenac was removed under a 35 W
Xenon light irradiation, at an applied voltage of 0.4 V (vs SCE), at pH 5.0, 0.10 M
Na 2 SO 4 and with a 5 mg/L diclofenac initial concentration [83].
4.3 Zinc Oxide (ZnO) Nanostructures
Zinc oxide is a semiconductor suitable for electrochemical techniques and has shown
to be effective in the degradation of dyes and pharmaceuticals. Its properties include
great chemical stability, environmental friendliness, low cost, high adsorption coefficients, high electron mobility, and electron communication features [84, 85]. However, it has a high isoelectric point of 9.5; therefore in acidic conditions, the charged
ZnO nanoparticles might repel the pharmaceuticals to be degraded, reducing the
removal efficiency. On the other hand, experiments made by Tashkourian et al.
evidence that the incorporation of ZnO nanoparticles and CNTs increase the surface
area of the electrode and allow a greater electron transfer, therefore enhancing
naproxen degradation [77].
Another example of implemented ZnO and TiO 2 nanostructures in the degradation of NSAIDS is the work published by Gomes et al. where it is shown that the
effect of adding the semiconductors together increases the surface area, and, therefore, a better response is obtained, compared to electrodes consisting of just zinc or
titanium oxide. In this work, a degradation of ibuprofen was achieved by a
photoelectrochemical method, and the total organic carbon removal was of
23% [86].
4.4 Platinum Nanoparticles (PtNPs)
Platinum is a noble metal widely required as an electrocatalyst due to its incredible
chemical durability, as well as its physical and chemical properties. Platinum
nanoparticles in particular have attracted the attention of researchers for applications
292
M. Cerro-Lopez et al.
degrade pharmaceuticals in the absence of light. However, its electrochemical
hydroxyl radical production is scarce but can be improved by annealing the
nanotubes in a reducing atmosphere that will create defects that allow the material
to generate oxidizing species by applying a voltage. Notably Carlson et al. were able
to degrade ibuprofen up to 50% in 15 min by using TiO 2 NTs annealed under a
reductive atmosphere [81].
Titanium dioxide nanotubes can also be used as support materials to hold
electroactive [82] or photoactive species. They have been employed to support
TiO 2 nanoparticles (TiO 2 NPs), N,S decorated TiO 2 nanocrystallites (N, S-TiO 2
NCs), reduced graphene oxide (RGO), and Pd nanoparticles (Pd NPs). Among these,
the photoelectrode that had shown the best photoelectrochemical performance, the
N,S co-doped TiO 2 nanocrystallites decorated TiO 2 nanotube arrays (N, S-TiO 2
NCs/TiO 2 NTAs) was used to carry on the photoelectrochemical degradation of
diclofenac. With this electrode, about 70% diclofenac was removed under a 35 W
Xenon light irradiation, at an applied voltage of 0.4 V (vs SCE), at pH 5.0, 0.10 M
Na 2 SO 4 and with a 5 mg/L diclofenac initial concentration [83].
4.3 Zinc Oxide (ZnO) Nanostructures
Zinc oxide is a semiconductor suitable for electrochemical techniques and has shown
to be effective in the degradation of dyes and pharmaceuticals. Its properties include
great chemical stability, environmental friendliness, low cost, high adsorption coefficients, high electron mobility, and electron communication features [84, 85]. However, it has a high isoelectric point of 9.5; therefore in acidic conditions, the charged
ZnO nanoparticles might repel the pharmaceuticals to be degraded, reducing the
removal efficiency. On the other hand, experiments made by Tashkourian et al.
evidence that the incorporation of ZnO nanoparticles and CNTs increase the surface
area of the electrode and allow a greater electron transfer, therefore enhancing
naproxen degradation [77].
Another example of implemented ZnO and TiO 2 nanostructures in the degradation of NSAIDS is the work published by Gomes et al. where it is shown that the
effect of adding the semiconductors together increases the surface area, and, therefore, a better response is obtained, compared to electrodes consisting of just zinc or
titanium oxide. In this work, a degradation of ibuprofen was achieved by a
photoelectrochemical method, and the total organic carbon removal was of
23% [86].
4.4 Platinum Nanoparticles (PtNPs)
Platinum is a noble metal widely required as an electrocatalyst due to its incredible
chemical durability, as well as its physical and chemical properties. Platinum
nanoparticles in particular have attracted the attention of researchers for applications
292
M. Cerro-Lopez et al.