TiO₂-Ag Photocatalysts for Degradation of Dyes and Antibiotics from Aqueous Solution ^†

Titanium dioxide, TiO₂, plays an important role due to its catalytic properties and high chemical stability being non-toxic and commercially available. Over time, several synthesis protocols have been established to control the distribution of nanoparticle sizes, surface properties, or morphologies [1]. In our studies, we developed a variety of synthesis methods in which titanium precursors and surfactants influence the properties of the obtained titanium oxide. Doping of TiO₂ with noble metals such as Ag, Au, Pt, or Pd improves photocatalytic efficiency in visible light by acting as an electron trap, promoting interfacial charge transfer and therefore delaying recombination of electron pairs [2]. In this paper, TiO₂ mesoporous oxides were obtained by the sol–gel method in the presence of different surfactants and precursors. TiO₂-Ag photocatalysts were obtained by impregnation of the obtained mesoporous titania and their activity was evaluated in the degradation of Brilliant Blue dye and the ciprofloxacin as an antibiotic.

TiO₂ was obtained by the sol-gel method using titanium isopropoxide (TP), titanium ethoxide (TE), and titanium butoxide (TB) as Ti precursors and surfactants BRIJ 58 (Bj) and CTAB (Cb). The obtained samples (TPBj, TEBj, and TBCb) were doped with silver (1, 2% Ag) by impregnation of TiO₂ with an AgNO₃ aqueous solution. The obtained samples were characterized using various techniques, such as X-ray diffraction, N₂ adsorption-desorption, scanning electron microscopy, UV-Vis, and photoluminescence spectroscopy. The photocatalytic activity of the obtained samples was tested in the degradation of the Brilliant Blue dye and Ciprofloxacin under UV (254 nm) and visible (532 nm) light (laser) conditions.

X-ray diffraction (XRD) of the TPBj-Ag material showed diffraction peaks for anatase without other impurities. A small rutile fraction besides anatase was evidenced for TEBj-Ag and TBCb-Ag samples. Ag species were not detected in XRD. It is probably the result of their good dispersion [3], low concentration, and the possible formation of amorphous species such as Ag oxide. Adsorption-desorption isotherms showed that TiO₂-Ag materials are mesoporous with typical IV isotherms and hysteresis II type. UV-Vis spectra indicated an insignificant effect of the synthesis conditions on the absorption bands and highlighted the effect of silver with the appearance of the plasmonic effect. Photoluminescence spectroscopy evidenced the increase of intensity for the emission peak located at 425 nm, indicating the increase in time of •OH radical concentration.

The results obtained encouraged the use of TiO₂-Ag nanoparticles for the degradation of dyes and the antibiotics under UV and visible light conditions. The best results in Brilliant Blue dye and ciprofloxacin photo-degradation were obtained for the TiO₂-Ag sample under UV light irradiation. Thus, the removal efficiency was 98.22% for dye and 90.66% for antibiotics after 150 minutes of irradiation. Photocatalytic tests under visible light showed a good efficiency (90%) after 18 hours in the degradation of ciprofloxacin for all the obtained materials.