The effect of solvent in the sol–gel synthesis of visible light-activated, sulfur-doped TiO2 nanostructured porous films for water treatment

Abstract

The effects of solvent on the synthesis of visible light-activated, sulfur-doped TiO2 (S–TiO2) films were studied. Four different polar, protic solvents, isopropanol, 1-butanol, ethanol, and methanol (iPrOH, BtOH, EtOH, and MeOH), were chosen as the solvent in four titania sol–gel preparations. The films were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), environmental scanning electron microscopy (ESEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), Ultraviolet (UV)–vis diffuse reflectance, and porosimetry. The structural, morphological, and porous characteristics of the sulfur-doped TiO2 films were correlated with solvent physical properties such as the dielectric constant (D-value) and the saturated vapor pressure. According to XPS and FT-IR, S6+/S4+ cations replaced Ti4+ ions in the lattice of TiO2, resulting in the formation of localized states within the bandgap of TiO2. The optical absorption edge for all S–TiO2 films was significantly shifted toward the visible light region. The solvent D-value has a negligible effect on the bandgap energy change and the doping states of the prepared S–TiO2 samples. S–TiO2 films synthesized using MeOH (S–TiO2–MeOH), despite their lower Brunauer, Emmett, and Teller (BET) surface area and porosity compared to the other films, showed the highest photocatalytic activity for the degradation of the hepatotoxin microcystin-LR (MC-LR) under visible light irradiation due to their high surface roughness and large pore size. The tailor-designed structure of the S–TiO2–MeOH film contributed to the high photocatalytic degradation rates of MC-LR. The larger pore size of the S–TiO2–MeOH films allowed easier transport of MC-LR inside the porous film, while the higher film surface roughness could increase nano-interfacial interactions between MC-LR and surface active sites. These results indicate that the structural and morphological properties of S–TiO2 photocatalysts can be tailor-designed using different solvents in the sol–gel synthesis, while inducing negligible effects on the sulfur doping and the visible light activation of TiO2. Therefore, the enhancement of photocatalytic activity of S–TiO2 films can be achieved by judicious choice of the main solvent for the sol–gel method.