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

摘要

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-1R), 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, S~(6+)/S~(4+) cations replaced Ti~(4+) 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.