Preparo de dióxido de titânio cristalino dopado com enxofre a partir de oxissulfeto de titânio amorfo para reações de foto-oxidação

Abstract

Titanium dioxide (TiO2) is a semiconductor extensively used in solar energy conversion processes, which has long-term stability against corrosion, low toxicity and cost. In recent years, research has been carried out to modify the optical and catalytic properties of TiO2, with the aim of converting visible light. In this sense, films of S-doped titanium dioxide (S-TiO2) were prepared from the oxidation of titanium oxysulfide (TiOxSy) by thermal treatment. The chemical composition of the synthesized materials was investigated by X-ray photoelectron spectroscopy (XPS) and showed that the sample heat treated at 600 ºC has an O/Ti ratio of 2.26, characteristic of TiO2. The XPS signal in the binding energy region of 163.47 eV was attributed to S2- do amorphous TiOxSy bound to titanium, indicating the formation of TiOxSy. This signal, in the high-resolution XPS spectrum, decreased in intensity from 300°C onwards and there is a significant reduction from 400°C, suggesting that sulfur is removed from the material during heat treatment. Measurements of Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) suggest the occurrence of phase transitions for the material, at temperatures between 300 and 600 ºC. The phase transition was shaped by X-ray diffraction (XRD) measurements. The films calcined at 300, 400, 500 and 600 ºC showed a pure anatase phase, while the uncalcined TiOxSy sample is amorphous. Raman spectroscopy studies indicated a greater structural organization of the oxide with the increase in the material's thermal treatment temperature. The optical characterization carried out by UV-Vis absorption spectroscopy showed that the sulfur present as a dopant changed the radiation absorption region of the material from the ultraviolet to the visible region. Compared to commercial TiO2, which had an band gap energy (Ebg) of 3.14 eV, S-TiO2 had an Ebg corresponding to light absorption in the visible region, estimated at 2.98 eV. Photoelectrochemical studies carried out under polychromatic irradiation revealed that S-TiO2 is an n-type semiconductor, with higher photocurrent values for the sample calcined at 600 ºC. The relative positions of the S-TiO2 bands indicate that the material is suitable for photo-oxidation processes of organic pollutants or other photocatalytic processes under visible irradiation.