Photocatalytic degradation of azo dyes by sol–gel TiO2 films: effects of polyethylene glycol addition, reaction temperatures and irradiation wavelengths

Abstract

Two nanostructured sol–gel TiO2 films were prepared on a glass substrate by means of the dip-coating technique with titanium (IV) isopropoxide as a precursor with and without the addition of polyethylene glycol (PEG) as a structure-directing agent. The synthesized films were characterized by using thermal gravimetry, differential scanning calorimetry, micro-Raman spectroscopy, and atomic force microscopy (AFM). Results of the AFM analysis revealed that both films are nanostructured and that the TiO2 film prepared with the addition of PEG has higher values of roughness. The photocatalytic activity of the films was evaluated by the photocatalytic degradation of the methyl orange monoazo dye and the congo red diazo dye with predominant radiation wavelengths of 365 nm (UV-A) and 254 nm (UV-C). The effects of temperature (17.5, 25 and 35 °C) on the film stability and on the degradation process were also followed. The TiO2 film created with the addition of PEG showed heightened photoactivity at all reaction temperatures and higher degradation rates of both dyes were observed with the UV-C than with the UV-A radiation. In some cases, the total decolorization process was complete in 90 or 120 min, but the total mineralization of the dye solutions was not achieved after 120 min. The TiO2 films were stable at all three temperatures after more than 50 working hours. The degradation processes of dyes were monitored by means of the UV/VIS spectrophotometry and the liquid chromatography mass spectrometry together with the total organic carbon.