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New photoanodes for recalcitrant pollutant degradation should harvest photons from a wide region of the solar spectrum and promote a direct water oxidation reaction towards the hydroxyl radical generation. For this purpose, sulfur, nitrogen-codoped titanium dioxide (S,N-TiO2) films were prepared with different sulfur percentages (1.0, 2.5 and 5.0 at.%) on Ti expanded meshes by sol-gel dip-coating followed by thermal treatment at 400 °C. The morphology, composition, oxidation states of dopants, structural features, optical and semiconducting properties of S,N-TiO2 were used for the materials characterization. While sulfur was found as S4+ or S6+ cations related to surface doping, nitrogen appears as N3− anion occupying substitutional positions in TiO2. This caused an improvement in the photoelectrochemical response of TiO2 related to a decrease of up to 0.25 eV in the optical band gap and the alignments of the TiO2 band structure. S,N-TiO2 films exhibit significantly higher photoelectrocatalytic activity towards ·OH generation under UV–vis light irradiation than the undoped TiO2 films. Using 2.5 at.% S,N-TiO2 film, an increase of 166% in current efficiency was obtained at 0.5 V vs Ag/AgCl, in regard to undoped TiO2 films, together with an enhance of 8.8 times in the initial ·OH generation rate.

The photoelectrocatalytic oxidation of an anionic surfactant (internal olefin sulfonate C20-C24, IOS) in synthetic oilfield wastewater was investigated by photocurrent measurements using a sulfur and nitrogen co-doped titanium dioxide electrode (S,N-TiO 2 /Ti). For the electrode preparation, S,N-TiO 2 films were supported on Ti expanded meshes by sol-gel dip-coating, followed by thermal treatment at 400 °C. Photocurrent measurements were performed by linear sweep voltammetry (LSV) under UV-Vis irradiation using different IOS concentrations (20-200 ppm). The photocurrent values obtained at 0.5 V vs Ag/ AgCl (i ph ) for each IOS concentration (C IOS ) were used to plot the 1/i ph vs 1/C IOS graph, in which two trends were identified. For high IOS concentrations (70-200 ppm), the values fitted well to the unimolecular Langmuir-Hinshelwood (LH1) kinetic model (R 2 = 0.973), which can be associated with the direct oxidation mechanism. For low IOS concentrations (20-70 ppm), the values fitted better to a linear combination of both unimolecular and bimolecular Langmuir-Hinshelwood (LH1 and LH2) kinetic models (R 2 = 0.854), which can be associated with direct and indirect oxidation mechanisms, respectively. These results suggest that at high IOS concentrations the IOS adsorption plays the main role on the photoelectrocatalytic oxidation, while at low IOS concentrations both IOS adsorption and H 2 O adsorption (surface hydrophilicity) play important roles on the photoelectrocatalytic oxidation.