C–N–S tridoped TiO2 for photocatalytic degradation of tetracycline under visible-light irradiation Original Research Article

C–N–S tridoped TiO2 was synthesized using a facile, cost-effective and easily scaled-up sol–gel method with titanium butoxide (Ti(OC4H9)4) as titanium precursor and thiourea as the dopant source. It was found that thiourea could suppress the crystal growth of the anatase TiO2 and inhibit its transformation from anatase to rutile phase. X-ray photoelectron spectroscopy (XPS) analysis revealed that carbon substituted some of the oxygen to form Ti–C bonds, nitrogen was interstitially and substitutionally doped into the TiO2 lattices to form Ti–N–O, Ti–O–N and O–Ti–N, and S6+ substituted for the lattice Ti4+ to result in cationic sulfur doping. The photocatalyst with the thiourea-to-Ti molar ratio of 0.05:1 and calcined at 450 °C (T0.05–450) possessed the optimum surface elemental contents of C (12.56 at.%, excluded adventitious carbon at 284.8 eV), N (0.54 at.%) and S (1.60 at.%) based on the XPS analysis, and exhibited the highest photocatalytic degradation efficiency of tetracycline (TC) under visible-light irradiation. This was attributed to the synergistic effects of TC adsorption on T0.05–450 due to its high specific surface area, band gap narrowing resulting from C–N–S tridoping, presence of carbonaceous species serving as photosensitizer, and well-formed anatase phase. The slightly alkaline pH condition and solar irradiation were more favorable for both the photocatalytic degradation and mineralization of TC. Microtox assay indicated that the extended solar photocatalysis was efficient in the detoxification of TC solution.