C-N-S tridoped TiO₂ for photocatalytic degradation of tetracycline under visible-light irradiation

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.