Doped photocatalyst immobilization on tubular surface: continuous photocatalysis of pharma drugs under visible light

Abstract

This study presents the fabrication and characterization of nitrogen-doped TiO₂ (N-TiO₂) thin films deposited on cylindrical quartz tubes for photocatalytic applications. A binder-free modified dip-coating technique is developed to uniformly immobilize the TiO₂ films, using a sol-gel process with amine-based precursors to control nitrogen doping levels. Thermal annealing ensured strong adhesion of the films to the substrate. Characterization by XRD, UV-vis spectroscopy, FTIR, and XPS confirmed nitrogen incorporation, phase formation, and modifications in band gap and surface chemistry. XPS further detailed the elemental composition and electronic states, highlighting the role of nitrogen in enhancing photocatalytic properties. The visible-light-driven photocatalytic performance is evaluated through the degradation of ciprofloxacin in aqueous medium, with more than 85% degradation efficiency. The improved activity is attributed to effective nitrogen doping and robust film adhesion. This scalable method offers a promising route to producing durable, high-performance photocatalysts for sustainable water treatment technologies.

Fig. 1. Schematic representation of the sol–gel synthesis process for nitrogen-doped TiO₂ powder preparation and thin film coating procedure.

Fig. 2. Schematic of the photocatalytic degradation system for ciprofloxacin (1 ppm) treatment employing a 12-watt visible light LED array as the illumination source.

Fig. 3. (a) UV-vis absorption spectra comparing commercial TiO₂ and N–TiO₂ photocatalyst powders, showing enhanced visible-light absorption (400–800 nm) for the doped sample relative to the undoped material. (b) Tauc plot analysis comparing the optical bandgap energies of commercial TiO₂ and N–TiO₂ powders, derived from UV-vis absorption data.

Fig. 4. XRD patterns of commercial TiO₂ and N–TiO₂ samples synthesized using different nitrogen precursors, with all doped materials calcined at 400 °C.

Fig. 5. Comparative FT-IR spectra of TiO₂ and nitrogen-doped TiO₂ samples prepared with different nitrogen precursors, showing characteristic vibrational modes associated with surface modifications.

Fig. 6. (a) XPS full survey spectra of pure and nitrogen-doped TiO₂ samples, showing characteristic binding energy peaks corresponding to different elements chemical states. (b) XPS spectra of the N 1s region for nitrogen-doped TiO₂ samples, showing characteristic binding energy peaks corresponding to different nitrogen chemical states.

Fig. 7. O 1s XPS spectra of nitrogen-doped TiO₂ samples, showing characteristic binding energy peaks corresponding to different oxygen chemical environments.

Fig. 8. Ti2p XPS spectra of nitrogen-doped TiO₂ samples, showing spin–orbit split doublet peaks (2p_3/2 and 2p_1/2) characteristic of titanium oxidation states.

Fig. 9. FESEM micrographs of N-doped TiO₂ coatings on quartz tube substrates shown at 50 000× magnification, demonstrating the nanoscale surface topography and coating uniformity.

Fig. 10. Rheological analysis of sol–gel prepared nitrogen-doped TiO₂ coating solution, behaviour assessment and viscosity calculation. The experimental data is shown in symbols and the solid line represent the linear fit, to estimate the viscosity.

Fig. 11. (a) Schematic of the continuous-flow photocatalytic degradation system for ciprofloxacin, featuring N-doped TiO₂-coated tubular reactor, 12W visible LED array illumination source and sequential pollutant transformation mechanism. MS spectrum of the (b) CIP, and the (c) reaction intermediates obtained at the end of 9 hours of experiment.

Fig. 12. (a) Continuous-flow and (b) batch photocatalytic degradation of ciprofloxacin (CIP), in an annular reactor system employing N–TiO₂-coated quartz tubes. The errors bars represent the uncertainty of the experimental measurements. The solid line shows the trend behaviour.

Fig. 13. Catalyst reuse study for photocatalytic degradation of ciprofloxacin (CIP) in a continuous flow annular reactor system employing N–TiO₂-coated quartz tubes. The errors bars represent the uncertainty of the experimental measurements. The solid line shows the trend behaviour.