Enhanced photocatalysis and bacterial inhibition in Nb2O5 via versatile doping with metals (Sr, Y, Zr, and Ag): a critical assessment

Abstract

Unique optical properties render semiconductor Nb₂O₅ nanoparticles suitable for light harvesting and photocatalytic applications. This study focuses on determining optical properties such as the band gap, conduction band edge, valence band edge and work function of as-prepared solution combustion synthesized Nb₂O₅ nanoparticles with the help of UV-vis Diffuse Reflectance spectroscopy (DRS) and ultraviolet photoelectron spectroscopy (UPS) techniques. Phase purity and the oxidation states of the elements present in the material were confirmed from X-ray diffraction (XRD) patterns and X-ray photoelectron spectra (XPS), respectively. Doping semiconductors with different metal ions impacts the activity of the material, and therefore efforts were made to understand the effect on the photocatalytic performance of Nb₂O₅ due to the incorporation of metal dopants viz. Sr, Y, Zr, and Ag. Lattice parameters were obtained from Rietveld refinement of the XRD patterns. Parameters which are closely related to the photoactivity of the catalysts such as the presence of surface defects, oxygen vacancies, surface area, and charge carrier dynamics were determined from photoluminescence (PL) analysis, Brunauer-Emmett-Teller (BET) surface area measurements and time-resolved fluorescence (TRF) analysis respectively. In addition, the dopant concentrations were optimised for enhanced photocatalytic activity. The doped Nb₂O₅ nanoparticles showed significant activity towards targeted degradation of organic pollutants like 2-chlorophenol (2-CP) and dye contaminants like methylene blue (MB), orange G (OG) and indigo carmine (IC). This strategy yielded a robust response towards inactivation of E. coli and S. aureus as well. Adsorption and photodegradation of MB followed Lagergren's pseudo 1^st order reaction model and the Langmuir Hinshelwood model respectively. Bacterial inactivation and OG, IC and 2-CP photodegradation followed 1^st order kinetics. The reusability of the catalyst for 5 cycles was demonstrated. Finally, a plausible mechanism is proposed based on radical trapping experiments and combined analysis of the characterization techniques.

Fig. 1. XRD patterns of Nb₂O₅ doped with Sr, Y, Zr, and Ag of 0.25 atom%.

Fig. 2. XPS spectra of (a) Nb-3d and (b) O-1s.

Fig. 3. UV-vis absorbance spectra of pristine and doped Nb₂O₅ and the inset presents the transformed Kubelka–Munk function vs. photon energy.

Fig. 4. UPS spectra of Nb₂O₅.

Fig. 5. PL spectra of the doped and pristine Nb₂O₅.

Fig. 6. TRF decay curve of Nb₂O₅.

Fig. 7. (a) SEM image of Nb₂O₅, (b) SEM image of Sr (0.25%)–Nb₂O₅, (c) HRTEM image of Nb₂O₅ showing the d spacing of the (1 0 0) plane and the inset shows the FFT image, and (d) HRTEM image of Nb₂O₅ showing the d spacing of the (0 0 1) plane and the inset presents the FFT image.

Fig. 8. Effect of dopants in Nb₂O₅ for MB degradation and the inset represents rate kinetics.

Fig. 9. Effect of dopants in Nb₂O₅ for OG degradation and the inset presents rate kinetics.

Fig. 10. Effect of dopants in Nb₂O₅ for IC degradation and the inset presents rate kinetics.

Fig. 11. Effect of dopants in Nb₂O₅ for 2-CP degradation and the inset presents rate kinetics.

Fig. 12. Effect of dopants in Nb₂O₅ for E. coli inactivation and the inset presents rate kinetics.

Fig. 13. Effect of dopants in Nb₂O₅ for S. aureus inactivation and the inset represents rate kinetics.

Fig. 14. Radical trapping experiment with scavengers and MB as the pollutant.

Fig. 15. Photocatalysis mechanism in metal-doped Nb₂O₅.