Photosensitization of TiO₂ nanofibers by Ag₂S with the synergistic effect of excess surface Ti³⁺ states for enhanced photocatalytic activity under simulated sunlight

TiO₂ nanofibers, with mean diameter ∼200 nm, were fabricated by electrospinning and successfully photosensitized with low bandgap Ag₂S nanoparticles of 11, 17, 23 and 40 nm mean sizes, with corresponding loading of 4, 10, 18 and 29 wt.% Ag₂S, respectively. 17 nm Ag₂S@TiO² nanofibers exhibited optimal activity in the photodegradation of methylene blue under simulated sunlight with pseudo-first order rate constant of 0.019 min-1 compared to 0.009 min^-1 for pure TiO² nanofibers. In spite of greater visible-light absorption and reduced bandgap, larger than 17 nm Ag₂S nanoparticles exhibited sluggish photodegradation kinetics probably due to less photo-induced carriers generation in TiO₂ and reduced electron injection rates from the larger sized Ag₂S into TiO². Furthermore, a UVO₃ surface treatment induced excess Ti³⁺ surface states and oxygen vacancies which synergistically enhanced the photodegradation rate constant to 0.030 min-1 for 17 nm Ag₂S@TiO₂ sample which is ∼70% better than the previously reported for Ag₂S/TiO² hierarchical spheres. This was attributed to the efficient charge separation and transfer driven by increased visible-light absorption, bandgap narrowing and reduced electron-hole recombination rates. The present study demonstrate the potential utilization of Ag₂S@TiO² nanofibers in filtration membranes for removal of organic pollutants from wastewater.