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

Abstract

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 UV-O3 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.

Photo-reduction of heavy metal ions and photo-disinfection of pathogenic bacteria under simulated solar light using photosensitized TiO2 nanofibers

“…1(bd) indicates successful deposition of the Ag NPs and their subsequent sulfurization to Ag 2 S which was also conrmed previously by the X-ray photoelectron spectroscopy (XPS) and Xray diffraction (XRD) studies. 46 The number-averaged size of the Ag 2 S NPs is 11, 17, and 23 nm for the dipping times of 0.5, 1.0, and 2.0 min, respectively. These sizes were statistically measured from at least 100 NPs by measuring their areas and calculating the corresponding area-equivalent diameters assuming circular shape.…”

“…1(b-d), characteristically shows log-normal distribution. 46 The weight percentages of the Ag 2 S in the NFs was estimated by inductively coupled plasma optical emission spectroscopy (ICP-OES) by calibrating the emission intensity of Ag using a standard AgNO 3 salt solution of 1-10 ppm. The weight fractions were 4, 10, and 18% for 11, 17, and 23 nm Ag 2 S/TiO 2 NFs, respectively.…”

“…The Ag 2 S/TiO 2 NFs in the present study, when irradiated under simulated sunlight, produces hydroxyl radicals (cOH) by the reaction of the positively charged hole (h + ) with the hydroxide ion (OH À ). 46 The hydroxyl radicals are known to destruct the bacterial cell of Gram-positive and Gram-negative bacteria by breaking the covalent bonds in the peptidoglycan layer.…”

“…Introduction of the sulfate ions in the solution probably interferes with the sorption of the Cr(VI) over the NFs and, thus, suppresses the overall photo-reduction process. Thus, excess Ti 3+ was benecial for photodegradation of methylene blue (MB) dye, 46 but not for the photo-reduction of Cr(VI) here. However, as shown below, the excess Ti 3+ chemical states enhances the antibacterial performance of these materials.…”

“…In addition, the Ti 3+ states introduced aer UV-O 3 treatment further promotes the lifetime of the charges. 46 The photo-generated electrons and holes react with O 2 and H 2 O to produce the reactive oxygen species (ROS) such as peroxide (O 2 À c) and hydroxide (cOH) radicals. These ROS diffuse in the cell wall of bacteria and react with the cell membrane and DNA leading to death of the bacteria.…”