Multifunctional Composite Nanofiber Membranes with Enhanced Photodegradation and Antimicrobial Performance for water Remediation

Abstract

Our society has made significant advancements in technology as it continues to grow in size which in turn has led to an accumulating amount of toxic threats. Some types of harmful pollution our society is currently facing includes industrial waste such as non biodegradable organic compounds, heavy metals and pathogens. Therefore, it is imperative to develop green and efficient technologies to control and reduce the growth of environmental hazards. In recent years, photocatalysis using titanium dioxide (TiO2) has become a promising route to degrading organic pollutants. However, Anatase phase of TiO2 has a band gap of 3.2 eV. This limits its practical application under sunlight because the light with energy greater than 3.2 eV constitutes only 3 ~ 4 % of the total solar energy reaching the earth. Therefore, modifications of TiO2 are needed to allow TiO2 to efficiently utilize the solar spectrum. Electrospun TiO2 nanofibers present a unique class of active materials with optimized photoactivity and cost efficiency due to ease of synthesis and fabrication in bulk. The high aspect ratios of these nanostructured materials shorten the transportation length of electrons and holes from the crystal interface to the surface, thus accelerating their migration to the active surface sites. The primary goal of this dissertation is to develop TiO2 nanofibers as an efficient and cost-effective catalyst for practical and multi-purpose application in water remediation. To achieve this, various strategies were employed including doping, photosensitization with a low bandgap material, modification of the surface chemical states, and incorporating second-phase materials in TiO2 nanofibers. The detailed characterization of the prepared nanofibers was carried out by SEM, TEM, XRD, XPS, UV-vis DRS, FTIR and PL. TiO2 nanofibers were prepared through sol-gel solution followed by electrospinning and calcination treatment. The electrospun nanofibers were successfully doped by phosphorus and the surface of nanofibers were decorated by silver nanoparticles. The synergistic effect of P-doping and Ag NPs resulted in a decrease in the bandgap and enhanced charge separation. Consequently, the rate constant of Cr(VI) photoreduction by Ag-PTNFs was 96 % higher than unmodified nanofibers and the rate constant of MB photoreduction was 83 % higher than that of the unmodified nanofibers. Another strategy was to make composite TiO2 nanofibers by incorporation of g-C3N4 in TiO2 nanofibers and the effect of making heterojunctions with Ag NPs was studied. The prepared composite nanofiber exhibited remarkable photocatalytic activity for degradation of MB, reduction of Cr(VI) and antibacterial activity against E. coli and S. aureus under simulated solar irradiation. TiO2 nanofibers were also successfully photosensitized with low bandgap Ag2S nanoparticles of 11, 17, 23 and 40 nm mean sizes. 17 nm Ag2S@TiO2 nanofibers exhibited optimal activity in the photodegradation of methylene blue and photoreduction of Cr(VI) under simulated sunlight. Whereas, 11 nm Ag2S@TiO2 nanofibers displayed excellent bactericidal activity under dark and simulated solar irradiation. Furthermore, a UV-O3 surface treatment induced excess Ti3+ surface states and oxygen vacancies which synergistically enhanced the photocatalytic activity. This was attributed to the efficient charge separation and transfer driven by increased visible-light absorption, bandgap narrowing and reduced electron-hole recombination rates. This dissertation demonstrates the potential utilization of modified TiO2 nanofibers in multifunctional filtration membranes for remediation of pollutants from wastewater under solar irradiation.