Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Rohani, Sohrab

Abstract

The fast diminishing of fossil fuels in the near future, as well as the global warming caused by increasing greenhouse gases have motivated the urgent quest to develop advanced materials as cost-effective photoanodes for solar light harvesting and many other photocatalytic applications. Recently, titania nanotube arrays (TNTAs) fabricated by anodization process has attracted great interest due to their excellent properties such as: high surface area, vertically oriented, highly organized, one-dimensional, nanotubular structure, photoactivity, chemical stability and biocompatibility. This unique combination of excellent properties makes TNTAs an excellent photoanode for solar light harvesting. However, the relatively wide band gap energy of titania limits its photoactivity to the UV spectra which accounts only for 5 % of solar light spectra. The specific objectives of this thesis are to: First, fabricate reproducible well-organized, vertically-oriented TNTAs in different viscous electrolytes and optimize the fabrication parameters. Second, modify the TNTAs by doping nitrogen and carbon and study the effect of modification on optical properties and photoelectrochemical performance. And third, functionalizing the TNTAs surface by monodispersed magnetic ferrite nanoparticles for improved solar light harvesting and drug delivery application in cancer treatment. The effect of each fabrication parameter such as electric potential, pH, water content, anodization time and electrolyte composition was discussed. TNTAs were successfully fabricated in an inexpensive viscous electrolyte composed of 2 wt.% sodium carboxy methylcellulose (CMC). TNTAs were successfully fabricated on both sides of a Ti disc with total tube length of 9.5 µm with a unique structure composed of conducting Ti metal sandwiched between two semiconducting layers of TNTAs on each side with a new potential electronic and photocatalytic applications.

A new, facile, low cost, environment-friendly and nanoarchitecture-safe method was introduced to fabricate N- and C-modified TiO2 nanotube arrays. Modified optical properties with narrow band gap energy, Eg, of 2.65 eV was obtained after annealing the modified TNTAs at 550°C. Modified TNTAs showed enhanced photoelectrochemical performance. Photoconversion efficiency (PCE) was increased from 4.35% for pristine (unmodified) TNTAs to 5.18% for modified TNTAs, an increase of 19%. Effect of nanotubes length of modified TNTAs on photoelectrochemical performance was also studied. Photocurrent density and PCE were increased by increasing nanotube length with a maximum PCE of 6.38% for nanotube length of 55 µm. This implies an excellent light penetration up to 55 µm depth into photoanode which is about 3.6 times higher than the maximum penetration depth (15 µm) in the nanoparticulate photoanode. This increasing pattern of photoconversion efficiency with increasing nanotubes length also implied a high charge separation rate and lower charge recombination rate. This high PCE value was attributed to: band gap reduction due to N- and C-modification of TNTAs surface, increased surface area of long TNTAs compared with short TNTAs, investigated in previous studies, and the excellent light penetration and harvesting properties.

Ferrite NPs-encapsulated TNTAs were fabricated for the first time using a facile and efficient method. Ferrite nanoparticles of 13 ± 3 nm diameters were successfully distributed all over the top and inner surface of the nanotubes. UV-Vis reflectance spectra showed excellent visible light absorbance up to wave length of 660 nm (Eg = 1.88 eV). The prepared magnetic nanocomposite showed their potential capability to controlling the drug release of an anti-cancer drug (5-fluorouracil). The drug release of 5-fluorouracil by diffusion was sustained with controlled initial burst effect. The suitability of magnetic nanocomposite for cancer drug delivery was confirmed by in vitro cytotoxicity study.

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