Sol-Gel Synthesis and Characterization of Neodymium-Ion Doped Nanostructured Titania Thin Films
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SOL-GEL SYNTHESIS AND CHARACTERIZATION OF NEODYMIUM-ION DOPED NANOSTRUCTURED TITANIA THIN FILMS Andrew Burns(1), W. Li(1), C. Baker(1) and S.I. Shah(1,2) (1)
Department of Material Science and Engineering Department of Physics and Astronomy University of Delaware, Newark, DE 19716 (2)
ABSTRACT Nd doped TiO2 nanostructured thin films were prepared by sol-gel technique on quartz and Si substrates using TiCl4 precursor. As-deposited amorphous films were annealed to form anatase phase in the thin films. The film grain size increased with annealing temperature. Above 800°C, rutile began to segregate and the grain size decreased slightly. The photodegradation of 2-chlorophenol (2-CP) was studied. Doping TiO2 with Nd+3 reduced the photodegradation time. The difference in the ionic radii of Nd+3 and Ti+4 and the oxygen affinities of Nd and Ti were responsible for this effect. These differences help promote electron trapping, thereby increasing the lifetime of the holes which are responsible for the oxidation of 2-CP. 1. INTRODUCTION Contaminants from industrial waste pose a major environmental threat to air and water. As of 1990, the EPA estimated that 10,000 U.S. public water sources contained pesticides or other contaminants linked to cancer and to kidney and nervous system damage. A method is needed to effectively neutralize these and other pollutants. Semiconductor photocatalysis offers a promising solution. Nanostructured semiconductors effectively catalyze aqueous reactions, which break down harmful organic pollutants to relatively harmless constituent chemicals. Semiconductor photocatalysis takes advantage of the valence/conduction bandgap specific to semiconductor molecules. Incoming photons with energies at or above the bandgap will cause valence electrons to become excited and move to the conduction shell, leaving holes in the valence band. These excited charge carriers can react with molecules adsorbed on the semiconductor surface, thus acting as catalysts [1]. There are several competing effects, which limit the effectiveness of the catalysts. Most of the activated charge carriers will undergo recombination before reaching the surface to interact with adsorbed molecules. Up to 90% of the generated carriers are lost within a nanosecond of generation. As the grain size decreases, the probability of volumetric recombination also decreases, along with creating greater surface area for adsorption, thus making nanostructured systems particularly viable. However, there is an optimum in the particle size. As the particle size decreases, the probability of surface recombination increases, thereby limiting the minimum particle size. Among the most viable nanoparticles for photocatalysis applications is titanium dioxide. TiO2 is stable in aqueous media and is tolerant of both acidic and alkaline solutions. It is inexpensive, recyclable, reusable and relatively simple to produce. It also forms nanostructures more readily than other catalysts. Furthermore, its bandgap V5.2.1
includes the redox potential for the H2O/•OH r
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