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Highly-Ordered Titanium Dioxide Nanotube Arrays and Their Energy Band Modification by N Doping

Author: LiShiPu
Tutor: ChenYong; LinShiWei
School: Hainan University
Course: Materials Physics and Chemistry
Keywords: titania nanotube arrays anodic oxidation controllable preparation N-doping electrochemical deposition photoelectrical properties
CLC: TB383
Type: Master's thesis
Year: 2012
Downloads: 37
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Abstract


TiO2is one of the most promising oxide semiconductors for photoelectrochemical applications, particularly due to its low cost, non-toxicity, and stability against photocorrosion. In contrast to nanoparticle, highly-ordered TiO2nanotube (TNT) arrays not only have high surface-to-volume ratios and adsorptive capacity, but also have good photocatalytic properties and high photoelectrical conversion efficiency. However, due to the comparably large band gap (anatase, Eg-3.2eV), TiO2can only respond to UV light irradiation (λ<380nm) which possesses about4%of the solar spectrum. This greatly limits the application prospects. To overcome this problem, two different strategies have been typically taken:element doping and surface modification. In this work, we want to develop a simple and efficient nitrogen-doping approach.Titania nanotube arrays were fabricated in deionize water and glycerol mixed electrolyte containing a certain amount of NH4F. Three different polishing methods were used for pretreatment of Ti substrates. The morphology of three different samples was imaged by scanning electron microscopy, and their photoelectrical properties were studied as well. Experimental results showed that Titania nanotube arrays grown on the Ti substrate and polished by polishing fluid have highly-ordered and well-defined nanotubural structure. The effects of anodization potential and duration on synthesis of highly-ordered TiO2nanotubes were also studied. Both the layer thickness and nanotube diameter linearly increase with the increasing potential. The layer thickness also increases with prolongation of anodization time. By optimizing the preparation conditions, we can successfully control the geometrical structure of TiO2nanotube arrays with diameters in the range between50and200nm and the layer thickness between800and3200nm. The photoelectrical properties of highly-ordered TNT arrays have been systematically and quantitatively studied and found to be closely related to their geometric structure. A geometric roughness factor has been applied to describe the combinative effect of the geometric characteristics. The TNT sample with the geometric roughness factor of125.32shows the superior photoconversion efficiency of13.2%.N-doped TiO2nanotube arrays were prepared by ammonia solution immersion. But the doping effect was not apparent. And the photoelectrical properties had little enhancement. We further tried to improve the N doping effect by acid treatment before ammonia solution immersion. However, the photoelectrical properties enhancement remained neglectable.N-doped TiO2nanotube arrays were prepared by electrochemical anodization in glycerol electrolyte, followed by electrochemical deposition in NH4Cl solution. An orthogonal experiment was used to optimize the doping conditions. Electrolyte concentration, reaction voltage and reaction time were the main factors to influence the N doping effect which was the determinant of the visible range photoresponse. The optimal N-doping conditions were determined as follows:reaction voltage is3V, reaction time is2h and electrolyte concentration is0.5M. The maximal photocurrent enhanced ratio was30%under the white-light irradiation. About58%improvement of photocatalytic efficiency was achieved in the Rhodamine B degradation experiment by N doping. The kinetic constant of the N-doped TNT sample was almost twice of the un-doped sample. Further analysis by X-ray photoelectron spectroscopy supported that electrochemical deposition is a simple and efficient method for N doping into TiO2nanotube arrays.

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