| Summary: | 碩士 === 國立交通大學 === 材料科學與工程學系所 === 105 === TiO2 nanotube arrays (TNAs), a semiconductor material and a catalyst, exhibits good performance in ultraviolet region, but limited performance in visible region due to its wide band gap of 3.2 eV. In contrast, TNAs thin film can provide an economic and environmentally benign solution as the photo-catalyst for photo-degradation of various pollutants. Recently, many research focuses on developing TNAs structure with nitrogen doping to enhance the photocatalytic activity in the visible region. In this study, nitrogen-doped (N-doped) TNAs was fabricated by facile annealing under N2 of TNAs prepared by a two-step anodization using a NH4F/aqueous ethylene glycol solution. Nitrogen doping contents (5.7 to 9.97 at%) were controlled by annealing TNAs at 450oC using N2 at various flow rates.
The crystal structure and orientation of TNAs and N-doped TNAs were characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis of as-fabricated and nitrogen-doped TNAs was also carried out to understand the chemical bonding and changes from Ti 2p, O 1s, N 1s emission spectra. Morphology and the tube dimension of as-fabricated and nitrogen-doped TNAs were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. And the electron charge transfer was analyzed by photoluminescence spectroscopy. N-doped TNAs in this work exhibited anatase crystalline structure with a strong preferential orientation of (004) by X-ray diffraction (XRD) analysis, compared to (101) orientation in the as-fabricated TNAs. The energy bandgap of N-doped TNAs was reduced from 3.22 eV (TNAs) to 3.0 eV.
Photocatalytic performance of as-fabricated TNAs and N-doped TNAs was measured by monitoring methylene blue degradation under visible light (λ > 400 nm) illumination at 120 mW.cm-2. N-doped TNAs exhibited appreciably enhanced photocatalytic activity compared to as-fabricated TNAs. The rate constant for N-doped TNAs (10.0 at% N) was 3.4 x 10-3 min-1, which is 81% improvement over that of TNAs (1.9 x10-3 min-1). The photocatalytic activity enhancement of N-doped TNAs can be attributed to the reduced bandgap, reduction the recombination rate and TNA morphology. The role of the nitrogen atom in the reducing energy bandgap of nitrogen-doped TNAs is also discussed.
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