利用簡易退火製程提升氮摻雜二氧化鈦奈米管陣列之光催化性能

Abstract

二氧化鈦奈米管陣列（TNAs）是一種半導體材料和催化劑，在紫外光區域中具有良好的性能，但是受限於其3.2eV的能帶間隙、不能利用可見光域。與其他材料相比之下，TNAs薄膜是同時具有經濟性及環境友好的材料，以其作為光催化劑，可以進行光降解各種污染物。最近，許多研究集中於開發氮摻雜TNAs及其結構，使其在可見光區域的光催化活性得以增強。在本研究中，首先使用NH4F /乙二醇水溶液的兩步陽極氧化製備TNA,進而、在不同氮氣流量、450⁰C下退火3小時、製備氮摻雜TNAs (N-TNAs) 、其氮原子含量在5.7%至9.97％。 本研究透過X射線繞射（XRD）分析和穿透式電子顯微鏡（TEM）分析TNA和氮摻雜的TNA的晶體結構和取向。並藉由的X射線光電子能譜（XPS）分析參雜氮與否的TNA，探討Ti 2p，O 1s，N 1s發射光譜的化學鍵和變化。亦透過掃描式電子顯微鏡（SEM）和穿透式電子顯微鏡（TEM）去確定氮摻雜前後的TNA的形態和奈米管管尺寸，最後透過光致發光光譜分析電子電荷轉移情形。在X射線繞射（XRD）分析顯示氮摻雜的TNA為銳鈦礦晶體結構，且具有（004）的優選方位而未參雜氮的TNA優選方位為（101）取向。 氮摻雜前後的TNA光催化表現是藉由監測以120mW.cm-2的可見光（λ> 400nm）照射下、亞甲基藍降解情形來定義。氮摻雜TNA比起未摻雜氮的TNA有明顯增強的光催化活性，此結果可歸因於減少的能帶間隙、降低重組速率和TNA形態。氮摻雜的TNA（9.97 at％N）的速率常數為3.4×10-3 min-1，比未摻雜氮TNAs（1.9×10-3 min-1）改進81％。

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.