以原子層沉積法製備陣列式多層二氧化鈦奈米管及其銀之改質與光催化性質研究

Fabrication of Ag-loaded Multiple-walled TiO2 Nanotube Arrays by Template-assisted Atomic Layer Deposition and Their Photocatalytic Activity

張雯婷(Wen-ting Chang)

指導教授 ： 彭宗平

國立清華大學/工學院/材料科學工程學系/碩士(2012年)

摘要

本論文主要係研究以銀改質之陣列式二氧化鈦多壁奈米管及其光催化性質。結合陽極氧化鋁(anodic aluminum oxide, AAO)與原子層沉積技術(atomic layer deposition, ALD)之模板法製備陣列式二氧化鈦多壁奈米管，以震盪輔助光化學還原法(ultrasound-aided photochemical reduction)沉積銀奈米顆粒改質陣列式二氧化鈦多壁奈米管之表面。多壁奈米管係由兩個以上直徑不同的同心奈米管組成，相較於傳統的單壁奈米管，多壁奈米管提供更多反應表面積，且置入更多奈米管時仍能同時保有高比表面積性質，以利反應載子傳輸。
自製陽極氧化鋁之圓柱孔洞尺寸可由電解質種類和組成濃度、工作電壓、反應溫度及擴孔時間控制，本研究製備圓柱孔洞尺寸範圍為20奈米至150奈米，而其中孔洞尺寸110奈米之陽極氧化鋁用為製備多壁奈米管之模板。利用高階梯覆蓋性、具極佳厚度控制之原子層沉積技術鍍覆二氧化鈦及氧化鋁薄膜，四氯化鈦(titanium (IV) tetrachloride)、三甲基鋁(trimethylaluminum)與水為原子層沉積技術之氣態前驅物，前驅物交替反應沉積二氧化鈦與氧化鋁奈米疊層於陽極氧化鋁高密度陣列圓柱孔洞，以得陣列結構二氧化鈦多壁奈米管。二氧化鈦奈米管管壁厚度及多層壁間距可分別由二氧化鈦及氧化鋁原子層沉積循環數(cycle number)精準控制。銀改質製備係利用紫外光觸發二氧化鈦高電子電洞分離效率，還原硝酸銀中銀離子至銀原子之光化學還原法。
純二氧化鈦及其銀改植多壁奈米管由X光繞射分析儀(XRD)分析其製程結構及成長均勻性，觀察到二氧化鈦多壁奈米管為混相結構，包含銳鈦礦(anatase)、金紅石(rutile)及板鈦礦(brookite)，表示二氧化鈦多壁奈米管含有多重異質接面。利用掃描式電子顯微鏡(SEM)及高解析穿透式電子顯微鏡(HRTEM)觀察其形貌，發現二氧化鈦鍍膜速率接近理論值約0.545 Å/ cycle。使用紫外光與可見光分光光譜儀(UV-vis spectrometer)量測試片吸收光譜，發現吸收光譜較本體二氧化鈦(Eg=3.2 eV)具藍移現象，可能為量子效應影響，但吸收光譜分布情形並未受多層壁管壁數多寡有大幅變動。
為檢測試片之光催化性質，單壁、雙壁及三壁二氧化鈦奈米管與以銀改質對應試片和純二氧化鈦奈米柱為光觸媒測試對象，使用10 W紫外光燈觸發二氧化鈦光觸媒特性降解濃度2 ppm、10 ml之亞甲基藍溶液，試片反應面積固定為 π/2 cm2。實驗結果指出在純二氧化鈦試片中三壁二氧化鈦奈米管有最佳表現，其降解效率高於觸媒含量最多之二氧化鈦奈米柱，三壁二氧化鈦奈米管將亞甲基藍溶液濃度降解至原本的10 %，其效果與銀改質單壁及雙壁二氧化鈦奈米管相近，推論為大反應面積、多重異質接面結構及銀改質皆能大幅提升光觸媒效率。

關鍵字

原子層沉積技術 ； 光催化 ； 陽極氧化鋁 ； 二氧化鈦 ； multiple-walled nanotube arrays, MWNTAs ； 銀

並列摘要

Bare and silver-loaded TiO2 multiple-walled nanotube arrays (MWNTAs) were prepared by template-assisted atomic layer deposition (ALD), and ultrasound-aided photochemical reduction was employed to synthesize silver nanoparticles on the nanotubes. The MWNT contains more than one concentric tubes with different diameters. They offer larger reaction surface area than conventional single-walled tubes. Increasing the wall number of nanotubes (i.e., more material) still keeps their high specific surface area characteristic.
The pore size of home-made anodized aluminum oxide (AAO) can be tuned by the applied voltage, electrolyte composition, reaction temperature, and pore-widening time. The home-made AAO was chosen as the template to fabricate TiO2 MWNTAs, due to its highly ordered nanoporous array structure. The pore size ranging from 20 nm to 150 nm was successfully fabricated. To fully duplicate the AAO structure, ALD with excellent conformability and thickness control ability was employed to deposit TiO2 into the pores of AAO. Gaseous precursors titanium (IV) tetrachloride (TiCl4), trimethylaluminum (TMA), and H2O were selected for the ALD process. They were alternatively introduced into the chamber and chemically reacted with each other to form nanolaminate of TiO2 and Al2O3 on AAO. The thickness of nanotube wall and the gap span of MWNTAs were accurately controlled by the ALD cycle numbers of TiO2 and Al2O3. Silver nanoparticles were further deposited on the TiO2 nanotubes by ultrasound-aided photochemical reduction method under UV light.
The TiO2 MWNTAs were characterized by X-ray diffraction for their crystallinity and phase, by scanning electron microscopy and high resolution TEM for their morphology, and by UV-visible light absorption spectroscopy for the optical property.
To examine the efficiency of both bare and Ag-loaded TiO2 MWNTAs in photodecomposition test, single-, double- and triple-walled TiO2 MWNTAs with or without Ag-loading and TiO2 nanorod arrays were selected as photocatalysts with the same reaction area (π/2 cm2). A solution of 10 ml containing 2 ppm methylene blue (MB) was tested for photodegradation. After the test, the concentration of MB was finally reduced to less than 10 % of the original concentration by triple-walled TiO2 MWNTAs and Ag-loaded single- and double-walled ones. It was demonstrated that the triple-walled TiO2 MWNTAs showed the best activity among the unloaded TiO2 samples. It could be attributed to the multi-heterojunction of three coexisting phases (anatase, rutile, and brookite) and larger reaction surface area. The result also reveals that Ag does enhance the efficiency of photocatalyst. It is attributed to Ag particles serving as electron traps to retard recombination rate of electrons and holes.

並列關鍵字

Atomic layer deposition ； Photocatalysis ； anodic aluminum oxide, AAO ； TiO2 ； 陣列多壁奈米管 ； Silver

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