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研究生： 郭維力
Vitaly Gurylev
論文名稱： 氧化鈦及氧化鋅奈米結構缺陷工程：設計製備，特性及應用
Design, Fabrication, Characterization, and Application of TiO2 and ZnO Nanostructures: A Defect Engineering Approach
指導教授： 彭宗平
Tsong-Pyng Perng
口試委員: 陳學仕
黃嘉宏
葉君棣
柯志忠
學位類別： 博士
Doctor
系所名稱： 工學院 - 材料科學工程學系
Materials Science and Engineering
論文出版年： 2016
畢業學年度： 104
語文別： 英文
論文頁數： 212
中文關鍵詞： 原子層沉積技術 、TiO2 、ZnO 、光催化反應
外文關鍵詞： Atomic Layer Deposition, TiO2, ZnO, Photocatalyst
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    • TiO2與ZnO的缺陷工程在近年來引起了廣泛的討論，主要是因為可以調整並明確說明這些材料的性質。舉例而言，在缺氧的氣氛下經熱處理之後，TiO2中氧原子空位在許多應用上如燃料電池、光催化反應、光感應器等有正面的影響。熱處理的條件是缺陷形成的關鍵因素，引入缺陷後，必須使用各種的儀器來分析金屬氧化物半導體的性質。然而，在許多文獻中，大多數研究只致力於此結構的應用，但是以系統化的方式來研究缺陷的行為，卻往往被忽視或只是輕描帶過。簡單來說，對於TiO2與ZnO材料的缺陷工程，如果缺乏基本概念的理解與紮實的背景知識，往往只能以不斷的試誤法來嘗試。
    本研究主要的動機係討論並研究TiO2與ZnO材料中的缺陷，所獲得的結果顯示，我們可以遵循明確的方式與途徑來預測材料的性質，並予以精準地調整，使其具有前瞻性的應用。
    第一部分，以原子層沉積技術(atomic layer deposition, ALD)製備TiO2薄膜，於350~500 oC氫氣中退火。由於氫化導致原子出現扭曲排列以及產生氧原子空位的緣故，誘發表面變得更粗糙。除此之外，大部分的氧原子空位形成於材料表面，當溫度上升時，其濃度與分布十分均勻。當退火溫度為500oC時，氧原子空位分布最均勻，同時表面的空位也具有最高的密度，與改良過後的TiO2具低電子-電洞再結合率及高光催化活性之結果一致。
    雖然在提升光催化效率上，氫化TiO¬2被視為簡單又有效的應用。但是對於雜亂相不易精確控制其厚度，在不同形貌上缺乏良好的共形性，限制了此應用的發展。為了解決這個問題，第二部分介紹了在結晶TiO2薄膜上，以ALD單一步驟的低溫沉積法製備額外的TiO2非晶層。這個複合結構對於提升光催化性質有益，因為所沉積的非晶層表現出獨特的性質，如分子局部雜亂性以及氧原子空位的生成，此與藉由氫化產生雜亂排列的殼層有異曲同工之妙。除此之外，改變ALD製程的循環數，雜亂排列的殼層與結晶基材的厚度可在原子級的尺度精確地控制，此結果對於調控TiO2複合物的光催化效率上極有助益。
    第三部分，以ALD製備ZnO薄膜，研究在10bar氫氣與350~450 oC退火溫度下，其缺陷的分布情形與配置。氫化同時造成氧原子與鋅原子的空位，其濃度與退火溫度具有高度相關性。以共聚焦光激發螢光圖譜分析這些缺陷在空間上的分布，發現彼此之間具有對應的關聯性，也顯示出奈米機械性質與彈性係數分布，可以被應用於估計ZnO頂端表面所累積的缺陷分配情形與個別少數原子層，建立表面形貌與缺陷的相關性。
    第四部分係將 ZnO奈米線成長於透明導電膜(ITO)上，並於10bar氫氣與350 oC退火氫化，導致能隙縮小，並提升光電化學效率。主要原因係在表面產生新的氧原子與鋅原子空位，這些空位的出現歸因於氫原子填充於晶格內並破壞Zn－O的鍵結。因此，單一類型的缺陷對於提升無序排列ZnO所呈現的效果應該重新考慮。
    最後一部分，係於低溫下以氫電漿處理ZnO，在表面形成不同濃度之電洞，而得到p-型導電ZnO薄膜。在薄膜最頂層以幾個原子層的深度分辨率評估電荷載子，可消除電漿製程對內部的影響，並建構一個理論架構，證明其p-型表面導電度，也解釋其與電漿處理時間的相關性。

    The defect engineering of TiO2 and ZnO has attracted immense interest in recent years since it was reported to be an efficient tool to adjust and specify the properties of these materials. For instance, it was demonstrated that the appearance of oxygen vacancies in TiO2 after thermal treatment in oxygen-deficient atmosphere has a positive impact on many applications such as fuel cell, photocatalyst, photosensors. It was determined that the condition of treatment crucially influences the formation of defects. Various instruments were used to analyze the properties of metal oxide semiconductors after introduction of defects. However, most of studies available in the literature were concentrated on the application of the structures, and systematic approach to investigate the behavior of defects has been often overlooked or only slightly covered. Simply to say, without basic knowledge and solid background about the defect engineering of TiO2 and ZnO those attempts can be referred to “trial-and-error pathway”.
    The main purpose of this research is to discuss and investigate the introduction of defects in TiO2 and ZnO. The obtained results would allow to predict the perspective properties of these materials and accurately tune some of their useful and promising applications following a certain pattern and road map.
    In the first part, TiO2 thin film prepared by atomic layer deposition (ALD) was subjected to annealing in hydrogen at 350-500 oC. Hydrogenation resulted in appearance of disordered states and oxygen vacancies which induced the increased surface roughness. Furthermore, it was revealed that oxygen vacancies were mostly formed on the surface, and their concentration and distribution uniformity increased with increasing temperature. The most uniform distribution of oxygen vacancies with the highest surface density was obtained after annealing at 500 oC, which is consistent with the lowest electron-hole recombination rate and highest photoactivity of the modified TiO2.
    Although hydrogenation of TiO2 is considered as a simple and effective approach to improve the photoefficiency, inability to precisely control the thickness of disordered phase and lack of good comformality over different morphologies limits the wide application of this technique. In order to solve this problem, in the second part it is shown a simple one-step and low temperature process to deposit an amorphous titanium dioxide overlayer by ALD on the crystalline TiO2 film. This composite structure was beneficial for improved photocatalytic properties since the as-deposited amorphous layer showed unique properties such as local disorder and presence of oxygen vacancies which are similar to disordered shell created by hydrogenation. Furthermore, it was demonstrated that by changing the cycle number of ALD process the thicknesses of disordered shell and crystalline substrate could be precisely controlled with an accuracy of atomic scale. It allows to tune the photoefficiency of TiO2 composite.
    In the third part, ZnO prepared by ALD and annealed in hydrogen at 10 bar and 350-450 oC was investigated in terms of defect distribution and allocation. Hydrogenation induced simultaneous formation of oxygen and zinc vacancies whose concentrations were closely related to the temperature of treatment. Spatial distributions of these defects were analyzed by photoluminescence confocal mapping which revealed that their localized appearances were linked to each other. It was also demonstrated that nanomechanical mapping of elastic modulus distribution could be used to assess the allocation of accumulated defects on the topmost surface of ZnO with a depth resolution of only several atomic layers. The correlation between the surface morphology and the accumulated defects was established.
    The fourth part is dedicated to ZnO nanorods grown on ITO and annealed in hydrogen at 10 bar and 350 oC. Hydrogenation resulted in narrowed band gap and enhanced photoelectrochemical efficiency. The origin of such improvement is discussed in terms of newly generated surface oxygen and zinc vacancies whose formation is attributed to initial filling of native defects with hydrogen atoms and subsequent breaking of Zn-O bonds. Hence, the effect of dominant role of only one type of defect on the improved performance of disordered ZnO should be reconsidered.
    In the last part of this work, low-temperature hydrogen plasma treatment with different lengths of time was used to fabricate a p-type surface conductive ZnO film with controlled concentration of holes. The distribution and concentration of charge carriers on the topmost surface of the film was assessed with a depth resolution of several atomic layers that allowed to eliminate any influence from the bulk. A theoretical framework was constructed to provide a rationale of the p-type surface conductivity and justify its relation to the treatment time.

    Table of Contents Chapter 1 Introduction 1.1 Titanium Oxide (TiO2) 1 1.2 Zinc Oxide (ZnO) 3 1.3 Methods to prepare TiO2 5 1.4 Methods to prepare ZnO 7 1.5 Defect engineering of TiO2 9 1.6 Defect engineering of ZnO 11 1.7 Bulk vs. surface defects 12 1.8 Atomic layer deposition 14 1.9 Motivation of this research 16 Chapter 2 Literature Survey 2.1 Synthesis, characterization, properties, and applications of disordered TiO2 24 2.1.1 Synthesis of disordered TiO2 24 2.1.1.1 Hydrogenation of TiO2 24 2.1.1.2 Other methods to create defects in TiO2 25 2.1.2 Characterization of disordered TiO2 27 2.1.1.1 Bulk analysis techniques 27 2.1.1.2 Surface analysis techniques 30 2.1.3 Properties of disordered TiO2 32 2.1.3.1 Structural properties 32 2.1.3.2 Optical properties 34 2.1.4 Applications of disordered TiO2 36 2.1.4.1 Photocatalytic and photoelectrochemical applications 36 2.1.4.2 Other applications 37 2.2 Synthesis, characterization, properties and applications of defective ZnO 39 2.2.1 Synthesis of defective ZnO 39 2.2.1.1 Hydrogenation of ZnO 39 2.2.1.2 Other methods to create defects in ZnO 41 2.2.2 Characterization of defects in ZnO 42 2.2.2.1 Bulk analysis techniques 42 2.2.2.2 Surface analysis techniques 44 2.2.3 Properties of defective ZnO 45 2.2.3.1 Structural properties 45 2.2.3.2 Optical and electrical properties 48 2.2.4 Applications of defective ZnO 50 2.2.4.1 Photocatalytic and photoelectrochemical applications 50 2.2.4.2 P-type conductive ZnO 52 2.2.4.3 Other applications 53 2.3 Defective engineering of other wide-band gap semiconductors 54 2.3.1 Surface defects 54 2.3.2 Bulk defects 56 Chapter 3 Surface Reconstruction, Oxygen Vacancy Distribution and Photocatalytic Activity of Hydrogenated Titanium Oxide Thin Film 3.1 Introduction 66 3.2 Experimental procedures 69 3.3 Results and discussion 70 3.3.1 Structural characterization 70 3.3.2 Raman spectroscopic analysis 77 3.3.3 Photoluminescence 89 3.3.4 UV-visible spectroscopy 92 3.3.5 Photocatalysis 94 3.4 Conclusion 99 References 100 Chapter 4. Enabling Higher Photoelectrochemical Efficiency of TiO2 via Controlled Formation of Disordered Shell: an Alternative to the Hydrogenation Process 4.1 Introduction 106 4.2 Experimental procedures 108 4.3 Results and discussion 110 4.4 Conclusion 127 References 127 Chapter 5. Distribution pattern and allocation of defects in Hydrogenated ZnO Thin Films 5.1 Introduction 130 5.2 Experimental procedure 133 5.3 Results and discussion 134 5.4 Conclusion 147 References 148 Chapter 6. Formation Mechanism and Nature of Defects in Hydrogenated ZnO Nanorods: the Origin of Enhanced Photoelectrochemical Efficiency 6.1 Introduction 153 6.2 Experimental procedures 156 6.3 Results and discussion 157 6.3.1 Electron microscopic and X-ray diffraction analysis 157 6.3.2 Resonant Raman spectroscopic analysis 161 6.3.3 Photoluminescence spectroscopic analysis 165 6.3.4 Visible Raman spectroscopic analysis 170 6.3.5 UV-vis spectroscopic analysis 172 6.3.6 Photoelectrochemical performance 174 6.4 Conclusion 178 References 178 Chapter 7. Hydrogenated ZnO with p-Type Surface Conductivity 7.1 Introduction 183 7.2 Experimental procedures 185 7.3 Results and discussion 186 7.4 Conclusion 200 References 200 Chapter 8. Conclusions 204 Chapter 9. Suggested Future Work 208 Publication List 211

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