The Structural, Photocatalytic and Photoelectric Properties of Oxide-Based Heterojunction Nanostructures

使用光觸媒材料在太陽光下進行”光催化分解水產氫”已是近來太陽能應用中重要且熱門的研究之一，如何提升光觸媒的催化活性進而提升能量轉換效率則是此類研究的重點。本研究中提出了多種二氧化鈦奈米管陣列以及以奈米管陣列為基底的異質接面氧化物複合材料來做為光電極，並探討離子缺陷和半導體異質接面對這些光觸媒材料其光催化及光電性質的影響。 首先，利用陽極氧化法搭配不同的電解液製備不同類型的二氧化鈦奈米管陣列(TiO2 nanotube array)，並將這些二氧化鈦奈米管陣列於450、550、650和750 °C下進行長時間退火處理，產生二氧化鈦氧缺乏的狀態。由SEM影像以及XRD圖譜得知，隨著退火溫度上升，銳鈦礦(anatase)結構的二氧化鈦奈米管開始坍塌並開始形成金紅石(rutile)相的二氧化鈦厚膜，且整體氧化物的厚度明顯提升，結晶結構也從純銳鈦礦結構轉變成由金紅石相主導的混相結構。XPS縱深分析中，在元素濃度穩定區域(the stable zone)中的氧鈦元素比(O/Ti atomic ratio)顯示各二氧化鈦試片的氧缺乏程度：在水基電解液製備之二氧化鈦奈米管陣列(縮寫為TiO2(aq))中，高溫(650和750 °C)退火的試片比低溫(450和550 °C)退火的試片擁有明顯小於2的氧鈦元素比；但在有機電解液製備之二氧化鈦奈米管陣列(縮寫為TiO2(EG))中，無論退火溫度高低，其氧鈦元素比都約為1.5，綜合以上兩種試片的氧鈦元素比結果，顯示氧空缺的濃度除了受退火溫度影響之外，也受到陽極處理時電解液種類的影響。在光吸收光譜分析中，大量氧空缺在高溫退火的TiO2(aq)試片中產生，這些氧空缺的存在造成長波長吸收率明顯提升、以及試片光吸收限的藍移現象；另外，在退火溫度高於550 °C的 TiO2(EG)試片中觀察到因為碳元素摻雜效應造成在波長400-600 nm範圍間光吸收率的上升、以及光吸收限的紅移現象。在光電流量測中，低溫退火的二氧化鈦奈米管陣列擁有較完整的奈米管結構以及較大的比表面積來進行化學反應，因此相較於高溫退火試片，在全光譜白光或是單純紫外光下展現較大的光電流反應；而這些高溫退火後的二氧化鈦試片，因為擁有極高濃度的氧空缺，所以在可見光(> 500 nm)光源下表現較強的光電流反應。 為了在二氧化鈦奈米管陣列中引入半導體異質接面的概念，二氧化鈦奈米管的表面利用水熱法披覆上鈦酸鍶或氧化鈰的奈米顆粒，藉此形成奈米管綴以奈米顆粒的複合材料。利用UV-Vis反射光譜及UPS分析，”二氧化鈦-鈦酸鍶”和”二氧化鈦-氧化鈰”兩種奈米異質接面的能帶結構得以被建構。另外，藉由製程參數的改變來調整氧空缺在複合材料內部的濃度，得以應證異質接面的能帶結構及其光電表現深受氧空缺存在的影響。而相較於單純的二氧化鈦奈米管陣列，這些奈米異質接面複合材料擁有較為優異的光電流以及光催化分解水產氫之表現，如此劇烈的光電流及光催化能力提升則是歸功於這些半導體異質接面產生的電位能差異，此電位能差異能加速光致電子電洞對分離，降低光致電子電洞對再結合的機率。最後，二氧化鈦-氧化鈰之奈米異質接面複合材料作為光電極材料運用於”光催化分解水產氫”為本研究之首創。In order to improve the efficiency of water splitting in photocatalysis, a series of photoelectrodes based on TiO2 nanostructures were proposed in this study. The optical and photoelectric properties of these photocatalysts influenced by ionic defects and semiconductor-composite heterojunctions were investigated. The dopant-free oxygen-deficient TiO2 nanotube arrays were prepared by electrochemical anodization in the aqueous and organic electrolytes, respectively yielding TiO2(aq) and TiO2(EG) nanotube arrays, followed by long-time annealing at four temperatures – 450, 550, 650, and 750 °C. The evolution of architectures (i.e., anatase nanotubes and rutile film) in TiO2 nanotube arrays is confirmed by XRD patterns and SEM micrographs. The depth profiles of these annealed TiO2 samples are obtained from XPS analysis, and the elemental-concentration stable zones within the TiO2 nanostructures show the approximate O/Ti atomic ratios, revealing the extent of oxygen deficiency. The TiO2(aq) samples annealed at high temperatures (i.e., 650 and 750 °C) have O/Ti atomic ratios significantly less than 2 compared to the low-temperature-annealed TiO2(aq) samples, and the TiO2(EG) samples annealed at these four temperatures show extreme O/Ti atomic ratios around 1.5, revealing that the oxygen vacancy concentration in TiO2 nanotube arrays is governed by the annealing temperature and the experimental conditions in the anodization procedure. The optical absorption spectra demonstrate quite different behavior between these two kinds of TiO2 nanotube arrays: a blue shift in absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies is observed in TiO2(aq) samples; on the other hand, a red shift in absorption edge and an increase in absorbance within the wavelength region of 400-600 nm both result from the carbon doping effect, and are examined in TiO2(EG) samples. For the photocurrent density measurement under controlled light irradiation, the low-temperature-annealed TiO2 samples exhibit large photocurrent responses under light sources containing UV because the high specific surface area provides a large number of active sites for chemical reactions. A strong photocurrent response is found for high-temperature-annealed TiO2 samples under filtered white light (visible light range, λ > 500 nm), which is attributed to the presence of a high concentration of oxygen vacancies. Nanostructured composites composed of TiO2 nanotube arrays and SrTiO3 or CeO2 nanoparticles were fabricated, forming an array of TiO2(EG) nanotubes coated with SrTiO3 or CeO2 nanoparticles. The UV-Vis and UPS spectra were adopted to identify the band structures of the TiO2-SrTiO3 and TiO2-CeO2 heterojunctions. The oxygen vacancy concentration, which can be modified by adjusted the experimental parameters, in composites strongly influenced the band structure of the heterojunction and the photoelectric properties of the composite samples. Compared to the TiO2(EG) nanotube arrays, the photocurrent densities and the capability of photocatalytic water splitting for these composite samples under irradiation are enhanced because the semiconductor heterojunctions in the composites promote the separation of the photo-induced e-/h+ pairs.致謝 I 中文摘要 IV ABSTRACT VI CONTENTS IX LIST OF FIGURES XIV LIST OF TABLES XXVI Chapter 1 Introduction 1 1.1 Motivation 1 1.2 The Outline of the Dissertation 6 Chapter 2 Literature Review 9 2.1 Photoelectrochemical Water Splitting 9 2.1.1 Fundamental Theories of Water Splitting 9 2.1.2 Photoelectrochemical Cell (PEC) 15 2.1.2.1 Band Structure 16 2.1.2.2 The Semiconductor/Electrolyte Interface 24 2.1.2.3 Designs of Photoelectrode 29 2.2 Improvements in Photocatalytic Activity of Photoelectrode 34 2.3 Photoelectrode Materials 39 2.3.1 TiO2 41 2.3.1.1 Electrochemically Anodized TiO2 Nanotube Arrays 46 2.3.1.2 Defect Chemistry 49 2.3.2 SrTiO3 51 2.3.2.1 TiO2-SrTiO3 Composites 53 2.3.3 CeO2 55 2.3.3.1 TiO2-CeO2 Composites 56 Chapter 3 Experimental Procedure 58 3.1 Anodic TiO2 Nanotube Arrays 59 3.1.1 Anodization in Aqueous Electrolyte 59 3.1.2 Anodization in Organic Electrolyte 60 3.2 TiO2-SrTiO3 Composite Thin Films and Nanostructures 60 3.2.1 Synthesis of TiO2 Sol and Thin Film 60 3.2.2 Synthesis of SrTiO3 and TiO2-SrTiO3 Thin Films 61 3.2.3 Synthesis of TiO2-SrTiO3 Composite Nanostructures 62 3.3 TiO2-CeO2 Composites Thin Films and Nanostructures 63 3.3.1 Synthesis of CeO2 Sol and Thin Film 63 3.3.2 Synthesis of TiO2-CeO2 Composite Nanostructures 64 3.4 Characterization of Properties 65 3.4.1 Microstructure and Crystalline Phases Analysis 65 3.4.2 Optical Properties Measurement 66 3.4.3 Atomic Composition Determination 66 3.4.4 Photocurrent Density Measurement 67 3.4.5 Hydrogen Production Measurement 73 Chapter 4 Results 75 4.1 Aqueous-Based TiO2 Nanotube Arrays 75 4.1.1 Crystalline Phases and Microstructure 75 4.1.2 O/Ti Atomic Ratio 80 4.1.3 Optical Absorption and Band Gap 83 4.1.4 Photocurrent Density 86 4.2 Organic-Based TiO2 Nanotube Arrays 88 4.2.1 Crystalline Phases and Microstructure 88 4.2.2 O/Ti Atomic Ratio 92 4.2.3 Optical Absorption and Band Gap 95 4.2.4 Photocurrent Density 96 4.3 TiO2-SrTiO3 Composite Thin Films and Nanostructures 98 4.3.1 TiO2-SrTiO3 Composite Thin Films 98 4.3.1.1 Crystalline Phases and Microstructure 98 4.3.1.2 Band Structure of the Heterojunction 100 4.3.1.3 Photocurrent Density 104 4.3.2 TiO2-SrTiO3 Composite Nanostructures 105 4.3.2.1 Crystalline Phases and Microstructure 105 4.3.2.2 Band Structure of the Heterojunction 109 4.3.2.3 Photocurrent Density 112 4.3.2.4 Modification of the Heterojunction 113 4.4 TiO2-CeO2 Composite Thin Films and Nanostructures 119 4.4.1 TiO2- CeO2 Composite Thin Films 119 4.4.1.1 Crystalline Phases and Microstructure 119 4.4.1.2 Band Structure of the Heterojunction 121 4.4.1.3 Photocurrent Density 124 4.4.2 TiO2- CeO2 Composite Nanostructures 125 4.4.2.1 Crystalline Phases and Microstructure 125 4.4.2.2 Band Structure of the Heterojunction 135 4.4.2.3 Photocurrent Density 141 4.4.2.4 Hydrogen Production 145 Chapter 5 Discussion 148 5.1 Aqueous-Based TiO2 Nanotube Arrays 148 5.1.1 Crystalline Phases and Microstructure 148 5.1.2 O/Ti Atomic Ratio 150 5.1.3 Optical Absorption and Band Gap 155 5.1.4 Photocurrent Density 159 5.2 Organic-Based TiO2 Nanotube Arrays 162 5.2.1 Crystalline Phases and Microstructure 162 5.2.2 O/Ti Atomic Ratio 162 5.2.3 Optical Absorption and Band Gap 164 5.2.4 Photocurrent Density 167 5.3 TiO2-SrTiO3 Composite Thin Films and Nanostructures 169 5.3.1 TiO2-SrTiO3 Composite Thin Films 169 5.3.1.1 Crystalline Phases and Microstructure 169 5.3.1.2 Band Structure of the Heterojunction 169 5.3.1.3 Photocurrent Density 170 5.3.2 TiO2-SrTiO3 Composite Nanostructures 170 5.3.2.1 Crystalline Phases and Microstructure 170 5.3.2.2 Band Structure of the Heterojunction 171 5.3.2.3 Photocurrent Density 172 5.3.2.4 Modification of the Heterojunction 175 5.4 TiO2-CeO2 Composite Thin Films and Nanostructures 178 5.4.1 TiO2-CeO2 Composite Thin Films 178 5.4.1.1 Crystalline Phases and Band Structures 178 5.4.1.2 Photocurrent Density 181 5.4.2 TiO2-CeO2 Composite Nanostructures 181 5.4.2.1 Crystalline Phases and Microstructure 181 5.4.2.2 Band Structure of the Heterojunction 184 5.4.2.3 Photocurrent Density 186 5.4.2.4 Hydrogen Production 190 Chapter 6 Conclusion 193 Chapter 7 Future Work 196 REFERENCE 19