自組裝BiFeO3-ε-Fe2O3/STO垂直異質磊晶＆半透明柔性氧化物Fe2O3/ZnO/Mica異質磊晶製備與可見光光電化學研究

近年來，光電化學水分解被視為一種具有前景的太陽能分解水產生氫氧燃料的技術，而此項技術同時擁有環保和可再生能源的特性，再加上奈米技術的發展，為光電化學水分解提供了新穎且高效能的光電極材料，使得此技術變得更具競爭力。目前許多研究致力於開發高效而且穩定的光電極，其中，結合兩種以上材料的異質結構為設計高效活性光電極提供了新的方法，而這是單一材料無法達到的，由於其獨特的性質，例如：高界面比，結構上與物理和化學的相互作用現象，人們已經開發並探索了新型異質結構光電極的重大研究。在本論文中，我們透過利用兩相之間密切接觸的優點來設計和研究新穎的異質結構，從而引起可調諧的電荷相互作用，為光電極的優化提供了新的配置型態。本論文將分成兩部分進行討論：應用於光電化學水分解之 (1) 自組裝垂直BiFeO3-ε-Fe2O3（BFO-FO）異質結構光電極的製備與表徵，(2) 柔性Fe2O3 / ZnO /Mica異質磊晶光電極的製備與表徵。在第一部分中，我們利用脈衝雷射沉積(Pulsed laser deposition, PLD)、X射線繞射（X-ray diffraction, XRD）、原子力顯微術（Atomic force microscopy, AFM）和穿透電子顯微術（Transmission electron microscopy, TEM）成長BFO-FO異質結構並研究其微觀結構，詳細的結構顯示出BFO和FO在SrTiO3（111）基板上交替生長，並具有垂直排列的柱狀結構。最重要的是，與單一BFO和FO相比，具有高界面-體積比的BFO-FO異質結構表現出顯著增強的光活性，我們進一步透過以下技術：泵探針光譜學(Pump-probe spectroscopy)、X射線光電子能譜學（X-ray photoelectron spectroscopy, XPS）、紫外光-可見光譜學（Ultraviolet-visible spectroscopy ）以及電化學阻抗譜法(Electrochemical impedance spectroscopy, EIS)去研究熱平衡與光激發態下兩相之間的電荷傳輸，所得的結果顯示電荷分離的鬆弛時間增加，抑制了電子-電洞復合。在第二部分中，我們開發了用於光電化學水分解的柔性異質磊晶光電極。眾所周知地，柔性電子器件技術的進步最近因其高存儲量、輕量、低成本和可彎曲性而引起了極大的科學興趣，為了開創以太陽能驅動電解之應用的可能性，極需對新型柔性透明光電化學水分解裝置進行研究。傳統的柔性基板，如：聚對苯二甲酸乙二酯、碳布和石墨烯已經被提出，但是由於其多晶結構、不穩定的化學物質以及脆弱且昂貴等特性；因此，希望可以提供一個新的平台來開發具有優異柔韌性、耐用性與化學穩定性的柔性透明光電化學水分解電池。在這項研究中，我們成長Fe2O3/ZnO/Mica異質磊晶氧化物，作為柔性透明光電極，並且研究其晶體結構、光學性質與光活性，再藉由使用時間解析光致發光（Time-resolved photoluminescence, TRPL）和超快動態量測來研究電荷載體和電荷分離。此外，在兩種不同的彎曲條件下測試了Fe2O3/ZnO/AZO/Mica的操作穩定性和機械耐久性，結果顯示Fe2O3/ZnO光電極不僅具有顯著的光電化學增強作用，而且也具有良好的柔韌性和耐久性。

關鍵字

BiFeO3 ； Fe2O3 ； ZnO ；異質磊晶；光光電化學研究

並列摘要

Recently, photoelectrochemical (PEC) water splitting has attracted much attention as a promising technique for production of hydrogen and oxygen fuel from solar energy and water, an environmental-friend and renewable sources. With the development of nanotechnology, which offers novel and efficient photoelectrode materials, the PEC technique could become more competitive. A significant research effort has been done in the development of efficient and stable photoelectrode materials. Currently, heterostructures, combining two or more than two materials, have paved a new route for designing highly active photoelectrodes that cannot be achieved with single component photocatalysts. Significant researchers have been developed and explored on the novel photoelectrodes based on heterostructures due to its unique properties such as high interface-to-surface ratios, structural, physical, and chemical interaction phenomena. In this thesis, we designed and investigated the novel heterostructures by taking the advantages of the intimate contact between two phases that can lead to a tunable charge interaction, providing a new configuration for the optimization of photoelectrodes. It will be discussed in two parts: (1) fabrication and characterization of self-assembled vertical BiFeO3- ε-Fe2O3 (BFO-FO) heterostructural photoelectrode, (2) fabrication and characterization of flexible Fe2O3/ZnO/Mica heteroepitaxial photoelectrode for photoelectrochemical (PEC) water splitting. In the first part, we fabricated and investigated the microstructure of the BFO-FO heterostructure by using pulsed laser deposition, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The detailed structures reveal that BFO and ε-FO have been grown alternatively on STO(111) substrate with a vertically aligned columnar structure. Most importantly, the BFO-FO heterostructures with a high interface-to-volume ratio exhibit a significant enhancement in photoactivity comparing to single BFO and ε-FO phases. We then focused on investigating charge transport between two phases at thermal equilibrium and under light excitation states by taking several techniques, including the pump-probe spectroscopy, x-ray photoelectron spectroscopy (XPS), Ultraviolet-visible spectroscopy (UV-vis), and electrochemical impedance spectroscopy (EIS). The obtained results indicated an increased relaxation time of charge-separation, suggesting a suppression of electron-hole recombination. In the second part, we developed a flexible epitaxial photoelectrode for PEC water splitting. It is well-known that the advances in technology for flexible electronic devices have recently received great scientific interests due to its high storage, lightweight, low cost, and bendability. In order to open new opportunities in the development of solar-driven electrolysis, investigation of novel flexible and transparent PEC device is highly on demand. Traditional flexible substrates such as polyethylene terephthalate, carbon cloth, exfoliated graphene have been proposed, but they are polycrystalline, unstable chemical, fragile and costly. Therefore, it is very desirable to deliver a new platform to develop soft transparent PEC cells with superior pliancy, durability, and chemical stability. In this study, we fabricated a promising flexible transparent photoelectrode in the harvest of solar energy based on flexible Fe2O3/ZnO/Mica oxide heteroepitaxy. We have successfully fabricated and investigated the crystal structures, optical and photoactivity properties. The charge carrier and charge separation were also investigated by using time-resolved photoluminescence (TRPL) and ultrafast dynamic measurements. Furthermore, the operational stability and mechanical durability of Fe2O3/ZnO/AZO/Mica were tested under two different bending conditions. The results show that the Fe2O3/ZnO photoelectrode not only exhibits remarkable PEC enhancement but also presents good flexibility and durability. The thesis consists of 5 chapters and is organized as follows: Chapter 1 presents the background and literature review for water splitting and the challenge in design photo-electrode structures. Chapter 2 describes the key experimental methods for structure characterization, electrical, magnetic, and electrochemistry analyses. The main results are presented in chapter 3 & 4. Chapter 3 shows self-assembled BiFeO3-ε-Fe2O3 vertical heteroepitaxy for visible light photoelectrochemistry. Chapter 4 presents semitransparent photoelectrode based on flexible oxide Fe2O3/ZnO/Mica heteroepitaxy for photoelectrochemical water splitting. Finally, Chapter 5 shows the summary of the present study.