釩酸鉍異質結構-用於可見光觸媒及光電化學等應用

BiVO4-Based heterostructures for Visible-Light Photocatalytic and Photoelectrochemical Applications

溫凡健(Nguyen, Van Chien)

指導教授 ： 朱英豪

國立交通大學/工學院/材料科學與工程系所/博士(2017年)

摘要

通過氧化物半導體光催化收穫太陽能提供了一種期望的方法，不僅解決替代的乾淨能源供應，而且還解決環境污染。光催化被廣泛用於有害物質的光分解或光氧化，人工光合作用，光催化水分裂以產生氫氣和氧氣。用於這些應用的材料對於具有有效的光吸收，良好的電荷分離，電荷傳輸性質，高穩定性，高活性和環境相容性是相當重要的。基於這些要求，由於釩酸鉍BiVO4（BVO）具有良好的能帶位置，窄帶隙和良好穩定性，此材料被視為可用於在可見光照射下釋放氧氣和光電化學水裂解的最有希望的光催化劑之一。這是在標準AM 1.5的太陽光照射下，BVO的理論太陽能 - 氫轉化效率達到9.2％，最大光電流為7.5
mA cm-2。然而，到目前為止， BVO較差的光生電荷載流子傳輸和水氧化的慢動力學使得其距離實現理論效率仍有一大段距離。單晶可以通過元素摻雜，晶面工程，形態控制以及宏觀/中孔結構構造以及通過均質/異質結構造和共催化劑負載製造的複合材料來克服這些限制。在本論文研究中，BVO被選擇作為一個模型系統，以演示提高光活性的方法，並了解基於氧化物半導體的磊晶異質結構基於磊晶生長的相關機制。通過使用磊晶生長，可以實現薄膜晶向的精確控制，組成之間的明確界面，以及排除諸如結構缺陷，晶界和雜質相的意外因素，確立理想納米複合光電極之設計。
首先，我們建立和分析異質磊晶與高質量的界面組成單晶單斜晶體BVO薄膜金納米晶體顆粒。使用脈衝激光沉積（PLD）在磊晶SrRuO3緩衝層和YSZ襯底上生長磊晶BVO膜。然後，通過使用去濕技術，具有各種尺寸和密度的金納米晶體顆粒均勻地生長在BVO的{001}面上。通過X射線光電子能譜和超快動力學的組合以及它們與光活性行為之間的相關性來研究金納米晶體顆粒尺寸和密度依賴性的能帶對齊和Au / BVO的動態弛豫過程。此外，基於3D有限差分時域仿真研究了表面等離子體共振（SPR）激發對Au / BVO的增強的光活性的貢獻。
接下來，我們製造了由氧化鎢WO和BVO相組成的自組裝晶相嵌入系統，並研究了界面處的電荷相互作用及其光催化活性。通過對結構特徵，能帶對齊和PEC性能的詳細研究，可以極大地改善嵌入晶體的異質結構的光活性。這種改進歸因於通過X射線光電子能譜（XPS），超快速動力學，光致發光（PL）光譜和電化學阻抗光譜（EIS）的組合證明的異質結處的電荷相互作用。我們的研究表明異質磊晶方法可以作為一個基本的平台，研究和了解金屬複合氧化物和/或晶體內嵌的複雜氧化物異質結構的基本光化學。

關鍵字

電荷轉移 ； Au-BiVO4 heterostructure ； WO3-BiVO4 heterostructure ； 光電化學水分裂 ； 異質外延 ； 自組裝納米複合材料

並列摘要

Harvesting solar energy by oxide-based semiconductor photocatalysis offers a desirable approach to solve not only the alternative clean energy supplies but also the environmental pollution. The photocatalysis is widely used for photo-decomposition or photo-oxidization of hazardous substances, artificial photosynthesis, photocatalytic water splitting to produce H2 and O2. The materials used for these applications are important to have efficient light absorption, good charge separation and charge transport properties, high stability, high activity and environmental compatibility. Based on these requirements, bismuth vanadate BiVO4 (BVO) has demonstrated as one of the most promising photocatalysts for the evolution of O2 and photoelectrochemical water splitting under the visible light irradiation owing to the favorably positioned band edges, the narrow band gap, and the good stability. The theoretical solar–to-hydrogen conversion efficiency of the BVO reached 9.2% with a maximum photocurrent of 7.5 mA cm-2 under standard AM 1.5 solar light irradiation. However, its achieved efficiency to date is far away from the theoretical efficiency maily due to the poor photo-generated charge carrier transportation and the slow kinetics of water oxidation. Single crystals can overcome these limitations by element doping, facet engineering, morphology control, as well as macro/mesoporous structure construction, and composites fabricated by homo/hetero-junction construction and co-catalyst loading. In this thesis study, the BVO has been chosen as a model system to demonstrate an approach to both improve photoactivity and understand the related mechanism of the oxide semiconductor-based epitaxial heterostructures based on epitaxial growth. By using the epitaxial growth, a precise control of film orientation, a well-defined interface between constituent phases, and the preclusion of unexpected factors such as structural defects, grain boundaries, and impurity phases, can be achieved, which are crucial for the rational design of ideal nanocomposite photoelectrodes.
Firstly, we established and characterized the heteroepitaxy with a high quality interface consisting single crystal monoclinic BVO thin film and Au nanocrystal particle (NPs). The epitaxial BVO film was grown on epitaxial SrRuO3 buffer layer and YSZ substrate using pulsed laser deposition (PLD). And then, the Au (NPs) with various sizes and densities were uniformly grown on {001} facets of BVO by using the dewetting technique. The Au NPs size and density dependencies of band alignment and the dynamic relaxation processes of Au/BVO were examined by a combination of X-ray photoelectron spectroscopy and ultrafast dynamics as well as the correlation between them and the photoactivity behaviors are discussed. In addition, the contribution of surface plasmon resonance (SPR) excitation on enhanced photoactivity of Au/BVO was explored based on the 3D finite-difference time domain simulation.
Next, we fabricated the self-assembled mesocrystal embedded system composed by WO and BVO phases and investigated the charge interaction at the interface and its photocatalytic activity. The photoactivity of the mesocrystal-embedded heterostructure can be greatly improved proved by a detailed investigation of the structural feature, band alignment, and PEC performance. This improvement is attributed to the charge interaction at the heterojunction evidenced by a combination of x-ray photoelectron spectroscopy (XPS), ultrafast dynamics, photoluminescence (PL) spectroscopy, and electrochemistry impedance spectroscopy (EIS). Our study suggests that the heteroepitaxial approach can serve as a fundamental platform to study and understand the fundamental photochemistry of metal-complex oxide and/or mesocrystal-embedded complex oxide heterostructures.

並列關鍵字

Charge transfer ； Au-BiVO4 heterostructure ； WO3-BiVO4 heterostructure ； Photoelectrochemical water splitting ； Energy band alignment ； Self-assembled Nanocomposites ； Heteroepitaxy

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