利用同步輻射X光技術、光學方法與第一原理計算對氧化物奈米晶體之研究

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研究生：	吳泰興 Wu, Tai-Sing
論文名稱：	利用同步輻射X光技術、光學方法與第一原理計算對氧化物奈米晶體之研究 Studies of Nanocrystal Oxide Materials Using Synchrotron-Radiation X-ray Techniques, Optical Methods, and First-principles Calculations
指導教授：	蘇雲良 Soo, Yun-Liang
口試委員:	諸柏仁 Chu, Peter Po-Jen 鄭弘泰 Jeng, Horng-Tay 黃玉山 Huang, Yu-Shan 湯茂竹 Tang, Mau-Tsu
學位類別：	博士 Doctor
系所名稱：	理學院 - 物理學系 Department of Physics
論文出版年：	2017
畢業學年度：	105
語文別：	英文
論文頁數：	64
中文關鍵詞：	氧化鈰、氧缺陷、能隙工程
外文關鍵詞：	Co-doped CeO2
相關次數：	點閱：2 下載：0
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在含有氧化鈰的觸媒材料中，其催化能力受到所含Ce3+濃度與材料的能隙所影響，許多研究也致力於提高材料中Ce3+濃度與縮小材料的能隙。
在本論文的前半部，我們探討了鈰原子在金屬氧化物中的光致還原現象。我們發現當照射到X光時，材料中的鈰離子會由四價轉變成三價，其催化能力亦顯著提升。透過X光吸收光譜的量測與第一原理計算，我們提出了由價帶電洞驅動的機制來解釋此一觀察到的現象。在論文後半部，我們提出了兩種操控氧化鈰能隙的方法。我們發現在鈷摻雜的氧化鈰中，經高溫退火後材料能隙大幅變小。藉由X光吸收光譜的量測配合理論計算，發現經高溫退火後鈷原子周圍的氧配位結構由六配位轉變成四配位，使得鈷3d軌域由原先的eg與t2g能級分裂變得更加雜化，因而造成材料能隙的減小並使材料的催化能力增加。此外我們也展示了在未鍛燒樣品、鈷周圍結構仍維持六配位的情況下，透過釔與鈷原子的共摻雜來操控材料能隙。隨著共摻雜的釔濃度上升，材料的能隙逐漸變小，透過第一原理理論計算，我們發現隨著材料當中氧缺陷濃度的增加，氧的2p軌域對於鈷的3d軌域的斥力減少，使得鈷3d軌域的eg與t2g能級分裂減小，而導致材料能隙變窄。

Owing to the fact that the catalytic activity of ceria-containing catalysts is largely affected by its Ce3+ concentration and band gap value, in this dissertation, we decided to study the photoreduction effect on Ce ion of oxide materials, using x-ray techniques and the first-principles calculations based on density functional theory. When irradiated under hard x-ray, the Ce4+ ions of the materials are gradually reduced to Ce3+ ions. The increased Ce3+ concentration enhanced the catalytic activity of the materials. A valance-hole-dominated mechanism is proposed to explain the observed photoreduction effect.
We also report the observed dramatic band gap narrowing of Co-doped CeO2 nanocrystals after thermal annealing. As demonstrated by x-ray absorption fine structures, thermal annealing causes an oxygen coordination rearrangement around Co atoms from an octahedral coordination to a square-planar coordination. First-principles calculations reveal two stable oxygen coordination types surrounding Co, consistent with the experimental observation. The band gap reduction is accompanied by a substantial enhancement of catalytic activity.
Finally, we demonstrate a controllable modulation of energy band gaps of CeO2 nanocrystals by incorporating heterovalent dopant elements into the material. In (Co, Y) codoped CeO2, the Co dopant atoms were found to act as a switch that turns on the dormant effect of Y-modulated band gap reduction. As revealed by density functional theory calculations, a Co 3d band that hybridizes with Ce 4f band was lowered due to reduced O 2p repulsion, which arose from oxygen vacancies. The Y doping gave rise to the observed band gap narrowing effect.

Introduction
1 X-ray Absorption Spectroscopy    2
1.1 XANES and EXAFS    2
1.2 The FEFF codes    4
1.3 The FDMNES codes    4
2 Density Function Theory    5
2.1 Hohenberg-Kohn Theorem    6
2.2 Kohn-Sham Equations    7
2.3 Local Density Approximation    7
2.4 Generalized Gradient Approximation     8
2.5 LDA (GGU)+U    9
Photoreduction of Cerium ions in Undoped and Y-doped CeO2 Nanocrystals
1 Sample Preparation & Characterization    10
2 X-ray Irradiation Experiments    11
2.1 Irradiation Time    13
2.2 The Y Doping effect    15
2.3 Photon Energy    15
2.4 Photon Flux    16
3 Sample Stability Under X-ray Irradiation    17
4 UV-light Irradiation Experiments    18
5 DFT Calculations    21
6 CO Oxidation Experiments    24
Photoreduction of Cerium Ions in Ce-doped TiO2 Nanocrystals
1 Sample Preparation & Characterization    26
2 X-ray Irradiation Experiments    27
3 DFT Calculations    30
4 UV-light Irradiation Experiments    35
Photon Irradiation Incurred Cobalt Dopant Coordination Rearrangement in (Y, Co) Codoped CeO2 Nanocrystals
1 The Co-doped CeO2    37
1.1 Sample Preparation & Characterization    37
1.2 Co K-edge XANES & FDMNES Simulations    39
1.3 Co K-edge EXAFS    40
1.4 UV-vis Diffuse Reflectance Measurements    43
1.5 DFT Calculations    43
2 The (Y, Co) Codoped CeO2    46
2.1 Sample Preparation & Characterization    46
2.2 Co K-edge XANES & EXAFS    47
2.3 X-ray Irradiation Experiments    48
2.4 UV-light Irradiation Experiments    51
Heterovalent-Doping-Enabled Band Gap Engineering in (Y, Co)-Codoped CeO2 Nanocrystals
1 Sample Preparation & Characterization    53
2 Co K-edge & Y K-edge EXAFS    54
3 UV-vis Diffuse Reflectance Measurements    57
4 DFT Calculations    58
Conclusions
1 Conclusions    63

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