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.

1 Introduction
1.1 X-ray Absorption Spectroscopy 2
1.1.1 XANES and EXAFS 2
1.1.2 The FEFF codes 4
1.1.3 The FDMNES codes 4
1.2 Density Function Theory 5
1.2.1 Hohenberg-Kohn Theorem 6
1.2.2 Kohn-Sham Equations 7
1.2.3 Local Density Approximation 7
1.2.4 Generalized Gradient Approximation 8
1.2.5 LDA (GGU)+U 9
2 Photoreduction of Cerium ions in Undoped and Y-doped CeO2 Nanocrystals
2.1 Sample Preparation & Characterization 10
2.2 X-ray Irradiation Experiments 11
2.2.1 Irradiation Time 13
2.2.2 The Y Doping effect 15
2.2.3 Photon Energy 15
2.2.4 Photon Flux 16
2.3 Sample Stability Under X-ray Irradiation 17
2.4 UV-light Irradiation Experiments 18
2.5 DFT Calculations 21
2.6 CO Oxidation Experiments 24
3 Photoreduction of Cerium Ions in Ce-doped TiO2 Nanocrystals
3.1 Sample Preparation & Characterization 26
3.2 X-ray Irradiation Experiments 27
3.3 DFT Calculations 30
3.4 UV-light Irradiation Experiments 35
4 Photon Irradiation Incurred Cobalt Dopant Coordination Rearrangement in (Y, Co) Codoped CeO2 Nanocrystals
4.1 The Co-doped CeO2 37
4.1.1 Sample Preparation & Characterization 37
4.1.2 Co K-edge XANES & FDMNES Simulations 39
4.1.3 Co K-edge EXAFS 40
4.1.4 UV-vis Diffuse Reflectance Measurements 43
4.1.5 DFT Calculations 43
4.2 The (Y, Co) Codoped CeO2 46
4.2.1 Sample Preparation & Characterization 46
4.2.2 Co K-edge XANES & EXAFS 47
4.2.3 X-ray Irradiation Experiments 48
4.2.4 UV-light Irradiation Experiments 51
5 Heterovalent-Doping-Enabled Band Gap Engineering in (Y, Co)-Codoped CeO2 Nanocrystals
5.1 Sample Preparation & Characterization 53
5.2 Co K-edge & Y K-edge EXAFS 54
5.3 UV-vis Diffuse Reflectance Measurements 57
5.4 DFT Calculations 58
6 Conclusions
6.1 Conclusions 63

References

1. J. Tian, Y. Sang, Z. Zhao, W. Zhou, D. Wang, X. Kang, H. Liu, J. Wang, S. Chen, H. Cai, and H. Huang, Small 9, 3864 (2013).
2. W. Shi, Y. Li, J. Hou, H. Lv, X. Zhao, P. Fang, F. Zheng, and S. Wang, J. Mater. Chem. A 1, 728 (2013).
3. N. J. Lawrence, J. R. Brewer, L. Wang, T.-S. Wu, J. Wells-Kingsbury, M. M. Ihrig, G. Wang, Y.-L. Soo, W.-N. Mei, and C. L. Cheung, Nano Lett. 11, 2666 (2011).
4. Z. Wu, M. Li, and S. H. Overbury, J. Catal. 285, 61 (2012).
5. W. Lei, T. Zhang, L. Gu, P. Liu, J. A. Rodriguez, G. Liu, and M. Liu, ACS Catal. 5, 4385 (2015).
6. D. Jiang, W. Wang, L. Zhang, Y. Zheng, and Z. Wang, ACS Catal. 5, 4851 (2015).
7. G. Hua, L. Zhang, G. Fei, and M. Fang, J. Mater. Chem. 22, 6851 (2012).
8. A. Younis, D. Chu, Y. V. Kaneti, and S. Li, Nanoscale 8, 378 (2016).
9. M. Zeng, Y. Li, M. Mao, J. Bai, L. Ren, and X. Zhao, ACS Catal. 5, 3278 (2015).
10. J. Zou, Z. Si, Y. Gao, R. Ran, X. Wu, and D. Weng, J. Phys. Chem. C 120, 29116 (2016).
11. T. S. Wu, H. D. Li, Y. W. Chen, S. F. Chen, Y. S. Su, C. H. Chu, C. W. Pao, J. F. Lee, C. H. Lai, H. T. Jeng, S. L. Chang, and Y. L. Soo, Sci. Rep. 5, 15415 (2015).
12. T.-S. Wu, Y. Zhou, R. F. Sabirianov, W. N. Mei, Y.-L. Soo, and C. L. Cheung, Chem. Commun. 52, 5003 (2016).
13. A. D. Liyanage, S. D. Perera, K. Tan, Y. Chabal, and K. J. Balkus Jr., ACS Catal. 4, 577 (2014).
14. M. Nakayama, and M. Martin, Phys. Chem. Chem. Phys. 11, 3241 (2009).
15. A. Bianconi, A Marcelli, H. Dexpert, R. Karnatak, A. Kotani, T. Jo, and J. Petiau, Phys. Rev. B 35, 806 (1987).
16. G. Kaindl, G. Schmiester, E. V. Sampathkumaran, and P. Wachter, Phys. Rev. B 38, 10174 (1988).
17. A. Kotani, J. Elec. Spec. Rel. Phenom. 100, 75 (1999).
18. M. J. Lipp, J. R. Jeffries, H. Cynn, J.-H. Park Klepeis, W. J. Evans, D. R. Mortensen, G. T. Seidler, Y. Xiao, and P. Chow, Phys. Rev. B 93, 064106 (2016).
19. T. Duchoň, M. Aulická, E. F. Schwier, H. Iwasawa, C. Zhao, Y. Xu, K. Veltruská, K. Shimada, and V. Matolín, Phys. Rev. B 95, 165124 (2007).
20. M. Drescher, M. Hentschel, R. Kienberger, M. Uiberacker, V. Yakovlev, A. Scrinzi, Th. Westerwalbesloh, U. Kleineberg, U. Heinzmann, and F. Krausz, Nature 419, 803 (2000).
21. C.-Y. Ruan, F. Vigliotti, V. A. Lobastov, S. Chen, and A. H. Zewail, Proc. Natl. Acad. Sci. 101, 1123 (2004).
22. G. Sciaini, M. Harb, S. G. Kruglik, T. Payer, C. T. Hebeisen, F.-J. Meyer zu Heringdorf, M. Yamaguchi, M. Horn-von Hoegen, R. Ernstorfer, and R. J. D. Miller, Nature 458, 56 (2009).
23. M. Eichberger, H. Schäfer, M. Krumova, M. Beyer, J. Demsar, H. Berger, G. Moriena, G. Sciaini, and R. J. D. Miller, Nature 468, 799 (2010).
24. M. Gulde, S. Schweda, G. Storeck, M. Maiti, H. K. Yu, A. M. Wodtke, S. Schäfer, and C. Ropers, Science 345, 200 (2014).
25. T. Frigge, B. Hafke, T. Witte, B. Krenzer, C. Streubühr, A. Samad Syed, V. Mikšić Trontl, I. Avigo, P. Zhou, M. Ligges, D. von der Linde, U. Bovensiepen, M. Horn-von Hoegen, S. Wippermann, A. Lücke, S. Sanna, U. Gerstmann, and W. G. Schmidt, Nature 544, 207 (2017).
26. K. Ozawa, M. Emori, S. Yamamoto, R. Yukawa, S. Yamamoto, R. Hobara, K. Fujikawa, H. Sakama, and I. Matsuda, J. Phys. Chem. Lett. 5, 1953 (2014).
27. G. Kresse, and J. Furthmüller, Phys. Rev. B 54, 11169 (1996)
28. J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996).
29. C. W. M. Castleton, J. Kullgren, and K. Hermansson, J. Chem. Phys. 127, 244704 (2007).
30. D. A. Andersson, S. I. Simak, B. Johansson, I. A. Abrikosov, and N. V. Skorodumova, Phys. Rev. B 75, 035109 (2007).
31. T. Zacherle, A. Schriever, R. A. De Souza, and M. Martin, Phys. Rev. B 87, 134104 (2013).
32. H. J. Monkhorst, and J. D. Pack, Phys. Rev. B 13, 5188 (1976).
33. E. Wuilloud, B. Delley, W. D. Schneider, and Y. Baer, Phys. Rev. Lett. 53, 202 (1984).
34. A. Pfau, K. D. Schierbaum, Surf. Sci. 321, 71 (1994).
35. J. F. Jerratsch, X. Shao, N. Nilius, H. J. Freund, C. Popa, M. V. Ganduglia-Pirovano, A. M. Burow, and J. Sauer, Phys. Rev. Lett. 106, 246801 (2011).
36. X. Shao, J. F. Jerratsch, N. Nilius, and H. J. Freund, Phys. Chem. Chem. Phys. 13, 12646 (2011).
37. T. S. Wu, Y. C. Chen, Y. F. Shiu, H. J. Peng, S. L. Chang, H. Y. Lee, P. P. Chu, C. W. Hsu, L. J. Chou, C. W. Pao, J. F. Lee, J. Kwo, M. Hong, and Y. L. Soo, Appl. Phys. Lett. 101, 022408 (2012)
38. Y. L. Soo, T. S. Wu, C. S. Wang, S. L. Chang, H. Y. Lee, P. P. Chu, C. Y. Chen, L. J. Chou, T. S. Chan, C. A. Hsieh, J. F. Lee, J. Kwo, and M. Hong, Appl. Phys. Lett. 98, 031906 (2011)
39. M. Newville, P. Lïvins, Y. Yacoby, J. J. Rehr and E. A. Stern, Phys. Rev. B 47, 14126 (1993).
40. J. J. Rehr, J. Mustre de Leon, S. I. Zabinsky and R. C. Albers, J. Am. Chem. Soc. 113, 5135 (1991).
41. A. R. Albuquerque, A. Bruix, Iêda M. G. dos Santos, J. R. Sambrano, and F. Illas, J. Phys. Chem. C 118, 9677 (2014)
42. P. Kubelka, and F. Munk, Z. Tech. Phys. (Leipzig) 12, 593–601 (1931).
43. J. Tauc, and A. Menth, J. Non-Cryst. Solids 8, 569-585 (1972).
44. O. Bunău, and Y. Joly, J. Phys.: Condens. Matter 21, 345501 (2009).
45. J. P. Perdew, A. Ruzsinszky, G. I. Csonka, O. A. Vydrov, G. E. Scuseria, L. A. Constantin, X. Zhou, and K. Burke, Phys. Rev. Lett. 100, 136406 (2008).
46. C. Loschen, J. Carrasco, K. M. Neyman, and F. Illas, Phys. Rev. B 75, 035115 (2007).