本研究採用溶膠凝膠法(sol-gel method)，以NH4VO3和Ti(OC3H7)4為前驅物(precursor)合成V1-xTixO2，目的是為了提升整體的化學均勻性，並著重於探討VO2-TiO2的相圖。其中，在VO2-TiO2系統中，在臨界溫度以下會發生可逆的spinodal相分離，而高於此溫度則會轉變為均勻的固溶體。藉由分析X光繞射圖，計算單相V1-xTixO2的晶格常數和Ti4+摻雜量的關係式，並由V0.7Ti0.3O2在不同溫度熱處理後發生spinodal相分離的結果，計算出相圖中spinodal line的臨界溫度為802K、臨界成分為35 mol % Ti。透過穿透式電子顯微鏡觀測顯微結構，結果顯示在發生spinodal相分離的成分中，有V-rich相和Ti-rich相組成的層狀結構，且其波長介於20-48 nm；而在發生成核成長的成分中，則會在基材中生成圓棒狀的不連續相。在電性表現方面，若發生spinodal相分離，在溫度介於318-341K有半導體急遽轉變成導體的現象，且隨著摻雜的Ti4+含量增多，其相變化溫度有降低的趨勢。

Spinodal decomposition with a reversible transformation between single and multiple phases takes place below a critical temperature in the system of vanadium-titanium oxide. To enhance chemical homogeneity, a sol-gel method using a mixture of ammonium vanadate and titanium isopropoxide is applied. Changes in the lattice parameter of single-phase state are analyzed by X-ray diffraction. The spinodal line, determined by annealing V0.7Ti0.3O2 at different temperatures in argon, has a critical temperature of 802K at a composition of 35 mol % Ti. Transmission electron micrographs reveal the spinodally decomposed samples have the lamella structures consisted of V-rich and Ti-rich layers with a wavelength of 20-48 nm, and the samples with nucleation & growth form the rod-like phase within the matrix. The metal-insulator transition takes place at reduced temperatures of 318-341K and a decrease in the metal-insulator transition temperature is observed with increasing amount of Ti4+ for the spinodally decomposed samples.

1. Y. Wu, L. Fan, W. Huang, S. Chen, Shi Chen, F. Chen, C. Zou, Z. Wu, "Depressed Transition Temperature of WxV1-xO2: Mechanistic Insights from the X-ray Absorption Fine Structure (XAFS) Spectroscopy," Physical Chemistry Chemical Physics, 16, 17705 (2014)

2. F. J. Morin, "Oxides Which Show a Metal-to-Insulator Transition at the Neel Temperature," Physical Review Letters, 3, 34 (1959)

3. X. Xu, X. He, G. Wang, X. Yuan, X. Liu, H. Huang, S. Yao, H. Xing, X. Chen and J. Chu, "The Study of Optimal Oxidation Time and Different Temperatures for High Quality VO2 Thin Film Based on the Sputtering Oxidation Coupling Method," Applied Surface Science, 257, 8824 (2011)

4. J. B. Goodenough, "The Two Components of the Crystallographic Transition in VO2," Solid State Chemistry, 3, 490 (1971)

5. M. M. Qazilbash, M. Brehm, B. G. Chae, P. C. Ho, G. O. Andreev, B. J. Kim, S. J. Yun, A. V. Balatsky, M. B. Maple, F. Keilmann, H. T. Kim and D. N. Basov, "Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging," Science, 318, 1750 (2007)

6. R. M. Wentzcovitch, W. W. Schulz and P. B. Allen, "VO2: Peierls or Mott-Hubbard? A View from Band Theory," Physical Review Letters, 72, 3389 (1992)

7. J. M. Booth and P. S. Casey, "Anisotropic Structure Deformation in the VO2 Metal-Insulator Transition," Physical Review Letters, 103, 086402 (2009)

8. X. Tan, T. Yao, R. Long, Z. Sun, Y. Feng, H. Cheng, X. Yuan, W. Zhang, Q. Liu, C. Wu, Y. Xie and S. Wei, "Unraveling Metal-Insulator Transition Mechanism of VO2 Triggered by Tungsten Doping," Science Report, 2, 466 (2012)

9. T. L. Wu, L. Whittaker, S. Banerjee and G. Sambandamurthy, "Temperature and Voltage Driven Tunable Metal-Insulator Transition in Individual WxV1-xO2 Nanowires," Physical Review B, 83, 073101 (2011)

10. M. Marezio, D. B. Mcwhan, J. P. Remeika and P. D. Dernier, "Structural Aspects of the Metal-Insulator Transitions in Cr-Doped VO2," Physical Review B, 5, 2541 (1972)

11. E. Strelcov, A. Tselev, I. Ivanov, J. D. Budai, J. Zhang, J. Z. Tischler, I. Kravchenko, S. V. Kalinin and A. Kolmakov, "Doping-Based Stabilization of the M2 Phase in Free-Standing VO2 Nanostructures at Room Temperature," Nano Letters, 12, 6198 (2012)

12. L. L. Fan, S. Chen, Z. L. Luo, Q. H. Liu, Y. F. Wu, L. Song, D. X. Ji, P. Wang, W. S. Chu, C. Gao, C. W. Zou and Z. Y. Wu, "Strain Dynamics of Ultrathin VO2 Film Grown on TiO2 (001) and the Associated Phase Transition Modulation," Nano Letters, 14, 4036 (2014)

13. Y. Muraoka and Z. Hiroi, "Metal-Insulator Transition of VO2 Thin Films Grown on TiO2 (001) and (110) Substrates," Applied Physics Letters, 80, 583 (2002)

14. Z. Hiroi, H. Hayamizu, T. Yoshida, Y. Muraoka, Y. Okamoto, J. I. Yamaura and Y. Ueda, "Spinodal Decomposition in the TiO2-VO2 System," Chemistry of Materials, 25, 2202 (2013)

15. J. W. Gibbs, In Collected Works, Vol. 1, Yale University Press: New Haven, Conneticut (1948)

16. C. Hu, H. Xu, X. Liu, F. Zou, L. Qie, Y. Huang and X. Hu, "VO2/TiO2 Nanosponges as Binder-Free Electrodes for High-Performance Supercapacitors," Scientific Reports, 5, 16012 (2015)

17. R. Shannon, "Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides," Acta Crystallographica Section A, 32, 751 (1976)

18. J. W. Cahn, "On spinodal decomposition," Acta Metallurgica, 9, 795 (1961)

19. R. Lopez, L. A. Boatner, T. E. Haynes, L. C. Feldman and R. F. H. Jr., "Synthesis and Characterization of Size-Controlled Vanadium Dioxide Nanocrystals in a Fused Silica Matrix," Journal of Applied Physics, 92, 4031 (2002)