A Structural Study of Iridium Oxide Grown on Ir(100) under Ambient Pressure Conditions

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/92529

Title:	A Structural Study of Iridium Oxide Grown on Ir(100) under Ambient Pressure Conditions
Authors:	李律昕;Lee, Lu-Hsin
Contributors:	物理學系
Keywords:	氧化銥;穿隧電子掃描儀;反射式高能電子繞射;近常壓X光光電子能譜儀
Date:	2023-04-17
Issue Date:	2024-09-19 15:54:44 (UTC+8)
Publisher:	國立中央大學
Abstract:	本實驗透過使用高能電子繞射(RHEED)、X-ray 光電子能譜(XPS)及穿隧電子掃描儀(STM)研究氧化銥薄膜在銥(001)樣品上的晶體結構及電子結構在高真空到近常壓環境下的成長機制，氧氣氣壓及曝量為控制氧化銥成長之主要之變因且樣品之成長溫度控制在 775 K 以利氧化銥成長。在氧氣曝量小於1.8 × 107 L時，銥(001)樣品上因被氧氣吸附而產生2 × 1的繞射結構，且在光電子能譜中銥金屬之 4f 電子軌域並無氧化的跡象，由此跡象推斷氧氣分子只是吸附在銥金屬表面，並無真正的變成金屬氧化物。增加氧氣曝量至1.8 × 108 L時，繞射結構從原本的2 × 1轉變至具有立體結構之繞射點，且繞射點的結構可以被解析為二氧化銥(IrO2)之(001)晶面朝上，在穿隧電子掃描儀之實驗中也可發現大量具一定高度之團簇(cluster)出現在表面，光電子能譜中的銥金屬之 4f 電子軌域開始些微氧化。增加氧氣曝量至6 × 108 L時，電子繞射實驗中出現二氧化銥(102)和(102̅)之兩相共存結構，繞射點間也開始漸漸出現繞射線，表示除了立體結構外，表面正在漸漸變得平整，穿隧電子掃描儀之實驗中也可以發現此變化，在光電子能譜中，銥金屬之 4f電子軌域開始劇烈氧化，經過計算，氧化層厚度可達 6 埃米並飽和不再增加。二氧化銥之(110)晶面只能在高於 1 torr 的氧氣環境中成長且反應時間需大於 30 分鐘，電子繞射實驗中表面出現明顯的立體結構與平面結構之共存態，隨著氧化層厚度增加至 18 埃米，因吸附物與金屬樣品之距離變遠，其交互作用應該要變小，因此我們懷疑其交互作用是來自於表面剛成長的氧化物而不是銥金屬基板，另外在穿隧電子掃描儀之實驗中，舉凡條紋或觸鬚狀結構出現在表面上，應證了我們上述的論點。為何二氧化銥之(110)晶面只能在高於 1 torr的氧氣環境中成長且反應時間需大於 30 分鐘?從熱力學第二定律出發，我們認為從銥金屬到反應形成氧化銥的過程應為不可逆反應，因此系統做功(系統亂度entropy)應隨著時間增加，使在中間態之氧化銥金屬能在氧化結束時找到適合其當下系統能量的結構，一旦結構因反應結束而被定義，更進一步的氧化既有的氧化層並不會進入原本不可逆之反應途徑，轉而進入另一條可逆反應中，因此二氧化銥(102)和(102̅)之兩相共存結構無法藉由更進一步的氧化變成二氧化銥之(110)晶面，同時兩種晶面都有出現飽和狀態，也就是在可逆反應中所說的平衡態，可以驗證我們的猜想。由此實驗可知，氧化銥的成長對於氣壓的改變非常敏感，其中反應機制值得探討，也希望對於氧化銥之成長機制的研究，能幫助未來在有機化合物於高反應性之氧化銥表面的催化反應的研究上有所幫助。 ;We used reflective high energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM) to investigate the structure and electronic state of iridium oxide layers grown by oxidizing Ir(001) at 775 K with oxygen pressure in between high vacuum to 5 torr. The result shows the surface only forms 2×1 oxygen adsorbed Ir(001) for oxygen dosage smaller than 1.8×〖10〗^7 L, indicating that Ir(001) is hard to be oxidized under this environment. The structure transforms to IrO2(001) with 3D structure form on the surface, where the oxide state can be observed in the valence band structure as the oxygen dosage increase to 1.8×〖10〗^8 L. Further increasing the oxygen dosage to 6×〖10〗^8 L results in the formation of the double-phase IrO2(102) & (102 ̅) surfaces, which has a huge oxide state signal in the valence band and shows the 3D structure with two symmetry phases coexisting on the surface. The IrO2(110) surface can be obtained if the pressure is larger than one torr. A flat surface coexisting with the 3D structure indicates that high surface-adsorbate interaction happens, which is counterintuitive to the general concept. The phenomenon can be elucidated as the strong surface oxide-adsorbate interaction, in which the dendritic island and stripe can form on the surface shown in the STM image. The XPS spectrum shows IrO2(110) eventually saturated at 18 Å with a dramatic enhancement of the oxide state. According to the second law of thermodynamics, the oxidation of clean Ir(001) could be an irreversible process so that the work of the intermediate state might determine the structure it will be transformed. Further oxidation on the oxide surface transfers to a reversible reaction so that the surface becomes equilibrium, eventually. On the other hand, the transformation from double-phase IrO2(102) & (102 ̅) to IrO2(110) is limited. This study demonstrated the step-by-step growth mechanism of iridium oxide under different oxygen dosages and pressure. The structure of iridium dioxide is very sensitive to oxygen pressure in the near ambient pressure environment. We hope this study can shed light on the study of organic compound reactions on stoichiometric IrO2.
Appears in Collections:	[Graduate Institute of Physics] Electronic Thesis & Dissertation

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