應用飛行時間-二次離子質譜(ToF-SIMS)研究多孔有機奈米框架結構(COF)中客體分子之擴散行為

Abstract

新型多孔有機奈米材料-共價有機框架(Covalent Organic Framework, COF)，因具有完全的有機結構、可調孔徑和優異的耐熱性等，而逐漸引起人們的關注，可被應用在半導體元件、藥物傳遞、能源儲存及轉換、CO2減排或催化H2生成等領域上。COF內部分子的吸收、釋放、擴散亦或是傳輸是許多應用和決定分子性質的重要關鍵因素，然而，目前難以確定已負載物種的分佈以及合成改質後(Post-Synthetic Modification, PSM)分子結構的變化。儘管可通過分析技術(如:固態核磁共振)來確認已負載物質的存在，並根據化學位移確定吸附物種的化學環境，但很難直接測量外表面(Outer surface)和內部分子在尺度為奈米等級下的空間分佈差異。因此，尋找可成功辨識在奈米尺度下外表面及內部分子的空間分布差異以建構正確結構資訊之方法極為重要。 本研究首先透過掃描式電子顯微鏡(SEM)、高功率X光繞射分析儀(XRD)、傅立葉轉換紅外光譜儀(FTIR)及X光光電子能譜儀(XPS)來確認COF的表面形貌、晶相、官能基、化學組態及元素含量等資訊，挑選出一個最適合用於在ToF-SIMS分析中的COF製備參數。接著，使用高靈敏度之表面分析儀器-飛行時間-二次離子質譜儀(ToF-SIMS)，透過各種不同的濺射離子源及不同能量密度的濺射參數調整，尋找出對於COF薄膜最佳的濺射參數，以建構正確的縱深資訊來分析COF之分子結構及其內部孔隙對於客體分子的擴散行為，期望透過此分析技術對於有機奈米框架結構-COF有更完整的了解。 於SIMS的研究中，我們使用C60+作為分析離子源，濺射離子源則以不同能量的Ar+、GCIB (Ar2500+)及C60+來分析，最終找到15 kV (E/n = 6) 或20 kV (E/n = 8)的GCIB (Ar2500+)為最適合分析COF薄膜的最佳濺射條件。接著本研究使用常見於汙水中的汙染物-甲基藍及結晶紫來做為客體分子，搭配適合客體分子之最佳濺射參數，研究其在COF孔隙中的擴散行為，經分析結果顯示，甲基藍(1.4 nm×1.0 nm, 長方形狀)在COF孔隙中的擴散率大於結晶紫(1.4 nm×1.4 nm, 正方形狀)，顯示出客體分子在COF孔隙中會因為本身的分子大小與形狀，而影響其擴散率。 總結來說，本研究選擇一最適用於SIMS分析的COF製程條件，並找出適合COF薄膜分析的最佳濺射參數，為COF建構了良好的縱深分析資訊，且最後利用建構好的縱深分析參數搭配適合客體分子之最佳濺射參數，來分析客體分子於COF孔隙中的擴散行為，於此研究中，我們建立了一個正確、完整且有效的縱深分析資訊，為往後進行COF分析時提供一個可以參考的研究方向。

Covalent Organic Framework (COF), a novel porous organic nanomaterial, have garnered attention due to their fully organic structure, tunable pore sizes, and excellent thermal stability. COF hold potential in areas such as semiconductor devices, drug delivery, CO2 reduction, and catalytic H2 generation. Understanding the absorption, release, and transport of molecules within COF is critical for these applications. However, determining the spatial distribution of loaded species and post-synthetic modifications (PSM) remains challenging. While techniques like solid-state NMR can confirm the presence of substances and their chemical environment, they struggle to measure nanoscale spatial differences between surface and internal molecules. In this study, SEM, XRD, FTIR, and XPS were first employed to examine the surface morphology, crystalline phase, functional groups, chemical composition, and elemental content of covalent organic framework (COF), in order to identify the most suitable synthesis conditions for subsequent ToF-SIMS analysis. A high-sensitivity surface analysis technique—time-of-flight secondary ion mass spectrometry (ToF-SIMS)—was then applied, utilizing various sputtering ion sources and energy densities to determine the optimal sputtering parameters for COF thin films. This enabled the construction of accurate depth profiles for analyzing the molecular structure of COF and investigating how their internal pores influence the diffusion behavior of guest molecules, thereby offering a more comprehensive understanding of these porous organic frameworks. In the SIMS investigation, C60+ was used as the acquisition beam, while Ar+, GCIB (Ar2500+), and C60+ with varying energies were employed as sputtering ion sources. The optimal sputtering condition for COF thin films was determined to be GCIB (Ar2500+) at 15 kV (E/n = 6 eV/atom) or 20 kV (E/n = 8 eV/atom). Subsequently, methylene blue (MB) and crystal violet (CV), two commonly encountered pollutants in wastewater, were introduced as guest molecules. By combining optimized sputtering parameters specific to these guest molecules, the diffusion behavior within the COF nanopores was investigated. The results indicated that methylene blue (1.4 nm×1.0 nm, rectangular shape) exhibited a higher diffusion rate than crystal violet (1.4 nm×1.4 nm, square shape), suggesting that molecular size and geometry play a critical role in governing the diffusion efficiency of guest species in confined COF structures. In summary, this study successfully identified the most suitable COF synthesis conditions for SIMS analysis and established the optimal sputtering parameters for COF thin film depth profiling. A reliable and accurate depth profiling strategy was developed and further utilized to evaluate the diffusion dynamics of guest molecules within COF nanopores. The methodology presented herein offers a comprehensive and validated framework for future investigations into COF systems and sets a valuable precedent for subsequent surface and interface studies involving organic frameworks.