金屬有機框架缺陷工程:提升混合質子電子導電度之策略

Abstract

金屬有機框架（MOFs）具有優異的可調性、高比表面積和孔隙率，使其在催化、吸附和分離等眾多領域中可以廣泛的應用。優異的可調性，讓MOFs在合成過程中改變合成參數就可能產生結構中的缺陷，或是在合成中快速結晶化也有造成結構錯位機會。MOFs有兩種缺陷，分別是連接體缺失(Missing linker)、簇團缺失(Missing cluster)，其中連接體缺失在文獻上指出可以提供活性位點、布忍斯特與路易斯酸位點、提高親水性，這讓MOFs在催化、吸附、質子傳輸上大放異彩。 在本研究中以鋯金屬與對苯二甲酸(H2BDC)合成出的UiO-66作為材料，Zr-O的配位鍵結很強導致UiO-66穩定性高，對水、酸性溶液、有機溶劑等的溶劑容忍性極高，甚至結構中產生缺陷也不會崩塌。UiO-66是質子導體，此特性有利於研究缺陷對質子傳輸造成的影響。為了創造出連接體缺失，我們在合成中加入調節器去跟對苯二甲酸(H2BDC)競爭與鋯金屬鍵結的機會，產生以醋酸鹽及OH/OH2形式存在的連接體缺失。從本研究的實驗結果發現有連接體缺失的UiO-66與文獻相比質子導電率有超過兩個數量級的差別，這證實連接體缺失提高親水性讓水分子更容易吸附在結構中，對質子傳輸有很大的幫助。另外，在紫外光可見光光譜中，發現有連接體缺失的UiO-66與文獻理論計算出的無缺陷UiO-66相比能隙低0.6 eV，因此也證實了有連接體缺失存在的UiO-66會改變原本無缺陷的電子結構。但是同時我們也發現並不是連接體缺失越多就會有越高的質子導電率及較低的能隙，這是因為當連接體缺失越多導致結構中的孔徑體積會越大，即使有連接體缺失會吸附更多水分子幫助質子傳輸，還是無法讓整個孔徑佈滿水分子，因此阻礙了質子傳輸的途徑。孔徑體積越大不只影響質子傳輸的途徑，也會影響電子穿過空間的傳輸途徑(through-space pathway)。而電子結構的部分則是因為在加入調節器產生連接體缺失時，同時也產生未配位的鋯金屬位點，但是為了維持電中性及結構穩定，調節器會變為封蓋劑與未配位的鋯金屬位點結合，導致最終有連接體缺失的UiO-66電子結構不受到連接體缺失的數量影響。 本研究著重於連接體缺失對於質子、電子導電率的影響，也經由連接體缺失與孔徑體積之間的關係進一步解釋質子、電子傳輸途徑的影響，對於研究金屬有機框架的載子最佳傳輸途徑提供了材料設計的指引。

Metal-organic frameworks (MOFs) possess excellent tunability, high specified surface area, and porosity, making them widely applicable in fields such as catalysis, adsorption, and separation. Their tunability allows the synthesis parameters to be adjusted, potentially creating defects in the structure or causing structural misalignment during rapid crystallization. MOFs exhibit two types of defects: missing linker and missing cluster. As reported in the literature, missing linkers can provide active sites, Brønsted and Lewis acid sites, and increase hydrophilicity, which greatly enhances MOFs' performance in catalysis, adsorption, and proton transport. In this study, UiO-66, synthesized from zirconium metal and terephthalic acid (H2BDC), was used as the material. The strong Zr-O coordination bonds lead to high stability and remarkable tolerance to water, acidic solutions, and organic solvents. Even with defects, the structure does not collapse. UiO-66 is an excellent proton conductor, beneficial for studying the effects of defects on proton transport. To create missing linkers, we introduced modulators during synthesis to compete with H2BDC for binding with zirconium metal, resulting in missing linkers in the form of acetate groups and OH/OH2 groups. Experimental results showed that UiO-66 with missing linkers exhibited proton conductivity more than two orders of magnitude higher than reported in the literature, confirming that missing linkers increase hydrophilicity and facilitate water molecule adsorption, greatly aiding proton transport. Additionally, UV-visible spectroscopy revealed that UiO-66 with missing linkers has a bandgap 0.6 eV lower than the defect-free UiO-66 calculated theoretically, indicating that missing linkers alter the original electronic structure of the defect-free UiO-66. However, we also found that an increasing number of missing linkers does not necessarily lead to higher proton conductivity and lower band gaps. This is because an increasing number of missing linkers results in larger pore volumes within the structure. Although the missing linkers enhance water molecule adsorption, facilitating proton transport, the pores cannot be entirely filled with water molecules, thereby obstructing the proton transport pathways. Larger pore volumes also impact the through-space pathway for electron transport. Regarding the electronic structure, the introduction of a modulator to create missing linkers also results in the formation of uncoordinated zirconium sites. To maintain charge neutrality and structural stability, the modulator acts as a capping agent, binding with these uncoordinated zirconium sites. Consequently, the electronic structure of UiO-66 with missing linkers is not influenced by the number of missing linkers. This study focuses on the impact of missing linkers on proton and electron conductivity. Additionally, it elucidates the relationship between missing linkers and pore volume, further explaining the effects on proton and electron transport pathways. These findings will aid future research in optimizing carrier transport pathways.