鎳有機金屬骨架薄膜之長晶機制並應用於染料敏化太陽能電池之電觸媒

Abstract

藉由原位配基合成的方法，以6,6’-二硫二菸酸(H2dtdn)作為初始物形成mdn配基，成功合成一種結構為[Ni(mdn)(H2O)2]n的鎳有機金屬骨架材料(MOF)。利用2-巰基-3-吡啶甲酸所形成的自組裝單層，可使[Ni(mdn)(H2O)2]n薄膜生長在不同的導電基材上，例如FTO導電玻璃、碳布、發泡鎳。MOF薄膜由基材底部開始生長的反應機制可分為以下的幾個步驟：(1) 2-巰基-3-吡啶甲酸錨定在基材上；(2) 在2-巰基-3-吡啶甲酸上的硫醇基和質子裂解後，分別形成3-吡啶羧酸根自由基和去質子的2-巰基菸酸根；(3) 從2-巰基-3-吡啶甲酸得到的3-吡啶羧酸根自由基會與從6,6’-二硫二菸酸得到的去質子6-巰基菸酸根形成碳-硫鍵結；從2-巰基-3-吡啶甲酸得到的去質子2-巰基菸酸根與從6,6’-二硫二菸酸得到的3-吡啶羧酸根自由基形成碳-硫鍵結；(4) 兩種鍵結都會形成6-(3-carboxylatopyridin-2-ylthio)nicotinate作為錨定基團；(5) 鎳離子(II)會與基材上的錨定基團配位；(6) mdn與鎳離子(II)的配位不斷延伸而形成[Ni(mdn)(H2O)2]n。當上述[Ni(mdn)(H2O)2]n@CC及[Ni(mdn)(H2O)2]n@NF作為染料敏化太陽能電池(DSSCs)的對電極時，MOF薄膜對於I3–還原的催化活性越高，染敏電池的光電轉換效率就會越好。通過調整反應時間進而調整薄膜的粗糙度、厚度以及中孔率可獲得最優化的[Ni(mdn)(H2O)2]n@CC (標示為D12h@CC)。有鑑於發泡鎳在MOF生長過程中會伴隨蝕刻反應，調整了薄膜的粗糙度、厚度以及附著力可獲得最佳的[Ni(mdn)(H2O)2]n@NF (標示為D06h@NF)。在染敏太陽能電池的測量上，D12h@CC與D06@NF 分別在一個太陽光下達到了9.30%與8.42%的效率。這些電極表現出的電化學性能比稀貴的白金碳布電極(9.36%)與白金發泡鎳電極(8.78%)來說更有競爭力。而在昏暗的低照度下，由D12h@CC組裝的染敏電池在6000流明達到10.80%，以及在3000流明達到15.52%的效率，顯示此材料對於染敏電池加入互聯網元件有良好的潛力。A nickel-based metal-organic framework (MOF) of [Ni(mdn)(H2O)2]n (mdn=6,6’-mercaptodinicotinate) was successfully synthesized via an in-situ mdn ligand formation from the starting material, 6,6’-dithiodinicotinic acid (H2dtdn), followed by ligand coordination reaction. With the presence of a self-assembly monolayer of 2-mercaptopyridine-3-carboxylic acid (2-mna), various [Ni(mdn)(H2O)2]n thin films were decently deposited on different conducting substrates, e.g., fluroine-doped tin oxide (FTO), carbon cloth (CC), and nickel foam (NF). A bottom-up crystal growth mechanism was followed: (1) anchoring of 2-mna on the substrate; (2) cleavages of thiols or protons of 2-mna to afford 3-pyridinecarboxylate radical and deprotonated 2- mercaptonicotinate radical; (3) C-S coupling between 3-pyridinecarboxylate radical and deprotonated 6-mercaptonicotinate radical; (4) formation of 6-(3-carboxylatopyridin-2-ylthio)nicotinate as the anchoring group; (5) Ni(II) ion coordination with the two pyridines of the anchoring group; (6) the mdn and Ni(II) coordination to extend [Ni(mdn)(H2O)2]n building block. When these [Ni(mdn)(H2O)2]n@CC and [Ni(mdn)(H2O)2]n@NF electrodes were applied as the counter electrodes in dye-sensitized solar cells (DSSCs), better electro-catalytic activity of a film toward I3– reduction, could lead to the better cell conversion efficiency. As etching of NF may occur during operation, the roughness thickness and adhesion strength of the film have to be adjusted for optimal [Ni(mdn)(H2O)2]n@NF(D06@NF) electrode.The DSSCs coupled with D12h@CC and D06h@NF reached 9.30% and 8.42% at 1 sun, repectively. Therefore, these electrodes had performance very competitive with the expensive Pt@CC (9.36%) and Pt@NF (8.78%). Under a dim light illumination, the cell with D12h@CC provided 10.80% at 6 klux and 15.52% at 3 klux, implying its good potential for applications in Internt of Things.