靜電紡絲法製備不同形貌之高比表面積銳鈦礦相二氧化鈦奈米纖維及其在染料敏化太陽能電池的應用

Preparation of anatase phase TiO2 nanofibers with high specific surface area in different morphologies via electrospinning method and its application on dye sensitized solar cells

林鈺儐(Yu-Pin Lin)

指導教授 ： 陳玉惠

中原大學/理學院/化學研究所/博士(2013年)

https://doi.org/10.6840/cycu201300805

摘要

本論文以靜電紡絲法為主要實驗方法製備不同形貌及高比表面積之銳鈦礦相TiO2奈米纖維，在不同製備條件下，探討對其形貌與比表面積之影響，並將所製備之不同形貌及高比表面積之銳鈦礦結晶相TiO2奈米纖維應用作為染料敏化太陽能電池(DSSC)光陽極材料，進一步研究對效能之影響。其內容主要分下列兩部分:
第一部分
本部分研究先以溶液性質與加工參數作為變因，找出能成功紡出均勻性TiO2/PVP奈米複合纖維的電紡絲溶液與靜電紡絲最適當實驗條件，再以此最適當實驗條件搭配室溫離子熔液[Bmim+][BF4-]為模板並以不同含量(0, 0.5, 1, 2及3 wt%)之比例，利用靜電紡絲法製備中孔洞TiO2奈米纖維(TNFx, x= 0~3)。藉由FE-SEM、XRD、TEM與BET等方法對TNFx進行分析與鑑定。由FE-SEM結果顯示，隨著[Bmim+][BF4-]添加量的不同，經鍛燒後的TNFx其纖維直徑大小、均勻性與表面粗糙度會隨之改變，在添加量為1 wt%時得到直徑最小且均勻的纖維，而隨著[Bmim+][BF4-]含量增加到3 wt%時，纖維呈現嚴重扭曲變形且具有最大表面粗糙度。XRD結晶性分析顯示，添加[Bmim+][BF4-]的TNFx (x= 0.5~3)相較於未添加[Bmim+][BF4-]的TNF0具有良好的銳鈦礦結晶相以及較佳的熱穩定性。此外，由TEM與BET結果得知，TNFx (x= 0.5~3)所產生的中孔洞結構屬於互相連通之蟲洞結構，且有較高的比表面積值；隨著[Bmim+][BF4-]的含量增加，蟲洞結構越趨完整與均勻，並在[Bmim+][BF4-]添加量為2 wt%時得到最大的比表面積值91.4 m2g-1，約為TNF0 (41.5 m2g-1)的2.2倍。簡言之，本部份研究製備出具有良好的銳鈦礦結晶性，且含有特殊的蟲洞結構及較高比表面積之TiO2奈米纖維，TNFx (x= 0.5~ 3)。唯此紡絲條件偶有針頭產生凝膠化而造成之塞針現象有待改進。
將TNFx( x= 0~3)系列TiO2奈米纖維應用作為DSSC光陽極材料，效能分析結果顯示，具有最細小且筆直纖維形貌、高比表面積與良好銳鈦礦結晶相的TNF1最適合作為DSSC的光陽極材料，其元件不但具有較佳的染料吸附量，並且擁有快速電子傳遞速率與較高的電荷收集效率，因此得到最高的光電轉換效率值5.64 %，相較於以未使用[Bmim+][BF4-]為模板的TNF0之DSSC元件效率值(3.75 %)提升了~50.4 %。
第二部分
本部份研究首先係為改善第一部分之TiO2電紡絲溶液與電紡絲參數，在製備過程針頭因電紡絲溶液凝膠化而造成塞針現象之疑慮，於TiO2電紡絲溶液中添加醋酸作為催化劑，解決前述塞針問題，並適當微調參數後，成功製備出均勻性TiO2/PVP奈米複合纖維。接著，經鍛燒後製得之TiO2奈米纖維(TNF)，於固定濃度10M之氫氧化鈉水溶液中進行水熱反應，藉由反應時間與溫度的調控及鍛燒處理，成功製備出不同形貌之TiO2奈米纖維，並利用FE-SEM、XRD、TEM與BET等方法針對不同形貌之TiO2奈米纖維進行分析與鑑定。結果顯示，當反應時間與溫度改變時，會產生不同層次結構表面及比表面積之TiO2奈米纖維；其中，最適當水熱條件為150 ˚C下反應12小時，所得的為刺棘狀多層次TiO2奈米纖維(HTF)，具有良好的銳鈦礦結晶性與中空管狀層次結構，且有154 m2g-1之最高比表面積值，較TNF (36 m2g-1)約提升4.3倍。
以HTF應用作為DSSC光陽極材料，其效能結果顯示雖然HTF光陽極具有較高的染料吸附量與較佳的光散射性，但表現並不如預期，元件所得到的光電轉換效率只有3.98 %較TNF為主的元件(4.09 %)低；此結果主要歸因於HTF的多層次獨特結構，造成與FTO玻璃間的介面貼附性不良之故。然而，藉著奈米粒子層的導入HTF層下方，提升與FTO玻璃間的介面貼附性後，有效的提升電子傳遞與收集效率，將HTF的高染料吸附量與較佳的光散射能力展現出來，使光電轉換效率大幅度提升到7.86 %，相較於同樣導入奈米粒子層的TNF元件輸出效率(6.24 %)提升了~ 26 %。
總體而言，本論文研究提供以靜電紡絲法於不同實驗條件下，製備一系列高比表面積之銳鈦礦結晶相TiO2奈米纖維，並顯示能有效提升DSSC之效能，為有潛力之光陽極材料。

關鍵字

靜電紡絲 ； 染料敏化太陽能電池 ； 二氧化鈦

並列摘要

In this study, anatase phase TiO2 nanofibers with high specific surface area in different morphologies were prepared under various condition via electrospinning method and the their effect on the performance of dye sensitized solar cell (DSSC) were investigated. This study consists of two parts as following:
Part 1
In this part, the electrospinning were used firstly to obtain uniform TiO2/PVP composite nanofibers under proper condition, subsequently, the mesoporous TiO2 nanofibers (TNFx, x=0~3) were prepared using the different content (0, 0.5, 1, 2 and 3 wt%) of room temperature ionic liquid, [Bmim+][BF4-], as the mesopore formation template. The TNFx materials were characterized by FE-SEM, XRD, TEM and BET measurements. From FE-SEM results, it was found that the fiber diameter、uniformity and surface roughness were varied with different content of [Bmim+][BF4-], and a uniform nanofiber with smallest diameter of nanofiber was obtained from 1 wt% of [Bmim+][BF4-] template, furthermore, a distortion with higher surface roughness of nanofiber was observed as [Bmim+][BF4-] content was increased to 3 wt%. From XRD results, it was found that TNFx (x=0.5~3) were thermally more stable than those spun without RTIL, TNF0. In addition, the TEM and BET results indicated the interconnected framework wormhole mesopores with high specific surface area in the TFNx (x=0.5~3). It was also noticed that the wormhole mesopores gradually became homogeneous as [Bmim+][BF4-] increased and the surface area of TNF2 was 91.4 m2g-1, which was more than 2.2 times higher than that of TNF0 (41.5 m2g-1). In summary, the TNFx (x=0.5~3) nanofibers not only exhibited a well anatase crystalline form but also a unique wormhole mesopore structure and a higher surface area. Nevertheless, the spinneret was often blocked by gelation of Ti precursor spinnable solution in this study.
The as-prepared TNFx fibers were used as electrode materials to form various photoanodes of DSSC, and their performances were then investigated. The results indicated that the unique morphology of TNF1, being a straight, large surface, wormhole mesoporous anatase nanofiber with the smallest average fiber diameter, was the main reason leading to the largest improvement in its dye loading, electron transport, and charge collection efficiencies among the series of TNFx-based photoanodes. This study demonstrated TNF1 is a promising photoanode material for DSSC; specifically, the DSSC fabricated with TNF1 showed a largest improvement (~50.4%) in energy conversion efficiency (5.64%) when compared with that fabricated with TNF0 electrospun without [Bmim+][BF4-] (3.75%).
Part 2
In this part, the acetic acid was used as the catalyst in the Ti precursor spinnable solution to overcome the blockage of spinnert, and the electrospinning parameters were further adjusted to fabricate the uniform TiO2/PVP composite nanofibers. Subsequently, the pure electrospun TiO2 nanofiber (TNF) was fabricated by calcination of the electrsopun TiO2/PVP composite nanofiber. A series of TiO2 nanofiber materials with different morphologies were synthesized by hydrothermally treated TNF in a 10M NaOH(aq) in various condition and calcination process. The TiO2 nanofiber with different morphologies were characterized by FE-SEM、XRD、TEM and BET measurements. It was found that the surface morphologies and surface area were varied with different hydrothermal comdition. As the reaction condition was fixed at 150 ˚C and 12 h, a prickle-like hierarchical anatase TiO2 nanofiber (HTF) was obtained. In addition, it was found that dense nanotube prickle-like branches were grown on the HTF with anatase crystalline phase and a surface area of up to 154 m2g-1, which was ~ 4.3 times larger than that of the TNF (36 m2g-1).
For application as the photoanode of DSSC, the one-layer device, HTF-D, showed slightly lower performance (3.98 %) than TNF-D (4.09 %) because of the poor contact at the HTF/FTO interface. However, the introduction of nanoparticles as a lower layer in a bi-layer HTF-based photoanode (BHTF-D), resulted in ~26 % higher power conversion efficiency (7.86 %) than the bi-layer TNF-based device, BTNF-D (6.24 %). This increase is attributed to the reduced contact resistance, unique morphology of HTF, leading to more dye loading, higher light scattering ability, faster electron transport, and greater charge collection efficiency.
In summary, anatase crystalline phase TiO2 nanofibers with high specific surface area in different morphologies were prepared under various condition via electrospinning method. The results of the TiO2 nanofibers prepared in this study exhibited large improvement on performance of DSSC, hence, it is believe that these TiO2 nanofibers will be high potential photoanode materials of DSSC.

並列關鍵字

dye sensitized solar cells ； TiO2 ； Electrospinning

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