二氧化鈦奈米晶體之合成與鑑定：染料敏化太陽電池應用

Synthesis and Characterization of TiO2 Nanocrystals: Application in Dye-Sensitized Solar Cells

彭嘉德(JIA-DE PENG)

指導教授 ： 何國川

國立臺灣大學/工學院/化學工程學研究所/博士(2016年)

https://doi.org/10.6342/NTU201601473

摘要

本論文主要研究方向著重於設計與發展二氧化鈦奈米晶粒以快速的步驟合成並作為高效率染料敏化太陽能電池的光電極材料。
近年來，文獻指出（001）晶面相比於（101）晶面的二氧化鈦能更進一步提升染料附載量，所以本論文合成並鑑定（001）晶面的二氧化鈦奈米片及它們在染料敏化太陽能電池之應用。結果顯示所合成的二氧化鈦奈米片雖有較小的比表面積但卻具有較高的染料附載能力（每克二氧化鈦的染料附載量/二氧化鈦的比表面積）。二氧化鈦奈米片可得到 8.77%的最佳光電轉化效率，相比於商用品 P25 的 6.92%（第三章）。此外，本論文使用磷鎢酸來修飾二氧化鈦奈米片。結果顯示二氧化鈦奈米片經磷鎢酸修飾後相比於未修飾者可提升 73.4mV 的開環電壓，進而提升 14.8%的光電轉化效率（9.32%）。此外，在最佳化之奈米片中混摻金奈米粒子，光電轉化效率可進一步提升至 10.03%，這是由於金粒子的表面電漿共振效應所造成（第四章）。
然而，由於奈米片彼此間容易相互堆疊，換言之，由於在﹝001﹞方向上的厚度，使此類結構難以具有高的比表面積。減少奈米片在﹝001﹞方向上的厚度並且增加（001）晶面二維結構的尺寸大小，同時使奈米片狀能自組裝形成階層式的三維結構是解決堆疊問題的最佳方式之一。此方法不但可提升二氧化鈦片狀物的比表面積，同時（001）晶面的所占比例也大幅增加。然而，有次序性的奈米片狀堆疊結構仍尚未被提出。在第五章，具有高（001）晶面比例（約 82%）、高比表面積（112.2 m2/g）、優異散射能力、三維的孔洞網絡及合適中孔洞大小的單分散二氧化鈦球，以原位晶面控制法成功被合成出並作為染料敏化太陽能電池的光電極材料。此染敏電池具有 11.13%的光電轉化效率，相比於對照組（8.11%）光電轉化效率提升了37.2%。在第六章，由於二氧化鈦球間的巨大孔隙，形成了一條能讓電解質通過的快速孔道，成功地解決了鈷離子電解質的質傳問題。然而，球間的巨大孔隙雖舒緩了電解質的質傳問題，往往伴隨著染料附載量下降的問題，這是由於球體所組成的二氧化鈦膜具有高孔隙度的原因。然而令人訝異的，本論文所遇到的情況中 S84-TiO2 膜展現了獨有的特性，它具有非常高的膜孔隙度（εfilm = 0.73）有利於鈷離子電解質的傳遞，同時具有極高的染料附載量（1.06×108 mol/(cm2μm)）。這項獨特性質使得 S84-TiO2 在眾多文獻所提及的二氧化鈦膜中具有極高的辨識度。此染敏電池具有 11.43%的光電轉化效率，相比於對照組（8.81%）光電轉化效率提升了 29.7%。
在第七章，本論文新合成具有（101）晶面的二氧化鈦球並著重它在室溫型離子液體染料敏化太陽能電池之應用。由於二氧化鈦膜中巨孔的存在，由高解析掃描式電子顯微鏡以及水銀測孔儀所證實，有助於離子液體電解質注入二氧化鈦膜中，同時它的高比表面積（108.1 m2/g）利於獲得高的染料附載量。此外，巨大的顆粒尺寸增加了對入射光的散射能力以至於獲得的光子數量大幅提升。新的雙離子液體被成功製備出，包含了 1–propyl–3–methylimidazolium iodide (PMII)以及 triethylmethylammonium methyl sulfate (TEMAMS) (65:35 = v/v)，具有特殊的熱塑性質並在室溫下維持膠態性質。量測結果顯示以二氧化鈦球作為光電極的染料敏化太陽能電池（6.18%）優於商用品透光層（5.24%）及商用品散射層 （4.99%）的光電轉化效率。染料敏化太陽能電池使用 PMII/TEMAMS 雙離子液體電解質，經由一千兩百小時長效穩定性測試結果發現具有無衰退之穩定性。

關鍵字

染料敏化太陽能電池 ； 二氧化鈦 ； 光電極 ； 奈米結構 ； 離子液體 ； 電解質

並列摘要

This dissertation aimed to develop TiO2 nanocrystals with facile synthesis process and used as the materials of photoanodes for dye-sensitized solar cells (DSSCs) with high cell efficiencies (η`s).
In recent years, literatures mentioned that (001)-facets TiO2 can further increase the dye loading as compared to (101)-facets TiO2, so we synthesized and characterized (001)-facets TiO2 nanosheets, and their applied in DSSCs. The results indicate that the synthesized TiO2 nanosheets have a smaller surface area and yet with a higher dye-loading capacity (dye molecules per gram and per surface area of TiO2). An optimal η of 8.77% is obtained for the DSSC with the TiO2 nanosheets film, as compared to that of the cell with the P25 film (6.92%) (Chapter 3). Besides, we used phosphotungstic acid (PWA) to modify TiO2 nanosheets (PWA-TiO2-NS). The results indicate that PWA modified TiO2 nanosheets have 73.4 mV higher open–circuit voltage as compared to the unmodified one, resulting in a 14.8% higher η for the corresponding DSSC (9.32%). Moreover, by blending gold particles with the optimized content of PWA-TiO2-NS, the η of the pertinent DSSC could further be enhanced to 10.03%, owing to the plasmonic effect of the gold particles (Chapter 4).
However, because of the vulnerability of nanosheets to overlap on each other, in other words, due to the relatively large thickness in the [001] direction, these structures are not expected to show a high surface area. Reduction of thickness of the nanosheets in [001] direction and increase of 2D lateral size of (001) planes, while simultaneously allowing the sheets to self-assemble into a hierarchical 3D architecture is one of the optimal solutions for preventing this aggregation. In this way, not only the surface area of the sheet–like TiO2, but also the percentage of its exposed (001)-facets can be largely increased. However, ordered structures of this type are yet to be obtained for TiO2. In Chapter 5, the mono–dispersed TiO2 microspheres with highly exposed (001)-facets (ca. 82%), high surface area (112.2 m2/g), good scattering ability, self–ordered 3D porous network and suitable mesopores in microspheres had been successfully synthesized by an in situ facet–controlling approach and employed as the photoanode material for the DSSCs. The pertinent DSSC exhibits a η of 11.13%, which represents a 37.2% improvement over that obtained for a DSSC containing the common Ref–TiO2 (η = 8.11%). In Chapter 6, owing to the presence of large pores among the TiO2 microspheres, formed a freeway to facilitate the penetration of the electrolyte solution, it successfully coped with the mass transport problem of a cobalt-based electrolyte. However, the enhancement in mass diffusion due to large voids among the microspheres is expected to be accompanied by a large decrease in the dye loading, on account of the associated high porosity of the spheres-based TiO2 film. Surprisingly, in our case, the film of S84-TiO2 shows an unique property, i.e., its porosity is very high (εfilm = 0.73) for facilitating the diffusion of the cobalt electrolyte, while the associated dye loading is also very high (1.06×1010 mol/(cm2μm)). This unique property makes our S84-TiO2 film be distinguishable from iother TiO2 films reported in the literature for use in cobalt-based DSSCs. An efficiency of 11.43% was obtaned for the DSSC with S84-TiO2; this value is 29.7% higher than that obtained for the DSSC with the Ref–TiO2 (8.81%).
In Chapter 7, we newly synthesized (101)-facets TiO2 microspheres (MSs) and focused on its application in room-temperature ionic liquids (RTILs)-based DSSCs. Presence of macropores in the MS film, as confirmed by high resolution scanning electron microscopy (HR–SEM) and mercury porosimeter, facilitates the penetration of IL-based electrolytes into the thin film, whereas its high surface area (108.1 m2/g) helps for high dye loading. Further, the large particle size increases the scattering ability of incident light leading to an excellent increment in the number of photons. Moreover, a new bi–ionic liquid (bi–IL) containing a mixture of 1–propyl–3–methylimidazolium iodide (PMII) and triethylmethylammonium methyl sulfate (TEMAMS) (65:35 = v/v) is prepared, which possesses unique thermal plastic characteristics and shows gel–state at room temperature.The results demonstrated that DSSCs containing the photoanode made of a MS structure show a superior η (6.18%) than that of the commercial transparent layer based (5.24%) and commercial scattering based TiO2 films (4.99%). The DSSCs with PMII/TEMAMS shows the extraordinary durability and unfailing stability of the cells for 1,200 h.

並列關鍵字

Dye-sensitized solar cells ； TiO2 ； photoanodes ； nanosheets ； ionic liquids ； electrolytes

參考文獻

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