利用直鏈醇提升揮發誘導自組裝法製備的規則中孔洞二氧化鈦之規則度

Use n-alkyl alcohols to improve regularity of ordered mesoporous TiO2 synthesized by evaporation-induced self-assembly

張嘉軒(Chia-Hsuan Chang)

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/碩士(2012年)

摘要

中孔洞材料是現在很熱門的話題，而將二氧化鈦這個用途廣泛的光電奈米材料製備成中孔洞二氧化鈦更是熱門，它具有高比表面積、高孔隙率、高熱穩定性等優點，可以將它應用於染料敏化太陽能電池、光觸媒、產氫等。本研究使用揮發誘導自組裝(evaporation-induced self-assembly)法，利用浸澤塗佈法(dip-coating)及舖盤法(coating on Petri dish)塗佈薄膜，合成規則中孔洞二氧化鈦，在40度及50%相對溼度下熟化，並於350度下煅燒，可獲得規則中孔洞二氧化鈦。以小角度及廣角度X光繞射儀(SXRD、XRD)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、氮氣吸脫附等溫曲線 (N2 adsorption/desorption)對樣品進行檢測。
本研究導入共表面活性劑(co-surfactant)的概念，於溶膠溶液中，加入常做為共表面活性劑及擴孔劑的直鏈醇，利用直鏈醇同時具有疏水端和親水端的特性，使其能在表面活性劑形成微胞的疏水端和親水端介面上，穩定微胞結構，提升規則孔洞結構規則度，並增加疏水端體積，擴大孔徑。實驗結果顯示藉由浸澤塗佈法製備的樣品之規則孔洞結構為2D hexagonal排列，孔徑10~14 nm，藉由舖盤法製備的樣品之規則孔洞結構也是2D hexagonal排列，孔徑5~7 nm，比表面積155~190 m2/g，晶相為銳鈦礦。
加入直鏈醇確實能夠增加規則度及孔徑。隨著同種直鏈醇用量越多，比表面積降低、孔徑增加、孔洞體積降低、孔壁越厚。隨著加入直鏈醇的碳鏈長越長，比表面積較小、孔徑較小、孔洞體積也較小、孔壁越厚。其中C8H17OH對規則度的增益最佳，這是因為C8H17OH沸點接近表面活性劑熱分解的溫度，到達沸點快速離開時，還有表面活性劑在結構裡固定結構，所以煅燒過程中，對規則孔洞結構破壞程度較小。C16H33OH沸點接近TiO2成晶溫度，離開結構時，因為已超過表面活性劑熱分解溫度，沒有表面活性劑可以固定結構，加上TiO2聚集成晶，所以對規則孔洞結構破壞程度較大。擴孔效果也是C8H17OH比較好，因為碳鏈較長的C16H33OH分子有捲曲行為，而表面活性劑的疏水端分子結構具有立體障礙，捲曲分子因為立體障礙被推往親水端，造成疏水端體積增加得比C8H17OH還少，而因為羥基較接近外圍，所以親水端吸引力較強，造成C16H33OH的孔壁較厚。
除此之外，不同的塗佈方式也會影響規則度的排序。浸澤塗佈法得到的膜厚大約1~2μm，而舖盤法得到的膜厚大約1 mm。膜厚較厚的情況下，碳鏈較長的C16H33OH分子會有許多不同方向的捲曲行為，使得有些分子受到立體障礙影響較小，還能在介面上，但數量較少，增益效果較小，有些則受到立體障礙影響較大，被推到親水端內。在煅燒時，橫向捲曲的分子對規則結構破壞較小，但其它捲曲方向的分子，不論在介面上或是原先的親水端內，都會對規則孔洞結構造成更嚴重的破壞。C8H17OH則因為碳鏈短，不會有捲曲，所以對結構的規則依舊有增益。因此膜厚不同，規則度排序有些許不同。
浸澤塗佈法規則度排序如下：C8OH-5.3:1 > C8OH-1:1 > C8OH-1:5.3 > C16OH-1:5.3 > C16OH-1:1 > C16OH-5.3:1 > C2OH。舖盤法規則度排序如下：C8OH-Petri dish-5.3:1 > C8OH-Petri dish-1:1 > C8OH-Petri dish-1:5.3 > C2OH-Petri dish > C16OH-Petri dish-1:5.3 > C16OH-Petri dish-1:1 > C16OH-Petri dish-5.3:1。

關鍵字

二氧化鈦 ； 規則中孔洞材料 ； 規則度 ； 中孔洞 ； 直鏈醇 ； 浸澤塗佈法 ； 舖盤法 ； 共表面活性劑 ； 擴孔 ； 揮發誘導自組裝法

並列摘要

Mesoporous materials are a popular research topic. Mesoporous TiO2 has unique properties such as high specific surface areas, high porosities, and high thermal stability. They can be applied in dye sensitized solar cells, photocatalysis, phtocatalytic hydrogen production, to name just a few. In this study, we synthesized ordered mesoporous TiO2 based on an evaporation-induced self-assembly process by dip-coating and coating on Petri dish. The resulting thin films were aged at 40℃ and 50%RH, followed by calcination at 350℃ to afford the final product. The characteristics of mesoporous TiO2 were investigated by small-angle, wide-angle X-ray diffraction (SXRD, XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption/desorption isotherms .
This study introduced the concept of co-surfactant addition in this system. We added n-alkyl alcohols into the sol solution. The n-alkyl alcohol has both hydrophilic and hydrophobic groups, so it can situate at the hydrophilic-hydrophobic interface of the micelles to help stabilize ordered mesoporous structure, improving the regularity and increasing the volume of hydrophobic core to expand the pore. The pore structure obtained by dip-coating was 2D hexagonal structure and the pore size was about 10~14 nm. The pore structure by coating on Petri dish was also 2D hexagonal structure. The pore size was about 5~7 nm, and the specific surface area was about 155~190 m2/g.
The result showed that adding n-alkyl alcohols significantly improved the regularity and expanded the pores. With increasing the amount of the same n-alkyl alcohol, the specific surface area and pore volume decreased, and pore size and pore wall thickness increased. As the n-alkyl chain length got longer, the specific surface area, pore size, and pore volume decreased. Among these n-alkyl alcohols, the best improvement came from C8H17OH. Because the boiling point of C8H17OH is near the decomposition temperature of the surfactant and the surfactant would stay in the structure when C8H17OH left. The boiling point of C16H33OH is near the sintering temperature of TiO2. When the co-surfactant molecules leave the structure, there is no surfactant left to support the structure.Furthermore, TiO2 crystals grew to destroy the structure. The pores obtained from addition of C8H17OH were larger than those from addition of C16H33OH. The n-alkyl chain of C16H33OH is longer than that of C8H17OH, and the chain may bend. There is also a steric effect in the hydrophobic core of the surfactant. The bending molecules may be pushed to the hydrophilic corona by the steric effect. It caused less increases in the hydrophobic core size, less expansion in pore sizes and more increases in pore wall thickness of C16H33OH than C8H17OH.
The way of coating affected the thickness of the thin film, and the thickness would then affect the structure regularity. The thickness of thin films by dip-coating was about 1~2μm, and that by coating on Petri dish was about 1 mm. When the thickness was large, n-alkyl chains of C16H33OH would bend in random directions. It caused the molecules move away from the hydrophilic-hydrophobic interface. In the calcination process, bending molecules in random directions would damage the structure regularity significantly. C8H17OH did not bend because of its shorter n-alkyl chains, and so its existence showed improvements in structure regularity.
The structure regularity ranking with the dip-coating process went as: C8OH-5.3:1 > C8OH-1:1 > C8OH-1:5.3 > C16OH-1:5.3 > C16OH-1:1 > C16OH-5.3:1 > C2OH。The structure regularity ranking with the coating on Petri dish process went as: C8OH-Petri dish-5.3:1 > C8OH-Petri dish-1:1 > C8OH-Petri dish-1:5.3 > C2OH-Petri dish > C16OH-Petri dish-1:5.3 > C16OH-Petri dish-1:1 > C16OH-Petri dish-5.3:1。

並列關鍵字

TiO2 ； ordered mesoporous material ； regularity ； mesopore ； n-alkyl alcohol ； dip-coating ； coating on Petri dish ； cosurfactant ； pore expansion ； evaporation-induced self-assembly

參考文獻

Antonelli, D. M. & Ying, J. Y. (1995) Synthesis of Hexagonally Packed Mesoporous Tio2 by a Modified Sol-Gel Method. Angew Chem Int Edit, 34 (18), 2014-2017.

Brinker, C. J., Lu, Y. F., Sellinger, A. & Fan, H. Y. (1999) Evaporation-induced self-assembly: Nanostructures made easy. Adv Mater, 11 (7), 579-585.

Brunauer, S., Deming, L. S., Deming, W. E. & Teller, E. (1940) On a theory of the van der Waals adsorption of gases. J Am Chem Soc, 62, 1723-1732.

Chen, C.-Y., Burkett, S. L., Li, H.-X. & Davis, M. E. (1993) Studies on mesoporous materials II. Synthesis mechanism of MCM-41. Microporous Materials, 2 (1), 27-34.

Chmelka, B. F., Zhao, D. Y., Feng, J. L., Huo, Q. S., Melosh, N., Fredrickson, G. H. & Stucky, G. D. (1998) Triblock copolymer syntheses of mesoporous silica with periodic 50 to 300 angstrom pores. Science, 279 (5350), 548-552.