不同晶相二氧化鈦之合成、特性鑑定與應用

Syntheses, Characterizations and Applications of Rutile, Anatase and Brookite Titanium Dioxide

楊閔涵

指導教授 ： 李紫原

國立清華大學/工學院/材料科學工程學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00050

摘要

近年來，二氧化鈦被廣泛的應用在各式的用途，如塗料、光觸媒、染料敏化太陽能電池以及鋰離子儲能裝置上。二氧化鈦的晶相、形貌以及晶體尺寸為影響材料性能三個重要的因子。本研究利用水熱法，以鈦酸鈉作為前驅物添加不同的鹼金屬鹽類並且調控反應酸鹼值，以合成出不同形貌及大小的銳鈦礦相及板鈦礦相二氧化鈦並探討其性質；此外，我們利用溶液熱法，將金紅石相奈米線成長於透明導電玻璃上，探討其電致變色的表現。
在銳鈦礦相二氧化鈦合成方面，我們發現鹼金屬鹽類對於銳鈦礦的晶體形貌有很大的影響，在氯化鋰溶液中，可以合成具有{101}晶面裸露的八面體形貌；在氯化鈉溶液中，可以合成具有{301}晶面裸露的紡錘狀形貌；而在氯化鉀溶液中，則是可以合成出針狀且頂端部分裸露出{301}晶面的銳鈦礦晶體。本實驗中所合成的{301}裸露面為首次被報導出來。由於鋰、鈉及鉀離子的電荷密度不同，對於銳鈦礦晶體在{100}和{001}基本晶面成長的抑制能力有所不同。高電荷密度的鋰離子會造成銳鈦礦<100>和<001>方向堆疊嚴重的抑制，而產生粒徑較小且低長寬比的晶體；而低電荷密度的鉀離子，對於銳鈦礦<001>方向的抑制能力弱，會產生粒徑較大且高長寬比的晶體。另外，我們藉由鹽酸調控前驅物溶液中的酸鹼值，可以得到不同尺寸的二氧化鈦晶體，當反應環境的pH下降時，會加速前驅物鈦酸鈉的溶解，而產生大量的銳鈦礦成核點，使得晶體尺寸縮小。由此我們可以藉由添加不同的鹼金屬鹽類以及控制反應的酸鹼值，獲得兼具形貌及尺寸調控的銳鈦礦晶體。在光降解反應中，我們發現{101}晶面裸露的銳鈦礦擁有最佳的光催化表現。此外，我們發現次微米級的二氧化鈦具有良好的結晶性，其光催化表現高於奈米級的二氧化鈦。而在染料敏化太陽能電池的電極測試中，我們利用混合奈米及次微米的銳鈦礦晶體作為電極，以提供高的染料吸附面積及良好的散射能力，可獲得最佳的光電轉換效率6.9%，為商用P25(5.3%)的1.3倍。
在板鈦礦相二氧化鈦合成方面，我們發現在反應溶液中添加氟化鈉，會產生板鈦礦晶體，且可經由不同的氟化鈉溶液濃度，以獲得不同的板鈦礦及銳鈦礦比例，在1M的氟化鈉溶液中可以得到近乎純相的板鈦礦晶體。由於前驅物鈦酸鈉的結構與板鈦礦有部分相似，因此我們認為氟化鈉可以穩定鈦酸鈉分解時所產生的板鈦礦結構單元，進而長成板鈦礦晶體。此外，不同尺寸及形貌的板鈦礦可藉由對鈦酸鈉進行不同程度的酸處理以達成。當酸處理程度低時，鈦酸鈉溶解速度慢，板鈦礦成核點少，會得到微米花狀的板鈦礦；而當酸處理程度提高時，會得到次微米海膽狀的板鈦礦；而當酸處理的程度太高時，鈦酸鈉溶解速度加快，會使板鈦礦的成核點增加，導致部分的板鈦礦結構單元被破壞，形成板鈦礦奈米棒及銳鈦礦奈米粒子的混合產物。將合成的板鈦礦進行不同溫度下的相轉變測試，當溫度提高到800℃時，微米花狀的板鈦礦無相轉變的情況發生；而次微米海膽狀的板鈦礦，在750℃時開始轉變成金紅石結構。
在金紅石相二氧化鈦合成方面，我們嘗試將金紅石二氧化鈦成長於導電玻璃上，利用四異丙醇鈦作為前驅物，在鹽酸與異丙醇的混合溶液中，以FTO玻璃作為基材進行溶液熱反應。在此反應條件，可獲得沿[001]方向成長的金紅石奈米線垂直成長於FTO上，且由此方法所合成的金紅石奈米線的邊長尺寸為在鹽酸水溶液中反應的十分之一。在電致變色的測試中，相較於用P25以及金紅石奈米線塗佈在FTO作為電極，我們發現此電極擁有良好的光學密度差以及鋰離子儲存能力。此優異表現可歸因於金紅石二氧化鈦，在[001]方向擁有極高的離子擴散速度，當與電場方向平行時，可以使離子快速的嵌入二氧化鈦晶體。此外，我們也利用鈉離子進行離子嵌入測試，首次發現二氧化鈦電極在-1V電位下有大量的鈉離子嵌入。

關鍵字

二氧化鈦 ； 銳鈦礦 ； 金紅石 ； 板鈦礦 ； 光催化 ； 染料敏化太陽能電池 ； 電致變色

並列摘要

Recently, titanium dioxide (TiO2) has been extensively used in various applications such as pigment, photocatalysis, DSSC and Li ion storage devices. In current publications, there are three well known important factors, the phase, morphology and particle size, affecting the performance of TiO2. The research is focused on the syntheses of anatase and brooktie TiO2 with various morphologies and sizes by hydrothermal method using sodium titanate as precursors, alkali metal salts as additives in different pH environment. In addition, the rutile nanowires grown on transparent conductive substrate, was synthesized with solvothermal process. The electrochromic properties of rutile TiO2 nanowires were investigated.
Anatase TiO2 with various shapes were synthesized by using sodium titanate as a precursor and different alkali metal salts as additives. {101}-exposed and {301}-exposed anatase TiO2 were synthesized in LiCl and NaCl solution, respectively. The TiO2 with exposed high-index {301} facet has never been reported. The reaction proceeding in KCl solution, the anatase TiO2 with a high aspect ratio elongated along <001> direction were obtained. Anatase TiO2 with a high aspect ratio was obtained in the presence of low charge density high atomic number alkali metal ions, which are not easily absorbed by the {001} planes of anatase, so growth along <001> direction was not restrained. Moreover, the size of anatase could be adjusted by pH values of precursor solution. Reducing pH value of the precursor solution accelerated the dissolving rate of precursor, releasing large amount of nuclei for anatase TiO2, which results in small sized anatase formation. The photodegradation of methylene blue assisted by anatase TiO2 with various exposed faces, reveals that the photoactivity of anatase {101} planes is higher than that of {301} planes. Moreover, submicron-sized TiO2 with high degree of crystallinity has the better photocatalytic performance than that of nano-sized TiO2 with a large amount of defects. With respect to the DSSC fabricated from the various TiO2, the best performance was achieved by using the mixture of nano-sized and submicron-sized rod-like TiO2 as the electrode exhibiting 1.3 times larger efficiency (η = 6.9%) than that of commercial P25 (η = 5.3%).
Brookite and anatase TiO2 was synthesized by using sodium titanate as a precursor in sodium fluoride aqueous solution. The ratio of brookite and anatase can be tailored by NaF concentration, and the high quantity brookite TiO2 was acquired in 1M NaF solution. Additionally, the morphology and size of brookite TiO2 can be adjusted by using various acid treated titanates. Micro-sized flower-like brookite was obtained from a weak acid treated titanate, whereas the submicron-sized urchin-like brookite was acquired from a middle acid treated titanate. When a strong acid treated titanate was used, brooktie nanorods with some anatase nanoparticles were obtained. With respect to the phase transformation of brookite with temperature, the micro-sized flower-like brookite remained the same shape and phase even at 800 ℃, but the submicron-sized urchin-like brookite transformed to rutile phase above 750 ℃.
Rutile nanowires were grown along the [001] direction and perpendicular to the FTO substrate (R⊥) by solvothermal process using titanium isopropoxide as a precursor in the mixture solution of HCl and IPA. The edge length of the nanowires obtained in IPA is ten times less than that in water. Compared to P25 and rutile nanowire powder coated on FTO, R⊥ have superior electrochromic performance due to its high ionic diffusion rate along rutile [001] direction. Furthermore, a large amount of sodium cations intercalation into TiO2 was first observed at -1.0 V.

並列關鍵字

TiO2 ； anatase ； rutile ； brookite ； photocatalysis ； DSSC ； electrochromism

參考文獻

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