- 學位論文
二氧化鈦奈米管負載銀之特性鑑定及其處理酸性染料與亞硝酸鹽廢水之研究
Synthesis and Characterization of Silver-Loaded Titanium Dioxide Nanotubes for the Treatment of Acid Dye and Nitrite Wastewaters
摘要
近年來，半導體觸媒的異相光催化反應在環境污染防治的研究上相當廣泛，其中奈米二氧化鈦因具有高活性、化學穩定性、無毒性及容易取得之優點，故應用性極具潛力，因此，本論文之主要目的乃使用水熱法來製備二氧化鈦奈米管(titanium nanotubes, TNs)，不需繁複的設備及步驟即可得到大量的產物，同時討論TNs之合成動力，將不同合成時間之奈米管對其作特性分析，以探討其成長機制；再以不同濃度之酸洗溶液酸洗合成出之奈米管，探討在酸洗過程中之離子交換對於奈米管結構及晶相之影響。但二氧化鈦卻由於需要高能量之UV光作為光源激發，且其電子電洞對再結合率也較高，因此本論文乃將二氧化鈦奈米管負載金屬銀，以改善上述之缺點，增進光催化之效果，並將對反應動力及參數取得及其應用於處理酸性染料及亞硝酸鹽廢水。 實驗主要部分包括：使用前驅物二氧化鈦銳鈦礦，以不同反應時間1 ~ 50 h，固定NaOH濃度，且不經酸洗過程製備奈米管，經場發掃描式電子顯微鏡(FE-SEM)及穿透式電子顯微鏡(TEM)觀察發現，TNs於合成時間前5 h為顆粒/片狀/管狀物質同時存在，在6 h之後皆為奈米管結構，隨著時間增長，管長可由400 nm成長至1000 nm不等，管徑約10 ~ 15 nm，經X光粉末繞射儀(XRPD)可得知其組成由二氧化鈦銳鈦礦的晶相逐漸轉變為鈦酸鹽之晶相。並以延伸細微結構X光吸收光譜(EXAFS)及X光吸收邊緣結構光譜(XANES)觀察奈米管結構隨合成時間不同的改變，由結構參數可知配位數的變化為一開始合成1 h為4，合成至4 h時形成奈米管變為2，再延長時間至42 h時形成奈米棒配位數轉為3。 以不同酸洗濃度酸洗合成出之奈米管，來解釋奈米管由鈦酸鹽結構轉為銳鈦礦結構之臨界條件。經FE-SEM及TEM觀察發現，TNs管長約80 ~ 150 nm，管徑約10 nm，經XRPD可得知其組成隨酸洗程度不同，鈦酸鹽特徵峰會逐漸消退而銳鈦礦特徵峰隨之明顯，可對應酸洗時之pH值不同對晶相之影響。並以恆溫氮氣吸脫附測定其表面積及孔徑分佈，可知酸洗條件在pH =1.6 ~ 2間及酸洗24 h可得最大表面積。另以Raman光譜對照酸洗前後之差異，可知在酸洗前其特徵峰的位置屬於Na2Ti3O7之結構，酸洗後會移回銳鈦礦之波峰位置。 以金屬銀負載二氧化鈦奈米管(Ag/TNs)改善二氧化鈦本身具有之缺點，延長電子電洞對再結合之時間，並降低本身激發所需的能量。將所製備之Ag/TNs，由FE-SEM及TEM結果可知，負載銀之粒徑約為3 ~ 4 nm，且負載過程不影響奈米管之管長與管徑。能量散射光譜儀(EDX)可證實於奈米管表面確實具有銀顆粒分佈。並以ESCA對其表面元素作分析，負載銀比例越多則強度越強，且氧含量之百分比會稍微下降，而對鈦元素之含量則無影響。由UV/Visible光譜分析結果可知，Ag/TNs在450 ~ 550 nm之間會有強烈的吸收峰，且銀含量越多，吸收峰位移至可見光區域越明顯，以KM方程式換算bang gap energy則銀含量越多其能隙值越低。透過XANES及EXAFS光譜探討Ag/TNs之配位數及鍵長，對奈米管之第一層Ti-O分析，配位數為2，鍵長約1.95 Å；對負載Ag之第一層Ag-O分析，配位數為1.6，鍵長約2.28 Å。 另外，以不同電洞捕捉劑測試於光催化系統中對亞硝之去除效率，其結果顯示甲酸效果較草酸為佳。而每克觸媒對不同濃度亞硝酸鹽溶液最終去除率分別為5 ppm (25 mg/g•L)，10 ppm (43.35 mg/g•L)及15 ppm (42.85 mg/g•L)，但每克去除亞硝酸鹽降低，並以10 ppm之值最高，故濃度越低反應速率越快。以Ag/TNs處理亞硝酸鹽廢水，由於奈米管上負載之Ag會阻隔UV光與TNs之激發，故效率反而降低。處理不同濃度酸性染料AB 9方面，由於高濃度染料會阻隔光照，故濃度越高去除效率越差。光催化動力模式使用簡化之Langmuir-Hinshelwood動力學模式ln(Ca/C0) = -kt，探討其反應速率，不同濃度5、10、20及30 ppm之速率常數分別為1.03×10-3、5.39×10-4、4.21×10-4及1.46×10-4 (min-1)。
並列摘要
Recently, titanium dioxide has been recognized as an excellent photocatalyst material applied on many fields especially for environmental science or engineering. Titanium nanotubes (TNs) with high specific surface area have been also studied due to its excellent catalytic activities, long-term stability, nontoxicity, and low cost. Therefore, the main objectives of the present study were to prepare titanium dioxide nanotubes in large quantities by hydrothermal routes and to characterize the photocatalytic properties, fine structures, formation mechanisms of TNs, and loaded with silver to modify the photoactivity applied to removal efficiency of nitrite and acid dye pollutants in wastewaters. Anatase TiO2 nanoparticle was used as a precursor for nanotubes synthesis. The nanotubes fabricated in different reaction times (1 ~ 50 h), at the temperature of 150°C, NaOH concentration of 10 M, and without acid-wash were characterized by FE-SEM and TEM techniques. The results showed that the length and diameter of nanotubes ranged of 400 ~ 1000 nm and 10 ~ 15 nm, respectively. The XRPD patterns showed that the structure of nanotubes were titanate-type after heat treatment for 6 h at 150°C. X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS) spectroscopies were performed to identify the fine structure of nanotubes in synthesis processes. Nanotubes were washed in acid solution with different concentrations and times to identify the transition of structure from titanate to anatase. XRPD patterns showed the transition structure from titanate to anatase crystal based on pH value of acid solution about 1.6 ~ 2, and washed for 24 h could obtain maximum surface area about 300 m2/g. By using Raman spectra, the structure was identifyied as Na2Ti3O7 species before being washed in acid solution, and after acid treatment the anatase structure would reveal. Silver-loaded titanium nanotubes (Ag/TNs) characterized by FE-SEM and TEM showed that the length and diameter of nanotubes ranged of 100 ~ 150 nm and 10 nm respectively, and the diameter of silver particles was 3 ~ 4 nm. ESCA and EDX data indicated that Ag/TNs consist of Ti, O, and Ag atoms and therefore the percentage of oxygen would decrease by increasing Ag amount. The UV/Vis spectra revealed that the critical wavelength of TNs was 410 nm. But after loading silver on TNs, the critical wavelength of UV/Vis for Ag/TNs increased. By using XANES spectra, the valency and framework of Ag/TNs were Ti(VI) with anatase-type structure, and Ag-O bonding for silver particles loaded on TNs. The EXAFS data revealed that Ag/TNs have a first shell of Ti-O bonding with bond distances of 1.95 Å and coordination number was 2. From the experimental data of the nitrite removal under ultraviolet light irradiation, two kind of hole scavengers showed that the photodegradative efficiency of formic acid was higher than the one of oxilic acid. From the comparison of different concentrations of nitrite, the photodegradative efficiencies were 5 ppm (25 mg/g•L), 10 ppm (43.35 mg/g•L), and 15 ppm (42.85 mg/g•L). The photocatalytic efficiency of Ag/TNs was lower than TNs because of the over load of silver nanoparticles. The photoillumination of TNs was obstructed by the Ag particles. In the photocatalytic experiments of different concentrations of dye solutions (Acid blue 9), it was found that the higher dye concentration had lower photodegradative efficienciy. Finally, a simple Langmuir-Hinshelwood model was used and calculated the relationship of ln(Ca/C0) versus time to describe the reaction constant. The reaction constants (k) of 5, 10, 20, and 30 ppm were 1.03×10-3, 5.39×10-4, 4.21×10-4, and 1.46×10-4 (min-1), respectively.