奈米氧化鈦的製備及其在氣體儲存與電化學電池上之應用

Abstract

本研究利用商用Merck TiO2粉末作為前驅物，在10 M 的氫氧化鈉水溶液中以水熱法製得氧化鈦奈米管(Tnt)，並在製備過程中利用 0.1 M的硝酸水溶液進行不同次數之酸洗製備出Tnt-5、Tnt-7、Tnt-11以及Tnt-13四個樣品，數字部分為酸洗後之濾液的pH值。HRTEM影像顯示Tnt形貌為末端開口(open-end)且具有多層管壁(multi-layered)的一維管狀結構，其外管徑約8-10 nm，內管徑約3-5 nm，管長約為數百個奈米。EDX數據顯示，上述樣品之鈉含量會因酸洗次數增加而減少，由9.63 wt% (Tnt-13)下降到4.30 wt% (Tnt-5)。由氮氣等溫吸脫附結果得知，樣品的BET表面積會隨著酸洗次數的增加而變大，由168 m2/g (Tnt-13) 提升至291 m2/g (Tnt-5)。將Tnt應用於二氧化碳氣體吸附研究中發現，在298 K且32大氣壓下，吸附量分別為102.0 mg/g (Tnt-13)、109.0 mg/g (Tnt-11)、121.5 mg/g (Tnt-7)與136.2 mg/g (Tnt-5)。同時由二氧化碳脫附實驗結果中我們發現，二氧化碳之化學吸附比例會隨著鈉含量上升而增加，而物理吸附比例則隨著BET表面積增加而上升。其中以具有最大BET表面積之Tnt-5，在298 K且32大氣壓下有最高的二氧化碳吸附量表現。另外，研究中也利用光化學沉積法將鉑與鈀金屬奈米粒子沉積在有完整多層管壁結構之Tnt-13表面，並應用於氫氣儲存實驗。在298 K且32大氣壓下，樣品之氫氣吸附量分別為18.3 mg/g (Tnt-13)、68.0 mg/g (Pt/Tnt-13)以及87.5 mg/g (Pd/Tnt-13)，結果顯示負載上貴重金屬之樣品Pt/Tnt-13及Pd/Tnt-13，其氫氣儲存量皆有明顯提升，其中以Pd/Tnt-13表現最佳。 在電化學電池之研究中，我們利用氧化鈦奈米管經由酸處理相轉換後所得到的二氧化鈦奈米顆粒(TNP)作為電子傳導層材料，並以兩步驟溶劑熱法製備二氧化鈦空心微米球(THMS)作為散射層材料。首先利用刮刀成膜(Doctor blade)法將二氧化鈦漿料塗佈於FTO上製備出二氧化鈦電極，再以連續離子層吸附與反應(SILAR)法將硫化鎘(CdS)以及硒化鎘(CdSe)敏化劑負載於二氧化鈦電極上，即得CdSe/CdS敏化太陽能電池之光陽極。本研究中所製備之光陽極皆以SILAR沉積上ZnS作為敏化劑之保護層。在AM 1.5模擬太陽光(100 mW/cm2)照射下進行太陽能電池測試，實驗結果顯示添加THMS作為散射層後，光電流密度(Jsc)有明顯的增加，由8.6 mA/cm2 (5CdSe/7CdS/3TNP)提升至13.1 mA/cm2 (5CdSe/7CdS/THMS/3TNP)，而光電轉換效率(η)也由2.87%增加至3.99%。添加散射層之光陽極(5CdSe/7CdS/THMS/3TNP)其光電轉換效率相較於無添加散射層之光陽極(5CdSe/7CdS/3TNP)增加約40%。證實散射層確實可以延長入射光在光陽極中的光徑，增加入射光的利用率，並使得整體光電轉換效率提升。在太陽光水分解實驗中，我們利用SILAR法將CdSe 敏化劑負載於7CdS/THMS/3TNP光陽極上，製備出一系列的CdSe/CdS敏化光陽極。結果顯示，7CdS/THMS/3TNP光陽極之電流密度值為8.8 mA/cm2，而4CdSe/7CdS/THMS/3TNP光陽極的電流密度值為12.2 mA/cm2。結果顯示，添加CdSe作為光陽極的共敏化劑，可以有效增加長波長之可見光的利用，使整體效率獲得提升。

Titanate nanotubes (Tnts) were prepared by a hydrothermal method with Merck TiO2 powders immersed in concentrated NaOH solution. The white precipitate was washed with different times of 0.1 M HNO3(aq) in the preparation process. The prepared Tnts were denoted as Tnt-5, Tnt-7, Tnt-11, and Tnt-13 to signify the obtained pH values of the washing eluate at 5, 7, 11, and 13, respectively. The open-end and multi-layered feature of the Tnts with outer diameter in the range of 8-10 nm, inner diameter in the range of 3-5 nm, and up to several hundred nanometers in length can be observed by HRTEM. With increasing the cycles of acidic treatment, the sodium content decreased from 9.63 wt% in Tnt-13 to 4.30 wt% in Tnt-5, whereas the BET surface aera (SBET) increased from 168 m2/g in Tnt-13 to 291 m2/g in Tnt-5. In CO2 storage measurements, the CO2 storage capacity of Tnts, which based on the weight of adsorbent at 298 K and 32 atm, were 102.0 mg/g, 109.0 mg/g, 121.5 mg/g, and 136.2 mg/g for Tnt-5, Tnt-7, Tnt-11, and Tnt-13, respectively. The percentage of physisorption was mainly dependent on the SBET of the Tnts. Higher SBET corresponded to the higher amount of physisorption. The chemisorption proportion rised with increased the Na content. The Tnt-5 of the highest surface area was shown the best CO2 storage performance under 32 atm. In the H2 storage study, Pt/Tnt-13 and Pd/Tnt-13 with the noble metallic nanoparticles deposition on Tnt-13 was carried out by photochemical deposition method. The bare Tnt-13 presented 18.3 mg/g H2 storage capacity at 298 K and under 32 atm. The Pt/Tnt-13 and Pd/Tnt-13 sample shown the adsorption capacity were 68.0 mg/g and 87.5 mg/g, respectively. In the photoelectrochemical cells study, we prepared the TiO2 nanoparticles (TNP) as an electron-conducting layer material from the transformations of titanate nanotube in acidic environment, and the TiO2 hollow microspheres (THMS) as scattering layer material by two-step solvothermal method. The TiO2 electrodes were prepared by coating TiO2 paste onto FTO glass by doctor-blade method. Successive ionic layer adsorption and reaction (SILAR) method were used to assemble the CdSe/CdS sensitized photoanodes. All of the prepared photoanodes were deposition of ZnS as a passivation layer. The photocurrent density (Jsc) and the solar energy conversion efficiency (η) of CdSe/CdS sensitized solar cells were increased from 8.6 mA/cm2 and 2.87 % of 5CdSe/7CdS/3TNP to 13.1 mA/cm2 and 3.99 % of 5CdSe/7CdS/THMS/3TNP, respectively. The η of 5CdSe/7CdS/THMS/3TNP was significantly enhanced by nearly 40 % as compared to 5CdSe/7CdS/3TNP which without THMS layer under AM 1.5 simulated solar irradiation (100 mW/cm2). The result indicated the light scattering layer can elongate the path length of incident light in photoanodes. For solar water splitting study, the photocurrent density of 7CdS/THMS/3TNP which only sensitization with CdS sensitizer was 8.8 mA/cm2. The photocurrent density of 4CdSe/7CdS/THMS/3TNP was increased to 12.2 mA/cm2. The results confirmed that adding CdSe as a co-sensitizer, the absorption of visible light can be effectively increased and the overall efficiency was improved.