- 學位論文
板鈦礦晶相二氧化鈦之製備、鑑定與光化學應用
Preparation, characterizations and photochemical applications of brookite-phase TiO2
摘要
本研究探討四氯化鈦(TiCl4)在高濃度硝酸(5 M HNO3)水溶液系統中，鈦離子濃度([Ti4+])及反應溫度對生成二氧化鈦晶相[銳鈦礦(anatase)、板鈦礦(brookite)及金紅石(rutile)]的影響。結果顯示在100 oC下，在我們所探討的[Ti4+]區間內(0.3 - 0.6M)，產物晶相皆為brookite/rutile混相。在70 oC下，當鈦離子濃度為0.3, 0.6及0.8 M時，產物晶相分別為brookite/rutile混相，anatase/brookite/rutile混相及anatase/rutile混相。我們將上述混相樣品與乙醇混合、離心後，可進一步地分離出anatase、brookite、rutile個別的純相二氧化鈦。亞甲基藍光脫色反應測試顯示三種晶相中以brookite催化活性最佳，anatase次之，rutile最差。 我們將anatase、brookite、rutile三種純相二氧化鈦在450 oC下鍛燒30分鐘(同染料敏化太陽能電池及光電催化水分解的電極製備條件)後進行特性鑑定。高解析電子顯微鏡(HRTEM)觀察顯示anatase呈顆粒形貌，brookite呈平板狀，而rutile呈棒狀。N2等溫吸附實驗顯示anatase、brookite及rutile的BET表面積分別為91、76及32 m2/g。紫外光可見光光譜(UV-vis)顯示三種晶相中以rutile光散射能力最佳，brookite次之，anatase最差。 染料敏化太陽能電池測試(模擬太陽光AM 1.5, 100 mW/cm2)顯示在相同的膜厚下，由於anatase電池所吸附染料最多(N719ads = 0.060 μmol cm-2)，故其光電轉換效率最佳(η = 4.26%)，明顯優於brookite電池的2.50% (N719ads = 0.041 μmol cm-2)以及rutile電池的1.55% (N719ads = 0.016 μmol cm-2)。 由於rutile及brookite擁有不錯的光散射能力，我們嘗試以溶膠凝膠(sol-gel)法製得anatase奈米顆粒(SG)作為染料敏化太陽能電池電極活性層(active layer)，探討rutile及brookite作為散射層(scattering layer)的影響。結果顯示塗佈rutile及brookite作為散射層，可使SG電池的光電轉換效率由7.09% 分別大幅提升至8.44% 及9.10%。這是因為brookite除了光散射特性佳可提高光使用率(light- harvesting efficiency)外，也有不錯的染料吸附能力。 光電催化分解水反應測試結果顯示anatase、brookite及rutile純相二氧化鈦電極的光電轉換效率分別為0.43%、0.87% 及0.80%。入射單色光子─電子轉換效率(incident photon-to-current efficiency, IPCE)量測顯示brookite及rutile電極的光使用率明顯優於anatase。我們由光電流隨時間衰退變化求出anatase、brookite及rutile的暫態時間(transient time)分別為0.20、0.71及0.42秒，顯示電子在brookite電極間應有較長的壽命。 我們利用同步輻射中心的原位X光繞射光譜(in-situ XRD)探討硫酸濃度在TiCl4/HNO3水溶液系統中對生成二氧化鈦晶相的影響。結果顯示在此系統中添加硫酸除了可抑制brookite/rutile生成及促進anatase生成之外，改變硫酸濃度亦能控制rutile及anatase生成時間順序。我們利用這種特性成功地製得以anatase作為活性層、rutile作為散射層的染料敏化太陽能電池電極(即anatase早於rutile生成於導電玻璃)，電池光電轉換效率可達4.11%。此一新穎二氧化鈦電極製備方法預期可大幅改善文獻中刮刀成膜(doctor-blade)法所需的多次重覆塗佈鍛燒製程。
並列摘要
In this study, the influences of Ti4+ concentration and reaction temperature on TiO2 phases (anatase, brookite and rutile) formed from hydrolysis of TiCl4 in 5 M HNO3(aq) were investigated. It is found that at the temperature of 100 oC, in the [Ti4+] range between 0.3 and 0.6 M, only brookite/rutile mixed phases form. While at the temperature of 70 oC, as [Ti4+] equals to 0.3, 0.6 and 0.8 M, respectively, brookite/rutile, anatase/brookite/rutile and anatase/rutile mixed phases are obtained. Separation of mixed phases into individual pure anatase, brookite and rutile was achieved via mixing as-synthesized samples with C2H5OH followed by centrifugation. Among them, brookite exhibits the best photocatalytic activity in the photobleaching of methylene blue under 300 nm UV illumination. Pure anatase, brookite and rutile samples were characterized via various instruments after calcination at 450 oC for 30 min (condition the same as the fabrication of electrode for dye-sensitized solar cell and water splitting). As revealed by HRTEM, the three phases exhibit their distinctive morphologies: nanoparticle for anatase, nanoplate for brookite and nanorod for rutile. The BET surface areas are 91, 76 and 32 m2/g, respectively. UV-vis spectra showed that the scattering abilities are in the order of rutile, brookite and anatase. Photovoltaic performance of dye-sensitized solar cells made up of anatase, brookite and rutile with the same thickness were measured under AM 1.5 (100 mW/cm2). Due to the superior capability for N719 dye adsorption (N719ads = 0.060 μmol cm-2), anatase-based cell exhibits the highest photoconversion efficiency (η = 4.26%), as compared to those of brookite- (2.50%, N719ads = 0.041 μmol cm-2) and rutile-based (1.55%, N719ads = 0.016 μmol cm-2) cells. Anatase nanoparticles prepared via sol-gel method (SG) were utilized as active layer and brookite/rutile, due to their better scattering abilities, as scattering layer for DSSC photoanodes. Photoconversion efficiencies were increased from 7.09% without scattering layers to 8.44% and 9.10% with rutile/brookite as scattering layers, respectively. Brookite can not only adsorb more dye but scatter incident light. Photocatalytic water splitting reaction indicated photoconversion efficiencies of pure anatase-, brookite- and rutile-based working electrodes are 0.43%, 0.87% and 0.80%, respectively. Brookite and rutile exhibit better light-harvesting efficiencies than anatase in incident photon-to-current efficiency (IPCE) measurements. Transient times of anatase, brookite and rutile calculated by photocurrent relaxation with time are 0.20, 0.71 and 0.42 s, respectively, which revealed brookite electrode has the longest electron lifetime than the other two polymorphs. The effect of H2SO4 concentration on TiO2 phases formed in TiCl4/HNO3(aq) system was studied via in-situ XRD, NSRRC. It is showed that addition of H2SO4 enables the suppression of brookite/rutile and promotion of anatase in this system, and formation orders of anatase and rutile can be well-controlled by adjusting [H2SO4]. The case that anatase emerges before rutile was chosen to fabricate TiO2/TCO electrode. Such bilayer microstructure is adopted to utilize smaller anatase to adsorb more dye and larger rutile particles to scatter incident light. The photoconversion efficiency is 4.11%, and the result demonstrates that this facile process is very promising to fabricate TiO2/TCO electrode for efficient photovoltaic devices.