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  • 學位論文

以原子層沉積法製備奈米複合觸媒應用於光觸媒水解產氫與燃料電池

Fabrication of Nanocomposite Catalysts by Atomic Layer Deposition for Photocatalytic Water Splitting and Fuel Cell

指導教授 : 彭宗平
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摘要


由於全球能源短缺,John Bockris博士於1970年即在美國通用汽車公司(General Motors)技術中心的演講中提出了氫經濟(hydrogen conomy)的概念,當時發生第一次能源危機,其主要勾勒未來氫氣取代石油成為支撐全球經濟的主要能源架構,包括整個氫能源生產、配送、貯存及使用的市場運作,以達到環境保護的目標。而所謂的氫能源即是利用氫氣經過化學反應後產生能量、電力,例如燃料電池,其中最大的課題就是如何降低氫氣取得的生產成本以及提升利用氫氣產生能源的效率。 本論文的研究目的係以此為出發點,首先,經由原子層沉積技術(atomic layer deposition, ALD)製備多種高性能奈米複合氧化物,作為光催化與水解產氫之觸媒,並以創新的奈米層疊結構(nanolaminate)取代傳統水相摻雜技術,俾降低相分離現象,增加摻雜的均勻性,提升光催化與水解產氫的效能。除此之外,亦利用原子層沉積技術製備白金以及氮化銅奈米晶粒,分別作為氫燃料電池與甲醇燃料電池中陰陽兩電極觸媒,除了降低觸媒使用量,亦提高燃料電池的效率。本論文分為兩部分,分別對不同的奈米複合物觸媒應用於光催化水解產氫和燃料電池電極觸媒層進行研究。 第一部分,利用奈米層疊結構製備鋁或鋅摻雜的二氧化鈦材料,使用原子層沉積技術將二氧化鈦/氧化鋁與二氧化鈦/氧化鋅依不同次序層疊地鍍覆在聚碳酸酯(polycarbonate)高分子薄膜上,在450 oC下加熱去除高分子薄模,製備長20 μm、直徑220 nm的鋁或鋅摻雜二氧化鈦奈米管,鋅與鋁的摻雜濃度可簡單地藉由前驅物循環比 (precursor cycle ratio) 控制,摻雜離子均勻地分佈於縱向,當氧化鋁或氧化鋅對二氧化鈦前驅物循環比為0.04,摻雜濃度可達到7-8 at.%,表面濃度甚至可高達16-18 at. %,由穿透式電子顯微鏡觀察,發現二氧化鈦晶格因高濃度的摻雜而嚴重的變形進而產生局部區域的固態非晶化。 此外我們將高濃度的鋁或鋅摻雜的二氧化鈦奈米管應用在光催化水解產氫與染料光催化降解反應,發現鋁的摻雜會降低光催化效率,電子電洞對再結合率因為鋁的摻雜而增加,因此降低溶液中的氫氧自由基的形成。相反地,鋅摻雜的二氧化鈦奈米管在一個適當的摻雜濃度(0.01)可得到較好的光催化活性與光電轉化效率,氫氣產量可以提升為商用二氧化鈦P25的六倍,除了歸因於較高的反應表面積外,藉由即時電子順磁共振光譜分析,亦發現觸媒表面分布的Ti3+與氧空缺對於光催化反應與光電轉換效率有決定性的影響。 除了二氧化鈦複合材料外,本論文亦製備新型態光催化奈米反應器系統,由氧化鋅奈米管陣列組成,並於管內自組裝金奈米晶粒,使用硫辛酸(thioctic acid)修飾大小約十奈米的金奈米晶粒。因為具有長碳鏈的硫辛酸可以避免金奈米晶粒團聚且使之均勻的覆蓋在氧化鋅奈米管內壁,此特殊的結構對於降解染料具有很好的催化活性,由Damköhler number分析,此薄膜型奈米反應器因具有極大的表面容積比(surface-to-volume ratio),因此有很好的質傳效能,幾乎可以完全忽略質傳阻力,所以可以在26.88 ms即可達到63%的染料降解率。 論文第二部分為燃料電池電極觸媒的應用。首先在碳布上利用水熱法直接成長直徑25奈米的一維氧化鋅奈米柱陣列,使用原子層沉積技術於光致親水性的氧化鋅奈米柱陣列上成長白金奈米晶粒,並將之作為陰極甲醇氧化反應之電極觸媒,由於氧化鋅奈米柱表面富含氫氧基,有利於將毒化物一氧化碳去除氧化,即進行所謂雙功能機制(bi-functional mechanism)。發現白金奈米晶粒與氧化鋅奈米柱之間亦具有交互作用而導致電子傳遞,提升甲醇氧化效率。此外,亦進行紫外光照射甲醇氧化的測試,其效能提高62%,這是因為高比表面積的氧化鋅奈米柱對於紫外光有很好的光響應。 本研究開發另一種複合型電極觸媒,係使用電漿輔助原子層沉積技術於奈米碳管上成長氮化銅奈米晶粒(copper (I) nitride, Cu3N),氮化銅奈米晶粒藉由島狀成長的機制,均勻地成長在奈米碳管表面,並可藉由原子層沉積控制氮化銅奈米晶粒的大小,此複合型電極觸媒具有很好的氧氣還原反應的催化效能,且其催化效能與晶粒大小有關,氧化還原的質量活性與傳統白金觸媒相當。由Koutecky–Levich分析發現,此複合型電極觸媒是進行二個電子與四個電子傳遞的複合反應。

並列摘要


The global demand for energy has been growing tremendously. A term of hydrogen economy were coined by Prof. John OʼM. Bockris during a talk he gave at General Motors (GM) Technical Center in 1970. The hydrogen economy is a proposed system of delivering energy and generating electrical power using hydrogen instead of traditional fossil fuel. Hydrogen is a non-emission and pollution free energy carrier which can be used as a fuel to produce electricity via chemical reactions such as fuel cell. However, the major issues are how to establish the efficient and cost-effective way to produce clean hydrogen gas and how to improve the utilization efficiency of hydrogen energy. It is the purpose of this research to explore nanocomposite materials as the highly active catalysts by atomic layer deposition (ALD) for photocatalysis and photocatalytic water splitting. We have employed the novel nanolamination of ALD process instead of conventional solution-based method to fabricate TiO2-based materials. It was attempted to achieve homogeneous doping distribution and to inhibit unwanted phase segregation. Furthermore, Pt and Cu3N nanoparticles as the anode and cathode electrocatalysts for application in fuel cell were deposited on innovative heterostructures by ALD. It not only reduced the usage amount of catalysts but also enhanced the efficiency of fuel cell. Therefore, this dissertation is divided two parts, which give insight into the fundamental understanding of correlation between lattice structure, morphology, surface chemistry, and electrochemical and photocatalytic properties of those novel nanocomposite catalysts. In the first section, highly homogeneous Al- and Zn-doped TiO2 nanotubes were fabricated by ALD via nanolaminated stacks of binary layers of Al2O3/TiO2 and ZnO/TiO2, respectively. The bilayers were alternately deposited on the polycarbonate (PC) membrane template by ALD with various cyclic sequences. The nanotubes in a length of 20 μm and a diameter of 220 nm were obtained after removal of the PC membrane by annealing at 450 oC. The doping concentrations of Al2O3 and ZnO in TiO2 depended on the precursor cycle ratios of Al2O3 and ZnO to TiO2. From the depth profiles measured by secondary ion mass spectrometry, Al and Zn are uniformly distributed across the thickness. With the precursor cycle ratios of Al2O3 and ZnO to TiO2 at 0.04, uniform bulk solubilities of ~7-8 at. % were obtained, and the surface concentrations were even higher, ~16-18 at. %. From the transmission electron microscopic observation, the highly doped anatase TiO2 exhibited some regions of severe deformation that resulted in localized solid-state amorphization. In addition to characterizations, the effects of doping composition on the photocatalytic and photoelectrochemical (PEC) activities were investigated. Increasing the Al doping reduced the photocatalytic activity of TiO2 due to formation of charge recombination sites and reduction of hydroxide radicals. In contrast, there was an optimal range of Zn doping to get enhanced photocatalytic activity and higher PEC efficiency. With a doping ratio of 0.01, the hydrogen production rate from water splitting was 6 times higher than that of commercial P25 TiO2. The photoinduced trapped electrons and holes were detected in Zn-doped TiO2 by in-situ electron paramagnetic resonance spectroscopy, which revealed that Ti3+ sites on the surface and surface oxygen vacancies played a key role in promoting the photocatalytic process. We have demonstrated a photocatalytic Au@ZnO@PC nanoreactor composed of monolayered Au nanoparticles chemisorbed on conformal ZnO nanochannel arrays within the PC membrane. Commercial PC membrane was used as the template for deposition of ZnO shell into the pores by ALD. Thioctic acid with sufficient steric stabilization was used as molecular linker for functionalization of Au nanoparticles in a diameter of 10 nm. High coverage of Au nanoparticles anchored on the inner wall of ZnO nanochannels greatly improved the photocatalytic activity for degradation of rhodamine B. The membrane nanoreactor achieved 63% degradation of rhodamine B within only 26.88 ms of effective reaction time owing to its superior mass transfer efficiency based on Damköhler number analysis. Mass transfer limitation could be eliminated in the present study due to extremely large surface-to-volume ratio of the membrane nanoreactor. Second part is to develop highly active catalysts for fuel cell application. A heterostructured electrocatalyst consisting of ZnO nanorods in a diameter of 25 nm on carbon cloth (CC) was synthesized by combining ALD and hydrothermal methods. Platinum nanoparticles were then deposited on photo-induced hydrophilic surface of ZnO nanorods by ALD. Electrochemical performance of the nanocomposite catalyst (Pt@ZnO@CC) for methanol oxidation reaction with or without UV irradiation was evaluated. The surface of ZnO nanorods rich in hydroxide species was more favorable for removal of CO via the so-called bi-functional mechanism. Additionally, the charge transfer occured between the ZnO nanorods and the Pt nanoparticles. UV light irradiation on the catalyst surface increased the chronoamperometric response by 62%, which was attributed to a synergistic effect of large surface area and strong light absorption in the UV region by the presence of ZnO nanorod arrays. A hybrid electrocatalyst consisting of copper (I) nitride (Cu3N) nanoparticles grown on carbon nanotubes (CNTs) by plasma enhanced ALD is presented as well. Island growth mechanism during ALD led to the formation of uniformly distributed Cu3N nanoparticles on the surface of CNTs. The size of copper nitride particles strongly influenced the electrocatalytic properties, and it could be precisely tuned by controlling the cycle number of ALD. The Pt-free non-precious nanocrystals coupling with CNTs exhibited pronounced electrocatalytic activity for oxygen reduction reaction (ORR). Koutecky–Levich analysis on the ORR current densities indicated that the Cu3N@CNT electrodes in alkaline media followed a mixed two- and four-electron transfer ORR pathway, whose mass activities were comparable to that of typical Pt/C electrode. A facile process to fabricate well-dispersed metal nitride on a selected support material as an ORR catalyst could raise the catalytic activity by synergistic chemical coupling effects.

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