具形狀序列演變與大小控制的金-鈀核殼奈米晶體之合成、光學調控、氫氣感測及自組裝成超級晶體結構

Synthesis of Au–Pd Core–Shell Nanocrystals with Tunable Shapes and Sizes for Examination of Their Optical and Hydrogen Sensing Properties, and Their Self-Assembly into Supercrystals

邱淳雅(Chun-Ya Chiu)

指導教授 ： 黃暄益

國立清華大學/理學院/化學系/博士(2014年)

摘要

此研究以金-鈀核殼雙層金屬結構材料探討精準的粒子形狀與大小控制對其光學性質的影響、及用這些粒子作為光學式的氫氣感測器與其自組裝成超級晶體材料。利用水相的化學合成法製備出金-鈀核殼的奈米結構，利用金八面體作為模板並使用植晶法在水相中進行金-鈀核殼異質結構的合成。在50 ℃下以溴化十六烷基三甲基銨鹽作為保護劑，將其與金八面體，四氯合鈀酸及維生素C水溶液混合並於兩小時內完成反應。藉由改變反應速率的條件與形成的機制，合成出具有單晶結構的從晶面(100)的立方體演變到晶面(111)的八面體金-鈀的核殼結構。這些形狀均一且具有不同殼層厚度的金-鈀核殼異質結構使我們能對其局部表面電漿共振性質進行研究及探討。當殼層厚度在較薄的情況下，光譜中出現金-鈀核殼異質結構中金的局部表面電漿共振吸收，而受到外層鈀介電常數的影響，相較於金八面體的吸收，其具有藍位移的現象。由於殼層厚度會影響奈米粒子整體中鈀與金之間的比例與鈀對金的影響，因此當金鈀核殼異質結構由立方體轉變成八面體時，金的局部表面電漿共振吸收會逐漸的紅位移並變得更加明顯。相較於過去的文獻，這是首次對於不同形狀及不同大小的雙金屬核殼異質結構進行其光學性質的探討。
藉由雙層金屬的混成作用，此核殼結構吸收在可見光-紅外線頻段，此結構內的奈米金作為奈米光學天線的作用，奈米光學天線一般是指金屬（金、銀、銅等）奈米顆粒及其相同結構的不同組合所構成，這些金屬奈米結構在特定波長光激發下產生局域表面電漿共振（LSPR），在奈米尺寸上能對可見光-紅外線具有場增強功能。藉由通過改變介質的介電常數主動控制模式對奈米光學天線進行調控。利用過渡金屬-鈀做為氫氣感測器之催氫金屬，氫氣感測機制藉由氫氣被吸附於此催氫金屬之表面，經過解離作用將氫分子解離為氫原子，接著氫原子將快速擴散通過催氫金屬晶格內層而到達感測平台。進而探討不同形狀與厚度的金-鈀核殼結構對氫氣感測表現之影響。研究結果顯示， Tetrahexahedral (THH) 結構對所得感測元件之氫氣感測性能影響甚為靈敏，其吸收波長改變量可達六十奈米以上，故能提高氫氣感測之靈敏度，使偵檢範圍變寬。金-鈀核殼的厚度對於感測元件偵測也有所影響。而當溶液中金-鈀之粒徑減小至一百奈米以下時，感測效果可輕易由肉眼觀察其顏色變化，同時也可重複吸附-脫附氫氣達多次以上，此簡易且方便的奈米氫氣感測元件在運用上具有極大的潛能。
研究中也進而利用「溶劑-揮發所誘導的介面活性劑濃度改變」與「界面活性劑擴散方法」的作用力進行金-鈀核殼雙層結構奈米粒子的自組裝，以植晶法所開發出來的奈米鈀包金核殼立方體(100)到八面體(111)來製備出3D超級晶體結構。可得到微米級且具有立方體、金字塔、八面體及菱形十二面體等不同形貌以及不同大小調控的超級晶體。而不同奈米晶體的緊密堆疊，多面體會受形狀效應主導它們的大規模排列。同時也對超級晶體的排列模式透過SEM與TEM加以分析探討，我們也由聚焦離子束進行超級晶體內層的分析，由剖面圖可以清楚看到奈米粒子規則的排列結構。藉由SAXS儀器分析也可以得到晶體間的排列間隙。超級晶體為一群規則排列的奈米粒子所構成，由於金屬奈米粒子具有LSPR特性，群聚的金屬奈米粒子則產生集體效應(Collective)成為類介質(dielectric-like)結構，因而我們將這些奈米粒子所排列而成的超級晶體結構進行電性與光學上的分析。這些類介質的超級晶體在電性量測上也顯現其非線性I-V特徵，而在紅外線頻段上有良好的共振反應(米式共振)。
在本篇論文中，我們研發出具形狀序列演變與大小控制的金-鈀核殼奈米晶體，且可控制其光學性質，運用在可調控且可重複使用的核殼奈米氫氣感測器，其感測具有反應靈敏、步驟簡單、具高重複利用性、低成本以及容易量產等優勢。更進一步自組裝金-鈀核殼奈米晶體成3D超級晶體結構，具有特別的電學與光學上的特性，其新材料的性質與應用將具有莫大的潛力效能。

關鍵字

核殼奈米晶體 ； 超級晶體 ； 表面電漿共振 ； 集體共振 ； 氫氣感測 ； 鈀 ； 金

並列摘要

Core−shell nanoparticles are highly functional materials with modified properties by changing either the constituting materials or the core to shell ratio. Various core−shell nanostructures have been synthesized such as Au−Cu2O, Au−Ag, Au−Cu, Pt−Pd, and Au−Pd core−shell nanoparticles. Among them, Au−Pd core−shell nanostructures have efficient catalytic properties for a variety of reactions and plasmonic gas sensing upon exposure to H2 as reversible H2 uptake from the Pd shell occurs. Furthermore, synthesis of well-defined Au−Pd core−shell nanocrystals with systematic shape evolution is still challenging by virtue of long reaction time.
In Chapter 1, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures in just 0.5–2 h at 50 ºC. Octahedral gold nanocrystals were used as cores. Since an important purpose of this work is to systematically examine the plasmonic properties of these particles as a function of particle size, shape, and shell thickness, octahedral Au nanocrystal cores with sizes of 35, 45, 74, and 92 nm have been employed as the templating cores. Au–Pd core–shell cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra with precisely tuned particle morphology and shell thickness have been achieved, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals for the first time. Nanoparticles with uniformly thin shell thicknesses, particularly the core–shell octahedra, exhibit the most pronounced plasmonic band derived from the gold cores.
In Chapter 2, we employed Au–Pd core–shell THH particles, octahedra, and nanocubes as hydrogen sensing materials. The nanocrystals were dispersed in an aqueous solution and hydrogen gas was introduced into a 10-mL flask through a syringe-attached balloon. Presence of dissolved hydrogen was detected. All of these nanocrystals were found to be excellent plasmonic hydrogen sensors producing very large spectral red-shifts after hydrogen absorption. THH nanocrystals exposing high-index facets displayed the largest spectral shifts. All these particles are highly selective to hydrogen. The spectral shifts are almost fully reversible with successive hydrogen absorption and desorption cycles. For smaller core–shell octahedra, the spectral changes can be visually observed. Larger particles with thicker Pd shells give the largest spectral red-shift. With all these advantages, these Au–Pd core–shell nanocrystals should find broad applications in which simple detection of hydrogen presence is desirable.
In Chapter 3, we utilized gold, gold-palladium, gold-silver, and lead sulfide nanocrystals with cubic and octahedral structures as building blocks to fabricate supercrystals by solvent evaporation and surfactant diffusional methods. The supercrystals prepared were characterized by SEM, TEM, and XRD techniques. The microstructures were studied by small-angle X-ray scattering (SAXS) technique. The growth process was investigated. Shapes and sizes of various supercrystals were also controlled. These supercrystals are considered novel superstructures and may show interesting optical and electrical properties which may be used for the fabrication of metamaterials and photonic devices.

In these works, we have developed a facile aqueous solution method to synthesize Au–Pd core–shell nanocrystals with systematic shape evolution from cubic to octahedral structures, allowing a thorough and detailed analysis of the plasmonic band appearance and shifts of these core–shell nanocrystals. We also employed polyhedral Au–Pd core–shell nanocrystals as hydrogen sensing materials. All these particles are highly responsive and reusable hydrogen sensors in aqueous solution. Furthermore, we utilized polyhedral Au–Pd core–shell nanocrystals as building blocks to fabricate 3D supercrystals, which are considered novel superstructures and may have opportunities for the fabrication of metamaterials and photonic devices.

並列關鍵字

core–shell nanocrystals ； supercrystals ； surface plasmon resonance ； collective resonance ； hydrogen sensing ； palladium ； gold

參考文獻

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