介尺度氧化鋅晶體與金為基礎之奈米粒子

Abstract

本研究概念為-以操控奈米粒子為基礎，進行特異結構之同質(homogeneous)與異質(heterogeneous)材料的合成。 在同質材料部分，以氧化鋅奈米粒子作為單元，進行介尺度氧化鋅晶體的合成並對其成長機制與催化應用進行探討。本實驗採用阿拉伯膠作為形貌控制劑於溫和水熱環境下(80 oC)製備出孿生結構之介尺度氧化鋅晶體，稱為Twin-Brush ZnO mesocrystals (TB-ZnO)；對其研究，得知其由眾多氧化鋅奈米粒子堆疊而成，且具有多孔結構並呈現單晶特性，再以氮氣吸脫附儀分析，知其多孔結構具有較高表面積ca. 22 m2/g；為了進一步操控合成，進而分析不同反應階段的粒子來推敲生長機制，發現其係由微小奈米粒子進行靠攏堆疊先形成平板，再經由晶體固有偶極場的誘導下生長成此結構特殊且具有較高表面積的介尺度氧化鋅晶體。 應用方面，TB-ZnO作為載體來製備高負載金屬觸媒Au/TB-ZnO可從ca. 18 wt% 到ca. 0.5 wt%不等，其一氧化碳氧化的催化活性評估是相當好的相較於一般Au/ZnO觸媒；其可在低溫及高氣體流速下(-10 oC, GHSV=490,000)仍擁有絕佳活性約為0.13 molco/molAu•s；透過HAADF-STEM的鑑定發現有半數金原子是原位取代氧化鋅上的鋅，且形成金原子團簇，而另半數的金則座落在氧化鋅外表面呈現 ca. 2 nm的分布，且其在金與氧化鋅的強作用(Strong-Metal-Support-Interaction)下，即便多次高溫處理，金屬顆粒大小仍維持一定，呈現出良好的催化穩定性。 在異質奈米材料研究上，奈米金球藉由微量二氧化矽修飾製備出非均勻披覆的Au@SiO2 奈米粒子，並以其裸露的金表面作為成長點進行奈米銀的沉積，生成非等向性金-銀 雙面結構之奈米粒子(Au-Ag Janus nanoparticles)。而操控溶液鹼性程度，可調整銀還原及沉積速度，進而獲得以金為端點進行成長的銀奈米短棒(Au-tipped Ag nanorods)；透過STEM-EDX線掃描分析得知，銀原子擴散進奈米金球內且含量從金-銀界面開始遞減至奈米金球邊界，推斷非等向性之金-銀 雙面結構奈米粒子的形成機制確實是由於非等向性銀沉積所致。

Using nanoparticles as building units to form a unique homo-structural or hetero-structural material is an interesting topic for the design of novel materials. Herein, two individual cases were demonstrated by means of different strategies. One is ZnO mesocrystals prepared by a biomimetic method. The other is Au-Ag Janus nanoparticles prepared by a post-modification and deposition method. Mesocrystals of ZnO were successfully synthesized under a mild hydrothermal condition by using gum arabic as a structure-directing agent. The hierarchical structure is a unique twin-brush form consisted of vertically aligned nanorods in a single crystalline porous form. The formation mechanism of the twin-brush ZnO (TB-ZnO) was investigated by quenching a time series of samples and their examination by TEM, SEM and XRD. It was also found the alignment of ZnO crystal units can be modulated by adding simple salts such as KCl to change from nanorods to nanoplates. Moreover, the hierarchical structures of ZnO mesocrystal can also be tuned by adding amounts of simple salts. These can be explained by the screening of dipolar force of the polar crystal. Spatially analyzed cathodoluminescence of the twin-brush ZnO was investigated to follow the local structure changes, where intensity variations at ca. 380 nm show a straightforward evidence to prove the ZnO mesocrystal is definitely a polar crystal. TB-ZnO was utilized as support for the preparation of catalyst, namely Au/TB-ZnO. Catalytic performance of the Au/TB-ZnO catalyst was examined in CO oxidation reaction, resulting in an excellent catalytic activity. Characterization of this catalyst was carried out by using these techniques of conventional HRTEM, HAADF-STEM, X-ray absorption spectroscopy and DRIFT spectroscopy. The results indicate the origin of catalytic activity is originated from high active isolated gold atoms and ca. 2 nm gold nanoparticles. The catalyst can also mitigate the growth of particle sintering against high temperature pretreatments. For the hetero-structural material, Au-Ag Janus nanoparticles were synthesized via asymmetric modification and deposition. Firstly, Au nanoparticles with an uneven SiO2 coating were prepared as a platform for silver deposition. Subsequently, the silver can effectively deposit on the exposed gold surface, resulting in an anisotropic bimetallic nanomaterial in the form of Au-Ag eccentric nanoparticles. Furthermore, silver deposition of Au-Ag eccentric nanoparticles can be facilely elongated by increasing the basicity of reaction solution, forming Au-tipped Ag nanorods. Through the analyses of STEM-EDX, it can be realized that some silver atoms diffuse into the whole gold nanoparticle, forming AuAg gradient alloy.