| 摘要: | 多數金屬成品生產後的表面並不具有純淨的金屬表面特性，而是在空氣中被氧化後形成一層金屬氧化物附著於表面上，所以汽車汽缸表面結構及其表面處理有很大的關係，另方面金屬/氧化物介面對其他工業如電子零件，金屬陶瓷探測器(sensor)，以及氧化物為底層的過度金屬催化劑，具有很重要的地位，例如不同催化劑具有不同表面物理特性，具有不同催化活性，所以表面物理特性的研究是很廣泛及重要，尤其微觀及nano-scale 下，其表面物理特性更具有特別量子效應，電子特性，物理特性，我們的工作計畫：從事鈷奈米晶體成長有規律的氧化鋁表面，藉此研究奈米晶體的相 (crystalline phase)以及 epitaxial growth-orientation 與其底層表面結構(substrate, matrix) surface structure 的相瓦關係，分析高能電子繞射圖RHEED patterns 從其電子入射方向，即zone-axis，[0-10] 所獲得的RHEED Pattern,可獲得Co nano-clusters 的reciprocal lattice points 排列於the zero-level (zero Laue zone) 上, →推論the reciprocal lattice of the Co nano-clusters, → Fourier transform 可得到Co-奈米的晶體結構，我們從reciprocal lattice plane 可推論 Co-nano-clusters 是fcc 晶相，並非bcc 出hcp 晶向. 我們探討 為何鈷奈米晶體優先選擇 fcc 相而不是hcp 相成長於氧化膜上(Θ-Al2O3). 另外奈米的結構及其磊晶成長(epitaxial growth)的分析及了解有助於研發(開啟)微電子零件，催化劑及metal-ceramic-based sensors。如CO 一氧化碳及甲醇(methanol)在陽極表面氧化過程的研發有助益於發展及製造modified 的陽極材料。利用高能電子繞射(Reflection High Energy Electron Diffraction) (RHEED)的特殊技術可測定奈米顆粒(nano-clusters)晶體的晶相(crystalline phase)及在金屬或金屬氧化物基質 (substrate)表面上磊晶成長(epitaxial growth)的(h k l)結晶面(crystalline orientation) (refer to Fig. 1)。研究觀測不同的系統(systems)，如鈷(Co)、鉑(Pt)、鈀(Pd)等奈米顆粒在2 3 Al O 及金(Au)的(100)表面上，我們發現奈米晶體的晶相及成長面 (h k l) (orientation)與基質的表面結構 (substrate surface structure)有很密切的關係：奈米晶相及磊晶(h k l)成長平面 (crystalline phase and orientation)相對於基質(substrate)表面結構，會選擇它們間的晶格常數差值 ( lattice mismatch) 的最小值，以便降低表面自由能 (surface free energy). While the morphology of cobalt clusters as well as the oxide film on NiAl(100) substrate have been studied by STM, there exists a lack of knowledge with respect to the matrix dependent orientation and crystalline phase of metal clusters, which may have strong effects on magnetic, electronic and catalytic properties. On the other hand, the metal-oxide interface is of extreme importance in many technological applications including microelectronic devices, oxide-supported transition metal catalysts and metal-ceramic-based sensors. For example, the Co nanoclusters embedded in an insulating matrix can serve as microscopic magnetic reading heads or sensors, and magnetically operated logic nanogates. The magnetic properties may strongly depend on the particle size and epitaxial growth of Co clusters on oxide. The latter may be strongly influenced by surface structure of oxide. The features of the epitaxial growth of cobalt nanocrystals on alumina oxide/NiAl(100) are significant because they may have various physical and chemical properties, which have not been reported so far. It is a fundamental interest to study the heterogeneous catalytic reaction of small organic molecules such as methanol, carbon monoxide and formic acid (1, 2). Small metal particles supported on oxide surfaces play a major role in the heterogeneous catalysis, in which supported metals serve as reaction centers. Nano-clusters of Pt deposited onto Ru-anode may lead to enhanced catalytic activity towards methanol electro-oxidation. For instance, the anode material in direct (DMFC) and indirect (IMFC) methanol fuel cells are in principle very attractive energy sources to be used in mobile systems, because the fuel is liquid at room temperature and the cell are operating at rather low temperature compared to phosphoric acid fuel cells. During the oxidation process, the hydrogen-rich fuel passes the anode and CO impurities are strongly adsorbed on the Pt, thereby blocking the adsorption sites needed for the main reactions, namely the hydrogen oxidation, and thus decreasing the efficiency of the cell dramatically. The poising problem can be reduced by using a PtRu alloy anode instead of pure Pt. Therefore it is interesting to study the promotion effects due to the modified anode toward methanol oxidation. In order to understand the mechanism of CO/methanol electrooxidation on bi-metal catalysts, more work on the surface structure and the relationship between the surface structure and the reactivity of catalysts are necessary. The understanding of the improved catalytic activity due to the modified electrode is extremely useful for the development of an efficient fuel cell. 研究期間：9908 ~ 10007 |
