STM and RHEED Studies of Vanadium Nanoclusters Grown on the θ-Al2O3/NiAl(100)

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题名:	STM and RHEED Studies of Vanadium Nanoclusters Grown on the θ-Al2O3/NiAl(100)
作者:	蕭凱仁;Hsiao, Kai-Jen
贡献者:	物理學系
关键词:	釩;氧化鋁;高能電子繞射儀;掃描穿隧式顯微鏡顯微鏡;STM;RHEED;Vanadium;Al2O3
日期:	2018-03-28
上传时间:	2018-04-13 11:08:59 (UTC+8)
出版者:	國立中央大學
摘要:	在催化過程中的反應性和選擇性可以透過控制金屬奈米團簇的大小來做改變，尺寸效應因此在奈米團簇的催化性質研究中扮演重要角色，為了說明尺寸效應，了解納米團簇的原子結構知識是必要的。目前的工作致力於研究Vanadium (V)金屬奈米團簇生長在Al2O3/NiAl(100)上的形貌與原子結構。我們在超高真空的條件下以掃描穿隧式顯微鏡顯微鏡(Scanning Tunneling Microscope, STM)和高能電子繞射儀(Reflection High Energy Electron Diffraction, RHEED)來研究，主要研究釩奈米團簇在室溫下的成長模式以及加熱對奈米團簇造成的影響。在室溫下形成的V奈米團簇隨著鍍量的增加，奈米團簇的尺寸會成長:直徑從1.56 nm增加到2.07 nm，高度從0.46 nm增加到0.66 nm。在低覆蓋率(< 0.88 ML)，奈米團簇的密度與尺寸都隨著覆蓋率增加；在高覆蓋率(> 0.88 ML)，直徑不在變化，高度則有明顯的變化，伴隨著奈米團簇密度的稍微增加；加熱到450 K 之後 V 奈米團簇會變小。當溫度加熱到570 K和700 K 後，V 奈米團簇會形成更大的尺寸。V 金屬的奈米團簇有很好的排列行為，有著 bcc 的結構並且沿著平行Al2O3(100) 表面以 (110) 面方向成長，其 V 金屬奈米團簇的 [001] 方向平行於Al2O3(100) 的 [0-10] 方向 (V(110)[001]//Al2O3(100)[0-10])，晶格常數會隨著鍍量上升而有所上升，但會隨退火溫度上升而有所下降。;Reactivity and selectivity of supported metal clusters in catalytic processes can be varied by controlling the cluster size. The size effect is thus centered in the studies of the nanocluster’s catalytic properties. To reveal origin of the size effect, a knowledge of the atomic structures of the nanoclusters is essential. The present work is devoted to controlling the size and resolving the structures at atomic level of vanadium (V) nanoclusters supported by ordered θ-Al_2 O_3 thin film grown on NiAl(100). We investigated the growth and thermal stability of V nanocluster grown on deposition of V vapor onto thin film θ-Al_2 O_3/NiAl(100) at 300 K, under ultrahigh vacuum conditions and with scanning tunneling microscope (STM) and reflection high energy electron diffraction (RHEED).The size of the V clusters increased with coverage: the mean diameter increased from 1.56 nm up to 2.07 nm, and the mean height from 0.46 nm to 0.66 nm at 300k. At low coverage (< 0.88 ML), the island density and average size of clusters increases with the coverage; at larger coverages (> 0.88 ML), the diameter does not change while the height increases significantly, accompanied with a slightly increase of cluster density. V nanoclusters become smaller after annealing to 450 K. With the temperature increasing to 570 K and 700 K, V atoms form larger clusters. The V clusters are structurally ordered, having a bcc phase, with their (110) plane parallel to the Al2O3(100) surface, and with [001] axes parallel to [0-10] direction of the Al2O3(100) (V(110)[001]//Al2O3(100)[0-10]). The lattice constant increases with the coverage and decreases with the annealing temperature.
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