- 學位論文
低壓扁平焰結合噴霧熱分解技術合成奈米結構白金中空球體與薄膜
Formation of Nanostructured Platinum Hollow Spheres and Thin Films by Spray Pyrolysis in Low Pressure Flat Flame
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摘要
本研究主旨在探討利用低壓扁平焰輔助之噴霧熱分解系統直接合成白金金屬奈米微粒與奈米結構之薄膜。研究中將探討奈米微粒的形成機制，以及薄膜的成長機制。研究中亦將探討燃料/氧化劑, 腔壓, 先驅物濃度, 操作時間以及火焰溫度等對微粒或薄膜性質之影響。本研究所合成的奈米微粒與薄膜之鑑定係藉由場發射電子顯微鏡(FESEM)與穿透式電子顯微鏡(TEM)來獲得微粒與薄膜之粒徑與形貌, 以X-光繞射儀(XRD)獲得結晶相與晶粒大小, 利用X-光吸收光譜(XAS)探討生成微粒之氧化組態, 原子結構與中心原子, 與其鄰近原子之平均配位數。 利用TEM與HRTEM觀察低壓扁平焰所合成之白金金屬多為具有奈米結構薄殼之次微米中空白金球體，操作時間增加，奈米結晶粒徑略為增加，推測可能是由於火焰傳遞熱輻射，施予熱能於微粒上所導致的結果。XAS研究結果顯示，生成微粒之first-shell鍵結為Pt-Pt。除了最低溫度1078K， 低壓扁平焰所合成之白金金屬其X光吸收近邊緣結構(XANES)呈現與白金箔(Pt foil)相似的吸收與震盪。 當火焰溫度高於1378K，即使是在氧氣過量的操作條件下，所生成之奈米結構薄膜經XRD分析為面心立方(fcc)晶體結構，晶粒大小隨反應溫度以及先驅物濃度的升高而增加，亦隨操作時間略為增加。腔壓對晶粒大小的影響則並不顯著，但火焰溫度對所生成微粒之晶粒大小卻有顯著的影響。 本研究提出中空球體與奈米結構薄膜可能之形成機制，推測是由於奈米結晶中空球體產生微爆裂現象後，形成奈米結構薄膜。
並列摘要
One-step synthesis of platinum nanoparticles and nanostructured thin film by spray pyrolysis in low-pressure flat flame was performed here. The particle formation mechanisms were also examined. The effects of fuel/oxidant ratio, chamber pressure, precursor concentration, operation time and flame temperature on particle or thin film characteristics were investigated. The flame-derived particles and thin film were characterized for size and morphology by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The crystal phase and grain size were analyzed by X-ray diffraction (XRD). The X-ray absorption spectroscopy (XAS) was used to investigate the oxidation state, the local atomic structure, average local coordination number, and to identify neighboring atoms of the particle samples. The TEM and HRTEM results suggest that the flame-derived particles are submicrometer spherical hollow particles with nanocrystalline crusts. Increasing the operation time resulted in slightly increased nanocrystals size, due probably to the extended radiative heating of the deposited particles by the flame. The XAS results suggested a Pt-Pt bonding from the first shell data of the as-derived particles. The oscillation by XANES at high energy side of the absorption edge for flame-derived particles is very similar to that of Pt foil, indicating a metallic state of the as-derived particles, except those prepared with the lowest flame temperature, 1078K. The unusual nano-thin films derived are composed of metallic platinum nanoparticles of face-center-cubic crystal structure when flame temperature is 1378K and above, even with an oxygen-enriched flame. The volume-averaged crystallite size increased strongly with flame temperature and precursor concentration, but slightly with operation time. The XRD patterns of the flame-derived thin film samples were identified to match those of f.c.c. platinmu in all cases except those obtained at 1078K. The chamber pressure does not have a direct impact on crystallite size but affects the flame temperature significantly, which in turn affects the crystallite size of the particles thus prepared. The formation mechanisms were proposed for both the hollow spheres and thin films. Micro-explosion of the nanocrystalline hollow spheres was suspected to have resulted in the unusual nano-thin films by the combined technique.
參考文獻
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