| 摘要: | 基於奈米結構與奈米顆粒的薄膜對於燃料電池、熱電及光電原件科學及技術領域擁有極大的重要性。在本博士研究中發展及使用了兩個創新的光控製材料製程方法，首先是藉由惰性氣體控制脈衝雷射沉積(PLD)薄膜的羽流力學來產生奈米顆粒；第二個為藉由掃描脈衝雷射退火動態的驅使材料的相變化。
做為第一種發法的應用，在氣體環境中的PLD使用於成長堆疊的鉑奈米顆力所構成的奈米多孔薄膜，並應用於質子交換膜燃料電池。在氬氣填充的環境下，一道10ns的532nm脈衝雷射被用來燒蝕鉑靶材，藉由改變氬氣壓力來同時優化奈米顆粒尺寸及在基板上的分散成度，進而增強PEM燃料電池的功率密度。然後將生長的催化劑層封裝成薄膜電及組件並測量其單電池性能。最一開始，PLD成長的催化計僅用於燃料電池的陽極，在鉑附載為17μgcm-2，燃料電池的電流密度再0.6V時達到1.08Acm-2，這接近於有著以E-TEK Pt/C製備200μgcm-2白金附載的陽極的電池。因此，鉑的使用減少了12倍。
接下來，PLD被使用來成長用於陰極的催化劑。相較於陽極的氫氧化反應(HOR)，由於陰極的氧化還原反應(ORR)本質上非常緩慢，所以陰極比陽極需要更多的催化劑。因此，通常陰極的ORR是個瓶頸與減少陰極側鉑使用量是降低燃料電池的成本的關鍵，當在燃料電池陰極側使用以PLD成長的催化劑，有著鉑附載為100μgcm-2時，單電池的電流密度再0.6V時達到1.2Acm-2，這接近以E-TEK Pt/C製備的00μgcm-2白金附載。此外，以PLD製備的電極，與陰陽極鉑附載總合為117μgcm-2的PEM燃料電池，再0.6V下的總鉑質量公率密度達到7.43kWg-1，約為E-TEK Pt/C電極的燃料電池的五倍。最後，藉由基於CV加速降解測試來評估催化劑/載體的電化學耐久性，發現使用PLD製備的樣品在5000個電位循環後保持其原始電化學表面積的60%，遠高於僅保留原始電化學表面積7%的E-TEK Pt/C樣品。
第二個光控製材料製成的方法使用於在基板上製造半導體量子點以應用於紅外光偵測器及熱電材料。使用具有線輪廓的脈衝雷射(λ = 532 nm, pulse width = 10 ns)通過光熱應變來動態的控制自發性量子點的誘發以產生均勻的量子點層。有著適當的設定，可再大於4mm2的面積中型成平均高度為2.9nm、平均直徑為25nm和密度為6×1010cm-2的鍺/矽量子點。基於觀察到的量子點特性與雷射參數的關係提出一個包含雷射誘發應力、表面擴散以及Ostwald熟化的模型，以解釋鍺/矽量子典型成的機制。
;Nanostructured and/or nanoparticles based thin film are of paramount importance for very diverse fields of science and technology such as fuel cells, thermoelectric and optoelectronic devices. Two innovative methods of light controlled material fabrication developed and im- plemented during this PhD research work are outlined in this thesis. First is the pulsed laser deposition (PLD) of thin films with the plume dynamics controlled by noble gases to generate nanoparticles. Second one is the kinetically controlled phase transition of material driven by scanning pulse laser annealing.
As a demonstrative applications of the first method, PLD in a gas atmosphere was used to grow nanoporous thin films composed of stacked nanoparticles of platinum on gas diffusion layer for application to proton-exchange membrane fuel cells. The 10 ns pulsed laser radi- ation of 532 nm wavelength was used to ablate Pt target in Ar filled chamber. Argon pressure was varied to simultaneously optimize both the particle size and dispersion of nanoparticles on the substrate to raise the electrochemical surface area of the platinum nanoparticles, which in turn resulted in the enhanced power destiny of PEM fuel cell. The grown catalyst layer was then assembled into a membrane electrode assembly and it’s single cell performance were measured. Initially the PLD grown catalyst was only used on the anode side of the fuel cell. In this case, with a Pt loading of 17 μg cm−2 the fuel-cell current density at 0.6 V reaches 1.08 A cm−2, which was close to that of a cell with the anode made by using E-TEK Pt/C of 200 μg cm−2 Pt loading. Thus the usage of Pt was decreased by 12-fold.
Next, PLD was used to grow catalyst for the cathode side of the fuel cell. Since the oxygen reduction reaction (ORR) at the cathode is very sluggish in nature compared to hydrogen oxidation reaction (HOR) at the anode, the cathode requires a much larger amount of catalyst compared to the anode. Therefore, generally the ORR at the cathode is the bottleneck and the reduction of Pt usage at the cathode holds the key to lowering the overall cost of the fuel cell. When PLD grown catalyst was used on cathode side of the fuel cell, the current density of a single cell reached 1.2 A cm−2 at 0.6 V, at the Pt loading of 100 μg cm−2, which was close to that of a single cell using E-TEK Pt/C electrode with a cathode Pt loading of 100 μg cm−2. Further- more, for a PEM fuel cell with both electrodes prepared by PLD and a total anode and cathode Pt loading of 117 μg cm−2, the overall Pt mass-specific power density at 0.6 V reached 7.43 kW g−1, which was about 5 times higehr than that of a fuel cell with E-TEK Pt/C elec- trodes. Finally, the electrochemical durability of the catalyst/support was evaluated by using accelerated degradation test based on CV. It was found that the pulsed laser deposition sample retains 60% of its initial electrochemical surface area after 5000 potential cycles, much higher than that with E-TEK Pt/C, which retains only 7% of its initial electrochemical surface area.
The second method of light controlled material fabrication devised during this study was used to produce semiconductor quantum dots on a substrates for application to infrared photodetector and ther- moelectric materials. A scanning pulsed laser beam (λ = 532 nm, pulse width = 10 ns) with a line profile was implemented for kinet- ically controlled induction of self-assembly of quantum dots (Ge/Si) via photo-thermal strain so as to produce a homogeneous quantum dot layer. it was demonstrated with suitable setting, Ge/Si quantum dots with a mean height of 2.9 nm, a mean diameter of 25 nm, and a dot density of 6×1010 cm−2 could be formed over an area larger than 4 mm2. Based on the observed dependence of the characteristics of QDs on the laser parameters, a model consisting of laser-induced strain, surface diffusion, and Ostwald ripening was proposed for the mechanism responsible the formation of Ge/Si QDs. |
