Study on the growth process and formation mechanism of crystalline nanostructures for platinum-group metals
|Keywords:||鉑族金屬;鉑族奈米結構;奈米線;platinum-group metals;platinum nanostructures;platinum nanorods||Issue Date:||2011||Abstract:||
目前有關鉑族金屬奈米線材料成長，仍是極大困難。多數研究製程方式仍以濕式化學方式製程獲得。本論文之研究目的在於解決濕式製程鉑族奈米線之缺點，研究開發可於基板上直接成長鉑族奈米線及奈米板之新方法，並經由探討鉑族奈米線成長過程獲得共通成長機制。本論文第一部份簡述鉑族金屬奈米材料的製備與相關文獻回顧。第二部份是實驗研究的成果。在實驗部份，利用鉑族金屬在大氣下具備高溫穩定相之特性，進行實驗的設計與開發。首先，利用動力學成長控制的方式，使用添加鑽石及氧化釔粉末於鉑(Pt)金屬坩鍋內，利用物理蒸發方式，高溫1500 oC，在大氣環境下，加熱坩鍋成功製備出鉑金屬奈米線直接成長於藍寶石基板上。藉由成長鉑奈米線研究歸納經驗，簡化實驗添加物，重新設計鉑金屬坩鍋，利用單純鉑金屬堝於大氣下1350 oC，同樣使用物理蒸發方式直接成長鉑金屬桿材料於藍寶石基板上，透過鉑金屬奈米線生長動力學統計，研究發現鉑金屬奈米線成長趨分兩階段，第一階段為孕核抑制成長階段(Nucleation inhibited growth step)；第二階段為氣固相成長階段(Vapor-Solid growth step)。由活化能計算結果推論，成長反應速率決定步驟(Rate-determined step)為孕核抑制成長階段，該成長階段中鉑金屬原子沿基板進行表面擴散外並伴隨具揮發性氧化鉑氣相擴散，使完成鉑顆粒成長及刻面化(Faceting)過程。另外依據活化能計算結果，推論發現鉑金屬奈米桿成長是藉由鉑金屬原子而非揮發性氧化鉑沿鉑金屬顆粒表面擴散進行成長。
由於奈米元件發展蓬勃，Free-Standing鉑及銥金屬奈米線可直接成長在氧化鋁基板上，適於製作相關奈米元件。在本論文中，對鉑及銥奈米線的電性量測顯示出鉑與銥金屬奈米線分別為10 μΩ•cm及5.1 μΩ•cm，具備與塊材接近之電性。在鉑奈米桿場發射性質的量測顯示出，鉑奈米桿最低起始電壓(Turn-on field)約在5V/㎛。當操作電壓在33V/㎛可以得到最大電流密度4mA / cm2。
To date, the preparation of platinum-group metals (PGM) nanostructures is still a grand challenge. In general, PGM nanocrystals are synthesized in the presence of the capping agents via reduction of the PGM containing precursors, decomposition of an organometallic complex, or a combination of these two routes. The two main purposes of this dissertation are to solve the disadvantage of the solution-phase process and to find a brand new growth method, which makes the PGM nanostructures prepared on substrate directly. Meanwhile, through the specific growth method, we hope that we can learn the general growth mechanism of PGM nanostructures. The first part of this dissertation, we briefly introduce the research background and literature reviews. The second part of this dissertation is the results of experiments including the preparation of Pt nanobelts, Pt rods, Ir wires, and Pd rods. The transport properties of Pt naobelts, rods and Ir wires were investigated in the last part of this dissertation. As regarding to the experimental aspect, we utilized that PGM posses the property of highly thermal stability in high temperature of atmosphere. Based on this unique property, we designed and developed the experiments. Firstly, we break through the limitation of classical thermodynamics for the preparation of Pt nanobelts by using a simple physical evaporation method. Diamond and yttrium oxide powders added into the Pt crucible were directly heated up to 1500 oC in air. By means of the growth experience of Pt nanobelts, we redesign the Pt crucible without adding diamond and yttrium oxide powders. The Pt rods were obtained at 1350 oC through this work. In additional, we also studied the kinetics of the growth of Pt rods in this case. The outcomes of kinetics study appeared that the growth process of Pt rods could be divided into two steps. One is nucleation inhibited growth step; the other one is vapor-solid growth step. The results of this study also appeared that the rate-determined step is nucleation inhibited growth step. The growth and faceting processes of Pt particles were complete accompanied with the surface diffusions of Pt atoms and volatile PtO2 phase on the substrate. Furthermore, according to the calculated results of activation energy of Pt rods, the surface diffusion of Pt adatoms leads to the formation of Pt rods.
Due to that PGM, Pt, iridium (Ir), ruthenium (Ru), rhodium (Rh), osmium (Os), and palladium (Pd), possessed similar physical and chemical properties and based on accumulating experimental experiences from the preparation of Pt rods, I suspect that the route of prepared Pt rods via thermal annealing process would be useful and workable for preparing other platinum-group metals nanostructures via identical process. As expected, free-standing Ir nanostructures (wires, platelets) could be prepared successfully at 1250oC for 2 hours duration in air at 1 atm by simple annealing the sapphire substrate underneath an Ir crucible. Final work of the growth of PGM nanostructures, the preparation and kinetics study of free-standing palladium rods was conduct in this dissertation. In contrast to the above-mentioned Pt and Ir, Pd is necessary to take into account a factor hitherto ignored as being negligible for the other metals, the vapor pressure of palladium metal itself. Observed from the partial pressure of Pd in equipment with one atmosphere of oxygen, the vapor pressure increases very rapidly, rising from 0.0063x10-6 atm at 1000oC to 620 x 10-6 atm at 1475 oC. The vapor of palladium is about 10000 times higher over the range 1000 to 1600 oC than that of platinum. Therefore, I rationally infer that Pd vapor phase were mainly supplied from the Pd crucible under the specimen and follow by getting precipitates from supersaturated Pd vapor phase and further lead to the formation of aligned Pd rods. This outcome also implies that the PGM adatoms were, indeed, diffused on the surface of the PGM particles and further lead to the formation of platinum-group metals rods (PGM rods).
The room-temperature electron transport and the field emission properties of Pt and Ir nanostructures were also conducted. In the electrical measurements of Pt and Ir nanowires aspect, they exhibited the excellent electronic transport as well as that of bulk Pt (10.5 μΩ•cm) and Ir (5.1 μΩ•cm), respectively. For the Electron Field Emission (EFE) measurement, we observed the current density rapidly increased at an electric field ( Eto ) of ~5 V / ㎛ due to Field Emission (FE). The maximum current density achieved was 4mA / cm2 at an applied field of 33 V / ㎛.
|Appears in Collections:||材料科學與工程學系|
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