Study on the growth process and formation mechanism of crystalline nanostructures for platinum-group metals
Date Issued
2011
Date
2011
Author(s)
Ma, Dai-Liang
Abstract
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 / ㎛.
Subjects
platinum-group metals
platinum nanostructures
platinum nanorods
Type
thesis
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