Crystallizing Calculation For Palladium Grain From Atomic Packing

Crystallization is a universal phenomenon in the field of material science. As nanoscience and nanotechanology has developed rapidly, the range of this field has extended largely. Especially for shape-controlled synthesis of metal nanocrystals, current achievement of the desired shape has required a laborious trial-and-error approach to finding optimal set of reactants and reaction conditions. As a result, comprehensive understanding crystallization process and developing theoretical models that can yield insights into the underlying morphology evolution and offer guidelines for the rational synthesis manipulation is essential for the development of new material chemistries. Crystallization process can be regarded as a microscopic process related to bond formation and breaking and thus it's effective to describe and analyze this process from chemical bond viewpoint.Through analysis of the stability of different atomic packing structures, bonding percentage, which combined surface unsaturated sites density and surface unsaturated sites strength, has been confirmed to reflect the relative stability of different structures. Further, two new parameters that can reflect stability of particular plane have been derived. A model for the relative crystal growth rate has thus been established. Pd was chosen as an example to confirm our model because the largest number of shapes for this novel metal has been generated and several systematical experiments have been conducted for exploring the shape evolution mechanism which can examine our calculation results.We first estimated the ideal morphology of palladium single crystal and also considered nanosize effect to the fluctuation of ideal morphology. This model has been further employed to predict the favored shape in the presence of bromide or iodide, both of which have simple structures but always have significant decoration for crystal shape control. Owing to their distinct chemical adsorption phases that can radically change the original dynamic chemical bonding environment around nonequivalent crystallographic planes through altering unsaturated sites strength and density, bromide can aid the exposure of{100} facets dominating the cubic shape, while iodide can promote the formation of octahedron and rhombic dodecahedron in different cases. The theoretical predictions are in a good agreement with experimental observations. Our results also show that the formation of certain plane ({110} facets) is associated with the alteration of surface step orientation.The current work can deepen our understanding of the capping effect on the evolution of particular crystallographic planes. The agreement between predictions and experimental observations confirms the validity of this model and indicates that the investigation of crystal growth from chemical bond viewpoint is reasonable, which provides a novel method to analyze crystallizing process.