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Grant Title: Collaborative Research: Hierarchically Structured Polycrystalline Hollow Gold Nanoparticles - a Model System for Integrated Experimental and Multiscale Computational Nanomechanics Grant Number: 1000831 Program: Nano/Bio Mechanics PIs: Yaowu Hao and Efstathios Meletis Synthesis, Annealing and Mechanical Properties of Hollow Gold Nanoparticles Department of Materials Science and Engineering, University of Texas at Arlington, TX 76019 Introduction Polycrystalline hollow gold particle diameter nanoparticles Au deposition starts from -0.4V (vs Ag/AgCl). According to Nernst equation, wall thickness (HGNPs) have been synthesized in large amounts with a narrow size distribution using bubble template nanoparticles (<100 nm) have a polycrystalline the glass area and gold deposited only on silver strip surface. When the applied gold shell with tunable thickness in the range of 20 potential is -0.7V, well-defined gold nanoparticles formed on the glass area, and to 50 nm and a grain size around 5 nm. HGNPs as the overpotential increases the size of gold nanoparticles decreases. made of These hydrogen evolution are expected to happen simultaneously when the applied potentials are more negative than -0.55 V. At -0.5V. no particle was observed on nanostructures method. grain size hydrogen evolution equilibrium potential for the is -0.55 V. Gold deposition and hollow are synthesis Mechanical Properties Applied Potential nanocrystalline -0.7V -0.5V We have compared mechanical properties of solid gold nanoparticles with HGNPs. Nanoparticles were placed on a Au-coated silicon wafer surface, and Nanoindentation was performed on both solid Au nanoparticles and HGNPs. Force-displacement curves were recorded (shown in the figure below). Comparing to solid Au nanoparticles which are single-crystal in nature, there seems no elastic deformation in HGNPs, only plastic deformation. -0.9V materials, belonging to the growing class of nanostructures with inherent structural hierarchy [1] (multiple length scales existing within an individual structure: grain size, shell thickness and diameter TEM micrograph of a single HGNP of sphere). HGNPs provide a golden opportunity to explore the effects of structural hierarchy on deformation mechanisms of nanoscale materials. Due to their hollow nature, HGNPs can be easily observed for in-situ TEM nanoindentation without much sample preparation; individual HGNP can be easily probed. As the only metal without native oxide at its surface, Au eliminates the effects of boundary constraint due to oxide films passivating the surface[3], which can play a significant role in determining the 2μm Concentration of Ni ions in electrolyte Hydrogen evolution can be enhanced by adding a hydrogen evolution enhancer into the electrolyte. In this experiment, nickel sulfamate solution used as a hydrogen evolution enhancer. The number of gold nanoparticles increases with the concentration of Ni ions. It does not show significant change of particle sizes. 0.4M 0.2M mechanical properties at the nanoscale. 2μm 2μm 2μm. 0.6M We have conducted in situ TEM nanoindentation experiment. Both blunt and sharp tips were used. Using blunt tip, continuous deformation has been observed. No particle fracture occurred. When a sharp tip was used, the tip penetrated through the Au shell. These results sugggest a extremely high plasticity, which is in a good agreement with the nanoindentation results. Synthesis In order to explore the effect of shell thickness, grain size, and curvature on the mechanical behavior of the HGNPs, it is critical to obtain the key fabrication parameters for the control of the structural properties of HGNPs. 10μm 10μm 10μm Using a silver stripe-patterned working electrode, we have studied the effect of key process parameters on the structural properties of HGNPs. The silver stripe pattern working electrode which was fabricated on microscopy glass substrate by photolithography technique is composed of conductive and nonconductive areas, When a potential more negative than hydrogen evolution equilibrium potential is applied to the silver strips, the hydrogen evolution occurs. The concentration of hydrogen molecules will be increased and becomes supersaturated in the solution close to the electrode surface. The supersaturated hydrogen molecules are expected to form hydrogen nanobubbles on both silver strip and the glass area. This electrode provides a convenient way to observe the morphology of HGNPs synthesized with different process parameters. we have studied the effects of applied potential and concentration of hydrogen evolution enhancer on the morphology and structures of HGNPs. Annealing The variation of abovementioned parameters is limited. Annealing may be a more effective way to obtain a variation in grain size. In addition, HGNPs are nanocrystalline materials with a very small grain size (about 5nm), so a significant volume of HGNPs is a grain boundary. The grain growth process occurs to decrease the interfacial energy and Conclusions and ongoing work hence the total energy of the system. For HGNPs, the driving force of the grain growth is expected to be high. However, many reports in literature suggested that the grain growth Synthesis: It has been found that as the applied potential increases, the size of in nanocrystalline materials is very small (almost negligible) at a reasonably high nanoparticles decreases. Other synthesis parameters such as the concentration of gold temperature. Therefore, another objective of annealing experiments is to use HGNPs as ions and stabilizers in the electrolyte are currently being investigated. a model to understand the nanoscale grain growth mechanism of nanocrystalline metal. Annealing: This preliminary result shows that the nanocrystalline grains in HGNPs do In the annealing experiments, HGNPs was placed inside a tube furnace under not have obvious growth at 300ºC but grow significantly at 500ºC. Currently, we are argon atmosphere and heated up at different temperatures for different times. After conducting a systematic annealing study of grain growth of HGNPs. HGNPs are HGNPs were cooled down inside the furnace, they were observed in HRTEM. annealed at different temperatures (from 200 to 700 ºC ) and different times (from 10 Annealing at 300ºC for 1 hour Annealing at 500ºC for 1 hour minutes to 3 hours). Also, in situ HRTEM heating method will be used. Individual HGNP will be put on the heating sample holder. It will be heated up while simultaneously 25μm 25μm observing the grain growth process. Mechanical Properties: nanoindentation The preliminary nanoindentation and in situ TEM studies show that HGNP exhibits extremely high plasticity. The understanding of this phenomenon requires more experiments and computer simulation, which is currently under the way. The schematic of a three electrode electrochemical cell using silver stripepatterned working electrode for the investigation of effects of synthesis parameters on structural properties of HGNPs. Glass area (non-conductive) Ag strip (Working electrode) SEM micrograph of stripe pattern electrode after a potential of -0.7 V (vs Ag/AgCl) for 200s. References HRTEM micrographs of HGNPs after annealing at 300C and 500C F It shows that the nanocrystalline grains in HGNPs do not have obvious growth at 300ºC but grow significantly at 500ºC. 1. Huang, C.W., J.C. Jiang, M.Y. Lu, L. Sun, E.I. Meletis, and Y.W. Hao, Capturing Electrochemically Evolved Nanobubbles by Electroless Deposition. A Facile Route to the Synthesis of Hollow Nanoparticles. Nano Letters, 9(12): 4297-4301 (2009). 2. Dao, M., L. Lu, R.J. Asaro, J.T.M. De Hosson, and E. Ma, Toward a quantitative understanding of mechanical behavior of nanocrystalline metals. Acta Materialia, 55(12): 4041-4065 (2007).