Atomistic modeling of Au-Ag nanoparticle formation (original) (raw)
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Surface Science, 2019
A strategy to estimate the composition profile in the top few atom layers of a binary metal alloy nanoparticle is presented. The method is based on a combination of the X-ray Photoelectron Spectroscopy (XPS) and Monte Carlo (MC) simulations. Au-Ag NPs were synthesized via solution-phase co-reduction followed by reduction in gas phase at 400°C. The average composition at the NP surface was measured using XPS. The number of surface atomic layers whose compositions differ from bulk was identified using MC. XPS intensities were calculated using the number of independent layers as an input to the NIST database for the Simulation of Electron Spectra for Surface Analysis (SESSA). Upon analyzing the simulated XPS intensity we conclude that not one but several layer-by-layer Au/Ag composition profiles at the surface are consistent with the measured (experimental) XPS intensities. Finally, a procedure to identify the layer-by-layer composition profile consistent with both XPS analysis and MC simulations is proposed. The segregation free energy is calculated from the layer-by-layer composition profile identified. The resulting atomic scale information can guide first-principles prediction of catalyst activity where the elemental distribution at the surface is crucial.
Theoretical study of structure and segregation in 38-atom Ag-Au nanoalloys
The European Physical Journal D, 2007
Ag-Au bimetallic "nanoalloy" clusters with 38 atoms have been studied using a Gupta manybody potential combined with a genetic algorithm search technique. Clear changes in structure are observed as a function of Ag/Au composition and there is a clear tendency for surface segregation of the Ag atoms. Cluster stability is found to increase with increasing number of Au-Au and Ag-Au bonds and the segregation has been rationalised in terms of bonds strengths and elemental surface energies.
The Journal of Physical Chemistry C, 2018
The nucleation and growth of silver nanoparticles are modeled and simulated based on first-principles calculations. The formation energy of single-crystal and multiply-twinned particles are calculated to elucidate the thermodynamic properties of particles and modeled as a function of geometric parameters. Based on the calculated formation energy and the molecular collision theory, Kinetic Monte Carlo simulations are performed to trace the formation process of silver nanoparticles. In particular, the temporal change of size distribution and morphology are obtained and used to elucidate the governing mechanism in each stage of the formation process. It is demonstrated that the formation process is separated into four phases depending on the power-law time dependence of the particle formation and they are characterized by the size-difference between coalescent particles. The temperature-dependence of size distribution and morphology are also studied to elucidate the underlying mechanisms. The findings are compared with classical theories quantitatively and a strategy to control the morphology of silver nanoparticles is discussed.
Journal of Alloys and Compounds, 2009
X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) have been used to study the silver nanoparticles (AgNPs) deposited on Au(1 1 1). It is found out that the AgNPs form a layer which covers most of the Au surface. XPS has been used to observe the changes in the core levels of the AgNPs due to a high-temperature treatment. The XPS investigations clearly show that annealing of the AgNPs films at 620 K results in decreasing of the Ag 3d peaks intensity. At 770 K it is not possible to detect silver signal in the XPS spectra. However, after soft Ar + sputtering of the sample silver is again detectable. Our results show that during annealing of the AgNPs deposited on Au(1 1 1), the hydrocarbon-based ligands decompose and at higher temperatures diffusion of silver cores into gold substrate takes place. In the theoretical part of the work the Ag diffusion has been studied using a mean field theory for minimizing the mixing free energy with respect to the atomic configuration and local spatial relaxation. The present approach searches for energetically the most favorable equilibrium structure of the surface region described in terms of random-alloy within the Valenta model of the inhomogeneous systems. As a result, the layer resolved atomic configuration in the surface region in different temperatures has been obtained. Both results, experimental and theoretical, are consistent.
Noble and Precious Metals - Properties, Nanoscale Effects and Applications, 2018
In this chapter, experimental and theoretical studies on surface segregation in Ag-Au systems, including our own thermodynamic studies and molecular dynamics simulations of surface restructuring, on the basis of density functional theory are reviewed. The restructuring processes are triggered by adsorbed atomic O, which is supplied and consumed during catalysis. Experimental evidence points to the essential role of Ag impurities in nanoporous gold for activating O 2. At the same time, increasing Ag concentration may be detrimental for the selectivity of partial oxidation. Understanding the role of silver requires a knowledge on its chemical state and distribution in the material. Recent studies using electron microscopy and photoelectron spectroscopy shed new light on this issue revealing a non-uniform distribution of residual Ag and coexistence of different chemical forms of Ag. We conclude by presenting an outlook on electromechanical coupling at Ag-Au surfaces, which shows a way to systematically tune the catalytic activity of bimetallic surfaces.
Journal of …
Bimetallic nanoparticles are important because they possess catalytic and electronic properties with potential applications in medicine, electronics, and chemical industries. A galvanic replacement reaction synthesis has been used in this research to form bimetallic nanoparticles. The complete description of the synthesis consists of using the chemical reduction of metallic silver nitrite (AgNO 3) and gold-III chloride hydrate (HAuCl) salt precursors. The nanoparticles display round shapes, as revealed by highresolution transmission electron microscope (HRTEM). In order to better understand the colloidal structure, it was necessary to employ computational models which involved the simulations of HRTEM images.
Structure of Gold–Silver Nanoparticles
The Journal of Physical Chemistry C, 2017
Nanoparticles with nominal structures of Au@ Ag (core@shell) and Au@Ag@Au (core@shell@shell) were prepared using the sequential citrate reduction technique and characterized using routine characterization techniques, including transmission electron microscopy. X-ray absorption spectroscopy was then carried out on the samples, and extended X-ray absorption fine structure (EXAFS) analysis was used to determine the structure of the systems. The results of the routine techniques and the X-ray absorption spectroscopy were then compared. EXAFS analysis of the nanoparticles with the Au@Ag structure revealed very limited bimetallic interactions, supporting the assignment of a core@shell structure. EXAFS analysis of the nanoparticles with Au@Ag@Au structure showed an increased proportion of bimetallic interactions. Based on the colloid composition, the other characterization techniques and the chemistry of the system, these nanoparticles were interpreted as having an Au@Au/Ag-alloy structure. The EXAFS analyses corroborated the other characterization techniques and enabled the determination of the average-structure of the entire sample.
Investigation on Optical Properties of Ag-Au Alloy Nanoparticles
The outstanding chemical stability of Au and intense localized surface plasmon resonance of Ag make it possible to obtain a nanostructure with a good balance of good chemical stability and optical response. In this paper, we investigated the relationship between optical properties and the composition and size of Ag-Au alloy nanoparticle with numerical calculation by applying experimental data. Simplified empirical formulas are proposed through numerical simulation. The properties of extinction efficiency and the relative contribution of scattering and absorption efficiency to the extinction efficiency have been researched in detail. The calculated result and experimental data has been compared, and good agreement is obtained. Our work contributes greatly to catalysis application of Au-Ag alloy NPs in specific regions.
Structural and Energetic Properties of Au@Ag and Ag@Au Core-Shell Nanoparticles
—Using first principles density functional calculations, the stability and structural properties Au@Ag and Ag@Au core shell nanoparticles is explored. Our data shows that the growth of Au shell layers on an Ag core nanoparticle is more likely to happen than the growth of Ag shell layers on an Au core nanoparticle. Our structural analysis indicates that lower coordinated sites experience a bigger contraction when compared to the metallic 12 coordinated sites. In addition, our structural and energetic data for homonuclear Au and Ag nanoparticles indicate that dispersion forces do not decisively influence these properties.