Final Report Summary - PHDBINA (Computing Phase Diagrams of Binary Nanoalloys)
The geometrical structures and chemical ordering (mixing/segregation) of nanoparticles of two or more metals (nanoalloys) are critical in determining their physical properties and their chemical reactivity – including their catalytic activity. The variation of these properties with cluster size and elemental composition, as well as temperature, are therefore of significant technological interest. At thermodynamic equilibrium, these properties can be expressed by means of building nanoalloy phase diagrams, which typically serve as a reference point for consideration of various nonequilibrium processes. Nanoalloy phase diagrams exhibit an additional degree of complexity compared to bulk phase diagrams, because system size has to be considered as an additional variable. On the other hand, nanoparticle structures and mixing patterns are often qualitatively different of those of bulk alloys. All these require extensive and systematic investigation of different bimetallic systems carried out within a unified methodological framework. Because of the wide variety of methods which have been used in the field, and the restricted range of system properties commonly reported, results are often difficult to compare directly. A unified, systematic study of key cases of nanoalloy phase diagrams should, therefore, provide valuable understanding about the equilibrium properties of nanoalloys.
The aims of this project are twofold. First, by carrying out an extensive study of several key nanoalloy systems within a unified methodological framework we expect to gain insight into what are the factors governing the mixing patterns in nanoalloys. Identifying some of these factors is one of the primary goals of the project. The selected binary systems, Au-Pd, Pd-Pt, and Ag-Cu, span the range of mixing properties of nanoalloys and, at the same time, they exhibit a number of common features. By studying the equilibrium thermodynamic properties of these systems we aim to obtain a clear and representative picture of how these properties depend on nanoparticles size, morphology and composition and to rule out differences due to methodological artefacts by a unified approach to all systems. Our ultimate goal with this study is to make a classification of nanoalloy phase diagrams looking for different limiting behaviour and enable theoretical predictions for thermodynamic equilibrium states, important for technological applications.
The second major aim is to establish experimentally testable link between theoretical predictions and technological applications. Most technological applications of nanoalloys, such as catalysis, involve the immobilisation of the nanoparticles on a support (generally the surface of an oxide such as MgO or TiO2). Microscopic measurements are also performed on carbon or oxide supports. We aim, therefore, to investigate the effect that the support has on the structure, morphology and chemical ordering of nanoalloy particles, comparing phase diagrams of free nanoparticles with the same nanoparticles on a support.
As steps towards these goals, we set the following objectives: (1) To develop semi-empirical many-body potentials for the systems being studied and for their interaction with the supporting material (MgO) by fitting potential parameters to input data from Density Functional Theory (DFT) calculations and experiment. (2) To find zero temperature configurations of bimetallic clusters which minimize their energy for compositions across the whole range. (3) To study temperature dependence of structure and mixing for selected cases of isolated bimetallic clusters. (4) To study temperature dependence for selected key cases of supported bimetallic clusters. This will display important aspects of the effect of the supporter on the nanoalloy phase diagram. (5) To compare theoretically predicted nanoalloy phase diagrams with experimental data.
For the sake of better collaboration with the group of Dr. Ziyou Li (School of Physics & Astronomy, University of Birmingham), we changed one of the binary systems envisioned in the project. Instead of Pd-Pt, we focused our attention on the Au-Rh system, for which there were available data of electron-microscope measurements.
Two new empirical potentials have been developed for the purposes of this project - parametrisation of an Au-Rh potential, which was so far lacking in the literature and one new model for interactions of metal atoms with the MgO(100) substrate has been parametrised for Pd.
We developed the parametrisation of the Au-Rh potential in collaboration with Prof. R. Ferrando (University of Genoa, Italy). The potential has been fitted to experimental bulk properties. We applied our Au-Rh potential to simulate and compare Au/Rh and Au/Pd interfaces for Au nanorods coated with Rh and Pd, respectively. Our study emphasized the key role of kinetic processes of deposition on the degree of mixing at the interface between the two metals, which may lead to contra-intuitive results. This work resulted in two publications. Another study, which was more computationally focused, simulating the vapour deposition of Rh, Pd and Pt on Au clusters, is in progress.
A new empirical potential for modelling the interaction of metal atoms with a MgO(100) substrate was developed in collaboration with Prof. A. Fortunelli (CNR, Pisa, Italy). Important effects due to electron polarisation induced by the ionic surface have been taken into account in this model, namely, the sensitivity of the metal-oxide interaction on whether or not other metal atoms are present above the interacting ones. This new feature of the model allows DFT data from large enough clusters (80 atoms) to be effectively included in the fitting procedure ensuring better accuracy of the model for large systems. Another new feature of the model is that it is continuous with respect to the metal-metal coordination, which makes it suitable for molecular dynamics simulations. This work has been reported in one publication. The work for parametrising the new potential for Au and eventually for Au-Pd on MgO is ongoing. In the study of Au-Pd on MgO, we are also investigating the effect of hetero- as opposed to homo-metallic interactions on the strength of metal-MgO binding.: this work is being conducted in collaboration with Prof. C. Mottet (CINAM, Marseille, France).
Two central questions have been considered regarding the immiscible Cu-Ag nanoalloy phase diagram: First, how the thermal dissolution between the two elements depends on the size and composition of the nanoparticle. Second, what induces the structural transition between core-shell and two-faced (Janus) configurations. Both questions also address the relation between nanoalloy and bulk phase diagrams.
Regarding the first of these questions, we studied the temperature effect on large (1000-2000 atoms) Cu-Ag nanoparticles obtained as a result of global minimum search. Defining concentration in a finite system at the atomic level was a nontrivial problem we had to solve in order to do this. We identified a subtle but clear dependance of solid solution concentrations on the internal nanoparticle composition and size. This is a principal difference with bulk phase diagrams. Our approach allows us to attribute this effect to the deviation of the stress distribution within the nanoparticle from uniform. This work is reported in one paper which has been submitted for publication.
Our goal from fundamental point of view is to try to relate general characteristics of the nanoalloy system such as surface, interface energies and stress induced by the lattice mismatch with the obtained nanoparticle structures. Toward this end, we have been working in collaboration with Dr. Nils Warken (School of Metallurgy & Materials, University of Birmingham) aiming to develop a continuous (phase field) descriptions of a Cu-Ag nanoparticle parametrised from data obtained at the atomic level. We have calculated interface energies between Cu and Ag phases for several orientations at the atomistic level and have studied their dependence on the layer thickness. The appropriate phase field model for a nanoparticle in which these data will be used as input is under development by the group of Dr. Warken.
The aims of this project are twofold. First, by carrying out an extensive study of several key nanoalloy systems within a unified methodological framework we expect to gain insight into what are the factors governing the mixing patterns in nanoalloys. Identifying some of these factors is one of the primary goals of the project. The selected binary systems, Au-Pd, Pd-Pt, and Ag-Cu, span the range of mixing properties of nanoalloys and, at the same time, they exhibit a number of common features. By studying the equilibrium thermodynamic properties of these systems we aim to obtain a clear and representative picture of how these properties depend on nanoparticles size, morphology and composition and to rule out differences due to methodological artefacts by a unified approach to all systems. Our ultimate goal with this study is to make a classification of nanoalloy phase diagrams looking for different limiting behaviour and enable theoretical predictions for thermodynamic equilibrium states, important for technological applications.
The second major aim is to establish experimentally testable link between theoretical predictions and technological applications. Most technological applications of nanoalloys, such as catalysis, involve the immobilisation of the nanoparticles on a support (generally the surface of an oxide such as MgO or TiO2). Microscopic measurements are also performed on carbon or oxide supports. We aim, therefore, to investigate the effect that the support has on the structure, morphology and chemical ordering of nanoalloy particles, comparing phase diagrams of free nanoparticles with the same nanoparticles on a support.
As steps towards these goals, we set the following objectives: (1) To develop semi-empirical many-body potentials for the systems being studied and for their interaction with the supporting material (MgO) by fitting potential parameters to input data from Density Functional Theory (DFT) calculations and experiment. (2) To find zero temperature configurations of bimetallic clusters which minimize their energy for compositions across the whole range. (3) To study temperature dependence of structure and mixing for selected cases of isolated bimetallic clusters. (4) To study temperature dependence for selected key cases of supported bimetallic clusters. This will display important aspects of the effect of the supporter on the nanoalloy phase diagram. (5) To compare theoretically predicted nanoalloy phase diagrams with experimental data.
For the sake of better collaboration with the group of Dr. Ziyou Li (School of Physics & Astronomy, University of Birmingham), we changed one of the binary systems envisioned in the project. Instead of Pd-Pt, we focused our attention on the Au-Rh system, for which there were available data of electron-microscope measurements.
Two new empirical potentials have been developed for the purposes of this project - parametrisation of an Au-Rh potential, which was so far lacking in the literature and one new model for interactions of metal atoms with the MgO(100) substrate has been parametrised for Pd.
We developed the parametrisation of the Au-Rh potential in collaboration with Prof. R. Ferrando (University of Genoa, Italy). The potential has been fitted to experimental bulk properties. We applied our Au-Rh potential to simulate and compare Au/Rh and Au/Pd interfaces for Au nanorods coated with Rh and Pd, respectively. Our study emphasized the key role of kinetic processes of deposition on the degree of mixing at the interface between the two metals, which may lead to contra-intuitive results. This work resulted in two publications. Another study, which was more computationally focused, simulating the vapour deposition of Rh, Pd and Pt on Au clusters, is in progress.
A new empirical potential for modelling the interaction of metal atoms with a MgO(100) substrate was developed in collaboration with Prof. A. Fortunelli (CNR, Pisa, Italy). Important effects due to electron polarisation induced by the ionic surface have been taken into account in this model, namely, the sensitivity of the metal-oxide interaction on whether or not other metal atoms are present above the interacting ones. This new feature of the model allows DFT data from large enough clusters (80 atoms) to be effectively included in the fitting procedure ensuring better accuracy of the model for large systems. Another new feature of the model is that it is continuous with respect to the metal-metal coordination, which makes it suitable for molecular dynamics simulations. This work has been reported in one publication. The work for parametrising the new potential for Au and eventually for Au-Pd on MgO is ongoing. In the study of Au-Pd on MgO, we are also investigating the effect of hetero- as opposed to homo-metallic interactions on the strength of metal-MgO binding.: this work is being conducted in collaboration with Prof. C. Mottet (CINAM, Marseille, France).
Two central questions have been considered regarding the immiscible Cu-Ag nanoalloy phase diagram: First, how the thermal dissolution between the two elements depends on the size and composition of the nanoparticle. Second, what induces the structural transition between core-shell and two-faced (Janus) configurations. Both questions also address the relation between nanoalloy and bulk phase diagrams.
Regarding the first of these questions, we studied the temperature effect on large (1000-2000 atoms) Cu-Ag nanoparticles obtained as a result of global minimum search. Defining concentration in a finite system at the atomic level was a nontrivial problem we had to solve in order to do this. We identified a subtle but clear dependance of solid solution concentrations on the internal nanoparticle composition and size. This is a principal difference with bulk phase diagrams. Our approach allows us to attribute this effect to the deviation of the stress distribution within the nanoparticle from uniform. This work is reported in one paper which has been submitted for publication.
Our goal from fundamental point of view is to try to relate general characteristics of the nanoalloy system such as surface, interface energies and stress induced by the lattice mismatch with the obtained nanoparticle structures. Toward this end, we have been working in collaboration with Dr. Nils Warken (School of Metallurgy & Materials, University of Birmingham) aiming to develop a continuous (phase field) descriptions of a Cu-Ag nanoparticle parametrised from data obtained at the atomic level. We have calculated interface energies between Cu and Ag phases for several orientations at the atomistic level and have studied their dependence on the layer thickness. The appropriate phase field model for a nanoparticle in which these data will be used as input is under development by the group of Dr. Warken.