Article pubs.acs.org/JPCB
Free Energy of Nucleation and Interplay between Size and Composition in CuNi Systems Solene Bechelli, B. Gonzalez, Vincent Piquet, Ilham Essafri, Caroline Desgranges, and Jerome Delhommelle* Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202, United States ABSTRACT: Using molecular simulation, we shed light on the crystal nucleation process in systems of Cu, Ni, and their nanoalloy. For each system, we simulate the formation of the crystal nucleus along the entire nucleation pathway and determine the free energy barrier overcome by the system to form a critical nucleus. Comparing the results obtained for the pure metals to those for the nanoalloy, we analyze the impact of alloying on the free energy of nucleation, as well as on the size and structure of the crystal nucleus. Specifically, we relate the greater free energy of nucleation, and bigger critical nuclei, obtained for the nanoalloy, to the difference in size and cohesive energy between the two metals. Furthermore, we characterize the dependence of the local composition of the incipient crystal cluster on its size, which is of key significance for the applications of bimetallic nanoparticles in catalysis.
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INTRODUCTION The process of alloying copper with nickel has recently been shown to result in a dramatic modification of its properties. For instance, on the macroscopic scale, CuNi alloys exhibit an enhanced resistance to corrosion, strength, and hardness when compared to pure Cu.1,2 Furthermore, on the nanoscale, CuNi nanoalloys are extremely reactive and exhibit much improved catalytic activities than pure Ni nanoparticles.3,4 The properties of the CuNi alloys strongly depend on the conditions under which the crystal phase forms, such as the temperature at which crystal nucleation takes place.5−7 This is due to the fact that the nucleation process has a strong impact on the type of crystal structure that arises during crystallization, as well as on the local composition within the alloy.8−10 As a result, for both the microscopic and macroscopic scales, the control of the improved properties of CuNi alloys relies on a complete understanding of the mechanisms underlying crystal nucleation in this system. The aim of this work is to use molecular dynamics simulations to elucidate the nucleation pathway for nucleation in the pure metal systems Cu and Ni, as well as in the equimolar mixture CuNi. Previous work on CuNi systems has focused on studying the nucleation and growth under rapid and strong supercooling.7,11,12 A key parameter for the nucleation process is the free energy barrier that the system has to overcome to form a crystal nucleus of a critical size.13−15 To determine this parameter, and to identify the structural changes occurring during crystal nucleation, we combine molecular dynamics simulations with the umbrella sampling technique.16−18 This method allows us to study nucleation at a moderate supercooling and to calculate the free energy associated with the crystal nucleation process. To compare the results obtained between the pure metals systems and the alloy, we keep the same degree of supercooling for all systems and carefully analyze the changes in structure, composition and free energy throughout the nucleation process. In particular, we © 2017 American Chemical Society
characterize the interplay between the size of the incipient nucleus and its structure and composition. Furthermore, our goal is also to determine the role played by the surface energy and cohesive energies of Cu and Ni in the formation of the crystal nucleus and on potential segregation effects in the nanocrystallites. The article is organized as follows. In the next section, we discuss the simulation method as well as the model used for the CuNi systems. In particular, we introduce the reaction coordinate (or order parameter) that is used to span the nucleation pathway that spreads from the metastable supercooled liquid to the critical nucleus, and detail the structural analyses that allow us to identify the onset of the various crystalline structures, either close-packed (CP) or body centered cubic (BCC) structures. We then discuss the results obtained for the nucleation process in pure Cu, pure Ni, and in the CuNi equimolar mixture to shed light on the competition between surface and volume effects during the formation of the crystal nucleus. We finally draw the main conclusions of this work in the last section.
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SIMULATION METHODS When crystal nucleation takes place for a moderate supercooling, there is a large free energy barrier that the system needs to overcome to form a crystal nucleus. This free energy barrier is often interpreted, e.g., in the classical nucleation theory, as arising from the interplay between an unfavorable contribution, corresponding to the formation of a solid−liquid interface, and a favorable contribution, stemming from the conversion of atoms from the metastable liquid into the thermodynamically stable crystal.17 To allow the system to overcome this free energy barrier, we combine molecular Received: June 19, 2017 Revised: August 15, 2017 Published: August 16, 2017 8558
DOI: 10.1021/acs.jpcb.7b06028 J. Phys. Chem. B 2017, 121, 8558−8563
Article
The Journal of Physical Chemistry B
for the two atoms are correlated.17 Atom i is assigned a solidlike environment if it has at least 8 connexions with its nearest neighbors. Second, among the atoms identified as solid-like, we further analyze the local environment by looking at the values taken by the w6(i) order parameter for each atom i. Specifically, the atom i is CP-like if w6(i)
