Structure and environment effects
Because of the strong dependence of the physical and chemical properties on the atomic structure of nanoalloys, it is of prime interest to describe properly their different thermodynamic equilibrium phases, for what concerns structure/morphology and chemical ordering. This new type of size-dependent “phase diagram” is complex to determine for nanoscaled “bimetallic” systems, either weakly or strongly miscible, where the surface effects can be overwhelming. However, to be more realistic: therefore it is important to take into account the environment in which the nanoalloys are embedded. This can be a support, a solid matrix or a fluid medium (gas, liquid), which influences the nanoalloy structure and properties. Finally, the addition of a third metal brings a new degree of freedom to tune structure and chemical ordering of nanoalloys, increasing the complexity and the richness of the structural landscape and giving rise to new challenging problems both on the experimental and the modelling side.
- Size-dependent nanoalloy phase diagrams and transitions
State of the art
The nanoalloy phase diagram determination is not a trivial task knowing the wide spectrum of different possible atomic structures, in close connection to their finite size. For example, at small sizes, nanoparticles can present fivefold symmetries, as in icosahedra (Ih) and decahedra (Dh), while at large sizes crystalline structures (i.e. Truncated octahedral TOh, as fragments of bulk crystals) prevail [1]. Recent experiments have been able to determine size-dependent structures and chemical ordering in systems with tendency to mixing, such as CoPt [2,3], and in phase-separating systems, such as CuAg [4-6]. These experiments used a variety of techniques, such as high-resolution transmission electron microscopy, in situ small angles and wide angle x-ray scattering [4,5], energy filtered transmission electron microscopy, high resolution imaging and high-angle annular dark field imaging. Ordered phases in CoPt clusters have been observed, and the transition from core-shell to Janus structures has been observed in CuAg [2-6].
From the theoretical point of view, Monte Carlo simulations and global optimization searches allowed to characterize structural crossover sizes between different motifs (Ih, Dh and TOh) [7], and the morphological instability of core-shell metallic nanoparticles [8-11] causing symmetry breaking. The calculations have also demonstrated that the difference in atomic size between the two elements induces strain effects which can favor non periodic structures (for example Ih or poly-Ih) over to periodic bulk-like structures.
Perspectives for the next years
Most of the existing theoretical phase diagrams concern systems with limited miscibility [12] giving rise to core-shell or Janus structures. There is a study on a specific cubic CuAg structure of 405 atoms TOh in the full range concentration range [13]. This study has been performed using state of the art thermodynamic models showing all the physical richness of nanoalloys, but it is restricted to one size and one structure! Another study about CuAg considered some sizes from 1000 to 2000 atoms and different structures (Ih and FCC), but only focusing on the solubility limits on both sides of the phase diagram for nanoalloys as compared to bulk, whereas any facet of the system should present new interesting phases. Therefore the draft of a full theoretical phase diagram as a function of temperature, composition, shape and size is still lacking even for the most studied systems. Moreover, the existence of inhomogeneous sites in nanoparticles (core, facets edges, etc…), may require the characterization by individual phases, giving thus rise to multiple phase diagrams instead of the single phase diagram of bulk systems.On the experimental side, the characterization of the structure and chemical ordering of nanoalloys, including surface and its inner core, is still very challenging. Synergy between advanced microscopy techniques, X-ray experiments and modelling will be necessary to achieve breakthrough advances such as maps of chemical ordering with atomic resolution.
Finally the environment can be also a tool for formulation engineering of nanoalloys. The response of the nanoalloy structure to the post-formation addition of ligands thus becomes of particular interest (for example, the functionalization in cancer therapy or as MRI contrast agents for imaging).
References
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[10] R. Ferrando, Symmetry breaking and morphological instabilities in core-shell metallic nanoparticles, J. Phys.: Condens. Matter 27, 13003 (2015)
[11] I. Parsina and F. Baletto Tailoring the Structural Motif of AgCo Nanoalloys: Core/Shell versus Janus-like JPCC 114, 1504 (2010).
[12] Y. Wang, M. Hou, Ordering of bimetallic nanoalloys predicted from bulk alloy phase diagrams, J. Phys. Chem. C 116, 10814 (2012)
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- Effects of the environment
State of the art
Nanoparticles can play an important role in several different fields, such as biology [1] magnetism [2] and catalysis [3]. For industrial applications, the production of nanomaterials used as catalysts in fuel cells, batteries, etc… needs to be performed taking into account the real environment in which the nanoparticles must operate. For example, the exposure to the atmosphere is a problem rather ubiquitous in both the applications of nanoalloys. Some elements are much more amenable to oxidation than others, leading to dramatic changes in structure. A better understanding of atomic-level structure and structure-property relationships, is required in realistic conditions of use. Numerous studies have demonstrated the role of the chemical and physical environment on the control of size, shape and composition of metallic nanoparticles (in presence of gas or atmosphere, deposited on a support, in liquid and/or passivated by organic ligands) [4-6]. However, the preparation of well-defined nanoparticles is still challenging, despite of the large number of synthetic protocols in the literature. This is especially true for nanoalloys, since their multi-element character causes a high sensitivity of the structure to the environment, leading to unexpected atom arrangements: segregated, surface contaminated, strained. In chemical processing, the control of the purity of the chemical products is difficult but crucial. Similarly in the physical processing, the control of the interaction with the matrix or the substrate is crucial but not totally understood [7-9].
Perspectives for the next years
Nevertheless, despite all the important research efforts in the field of gas and liquid media, the understanding of the influence of the environment on nanomaterials and especially nanoalloys is still incomplete, because of the complexity of chemical reaction itself due to the number of components and of the role of the chemical byproducts. The influence of several physical and chemical parameters is still under debate [4-9]. For example, the effects of confinement (as in micelles or in porous matrices), solid supports, temperature, the concentration and nature of precursors, the role of organic ligands, the gas adsorption (CO, O2 or H2), on the nanoalloy structure are still poorly understood. Developing a theoretical framework that can predict the effects of ubiquitous environment exposure would strongly support experimental activities and very much strengthen the basis for experiment-theory comparisons which are crucial to the robust development of new theoretical methods and experimental directions. Especially, in the supported nanoalloys which constitute the majority of cases, the effect of roughness, nanostructuration, structure and chemistry of substrate must be taken into account [7,10]. This issue will be explored in the GDRI by state of the art techniques using in situ characterization combined with modelling to understand the critical parameters controlling size, crystallinity, shape and chemical ordering of nanoalloys [11-12]. These problems are strongly correlated with the nucleation and growth process of nanocrystals in solution or on a support, which can now be studied by in- and ex-situ measurements [13] (see section 2.1).
Finally the environment can be also a tool for formulation engineering of nanoalloys. The response of the nanoalloy structure to the post-formation addition of ligands thus becomes of particular interest (for example, the functionalization in cancer therapy or as MRI contrast agents for imaging).
References
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- Strain engineering in binary and ternary nanoalloys
State of the art
The electronic properties of a surface can be modified by applying strain, even without any change of the chemical composition of the two uppermost layers (surface and subsurface layers). This can have important effects on adsorption on molecules and surface chemical reactions [1]. In core-shell nanoalloys, strained shells are very often obtained [2] as a consequence of the lattice mismatch between the components, which can induce several types of stress-driven surface reconstructions [3]. There are several examples of chemical reactions occurring on the surface of core-shell nanoalloys, such as Ru@Pt, Pt-Cu@Pt and Pt-Co@Pt nanoalloys [4-6], in which strain effects are playing a crucial role in enhancing reactivity and/or selectivity.
Perspectives for the next years
A scientific breakthrough in this domain would be achieved by the ability of controlling strain at the atomic level, in order to design nanoalloys with the desired properties as catalysts for specific chemical reactions. This achievement would lead to a true strain engineering of nanoalloys. Some steps in this direction have been made for example in Ref. [7], in which the strain in the Pt shell of Fe-Pt@Pt nanoalloys was tuned first by changing chemical ordering in the Fe-Pt core from a disordered A1 phase to an ordered L10 phase, and then by replacing some Fe atoms in the core by Cu atoms. The use of a third metal can indeed offer many more possibilities for strain engineering, both in crystalline and in non-crystalline geometries such as icosahedra and decahedra. The development of strain engineering requires close collaboration between experimental and theory/modelling groups. Strain maps in nanoparticles can be experimentally obtained by electron microscopy [8] and compared to the calculation results [3], which can serve also in designing nanoalloys for specific needs.
References
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