5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

At the forefront of nanochemistry, there exists a research endeavor centered around intermetallic nanocrystals, which are unique in terms of long-range atomic ordering, well-defined stoichiometry, and controlled crystal structure. In contrast to alloy nanocrystals with no elemental ordering, it is challenging to synthesize intermetallic nanocrystals with a tight control over their size and shape. Here, recent progress in the synthesis of intermetallic nanocrystals with controllable sizes and well-defined shapes is highlighted. A simple analysis and some insights key to the selection of experimental conditions for generating intermetallic nanocrystals are presented, followed by examples to highlight the viable use of intermetallic nanocrystals as electrocatalysts or catalysts for various reactions, with a focus on the enhanced performance relative to their alloy counterparts that lack elemental ordering. Within the conclusion, perspectives on future developments in the context of synthetic control, structure-property relationships, and applications are discussed.

Intermetallic Nanocrystals: Syntheses and Catalytic Applications

Chemically ordered bimetallic nanoparticles are promising candidates for magnetic-storage applications. However, the use of sub-10 nm nanomagnets requires further study of possible size effects on their physical properties. Here, the effects of size and morphology on the order-disorder phase transition temperature of CoPt nanoparticles (T(C)(NP)) have been investigated experimentally, using transmission electron microscopy, and theoretically, with canonical Monte Carlo simulations. For 2.4-3-nm particles, T(C)(NP) is found to be 325-175 degrees C lower than the bulk material transition temperature, consistent with our Monte Carlo simulations. Furthermore, we establish that T(C)(NP) is also sensitive to the shape of the nanoparticles, because only one dimension of the particle (that is, in-plane size or thickness) smaller than 3 nm is sufficient to induce a considerable depression of T(C)(NP). This work emphasizes the necessity of taking into account the three-dimensional morphology of nano-objects to understand and control their structural properties.

/pdf/size-and-shape-effects-on-the-order-disorder-phase-1q38wtrcn2.pdf

Size and shape effects on the order–disorder phase transition in CoPt nanoparticles

Grain boundary segregation, chemical ordering and stability of nanocrystalline alloys: Atomistic computer simulations in the Ni–W system

Based on large-scale density functional theory calculations we provide a systematic overview of the size dependence of the energetic order and magnetic properties of various morphologies of FePt and CoPt clusters with diameters of up to 2.5 nm. For FePt, ordered multiply twinned icosahedra and decahedra are more favorable than the L1_(0) phase throughout the investigated size range. For CoPt, segregated morphologies predominate with considerably increased energy differences to the L1_(0) structure. The compositional trends are traced back to differences between the morphologies in the partial electronic density of states associated with the 3d element.

Multiply twinned morphologies of FePt and CoPt nanoparticles.

The formation of nanoscale self-assembled compositional patterns in monolayer bulk-immiscible alloy films is studied from first principles within the framework of a previously proposed hybrid atomistic-continuum model [V. Ozoli\ifmmode \mbox{\c{n}}\else \c{n}\fi{}\ifmmode \check{s}\else \v{s}\fi{} et al., Phys. Rev. Lett. 88, 096101 (2002)]. The details surrounding the parametrization of the model from first-principles calculations are described for both hexagonal (0001) and bcc (110) substrates, and we demonstrate how the theoretical model can be employed in Monte Carlo simulations as a predictive framework for modeling the structure and finite-temperature stability of compositional patterns. The methodology is applied in a comparative study of equiatomic FeAg pseudomorphic alloy films on Mo(110) and Ru(0001) substrates. Stripe patterns with periodicities of a few nanometers are predicted to be stable in both systems, which is in good agreement with available experimental data for FeAg/Mo(110). The regularity of the stripe patterns and their stability with respect to disordering are found to be substantially enhanced on the anisotropic Mo(110) substrate relative to the nearly isotropic Ru(0001) surface, despite the slightly stronger ordering energetics in the latter system. A comparison of the results of the present study to the predictions of continuum theories commonly employed to describe pattern formation on crystalline surfaces serves to highlight the limitations of such models in the application to patterns with periodicities with length scales of approximately ten atomic spacings.

First-principles theory of nanoscale pattern formation in ultrathin alloy films: A comparative study of Fe-Ag on Ru(0001) and Mo(110) substrates

Corrigendum to “The nature of A1–L10 ordering transitions in alloy nanoparticles: A Monte Carlo study” [Acta Mater. 54 (2006) 4201–4211]

Strong effect of substrate symmetry and pre-patterning on self-assembly of compositional patterns

The nature of A1–L10 ordering transitions in alloy nanoparticles: A Monte Carlo study

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First-principles theory of nanoscale pattern formation in ultrathin alloy films: A comparative study of Fe-Ag on Ru(0001) and Mo(110) substrates

Corrigendum to “The nature of A1–L10 ordering transitions in alloy nanoparticles: A Monte Carlo study” [Acta Mater. 54 (2006) 4201–4211]

Strong effect of substrate symmetry and pre-patterning on self-assembly of compositional patterns

The nature of A1–L10 ordering transitions in alloy nanoparticles: A Monte Carlo study