Materials Science

The Shape of Things to Come

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Science  05 Aug 2011:
Vol. 333, Issue 6043, pp. 677
DOI: 10.1126/science.333.6043.677-a
CREDIT: RINGE ET AL., NANO LETT. 11, 10.1021/NL2018146 (2011)

The Wulff construction uses thermodynamic arguments to predict the equilibrium shape of a crystal based on knowledge of the surface energies of the crystal faces. For single-component systems, the thermodynamic shape proves to be independent of the crystal size unless the crystal is highly strained. For an alloy system, where there is an additional degree of freedom due to the possibility of one of the species segregating to the surface, the size invariance of the thermodynamic shape is lost, and the infinite reservoir approximation used in the Wulff construction no longer holds true. Ringe et al. developed a modified Wulff construction for alloy systems to account for concentration gradients. The behavior of a particular system was shown to depend on the alloy strength, which is determined by the difference between the actual bulk free energy and a linear interpolation of the idealized values. For weak alloys, there is a strong segregation of one of the components, and the thermodynamic shape of the crystal is similar to that determined from a basic Wulff construction. For strong alloys, which include many bimetallic catalysts such as CuAu and AuPd, there is a competition between lowering the surface energy and the bulk energy, with segregation to the surface a significant factor for the smallest particles. The limited experimental data in the literature confirm the general trends seen by the authors.

Nano Lett. 11, 10.1021/nl2018146 (2011).

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