Comment on “Electrum, the Gold−Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules” Mingjin Cui, Haiming Lu,* Haiping Jiang, and Xiangkang Meng* National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, College of Engineering and Applied Sciences, Institute of Materials Engineering, Nanjing University, Nanjing, Jiangsu 210093, P. R. China n a recent paper, Guisbiers et al.1 published a nanothermodynamic study on the phase diagram of bimetallic Au−Ag nanoalloy through considering the effects of lattice structure, shape, and surface properties of the particle on its material properties. They also proposed two segregation rules according to the relationship among immiscibility, melting temperature, and solid surface energy for bimetallic alloys.1 However, their results and discussions are quite questionable. Their proposed first segregation rule says that the element with the highest melting temperature goes to the surface. The second segregation rule says that the element with the lowest surface energy will segregate to the surface, when the melting temperatures of both elements have more or less the same magnitude. In the case of total or partial miscibility, both the first and second rules apply, and in the case of total immiscibility, only the second segregation rule applies.1 As shown in Table 2 of ref 1, Pt is miscible with Ni, and thus both the first and second rules apply to Pt−Ni alloy. However, both the melting temperature (2041 K) and surface energy (2.30 J/ m2) of Pt are larger than those of Ni (1728 K and 2.01 J/m2).2,3 Namely, the segregation of Pt obeys the first rule while is in violation of the second rule. According to their thermodynamic calculations based on the Williams−Nason’s model and their energy dispersive X-ray (EDX) spectroscopy line scan result,4 the authors of ref 1 also pointed out that Ni segregated to the surface of Cu−Ni alloy. It is noteworthy that the surface composition determined by the Williams−Nason’s model (eqs 3 and 4 of ref 4) should correspond to that of the element with the highest sublimation enthalpy, i.e., Ni in Cu−Ni alloy.5 In Figure 23 of the paper by Williams and Nason,5 we found that the surface composition of Ni is lower than its bulk atom fraction. Figure 1 shows the bulk phase diagram of Cu−Ni alloy without and with surface segregation determined by the Williams−Nason’s model. It is obvious that the segregation element is Cu rather than Ni in Cu−Ni alloy. The same conclusion can be found in Hamilton’s theoretical prediction and previous experimental data.6 Using the general concept of segregation calculated via ab initio density functional theory, Wang and Johnson obtained a positive segregation energy for Ni-core and Cu-shell configuration, indicating that Cu-shell was preferred.7 Tomanek et al. also calculated a positive segregation energy of 35.5 kJ/mol based on a bond model for the interatomic interactions, corresponding to segregation of Cu.8 It is known that surface segregation can be modified by some parameters, such as the size of nanoparticles, adsorbates, heat treatment, and support/
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© 2016 American Chemical Society
Figure 1. Bulk phase diagram of spherical Cu−Ni alloy without and with surface segregation.
substrate, etc.9,10 For example, Cu segregation to the surface of Ni/Cu(100) was observed during the hydrogenation of CO2. However, by adding CO to the reactant stream, such segregation was hindered due to the higher binding energy of CO to Ni as compared to Cu.11 Additionally, Ni surface segregation in NixCu1−x alloys in the presence of CO or O2 was predicted theoretically.8 Thus, the observed Ni segregation in their EDX line scan is understandable. Since Hamilton concluded that the surface energy difference was the dominant driving force while the strain energy and heat of solution played a minor role in determining segregation behavior for most binary alloys,6 we proposed two segregation rules based on the solid surface energy γs and atomic diameter d to predict the nature of the segregated element. The first rule says that the element with the lowest solid surface energy will segregate to the surface. When the difference of γs between two elements is < > < ∼ < < < <
segregated element Ag1 Au1 Pd1 Cu5−8 Pt1 Ag1 Au1 Au1 Ag1 Ag1
and γs(Co),12 the segregated element in Au−Ag, Ag−Cu, Ag− Ni, and Ag−Co alloys is Ag according to our first rule, in agreement with the corresponding experimental results.1 Similar conditions happen to Au−Cu, Au−Pt, Au−Ni, Pt− Pd, and Cu−Ni alloys. While for the Pt−Ni alloy with d(Pt) = 0.278 nm larger than d(Ni) = 0.249 nm,3 Pt segregates to the surface according to our second rule, since the difference between γs(Ni) and γs(Pt) is smaller than 5%.12 It is worth noting that the γs values taken for comparison are cited from the experimental data12 rather than the full-charge density calculations with the generalized gradient approximation (GGA),2 where the latter is employed in ref 1. Although GGA is considered to be the superior method for energetic calculations,2 there are often exceptions where the most closepacked surface does not have the lowest γs values, and it has been shown that GGA needs to be corrected due to the neglect of surface electron self-interactions.13−15
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
ACKNOWLEDGMENTS This project was supported by grants from the Fundamental Research Funds for the Central Universities, PAPD (50831004), FANEDD (201146), the National Natural Science Foundation of China (11174120 and 51371096), the FOK YING TUNG Education Foundation (141044), the Natural Science Foundation of Jiangsu Province (BK20131274 and BK20141234), and the State Key Program for Basic Research of China (2015CB659300). REFERENCES (1) Guisbiers, G.; Mendoza-Cruz, R.; Bazan-Diaz, L.; VelaazquezSalazar, J. J.; Mendoza-Perez, R.; Robledo-Torres, J. A.; RodriguezLopez, J. L.; Montejano-Carrizales, J. M.; Whetten, R. L.; JoseYacaman, M. Electrum, the Gold−Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules. ACS Nano 2016, 10, 188−198 and references therein. (2) Vitos, L.; Ruban, A. V.; Skriver, H. L.; Kollar, J. The Surface Energy of Metals. Surf. Sci. 1998, 411, 186−202. (3) http://www.webelements.com/ (access date June 4, 2016). (4) Guisbiers, G.; Khanal, S.; Ruiz-Zepeda, F.; de la Puente, J. R.; Jose-Yacaman, M. Cu−Ni Nano-Alloy: Mixed, Core−Shell or Janus Nano-Particle? Nanoscale 2014, 6, 14630−14635. (5) Williams, F. L.; Nason, D. Binary Alloy Surface Composition from Bulk Alloy Thermodynamic Data. Surf. Sci. 1974, 45, 377−408. 10619
DOI: 10.1021/acsnano.6b03701 ACS Nano 2016, 10, 10618−10619
