Dealloying processes

EDS compositional analysis with a focused 2nm electron beam of the FEG-TEM in the central part of mother cluster on the Ar-irradiated AuAg sample, gives an Au/ Ag ratio (measured at AuL and AgL) of 1.4 + 0.1, whereas the same ratio measured on the satellite clusters is 2.3+0.8. Similar ratios have been found from EDS analysis on AuAg sample irradiated with He, Ne, or Kr ions. EDS analysis reveals therefore a preferential extraction of Au atoms from the original cluster and this selective dealloying process is independent of the particular system investigated (we obtained similar results for Ne-irradiated AuCu cluster, as previously reported). 

As the Pd coverage increases, not only the number of Pd-Ni bonds decreases but there is also less opportunity for stress release. The stress release mechanism stabilizes the disordered surface alloy and as its likelihood diminishes, a dealloying process sets in. 

Other Cu-based alloys Dealloying phenomena have also been discussed for Cu alloys from the Cu—Ni, Cu—Mn, and Cu—Sn systems [45, 84, 85]. In the case of long-term corrosion of Sn-bronze (a-Cu—Sn) in natural environments, which is obscured by complex patina formation, it has been shown that the relevant dealloying process is decuprification rather than destannification (as formerly assumed). 

This results in metallic Sn particles that remain finely dispersed in the Li20 matrix. This produces a favourable geometry and facilitates a subsequent reversible reaction as indicated by the lithium alloying-dealloying process shown in Equation 7.11. 

However, since then, several mechanisms had been advanced to explain the dealloying process. They are volume diffusion, vacancy diffusion, surface diffusion, oxide formation, percolation, and ionization and re-deposition [4]. 

Fig. 20.9 KMC simulation of a dealloying process. A PtCo alloy nanoparticle of approximately 8 nm diameter is exposed to an acidic solution (represented by the small spheres surrounding the surface of the nanoparticle), (a), (b), and (c) represent snapshots taken at increasing times. Co segregates to the surface and dissolves. The lowest coordination sites are the first to dissolve causing a dramatic increase of the surface roughness (Callejas-Tovar and Balbuena, unpublished results)...

Fig. 6.2 Schemes of two methods to fabricate Pt (or Pd) skin on BMNCs. (a) Pt monolayer forms on the alloy of Pt and an early transition metal through the dealloying process during annealing, (b) A Cu monolayer forms through UPD, followed by the galvanic replacement reaction between the Cu and PtCL( or PdCU ions...

In the dealloying process, typically one of two mechanisms occurs alloy dissolution and replating of the cathodic element or selective dissolution of an anodic alloy constituent. In either case, the metal is left spongy and porous and loses much of its strength, hardness, and ductility. Table 2 lists some of the alloy-environment combinations for which dealloying has been reported. By far the two most common forms of dealloying are dezincification and graphitic corrosion. 

Wang, J. Sattayasamitsathit, S. Gu, Y Kaufmann, K. Polsky, R. Minteer, S. D. Tunable hierarchical macro/mesoporous gold microwires fabricated by dual-templating and dealloying processes. Nanoscale 2013, 5, 7849-7854. 

Alloying and dealloying processes involved in the preparation of metal nanoshells through a galvanic replacement reaction, Y. Sun and Y. Xia, Nano Lett, 2003, 3, 1569. 

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