**4. Conclusions**

elements are more than two orders lower than that of Cu element [47, 48], and the formation of the crystalline phase of G-II elements (Ni, Pd, Pt) was interrupted by the fast diffusion of Cu adatoms. Furthermore, the misfits in the atomic radius between Cu and Pd and Pt elements are ca. 8%, and the incorporation of Pd/Pt into the Cu lattices appears more easily. The changes in the lattice constants of the NPCs stabilized by G-I and G-II indicated by XRD patterns in **Figure 1** and Refs. [27, 28] support the present hypothesis. As has presented above, the formation of the Ag and Au crystalline phases after dealloying is considered to be due to the difference in self-diffusion behaviors and misfits of the atomic radius. The minor addition of Group-I LSDs causes the formation of the crystalline Ag/Au phases, and the refining factors of Group I were small. On the other hand, the micro-alloying by Group-II LSDs mainly resulted in the invasion of the Cu lattices, with some Cu atoms in the lattice substituted by Ni, Pd, and Pt atoms, forming solid solutions, and the refining factors for Group II were high. In some extent, these LSD atoms in the lattice are considered to be the main contributors for the

The diffusion in the interfacial regions between Cu, LSD adatoms, and Ti adatoms played a key role in the formation of ultrafine NPCs. For example, Evangelakis investigated the diffusion of Au adatoms on Cu and self-diffusion of Cu adatoms by using the molecular dynamic method [50]. They have found that diffusion of Cu adatoms takes place exclusively by hopping from one adatom position to an adjacent one and that multiple jumps are frequent at low temperatures. On the other hand, Au adatoms on the NPC ligaments hopped less frequently than in the case of Cu self-diffusion. The migration energy required for hopping of Au on Cu was almost twice that of the corresponding energy for Cu adatoms. While the diffusion of Au on Cu was more difficult than the diffusion of Cu on Cu, this phenomenon is compatible with the binding energy of Au adatoms, which is found to be 2.77 eV for Au adatoms and 2.26 eV for Cu adatoms. The Au adatoms relaxed at a distance −15% smaller than the bulk interlayer distance, but in the case of the Cu adatoms, it was −10% for the same quantity [50]. The Cu adatoms diffused quickly in and out by themselves; however, the Au adatoms at the activated sites diffused out slowly and hardly ever diffused back to the NPC ligaments. Consequently, Au adatoms gradually accumulated outside the ligaments during the dealloying [27, 30]. As a result, Au adatoms uniformly distributed outside the ligaments after dealloying for 43.2 ks via the hopping mechanism, as shown in **Figure 5**. When more Au was added to TiCu amorphous alloys, the surface coverage of Au adatoms increased during dealloying, suggesting that the accumulation rate of Au adatoms in the more concentrated solutions was higher due to the faster migration of Au and Cu adatoms [27]. Because the accumulated Au adatoms built up a continuous outmost diffusion barrier, the behaviors of diffusion and rearrangements of Cu and Au adatoms can be fulfilled to form ultrafine NPCs from both the LSD-substituted Ti60Cu40 amorphous precursor alloys in HF solutions. The diffusion behavior for other LSDstabilized nanoporous structures was considered to be similar. However, the accumulation of G-II Pt-group elements (Ni, Pd, and Pt) with much slower diffusion rates and Ni, Pd, and Pt cluster or grains with much smaller size less than 7 nm tended to form, which are not sensitive to X-ray [51]. As has been reported [52], a bimodal nanoporous structure with a pore size of 10 nm and 20 nm has been fabricated from Al75Pd17.5Au7.5 precursor alloys by succes-

sive dealloying. The initial heterogeneous microstructure consisting of Al<sup>2</sup>

intermetallics causes the formation of a bimodal nanoporous structure. The similar evolution of bimodal or multimodal nanoporosity on precursor alloys heterogeneous in microstructure

Au- and Al3

Pd-type

elaboration of NPCs.

48 New Uses of Micro and Nanomaterials

The 1 at% minor addition of low surface diffusive (LSD) elements in two groups (G-I, Au-group (Ag, Au), and G-II, Pt-group (Ni, Pd, Pt)) is able to elaborate the nanoporous Cu structure dealloyed from micro-alloyed Ti60Cu40 alloy efficiently. The chemical compositions of Ti60Cu40 alloys stabilized by the addition of G-II metals were shown to be more effective in refining nanoporous structure than the addition of G-I metals. Nanoporous Cu with a pore size of less than 7 nm was obtained from Ti60Cu39Pd<sup>1</sup> and Ti60Cu39Pt<sup>1</sup> ribbons after dealloying. The residue of dealloyed Ti60Cu39Pd<sup>1</sup> and Ti60Cu39Pt<sup>1</sup> ribbons was confirmed to be bimetallic solid solutions, such as fcc Cu(Pd) or Cu(Pt) solid solutions, and fcc Cu(Au), Cu(Ag), and Cu(Ni) solid solutions formed after dealloying. The refining factor increases approximately from 3.7 for the Ti60Cu39Ag<sup>1</sup> precursor alloy to 1780 for the Ti60Cu39Pt<sup>1</sup> precursor alloy. The elaboration was attributed to the dramatic decrease in the surface diffusivity during both preferential dissolution and rearrangement of Cu adatoms. The refinement efficiency of the micro-alloying of the G-II LSDs in Pt-group elements was almost one order higher than that of the G-I LSDs in Au-group elements. The homogeneous distribution of LSD elements in both of the amorphous precursor alloys and the final stabilized NPCs played a key role in refining the NPCs. The strategy outlined in this work has the potential to be applied to other alloy systems to obtain other ultrafine nanoporous metals with comparable nanoporosity to those high-cost catalysts.
