**2. Materials and methods**

so forth [1–4]. NPMs with a variety of superior physical-chemical properties arisen from their unique pore structure, large specific surface area, and high electrical conductivity have attracted great interests to explore their electrocatalytic properties and greatly extend their potential applications in catalysts, electrochemical sensing, and energy systems [1]. Dealloying primarily originated from the phenomenon of selective corrosion has attracted more attention recently because it has been regarded to be an effective method to fabricate NPMs with a three-dimensional bi-continuous interpenetrating ligament-channel structure at the nanometer scale [5–8]. Nanoporous copper (NPC) is cost-effective and readily fabricated via dealloying process due to the high electrochemical stability. NPCs with different morphologies have been obtained from numerous binary alloy systems including Zn-Cu [7, 9], Mg-Cu [10], Al-Cu [7, 11–16], Ni-Cu [7], Mn-Cu [17, 18], Zr-Cu [19], and

The characteristic pore sizes of NPCs obtained from the systems mentioned above are relatively larger than those of nanoporous gold (NPG) or nanoporous platinum (NP Pt) and change from few tenth nanometers to few hundredth nanometers, particularly for Zr-Cu system with a pore size of 500 nm [19]. The pore sizes have a significant effect on the mechanical properties of NPMs. The smallest size of nanopores is of order of 3.5 nm for the Cu-Pt system [21]. The yield strength of nanometer-sized NPG ligaments has been improved from ~880 MPa to 4.6 GPa as the NPGs' pore size was refined from 50 nm to 10 nm [22]. As has been reported, the rough NPCs have a relative lower yield strength (i.e., 128 ± 37 MPa with a ligament size of 135 ± 31 nm [17], 86 ± 10 MPa as the ligaments with a size of 300–500 nm of NPCs fabricated by one-step dealloying of the melt-spun Al-50 at.% Cu alloy [11]). It is thus of importance to fabricate NPMs with finer nanostructure with smaller pore size and ligament scales. On the other hand, the elaboration of NPCs is helpful for the enhancements of the catalytic performance and sensitivity for various gaseous phases or metallic ions. Effective ways to reduce the characteristic nanopore sizes have been reported to be dealloying at low temperature [23]; chemical composition design of the precursor alloys, for example, Ag-Au-Pt [24], Al-Pt-Au [25], Ti-Cu-Au [26, 27], Ti-Cu-Ag [28], Ti-Cu-Ni [29], and Ti-Cu-Pd/Pt [30]; and modification of the solution chemistry by using organic acids [31] and by introducing the macromolecules of polyvinylpyrrolidone [32, 33]. The chemical composition design is considered to be an effective way to change the surface diffusion and rearrangements of the adatoms of the noble elements in the precursor alloys since these noble elements take effect from inside to outside. However, the uniform nanoporosity of NPCs is of importance for enhancing the mechanical properties and catalytic performances. Final nanoporous structure is affected by many factors, such as the chemical compositions and initial microstructure of the precursor alloys, the solution chemistries of dealloying solutions, and the experimental conditions (i.e., temperatures, etc.) [12, 34, 35]. The crystalline precursor alloys are extensively dealloyed to prepare NPMs (i.e., coarsen crystalline Al-Cu alloys [11], nanocrystalline Ag-Au, Ag-Au-Pt alloys [22, 24]). While the intermetallic phases or secondary phases exist in the matrix, the final nanoporous structures inherit the characteristics of their initial microstructure of precursor alloys. The characteristics of the casting structures, the intermetallics, and the phase segregations

Ti-Cu [20].

40 New Uses of Micro and Nanomaterials

On the basis of binary Ti60Cu40 alloy, ternary alloys with nominal compositions of Ti60Cu39M<sup>1</sup> (M: Ni, Pd, Pt, Ag, Au) were designed and prepared by arc melting of high purity (purity > 99.99 mass%) of raw metals. The LSD elements were divided into two groups: G-I Au-group (Ag, Au) and G-II Pt-group (Ni, Pd, Pt). The surface diffusion coefficients have been reported to be 1.1 × 10−24 m s−2 for Pd and 3.6 × 10−26 m s−2 for Pt and 2.2 × 10−23 m s−2 for Au [38]. The surface diffusion coefficient of Ag in a vacuum is two orders lower than that of Cu as reported [12, 39, 40]. That of Cu is 1.1 × 10−18 m s−2, more than two orders higher than LSD elements. The surface diffusivity of Ni adatoms in the electrolyte was one tenth of that of Cu adatoms [41]. The ribbon samples were fabricated by melt spinning with a dimension of 20 μm in thickness and 2 mm in width. The starting Ti60Cu39M<sup>1</sup> amorphous precursor alloys were treated under the free immersion condition for 43.2 ks in 0.03 M HF solution at 298 K. The detailed information of the experimental procedures has been supplied in the previous publications [27–30]. An X-ray diffractometer (XRD, Rigaku 4200) was employed to identify the change in the lattice constants, crystalline states of dealloyed alloys and microstructure of precursor alloys and dealloyed alloys. The porous morphologies of the nanoporous NPC and LSD-stabilized samples were observed by a scanning electron microscope (JEOL, JIB-4610F). The chemical composition of the as-dealloyed samples was analyzed by an energy dispersive X-ray spectroscope (JEOL, JIB-4610F). Transmission electron microscopes (JEOL, HC2100, and ARM200) were used to observe the internal porous structure, the nanoporosity of dealloyed alloys, and crystalline characteristics of the LSD-stabilized Cu ligaments in high-resolution TEM modes. The nanoporosity was mainly analyzed on the basis of the average size of the nanopores and ligaments. The characteristic pore size of as-dealloyed alloys was confirmed by the single chord method for over 125 sites on SEM and TEM images.
