Tailored Dealloying Products of Cu-based Metallic Glasses in Hydrochloric Acid Solutions

Free dealloying of Cu-Hf-Al metallic glasses in HCl electrolytes are studied in this paper. The results show that the electrolyte concentration and dealloying time strongly influence the type of dealloying products. A superficial dealloying happens in diluted HCl electrolytes while a complete dealloying occurs in concentrated HCl electrolytes. The results present that Cu 2 O microparticles with regular morphology can be tailored on glassy surfaces in 0.05 M HCl solution by controlling the dealloying time. Furthermore, the designable products of nanoporous Cu, Cu 2 O nanoplates and CuO microwires can be fabricated in 1.2 M HCl electrolyte with the dealloying time. Due to a big difference of examined Cu-Hf-Al alloys in the electrolyte concentration and dealloying time, one or mixed dealloying products (Cu, Cu 2 O and CuO), which depend on the progress of relative chemical reactions and the different dealloying route, will finally be produced.


Introduction
Dealloying, or selective removal of one or more components from a binary or multicomponent alloy, has become a popular technique for rapid and direct generation of various nanoporous metals and alloys recently [1][2][3][4] . Nanoporous metals produced by dealloying have been applied in many fields, such us catalysis 5 , heat exchangers 6 , actuators 7 , energy storage 2,8 , fuel cells 9 , sensors 10 and radiation damage resistant materials 11 . Moreover, dealloying process can be extended to the fabrication of important transition metal oxide nanostructures with intricate structural properties rather than a porous structure as often seen during dealloying 12 . For instance, regular Fe 3 O 4 octahedra, Co 3 O 4 nanoplates and octahedral-like Mn 3 O 4 nanoparticles have been successfully produced by the in situ dealloying and spontaneous oxidation method 13,14 . Nowadays, not only nanoporous metals but also transition metal oxide can be fabricated by dealloying and extended methods.
Usually, the formation of a porous structure during dealloying in electrolytes needs to address some issues including the critical potential, the dealloying threshold, the initial alloy composition and so on 15 . Furthermore, Chen 16 found that a critical concentration of electrolytes required for the formation of nanoporosity. For example, nanoporous Cu (NPC) with different pore characteristics are able to be fabricated by electrochemically dealloying Cu 30 Mn 70 alloy in 0.025 M, 0.1 M and 0.5 M HCl electrolytes with different time. Instead of the formation of NPC, however, nanostructured cuprous oxide (Cu 2 O) is made on the ribbon surfaces when electrochemically dealloying of the same alloy in 0.001 M HCl solution. Therefore, we can see that the electrolytes concentration is one of the key factors to determine which kind of dealloying products can be created during the dealloying.
Up to now, lots of literatures have focused on the fabrication of nanoporous Cu by dealloying different precursors in HCl solutions [17][18][19] . Unfortunately, few studies focused on the other dealloying products obtained from these precursors in HCl solutions 20,21 . In this paper, in order to systematically investigate the correlation between the dealloying products and the dealloying conditions, free dealloying of Cu-Hf-Al metallic glasses in both diluted and concentrated HCl electrolytes for different time are carried out. Various dealloying products, including NPC, Cu 2 O, and CuO with interesting morphologies, are produced and tailored by controlling the dealloying conditions. The indepth understanding the chemical process during dealloying is discussed in this work, which guides for designing different useful dealloying products.

Experimental
Ternary alloys with nominal compositions of Cu 52.5 Hf 40 Al 7.5 (at.%) were prepared by arc melting a mixture of pure Cu (99.99 wt.%), pure Hf (99.99 wt.%) and pure Al (99.99 wt.%) in a high-purity argon gas atmosphere and using Ti getters. The melt spinning method was used to prepare amorphous Cu-Hf-Al ribbons with 20 μm in thickness and 2 mm in width.
Dealloying was performed in hydrochloric acid (HCl) solutions with a concentration of 0.05 M and 1.2 M in a free corrosion condition open to air at 298 K. After different immersion times, the dealloyed ribbons were taken out and well rinsed with deionized water for three times to remove the residual chemical reagents and then dried in a vacuum drying oven before a further testing.
The amorphicity of the as-spun Cu-Hf-Al sample and phase type of the dealloyed specimens were confirmed by X-ray diffractometer (XRD, Bruker D8) with Cu Kα *e-mail: chunlingqin@163.com; clqin@hebut.edu.cn radiation (λ = 0.15418 nm). The morphology of the asdealloyed samples was observed by scanning electron microscope (SEM, Hitachi S-4800). The volume fraction of the dealloying products, which means the surface coverage rate of the corrosion products (such as Cu 2 O and CuO particles), is measured by the software Image Tool.   Table 1. From the Table 1, it can be seen that the threedimensional sizes of Cu 2 O crystals do not change much in the first 8 hours of dealloying though they show different edge length. Then, the sizes of Cu 2 O crystals increase obviously after dealloying for 14 h, and get bigger with the increase of the dealloying time. It should be noted that, in this work, the volume fraction of Cu 2 O particles on the surface is low (less than 20%) and does not change much with the extension of etching time. Since the Cu 2 O crystals with different morphologies possess interesting physical, electrical and optical properties [22][23][24] , it is important to enhance the volume fraction of Cu 2 O particles on the glassy surface. Furthermore, metallic glasses are good carriers for Cu 2 O particles due to their high strength and high toughness. In the future, the as-prepared metallic glass/ Cu 2 O compounds with multiple properties are hopeful to be applied in broad fields.  Figure 2. The diffraction pattern for the as-spun alloy (dealloying for 0 h) exhibits a broad halo peak and has no Bragg peaks (Figure 2a), indicating a single homogeneous glassy structure. However, the broad peak disappears after dealloying for 12 h or longer time, which demonstrates that glassy structure is destroyed for the Cu 52.5 Hf 40 Al 7.5 metallic glasses dealloyed in concentrated HCl solution for a long time. When dealloying for 12 h, Figure 2b reveals that only peaks matching with This work [20] This work [20] This work [20] [20] [20] fcc Cu (111) Figure 3. Considering XRD results, as shown in Figure 2b, it is interestingly found that a nanoporous Cu structure in Figure 3a is produced by the accumulation  of Cu nanospheres with diameter of 70-90 nm during the 12-hour dealloying. In this situation, a complete dealloying has finished, which results from relatively high concentration of HCl electrolytes and long time of etching (12 h). We should note that the as-produced nanoporous structure shown in Figure 3a is not the same as the widely studied bicontinuous nanoporous structure during dealloying 25 . The different dissolution behavior of constituent elements in dealloying electrolytes seems to be an important factor to influence the reorganized way of residual noble metal during the dealloying. There must be complicated reasons for determining the type of NPC, which would arouse an interesting topic for discussion.

Dealloying in 1.2 M HCl solution
When the dealloying time prolongs to 14 h, as shown in Figure 3b Once a Cu-Hf-Al glassy ribbon (Figure 4a) is immersed into HCl electrolytes, the Hf and Al elements are selectively etched, and fresh copper layer is subsequently formed on the surface of the sample (Equation 1 in Figure 4b). As we know, the depth of a dealloyed layer is determined by the dissolution rate of the glassy alloys in HCl solutions besides dealloying time. Usually, the Cu 52.5 Hf 40 Al 7.5 glassy alloy presents very low dissolution rate in diluted HCl solution, whereas it shows enhanced dissolution rate in concentrated HCl solution. In this paper, Figure 1 shows the different particle structure formed on the surface of Cu-Hf-Al glassy substrate when etching in diluted HCl solution. On the other hand, Figure 3a exhibites typical Cu porous structure after dealloying in concentrated HCl solution, which implies that the constituent element Hf in the Cu-Hf-Al alloy shows more active chemical behavior in concentrated HCl solution.
Therefore, the dealloying process in diluted HCl solution might be retarded by the slow dissolution of Hf 4+ cations or increase of pH value. So, the chemical dealloying process of the Cu-Hf-Al metallic glasses are different between in diluted and concentrated HCl solutions, and can be classified into two routes.
A complete dealloying happens when dealloying in concentrated HCl electrolytes in Figure 4c (Route 1). As a result, the porous structures are formed at the initial stage of the chemical reaction. However, a superficial dealloying happens when dealloying in diluted HCl electrolytes in Figure 4d (Route 2). In this situation, the glassy structures are retained. With the increase of the dealloying time, the Route 1 and Route 2 will further proceed. The oxygen dissolved in the electrolyte is strongly adsorbed on the fresh copper surface (Equation 2). At the same time, part of the fresh Cu dissolves into the acid solutions to form Cu 2+ cations, acompanying with H 2 release (Equation 3). It seems that it would be easier to form CuO rather than Cu 2 O by the reaction of Cu 2+ cations and adsorbed oxygen. Actually, the Cu + cations will be preferentially obtained from a disproportionation reaction between fresh Cu and Cu  Figure 1 and Table 1). The key factor for forming Cu 2 O crystals with different shapes can be attributed to the value of R [22] , which represents the growth rate of Cu 2 O particles along the <1 0 0> direction relative to that of the <1 1 1> direction. In this situation, the content (time dependent) of adsorbed oxygen with Cu + cations strongly influence the value of R. As a result, Cu 2 O crystals with designable shapes can be tailored by controlling dealloying time, as shown in Table 1 and Figure 4f.
Moreover, with the further increase of the dealloying time, Cu 2 O is oxidized by adsorbed oxygen on the surface and change to CuO microwires (Equation 6 in Figure 4g). In summary, due to a big difference of examined Cu-Hf-Al alloys in the electrolyte concentration and dealloying time, one or mixed dealloying products (Cu, Cu 2 O and CuO), which depend on above chemical process, will finally be produced.
As we know, both Cu 2 O and CuO are p-type semiconductors. Cu 2 O have potential applications in gas sensors, solar energy conversion, lithium ion batteries, photocatalysts and so on [27][28][29][30] . CuO is known for its applications in optical switches, field emitters, gas sensors, Li-ion battery anode materials, and chemical conversion catalysts 31,32 . Hence, Cu 2 O and CuO particles with different size and morphologies are highly desirable for wide applications. In traditional chemical method, they are synthesized and stored in liquid solutions. By using a dealloying process, however, they can be fabricated on solid ribbon and easily stored or extracted from drying oven. According to different application requirement, amorphous/Cu x O 20,21 composites and porous Cu/Cu x O (x=1, 2) composites can be synthesized and tailored by controlling dealloying conditions and choosing different dealloying routes (Figure 4). More detailed studies about applications in various fields of these dealloying products need to be done in the future.

Conclusions
In this work, controllable corrosion products are designed by free dealloying Cu-Hf-Al metallic glasses using different technique parameters. The concentration of electrolytes and dealloying time strongly influence the type of dealloying products. A superficial dealloying happens when dealloying in diluted HCl electrolytes. Regular Cu 2 O microparticles with designable morphology can be tailored in 0.05 M HCl solutions by controlling dealloying time. A complete dealloying take place when dealloying in concentrated HCl electrolytes. Designable products of NPC, Cu 2 O, and CuO with interesting morphologies are tailored in 1.2 M HCl electrolytes with different dealloying time. A clear chemical process during dealloying is summarized, which guides for designing different useful dealloying products. Due to a big difference of examined Cu-Hf-Al alloys in the electrolyte concentration and dealloying time, one or mixed dealloying products (Cu, Cu 2 O and CuO), which depend on the progress of relative chemical reactions and the different dealloying route, will finally be produced.