Tunable Cu 2 O Nanocrystals Fabricated by Free Dealloying of Amorphous Ribbons

This work discovers that Cu2O nanocrystals with controllable structures can be synthesized on surfaces of nanoporous Cu and amorphous ribbons by free dealloying of Cu-based amorphous alloys in acidic solutions. Technological parameters, such as the acid, acid concentration, and dealloying time strongly influence the crystal size, structure and morphology of Cu2O. Cu2O nanocubes are fabricated on surfaces of nanoporous Cu in the hydrofluoric acid treated alloy, while various Cu2O particles are tailored on surfaces of amorphous alloys immersed in hydrochloric acid for different time. The increasing dealloying time and adsorbed oxygen improve the growth rates along the 〈1 0 0〉 direction of Cu2O crystals relative to that of the 〈1 1 1〉 direction, which is the key to change the shapes of Cu2O crystals. The understanding of morphology evolution of Cu2O nanocrystals in this work is helpful in tailoring Cu2O particles with designable shapes and controllable properties in application fields.


Introduction
Dealloying, which is a phenomenon of corrosion where the less noble metal is selectively removed from an alloy, has recently gained interest for tailoring nanoporous metallic materials [1][2][3][4].Nowadays, Chen et al. [5] found that, instead of formation of nanoporous copper, cuprous oxide (Cu 2 O) nanocubes and nanograins can be formed on the ribbon surfaces when electrochemically dealloying a Cu 30 Mn 70 alloy in hydrochloric acid (HCl) solutions.At the same time, octahedral metal oxide nanoparticles, such as Fe 3 O 4 and Mn 3 O 4 , are produced by dealloying binary alloys in NaOH solution [6].These existing examples clarify that dealloying method can be extended to the fabrication of metal oxide nanostructures with intricate structural properties.
Cu 2 O, which is an important p-type semiconductor with a direct band gap of 2.17 eV [7], has been widely studied as a promising material for applications in gas sensors [8], in solar energy conversion [9], as an electrode in lithium ion batteries [10], as a photocatalyst for the degradation of organic pollutants [11] and for the decomposition of water into H 2 and O 2 under visible light irradiation [12].Therefore, Cu 2 O particles with different sizes and morphologies are highly desirable for these applications.So far, Cu 2 O has been prepared by several different methods [13][14][15][16], such as sonochemical methods [11], solvothermal [17], and hydrothermal methods [18].In this paper, we present a new approach to design tunable-structured Cu 2 O nanocrystals by free dealloying Cu-based metallic glasses in hydrofluoric acid (HF) or HCl aqueous solutions and investigate the morphology evolution of Cu 2 O particles with different conditions.To our knowledge, this is first attempt to select metallic glasses as a precursor and carrier because metallic glasses have high strength, high toughness and free of crystalline defects, which are different from crystalline alloys.In addition, we simplify the fabrication process of Cu 2 O by using this route as compared to traditional chemical method [19,20].The most important contribution by this work is to produce the amazing compounds with multiple properties which are hopeful to be applied in broad fields in the future.Cu 2 O together with its carrier can be stored in solid form, but not in liquid anymore.

Experimental
Ternary Cu-Hf-Al ingots with nominal compositions of Cu 52.5 Hf 40 Al 7.5 were prepared by arc-melting Cu (99.99 mass%), Hf (99.99 mass%), and Al (99.99 mass%) metals in argon gas atmosphere and using Ti getters.Thin ribbons of Cu-Hf-Al alloy about 20 μm thick and 1.2 mm wide were prepared by melt-spinning with a linear velocity of the copper wheel of 40 m/s.Samples of 15 mm in length were cut from ribbons for free dealloying experiments at room temperature.One of the samples was etched in 0.5 M hydrofluoric acid (HF) solution for 4 min.The others were etched in 0.05 M HCl solution for 6 h, 8 h, 14 h, 20 h, and 24 h, respectively.The dealloyed samples were rinsed in deionized water for three times to remove the residual chemical substances and then dried in a vacuum drying oven.The X-ray diffraction (XRD) patterns were identified using a Bruker D8 X-ray diffractometer with Cu-Kα radiation.The microstructures of the dealloyed ribbons were characterized by a scanning electron microscope (SEM, Hitachi S-4800) equipped with energy-dispersive spectroscopy (EDS) analysis.Figure 2 shows SEM images of the Cu 52.5 Hf 40 Al 7.5 alloy at various dealloying conditions.It was reported that Cu 2 O nanocubes with a cube edge of 150-200 nm can be produced on ribbon surfaces when electrochemically dealloying a Cu 30 Mn 70 alloy in 0.001 M HCl solutions [5].In this paper, we fabricate Cu 2 O nanocubes with a finer size of 60-90 nm by free dealloying of a Cu-based amorphous alloy in 0.5 M HF solutions for 4 min (Figure 2 and morphology can be tailored by controlling dealloying conditions.

Results and Discussion
Up to now, a variety of Cu 2 O nanostructures have been synthesized.Xu and Xue [21] reported five branching growth patterns in the cubic crystal system of Cu 2 O.The five patterns are cube, truncated cube, cuboctahedron, truncated octahedron, and octahedron, respectively.In our work, we successfully synthesize four of them, as shown in Figure 3.It is well known that the growth rate of Cu 2 O particles along the 1 0 0 direction relative to that of the 1 1 1 direction, or the value of R, is the key reason to result in the morphology evolution of Cu 2 O cubes [21].For cubic, cuboctahedral, truncated octahedral, and octahedral Cu 2 O particles, their values of R are about 0.58, 0.87, 1.15, and 1.73, respectively [21].
By dealloying of the Cu-based amorphous alloy in acid solutions, active metals in the alloy are selectively dissolved into the solutions.During this etching process, the relatively inert metal atoms left behind undergo spontaneous oxidation at the metal/electrolyte interface to form metal oxides [6].Thus, Cu 2 O particles are formed by reaction between Cu atoms and dissolved oxygen.As the increase of dealloying time, absorbing more dissolved oxygen on the surface of Cu 2 O cubes results in a development in the growth rates along the 1 0 0 direction of Cu 2 O crystals relative to that of the 1 1 1 direction and subsequently leads to the morphology evolution of Cu 2 O cubes.In this study, the dealloying time is short for HF-treated alloy, whereas it is long for HCl treated alloys.Therefore, Cu 2 O cubes show their original morphology in HF-treated alloy but change to various shapes in HCl-treated alloys.
It is worth noting that Hf belongs to a corrosion-resistant element and shows a strong passivating ability in diluted HCl solution but easily dissolves in HF solution [22].When the Cu-based metallic glass is immersed in diluted HCl solution, the matrix of the alloys can still remain metallic glass structure, in addition to the surface of the samples is occupied by Cu and oxidation products Cu 2 O. On the Figure 4 shows the morphology evolution process of Cu 2 O octahedron.It was reported that the six corners of the octahedron represented its six activated corners [21].These activated corners would grow along their 1 0 0 direction of Cu 2 O octahedron.With the increasing dealloying time and adsorbed oxygen, Cu 2 O octahedron grows up to hexapods and then develops into dendrite or octahedron-detached hexapods.In general, dendritic Cu 2 O crystals can be formed  on a long-time-eroded ribbon surfaces.However, if the conditions in six growth directions are the same, a peculiar octahedron-detached hexapods can be synthesized.Characteristics of Cu 2 O crystals produced by free dealloying and spontaneous oxidation are listed in Table 1.Though the area percentage of Cu 2 O crystals compared to the whole area of sample is low, there are also some points of advantage by using this method compared with traditional chemical method [19,20].Firstly, this route is simpler with no need for extraction of craft from solutions, which is often used in traditional method.Secondly, considering uniform compositions and many good properties [23], amorphous alloys are selected for first time as carriers of Cu 2 O particles.Thus, the dilute HCl-dealloyed ribbon can still possess good ductility which cannot be obtained from crystal materials.Thirdly, two kinds of products, Cu 2 O nanocubes embedded on nanoporous Cu and Cu 2 O crystals covered on amorphous alloys, are prepared in this study.We can gain multiple properties by tailoring the area percentage of Cu 2 O particles from these amazing compounds with undiscovered properties in the future.Moreover, these solid products are easy to store or collect, which can promote their applications.Many promising work are worth doing in the future.The increasing dealloying time and adsorbed oxygen improve the growth rates along the 1 0 0 direction of Cu 2 O crystals relative to that of the 1 1 1 direction, which is the key to change the shapes of Cu 2 O crystals.Cu 2 O octahedron is able to grow up to special shapes, such as dendrite and octahedron-detached hexapods.Furthermore, two kinds of compounds produced in this paper are hopefully to be applied in broad fields for their multiple properties.

Figure 1
Figure 1 shows XRD patterns of as-spun Cu 52.5 Hf 40 Al 7.5 ribbon, the ribbon dealloyed in 0.05 M HCl solution for 14 h, and the ribbon dealloyed in 0.5 M HF solution for 4 min.The diffraction pattern for the as-spun alloy is broad and has no Bragg peaks, indicating a single homogeneous glassy structure.The XRD pattern of the HCl-treated ribbon exhibits a broad halo peak superimposed on sharp crystal peaks.These crystal peaks match with (111), (200), (220), and (311) crystal planes of Cu 2 O (JCPDS number 05-0667) and (111), (200), and (220) crystal planes of Cu (JCPDS number 04-0836), respectively.Moreover, the existence of a broad halo peak reveals that although the surface of the sample is rich in Cu 2 O and Cu, the inner part remains metallic glassy structure.Similar XRD patterns are also obtained in other HCl-immersed samples of this study.For the HF-treated ribbon, the amorphous pattern disappears.Only Cu and Cu 2 O peaks are identified, which indicates that HF selectively leaches the Hf and Al elements of the alloy, leaving Cu behind.Figure2shows SEM images of the Cu 52.5 Hf 40 Al 7.5 alloy at various dealloying conditions.It was reported that Cu 2 O nanocubes with a cube edge of 150-200 nm can be produced on ribbon surfaces when electrochemically dealloying a Cu 30 Mn 70 alloy in 0.001 M HCl solutions[5].In this paper, we fabricate Cu 2 O nanocubes with a finer size of 60-90 nm by free dealloying of a Cu-based amorphous alloy in 0.5 M HF solutions for 4 min (Figure2(a)).The inner ribbon shows clear nanoporous Cu structure after selectively leaching the Hf and Al elements of the alloy.Thus, the resultants of the HF treated alloy are Cu 2 O nanocubes embedded on nanoporous Cu.On the other hand, various shapes of Cu 2 O crystals are synthesized by free dealloying of the Cu 52.5 Hf 40 Al 7.5 amorphous alloy in 0.05 M HCl solutions at different time (Figures 2(b)-2(f)).With the prolonged dealloying time, various Cu 2 O crystals are observed on the ribbon surfaces.According to the XRD results, we can see that the final synthetic products of the HCl treated alloy are Cu 2 O particles and Cu metal coated on amorphous alloys.As a result, Cu 2 O nanoparticles with designable size, structure

Figure 1 :
Figure 1 shows XRD patterns of as-spun Cu 52.5 Hf 40 Al 7.5 ribbon, the ribbon dealloyed in 0.05 M HCl solution for 14 h, and the ribbon dealloyed in 0.5 M HF solution for 4 min.The diffraction pattern for the as-spun alloy is broad and has no Bragg peaks, indicating a single homogeneous glassy structure.The XRD pattern of the HCl-treated ribbon exhibits a broad halo peak superimposed on sharp crystal peaks.These crystal peaks match with (111), (200), (220), and (311) crystal planes of Cu 2 O (JCPDS number 05-0667) and (111), (200), and (220) crystal planes of Cu (JCPDS number 04-0836), respectively.Moreover, the existence of a broad halo peak reveals that although the surface of the sample is rich in Cu 2 O and Cu, the inner part remains metallic glassy structure.Similar XRD patterns are also obtained in other HCl-immersed samples of this study.For the HF-treated ribbon, the amorphous pattern disappears.Only Cu and Cu 2 O peaks are identified, which indicates that HF selectively leaches the Hf and Al elements of the alloy, leaving Cu behind.Figure2shows SEM images of the Cu 52.5 Hf 40 Al 7.5 alloy at various dealloying conditions.It was reported that Cu 2 O nanocubes with a cube edge of 150-200 nm can be produced on ribbon surfaces when electrochemically dealloying a Cu 30 Mn 70 alloy in 0.001 M HCl solutions[5].In this paper, we fabricate Cu 2 O nanocubes with a finer size of 60-90 nm by free dealloying of a Cu-based amorphous alloy in 0.5 M HF solutions for 4 min (Figure2(a)).The inner ribbon shows clear nanoporous Cu structure after selectively leaching the Hf and Al elements of the alloy.Thus, the resultants of the HF treated alloy are Cu 2 O nanocubes embedded on nanoporous Cu.On the other hand, various shapes of Cu 2 O crystals are synthesized by free dealloying of the Cu 52.5 Hf 40 Al 7.5 amorphous alloy in 0.05 M HCl solutions at different time (Figures 2(b)-2(f)).With the prolonged dealloying time, various Cu 2 O crystals are observed on the ribbon surfaces.According to the XRD results, we can see that the final synthetic products of the HCl treated alloy are Cu 2 O particles and Cu metal coated on amorphous alloys.As a result, Cu 2 O nanoparticles with designable size, structure

Table 1 :
Characteristics of Cu 2 O crystals produced by free dealloying.Cu 2 O nanocrystals with morphological control are successfully synthesized on surfaces of nanoporous Cu and amorphous ribbons by free dealloying of a Cu 52.5 Hf 40 Al 7.5 amorphous alloy in acidic solutions.Acids, acid concentration and dealloying time have significant effects on the particle size, structure and morphology of Cu 2 O. Differing from traditional powders products, the resultants of the HF treated alloy are Cu 2 O nanocubes embedded on nanoporous Cu materials, while the synthetic products of HCl-treated alloys are Cu 2 O particles and Cu metal covered on amorphous alloys.Additionally, various morphologies of Cu 2 O are presented on the ribbon surfaces in HCl solution by controlling dealloying time.