Synthesis of Cu xO(x = 1,2)/amorphous compounds by dealloying and spontaneous oxidation method

Synthesis of Cu_xO(x = 1,2)/amorphous compounds by dealloying and spontaneous oxidation method

Zhifeng Wang; Chunling Qin; Li Liu; Lijuan Wang; Jian Ding; Weimin Zhao^*

School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China

ABSTRACT

Cu_xO(x = 1,2)/amorphous compounds have been successfully synthesized by chemical free dealloying and spontaneous oxidation method. Technological parameters, such as the acid concentration and dealloying time strongly influence the crystal type, size and morphology of coppery oxide. The further study shows that with the increase of HCl concentration, the surface coverage rate of Cu₂O micro-flowers increases and the sizes of Cu₂O micro-flowers get bigger. Moreover, it is observed that cracks are formed on the etched ribbon surface and plentiful Cu₂O/CuO particles grow up from these crack walls if the dealloying time extends to long enough. Considering many fascinating properties of Cu₂O/CuO particles and the amorphous alloy carrier, potential application fields of these amazing compounds will be developed in future.

Keywords: dealloying, amorphous alloy, Cu₂O, CuO, compounds

1. Introduction

Dealloying, which refers to the selective dissolution of one or more components out of an alloy, is superior in the fabrication of nanoporous metals with open pores owing to its high reactivity of some alloying elements and controllability of chemical reactions¹. This method has been successfully adopted in the fabrication of nanoporous noble metals in different alloy systems^2-4. Nowadays, some studies reveal that dealloying method can be extended to the fabrication of metal oxide nanostructures with intricate structural properties. Fascinating nanostructures, such as Cu₂O nanocubes^1,5, octahedral Fe₃O₄ and Mn₃O₄ nanoparticles⁶, are successfully produced by dealloying method.

Cu₂O, 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 sensors⁸, in solar energy conversion⁹, as an electrode in lithium ion batteries¹⁰, as a photocatalyst for the degradation of organic pollutants¹¹ and for the decomposition of water into H₂ and O₂ under visible light irradiation¹². CuO is a p-type semiconductor with narrow band gap of 1.2 eV^[13], and is known for its applications in optical switches, field emitters, gas sensors, high temperature microconductors, Li-ion battery anode materials, and chemical conversion catalysts^14,15. Therefore, Cu₂O and CuO particles with different size and morphologies are highly desirable for these applications.

So far, Cu₂O and CuO have been prepared by several different methods. In our previous paper, we develop a new approach to produce Cu₂O/amorphous compounds by free dealloying Cu-based amorphous alloys and spontaneous oxidation method. In this study, we adjust the technical parameters to improve the Cu₂O surface coverage rate and produce Cu_xO(x = 1,2)/amorphous compounds by using this method. To our knowledge, amorphous alloys are good carriers for Cu_xO(x = 1,2) particles, because amorphous alloys exhibit high strength and high toughness. In addition, the Cu_xO(x = 1,2) particles formed in the amorphous alloy precursor are more easier to be stored or extracted as compared to traditional chemical method. Meanwhile, the fabrication process of Cu_xO(x = 1,2) particles are simplified in this route. 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 future.

2. Experimental

Cu-based ingots with nominal compositions of Cu_52.5Hf₄₀Al_7.5 (at%) were prepared by arc-melting Cu (99.99 mass%), Hf (99.99 mass%), and Al (99.99 mass%) metals in high-purity argon gas atmosphere and using Ti getters. Thin precursor ribbons of Cu-based alloys about 20 µm thick and 2 mm wide were prepared by melt-spinning with a linear velocity of the copper wheel of 40 m/s. The free dealloying was performed by immersing amorphous precursor ribbons (about 20 mm long) into HCl solutions with different concentration and immersing time open to air at room temperature. 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 microstructure and surface morphology of the dealloyed specimens were characterized by X-ray diffraction (XRD, Bruker D8, Cu-Kα radiation) and scanning electron microscope (SEM, Hitachi S-4800), respectively.

3. Results and Discussion

Figure 1 shows XRD patterns of the dealloyed Cu-Hf-Al amorphous alloys in 0.5 M HCl solution for 0 h, 8 h, 14 h and 20 h, respectively. The diffraction pattern for the as-spun alloy is broad and has no Bragg peaks, indicating a single homogeneous glassy structure. The XRD patterns of the HCl treated ribbons exhibit broad halo peaks superimposed on sharp crystal peaks. These crystal peaks are match with (111), (200), (220) crystal planes of Cu₂O (JCPDS No. 05-0667) and (002), (111), (202) crystal planes of CuO (JCPDS No.41-0254), respectively. Moreover, the existence of a broad halo peak reveals that although the surface of the sample is rich in Cu₂O and/or CuO, the inner part remains glassy structures. When the dealloying time reaches to 8 h, Cu₂O particles are synthesized on the surface of amorphous alloy. The further increasing dealloying time leads to the formation of more oxidation products of CuO instead of Cu₂O.

Figure 2 shows SEM images of the Cu_52.5Hf₄₀Al_7.5 alloys immersed in HCl solution with different concentration for 8 h. It is observed that Cu₂O particles formed in the amorphous alloy surface exhibit flower morphology when the dealloying time reaches to 8 h. The mean surface coverage rate of Cu₂O micro-flowers increases from 17.2% to 33.1% (as shown in Figure 3) with the increase of HCl concentration. The Cu₂O coverage rate in this paper has been improved compares with our previous study (13.9%, 0.05 M HCl for 8 h). That is because the increased HCl concentration promotes the dealloying process, including reaction speed and the reaction extent. As a result, the sizes of Cu₂O crystals gradually turn to bigger and Cu₂O crystals in regular polyhedral shapes⁵ cannot retain but grow up to micro-flowers. On the other hand, SEM micrographs of the Cu-Hf-Al amorphous alloys dealloyed in 0.5 M HCl solution for different time are shown in Figure 4. When the dealloying time extends from 8 h to 14 h, some cracks are formed on the surface of amorphous alloys and deeper cracks are found in the ribbon dealloying for 20 h. Furthermore, it is also observed that plentiful Cu₂O/CuO particles grow up from these crack walls. The morphology of Cu₂O/CuO particles generating from crack walls is no longer flower-like, but irregular shape. The white nanoparticles denoted by black arrow in Figure 4 (e and f) grow up based on the Cu₂O micro-particles, which is a powerful evidence to represent the oxide process from Cu₂O to CuO. With the increase of the dealloying time, the mean surface coverage rate of Cu_xO (x = 1,2) crystals increases gradually (as shown in Figure 5). In addition, more CuO crystals can be detected from both XRD (Figure 1) and SEM (Figure 4) images.

By dealloying in HCl acidic solutions, the formation of Cu₂O and CuO on the ribbon surface is probably through the following process: First, the thin oxidized surface layer of the Cu-Hf-Al alloy is removed in the acidic electrolyte and fresh alloy layer forms on the alloy surface. Subsequent, the constituent element of the alloy are selectively dissolved into the solutions to form Cu²⁺ cations along with the other alloy elements cations¹. Meanwhile the oxygen dissolved in the electrolyte also adsorbs on the fresh alloy surfaces. During the dissolution and adsorption process, Cu²⁺ cations react with metallic Cu to form Cu⁺ through a disproportionation reaction, these Cu⁺ cations are unstable and will rapidly react with the O–O_(adsorb) to form Cu₂O. As a result, the Cu₂O particles are formed through the disproportionation reduction of metallic Cu and Cu²⁺ accompanied by surface adsorbed oxygen. If the dealloying time is long enough, Cu₂O will further react with adsorbed oxygen in acidic conditions to form the final oxidation product CuO.

A formation schematic for Cu_xO(x = 1,2)/amorphous compounds is illustrated in Figure 6. Once the amorphous ribbon (Figure 6a) is immersed in HCl solution, the dealloying process begins. Because of the metal reactivity Al > Cu > Hf in dilute HCl solution, in principle Al and Cu elements will be selectively dissolved during dealloying process. However, the dissolution rate of Al element is much higher than that of Cu element. As a result, the ribbon retains its main part in dilute HCl solution and parts of Al elements on ribbon surface are selectively dissolved in the solution (Figure 6b). On the other hand, Cu atoms in dealloyed layer undergo self-assembly process on sample surface. It is, therefore, concluded that Cu₂O particles in the etched alloy surface are formed as a result of the disproportionation reduction of metallic Cu and Cu²⁺ accompanied by surface adsorbed oxygen (Figure 6c). If the dealloying time is extended to long enough, etching in local area speeds up. Then cracks are formed on the etched ribbon surface and plentiful Cu₂O/CuO particles grow up from these crack walls (Figure 6d).

4. Conclusions

Cu_xO(x = 1,2)/amorphous compounds were successfully synthesized by chemical dealloying and spontaneous oxidation method in HCl solutions. With the increase of HCl concentration, the volume fraction of Cu₂O micro-flowers improves and the size of Cu₂O micro-flowers gets bigger. The increasing dealloying time leads to the formation of more oxidation products of CuO instead of Cu₂O. In addition, it is noticed that cracks are formed on the etched ribbon surface and plentiful Cu₂O/CuO particles grow up from these crack walls when the dealloying time extends to long enough. Cu₂O/CuO particles possess many useful properties, while amorphous alloys have high strength, high toughness, and are good carriers for these oxide particles. These amazing compounds with multiple properties are hopeful to be applied in broad fields in future.

Acknowledgments

This work is financially supported by the "100 Talents Project" of Hebei Province, China (E2012100009) and Natural Science Foundation of Hebei Province, China (E2012202017, E2010000057).

Received: August 7, 2012

Revised: August 22, 2013

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Publication Dates

• Publication in this collection
  24 Sept 2013
• Date of issue
  Feb 2014

History

• Received
  07 Aug 2012
• Accepted
  22 Aug 2013