Transforming bulk metals into metallic nanostructures: a liquid metal

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Transforming bulk metals into metallic nanostructures: a liquid metal-assisted top-down dealloying strategy with sustainability Zhenbin Wang, Hui Gao, Jiazheng Niu, Chi Zhang, and Zhonghua Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05287 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Sustainable Chemistry & Engineering

Transforming bulk metals into metallic nanostructures: a liquid metal-assisted top-down dealloying strategy with sustainability Zhenbin Wang,1 Hui Gao,1 Jiazheng Niu,1 Chi Zhang,2 Zhonghua Zhang1,2,* 1Key

Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of

Education), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan, 250061, P.R. China 2School

of Applied Physics and Materials, Wuyi University, 22 Dongcheng Village, Jiangmen

529020, P.R. China *Corresponding author. Email: [email protected].

1

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ABSTRACT: Herein, we propose a liquid metal-assisted dealloying strategy to transform bulk metals into metallic nanostructures. Through dealloying of gallium (Ga)-based alloys, nanoporous metals with different dimensions (0D Au, 1D Ag and 2D Cu) were successfully fabricated. Moreover, the sacrificial element (Ga) was efficiently recycled in the form of liquid Ga (L-Ga) and GaOOH with the recovery rate of up to 94.5%. The proposed strategy addresses the sustainability issues of conventional dealloying, and paves a new way to fabricate nanostructured materials from bulk metals. KEYWORDS: liquid metal, nanostructured metals, dealloying, sustainability

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INTRODUCTION Liquid metals are the starting point of solidification, which has an ancient history of several thousand years. Over a long period of time, liquid metals have received wide concerns mainly from the viewpoint of castings (for engineering applications or as crafts). A few liquid metals with low melting points can be directly used (for example, mercury is normally utilized in thermometers). Most recently, liquid metals also show potentials

in

motors,1

electronics,2-7

sensors,8

biomaterials,9-11

batteries,12

catalysts,13-14 etc. For example, a molten 27% Ni–73% Bi alloy shows excellent catalytic activity towards pyrolysis of methane to pure hydrogen without CO2 or other by-products.13 Liu at al.15 successfully realized the electrochemical manipulation of liquid metals on a graphite surface. Additionally, Zavabeti at al.16 proposed a novel strategy to fabricate atomically thin metal oxides in a liquid metal environment at room temperature. Therefore, it would be great to build a bridge between liquid metals and advanced nanomaterials. Metallic nanostructures with different sizes, morphologies and dimensions have unique mechanical, physical and chemical properties,17-21 and show great potentials in applications like catalysis, sensing, actuation, energy storage, and so forth.22-25 A variety of methods have been reported to synthesize nanostructured metals, such as controllable reduction of metal salts,17 self-terminating rapid electrodeposition process,26 CO-confined growth method,23 carbothermal shock synthesis,27 and selective corrosion (dealloying).21,24-25 Lei at al.28 recently reported direct transformation of bulk alloys into metallic oxide nanowires, based upon which separators were fabricated and showed enhanced safety and rate capabilities for lithium ion batteries. However, it is still a great challenge to transform bulk metals into metallic nanostructures with diverse dimensions. Herein we developed a liquid metal-assisted top-down strategy to synthesize metallic nanostructures from bulk metals. Through solidification control of liquid gallium (L-Ga) alloys and subsequent dealloying, bulk metals (Au, Ag, Cu) were directly transformed into nanostructured metals with porous structures and different dimensions. Although dealloying is widely used to produce nanoporous (np) materials,29 sustainability has never been considered. Dealloying is a selective corrosion process during which the less noble element is etched into the corrosive solution,30 but the recycling of the less noble element is an enormous challenge. In 3



this work, we for the first time proposed the concept of sustainability in the field of dealloying, and achieved successful recycling of the less noble Ga element.

EXPERIMENTAL SECTION Sample Fabrication. The weight ratio 199:1 of the L-Ga and bulk metals (Au, Ag and Cu) were mixed at 150~200 oC for 10h to form GaX alloy melts. Then the GaX alloy melts were cooled down to 80 oC and kept at this temperature for use. A paper was put onto a heater with the temperature of 80 oC during a painting process. Subsequently, using a paintbrush, the GaX alloy melts were painted on the surface of the paper. The paper with liquid GaX films were further cooled and solidified (to form solid GaX film) at -15 oC in a refrigerator. The solid GaX films can be easily peeled off from the substrate. Then the free standing solid GaX films were dealloyed in a 1M HNO3 + 0.1 M HF aqueous solution at ambient temperature. The L-Ga was separated from the solution when the Ga phase in the solid GaX films was completely melted. The dealloying process was continuously conducted until no obvious bubbles emerged. Subsequently, the white flocculated precipitates were formed in the deallyed solution by adding a 5 M NaOH solution. The as-dealloyed samples and precipitates were washed with ultra-pure water and alcohol before being dried in vacuum. For the scenario of Ga-Ag precursors, the influence of different Ag contents (1 %, 5 % and 10 %, wt.%) on the dealloying process and the formation of nanoporous structure was investigated. In addition, to reveal the morphology of intermetallic phases in the GaX films, the Ga phase in the GaX precursors was etched away in the 1 M HNO3 + 0.1 M HF solution and the released intermetallic phases were obtained. Microstructural characterization. X-ray diffraction (XRD) measurements of all samples were performed on an XD-3 diffractometer (Beijing Purkinje General Instrument Co., Ltd, China) with Cu Kα radiation. The microstructures and chemical compositions of the as-dealloyed samples were characterized using a scanning electron microscopy (SEM, ZEISS SIGMA300), equipped with an energy dispersive X-ray (EDX) analyzer. The microstructure of the np-Au was characterized using a transmission electron microscopy (TEM, Philips CM 20) with selected-area electron diffraction (SAED). N2 adsorption-desorption isotherms of the as-dealloyed samples were measured at 77 K by using a surface area and porosity analyzer (Gold APP V-Sorb 2800P). The specific area was obtained based on the Brunauer-Emmett-Teller 4


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(BET) method. The pore size distribution was calculated from the adsorption branch of the isotherm using the corrected form of the Kelvin equation by the Barrett-Joyner-Halenda (BJH) model.

RESULTS AND DISCUSSION Ga has a melting point of only 29.8 oC. In a typical procedure (Scheme 1), bulk Au (Ag or Cu) was firstly dissolved into L-Ga to form GaX (X = Au, Ag or Cu) alloy melt at 150~200 oC. The liquid GaX alloy was cooled down to 80 oC and homogeneously painted onto a substrate (here paper, Figure S1a). The substrate-supported liquid GaX film was further cooled and solidified (to form solid GaX film, Figure S1b). The solid GaX film could be easily peeled off and free standing GaX sample was obtained (Figure S1c). Then the GaX film was dealloyed in the HNO3 + HF solution at room temperature (around 25 oC). During dealloying, X formed a nanoporous structure with diverse dimensions, and simultaneously Ga precipitated in a form of liquid metal. Through such a procedure, bulk X metals were transformed into metallic nanostructures and L-Ga was successfully recycled. To well demonstrate the sustainability of the proposed strategy, both the video and photographs of the dealloying process were recorded (Movie S1 and Figure 1). Figure 1 shows the scenario of Ga-Ag, and a series of photographs clearly present the dealloying phenomenon of Ga-Ag film at different times. The solid Ga-0.5wt.% Ag film is composed of Ga (PDF # 05-0601) and Ag3Ga (PDF # 47-0991) phases (Figure S2). When being immersed into the HNO3 + HF solution, the Ga-Ag film immediately reacted with the solution and a large amount of bubbles released (Figure 1a). With the progress of dealloying, the amount of Ga-Ag film became less and less (Figure 1b and 1c). After about 300 s, the Ga-Ag film disappeared and grey powders were observed (Figure 1d). Surprisingly, L-Ga balls could also be seen at the bottom of the beaker (enlarged view of Figure 1d, Figure S3). Melting did occur at the meanwhile of dealloying, and this phenomenon was observed for the first time. Moreover, these L-Ga balls aggregated into a large liquid drop with stirring (Figure 1e and 1f). The as-dealloyed product (grey powders, nanoporous Ag) was also obtained. Through such a dealloying strategy, the less noble element (Ga) could be recycled in the form of liquid metal. During dealloying, partial Ga dissolved into the HNO3 + HF solution in the form 5



of Ga3+ ions. We also considered the recovery of this Ga through the addition of alkaline solution. After dealloying, the solution was clear (Figure 2a), but immediately became opaque after the addition of 5 M NaOH (Figure 2b). A large amount of white flocculated precipitates formed in the solution. The precipitates could be easily separated and white powders were obtained after drying (inset of Figure 2c). The XRD result confirms that the obtained powders are GaOOH (PDF # 54-0910), Figure 2c. That is to say, the Ga3+ ions in the dealloying solution could be recovered in the form of GaOOH powders. In addition, the recovery rate (the recovered amount divided by the input amount, mass percentage) of Ga was evaluated to better show the sustainability of the present strategy (Figure 2d). The recovery rate of Ga in the form of L-Ga and GaOOH is 83.8 ± 1.7 % and 10.3 ± 3.0 %, respectively. Notably, the total recovery rate is as high as 94.5 %, indicating that most of the less noble element (Ga) could be recycled in the present dealloying strategy. We now analyze the microstructure of the as-dealloyed powders. With respect to the scenario of Au, the XRD pattern of the liquid Ga-0.5wt.% Au alloy supported on the paper shows a broad hump and a sharp peak at 25.7°(Figure S4a). The hump can be ascribed to the presence of liquid Ga(Au) and the crystalline peak corresponds to AuGa2 (PDF # 03-0969) which formed during the XRD detection at room temperature. After solidification, the solid Ga-Au film is composed of Ga and AuGa2 phases (Figure S4b). The morphology of the AuGa2 particles is micro-sized polyhedral (Figure S5). The atomic ratio of Au:Ga in the as-prepared particles is around 1:2 (Figure S5c), which further confirms the presence of AuGa2 phase. Figure 3 shows SEM images of the as-dealloyed powders. The as-obtained Au powders are micron-sized polyhedral particles, and some particles are regular octahedra (Figure 3a). Clearly, the morphology of the Au powders was inherited from that of the AuGa2 particles in the Ga-0.5wt.% Au precursor during dealloying. Noticeably, the high-magnification SEM image shows that the Au particles are nanoporous, having a typical three-dimensional bicontinuous ligament-channel structure with the average ligament size of 29.9 ± 5.6 nm (Figure 3b). These np-Au particles could be regarded as zero-dimensional (0D) materials, as schematically presented in Figure 3c. The specific surface area of np-Au was determined to be 1.55 m2 g-1 by the BET method (Figure S6). The pore size distribution (Figure S6b) mainly centers in the range of 30-60 nm, which is in good accordance with the SEM results. The length scale of the 6


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np-Au particles is less than 10 μm (Figure 4a), and the size distribution is shown in Figure 4b. The average particle size of np-Au is 5.2 ± 2.9 μm. The further compositional analysis shows that the Au content of np-Au is as high as 98.6 at.% and the residual Ga is only 1.4 at.%. One typical EDX spectrum is displayed in Figure 4c. Other nanostructured metals can also be fabricated from bulk counterparts by the same liquid metal-directed top-down strategy. The SEM images reveal the unique micro-sized wires of the Ag3Ga phase (Figure S7a,b). Similarly, individual freestanding Ag2Ga alloy nanoneedles were successfully fabricated at room temperature.31 In addition, the further compositional analysis shows that the Ag:Ga atomic ratio is around 2:1 (Figure S7c). After dealloying of the Ga-Ag foils, the as-dealloyed product is mainly composed of a face centered cubic (fcc) Ag phase (Figure S2c). The SEM images reveal the as-dealloyed powders with the same micron-sized wire morphology (Figure 3d and 4d). The high-magnification SEM image (Figure 3e) demonstrates that these Ag microwires are highly porous. The BET specific surface area of np-Ag was calculated to be 0.92 m2 g-1 (Figure S8). The pore distribution plot (Figure S8b) shows a size distribution of around 200-400 nm, which is consistent with the SEM result. The gradually enlarged SEM images (Figure S9) clearly show the irregular ligament-channel structure of np-Ag and the elongated ligaments along the np-Ag wires are 150-500 nm in size. Thus, the one-dimensional (1D) np-Ag microwires were obtained through such a dealloying strategy, and one schematic illustration is presented in Figure 3f. The average diameter of the np-Ag microwires is 3.4 ± 2.4 µm, as indicated by the size distribution in Figure 4e. Additionally, the residual Ga content is as low as 0.3 at.% in these np-Ag microwires (Figure 4f). Additionally, the Ga-Ag precursors with different Ag contents of 1%, 5% and 10% were dealloyed, and the microstructures of the as-dealloyed samples are shown in Figure S10. Both the SEM and EDX results verify the formation of np-Ag microwires in the as-dealloyed samples. The solid Ga-Cu film is composed of Ga and CuGa2 (PDF # 25-0275) phases, Figure S11. The morphology of the CuGa2 phase is micro-sized flake-like，which is confirmed by SEM in Figure S12. One typical EDX spectrum (Figure S12c) shows that the Cu:Ga atomic ratio is around 1:2, which indicates the presence of CuGa2 in the solid Ga-Cu film. Tang at al. fabricated CuGa2 through effectively packing the liquid metal with Cu particles.32 The microstructure of the as-dealloyed sample was 7



investigated by SEM. Interestingly, the as-obtained sample shows a rectangular flake-like morphology (Figure 3g and S13a). The high-magnification SEM image further reveals a typical bicontinuous ligament-channel structure with the ligament size of 20-55 nm in the flakes, Figure 3h. The specific surface area of np-Cu was determined to be 7.84 m2 g-1 by the BET method (Fig. S14). Figure S14b presents the pore size distribution of np-Cu. The two-dimensional (2D) np-Cu flakes are 1-2 μm in thickness, and a schematic illustration is presented in Figure 3i. Additionally, the composition of the np-Cu flakes was determined by EDX to be 92.4 at.% Cu and 7.6 at.% Ga (Figure S13b). The microstructure of the 0D np-Au particles was further characterized by TEM. The TEM images (Figure 5a and S15) well verify the bicontinuous ligament-channel structure with the average ligament size of 27.7 ± 0.7 nm, which is in good agreement with the SEM results. The high-resolution TEM (HRTEM) image also shows the nanoporous feature of np-Au (Figure 5b). Moreover, lattice fringes could be discerned and the interplanar spacing of 0.236 nm well matches with the (111) plane of fcc Au. In addition, the SAED pattern is composed of polycrystalline rings (Figure 5c), corresponding to (111), (200), (220), (311) and (400) reflections of fcc Au. Finally, we discuss the dealloying mechanism as well as the formation of liquid Ga during dealloying of GaX alloys in the HNO3 + HF solution, taking Au as an example (Figure 6). The Ga-Au precursor is composed of Ga and AuGa2 phases. The mixture of Ga and Au in AuGa2 on the atomic scale is a prerequisite for dealloying to form a nanoporous structure (Figure 6a). It is known that the formation of a nanoporous structure involves the selective dissolution of the less noble element and the re-organization of the more noble element along the alloy/electrolyte interface by surface diffusion during dealloying.30,33 At the beginning of dealloying, the Ga atoms in AuGa2 and Ga were selectively dissolved into the electrolyte (to form Ga3+ ions) due to different chemical activity between Au and Ga. Simultaneously, the temperature of the solution rose rapidly because of heat release (the dissolution of Ga was an exothermic reaction), as indicated by the red color in the thermometer in Figure 6b. The preliminary measurement by the thermometer showed that the temperature rise could reach up to 4 oC in the bulk solution. The temperature of the alloy/solution interface was higher than that of the bulk solution, and even exceeded the melting point of metallic Ga. Although the initial temperature of the dealloying solution was lower than the melting point of Ga, the significant temperature rise could 8


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result in the melting of solid Ga phase in the Ga-Au precursor (Figure 6b and 6c). Eventually, the aggregated melting Ga atoms formed liquid drop. The released Au atoms diffused and re-organized to form the nanoporous structure, accompanying the selective dissolution of Ga from AuGa2 (Figure 6c). Similar mechanism also applies to the dealloying of Ga-Ag and Ga-Cu films. In addition, the shapes of 0D Au, 1D Ag and 2D Cu inherited from those of intermetallic phases (AuGa2, Ag3Ga and CuGa2) in the GaX precursors (Figures S5, S7 and S12) due to the dealloying inheritance effect (DIE).34 On the one hand, AuGa2, Ag3Ga and CuGa2 formed micro-sized polyhedra (0D), wires (1D) and flakes (2D) embedded in the solid Ga phase during solidification of GaX alloys, respectively. On the other hand, the shapes of these intermetallic phases could be retained during dealloying. Noticeably, the addition of HF in the corrosive solution plays a significant role in the dealloying process as well as the recycling of L-Ga. Even in a 2 M HNO3 solution (without HF), the reaction of the Ga-Ag foils was quite slow (Figure S16). Moreover, L-Ga did not appear in the whole dealloying process. After dealloying of 20 h, only the np-Ag powders were obtained. Therefore, HF could effectively facilitate the selective dissolution of Ga from the GaX precursors during the dealloying in the HNO3 + HF solution, which accounted for the formation of L-Ga. Eventually, the addition of HF was frequently used in corrosion of metals and alloys.35-37

CONCLUSIONS In summary, we report a facile liquid metal-assisted top-down strategy to transform bulk metals into nanostructured materials. Through composition design of precursors and chemical dealloying, we successfully synthesized nanostructured metals with different dimensions (0D Au, 1D Ag and 2D Cu) and nanoporous structures. Furthermore, the active element Ga was recycled in the form of L-Ga and GaOOH powders, with the recovery rate of up to 94.5%. This is the first time to realize the high-efficiency recovery of the sacrificial metal in the field of dealloying. Except for cellulose paper, liquid GaX alloys can also be painted onto other substrates like Cu foil, plastic plate, and so forth (Figure S17), indicating great potentials for fabrication of self-supporting and flexible nanostructured materials. The proposed strategy addresses the sustainability issues of conventional dealloying, and paves a new way to fabricate nanostructured materials from bulk metals. 9



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ASSOCIARED CONTENT Supporting information The Supporting information is available in the online version of the paper. Pictures of Ga-Ag films and the dealloying process of Ga-Ag films; XRD Patterns, SEM images and EDX spectra; N2 adsorption-desorption isotherms; and TEM observations of np-Au.

AUTHOR INFORMATION Corresponding Author *Email: [email protected].

ORCID Zhong-Hua Zhang: 0000-0002-2883-4459 Notes There are no conflicts to declare.

ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by National Natural Science Foundation of China (51871133 and 51671115), and the support of Department of Science and Technology of Shandong Province for Young Tip-top Talent Support Project.

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Scheme 1. Schematic illustration showing the fabrication process of nanostructured metals (0D, 1D, 2D) from bulk counterparts (Au, Ag and Cu) through the liquid Ga-assisted alloying-dealloying strategy. During dealloying of GaX films, the melting of Ga occurred simultaneously.

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Figure 1. The photographs showing the dealloying process of the Ga-Ag alloy foils in the HNO3 + HF solution at different durations. The dashed ellipse in (e) is highlighted as an enlarged view in (f).

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Figure 2. The photographs showing the HNO3 + HF solution after (a) dealloying and (b) the addition of 5 M NaOH. The addition of NaOH facilitated the precipitation of Ga3+ in the form of GaOOH. (c) The XRD pattern of the recycled GaOOH sample and the corresponding photograph as an inset. (d) The histogram showing the recovery rate of Ga in the form of L-Ga and GaOOH.

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Figure 3. SEM images showing the microstructures of the as-prepared nanostructured metals including (a,b) np-Au, (d,e) np-Ag and (g,h) np-Cu. (c,f,i) Corresponding schematic illustrations of 0D Au, 1D Ag and 2D Cu.

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Figure 4. Low-magnification SEM images showing the (a) 0D np-Au particles and (d) 1D np-Ag microwires. The histograms showing the size distribution of (b) np-Au and (e) np-Ag. The typical EDX spectrums of (c) np-Au and (f) np-Ag.

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Figure 5. (a) TEM image, (b) HRTEM image and (c) SAED pattern of np-Au.

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Figure 6. Schematic illustrations of the dealloying mechanism (the scenario of Ga-Au alloy is taken as an example). The red color in the thermometer in (b) indicated the temperature rise of the dealloying solution.

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Table of contents (TOC) entry:

200 nm Through a liquid metal-assisted dealloying strategy, bulk metals were directly transformed into nanostructures with different dimensions and ~ 94.5% of the sacrificial element was recycled.

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Transforming bulk metals into metallic nanostructures: a liquid metal

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