Preparation of a nanoporous Cu–Ag solid solution with enhanced sono-Fenton-like catalytic activity

Abstract

Uniform 3D bi-continuous nanoporous Cu–Ag solid solution (NPCS) and nanoporous copper (NPC) were successfully synthesized by dealloying Cu₇₀Y₂₈Ag₂ and Cu₇₂Y₂₈ metallic glasses, respectively, which was confirmed by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM and TEM images show that the ligament size of NPCS (d_SEM = 65 nm, d_TEM = 45 nm) is much smaller than that of NPC (d_SEM = 402 nm, d_TEM = 370 nm), which reveals that the ligaments of NPC can be significantly refined by the substitution of 2 at% Ag for Cu in the amorphous precursor. The obtained NPCS exhibits much larger specific surface area and higher total pore volume (S_BET = 8.34 m² g⁻¹, V_p = 0.093 cm³ g⁻¹) compared to NPC (S_BET = 1.77 m² g⁻¹, V_p = 0.050 cm³ g⁻¹). Furthermore, the catalytic activities of the samples were evaluated by decomposing methyl orange (MO) dye under the irradiation of ultrasound. The results show that NPCS with an extreme fine microstructure displayed superior sono-Fenton-like catalytic activity compared to NPC and commercial copper foil.

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1. Introduction

Advanced oxidation process (AOP)^1–3 is one of the most popular approaches used for textile industry wastewater treatment. Among all AOPs, the photocatalytic method is usually adopted due to its convenience.^4,5 However, it is difficult to degrade opaque or translucent dye wastewater by this method because the penetrating ability of the light is limited.⁶ In consideration of the strong penetrating ability of ultrasound for any medium,⁷ a highly efficient sono-Fenton-like process⁸ induced by ultrasonic irradiation was carried out. Ma et al.⁹ used commercial zero-valent copper to build the sono-Fenton-like system; a significantly enhanced NOR removal was obtained and almost complete removal of NOR was achieved at 30 min. Vaishnave et al.¹⁰ reported that the azure-B dye can be efficiently and completely degraded into CO₂ and H₂O through the sono-Fenton-like process.

In the Fenton-like system, transition metal catalyst was need to react with hydrogen peroxide (H₂O₂) to produce high active hydroxyl radicals (·OH).¹¹ Unfortunately, traditional metal powder catalyst is hard to be recycled, while metal foil has low catalytic activity owing to its small specific surface area.⁶ Therefore, nanoporous metals (NPM) with both large specific surface area and high recovery can be an excellent choice for constructing sono-Fenton-like catalytic system. In recent decades, many methods (template, hydrothermal and dealloying) were used to fabricate nanoporous metals.^12–15 Among them, dealloying has been considered to be the most promising method because it is readily available for obtaining various nanoporous metals with 3D bi-continuous structures. For example, Rizzi et al.¹⁶ reported that uniform nanoporous gold (NPG) can easily be prepared by dealloying Au₄₀Cu₂₈Si₂₀Ag₇Pd₅ glassy alloys, Jin et al.¹⁷ reported that monolithic nanoporous silver (NPS) can be fabricated through dealloying of Ag_60.5Ca_39.5 amorphous ribbons, and Wang et al.¹⁸ reported that homogeneous nanoporous copper (NPC) can be synthesized via chemical dealloying Cu₇₄Ce₂₆ metallic glasses.

Compared with nanoporous noble metals (nanoporous gold (NPG),¹⁹ nanoporous silver (NPS),²⁰ nanoporous palladium (NPPd),²¹ nanoporous platinum (NPPt)²²), nanoporous copper (NPC) and nanoporous Cu-based solid solution material are more attractive owing to both cost-effectiveness and high catalytic activity.¹⁸ More importantly, as an efficient transition metal catalyst, copper as well as iron is often used in Fenton/Fenton-like catalytic process.^10,23,24 In view of the facts that iron can not be existed in the form of nanoporous metals (NPM), which are very applicable for the sono-Fenton-like catalytic process, nanoporous Cu-based material should be a better choice. Among all the nanoporous Cu-based material, nanoporous Cu–Ag solid solution can be the perfect candidate material to construct sono-Fenton-like catalytic system, which can be attributed to the following three points: (1) silver is a stable noble metal. Hence, nanoporous Cu–Ag solid solution has better oxidation resistance compared with pure nanoporous copper, thus increasing the stability of nanoporous metal materials. Moreover, silver is much cheaper than other stable noble metals (gold, palladium and platinum). (2) The surface diffusion coefficient of Ag atoms is much lower than that of Cu atoms, which can act as a diffusion barrier to restrict the long distance diffusion of Cu atoms in the dealloying process.²⁵ Consequently, nanoporous Cu–Ag solid solution fabricated by dealloying possesses very fine microstructure, thus leading to a high sono-catalytic activity.⁶ (3) Nanoporous Cu–Ag solid solution of uniform composition and microstructure can be obtained by dealloying: Ag and Cu elements have the same chemical properties, the substitution of Ag element for Cu element in Cu-based metallic glasses can improve its glass forming ability (GFA).²⁶ The resulting fully amorphous alloys have a unique disordered atomic structure without grain boundaries, dislocations and segregations,^27,28 which is readily available for dealloying synthesis of uniform nanoporous materials.

There are some reports about dealloying synthesis of nanoporous Cu–Ag alloys. Dan et al.²⁵ have synthesized nanoporous Cu–Ag alloys with Ag atoms concentrated at the grain boundary through dealloying of the Cu₃₉Ti₆₀Ag₁ and Cu₃₈Ti₆₀Ag₂ glassy alloys in hydrofluoric acid (HF) solutions. Unfortunately, highly toxic HF solution was used in this dealloying process, which can pollute the human environment and threaten people's health. Li et al.²⁹ have prepared a 3D nanoporous bimetallic Cu–Ag alloy with some Cu atoms separated from the metastable nanoporous matrix via chemical dealloying Mg₆₅Ag_12.5Cu_12.5Y₁₀ amorphous ribbons in sulphuric acid (H₂SO₄) aqueous solution. Which is yet unsatisfying is that the preparation procedure of Mg₆₅Ag_12.5Cu_12.5Y₁₀ alloy ingots is very complex, it involves the smelting of Cu–Y pre-alloy and the melting of volatile Mg, hence the alloy ingots need be produced by using both arc-melting furnace and high-frequency induction furnace. Furthermore, for above two dealloying techniques, the synthesized nanoporous Cu–Ag alloys are uneven, Ag or Cu atoms can be found in the dealloying products. In this work, easily obtained Cu₇₀Y₂₈Ag₂ metallic glasses were used as precursors and non-toxic dilute H₂SO₄ solution was used as aggressive medium to prepare uniform nanoporous Cu–Ag solid solution (NPCS), and then the fabricated NPCS was applied in the sono-Fenton-like catalytic degradation of methyl orange (MO). To the best of our knowledge, this is the first report about the sono-Fenton-like process of nanoporous Cu–Ag solid solution.

2. Experimental

The ingots of Cu_72−xY₂₈Ag_x (x = 0 and 2 at%, the alloy composition is represented in nominal atomic percentage of the pure elements mixture) were prepared by arc melting a mixture of pure Cu, Y and Ag elements with purities above 99.99 wt% in a Ti-gettered argon atmosphere, followed by a single roller spinning to obtain amorphous ribbons with a width of 2 mm and a thickness of 20–30 μm. Then the ribbons were cut into 3–4 cm in length and immersed in 1 M H₂SO₄ solution for 90 min. The solution was kept at 333 K by a water bath. After dealloying, the corrosion ribbons were rinsed with distilled water and dehydrated alcohol for 10 times and kept in vacuum chamber until characterizations.

The phase composition and chemical valence of the samples were characterized by X-ray diffraction (XRD) using a D8-Discover diffractometer with Cu-K_α radiation (λ = 1.5406 Å) and X-ray photoelectron spectroscopy (XPS) using a PHI-5000 Versa Probe spectrometer with Al-K_α radiation (hγ = 1486.6 eV), respectively. The microstructures of obtained dealloying specimens were investigated by FEI-3D scanning electron microscopy (SEM) and G2-20 transmission electron microscopy (TEM), respectively. The size of the metal ligaments was statistically measured by a single length chord method over 100 sites of SEM/TEM morphologies. In addition, a Quantachrome Autosorb-IQ2 analyzer was used to measure the specific surface area (S_BET) and total pore volume (V_p) of the nanoporous materials.

The sonocatalytic activities of the samples were evaluated by the degradation of MO under the irradiation of the ultrasound. In this experiment, 100 mL MO solution (20 mg L⁻¹), 50 mg catalyst and 0.5 mL 30 wt% hydrogen peroxide (H₂O₂) were put into a 250 mL Erlenmeyer flask. Prior to the irradiation, the whole suspension was stirred in the dark for 30 min to ensure the establishment of the adsorption–desorption equilibrium. Then, the degradation reactions were conducted in a serial-ultrasonic apparatus (KH-300DB, Kunshan Ultrasonic Apparatus Company, China) with a frequency of 40 kHz and output power of 4800 mW cm⁻² at 333 K. The concentration of MO solution was evaluated by measuring the change in maximum absorbance through TU-1810 UV-vis spectrophotometer.

3. Results and discussion

Fig. 1a shows XRD patterns of as-spun Cu_72−xY₂₈Ag_x (x = 0 and 2 at%) ribbons. Only a strong broad diffraction peak appeared around 39° in their patterns, indicating that the as-spun specimens have a fully amorphous structure. Fig. 1b displays the XRD patterns of as-dealloyed ribbons, the patterns exhibit crystalline diffraction peaks of (111), (200) and (220), which can be indexed to fcc Cu (JCPDS card no. 04-0836). It should be noticed that the peak positions of the sample synthesized by dealloying Cu₇₀Y₂₈Ag₂ amorphous ribbons have obvious left-shift comparing with those of the sample fabricated by dealloying Cu₇₂Y₂₈ ribbons. The phenomenon can be caused by the formation of fcc Cu-based solid solution in as-dealloyed Cu₇₀Y₂₈Ag₂ ribbons.

Fig. 1 XRD patterns of (a) Cu–Y(–Ag) metallic glasses and (b) the corresponding samples after dealloying in 1 M H₂SO₄ aqueous solution at 333 K for 90 min; (c) XPS Ag 3d spectra of as-dealloyed Cu₇₀Y₂₈Ag₂ ribbons.

In view of the facts that Ag element can not be detected in XRD patterns of as-dealloyed Cu₇₀Y₂₈Ag₂ ribbons, the high resolution XPS spectrum of Ag 3d was measured to confirm the phase composition of the dealloying products. Fig. 1c presents Ag 3d spectra of the sample prepared by dealloying Cu₇₀Y₂₈Ag₂, the binding energies of Ag 3d_3/2 and Ag 3d_5/2 are observed at 374.5 and 368.5 eV, respectively. Obviously, the splitting energy of 6 eV is in agreement with the standard spectrum of Ag^m.³⁰ Combining this result with the XRD result, it can be concluded that as-dealloyed Cu₇₀Y₂₈Ag₂ ribbons are in form of fcc Cu–Ag solid solution.

Fig. 2a and b depicts SEM images of the samples by dealloying Cu₇₂Y₂₈ and Cu₇₀Y₂₈Ag₂ amorphous ribbons, respectively. The images all exhibit 3D bi-continuous and interpenetrating nanoporous structures. Based on the results of SEM, XRD and XPS can confirm that nanoporous copper and nanoporous Cu–Ag solid solution have been successfully prepared. For convenience, the samples synthesized by dealloying Cu₇₂Y₂₈ and Cu₇₀Y₂₈Ag₂ are labeled as NPC and NPC and NPCS, respectively. It is obvious that the ligaments of NPCS is significantly finer than that of NPC, suggesting that the metal ligaments can be significantly refined by the substitution of 2 at% Ag element for Cu in Cu-based amorphous precursor. This is because that the surface diffusion coefficient of Ag atoms in a vacuum is two orders lower than that of Cu atoms.¹¹ Hence, Ag atoms which are uniformly distributed in amorphous precursor not only can recombine with Cu atoms to form the skeleton of the ligaments in the dealloying process, but also can act as a diffusion barrier to restrict the long distance diffusion of Cu atoms,²⁵ achieving the goals of refining the ligaments of nanoporous materials. Moreover, the EDS results show that the nanoporous material synthesized by dealloying Cu–Y metallic glasses is composed of Cu element, while the one fabricated by dealloying Cu–Y–Ag alloys consists of both Cu and Ag elements, indicating that Y has been completely removed from the precursor alloys and the remaining inert metal elements form a framework of nanoporous structure. The results agree well with the XRD and XPS results.

Fig. 2 SEM images of the samples prepared by dealloying (a) Cu₇₂Y₂₈ and (b) Cu₇₀Y₂₈Ag₂ metallic glasses in 1 M H₂SO₄ aqueous solution at 333 K for 90 min. TEM images of the samples: (c) NPC; (d) NPCS.

Fig. 2c and d presents the TEM images of NPC and NPCS. The images show that the samples consist of a bi-continuous pore-to-ligament nano-structure and NPCS has much finer ligament structure than NPC. The results are consistent with the SEM results. By measuring from TEM images, the sizes of ligaments of NPC and NPCS are estimated to be 370 and 45 nm, while the ones measured by previous SEM images are evaluated to be 402 and 65 nm, respectively (displayed in Table 1). The reason can be explained as follows: owing to the concentration gradient of H₂SO₄ solution in the corrosion ribbons, the dealloying samples have finer nanoporous structures in its internal region, which can be obtained by ion beam thinning and measured by TEM. Fig. 3 shows EDS element mapping of the NPCS sample, the images indicates that Cu and Ag elements are uniformly distributed in the as-dealloyed Cu₇₀Y₂₈Ag₂ ribbons and uniform Cu–Ag solid solution was formed. Table 1 shows that NPCS (S_BET = 8.34 m² g⁻¹, V_p = 0.093 cm³ g⁻¹) with finer microstructure has far larger specific surface area and higher total pore volume than NPC (S_BET = 1.77 m² g⁻¹, V_p = 0.050 cm³ g⁻¹), which are measured by BET method.

Table 1 The morphological features (ligament size measured by SEM (d_SEM), ligament size measured by TEM (d_TEM), specific surface area (S_BET) and total pore volume (V_p)) of the samples

Samples	dSEM (nm)	dTEM (nm)	SBET (m^2 g^−1)	Vp-total (cm^3 g^−1)
NPC	402 ± 37.1	370 ± 35.0	1.77 ± 0.58	0.050
NPCS	65 ± 11.9	45 ± 10.6	8.34 ± 1.31	0.093

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Fig. 3 (a) EDS element mapping of the NPCS sample; the distribution of each element: (b) Cu Kα, (c) Ag Lα.

To investigate the catalytic activity of nanoporous Cu–Ag solid solution, the experiment of methyl orange (MO) sono-degradation was carried out. Fig. 4a shows the sonocatalytic degradation curves of MO. It can be seen that only 35.2% and 32.3% of MO were degraded over the commercial copper foil and silver foil within 15 min, respectively, while up to 63.6% and 81.0% of MO were decomposed over the NPC and NPCS sample within the same time, respectively. Obviously, nanoporous Cu-based material has higher catalytic activity compared to the pure metal foil. The reason can be attributed to 3D bi-continuous nanoporous structure of NPC and NPCS.

Fig. 4 (a) Sonocatalytic degradation of MO versus reaction time, (b) linear transform ln(C/C₀) of the kinetic curves of MO degradation.

Fig. 4b demonstrates linear relationships between ln(C/C₀) and ultrasound irradiation time (t), revealing that these degradation processes obey pseudo-first-order kinetics. Therefore, Langmuir–Hinshelwood model (eqn (1))^31,32 can be used to calculate to the values of the reaction rate constant:

ln(C/C₀) = −kt (1)

where C is the concentration of MO at time t, C₀ is the initial concentration, k is the apparent reaction rate constant. The k values under different experiment conditions of no catalyst, NPCS without ultrasound, Ag foil, Cu foil, NPC and NPCS were evaluated to 0.004, 0.028, 0.034, 0.039, 0.086 and 0.131 min⁻¹. Evidently NPCS is the most effective, the reasons can be related to two points: (1) NPCS has much larger specific surface area compared with NPC and copper foil, which can provide more interfaces to facilitate interaction between the catalyst and dye molecules.³³ In addition, more cavitation bubbles can be formed on the surface of NPCS to produce ·OH because the surface of catalysts act as the nucleus for cavitation bubbles in the sono-catalytic process.³⁴ (2) NPCS has higher total pore volume than NPC and Cu foil, thus not only allowing the interior ligaments to maximize the absorption of acoustic energy and produce ·OH, but also providing more channels for diffusion and transportation of ·OH,³⁵ which play very important roles in the sono-catalytic degradation process. It should be noticed that the degradation rate of NPCS decreases extremely when the ultrasound was removed, implying that the degradation of MO dye in this experiment is a sono-catalytic process.

Fig. 5 shows the SEM images of NPC and NPCS after reaction. It can be seen that only a few metal nanoparticles break away from the nanoporous metal and the structure of nanoporous metal partly break apart, indicating that the catalysts have certain stability in the structure.

Fig. 5 SEM images of the nanoporous metal catalysts after catalytic reaction: (a) NPC; (b) NPCS.

The sono-Fenton-like process of NPCS is similar to that of NPC,^6,18 which can be described as follows: under the radiation of ultrasound, fine Cu–Ag ligaments react with H₂O₂ to generate Cu⁺, Ag⁺ and high activity hydroxyl radicals (·OH), subsequently the Cu⁺ which is produced in the previous step further react with H₂O₂ to form Cu²⁺ and ·OH. At the same time, Cu²⁺, Cu⁺ and Ag⁺ combine with electrons (e⁻) to be reduced back to Cu⁺, Cu and Ag, respectively, thus allowing the sono-Fenton-like process to go and return in following a circle. Finally, ·OH produced by this process decomposes MO molecules into oxidation product. The reactions can be described as eqn (2)–(7):

Cu–Ag ligaments + H₂O₂ + H⁺ + ))) (ultrasound) = Cu⁺/Ag⁺ + H₂O + ·OH (2)

Cu⁺ + H₂O₂ + H⁺ + ))) (ultrasound) = Cu²⁺ + H₂O + ·OH (3)

Cu²⁺ + e⁻ = Cu⁺ (4)

Cu⁺ + e⁻ = Cu (5)

Ag⁺ + e⁻ = Ag (6)

·OH + MO → oxidation product (7)

4. Conclusions

Nanoporous Cu–Ag solid solution (NPCS) with a 3D bi-continuous nanoporous structure was successfully prepared by dealloying Cu₇₀Y₂₈Ag₂ amorphous ribbons. The synthesized NPCS not only possesses much finer ligaments (d_SEM = 65 nm, d_TEM = 45 nm) than NPC fabricated by dealloying Cu₇₂Y₂₈ (d_SEM = 402 nm, d_TEM = 370 nm), but also presents higher sono-Fenton-like catalytic performance (k = 0.131 min⁻¹) compared to NPC (k = 0.086 min⁻¹) and Cu foil (k = 0.039 min⁻¹). The excellent sonocatalytic activity of NPCS can mainly be attributed to its larger specific surface area (8.34 m² g⁻¹) and higher total pore volume (0.093 cm³ g⁻¹). Thus it can be seen that NPCS with both high sonocatalytic activity and good oxidation resistance has great potential in organic industry wastewater treatment.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Advanced Talents Funds of Nanjing Institute of Technology (Grant No. YKJ201809), the Science Foundation for the Excellent Youth Scholars of Jiangsu Province (BK20180106) and the Innovation Funds of Nanjing Institute of Technology (Grant No. CKJA201703).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra03247a

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