An environmentally friendly route to synthesize Cu micro/nanomaterials with “sustainable oxidation resistance” and promising catalytic performance

Abstract

Practical application of nanostructured Cu has long been limited by the surface oxidation. Although conventional surface modification can slow down the oxidation rate, the formation of a surface oxide shell cannot completely be prevented. Here, we report an effective approach to achieve “sustainable oxidation resistance” for Cu micro/nanomaterials. Once the Cu is oxidized by the external environment, an ageing treatment would not only convert the oxidized sample back to an unoxidized state but also enhance the oxidation resistance. This approach takes advantage of the dual functions of the citrate group: one is its complexation with the Cu²⁺, which can facilitate the oxidative etching of Cu₂O; the other is its interaction with the Cu surface, which can effectively enhance the Cu oxidation resistance. In the ageing process, the oxide layer was etched by the oxygen, whereas the formed Cu⁰ was protected by the citrate group. Since there is no long-chain or hydrophobic molecule capped on the surface, the adsorption and desorption of the reactant on the Cu surface could proceed smoothly, enabling Cu to be a preferable catalyst. In the reduction of 4-nitro-phenol (4-NP), the rate constant of the reaction catalyzed by the Cu particles is estimated to be 3.85 × 10⁻² s⁻¹. By comparison, rate constants for Ag and Au particles are much lower, which are 1.03 × 10⁻² s⁻¹ and 2.73 × 10⁻³ s⁻¹, respectively. Since the Cu is significantly cheaper, this work provides a promising platform for the development of non-noble metal catalysts.

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Introduction

Nanostructured noble metals have recently attracted much interest, owing to their broad range of applications.^1–3 As the demand for micro/nanomaterials grows, the development of cost-effective alternatives to conventional noble metals has been of great importance. Cu has long been considered to be an ideal alternative, because of its low resistivity, cheap price and large reserves. For several years, researchers have demonstrated that nanostructured Cu can be widely applied in catalysis,^4–10 conductive ink,^11,12 electrodes,¹³ and surface enhanced Raman.^14,15 Nevertheless, because nanostructured Cu is prone to fast oxidation in ambient conditions, practical utilization of Cu micro/nanomaterials is still in a rudimentary state.

In attempts to improve the oxidation resistance, many researchers have been interested in the modification of Cu surface. A conventional method is to coat the Cu micro/nanomaterials surface with polymer such as poly(N-vinylpyrrolidone), poly(ethylene glycol) or poly(acrylamide).^11,12,14 More recently, via the chemical interaction between the carboxylate moiety and Cu atom, Jeong et al. have fabricated the surface-oxide free Cu NP by utilizing oleic acid as the capping molecule.¹⁶ Although above modifications can slow down the oxidation rate, the formation of a surface oxide shell cannot completely be prevented. And with increasing air exposure time, the sample has to be oxidized. Most significantly, when an extensive modification is implemented, the catalytic active site would no doubt be reduced. Thus, it is desirable to develop an effective approach to realize the “sustainable oxidation resistance” for Cu micro/nanomaterials.

Sodium citrate is generally recognized as safe for use in foods.¹⁷ Because of its safety and biodegradability, it is widely used in phosphate-free detergents. Advantages like lower costs and favourable side effect profile make it a desirable anticoagulant.¹⁸ In the chemical synthesis, sodium citrate is considered to be the prime example of an environmentally acceptable complexing agent. In contrast to oleic acid, sodium citrate is highly soluble in water and has three carboxylate moieties in its structure. Thus, we infer that the oxidation of Cu would be restrained if its citrate group is chemisorbed on their surface.

On the other hand, our previous work showed that the presence of citrate group would facilitate the Cu₂O dissolution. This can be accounted by the fact that the formation of [Cu₂(cit)₂]²⁻ (reaction (2)) would force the etching (reaction (1)) toward the right-hand side.¹⁹

2Cu₂O + O₂ + 4H₂O → 4Cu²⁺ + 8OH⁻ (1)

2Cu²⁺ + 2cit³⁻ → [Cu₂(cit)₂]²⁻ (2)

Utilizing the two functions of citrate group, in this work, we demonstrate an environmentally friendly route to synthesize Cu micro/nanomaterials with “sustainable oxidation resistance”. Firstly, pure Cu particles was prepared by a chemical reduction of Cu²⁺. In the synthesis process, NaCl, for the first time, was proved to be a promising additive that can induce the selective reduction of Cu²⁺ to Cu⁰. Sodium citrate, which could chemisorb its citrate group to the Cu surface, was introduced as the protective agent to improve the oxidation resistance. Cu particles obtained by this route could be stored under ambient conditions for long period of time, even after 120 days, no impurity phase such as Cu₂O or CuO can be detected from their X-ray diffraction. XPS (X-ray photoelectron spectroscopy) results revealed that the chemical interaction between citrate group and Cu atoms can effectively enhance the Cu oxidation resistance.

When the sample was oxidized by the elevated temperature, its anti-oxidation property could be restored by an ageing treatment. The ageing process occurs via two consecutive reactions: etching oxidative of Cu₂O and selective reduction of Cu²⁺ to Cu⁰. Both the reductant and Cl⁻, along with citrate group in the aging solution were vital to this process. The reductant could be used to recycle the Cu²⁺, while the presence of Cl⁻ could induce the selective reduction of Cu²⁺ to Cu⁰. For the environmental purpose, remaining solution in the synthesis process was reused as the aging solution because of its abundant citrate groups, Cl⁻ ions and glucose.

Moreover, without extensive modification, the Cu particles exhibit excellent catalytic performance. In the 4-NP reduction, rate constant of the reaction catalyzed by the Cu particles is much superior to that of Au and Ag particles prepared by the same route. Furthermore, in the degradation of dyes, the Cu particles also show their excellent catalytic activity.

Results and discussion

Synthesis of Cu particles with high purity

In a standard synthesis, 7.0 g of Na₂CO₃, 4.3 g of sodium citrate, 3.0 g of NaCl, 4 mL of 0.68 M CuSO₄ aqueous solution were added in 68 mL of deionized water. After all the reagents were dissolved, 30 mL of 1.5 M glucose were dropped into this mixture. The final solution was then transferred into an oil bath and heated at 100 °C for 15 min. As the reaction proceeded, the solution color changed from blue to red-brown, and a reddish precipitate appeared, which signify that Cu²⁺ has been reduced by glucose. The reddish precipitate was collected by a centrifugation. And the remaining preparation solution, which can then be utilized as the ageing solution, was preserved. One noteworthy advantage of this route is that the whole experiment does not need inert gas protection. The crystal structure of the obtained product was characterized by the powder XRD measurement. As exhibited in Fig. 1, the distinguishable diffraction peaks are ascribed to the (111), (200), (220), and (311) planes of face-centered cubic (fcc) Cu, which are in good agreement with JCPDS 85-1326. To demonstrate the oxidation resistance of the resulting Cu particles, we stored them in the ambient condition and monitored their time-dependent XRD patterns. It is interesting to observe that, even though the Cu particles have been stored for 120 days, no peak corresponding to other phases can be found from their XRD patterns. For clarity, the Cu particles that after being stored for 60 and 120 days were designated as Cu-60 days and Cu-120 days, respectively.

Fig. 1 Time-dependent XRD patterns of the resulting product.

The typical FESEM, STEM and HRTEM images of the product are given in (Fig. S1†). The resulting Cu particles present a spherical morphology, with an average size of 200–300 nm in diameter. The interplanar distance (d-spacing) is 2.09 Å, which agrees well with the d value of the (111) plane of the fcc Cu. Furthermore, the EDX spectrum provides an additional indication for the purity of product, in which only Cu characteristic peaks can be observed (Fig. S2†).

Analysis for the surface oxidation resistance

To further investigate the surface oxidation resistance of the resulting Cu particles, we collected their X-ray photoelectron spectroscopy (XPS). Fig. 2 shows the typical XPS Cu 2p core level spectrum of Cu-60 days and Cu-120 days. As shown in Fig. 2a, two distinct peaks at 932.3 eV and 952.2 eV are assigned to Cu (2p_3/2) and Cu (2p_1/2) core level, respectively. Here, Cu (2p_3/2) core level is used to analyse the degree of oxidation.²⁰ According to the curve-fitting results, the symmetric peak at Cu (2p_3/2) core level is assigned to Cu⁰, which indicate that no oxide layer formed even though the resulting Cu particles upon 60 days air exposure. As for the Cu-120 days (Fig. 2b), a satellite peak at 934.9 eV is assigned to the Cu(OH)₂, suggesting that an oxide layer has formed on its surface.²¹ However, because the oxide layer is very thin, this oxidation cannot be detected by the XRD analysis.

Fig. 2 Peak fitting of the Cu 2p_3/2 and Cu 2p_1/2 spectra for the: (a) Cu-60 days and (b) Cu-120 days.

Selective reduction of Cu²⁺ to Cu⁰

As we know, reduction of Cu²⁺ occurs via two consecutive reactions with Cu⁺ as the intermediate step. In this synthesis, Cl⁻ in the reaction solution is critical to ensure the purity of Cu particles. This can be proved by another experiment, in which NaCl was not added, the product is a mixture of Cu₂O and Cu (Fig. 3). Actually, this difference also can be observed from the FESEM images. Compared with pure Cu particles, impurity product mainly consist of cubic and spherical particles (Fig. S3†). The cubic particles can be derived from the presence of Cu₂O.

Fig. 3 XRD patterns of product obtained without additive. The signals marked with # indicate the peaks corresponding to Cu₂O.

These contrasts can be due to the anions in the reaction solution. Cl⁻ provided by the additive would combine with Cu⁺ to form the intermediate species.²² These species intermediate species evoke a stabilization of Cu⁺, which not only slow down reduction rate but then be converted into Cu⁰. Therefore, it could be conjectured that, in a mild reducing environment, Cl⁻ in the reaction solution would induce the reduction from Cu²⁺ to Cu⁰, thus leading to the selective reduction of Cu²⁺.

The dual role of sodium citrate

When there was no sodium citrate involved in the synthesis process, it is interesting to find that the product contain a large amount of oxide (Fig. S4†). We thus can infer that the sodium citrate here is considered to play a dual role: one is the complexing agent that combines with Cu²⁺ to form the cupric citrate (reaction (1)), which then be reduced by glucose (reaction (3)); the other is the protective agent to against the oxidation of Cu particles.

[Cu₂(cit)₂]²⁻ + C₅H₁₁O₅–CHO → Cu↓ + C₅H₁₁O₅COOH (3)

To verify that the citrate group has been chemisorbed on the Cu surface, XPS of the as-prepared Cu particles at O 1s and C 1s was also analyzed. According to the curve-fitting in Fig. 4a, two C 1s peaks at 284.8 and 288.2 eV are ascribed to the aliphatic chain (C–C) and the carboxylate moiety (–COO⁻).²³ Carboxylate moiety here can be result from the citrate group on the Cu surface. Regarding the O 1s spectra (Fig. 4b), the presence of both C–O and C=O bond implies that the carboxylate moiety in citrate group is bound to the Cu atoms through two inequivalent oxygen atoms, leading to a high surface coverage (Fig. 4c).¹⁶ This coverage by citrate group effectively reduces the contact between Cu atoms with atmosphere, thereby allowing a better stabilization of Cu⁰. Here, Na⁺ in sodium citrate is not responsible for the oxidation resistibility. When potassium citrate was used instead of sodium citrate, the Cu particles can still maintain their oxidation resistance (Fig. S5†).

Fig. 4 Peak fitting of the: (a) C 1s and (b) O 1s spectra for the Cu particles. (c) Schematic illustration of a citrate group bound to the Cu surface.

It is generally accepted that the sodium citrate would be dehydrated when the temperature reaches 150 °C. In order to further define the surface coverage by citrate group is the vital factor for the oxidation resistance, thermal decomposition behaviour of the synthesized Cu particles was examined by the TG analysis in dry air atmosphere (Fig. S6†). The TG curve shows a weight loss step in the temperature range of 150 to 170 °C, corresponding to the degradation of sodium citrate. When the temperature above 170 °C, a sharp weight increase can be observed, which indicate the generation of an oxide layer. This process was also verified by the fact that the Cu particles were oxidized after 1 hour annealed treatment at 200 °C. As shown in Fig. 5a, peaks corresponding to Cu₂O (JCPDS 05-667) were observed in the XRD patterns of the annealed Cu particles. It indicates that the Cu particles cannot maintain their oxidation resistance when the surface sodium citrate was dehydrated.

Fig. 5 (a) XRD patterns of the annealed Cu particles and the sample after ageing treatment. Insets: the digital image of the annealed Cu powder that before (right) and after (left) the ageing treatment. (b) Illustration for the whole process to realize the sustainable oxidation resistance of Cu particles. (c) The proposed schematic illustration of conversion between oxidized Cu particles and unoxidized Cu particles.

The proposed mechanism above is universal for the other complexing agent with carboxylate moiety in its structure. When the sodium citrate was replaced by potassium sodium tartrate, Cu particles with oxidation resistance can also be obtained (Fig. S7†).

The “sustainable oxidation resistance”

After the anneal treatment, the oxidized Cu particles exhibit a dark gray color. However, upon 10 days ageing in the remaining preparation solution (as the ageing solution), their color turns into reddish again. Based on the XRD analysis, we can conclude that the oxidized sample has been converted into the unoxidized state (Fig. 5a).

This conversion may result from the synergistic effect of oxidative etching for Cu₂O and selective reduction of Cu²⁺ to Cu⁰. Surplus reductant, sodium citrate, and Cl⁻ ions in the ageing solution are instrumental to the whole process. As the schematic illustration included in Fig. 5c, Cu₂O layer was first transformed into Cu²⁺ species by the etching process. As etching was continued, the concentration of Cu²⁺ species in the solution was subsequently sufficiently increased to allow the reduction by glucose to take place. In the presence of Cl⁻, Cu²⁺ species were finally converted into the Cu⁰ that capped by citrate groups on its surface. This argument is supported by the fact that the final Cu⁰ is not re-oxidized by the solution. Since the oxidation resistance of the final Cu⁰ is restored, we can conclude that the “sustainable oxidation resistance” of Cu micro/nanomaterials is realized though this conversion. Because treatment was realized by a chain of reactions that proceed at room temperature, a stirring of the remaining preparation solution would accelerate this conversion (Fig. S8†).

Compared with conventional processes that restore the oxide back to metal, the advantages of this ageing process are as follows: (1) the conversion, from the oxidized state to the unoxidized state, is not only an etching of the oxide layer but a “nanorecycling” for reuses the oxidized micro/nanomaterials. This can be justified by the fact that pure Cu₂O microcrystals can be converted to a mixture of Cu and Cu₂O after the same ageing treatment (Fig. S9†); (2) since the conversion from Cu₂O to Cu can proceed under room temperature, such “nanorecycling” process in this work has an advantage over previous processes based on thermal energy treatment.²⁴ (3) The aging solution here may be further utilized as an environmentally friendly solution to storage the nanostructured Cu or purify the oxidized Cu micro/nanomaterials.

The catalytic performance

To evaluate the catalytic performance of the resulting Cu particles, we choose the reduction of 4-NP to 4-aminophenol as a model reaction. For this catalytic reduction experiment, the initial concentrations of 4-NP and NaBH₄ were kept at 0.1 mM and 40 mM, respectively. Detailed procedures can be found in the Experimental section.

As depicted in the Fig. 6a, 4-NP solution exhibits absorption at 310 nm under a neutral condition. Upon the addition of NaBH₄, the alkalinity of the solution increased, and the absorption for 4 nitro-phenolate (at 403 nm) would become the dominant.²⁵ Since NaBH₄ was exceeded in the reduction, concentration of BH⁴⁻ was considered as a constant. As shown in Fig. 6b, the relationship between the plot of ln(C/C₀) versus time for the reduction of 4-NP, in the presence of catalyst, is approximately linear. This indicates that the reaction followed the first-order kinetics. The reaction rate constant k,²⁶ defined as eqn (A), can be obtained from the slope of the fitting line.

ln(C/C₀) = −kt (A)

C₀ is the initial concentration of 4-NP.

Fig. 6 The time-dependent absorption spectra of the reaction solution in the presence of the: (a) Cu particles, (b) Ag particles, (c) Au particles; (d) the plot of ln(C/C₀) versus time for the reduction of 4-NP catalyzed by different metals.

In a catalytic reaction, rate constant is a critical parameter that reflects the catalytic activity. Generally, the reaction rate depends on the concentration of the reactants, reaction temperature and the catalyst. In this reaction, since the amount of the reactants and catalyst were unchanged, reaction temperature becomes the major factor that affects the reaction rate. At room temperature (18 °C), the rate constant of the reaction by the Cu particles is calculated to be 3.85 × 10⁻² s⁻¹. To investigate relationship between the catalytic activity and reaction temperature, control tests were also carried out at 23, 28 and 33 °C, respectively. The rate constants at different temperatures are listed in Fig. S10.† And the increase of temperature would facilitate this reduction.

Activation energy, pre-exponential factor and entropy of activation are three major kinetic parameters to evaluate the activity of catalyst. According to the Arrhenius equation and rate constants at different temperatures, we also give these parameters for this reaction catalyzed by the Cu particles (Table 1), and the details of calculations are described in the Fig. S11.†

Table 1 Summary of the kinetic parameters for the reaction that catalyzed by the Cu particles

	Activation energy (kJ mol^−1)	Pre-exponential factor (s^−1)	Entropy of activation (J mol^−1 K^−1)
Cu particles	10.55	2.93	8.94

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Compared with the noble metals and Cu micro/nanomaterials obtained in previous studies, catalytic performance of the Cu particles in this work is also preferable. As shown in Table 2, except the Au nanocages, the catalytic parameters for Cu particles are much superior to other materials. This excellent performance can be attributed to two aspects.

Table 2 Comparison of the catalytic parameters for Cu particles with other catalysts

Materials	Rate constants (min^−1)	Activation energy (kJ mol^−1)	Catalyst used (mg)	Concentration of 4-NP (mol L^−1)
(1) Au NPs@cellulose single nanofibers^28	0.35	—	—	1.5 × 10^−6
(2) Au nanocages^29	2.83	28.04	—	1.4 × 10^−4
(3) Au/graphene hydrogel^30	0.19	—	0.10	1 × 10^−4
(4) Pt–Au/graphene oxide nanosheets^31	0.23	—	0.005	2.1 × 10^−4
(5) Ag/fibrous nano-silica^3	0.6	—	0.20	1.2 × 10^−4
(6) AgNPs/eggshell membrane^32	0.25	27.16	10.0	3.2 × 10^−4
(7) Ni@Pd NP/fibrous nano-silica^33	1.22	—	0.40	1.2 × 10^−4
(8) Pt–Pd alloy NCs^34	0.0133	26.2	—	3.1 × 10^−4
(9) Pd NPs/carbon nanotube^26	0.63	—	0.005	5 × 10^−5
(10) Cu3Ni2 nanocrystals^35	0.58	—	4.00	1 × 10^−4
(11) Porous Cu microspheres^36	0.18	—	0.50	2 × 10^−4
(12) Cu polyhedrons^37	0.23	47.62	0.096	1 × 10^−4
This work	2.31	10.55	0.096	1 × 10^−4

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The first aspect can be attributed to the feasible modification for Cu surface. In terms of theory, a smaller catalyst particle size should lead to a decrease in reaction activation energy. However, the actual values of activation energy do not follow this tendency. As generally known, the synthetic process, a large amount of ligands have to be applied in to obtain a smaller size particle. The ligands on the Cu surface would hinder the electron transfer between the catalyst and reactant, resulting in the increase of reaction activation energy. Compared with conventional ligands, the citrate group is relatively short, enabling the adsorption and desorption of reactant proceed smoothly.

The second aspect is the fact that adsorption energy of 4-NP molecule onto the Cu surface is stronger than onto noble metals.²⁷ Since this reduction is pseudo-first-order, the catalyst with strong adsorption energy should lead to a higher reaction rate. To prove this point of view, we chose two kinds of metals (Au and Ag) that have lower adsorption energies and investigated their catalytic performances. Taking into consideration the ligand effects on the adsorption energy, both of Au and Ag were prepared by the same route as for Cu particles. As a result, morphology of prepared noble metals is aggregation of particles as well (Fig. S12†). As shown the successive spectra in Fig. 6c and d, it takes about 260 s and for the 4-NP to achieve the reduction in the presence of the as-synthesized Ag particles (k = 1.03 × 10⁻² s⁻¹). In the presence of the Au particles, this reduction needs 1000 s (k = 2.73 × 10⁻³ s⁻¹). The results are well consistent with the order of adsorption energy (Cu > Ag > Au). And it can be concluded that a mild surface modification for Cu particles enables them to be a promising catalyst. In addition to two aspects mentioned above, the morphology is another factor that determines catalyst performance as well. Accordingly, it is not difficult to understand why the Au nanocages show a superior catalytic performance.

In recent years, with the growing demand for textile products, the textile industry has become one of the major sources of water pollution. Developing a suitable and effective catalyst for degradation of dyes would be of great value. Herein, we detected the catalytic activity of the Cu particles toward the degradation of methyl orange, methylene blue, Congo red, and rhodamine B. In the degradation, the dyes would gradually fade and completely bleach upon the addition of catalyst. It should be noted that, without catalysts, the degradation would be much slower and incomplete (Fig. S13†). Fig. 7 shows the successive UV-vis spectra of the reaction mixture upon the addition of the resulting Cu particles, reflecting the degradation process of dyes. It was observed that maximal absorption peak of methyl orange, methylene blue, Congo red, and rhodamine B was at 465, 675, 495 and 552 nm, respectively. And degradation processes of the four dyes were varied. In the degradation of methyl orange, the maximal absorption peak at 465 nm decreases linearly as the reaction time increases and vanishes in about 182 s (Fig. S14†). Similarly, both of the degradation for methylene blue and Congo red need about 102 s (Fig. 7b and c). As illustrated in Fig. 7d, degradation for rhodamine B needs the longest time. This may because it requires a longer time to adsorb rhodamine B molecule onto the catalyst. Relationship between plots of maximum absorption peak (552 nm) versus time provides better understanding of this process (Fig. S15†). It is observed that, in the first 80 s, the absorption peak exhibits little variation, indicating a longer time for the rhodamine B to initiate its degradation. However, with the degradation proceeding, this peak drops rapidly and vanished in about 254 s. Mallick and co-workers demonstrated that the catalyst acts as an electron relay system for the reductant of dye, which can facilitate a faster dye reduction.³⁸

Fig. 7 The successive UV-vis spectra of reaction solution in the presence of Cu particles: (a) methyl orange; (b) methylene blue; (c) Congo red and (d) rhodamine B.

In the above degradations, the Cu particles transfer the electrons from BH⁴⁻ to the dyes, leading to an effective degradation of dyes.³⁹

Conclusions

In summary, this study reports an environmentally friendly approach to keep the anti-oxidation property of Cu micro/nanomaterials without implementing extensive surface modification. The viability of using sodium citrate for enhancing the Cu particles oxidation resistance was verified by experiments. Selective reduction of Cu²⁺ to Cu⁰ was achieved by introducing Cl⁻. Experimental results show that the anti-oxidation property of Cu particles can be successfully restored by a facile ageing treatment. Since oxide layer was finally converted to Cu⁰, an oxidized sample can be turned back into its unoxidized state repeatedly, which not only achieves the concept of “sustainable oxidation resistance” but recycles the oxidized micro/nanomaterials. Data in different systems clearly point to the fact that the resulting Cu particles can be utilized as a feasible catalyst. More broadly, we anticipate that low-cost Cu micro/nanomaterials would be practically used in other emerging applications.

Experimental

Reagents

Anhydrous sodium carbonate (Na₂CO₃), sodium citrate, copper sulfate pentahydrate (CuSO₄·5H₂O), sodium chloride (NaCl), sodium sulfate (Na₂SO₄), potassium sodium tartrate and glucose (C₆H₁₂O₆) were purchased from Guoyao Chemical Reagent Company (Shanghai, China). All the chemicals were of analytical grade and used without further purification.

Characterization

The obtained samples were characterized by powder X-ray diffraction (XRD, Shimadzu-6000) which used Cu Kα radiation at 40 kV and 40 mA. The ultraviolet-visible absorption (UV-vis) spectra were collected on an Ocean Optics QE 65000 spectrometer. The X-ray photoelectron spectroscope (XPS) was collected on the VG ESCALAB MKII spectrometer with an Mg Kr excitation (1253.6 eV). The shape and size of the as-obtained samples were characterized by the Magellan-400 Field emission scanning electron microscope (FESEM, FEI Company). Scanning transmission electron microscope (STEM), high resolution transmission electron microscope (HRTEM) images were acquired with a JEM-2200FS TEM operating at 200 kV. Energy-dispersive X-ray (EDX) spectroscopy was performed during TEM measurements. The thermogravimetry (TG) analysis was performed under static air atmosphere at 10 °C min⁻¹ (Linseis, STA PT-1750).

Catalytic reduction of p-nitro-phenol

In a typical process, 1.7 mL of nitro-phenol (0.1 mM) and 0.7 mL of NaBH₄ (40 mM) in were added into a 5 mL quartz cell. 0.1 mL of aqueous solution of resulting Cu particles (15 mM) was then added to the quartz cell. Held for 2–3 s and then monitored the reaction in quartz cell by the UV-vis spectrophotometer. Similar procedures were taken to the degradation of methyl orange, rhodamine B, Congo red, and methylene blue.

Acknowledgements

This work was supported by the NFSC (No. 91227202, 51202086 and 11504126), RFDP (No. 20120061130006), Changbai Mountain scholars program (No. 2013007), China Postdoctoral Science Foundation (No. 2014M561281), and Jilin Provincial Science & Technology Development Program (No. 20150520087JH).

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Footnote

† Electronic supplementary information (ESI) available: EDX of the as-prepared Cu particles; FESEM images of the obtained noble metals; 4-nitro-phenol reaction by noble metals; other absorption spectra and XRD patterns. See DOI: 10.1039/c6ra02039a

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