Size-controlled synthesis of copper nanoparticles in

chemical engineering research and design 9 8 ( 2 0 1 5 ) 36–43

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Chemical Engineering Research and Design journal homepage: www.elsevier.com/locate/cherd

Size-controlled synthesis of copper nanoparticles in supercritical water Lu Zhou a,∗ , Shuzhong Wang b , Honghe Ma a , Suxia Ma a , Donghai Xu b , Yang Guo b a

Department of Thermal Engineering, School of Electrical and Power Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China b Key Laboratory of ThermoFluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

In this work the particle size control method of copper nanoparticles synthesized in super-

Received 25 December 2014

critical water was investigated. The experiments were respectively carried out in a flow-type

Received in revised form 28 March

reaction apparatus and in a batch-type quartz tube apparatus. Results show that particle

2015

size of the products was very sensitive to the concentration of copper sulfate and alkalin-

Accepted 3 April 2015

ity of the reactant. When the concentration of copper sulfate increased from 0.05 mol/L to

Available online 11 April 2015

0.5 mol/L, the average particle size increased from 14 nm to 50 nm. When the molar ratio of NaOH to copper ion increased from 0:1 to 2:1, the average particle size decreased from 85 nm

Keywords:

to 14 nm. The homogeneity and dispersity of the products were improved by increasing the

Supercritical water

concentration of complexing agent ethylenediamine tetraacetic acid (EDTA). The adsorption

Hydrothermal synthesis

mechanism of EDTA on the surfaces of nanoparticles was researched by a Fourier transform

Reductive

infrared spectroscopy. It was speculated that EDTA adsorbed on the surfaces of nanoparti-

Copper nanoparticles

cles by the dehydration reaction of carboxyl groups in the molecule of EDTA and hydroxyl groups on surfaces of copper nanoparticles ( Cu OH). These results are expected to be of great interest as basic data for the preparation of size-controlled copper nanoparticles by supercritical hydrothermal synthesis method. © 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1.

Introduction

Copper nanoparticles are widely used in conductive coatings, metal surface wear resistant coatings, microelectronics, and lubricant additives (Xuan and Li, 2000; Lu et al., 2000). Compared with single atoms and bulk materials, copper nanoparticles have high application value because of their special size-related properties. As for the preparation of the copper nanoparticles, numerous methods have been developed, including electrolysis method, gas evaporation method, chemical reduction method, supercritical hydrothermal synthesis method and so on (Masoud and Fatemeh, 2009; Ding, 1996; Dang et al., 2011; Huaman et al., 2011).

∗

Among these methods, the supercritical hydrothermal synthesis method is unique. It has been widely used to produce metal oxide nanoparticles from metal salt aqueous solutions using supercritical water as the reaction medium (Noguchi et al., 2008). The extremely low dielectric constant of the supercritical water can lead to a high hydrothermal synthesis reaction rate and a low solubility of metal oxides, which help to produce fine particles. In addition, the supercritical water has specific property of high solubility of most reducing gases, which makes it suitable for the synthesis of metal nanoparticles (Adschiri et al., 2001, 1992; Sue et al., 2006a,b). So far, various metal (e.g. Ag, Ni, Pd, Fe, and Cu) nanoparticles have been synthesized using hydrogen as reducing agent

Corresponding author. Tel.: +86 29 6010281. E-mail address: [email protected] (L. Zhou). http://dx.doi.org/10.1016/j.cherd.2015.04.004 0263-8762/© 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.


at supercritical reaction conditions (Arita et al., 2011; Kubota et al., 2014). These findings suggest that the thermodynamic equilibrium constant of the reduction reaction may have significant impact on the phase composition of final product. Among these metal nanoparticles, pure zero-valent copper nanoparticles could be prepared with the least consumption of hydrogen (Arita et al., 2011). In the field of supercritical hydrothermal synthesis, control of the morphological structure and particle size of the nanoparticles is one of the research emphases. As reported in some previous research papers (Ziegler et al., 2001; Gadhe and Gupta, 2007; Arita et al., 2011; Seong et al., 2011; Kubota et al., 2014; Sue et al., 2006a,b), particle size control of metal oxide nanoparticles during the supercritical hydrothermal synthesis has been extensively studied. However, only a limited number of investigations deal with the particle size control of the metal nanoparticles during the reductive supercritical hydrothermal synthesis. This problem was tackled by Sue et al. (2004). They synthesized Ni fine particles in near-critical and supercritical water. During the reaction, 1,10-phenanthroline was used as complexing agent that can complex with nickel ion to form stable chelate compounds at lower temperatures, which may contribute to the control of the reduction rate of nickel ion and thus the particle size of final products. To the best of author’s knowledge, no systematic and experimental study has been found for investigating the effects of the synthetic parameters on the particle size of copper nanoparticles that are synthesized in supercritical water. In this study, it was aimed to determine the effect of reactant concentration on the particle size and morphology of copper nanoparticles that were produced under supercritical reaction condition. Ethylene diamine tetraacetic acid (EDTA), which is a carboxylate forming relatively stable complex with copper ion, was used as the complexing agent. The adsorption mechanism of EDTA on the surfaces of nanoparticles was researched by Fourier transform infrared spectroscopy, and a reductive hydrothermal synthesis scheme for the preparation of copper nanoparticles was speculated.

2.

Experiments

2.1.

Materials

All chemicals used in the experiments were of analytical reagent grade and were used without further purification. In all the experiments, copper sulfate (CuSO4 ·5H2 O, purity >99.9 wt%) was used as the copper ion source. Ethylenediamine tetraacetic acid disodium salt (EDTANa2 ·2H2 O, purity >99.0 wt%) was used as the complexing agent. Sodium hydroxide (NaOH, purity >99.5 wt%) was used to regulate the alkalinity of the starting material. Formaldehyde (HCHO, 30 wt%) was employed as the reducing agent. The reducing agent was not stable in supercritical water. It could decompose into reducing gases such as CO and H2 (Osada et al., 2004; Watanabe et al., 2003) which were miscible with the supercritical water to form a homogeneous reductive reaction system.

2.2.

Experimental apparatus and procedure

In this study, two kinds of experimental apparatuses were used in the experiments, namely, a flow-type reaction apparatus and a batch-type quartz tube apparatus. The flow-type apparatus was used for the experiments with copper sulfate

37

concentration of 0.05–0.2 mol/L, while the quartz tube reactor was used for the experiments with copper sulfate concentration of 0.5 mol/L. Initially, we also employed the flow-type apparatus for the experiments with the copper sulfate concentration of 0.5 mol/L. However, plugging of the lines by solid products frequently occurred. Thus, the quartz tube reactor was adopted as an alternative experimental apparatus.

2.2.1.

Continuous flow-type reactor

Details of the continuous flow-type apparatus and experimental procedure were described in a previous work (Zhou et al., 2014). For the present experiments, the reaction temperature and pressure were fixed at 400 ◦ C and 25 MPa, respectively. The flow rate was adjusted by the pump so that the residence time of reactants in the reactor was 3 min. Prior to the experiments, the aqueous solution of copper sulfate was firstly prepared, to which a certain amount of mixed solution containing EDTA, NaOH, and HCHO was slowly added with stirring at room temperature. After the experiments, the collected copper nanoparticles was alternately washed with pure water and alcohol. Finally, the products were dried in a vacuum dryer at 60 ◦ C for further examination.

2.2.2.

Batch-type reactor

The experiments with the batch-type quartz reactor were performed using the following procedure. Firstly, the quartz reactor was loaded with the prepared precursor solution (as described above). Then the open end of each quartz reactor was attached with a piece of rubber tube. After that all these quartz reactors were put into a horizontal tube chamber (1.0 m in length and 100 mm in i.d.), the two ends of which were connected respectively with a vacuum pump and a nitrogen cylinder. The air in the chamber was pumped out by the vacuum pump, and these quartz reactors were left under vacuum for over 10 h to ensure that the air was completely eliminated from these capillaries. Then the valve of nitrogen cylinder was opened and the chamber was filled by nitrogen. Before these reactors were removed from the chamber, the rubber tube on the reactor was rapidly fastened with a clip to prevent air entering the reactor. Thereafter, the quartz reactor (350 mm in length and 2 mm in i.d.) was sealed by using acetylene flame at a certain distance to the cutting edge, and placed in an electric furnace (preheated to 400 ◦ C). According to the density of loaded precursor solution at the reaction temperature, the pressure of reaction media was 25 MPa. The reaction was performed for 3 min and terminated by submerging the reactor in a water bath at room temperature for cooling. Finally, alcohol was introduced to the reactors to collect the produced nanoparticles.

2.3.

Characterization and product analysis

The concentrations of copper ion in the liquid products were measured by the biscyclohexanone oxalyldihydrazone (BCO) photometric method using individual Merck cell test on a Spectroquant NOVA60 instrument, with results reported directly in concentration units. Particle morphology was observed by transmission electron microscopy (TEM, JEM2100F). Average particle size and the standard deviation (SD) were determined on the basis of over 200 particles measured from TEM images. Powder XRD analysis was performed using an X-ray diffraction meter (Rigaku D/MAX-2400) with Cu K␣ radiation. The system was operated at 100 mA and 40 kV, and the scan speed was fixed at 10◦ /min. The surface functional

38


Fig. 1 shows the XRD profiles of the samples. When the molar ratio of formaldehyde to copper ion was 1:2, in addition to zero-valent Cu, small amounts of Cu2 O were also identified in the product. When the molar ratio increased to 1:1 or higher, pure zero-valent Cu was obtained. This result shows that the formaldehyde required to produce Cu nanoparticles by the supercritical hydrothermal synthesis method was much lower than that of the traditional chemical reduction method (Wen et al., 2003).

Fig. 1 – XRD profiles of the nanoparticles synthesized at different molar ratios of HCOH to Cu2+ ([HCHO]/[Cu2+ ]): (a) 1:2, (b) 1:1, and (c) 3:1. groups adsorbed on the surfaces of copper nanoparticles were determined by a Fourier transform infrared spectroscopy (FTIR, Vetex70).

3.

Results and discussion

Table 1 shows the experimental conditions and results. According to the analyses of the recovered solutions, the conversions of copper ion for all experimental runs were reached almost 100%.

3.1.

Production of copper nanoparticles

First of all, the amount of reducing agent required to produce zero-valent copper was determined. The effect of reducing agent was investigated by varying the molar ratio of formaldehyde to copper ion from 1:2 to 3:1 (see Run 1–3 in Table 1).

3.2.

Control of particle size

3.2.1.

Effect of copper sulfate concentration

The effect of copper sulfate concentration on the particle size of copper nanoparticles was studied (see Run 6 and 8–10 in Table 1). Fig. 2 shows the TEM images of the synthesized nanoparticles under different copper sulfate concentration. When the concentration of copper sulfate increased from 0.05 mol/L to 0.5 mol/L, the average particle size of the nanoparticles increased from about 14 nm to 50 nm. It seems that the nanoparticles may obtain sufficient growth at higher copper sulfate concentrations. This result is consistent with the findings of prior studies on the preparation of -AlO(OH) (Hakuta et al., 2005) and Fe2 O3 (Sue et al., 2010) nanoparticles by supercritical hydrothermal synthesis method. In addition, the particle size distribution of the products became wider as the concentration of copper sulfate increased (see Fig A.1 in Appendix A). When the copper sulfate concentration increased to 0.5 mol/L, the particle size of the products showed obvious heterogeneous distribution. It suggests that the copper sulfate concentration may have a significant impact on the crystallization kinetics of copper nanoparticles. At low copper sulfate concentration, primary nuclei were rapidly generated at supercritical water, which greatly reduced the concentration of copper ion and thus suppressed the formation of secondary nucleus. In this case, the growth time of nanoparticles was limited and small

Fig. 2 – TEM micrographs of copper nanoparticles synthesized at different copper sulfate concentrations: (a) 0.05, (b) 0.1, (c) 0.2, and (d) 0.5 mol/L.

39


Table 1 – Experimental conditions and results. Run 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. a b c

T (◦ C) 400 400 400 400 400 400 400 400 400 400 400 400

[Cu2+ ] (mol/L)

[EDTA]/[Cu2+ ]a

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.2 0.1 0.05 0.05 0.05

2:1 2:1 2:1 0:1 1:1 2:1 4:1 2:1 2:1 2:1 2:1 2:1

[NaOH]/[Cu2+ ]b 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 1:1 0:1

[HCHO]/[Cu2+ ]c 3:1 1:1 1:2 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1 2:1

Phase composition Cu Cu Cu2 O, Cu – – – – – – Cu Cu Cu2 O, Cu

Average size ± S.D. (nm) – – – – 97 ± 32 50 ± 17 49 ± 14 36 ± 8 27 ± 5 14 ± 3 24 ± 6 85 ± 33

Molar ratio of EDTA to copper ion. Molar ratio of NaOH to copper ion. Molar ratio of HCOH to copper ion.

particles with good uniformity were readily produced. Whereas, at high copper sulfate concentration, the nucleation period was prolonged. Repeated heterogeneous phase nucleation thus occurred, which ultimately resulted in heterogeneous distribution of nanoparticles.

3.2.2.

Effect of EDTA concentration

The effect of EDTA concentration on the particle size of copper nanoparticles was studied (see Run 4–7 in Table 1). Fig. 3 shows the TEM photographs of the samples. It can be seen that dispersible ellipsoidal copper nanoparticles were obtained. Fig A.2 in Appendix A shows that the ranges of particle size at molar ratios of EDTA to copper ion of 1:1, 2:1, and 4:1 were 24–405 nm, 17–114 nm, and 22–88 nm, respectively. The result indicates that increasing the concentration of EDTA was advantageous to the homogeneity and dispersity of the nanoparticles. This could be because increasing the concentration of EDTA provided enough atoms to coordinate with copper ions to form stable chelate compounds during the

heating process, which prevented premature nucleation of the nanoparticles.

3.2.3.

Effect of sodium hydroxide concentration

Previous researches have shown that alkalinity of reactant solution may be a key factor affecting the size of nanoparticles in supercritical hydrothermal synthesis reactions (Sue et al., 2006a,b; Sahraneshin et al., 2012; Lu et al., 2013). In this study, the alkalinity of reactant solution was adjusted by the concentration of NaOH. The TEM images of the products obtained at the different NaOH concentrations are shown in Fig. 4 (see Run 10–12 in Table 1). The corresponding particle size distributions are shown in Fig A.3 in Appendix A. When the molar ratio of NaOH to copper ion increased from 0:1 to 2:1, the particle size range of the products significantly decreased from 21–210 nm to 5–19 nm, and the corresponding average particle size decreased from about 85 nm to 14 nm. This finding is understandable because increasing alkalinity of the reactant solution led to the increase of the degree of the

Fig. 3 – TEM images of copper nanoparticles synthesized at different EDTA concentrations. (a) Without EDTA, (b) [EDTA]/[Cu2+ ] = 1:1, (c) [EDTA]/[Cu2+ ] = 2:1, and (d) [EDTA]/[Cu2+ ] = 4:1.

40


Fig. 4 – TEM images of copper nanoparticles synthesized at different NaOH additions. (a) [NaOH]/[Cu2+ ] = 2:1, (b) [NaOH]/[Cu2+ ] = 1:1, and (c) without NaOH.

Fig. 6 – Adsorption mechanism of EDTA on the surfaces of copper nanoparticles.

Fig. 5 – FT-IR spectra of copper nanoparticles synthesized at different NaOH additions. (a) Without NaOH, (b) [NaOH]/[Cu2+ ] = 1:1, and (c) [NaOH]/[Cu2+ ] = 2:1. supersaturation. This result agrees with Sue’s (2006) analysis in that particle size of the nanoparticles varies inversely to their solubilities or supersaturations in the supercritical water. Based on the above analysis, the particle size and distribution of the copper nanoparticles were affected by the above three factors, in which the alkalinity of reactant solution (NaOH concentration) affected it most obviously.

3.3.

Mechanism analysis

3.3.1.

Characterization of the nanoparticle surface

In this section, characterization of the nanoparticle surface was studied. Prior researches have shown that some organic

ligands can adsorb on the surfaces of nanoparticles through their polar functional groups (Rangappa et al., 2008; Lu et al., 2012; Mousavand et al., 2007). To investigate the chemical bonding between EDTA and surface of copper nanoparticles, Fourier transform infrared (FT-IR) analysis was conducted. Fig. 5 shows the FT-IR spectra of the copper nanoparticles synthesized with EDTA at different NaOH concentrations (see Run 10–12 in Table 1). The spectra showed broad hydroxyl ( OH) stretch peaks at 3400 cm−1 , even for the nanoparticles synthesized without NaOH. It indicates that the unsaturated bonds on the surfaces of nanoparticles tended to be combined with hydroxy that was hydrolyzed from water. The absorption bands, corresponding to CH2 and CH3 stretching vibration on the particles, were observed at 2885 cm−1 and 2947 cm−1 . The bands at 1047 cm−1 and 1640 cm−1 were the vibration bands of C OH and C O stretching vibration, which attributed to the adsorption of carboxyl group ( COOH). The bands at 620 cm−1 corresponded to the stretching vibrations of Cu O. Based on above analysis, the adsorption mechanism of EDTA on the surfaces of copper nanoparticles was proposed. As shown in Fig. 6, the complexing agent adsorbed on the surface of nanoparticles by the dehydration reaction of carboxyl groups in the molecule of EDTA and hydroxyl groups on surface of copper nanoparticles ( Cu OH).


Fig. 7 – XRD patterns of samples prepared at various addition amounts of NaOH. (a) Without NaOH, (b) [NaOH]/[Cu2+ ] = 1:1, and (c) [NaOH]/[Cu2+ ] = 2:1.

3.3.2.

Synthesis mechanism of the copper nanoparticles

In this section, the mechanism of the synthesis of copper nanoparticles in supercritical water was analyzed. Kubota et al. (2014) have investigated the reaction mechanism of the preparation of copper nanoparticles using hydrogen as the reducing agent in supercritical water. In their experiments, Cu2 O was detected in the final products and the content of which decreased with the reaction time. The reason, they suggested, is that the hydrothermal synthesis rate of CuO was fast while the releasing rate of hydrogen from the decomposition of formic acid was relatively slow. Seong et al. (2011) synthesized cobalt nanoparticles and they also believed that cobalt oxide was firstly produced by the hydrothermal synthesis reaction and then was reduced to metal cobalt by hydrogen. Unlike these earlier studies, however, formaldehyde rather

41

than formic acid was adopted as the source of reducing agent in this study. The experimental results in Section 3.1 have shown that the incomplete reduction product Cu2 O was detected in the sample when the formaldehyde concentration was low. It indicates that there existed the reduction process of CuO → Cu2 O or Cu2+ → Cu+ . To further explore the reaction mechanism, the phase compositions of the nanoparticles obtained at various NaOH concentrations were determined (see Run 10–12 in Table 1), where the concentration of NaOH was varied to adjust the synthesis rate of CuO. Fig. 7 shows the XRD profiles of these samples. It can be seen that Cu2 O was detected in the product that was obtained without NaOH (see Fig. 7a). Under this condition, the hydrothermal synthesis of CuO was suppressed due to the acidic reaction medium. Whereas, when the molar ratio of NaOH to copper ion increased to 1:1 or 2:1, pure zero-valent Cu was obtained (see Fig. 7b and c). These results suggest that zero-valent copper was readily produced when the formation rate of CuO was faster. Thus, it is speculated that the main reduction reaction path was CuO → Cu2 O → Cu in this study. According to the results above, the shape-controlled hydrothermal synthesis strategy of copper nanoparticles with EDTA as complexing agent is considered to progress as follows. The CuO precursors were rapidly produced by supercritical hydrothermal synthesis reaction (hydrolysis and dehydration). In this process, EDTA chelated copper ion to form stable chelate compound during the heating process, which helped the formation of homogenous nucleation in the reaction system. Then the reduction reaction occurred to form primary particles of zero-valent Cu. Subsequent growth of these primary particles resulted in the final copper nanoparticles with a particular size. In this process the adsorption of EDTA on the surfaces of nanoparticles formed coated layers and thereby inhibiting further growth of the nanoparticles.

Fig. A.1 – Particle size distribution of copper nanoparticles synthesized at different copper sulfate concentrations: (a) 0.05, (b) 0.1, (c) 0.2, and (d) 0.5 mol/L.

42

4.


Conclusions

In this work size-controlled copper nanoparticles were prepared by supercritical hydrothermal synthesis method. The average particle size of the products increased from about 14 nm to 50 nm when the concentration of copper sulfate

increased from 0.05 mol/L to 0.5 mol/L. Moreover, the results indicate that the uniformity of the particle size distribution became worse with the increase of copper sulfate concentration. Alkalinity of the reactant was found to be the most critical influence on the particle size. When the molar ratio of NaOH to copper ion increased from 0:1 to 2:1, the

Fig. A.2 – Particle size distribution of copper nanoparticles synthesized at different EDTA concentrations. (a) [EDTA]/[Cu2+ ] = 1:1, (b) [EDTA]/[Cu2+ ] = 2:1, and (c) [EDTA]/[Cu2+ ] = 4:1.

Fig. A.3 – Particle size distribution of copper nanoparticles synthesized at different NaOH concentrations. (a) [NaOH]/[Cu2+ ] = 2:1, (b) [NaOH]/[Cu2+ ] = 1:1, and (c) without NaOH.


average particle size significantly decreased from about 85 nm to 14 nm. An increase in the concentration of the complexing agent EDTA led to the formation of homogeneous and monodispersed nanoparticles, probably due to that EDTA prevented premature nucleation during the heating process. Test results of FT-IR showed that EDTA adsorbed on the surfaces of copper nanoparticles by dehydration reaction of carboxyl group ( COOH) in the molecule of EDTA and hydroxyl group on the copper surface ( Cu OH). The zero-valent Cu was speculated to be produced mainly by the reduction reaction path of CuO → Cu2 O → Cu.

Acknowledgements The authors acknowledge the financial supports from the Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (no. tyut-rc201430a) and (no. tyut-rc201316a), the Young Foundation of Taiyuan University of Technology (no. 2013Z042), and the Program for New Century Excellent Talents in University of Chinese Education Ministry (Grant NCET-070678).

Appendix A. Here below are the figures about the effects of the concentrations of copper sulfate, EDTA, and NaOH on the particle size distribution of the synthesized copper nanoparticles (particle number fraction vs. particle size). The particle size distributions were statistically calculated on the basis of over 200 nanoparticles measured from the TEM images of the products.

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