Trans. Nonferrous Met. Soc. China 22(2012) 117-123

Preparation of Cu nanoparticles with NaBH₄ by aqueous reduction method

LIU Qing-ming^{1, 2}, ZHOU De-bi^{1, 2}, Yuya YAMAMOTO², Ryoichi ICHINO², Masazumi OKIDO²

1. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;

2. Department of Materials Science, Graduate School of Engineering, Nagoya University, Nagoya 4648603, Japan

Received 12 January 2011; accepted 16 May 2011

Abstract: Cu nanoparticles were prepared by reducing Cu²⁺ ions with NaBH₄ in alkaline solution. The effects of NaBH₄ concentration and dripping rate on the formation of Cu nanoparticles were studied. The optimum conditions are found to be 0.2 mol/L Cu²⁺, solution with pH=12, temperature of 313 K and 1% gelatin as dispersant, to which 0.4 mol/L NaBH₄ is added at a dripping rate of 50 mL/min. NH₃·H₂O is found to be the optimal complexant to form the Cu precursor. A series experiments were conducted to study the reaction process at different time points.

Key words: Cu nanoparticles; aqueous reduction method; precursor; reaction process

1 Introduction

In recent years, the preparation of Cu nanoparticles has become an intensive area of scientific research as Cu nanoparticles exhibit many excellent physical and chemical properties such as high electrical conductivity and chemical activity. Cu nanoparticles are considered possible replacements for Ag and Au particles in some potential applications, such as in catalysts and conductive pastes [1-4]. There are many well-known procedures for preparation of Cu nanoparticles, such as the radiation method [5], microemulsion technique [6], supercritical technique [7], thermal reduction [8], sonochemical reduction [9], laser ablation [10], metal vapor synthesis [11], vacuum vapor deposition [12] and aqueous reduction method [13]. Among these methods, aqueous reduction method is most widely employed because of its advantages such as simple operation, high yield and quality, limited equipment requirements and ease of control. Because of its strong reducing ability, NaBH₄ is widely used as a reductant for this aqueous reduction process. According to the previously reported research, much work has been done to explore the optimum conditions for preparation of copper nanoparticles [14]. However, the reaction mechanism has been seldom reported. In this research, the mechanism of reaction process was investigated based on the optimizing of the reaction conditions.

2 Experimental

All the reagents used in the experiments were of analytical grade and obtained from Nacalai Tesque (Kyoto). The flowchart of the experimental process is shown in Fig. 1. Prior to carrying out the experiments,  50 mL CuSO₄ solution and NaBH₄ solutions were prepared, and argon was bubbled through both the solutions for 30 min. Each pH value was adjusted to the same value using H₂SO₄ and NaOH solutions, respectively. Then 1% gelatin (mass fraction) was added into the CuSO₄ solution as a dispersant. The NaBH₄ solution was then added dropwise to the CuSO₄ solution in a beaker at 313 K with magnetic rod stirring. The color of the mixture was changed from blue to brown, indicating the precipitation of Cu nanoparticles. When the reaction was completed, a small quantity of the slurry was collected for size distribution measurements using an electrophoretic light scattering spectrophotometer (Model: ELS-8000NS, Otsuka Electronics Co. Ltd., Japan). Cu particles were formed by precipitation, which were separated by centrifugation, washed several times with distilled water and ethanol, and finally dried in a vacuum stove at room temperature for several days. The SEM images were obtained by using a scanning electron microscope (model: S-800, Hitachi Co. Ltd., Japan) and the XRD patterns of the specimen were recorded using an X-ray diffractometer (model: XRD-6000, Shimadzu Co. Ltd., Japan) with Cu K_α radiation.

Fig. 1 Flow chart of experiment process

3 Results and discussion

3.1 Effect of NaBH₄ concentration on Cu nanoparticles preparation

In aqueous solution, the reaction takes place as

=   (1)

In theory, the stoichiometric ratio of Cu²⁺ ions to NaBH₄ is 4:1. In the experiment, Cu²⁺ concentration is fixed at 0.2 mol/L, thus, the NaBH₄ concentration should ideally be 0.05 mol/L. With a fixed concentration (1%, mass fraction) gelatin as the dispersant and solution pH of 12, the effect of NaBH₄ concentration on the Cu particles was investigated. The results are shown in Fig. 2. It is observed that the average size of the Cu nanoparticles decreases with increasing NaBH₄ concentration. When the NaBH₄ concentration is     0.4 mol/L (8 times greater than the stoichiometric dosage), Cu nanoparticles with an average size of 37 nm are obtained. The XRD patterns show that at a low NaBH₄ concentration, the resultant particles contain Cu(OH)₂ and Cu₂O. At a higher NaBH₄ concentration, the Cu(OH)₂ contaminant disappears, but Cu₂O disappears only when the NaBH₄ concentration reaches several times the stoichiometric value. The analysis by XRD reveals that Cu(OH)₂ and Cu₂O contaminants are the intermediate products of the reduction process.

  3.2 Effect of NaBH₄ dripping rate on Cu nanoparticles preparation

In the experiments, it is found that the dripping rate has a significant effect on the average size and shape of the Cu nanoparticles. The effect of the NaBH₄ dripping rate was investigated under constant condition of 0.2 mol/L Cu²⁺, 0.4 mol/L NaBH₄, 1% gelatin and pH=12. The Cu particles were prepared with the NaBH₄ dripping rate of 5 and 50 mL/min, respectively. The results of this experiment are shown in Fig. 3. The average size of Cu particles prepared at a dripping rate of 5 mL/min is larger than that obtained at 50 mL/min. Traces of Cu(OH)₂ are also found, indicating that the reduction reaction is incomplete. The Cu nanoparticles obtained at dripping rate of 50 mL/min are smaller due to the occurrence of explosive nucleation when the two solutions are combined. According to the classical theory of nucleation, formation of Cu nanoparticles usually undergoes three stages: pre-nucleation, nucleation and crystal nucleus growth [15]. Explosive nucleation involves the generation of a large number of nuclei during the initial stages of nucleation. Since most of the Cu²⁺ ions are consumed for nucleation, the aggregation is limited. Thus, Cu nanoparticles with very small size can be obtained.

3.3 Effect of complexant on Cu nanoparticles preparation

Appropriate complexants in the CuSO₄ solution could not only eliminate the agglomeration of the Cu particles, but also change the morphology of the resultant particles. In this work, three types of complexants were adopted in the experiments, NH₃·H₂O, potassium sodium tartrate (KNaC₄H₄O₆) and trisodium citrate (C₆H₅O₇Na₃). The reduction reactions can be represented by the following equations.

When NH₃·H₂O is adopted as complexant,

=        (2)

=

                (3)

Fig. 2 SEM images and XRD patterns of copper particles obtained using NaBH₄ with different concentration: (a), (a′) 0.05 mol/L; (b), (b′) 0.1 mol/L; (c), (c′) 0.2 mol/L; (d), (d′) 0.4 mol/L

When KNaC₄H₄O₆ is adopted as complexant,

=            (4)

=

           (5)

    And when C₆H₅O₇Na₃ is adopted as complexant,

 =            (6)

=

             (7)

With fixed values of 0.2 mol/L Cu²⁺, 0.4 mol/L NaBH₄, 1% gelatin and pH of 12, the effect of different complexants on Cu nanoparticles preparation was investigated. The results are shown in Fig. 4. With 1.2 mol/L NH₃·H₂O, 0.6 mol/L KNaC₄H₄O₆ and 0.6 mol/L C₆H₅O₇Na₃ as the complexants, the average sizes of Cu particles obtained are 42, 115 and 108 nm, respectively. Thus, NH₃·H₂O is the optimal complexant for precursor formation.

Fig. 3 SEM images and XRD patterns of copper particles obtained using NaBH₄ at different dripping rates: (a), (a′) 5 mL/min; (b), (b′) 50 mL/min

Fig. 4 SEM images of copper particles obtained using different complexants: (a) 1.2 mol/L NH₃·H₂O; (b) 0.6 mol/L KNaC₄H₄O₆; (c) 0.6 mol/L C₆H₅O₇Na₃

3.4 Reduction process at different reaction time

A series experiments were conducted to study the reactions occurring during Cu nanoparticles formation. At fixed values of 0.2 mol/L Cu²⁺, 0.4 mol/L NaBH₄, 1% gelatin and pH=12, the size distribution, SEM images and XRD patterns of the Cu nanoparticles at different reaction time points (0, 0.5, 1, 3, 5, 10 and 60 min) were examined. The results are shown in Fig. 5. The SEM images and XRD patterns show that during the initial stages of the reaction, the majority of the particles is rod-shaped Cu(OH)₂. Within 5 min, Cu(OH)₂ disappeared. The XRD patterns at 10 and 60 min are similar to that obtained at 5 min, indicating the completion of the reduction reaction within 5 min. The XRD patterns also show that all the Cu²⁺ ions are transformed to Cu(OH)₂ before the two solutions are mixed. Cu(OH)₂ is then reduced to Cu nanoparticles by NaBH₄. The final reaction process can be represented as

=                     (8)

=       (9)

Fig. 5 SEM images and XRD patterns of copper particles obtained at different time points: (a), (a′) Before mixing; (b), (b′) After 0.5 min; (c), (c′) After 1 min; (d), (d′) After 3 min; (e), (e′) After 5 min; (f), (f′) After 10 min; (g), (g′) After 60 min

4 Conclusions

1) The average size of the Cu nanoparticles reduces with increasing excess of NaBH₄ to Cu²⁺. When the Cu²⁺ and NaBH₄ concentration are 0.2 and 0.4 mol/L, the dripping rate is 50 mL/min, gelatin concentration is 1%, pH=12 and solution temperature is 313 K, the finest Cu nanoparticles (37 nm) are obtained.

2) Among NH₃·H₂O, KNaC₄H₄O₆ and C₆H₅O₇Na₃, the optimal complexant is found to be NH₃·H₂O for precursor formation. The smallest Cu nanoparticles are obtained with 1.2 mol/L NH₃·H₂O.

3) During the reaction process, Cu²⁺ is transformed to Cu(OH)₂ before combination of the solutions and then this Cu(OH)₂ is reduced by the addition of NaBH₄ solution.

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NaBH₄的水性还原法制备纳米铜颗粒

刘清明^{1, 2}，周德璧^{1, 2}，山本雄也²，市野良一²，兴户正纯²

1. 中南大学 化学化工学院，长沙 410083，中国；

2. 名古屋大学 工学研究科 材料科学专业，名古屋 4648603，日本

摘 要：在碱性溶液中用NaBH₄还原Cu²⁺制备纳米铜颗粒，研究NaBH₄浓度和滴加速率对Cu纳米颗粒制备的影响。反应的最佳条件是：0.2 mol/L Cu²⁺，溶液pH 12，温度313 K，1%明胶作为分散剂，将0.4 mol/L NaBH₄溶液以50 mL /min的速率加入CuSO₄溶液中。氨水是最佳的络合剂。采用一系列实验研究不同时间点的反应进程。

关键词：纳米铜颗粒；水性还原法；前驱体；反应进程

(Edited by FANG Jing-hua)

Corresponding author: LIU Qing-ming; Tel: +81-52-789-3353; Fax: +81-52-789-3355; E-mail: luisman@126.com

DOI: 10.1016/S1003-6326(11)61149-7