Preparation of copper nanoparticles by chemical reduction method using potassium borohydride
ZHANG Qiu-li(张秋利)1, YANG Zhi-mao(杨志懋)2, DING Bing-jun(丁秉钧)2,
LAN Xin-zhe(兰新哲)1, GUO Ying-juan(郭莹娟)1
1. School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China;
2. School of Science, Xi’an Jiaotong University, Xi’an 710049, China
Received 6 July 2009; accepted 30 December 2009
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Abstract: High dispersive copper nanoparticles were prepared by chemical reduction method using potassium borohydride as reducing agent. The effects of reactant ratio, concentration of CuSO4, reaction temperature, and dispersant on the size of product and conversion rate were studied. The morphologies of copper nanoparticles were characterized by scanning electron microscopy. The results show that the optimum process conditions are as follows: the molar ratio of KBH4 to CuSO4 is 0.75 (3?4), concentration of CuSO4 is 0.4 mol/L, reaction temperature is 30 ℃, and dispersant is n-butyl alcohol. The average particles size of copper powders with spherical shape gained is about 100 nm.
Key words: copper; nanoparticles; potassium borohydride; chemical reduction
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1 Introduction
In the past few years, considerable interest has been focused on metal nanoparticles due to their special properties and potential applications in diverse fields. Among various metal particles, copper nanoparticles have attracted considerable attention because of their catalytic, optical, and electrical conducting properties. Several methods were developed for the preparation of copper nanoparticles, including thermal reduction, metal vapor synthesis, radiation methods, microemulsion techniques, laser ablation, mechanical attrition, and chemical reduction[1-9].
Most of the preparation methods have some factors that impede the use and development of the copper nanoparticles. For example, gas evaporation method presents the high cost of raw materials and complicated equipment. Copper nanoparticles synthesized by mechanical chemical method have wide particle size distribution and low purity. Among these methods, the solution method is simple and the most versatile for copper nanoparticles. Liquid reduction has its unique advantages such as simple equipment, short process and easy industrial production. Reducing agent is often used in present methods, including formaldehyde[10], ascorbic acid[11], sodium hypophosphite[12-13], and hydrazine hydrate[14].
However, some reducing agents are toxic and expensive, some have poor reducing ability, some have high costs, and some are easy to introduce other impurities to the process. Therefore, it is important to research more appropriate reducing agent and to study the chemical reaction system.
In the present study, potassium borohydride was used as the reducing agent to investigate the preparation of copper nanoparticles. Furthermore, the affecting factors were researched, including reactant ratio, copper sulfate concentration, reaction temperature, and dispersant.
2 Experimental
2.1 Preparation of copper particles
CuSO4?5H2O and KOH are analytical reagents, and KBH4 is chemical pure. The main equipment has an HH-4 type digital pot at a water bath temperature, a JJ-1 type blender from time to time, and a DZF-6050 vacuum drying oven.
CuSO4 and EDTA were dissolved in distilled water.KBH4 was added to the above solution as reducing agent, and then the solution was heated to reaction temperatures. After stirring at different temperatures for a certain time, the aqueous solution was spilled out and filtered. After that, the product was washed 3 times with ethanol and acetone separately, and finally dried in a vacuum drying oven.
2.2 Characterization
The morphologies of the copper particles were investigated via scanning electron microscope (SEM, TESCAN VEGA), and the particle size distributions were obtained by laser particle size analyzer (OMEC LS800).
3 Results and discussion
3.1 Effects of molar ratio of KBH4 to Cu2+
KBH4 is a strong reducing agent, stable in air and alkaline solution, and has the risk of burning when exposed with acid or oxidants. Hydrogen will be slowly emitted when KBH4 contacts with water. So, copper powder should be synthesized in alkaline environment when KBH4 is used as reducing agent.
KOH is used as alkaline medium in the experiment, and EDTA is added as ingredient agent in order to avoid the formation of copper hydroxide precipitate in alkaline medium.
The equation of KBH4 reacting with Cu2+ under alkaline conditions is as follows:
4CuSO4+KBH4+8KOH=4Cu+4K2SO4+KBO2+6H2O
It can be seen from the reaction that the theoretical molar ratio of KBH4 to Cu2+ is 1?4. Different molar ratios are selected in the experiment, with reaction time fixed to 0.5 h. Therefore, the effects of particle size of copper powder on the conversion rate are investigated. The results are shown in Figs.1 and 2.
Fig.1 Effect of molar ratio of KHB4 to CuSO4 on average particles size
Fig.2 Effect of molar ratio of KHB4 to CuSO4 on conversion rate
From Figs.1 and 2, it can be seen that reaction conversion rate of copper sulfate increases with the increase of molar ratio of KBH4 to CuSO4, that is, the increase of KBH4 amount. The reaction conversion rate is about 95% when the molar ratio is around 0.75 (that is 3:4), and then the increasing trend is not distinct. And size of copper is the smallest simultaneity. So, it has higher conversion rate and the KBH4 consumption is less when the molar ratio is around 0.75. Therefore, the optimal molar ratio of KBH4 to CuSO4 is 0.75.
3.2 Effects of initial concentration of Cu2+
There are two stages when copper nanoparticles generate in the solution. The first stage is to generate copper nuclei, and the second stage is the growth of copper. So, it is important to control preparation process that copper nuclei must generate faster and grow up slower, which requires better control of the initial concentration of Cu2+.
The molar ratio of KBH4 to CuSO4 is fixed to be 0.75 in the experiment, and the initial concentration of Cu2+ is changed. The effect of concentration of Cu2+ on the average particle size is studied. The results are shown in Fig.3.
Fig.3 Effect of concentration of copper ions on average particle size of copper
It can be seen that reaction conversion rate increases with the increase of initial concentration of Cu2+. With the increase of reaction conversion rate, the amount of the copper nuclei rises, and smaller particle size powders are obtained correspondingly. But an excess number of nuclei will be generated when the reactant concentration is too high. This results in the agglomeration of the nuclei and the growth of the particle size. So, the optimal initial concentration of Cu2+ is 0.4 mol/L.
3.3 Effects of reaction temperature
Fixing the molar ratio of KHB4 to CuSO4 to be 0.75, and the concentration of copper sulfate of 0.4 mol/L, we investigated the preparation of copper powders under different reaction temperatures. The results are shown in Fig.4.
Fig.4 Effect of reaction temperature on average particle size of copper
It can be seen from Fig.4 that the size of copper powder decreases when the temperature goes up, but the gained size grows up when increasing to a certain temperature. In the reaction system, the effect on the nucleation rate by temperature is greater than that on the growth rate, so the nucleation rate of growth is faster than the growth rate when the temperature increases, and the smaller powder particle size is gained. But the nuclei surface activity is enhanced when the temperature is too high, which makes that the nuclei are prone to collide and reunite.
HUANG and REN[1] indicated that the purity of product would be increased and the oxidation of copper powder will be reduced when reacting at a lower temperature. Therefore, the optimal reaction temperature is 30 ℃.
3.4 Effects of dispersing agent
It is a key point to disperse effectively in the process of nanoparticle preparation. The surface activity of nanoparticles makes reunite and form a larger size group with number of weak interfaces easily. Adding dispersing agent, which is a kind of polymer, can prevent their reunion. Dispersing agent has two base groups: its hydrophilic group is attached to the surface of copper powder particles, while the other queue hydrophobic group is exposed to the solution. So, the surface of copper is covered with a layer of organic thin film, which can reduce collision between the powder cores. Then it prevents the growth of reunion particles and improves the dispersion of particles. At the same time, it can hinder copper from contacting with oxygen and prevent them from being oxidized.
There are four dispersing agents chosen in the experiment, namely polyvinyl pyrrolidone (PVP), glycerol, n-butanol and PEG-400. The results are shown in Table 1.
Table 1 Effect of dispersing agents on average particle size
It can be seen from Table 1 that n-butanol and PVP are more suitable for dispersing agent in this system, and smaller particles are gained using n-butyl alcohol. Therefore, n-butanol is chosen as dispersing agent.
3.5 SEM images of samples
The optimal conditions for the experiment are as follows: molar ratio of KBH4 to CuSO4 is 0.75, concentration of CuSO4 is 0.4 mol/L, and reaction temperature is 30 ℃. Copper powders are synthesized using different dispersing agents respectively. The SEM images are shown in Fig.5.
It can be seen from Fig.5 that the gained powders reunite seriously but have some particles with smaller size when PVP is chosen as dispersing agent (shown in Fig.5(a)). When glycerol and PEG-400 are used as dispersing agent separately, the copper powders have a larger particle size and reunite seriously (Figs.5(b) and (d)). When n-butyl alcohol is used, the product possesses small size about 100 nm and even dispersive (Fig.5(c)). According to Ref.[15], copper nanoparticles can be gained when KBH4 is used as reducing agent and PVP and sodium dodecyl sulfate are used as dispersing agents, the gained powder will be both spherical and non-aggregation[15]; when formaldehyde is used as reducing agent, the prepared powders reunite seriously and the size is in the range of 20-400 nm. Although spherical-similar copper powders with size of 35 nm can be prepared by the use of hydrazine method[14], the method has the disadvantage of toxic resource materials and is unsuitable for large-scale application[1].
Fig.5 SEM images of copper powders prepared with different dispersing agents: (a) Polyvinyl pyrrolidone;(b) Glycerol propanetriol; (c) n-butyl alcohol; (d) PEG-400
4 Conclusions
1) The present study illustrates simple, convenient and significant methods for the synthesis of copper nanoparticles through the reduction of copper salts using KBH4 as reducing agent. Relatively monodisperse copper nanoparticles with a size range of 100 nm are obtained under optimum conditions.
2) The optimum conditions are as follows: the molar ratio of KBH4 to CuSO4 is 0.75 (3?4), concentration of CuSO4 is 0.4 mol/L, and reaction temperature is 30 ℃. The dispersing agent plays an important role in determining the copper particle size distribution and inhibiting the agglomeration.
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Foundation item: Project(50834003) supported by the National Natural Science Foundation of China; Project(09JK561) supported by Educational Commission of Shaanxi Province of China
Corresponding author: ZHANG Qiu-li; Tel: +86-29-82201248; E-mail: qiulizhang@126.com
(Edited by CHEN Wei-ping)
