Trans. Nonferrous Met. Soc. China 24(2014) 3739-3746

Effect of surfactants on dispersion property and morphology of nano-sized nickel powders

Guo-yong HUANG¹, Sheng-ming XU^1,2, Lin-yan LI¹, Xue-jun WANG¹

1. Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China;

2. Beijing Key Laboratory of Fine Ceramics, Tsinghua University, Beijing 100084, China

Received 21 December 2013; accepted 14 April 2014

Abstract: Fine face-centered cubic (FCC) nickel powders were synthesized by liquid phase reduction with different surfactants. The products were investigated by scanning electron microscopy (SEM), laser particle size analyzer and X-ray powder diffraction (XRD). The results indicate that the type, dosage and relative molecular mass of surfactants significantly impact the purity, dispersion property, particle size, size distribution and morphology of the products. The nonionic surfactants poly ethylene glycol (PEG) and polyethylene glycol sorbitan monostearate (Tween) showed better dispersing ability in the reaction system than the others. The optimal mass ratios of surfactant to Ni are 100 mg/g and 150 mg/g for PEG-600 and Tween-40, respectively. The products obtained in the optimal conditions have ideal morphology and narrow size distribution. Moreover, study on the relative molecular mass effect revealed that with the increase of the relative molecular mass of Tween, the morphology of nickel powders changed from sphere to spiny ball.

Key words: nickel powder; surfactant; dispersion; morphology; synthesis

1 Introduction

Due to the high surface activity, low resistivity, great thermal and electrical conductivities, nano-sized nickel powder has been widely used as magnetic material [1-3], electrical conductive paste [4,5], catalyst [6-8], electrode material [9] and binder for cemented carbide [10]. With respect to the application of nano-materials, a frequently encountered problem is the aggregation of the nano-sized particles, especially when the particle size decreases to a considerably small extent. During the past decades, many efforts have been directed to the development of nickel powders with well-defined morphology and favorable dispersion property.

Till now, several synthetic methods, such as laser-driven synthesis [11,12], spray pyrolysis [13,14], micro-emulsion [15], polyol process [16-18], vacuum pyrolytic decomposition [19,20], hydrothermal method [21] and liquid phase reduction [22-24], have been developed to prepare the nickel powders. Among these methods, liquid phase reduction has been intensively investigated because of its better control of the composition, low reaction temperature and simple procedure. For this method, amphiphilic surfactants like poly vinyl pyrrolidone (PVP) and poly ethylene glycol (PEG) have been widely used as dispersing agents to control the dispersion and morphology of the products [25-29]. The reason lies on the following two aspects. On the one hand, amphiphilic surfactants have unique ability to self-organize in solution, which can modify the interfacial property and enhance the compatibility between particles of different characteristics. On the other hand, amphiphilic surfactants dissolved in water are inclined to form thermodynamically stable supramolecular assemblies such as micelles and microemulsions, which can be used as nano-reactors for nickel particles with certain shapes and size distributions. Hence, the systematical research on the effect of surfactants on the dispersion property and morphology of nickel powder is much required.

In this work the effect of surfactants on the synthesis of nano-sized nickel powders was studied. The nickel powders were synthesized by reducing nickel sulfate in aqueous hydrazine solution with a variety of surfactants including cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), PVP, PEG, poly oxyethylene tert-octylphenyl (Trion X-100), triethanolamine (TEA) and polyethylene glycol sorbitan monostearate (Tween). The influence of surfactants on the purity, dispersion property, particle size and distribution of the nickel products was investigated. Meanwhile, the relationship between the relative molecular mass of surfactants and the morphology of the product was discussed.

2 Experimental

2.1 Materials and reagents

All reagents were analytical grade. NiSO₄·6H₂O (The relative molecular mass (M_r) is 262.85), 85% N₂H₄· H₂O (M_r=50.06) and NaOH (M_r=40.00) were used as source materials. And CTAB (C₁₉H₄₂BrN, M_r=364.45), SDS (C₁₂H₂₅NaO₄S, M_r=288.38), TEA ((HOCH₂CH₂)₃N, M_r≈149.19), PVP ((C₆H₉NO)_n, average M_r≈30000), PEG-600 (H(OCH₂CH₂)_nOH, average M_r≈600), Triton X-100 (C₁₄H₂₂O(OCH₂CH₂)_n, average M_r≈646.86), Tween-40 (C₆₂H₁₂₂O₂₆, M_r=1283.62), Tween-60 (C₆₄H₁₂₆O₂₆, M_r=1311.65) and Tween-85 (C₁₀₀H₁₈₈O₂₈, M_r=1838.54) were used as surfactants.

2.2 Preparation of nano-sized nickel powders

The method of preparing fine nickel powders by liquid phase reduction has been described in our previous work [30,31]. Nickel sulfate (6.57 g) and an appropriate amount of surfactant were dissolved in deionized water to obtain 50 mL solution, and it was transferred into a flask (250 mL); then, hydrazine (5.89 g) and NaOH (2.00 g) were mixed with deionized water to obtain the other 50 mL solution, and it was added dropwise into the flask under stirring at 60 °C. However, when the influence of different types of surfactants on the preparation of nickel powders was investigated, the dosages of all of the source materials were cut down a half. And the pH value of reaction system was controlled at 13.0-14.0 through adding a handful of saturated NaOH solution. In order to ensure Ni-ions to be reduced completely, the amount of hydrazine was 4 times of the theoretical value.

2.3 Characterization

The crystal phases of samples were characterized by X-ray diffraction (XRD) (Rigaku, D/Max-2000) using Cu K_α radiation with graphite monochromator. The primary grain sizes of the samples were calculated by the Debye-Scherer formula. The morphology and dispersion were examined by scanning electron microscope (SEM) (JEOL, JSM 4500). The particle size and size distribution were analyzed by the laser particle size analyzer (Malvern 2000, measurement range: 0.01- 1000 μm).

3 Results and discussion

3.1 Effect of different types of surfactants

To find the influence of different types of surfactants on the preparation of nickel powders, seven kinds of surfactants including CTAB (cationic type), SDS (anionic type) and five other nonionic ones, i.e. PVP, PEG-600, Trion X-100, TEA and Tween-40, were employed in the reduction reaction and the dosage of each surfactant (versus Ni) is 100 mg/g. The XRD patterns of the products are shown in Fig. 1. The diffraction peaks marked with () can be indexed as face-centered cubic (FCC) metallic Ni phase (JCPDS card No. 03-065-2865), and those marked with (#) can be indexed as hexagonal phase of Ni(OH)₂ (JCPDS card No. 00-014-0117). It can be seen that nickel powders with high purity were obtained using any of the nonionic surfactants. In contrast, the reduction systems with ionic surfactants CTAB and SDS resulted in incompletely reduced products which were mixed with Ni(OH)₂ impurity. This may be explained as follows. The Ni-ions are positively charged in the reaction solution. The cationic surfactant CTAB cannot effectively coat the Ni-ions due to the electrostatic repulsion. However, the intermediate products Ni(OH)₂ are neutral species, which are prone to be cladded by the CTAB micelles. This prevents further reduction of Ni(OH)₂ to attain the nickel powders. Regarding the anionic-type surfactant, it is known that SDS is also a foaming agent. A great amount of bubbles were formed in the reaction solution, which made it difficult to control the reduction conditions. So the products in SDS-emulsified system were also obtained with less purity.

Fig. 1 XRD patterns of Ni powders prepared with different surfactants

The morphology of pure Ni particles is shown in Fig. 2. Reference samples prepared without surfactants (Fig. 2(a)) easily agglomerate because of the van der Waals force and magnetic attraction. However, the introduction of nonionic surfactants, especially PEG-600 (Fig. 2(c)) and Tween-40 (Fig. 2(f)), obviously improve the dispersion property of the nickel particles. This can be attributed to the favorable stability of the nonionic surfactant micelles in aqueous solution, which results in a better interaction with the nickel particles and prevent the aggregation of the products.

Fig. 2 SEM images of samples prepared with nonionic surfactants

3.2 Effect of dosage of surfactants

The dosage of surfactants is also an important factor influencing the properties of the nano-sized nickel products. Due to the excellent dispersing ability, PEG-600 and Tween-40 were chosen as representatives for the investigation of dosage effect. The synthesis experiments were performed with different mass ratios of surfactant to Ni varying from 10 mg/g to 200 mg/g.

Fig. 3 XRD patterns of Ni powders prepared with different dosages of PEG-600

Fig. 4 SEM images of samples prepared with different dosages of PEG-600

3.2.1 Effect of dosage of PEG-600

The XRD patterns of samples prepared with different dosages of PEG-600 are shown in Fig. 3. The diffraction peaks marked with () can be indexed as FCC

nickel phase and it indicates that pure nickel powders were obtained. And the morphology of the corresponding samples is spherical and less-agglomerated, which is shown in Fig. 4. In addition, based upon the calculation by the Debye-Scherer formula, all of the primary grain sizes of nickel powders in different conditions are about 13 nm (Fig. 5). It is claimed that the dosage of PEG-600 almost does not affect the primary grain size of product. However, as the dosage of PEG-600 increased from 10 mg/g to 200 mg/g, the average particle size of samples first decreased from 430 nm to 214 nm, and then increased to 455 nm (Fig. 5). Moreover, the amount of PEG-600 could obviously affect the size distribution of the nickel particles. Figure 6 shows that too little (10 mg/g ~ 50 mg/g) or too much (150 mg/g- 200 mg/g) surfactant resulted in a wide size distribution. The optimal mass ratio of PEG-600 to Ni is 100 mg/g. Such phenomenon is understandable. A small amount of PEG-600 is not sufficient to coat the nickel particles, which may aggregate together in the reaction mixture. However, too much surfactant in the solution will increase the viscosity of the system, as well as the difficulty to control the morphology of the nickel particles. Both the situations cannot lead to a favorable result. Only under the optimal experimental conditions, nickel powders with narrow size distribution ranging from 170 nm to 410 nm and average particle size of 214 nm were obtained.

Fig. 5 Primary grain sizes and average particle sizes of samples prepared with different dosages of PEG-600

Fig. 6 Size distribution of samples prepared with different dosages of PEG-600

Fig. 7 XRD patterns of Ni powders prepared with different dosages of Tween-40

Fig. 8 SEM images of samples prepared with different dosages of Tween-40

3.2.2 Effect of dosage of Tween-40

XRD patterns of the samples prepared with different dosages of Tween-40 are shown in Fig. 7. The diffraction peaks are marked as Fig. 3 and pure FCC nickel powders were also obtained. The products have a spherical or near-spherical morphology, which is shown in Fig. 8. Primary grain size of the samples calculated based on the Debye-Scherer formula was between 12-14 nm (Fig. 9), suggesting that the dosage of Tween-40 has no evident influence on such parameter, which was similar to the situation of PEG-600. Meanwhile, similarity was also found in the dosage effect on the average particle size and the size distribution. When the mass ratio of Tween-40 to Ni increased from 10 mg/g to 200 mg/g, the average particle size of the nickel products decreased from 420 nm to 208 nm, followed by an increase to 415 nm (Fig. 9). For the size distribution, an optimal mass ratio of Tween-40 to Ni was found to be about 150 mg/g. In this condition, the nickel particles showed a narrow size distribution ranging from 100 nm to 350 nm. And the average particle size was found to be 208 nm (Fig. 10). Too much surfactant (200 mg/g) resulted in a wide size distribution of the product. The great similarity in the dosage effect of PEG-600 and Tween-40 is ascribed to the similar dispersion mechanism of the nonionic surfactants.

Fig. 9 Primary grain sizes and average particle sizes of samples prepared with different dosages of Tween-40

Fig. 10 Size distributions of samples prepared with different dosages of Tween-40

3.3 Effect of relative molecular mass of surfactants

The effect of relative molecular mass of Tween surfactant (Tween-40, Tween-60 and Tween-85) on the morphology of nickel powders was investigated. The XRD patterns of the samples are shown in Fig. 11. The diffraction peaks match well with the peaks in Fig. 3, so all of the samples are pure FCC nickel powders. With the increase of relative molecular mass of Tween surfactants, the morphology of particles changed from fine sphere to irregular shape like spiny ball (Fig. 12). The evolution can be explained by the larger viscosity and steric hindrance resulting from the use of Tween surfactant with higher relative molecular mass. In such circumstance, it is more difficult to form ideal cladding of surfactant micelles to the nickel particles, and the crystal nuclei can not grow up at the same rate in all directions. This produces nickel particles with irregular shape.

Fig. 11 XRD patterns of Ni powders prepared with different relative molecular mass of Tween surfactants

4 Conclusions

High-purity FCC nickel powders with favorable dispersion property were prepared by liquid phase reduction. A variety of surfactants were employed for the investigation of the effect of surfactants on the dispersion and morphology of the products. Among them, the nonionic surfactants PEG-600 and Tween-40 were found to show better performance. The detailed study revealed that the dosage of the two surfactants almost had no influence on the primary grain sizes of the products, but obviously affected the particle size and distribution. The optimal mass ratio of the surfactant to Ni was 100 mg/g and 150 mg/g for PEG-600 and Tween-40, respectively. Under the optimized conditions, nickel powders with ideal morphology and narrow size distribution could be obtained. Besides, the study on the effect of relative molecular mass showed that Tween surfactant with lower molecular mass was more inclined to result in nickel products with fine sphere morphology.

Fig. 12 SEM images of Ni powders prepared with different relative molecular mass of Tween surfactants

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表面活性剂对超细镍粉分散性及形貌的影响

黄国勇¹，徐盛明^1,2，李林艳¹，王学军¹

1. 清华大学 核能与新能源技术研究院，北京 100084；

2. 清华大学 精细陶瓷北京市重点实验室，北京 100084

摘  要：采用液相化学还原法制备超细镍粉时，颗粒容易团聚，需加入合适且适量的表面活性剂进行改进。通过SEM、XRD、激光粒度分析仪等表征手段，考察表面活性剂种类及用量对超细镍粉分散性的影响，并研究表面活性剂相对分子质量大小对颗粒形貌的影响。结果表明：非离子型表面活性剂PEG-600和吐温(Tween-40)能明显地改善超细镍粉的团聚现象，且其相对镍理论产量的最佳用量分别为100 mg/g和150 mg/g；并发现随着Tween相对分子质量的增大，镍粉颗粒由光滑的球形变成不规则形状。

关键词：镍粉；表面活性剂；分散；形貌；合成

(Edited by Sai-qian YUAN)

Foundation item: Projects (51074096, 51274130) supported by the National Natural Science Foundation of China

Corresponding author: Sheng-ming XU; Tel: +86-10-62773585; Fax: +86-10-62773585; E-mail: smxu@tsinghua.edu.cn

DOI: 10.1016/S1003-6326(14)63523-8