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Preparation of fine Ni powders from nickel hydrazine complex
Materials Chemistry and Physics 97 (2006) 371–378
Preparation of fine Ni powders from nickel hydrazine complex Jung Woo Park a,∗ , Eun H. Chae a , Sang H. Kim a , Jong Ho Lee a , Jeong Wook Kim a , Seon Mi Yoon b , Jae-Young Choi b a b
Materials R&D Group, Passive Component Division, Samsung Electro-Mechanics Co., Ltd., 314, Suwon 443-743, Republic of Korea Materials & Devices Research Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Republic of Korea Received 6 May 2005; received in revised form 9 August 2005; accepted 12 August 2005
Abstract Fine nickel powders with narrow size distribution have been prepared from the reduction of nickel hydrazine complexes in aqueous solution. The pure nickel hydrazine complexes, [Ni(N2 H4 )3 ]Cl2 were prepared with the molar ratio of N2 H4 /Ni2+ = 4.5, while a mixture of complexes, such as Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 were formed with N2 H4 /Ni2+ < 4.5. By the X-ray diffraction (XRD), FT-IR, and scanning electron microscopy (SEM) analyses, it was found that the reduction of Ni2+ to metallic Ni powder proceeded via the formation of nickel hydroxide which was reduced by hydrazine liberated from the ligand exchange reaction between the nickel hydrazine complex and NaOH. The standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex while the mean particle size increased. As the amount of hydrazine increased, the surface roughness of the particles was improved significantly due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle. It was found that average particle size could be controlled from 150 to 380 nm by adjusting the reaction molar ratio and temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: Nickel hydrazine complex; Fine nickel powder; Chemical reduction
1. Introduction There have been extensive studies on the synthesis of fine nickel powders in the past two decades due to their potential applications in optical, electronic, catalytic, magnetic materials, and so on [1–3]. Moreover, they have been a great deal of attention as an internal electrode in multi-layer ceramic capacitors (MLCCs). MLCCs are one of the most widely used passive components in electronics, such as computers, wire and wireless communication devices, etc. [4,5]. The continuing development of advanced electronic devices for high performance with miniaturized dimensions has lead to the improvements of MLCCs with larger capacitance in a smaller volume. High volumetric efficient MLCCs (large capacitance in small volume) can be achieved by control of the thickness and the lamination number of dielectric layers [6]. Especially, stacking a larger number of thinner dielectric ∗
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layers and thinner electrode layers is inevitable to achieve small-size capacitors with larger capacitance. Under these circumstances, the increasing amount of internal metal electrode has lead to the replacement of Pd or Pd/Ag by Ni as an electrode material due to its lower cost and lower resistivity [7–10]. However, it is more likely to cause not only a shortcircuit between the internal electrode layers but also structural defects, such as cracks and delamination in the highly integrated MLCCs with thinner dielectric and electrode layers [11]. Therefore, to circumvent these problems in the fabrication of the thinner internal electrodes, it is essential for the use of smaller size and non-agglomerated with the narrow size distribution Ni powder. There have been developed various kinds of synthetic methods, such as chemical vapor deposition, sonochemical decomposition, microwave-hydrothermal methods, polyol process, and electrochemical reduction controlled chemical reduction for the preparation of Ni fine powders [12–17]. Among those methods, the chemical reduction of nickel salts, such as NiCl2 and NiSO4 by a strong reducing agent in
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aqueous solution has been researched intensively due to not only economical and mass-productive aspects but also technological aspects, such as better structural control on the microscopic level, low reaction temperature, and the simple procedure [18–23]. In this chemical reduction method, hydrazine and alkali metal borohydride have been generally used as an external reducing agent. But in the case of alkali metal borohydride, borohydride ions are known to reduce metal cations to metal borides, particularly in aqueous systems [24–26]. On the other hand, hydrazine has more widely been used as reducing agent for preparation of metal nanoparticles because pH and temperature dependent reducing ability of hydrazine make controllability of reduction rate easy. Hydrazine is basic and the chemically active free ion is hydrazium cation, N2 H5 + . The standard reduction potential for the hydrazinium ion, N2 H5 + is −0.23 V (N2 + 5H+ + 4e− = N2 H5 + ) in acidic solution. But in basic solution, the standard reduction potential of hydrazine N2 H4 is −1.16 V (N2 H4 + 4OH− = N2 + 4H2 O + 4e− ) [19,27]. On the other hand, standard reduction potentials of Ni2+ and Ni(OH)2 are −0.25 and −0.72 V for Ni2+ + 2e− = Ni and Ni(OH)2 + 2e− = Ni + 2OH− , respectively. Therefore, it is necessary to maintain enough alkalinity of solution and avoid precipitation of agglomerated nickel hydroxide during the reduction process for inducing an efficient control of reduction rate, size, and morphology of metal particle. In this respect, nickel hydrazine complex is attractive starting material for preparation of fine nickel powder by chemical reduction method. The stabilization of nickel cation by hydrazine ligand may retard and prevent an abrupt formation and agglomeration of nickel hydroxide. Also, highly uniform distribution of hydrazine molecule around the nickel cation by liberation in high pH can induce homogeneous reduction process. Although considerable efforts have been made to study the change in the size and shape, there are no reports in the literature regarding the preparation of fine nickel powder by the controlled reduction of nickel hydrazine complexes in aqueous solution. In this paper, we report on the preparation of fine nickel powders from the nickel hydrazine complex and their reduction pathway. We have also investigated on the effect of nickel hydrazine complexes, reaction molar ratio and reaction temperature on the phase composition, and morphology of nickel powders.
chased from Daejung chemicals & Metals. The water used throughout this work was deionized with Human Power I+ water purification system (Human Co., Korea). 2.2. Synthesis of nickel hydrazine complex Typically, an appropriate amount of nickel chloride solution dissolved in deionized water was slowly added dropwise over 30 min to an appropriate amount of hydrazine monohydrate at 60 ◦ C. During the addition, blue, blue–violet, or pink precipitates depending on the reaction molar ratio of N2 H4 /Ni2+ were formed, and the resulting mixtures were stirred for 1–2 h. The crude products obtained by filtration were washed three times with 30 mL of deionized water and finally dried at 30 ◦ C for 24 h in a vacuum dry oven. The products were characterized by X-ray diffraction (XRD) and FT-IR. 2.3. Preparation of fine nickel powder To a nickel hydrazine complex solution prepared by above procedure, an appropriate amount of NaOH was poured at corresponding reaction temperature and the resulting mixture was stirred for 1 h. As the reduction reaction proceeded, the blue, blue–violet, or pink solution turned to black within 15 min, indicating a formation of metallic nickel. The resulting black slurry was carefully decanted and washed repeatedly with deionized water to remove by-products, such as NaCl and unreacted hydrazine or NaOH. The metallic powder, thus obtained was filtered and dried in a vacuum dry oven at 80 ◦ C for 24 h. 2.4. Characterization The nickel hydrazine complexes and nickel powders were characterized by X-ray diffraction (Rigaku, RINT2200HF+ ) using Cu K␣ radiation with graphite monochromator. The Ni particle size and morphology analyses were performed using scanning electron microscopy (SEM, JEOL, JSM-6700F with accelerating voltage of 10 kV). FT-IR spectra were recorded with a Nicolet Magna-IR 760 spectrometer.
3. Results and discussion 3.1. Synthesis of nickel hydrazine complexes
2. Experimental 2.1. Materials All chemicals used in this experiment were reagent grade and used without further purification. Nickel chloride hexahydrate (NiCl2 ·6H2 O) was received from Incheon Chemicals. 80 wt.% hydrazine monohydrate (N2 H4 ·H2 O) solution and 50 wt.% sodium hydroxide (NaOH) solution were pur-
It has been known that the reaction between hydrazine and NiCl2 with the molar ratio of hydrazine: Ni2+ = 1:2 or 1:3 in dilute ethanol solution yields bis(hydrazine) nickel(II) chloride (Ni(N2 H4 )2 Cl2 ) or tris(hydrazine) nickel(II) chloride ([Ni(N2 H4 )3 ]Cl2 ), respectively [28–30]. However, according to our preliminary experiments in concentrated aqueous solution, the resulting complexes exist as the mixtures of Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 of which compositions are highly dependent on the reaction
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Fig. 1. XRD patterns and FT-IR spectra of as-prepared nickel hydrazine complexes. The molar ratio of the hydrazine/Ni2+ are (a) 2, (b) 3, and (c) 4.5.
condition, such as the reaction temperature and the molar ratio of hydrazine/Ni2+ . When the reaction temperature was below 60 ◦ C, the product existed as mixture of three complexes, Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 regardless of the molar ratio of hydrazine/Ni2+ . On the other hand, at a reaction temperature of 60 ◦ C, the color change of the solution and the reaction products are quite different with the molar ratio of hydrazine/Ni2+ . The compositions of the products from the reaction molar ratio of hydrazine/Ni2+ = 2, 3, and 4.5 in aqueous solution at 60 ◦ C were determined by XRD and FT-IR (Fig. 1) analyses. When the molar ratio of hydrazine/Ni2+ = 2 and 3, the green color of the nickel chloride solution turned to purple upon the addition of hydrazine monohydrate. As the reaction proceeded, blue and blue–violet precipitates were formed. However, the initial green color of the nickel chloride solution turned to purple and finally pink precipitates were formed in the case of the reaction molar ratio of hydrazine/Ni2+ = 4.5. The XRD patterns in Fig. 1 shows that the reaction products have different compositions with the reaction molar ratio of hydrazine/Ni2+ . The diffraction peaks marked with () and () of the blue precipitates from the reaction molar ratio of hydrazine/Ni2+ = 2 are attributed to the Ni(N2 H4 )2 Cl2 (JCDPS no. 28-065) and [Ni(NH3 )6 ]Cl2 (JCDPS no. 76-1842), respectively, in Fig. 1a. Also, the FT-IR spectra in Fig. 1a shows the vibrational peaks arising from the Ni(N2 H4 )2 Cl2 and [Ni(NH3 )6 ]Cl2 complexes. The stretching vibrational mode (N–N) of hydrazine ligands at the frequency of 980 cm−1 and other vibrational peaks except the peaks in the region of 1458, 1398, 650, and 613 cm−1 are consistent with the values in the Ni(N2 H4 )2 Cl2 complex reported in the earlier literature [28,30,31]. The characteristic peaks, such as the rocking vibration (650 and 613 cm−1 ) and the symmetric distortion of the NH3 ligands in the [Ni(NH3 )6 ]Cl2 complex can be also found in Fig. 1a and are accordant with the reported values [32]. In the case of the reaction molar ratio of hydrazine/Ni2+ = 3, diffraction peaks and FT-IR spectra of the blue–violet precipitates show the reaction products are the mixture
of [Ni(N2 H4 )3 ]Cl2 , Ni(N2 H4 )2 Cl2 , and [Ni(NH3 )6 ]Cl2 as shown in Fig. 1b. In contrast, [Ni(N2 H4 )3 ]Cl2 are formed exclusively (with the negligible amount of Ni(N2 H4 )2 Cl2 and [Ni(NH3 )6 ]Cl2 ) from the reaction with the molar ratio of hydrazine/Ni2+ = 4.5. The diffraction peaks marked with (䊉) and the vibrational peaks including ν(N–N) of hydrazine ligands at the frequency of 974 cm−1 in Fig. 1c are comparable to the values in the [Ni(N2 H4 )3 ]Cl2 complex reported in the earlier literature [28,30]. Because the concentration of hydrazine monohydrate is always higher than that of the nickel chloride during the dropwise addition, the purple [Ni(N2 H4 )3 ]Cl2 complex was initially formed regardless of the molar ratio of hydrazine/Ni2+ . While the subsequent addition of the nickel chloride solution, the disproportionation reaction between initially formed [Ni(N2 H4 )3 ]Cl2 and nickel chloride would induce a formation of Ni(N2 H4 )2 Cl2 complex in the case of lower molar ratio hydrazine/Ni2+ = 2 and 3. In addition to this disproportionation, the decomposition reaction between Ni(N2 H4 )2 Cl2 and free hydrazine would proceed to form [Ni(NH3 )6 ]Cl2 complex. The hexakis(ammonia) nickel chloride complex formation from the decomposition reaction between Ni(N2 H4 )2 Cl2 and free hydrazine was reported by the Guo et al. in their synthesis of [Ni(NH3 )6 ]Cl2 nanotube [33]. As the increasing molar ratio of hydrazine/Ni2+ , [Ni(N2 H4 )3 ]Cl2 complex can be formed by the reaction of Ni(N2 H4 )2 Cl2 or [Ni(NH3 )6 ]Cl2 with additional excess hydrazine. In fact, the XRD patterns and FT-IR spectra in Fig. 1 shows that the portion of the Ni(N2 H4 )2 Cl2 or [Ni(NH3 )6 ]Cl2 complexes decrease and the portion of the [Ni(N2 H4 )3 ]Cl2 complex increases as the molar ratio of hydrazine/Ni2+ is raised from 2 to 4.5. It is notable that the different products are formed with the reaction condition. As the reduction reaction will proceed via these hydrazine complex precursors and the reduction potentials and chemical reactivity of the complexes are supposed to be different, respectively, synthesis of pure nickel hydrazine complex should be required to induce a homogeneous reduction reaction.
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formation of nickel hydroxide during steps I and II was identified by XRD (Fig. 4a and b) and FT-IR (Fig. 5a and b). The rapid color change of the solution upon the addition of the NaOH was due to the formation of nickel hydroxide by the ligand exchange of Cl− ion by OH− as following equation: [Ni(N2 H4 )3 ]Cl2 + 2NaOH → Ni(OH)2 + 3N2 H4 + 2NaCl (2)
Fig. 2. The solution temperature profile with reaction time. [Ni2+ ] = 1.585 M, molar ratio for N2 H4 /Ni2+ = 4.5, and NaOH/Ni2+ = 2.66.
3.2. Formation of Ni particles and their reduction pathway In conventional process, the reduction reaction of nickel ion with the hydrazine as a reducing agent in aqueous solution could be expressed as Eq. (1) 2Ni2+ + N2 H4 + 4OH− → 2Ni + N2 + 4H2 O
(1)
However, in the case of [Ni(N2 H4 )3 ]Cl2 complex, it has not been known the reduction pathway upon the addition of NaOH to the [Ni(N2 H4 )3 ]Cl2 complex solution. The temperature profile of the solution with the reaction time upon the addition of NaOH was represented in Fig. 2. When the NaOH (molar ratio for NaOH/Ni2+ = 2.66) was poured into the [Ni(N2 H4 )3 ]Cl2 complex solution (prepared with molar ratio for N2 H4 /Ni2+ = 4.5), the pink solution turned to sky blue immediately within 30 s (step I). As the reaction proceeded, the sky blue solution initially formed gradually changed to dark cyan, and finally gray blue in 2 min. The
The diffraction peaks marked with () in Fig. 3a and b can be indexed as hexagonal phase of Ni(OH)2 (JCDPS no. 14-0117). In FT-IR spectra shown in Fig. 3a and b, the sharp absorption peaks in the region of around 3600 cm−1 due to non-hydrogen bonded OH groups and the broad absorption band centered around 460 cm−1 due to the Ni O stretching vibration can be assigned to vibrational peaks of Ni(OH)2 . The broad absorption band centered around 3400 and 1600–1650 cm−1 are corresponding to the hydrogen bonded OH stretching and H O H bending vibrational mode of water. In the region of steps III and IV in Fig. 2, the color of the solution gradually changed to gray (step III) and black (step IV) with the appreciable evolution of nitrogen gases due to the reduction of Ni2+ as expressed in Eq. (1). The formation of metallic nickel particle in steps III and IV was evidenced by the diffraction peaks marked with (×) in Fig. 3c which are characteristic XRD patterns of face centered cubic metallic Ni phases (JCDPS no. 04-0850). The XRD patterns of the samples obtained at 8 min. Fig. 3d shows only metallic Ni phases. Also, there are no absorption peaks corresponding to Ni(OH)2 in the FT-IR spectra of this sample (Fig. 3d). Therefore, it can be thought that the reduction reaction was completed in the region IV. The SEM micrographs of the samples at each reaction time are represented in Fig. 4. The nanosized nickel hydroxide particles were formed upon the addition of NaOH as shown in Fig. 4b and c. According to the Scherrer’s equation [34], the mean particle sizes of Ni(OH)2 in Fig. 3a–c are calculated to be around 4 nm. The dissolution of these nanosized nickel hydroxides into the solution, and subsequent reduction by hydrazine would be occurred
Fig. 3. XRD patterns and FT-IR spectra of the samples at each reaction time. The samples were obtained at (a) 30 s, (b) 2 min, (c) 4 min, and (d) 6 min.
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Fig. 4. SEM micrographs of the samples at each reaction time. The samples were obtained at (a) 0 min ([Ni(N2 H4 )3 ]Cl2 complex), (b) 30 s, (c) 2 min, (d) 4 min, (e) 6 min, and (f) 1 h.
in the next steps. Although the metallic nickel particle cannot be distinguished from nickel hydroxide in Fig. 4d, the XRD patterns (Fig. 3c) of this sample shows the formation of metallic nickel phases in step III. Eventually, fine nickel powders as shown in Fig. 4e were formed by way of nucleation and growth steps. The crystallite sizes of metallic Ni powders in Fig. 4d and e are calculated to be about 20 nm. The particle size, the crystallite size, and the size distribution between the samples obtained at 6 min and 1 h are nearly identical. No difference in mean particle size and crystallite size between those two samples indicates that the growth of the nickel particles is completed within 6 min. It is notable that the catalytic decomposition reaction of remaining hydrazine at the surface of nickel particles cannot be ruled out in the (b–d) steps in Fig. 4. The decomposition of hydrazine at the surface of nickel nanoparticle is reported by Wu and Chen [16] as following equation: N2 H5 OH → 2H2 + N2 + H2 O
(3)
In fact, the generation of hydrogen gas in the course of our reduction reaction was evidenced by GC–MS analysis. The surface of nickel might be refreshed by this catalytic decomposition of hydrazine, which can induce surface smoothing of nickel particles. 3.3. The effect of nickel hydrazine complex concentration The effects of nickel hydrazine complex concentration on the formation of nickel powders were investigated. The reduction reactions were carried out with molar ratio for Ni2+ :N2 H4 :NaOH = 1:4.5:2.66 at 60 ◦ C for 1 h. The mean particle size with standard deviation and SEM micrographs are shown in Figs. 5 and 6. It was found that the standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex. Also, the mean particle size increased as the concentration of Ni2+ decreased from 1.585
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in 8 min), the solution temperature gradually rose to 68 ◦ C in 18 min for [Ni2+ ]=1.107 M. In these cases, the formation of smaller number of nuclei in the lower concentration due to the low solution temperature and low nucleation rate would induce growth of particles with broad size distribution. However, in the very low concentration, such as [Ni2+ ] = 0.528 M, the particle size decreased again with broad size distribution. Because the solution temperature maintained at 60 ◦ C over the reaction time, the reduction rate might be too low, and thus considerable amount of unreduced nickel species might be remained in the solution. Eventually, the final particle size decreased and the surface roughness increased due to the contamination of unreduced nickel hydroxide as shown in Fig. 6a. The contamination of unreduced nickel hydroxide in the case of [Ni2+ ] = 0.528 M can be found in XRD patterns in Fig. 6a. Fig. 5. The particle size and size standard deviation of nickel powders from the various nickel hydrazine complex concentration.
to 1.057 M but almost unchanged between [Ni2+ ] = 0.528 and 1.585 M. These results can be explained by the reaction rate on the nucleation. It is known that the average particle size decreases with increasing the nuclei concentration in the solution due to suppressing the growth of particles by the formation of many nuclei in the period of nucleation step. Additionally, when most nuclei are formed at the same time and grow at the same rate, the size distribution of resulting particles will be in the narrow range. In this work, the induction time for reduction decreased from 10 to 4 min with increasing the concentration of Ni2+ from 1.107 to 1.585 M. In comparison with the rapid solution temperature rising due to the exothermic redox reaction for [Ni2+ ] = 1.585 M (78 ◦ C
3.4. The effect of hydrazine concentration and reaction temperature To find the effect of the hydrazine concentration on the morphology of resultant nickel powders, the reduction reactions were carried out at 60 ◦ C for 1 h with the molar ratio for N2 H4 /Ni2+ = 2, 6, and 8 while maintaining NaOH/Ni2+ = 2.66. The SEM micrographs of resulting nickel powders are represented in Fig. 7. As the amount of hydrazine increased, the surface smoothness of the particles was improved significantly. The smoothening of the surface roughness with increasing the amount of hydrazine are probably due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle as mentioned in Section 3.2. However, the particle size was not changed apparently with increasing the amount of hydrazine as shown in Fig. 7. When the molar ratio of hydrazine to nickel chloride and the
Fig. 6. SEM micrographs and XRD patterns of nickel powders prepared from (a) [Ni2+ ] = 0.528 M, (b) [Ni2+ ] = 1.057 M, and (c) [Ni2+ ] = 1.585 M.
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Fig. 7. SEM micrographs of nickel powder from various hydrazine concentrations. [Ni2+ ] = 1.585 M, NaOH/Ni2+ molar ratio is 2.66. N2 H4 /Ni2+ molar ratio is (a) 2, (b) 6, and (c) 8.
Fig. 8. SEM micrographs of nickel powders from reaction temperature for (a) 55 ◦ C (Dmean = 378 ± 98.3 nm), (b) 52 ◦ C (Dmean = 304 ± 73.9 nm) (c) 45 ◦ C (Dmean = 201 ± 32.8 nm), and (d) 40 ◦ C (Dmean = 148 ± 19.7 nm). [Ni2+ ] = 1.585 M and reaction molar ratio for Ni2+ :N2 H4 :NaOH = 1:6.4:5.1.
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nickel ion concentration was large enough, the nucleation rate cannot be further raised, and the number of nuclei held almost constant with further increasing of hydrazine concentration. Therefore, the size of the resulting particles was kept at nearly constant value with the molar ratio of N2 H4 /Ni2+ = 2, 6, and 8. However, the concentration of hydrazine was too high (molar ratio for N2 H4 /Ni2+ = 8), the non-spherical in shape and hard agglomerated powders with broad size distribution were observed as in Fig. 7c. The effects of the reaction temperature on the resulting Ni powder were also investigated. As the reaction temperature was raised, the mean particle size of the Ni powder increased. This is probably due to the increasing growth rate at the higher reaction temperature. Eventually, 150, 200, 300, and 400 nm grade fine nickel powders with narrow size distribution and low surface roughness could be prepared with the different reaction temperatures as shown in Fig. 8.
4. Conclusion Fine nickel powders with narrow size distribution have been prepared from the reduction of hydrazine complexes in aqueous solution at 60 ◦ C. It was found that the phase and composition of hydrazine complex used as a precursor in this work were highly dependent on the synthetic condition. The pure hydrazine complexes, [Ni(N2 H4 )3 ]Cl2 were prepared with the molar ratio of N2 H4 /Ni2+ = 4.5, while a mixture of complexes, such as Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 were formed with N2 H4 /Ni2+ < 4.5. It was found that the reduction of Ni2+ to metallic Ni powder proceeded via the formation of nickel hydroxide upon the addition of NaOH to nickel hydrazine complex solution. The resulting nickel hydroxide was reduced by hydrazine liberated from reaction between the nickel hydrazine complex and NaOH. The standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex while the mean particle size increased. However, when the concentration of nickel hydrazine complex was too low, the particle size decreased again with the broad size distribution due to the slow reduction rate. As the amount of hydrazine increased, the surface roughness of the particles was improved significantly due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle. The particle size was not changed apparently with increasing amount of hydrazine because the high concentration of Ni2+ ion in the reaction condition of this work made the number of nuclei to hold constant. The advantages of this work for preparing the metallic powders lie in the narrow size distribution of the resulting powders, the high yield, and mild reaction conditions. Therefore, it offers an attractive and convenient process for mass production.
Acknowledgments We thank Dr. Hyun Chul Lee and Yong Gyun Lee (Samsung Advanced Institute of Technology) for fruitful discussions.
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Preparation of fine Ni powders from nickel hydrazine complex Jung Woo Park a,∗ , Eun H. Chae a , Sang H. Kim a , Jong Ho Lee a , Jeong Wook Kim a , Seon Mi Yoon b , Jae-Young Choi b a b
Materials R&D Group, Passive Component Division, Samsung Electro-Mechanics Co., Ltd., 314, Suwon 443-743, Republic of Korea Materials & Devices Research Center, Samsung Advanced Institute of Technology, P.O. Box 111, Suwon 440-600, Republic of Korea Received 6 May 2005; received in revised form 9 August 2005; accepted 12 August 2005
Abstract Fine nickel powders with narrow size distribution have been prepared from the reduction of nickel hydrazine complexes in aqueous solution. The pure nickel hydrazine complexes, [Ni(N2 H4 )3 ]Cl2 were prepared with the molar ratio of N2 H4 /Ni2+ = 4.5, while a mixture of complexes, such as Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 were formed with N2 H4 /Ni2+ < 4.5. By the X-ray diffraction (XRD), FT-IR, and scanning electron microscopy (SEM) analyses, it was found that the reduction of Ni2+ to metallic Ni powder proceeded via the formation of nickel hydroxide which was reduced by hydrazine liberated from the ligand exchange reaction between the nickel hydrazine complex and NaOH. The standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex while the mean particle size increased. As the amount of hydrazine increased, the surface roughness of the particles was improved significantly due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle. It was found that average particle size could be controlled from 150 to 380 nm by adjusting the reaction molar ratio and temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: Nickel hydrazine complex; Fine nickel powder; Chemical reduction
1. Introduction There have been extensive studies on the synthesis of fine nickel powders in the past two decades due to their potential applications in optical, electronic, catalytic, magnetic materials, and so on [1–3]. Moreover, they have been a great deal of attention as an internal electrode in multi-layer ceramic capacitors (MLCCs). MLCCs are one of the most widely used passive components in electronics, such as computers, wire and wireless communication devices, etc. [4,5]. The continuing development of advanced electronic devices for high performance with miniaturized dimensions has lead to the improvements of MLCCs with larger capacitance in a smaller volume. High volumetric efficient MLCCs (large capacitance in small volume) can be achieved by control of the thickness and the lamination number of dielectric layers [6]. Especially, stacking a larger number of thinner dielectric ∗
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0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.028
layers and thinner electrode layers is inevitable to achieve small-size capacitors with larger capacitance. Under these circumstances, the increasing amount of internal metal electrode has lead to the replacement of Pd or Pd/Ag by Ni as an electrode material due to its lower cost and lower resistivity [7–10]. However, it is more likely to cause not only a shortcircuit between the internal electrode layers but also structural defects, such as cracks and delamination in the highly integrated MLCCs with thinner dielectric and electrode layers [11]. Therefore, to circumvent these problems in the fabrication of the thinner internal electrodes, it is essential for the use of smaller size and non-agglomerated with the narrow size distribution Ni powder. There have been developed various kinds of synthetic methods, such as chemical vapor deposition, sonochemical decomposition, microwave-hydrothermal methods, polyol process, and electrochemical reduction controlled chemical reduction for the preparation of Ni fine powders [12–17]. Among those methods, the chemical reduction of nickel salts, such as NiCl2 and NiSO4 by a strong reducing agent in
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aqueous solution has been researched intensively due to not only economical and mass-productive aspects but also technological aspects, such as better structural control on the microscopic level, low reaction temperature, and the simple procedure [18–23]. In this chemical reduction method, hydrazine and alkali metal borohydride have been generally used as an external reducing agent. But in the case of alkali metal borohydride, borohydride ions are known to reduce metal cations to metal borides, particularly in aqueous systems [24–26]. On the other hand, hydrazine has more widely been used as reducing agent for preparation of metal nanoparticles because pH and temperature dependent reducing ability of hydrazine make controllability of reduction rate easy. Hydrazine is basic and the chemically active free ion is hydrazium cation, N2 H5 + . The standard reduction potential for the hydrazinium ion, N2 H5 + is −0.23 V (N2 + 5H+ + 4e− = N2 H5 + ) in acidic solution. But in basic solution, the standard reduction potential of hydrazine N2 H4 is −1.16 V (N2 H4 + 4OH− = N2 + 4H2 O + 4e− ) [19,27]. On the other hand, standard reduction potentials of Ni2+ and Ni(OH)2 are −0.25 and −0.72 V for Ni2+ + 2e− = Ni and Ni(OH)2 + 2e− = Ni + 2OH− , respectively. Therefore, it is necessary to maintain enough alkalinity of solution and avoid precipitation of agglomerated nickel hydroxide during the reduction process for inducing an efficient control of reduction rate, size, and morphology of metal particle. In this respect, nickel hydrazine complex is attractive starting material for preparation of fine nickel powder by chemical reduction method. The stabilization of nickel cation by hydrazine ligand may retard and prevent an abrupt formation and agglomeration of nickel hydroxide. Also, highly uniform distribution of hydrazine molecule around the nickel cation by liberation in high pH can induce homogeneous reduction process. Although considerable efforts have been made to study the change in the size and shape, there are no reports in the literature regarding the preparation of fine nickel powder by the controlled reduction of nickel hydrazine complexes in aqueous solution. In this paper, we report on the preparation of fine nickel powders from the nickel hydrazine complex and their reduction pathway. We have also investigated on the effect of nickel hydrazine complexes, reaction molar ratio and reaction temperature on the phase composition, and morphology of nickel powders.
chased from Daejung chemicals & Metals. The water used throughout this work was deionized with Human Power I+ water purification system (Human Co., Korea). 2.2. Synthesis of nickel hydrazine complex Typically, an appropriate amount of nickel chloride solution dissolved in deionized water was slowly added dropwise over 30 min to an appropriate amount of hydrazine monohydrate at 60 ◦ C. During the addition, blue, blue–violet, or pink precipitates depending on the reaction molar ratio of N2 H4 /Ni2+ were formed, and the resulting mixtures were stirred for 1–2 h. The crude products obtained by filtration were washed three times with 30 mL of deionized water and finally dried at 30 ◦ C for 24 h in a vacuum dry oven. The products were characterized by X-ray diffraction (XRD) and FT-IR. 2.3. Preparation of fine nickel powder To a nickel hydrazine complex solution prepared by above procedure, an appropriate amount of NaOH was poured at corresponding reaction temperature and the resulting mixture was stirred for 1 h. As the reduction reaction proceeded, the blue, blue–violet, or pink solution turned to black within 15 min, indicating a formation of metallic nickel. The resulting black slurry was carefully decanted and washed repeatedly with deionized water to remove by-products, such as NaCl and unreacted hydrazine or NaOH. The metallic powder, thus obtained was filtered and dried in a vacuum dry oven at 80 ◦ C for 24 h. 2.4. Characterization The nickel hydrazine complexes and nickel powders were characterized by X-ray diffraction (Rigaku, RINT2200HF+ ) using Cu K␣ radiation with graphite monochromator. The Ni particle size and morphology analyses were performed using scanning electron microscopy (SEM, JEOL, JSM-6700F with accelerating voltage of 10 kV). FT-IR spectra were recorded with a Nicolet Magna-IR 760 spectrometer.
3. Results and discussion 3.1. Synthesis of nickel hydrazine complexes
2. Experimental 2.1. Materials All chemicals used in this experiment were reagent grade and used without further purification. Nickel chloride hexahydrate (NiCl2 ·6H2 O) was received from Incheon Chemicals. 80 wt.% hydrazine monohydrate (N2 H4 ·H2 O) solution and 50 wt.% sodium hydroxide (NaOH) solution were pur-
It has been known that the reaction between hydrazine and NiCl2 with the molar ratio of hydrazine: Ni2+ = 1:2 or 1:3 in dilute ethanol solution yields bis(hydrazine) nickel(II) chloride (Ni(N2 H4 )2 Cl2 ) or tris(hydrazine) nickel(II) chloride ([Ni(N2 H4 )3 ]Cl2 ), respectively [28–30]. However, according to our preliminary experiments in concentrated aqueous solution, the resulting complexes exist as the mixtures of Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 of which compositions are highly dependent on the reaction
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Fig. 1. XRD patterns and FT-IR spectra of as-prepared nickel hydrazine complexes. The molar ratio of the hydrazine/Ni2+ are (a) 2, (b) 3, and (c) 4.5.
condition, such as the reaction temperature and the molar ratio of hydrazine/Ni2+ . When the reaction temperature was below 60 ◦ C, the product existed as mixture of three complexes, Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 regardless of the molar ratio of hydrazine/Ni2+ . On the other hand, at a reaction temperature of 60 ◦ C, the color change of the solution and the reaction products are quite different with the molar ratio of hydrazine/Ni2+ . The compositions of the products from the reaction molar ratio of hydrazine/Ni2+ = 2, 3, and 4.5 in aqueous solution at 60 ◦ C were determined by XRD and FT-IR (Fig. 1) analyses. When the molar ratio of hydrazine/Ni2+ = 2 and 3, the green color of the nickel chloride solution turned to purple upon the addition of hydrazine monohydrate. As the reaction proceeded, blue and blue–violet precipitates were formed. However, the initial green color of the nickel chloride solution turned to purple and finally pink precipitates were formed in the case of the reaction molar ratio of hydrazine/Ni2+ = 4.5. The XRD patterns in Fig. 1 shows that the reaction products have different compositions with the reaction molar ratio of hydrazine/Ni2+ . The diffraction peaks marked with () and () of the blue precipitates from the reaction molar ratio of hydrazine/Ni2+ = 2 are attributed to the Ni(N2 H4 )2 Cl2 (JCDPS no. 28-065) and [Ni(NH3 )6 ]Cl2 (JCDPS no. 76-1842), respectively, in Fig. 1a. Also, the FT-IR spectra in Fig. 1a shows the vibrational peaks arising from the Ni(N2 H4 )2 Cl2 and [Ni(NH3 )6 ]Cl2 complexes. The stretching vibrational mode (N–N) of hydrazine ligands at the frequency of 980 cm−1 and other vibrational peaks except the peaks in the region of 1458, 1398, 650, and 613 cm−1 are consistent with the values in the Ni(N2 H4 )2 Cl2 complex reported in the earlier literature [28,30,31]. The characteristic peaks, such as the rocking vibration (650 and 613 cm−1 ) and the symmetric distortion of the NH3 ligands in the [Ni(NH3 )6 ]Cl2 complex can be also found in Fig. 1a and are accordant with the reported values [32]. In the case of the reaction molar ratio of hydrazine/Ni2+ = 3, diffraction peaks and FT-IR spectra of the blue–violet precipitates show the reaction products are the mixture
of [Ni(N2 H4 )3 ]Cl2 , Ni(N2 H4 )2 Cl2 , and [Ni(NH3 )6 ]Cl2 as shown in Fig. 1b. In contrast, [Ni(N2 H4 )3 ]Cl2 are formed exclusively (with the negligible amount of Ni(N2 H4 )2 Cl2 and [Ni(NH3 )6 ]Cl2 ) from the reaction with the molar ratio of hydrazine/Ni2+ = 4.5. The diffraction peaks marked with (䊉) and the vibrational peaks including ν(N–N) of hydrazine ligands at the frequency of 974 cm−1 in Fig. 1c are comparable to the values in the [Ni(N2 H4 )3 ]Cl2 complex reported in the earlier literature [28,30]. Because the concentration of hydrazine monohydrate is always higher than that of the nickel chloride during the dropwise addition, the purple [Ni(N2 H4 )3 ]Cl2 complex was initially formed regardless of the molar ratio of hydrazine/Ni2+ . While the subsequent addition of the nickel chloride solution, the disproportionation reaction between initially formed [Ni(N2 H4 )3 ]Cl2 and nickel chloride would induce a formation of Ni(N2 H4 )2 Cl2 complex in the case of lower molar ratio hydrazine/Ni2+ = 2 and 3. In addition to this disproportionation, the decomposition reaction between Ni(N2 H4 )2 Cl2 and free hydrazine would proceed to form [Ni(NH3 )6 ]Cl2 complex. The hexakis(ammonia) nickel chloride complex formation from the decomposition reaction between Ni(N2 H4 )2 Cl2 and free hydrazine was reported by the Guo et al. in their synthesis of [Ni(NH3 )6 ]Cl2 nanotube [33]. As the increasing molar ratio of hydrazine/Ni2+ , [Ni(N2 H4 )3 ]Cl2 complex can be formed by the reaction of Ni(N2 H4 )2 Cl2 or [Ni(NH3 )6 ]Cl2 with additional excess hydrazine. In fact, the XRD patterns and FT-IR spectra in Fig. 1 shows that the portion of the Ni(N2 H4 )2 Cl2 or [Ni(NH3 )6 ]Cl2 complexes decrease and the portion of the [Ni(N2 H4 )3 ]Cl2 complex increases as the molar ratio of hydrazine/Ni2+ is raised from 2 to 4.5. It is notable that the different products are formed with the reaction condition. As the reduction reaction will proceed via these hydrazine complex precursors and the reduction potentials and chemical reactivity of the complexes are supposed to be different, respectively, synthesis of pure nickel hydrazine complex should be required to induce a homogeneous reduction reaction.
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formation of nickel hydroxide during steps I and II was identified by XRD (Fig. 4a and b) and FT-IR (Fig. 5a and b). The rapid color change of the solution upon the addition of the NaOH was due to the formation of nickel hydroxide by the ligand exchange of Cl− ion by OH− as following equation: [Ni(N2 H4 )3 ]Cl2 + 2NaOH → Ni(OH)2 + 3N2 H4 + 2NaCl (2)
Fig. 2. The solution temperature profile with reaction time. [Ni2+ ] = 1.585 M, molar ratio for N2 H4 /Ni2+ = 4.5, and NaOH/Ni2+ = 2.66.
3.2. Formation of Ni particles and their reduction pathway In conventional process, the reduction reaction of nickel ion with the hydrazine as a reducing agent in aqueous solution could be expressed as Eq. (1) 2Ni2+ + N2 H4 + 4OH− → 2Ni + N2 + 4H2 O
(1)
However, in the case of [Ni(N2 H4 )3 ]Cl2 complex, it has not been known the reduction pathway upon the addition of NaOH to the [Ni(N2 H4 )3 ]Cl2 complex solution. The temperature profile of the solution with the reaction time upon the addition of NaOH was represented in Fig. 2. When the NaOH (molar ratio for NaOH/Ni2+ = 2.66) was poured into the [Ni(N2 H4 )3 ]Cl2 complex solution (prepared with molar ratio for N2 H4 /Ni2+ = 4.5), the pink solution turned to sky blue immediately within 30 s (step I). As the reaction proceeded, the sky blue solution initially formed gradually changed to dark cyan, and finally gray blue in 2 min. The
The diffraction peaks marked with () in Fig. 3a and b can be indexed as hexagonal phase of Ni(OH)2 (JCDPS no. 14-0117). In FT-IR spectra shown in Fig. 3a and b, the sharp absorption peaks in the region of around 3600 cm−1 due to non-hydrogen bonded OH groups and the broad absorption band centered around 460 cm−1 due to the Ni O stretching vibration can be assigned to vibrational peaks of Ni(OH)2 . The broad absorption band centered around 3400 and 1600–1650 cm−1 are corresponding to the hydrogen bonded OH stretching and H O H bending vibrational mode of water. In the region of steps III and IV in Fig. 2, the color of the solution gradually changed to gray (step III) and black (step IV) with the appreciable evolution of nitrogen gases due to the reduction of Ni2+ as expressed in Eq. (1). The formation of metallic nickel particle in steps III and IV was evidenced by the diffraction peaks marked with (×) in Fig. 3c which are characteristic XRD patterns of face centered cubic metallic Ni phases (JCDPS no. 04-0850). The XRD patterns of the samples obtained at 8 min. Fig. 3d shows only metallic Ni phases. Also, there are no absorption peaks corresponding to Ni(OH)2 in the FT-IR spectra of this sample (Fig. 3d). Therefore, it can be thought that the reduction reaction was completed in the region IV. The SEM micrographs of the samples at each reaction time are represented in Fig. 4. The nanosized nickel hydroxide particles were formed upon the addition of NaOH as shown in Fig. 4b and c. According to the Scherrer’s equation [34], the mean particle sizes of Ni(OH)2 in Fig. 3a–c are calculated to be around 4 nm. The dissolution of these nanosized nickel hydroxides into the solution, and subsequent reduction by hydrazine would be occurred
Fig. 3. XRD patterns and FT-IR spectra of the samples at each reaction time. The samples were obtained at (a) 30 s, (b) 2 min, (c) 4 min, and (d) 6 min.
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Fig. 4. SEM micrographs of the samples at each reaction time. The samples were obtained at (a) 0 min ([Ni(N2 H4 )3 ]Cl2 complex), (b) 30 s, (c) 2 min, (d) 4 min, (e) 6 min, and (f) 1 h.
in the next steps. Although the metallic nickel particle cannot be distinguished from nickel hydroxide in Fig. 4d, the XRD patterns (Fig. 3c) of this sample shows the formation of metallic nickel phases in step III. Eventually, fine nickel powders as shown in Fig. 4e were formed by way of nucleation and growth steps. The crystallite sizes of metallic Ni powders in Fig. 4d and e are calculated to be about 20 nm. The particle size, the crystallite size, and the size distribution between the samples obtained at 6 min and 1 h are nearly identical. No difference in mean particle size and crystallite size between those two samples indicates that the growth of the nickel particles is completed within 6 min. It is notable that the catalytic decomposition reaction of remaining hydrazine at the surface of nickel particles cannot be ruled out in the (b–d) steps in Fig. 4. The decomposition of hydrazine at the surface of nickel nanoparticle is reported by Wu and Chen [16] as following equation: N2 H5 OH → 2H2 + N2 + H2 O
(3)
In fact, the generation of hydrogen gas in the course of our reduction reaction was evidenced by GC–MS analysis. The surface of nickel might be refreshed by this catalytic decomposition of hydrazine, which can induce surface smoothing of nickel particles. 3.3. The effect of nickel hydrazine complex concentration The effects of nickel hydrazine complex concentration on the formation of nickel powders were investigated. The reduction reactions were carried out with molar ratio for Ni2+ :N2 H4 :NaOH = 1:4.5:2.66 at 60 ◦ C for 1 h. The mean particle size with standard deviation and SEM micrographs are shown in Figs. 5 and 6. It was found that the standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex. Also, the mean particle size increased as the concentration of Ni2+ decreased from 1.585
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in 8 min), the solution temperature gradually rose to 68 ◦ C in 18 min for [Ni2+ ]=1.107 M. In these cases, the formation of smaller number of nuclei in the lower concentration due to the low solution temperature and low nucleation rate would induce growth of particles with broad size distribution. However, in the very low concentration, such as [Ni2+ ] = 0.528 M, the particle size decreased again with broad size distribution. Because the solution temperature maintained at 60 ◦ C over the reaction time, the reduction rate might be too low, and thus considerable amount of unreduced nickel species might be remained in the solution. Eventually, the final particle size decreased and the surface roughness increased due to the contamination of unreduced nickel hydroxide as shown in Fig. 6a. The contamination of unreduced nickel hydroxide in the case of [Ni2+ ] = 0.528 M can be found in XRD patterns in Fig. 6a. Fig. 5. The particle size and size standard deviation of nickel powders from the various nickel hydrazine complex concentration.
to 1.057 M but almost unchanged between [Ni2+ ] = 0.528 and 1.585 M. These results can be explained by the reaction rate on the nucleation. It is known that the average particle size decreases with increasing the nuclei concentration in the solution due to suppressing the growth of particles by the formation of many nuclei in the period of nucleation step. Additionally, when most nuclei are formed at the same time and grow at the same rate, the size distribution of resulting particles will be in the narrow range. In this work, the induction time for reduction decreased from 10 to 4 min with increasing the concentration of Ni2+ from 1.107 to 1.585 M. In comparison with the rapid solution temperature rising due to the exothermic redox reaction for [Ni2+ ] = 1.585 M (78 ◦ C
3.4. The effect of hydrazine concentration and reaction temperature To find the effect of the hydrazine concentration on the morphology of resultant nickel powders, the reduction reactions were carried out at 60 ◦ C for 1 h with the molar ratio for N2 H4 /Ni2+ = 2, 6, and 8 while maintaining NaOH/Ni2+ = 2.66. The SEM micrographs of resulting nickel powders are represented in Fig. 7. As the amount of hydrazine increased, the surface smoothness of the particles was improved significantly. The smoothening of the surface roughness with increasing the amount of hydrazine are probably due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle as mentioned in Section 3.2. However, the particle size was not changed apparently with increasing the amount of hydrazine as shown in Fig. 7. When the molar ratio of hydrazine to nickel chloride and the
Fig. 6. SEM micrographs and XRD patterns of nickel powders prepared from (a) [Ni2+ ] = 0.528 M, (b) [Ni2+ ] = 1.057 M, and (c) [Ni2+ ] = 1.585 M.
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Fig. 7. SEM micrographs of nickel powder from various hydrazine concentrations. [Ni2+ ] = 1.585 M, NaOH/Ni2+ molar ratio is 2.66. N2 H4 /Ni2+ molar ratio is (a) 2, (b) 6, and (c) 8.
Fig. 8. SEM micrographs of nickel powders from reaction temperature for (a) 55 ◦ C (Dmean = 378 ± 98.3 nm), (b) 52 ◦ C (Dmean = 304 ± 73.9 nm) (c) 45 ◦ C (Dmean = 201 ± 32.8 nm), and (d) 40 ◦ C (Dmean = 148 ± 19.7 nm). [Ni2+ ] = 1.585 M and reaction molar ratio for Ni2+ :N2 H4 :NaOH = 1:6.4:5.1.
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nickel ion concentration was large enough, the nucleation rate cannot be further raised, and the number of nuclei held almost constant with further increasing of hydrazine concentration. Therefore, the size of the resulting particles was kept at nearly constant value with the molar ratio of N2 H4 /Ni2+ = 2, 6, and 8. However, the concentration of hydrazine was too high (molar ratio for N2 H4 /Ni2+ = 8), the non-spherical in shape and hard agglomerated powders with broad size distribution were observed as in Fig. 7c. The effects of the reaction temperature on the resulting Ni powder were also investigated. As the reaction temperature was raised, the mean particle size of the Ni powder increased. This is probably due to the increasing growth rate at the higher reaction temperature. Eventually, 150, 200, 300, and 400 nm grade fine nickel powders with narrow size distribution and low surface roughness could be prepared with the different reaction temperatures as shown in Fig. 8.
4. Conclusion Fine nickel powders with narrow size distribution have been prepared from the reduction of hydrazine complexes in aqueous solution at 60 ◦ C. It was found that the phase and composition of hydrazine complex used as a precursor in this work were highly dependent on the synthetic condition. The pure hydrazine complexes, [Ni(N2 H4 )3 ]Cl2 were prepared with the molar ratio of N2 H4 /Ni2+ = 4.5, while a mixture of complexes, such as Ni(N2 H4 )2 Cl2 , [Ni(N2 H4 )3 ]Cl2 , and [Ni(NH3 )6 ]Cl2 were formed with N2 H4 /Ni2+ < 4.5. It was found that the reduction of Ni2+ to metallic Ni powder proceeded via the formation of nickel hydroxide upon the addition of NaOH to nickel hydrazine complex solution. The resulting nickel hydroxide was reduced by hydrazine liberated from reaction between the nickel hydrazine complex and NaOH. The standard deviation of the particle size decreased with the decreasing molar concentration of nickel hydrazine complex while the mean particle size increased. However, when the concentration of nickel hydrazine complex was too low, the particle size decreased again with the broad size distribution due to the slow reduction rate. As the amount of hydrazine increased, the surface roughness of the particles was improved significantly due to the catalytic decomposition of the excess hydrazine at the surface of the nickel particle. The particle size was not changed apparently with increasing amount of hydrazine because the high concentration of Ni2+ ion in the reaction condition of this work made the number of nuclei to hold constant. The advantages of this work for preparing the metallic powders lie in the narrow size distribution of the resulting powders, the high yield, and mild reaction conditions. Therefore, it offers an attractive and convenient process for mass production.
Acknowledgments We thank Dr. Hyun Chul Lee and Yong Gyun Lee (Samsung Advanced Institute of Technology) for fruitful discussions.
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