Materials Chemistry and Physics 78 (2003) 645–649

A simple synthesis of metallic Ni and Ni–Co alloy fine powdersfrom a mixed-metal acetate precursor

Syukri∗, Takayuki Ban, Yutaka Ohya, Yasutaka TakahashiDepartment of Applied Chemistry, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan

Received 9 November 2001; received in revised form 22 April 2002; accepted 23 April 2002

Abstract

A new process for the synthesis of metallic Ni, and Ni–Co alloy nanoparticles at a relatively low temperature is reported. The processconsists of heating a precursor containing a methyl hydrazine complex of the respective metal ions in nitrogen atmosphere at temperatureas low as 400◦C. The metallic Ni and Ni–Co alloy powders are characterized by scanning electron microscopy, powder X-ray diffrac-tion (XRD), differential thermal analysis (DTA), and thermogravimetric (TG) analysis. The crystalline nanoparticles showed a narrowdistribution in the particle size and homogeneous spherical form with mean diameter around 0.40�m.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Metal acetate; Nanoparticles; Nickel

1. Introduction

Nanosized metallic and alloy powders of uniform shapeand high purity are increasingly required for specific uses inmany technological areas, especially on the preparation ofelectronic materials for examples magnetic recording media[1], commercial batteries[2] and the formation of catalysts[3–5]. A variety of techniques have been used to producesuch particles of pure metal and alloy in different formssuch as sonochemical[6,7], microemulsion[8], chemicalreduction[9–12], gas-evaporation[13,14], and ball milling[15,16].

Because all the above applications require fine metallicpowders, an alternative inexpensive and simple method forproducing such powders is of considerable interest. Onepossible approach is to prepare the powders by sol–gel pro-cess. It is well known that sol–gel process has been widelyaccepted as an important process for the preparation ofnanoparticles. Potential advantages of the sol–gel processare controlled size and shape, molecular scale homogeneity,and low processing temperatures.

Nickel was chosen due to our interest in its magneticproperties and its industrial importance as a catalyst and theapplication to nickel paste. To our knowledge the produc-tion of fine particles of Ni and Co–Ni binary alloy by thesol–gel process has not been reported in the literatures. Chen

∗ Corresponding author.E-mail address: f3113005@guedu.cc.gifu-u.ac.jp (Syukri).

and co-workers[8] have synthesised nickel nanoparticles inwater in oil (w/o) microemulsion by the reduction of nickelchloride with hydrazine. Prozorov and co-workers[6,17]have reported the preparation of Ni and Co–Ni alloy bysonochemical decomposition of volatile organic precursors.They found that varying the initial precursor concentrationin solution could control the composition of Ni–Co alloyparticles. Toneguzzo et al.[18] reported the preparation ofsub-micrometer size crystalline Ni, Co–Ni and Fe–Co–Nipowders by precipitation in liquid polyol.

On the other hand, the chemical solution or spray pyroly-sis has been also used as another process of chemical methodfor preparation of Ni particle[19]. However, this systemmust provide an equipment system that accomplishes manyaspects such as atomizer, etc. In addition, to get metallicpowders by this process, hazardous hydrogen is the mostcommonly used as reducing agent.

The purpose of this study is to describe a new simpleway for the preparation of metallic Ni and Ni–Co binaryalloy fine powders by sol–gel combustion process usinginexpensive metal resources (metal acetates) and methylhydrazine as an additive and reducing agent. Pure nickeland binary alloy nickel–cobalt particles have been obtainedand characterized herein.

2. Experimental

Appropriate amounts of nickel acetate tetrahydrate[Ni(OAc)2·4H2O] and cobalt acetate tetrahydrate [Co(OAc)2

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0254-0584(02)00185-2

646 Syukri et al. / Materials Chemistry and Physics 78 (2003) 645–649

·4H2O] and methyl hydrazine (MH) were mixed in2-propanol solution at room temperature. The molar ratio ofMH to metal acetate was one and the concentration of metalacetate ranged of 0.2 to 1.0 mol/l. The mixture was stirredat room temperature for a day till the complete dissolutionof the metal acetates. The color of these solutions changedto blue or deep blue, indicating the substitution of aqualigands by MH. To obtain the dried gel powders, the ho-mogeneous solution was dried in the air atmosphere around50◦C or the solvent was evaporated by using a rotary evap-orator. Elemental analysis of this powder gave the results:H = 6.37%, C= 25.72%, N= 11.63%, Ni = 23.96% thatrelatively comparable to the composition of a compound,Ni(CH3COO)2(H2O)3(MH) [H = 6.58%, C = 24.52%,N = 11.44%, Ni = 23.96%]. This complex powder hashigh solubility in simple alcohols, making them suitablefor many applications. The complex powders were furtherheat-treated at various temperatures for 1 h under flowingnitrogen atmosphere (50–100 ml/min) to obtain pure metalor metal alloy fine powders.

The crystallographic transformation of the powders wasmonitored by X-ray diffraction (XRD) with graphite-mono-chromated Cu K� radiation at a scan rate of 2◦/min. In or-der to obtain the exact diffraction angles, Si powders whichwere derived from a single crystal were used as the internalstandard. Thermogravimetric and differential thermal anal-yses (TG-DTA) were performed on a Shimadzu DT-40 ana-lyzer system under air or nitrogen atmosphere at heating rateof range 2◦C to 10◦C/min. The morphology and mean par-ticle size were determined by scanning electron microscopy(SEM) (Phillip, XL 30).

3. Results and discussion

3.1. Ni powders

3.1.1. X-ray diffraction studiesIt is well known that there are two-nickel polymorphs.

The hexagonal close packed (hcp) form is stable at low tem-peratures below 300◦C, while the face centered cubic (fcc)form is the stable structure at higher temperature. The typi-cal nickel fine powders prepared in this work were found tohave both hcp and fcc phases as observed from XRD patternshown inFig. 1. At firing temperatures of 300◦C, the pow-ders were found to be crystallized into hcp nickel phase. De-tailed analysis of the XRD peak positions showed that latticeparameter ofa = 0.2649 nm andc = 0.4333 nm withc/aratio was 1.636, which was very similar to that reported inthe literature of hcp nickel[20]. An average crystallite sizeof around 10 nm was estimated from the (1 0 1) peak width.XPS studies were made to determine the chemical compo-sition using Ni films which were prepared by dip-coatingat 300◦C (hcp phase) and 400◦C (fcc phase) on glass sub-strates. The Ni 2p region consists of two peaks at binding en-ergies of 853 and 870 eV for both phases, suggesting that the

Fig. 1. XRD patterns of nickel powders fired at (a) 300◦C, (b) 400◦C,and (c) 500◦C. The symbols (�) and (�) denote hcp and fcc phase,respectively.

chemical shifts of Ni 2p of both films are the same. The car-bon contamination was found only 1–3%. We also analyzed,by XRD, Ni powders prepared by decomposition of nickeloxalate at 300◦C, because the oxalate can deposit very purenickel. It gave no hcp phase but only fcc phase with crystal-lite size of 36 nm. These findings strongly suggest that hcpformation must be due to the presence of carbon atom.

When heat-treated at higher temperature of 400◦C, threetypical peaks of fcc nickel noticeable by indices〈1 1 1〉,〈2 0 0〉 and〈2 2 0〉 were observed, indicating that the resultantparticles have exclusively transformed to fcc phase. The peakintensity increased with increasing in firing temperatures dueto the enhanced grain growth. This stable fcc phase habit isconserved up to the highest temperature (in this experimentof 900◦C).

In order to understand the phase transformation of thenickel powders the effect of firing time at the fixed tem-perature (300◦C) was examined as shown inFig. 2. Thisfigure indicates that when heated at 300◦C for 5 h the nickel

Fig. 2. XRD patterns of nickel powders heat-treated at 300◦C for (a)0.5 h, (b) 2 h, (c) 5 h, (d) 12 h, and (e) 24 h.

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Fig. 3. DTA curves of the dried precursor gels at different atmosphere.

powder started partly transformed to the fcc phase and itsratio increased with increasing firing time. After fired fora day the transition of hcp to fcc phase was complete. Atpresent the transformation mechanism of Ni fine powdersremains unclear.

3.1.2. TG-DTA studiesThe DTA of the as prepared powder (dry gel) in vari-

ous atmospheres is shown inFig. 3. The thermogram (a)shows the a weak endothermic around 100◦C followed bya very strong exothermic peak around 400◦C. This exother-mic peak suggests the combustion of organic residual and atthe same time the nucleation and growth of the crystallitesof NiO. In contrast, when the powders were treated undernitrogen atmosphere (b), the thermogram shows a board en-dothermic peak in the temperature range of 270–380◦C. Itsuggests the decomposition of organic complex residue andreduction of Ni2+ leading to a conversion to the hexagonalphase of Ni0, which was agreement with the XRD patternshown inFig. 4.

In order to understand the stability of the resultant Nipowders in air they were also characterized by using thermo-

Fig. 4. XRD patterns of powders obtained from heat-treated of TGA (onFig. 3) (a) nickel oxide and (b) pure nickel.

Fig. 5. TGA curve of the nickel powder.

gravimetry (TG). As shown inFig. 5, the pure nickel pow-der began to be oxidized around 400◦C, receiving a weightgain. It seems around 800◦C that it was fully oxidized toNiO. The final weight gain was around 27% that were al-most consistent to the theoretical weight gain (27.3%) forperfect conversion of pure Ni to NiO. This datum indicatesthat the particles consist of pure nickel. The weight gainfrom TG data for different sizes of Ni-powder when heatedin air up to 800◦C at rate of 10◦C/min is listed inTable 1.The particle with the crystallite size of 46 nm showed slowweight gain (4.1%) at 500◦C, while the particle smaller sizeof 31 nm, when treated at the same condition had weight in-crease of 12.1%. These data suggest that the increasing ofcrystallite size decrease the oxidation rate of particle Ni.

3.1.3. Morphology studiesScanning electron microscopy (SEM) studies gave a clear

indication of a good state of particle dispersion as shown inFig. 6a and b. The size of these particles was in the range of0.20–0.70�m. The typical image (Fig. 6a) demonstrates thatthe nickel particles were spherical in shape. And this imagealso suggests a narrow distribution of particle size with meandiameter around 0.40�m as shown in a histogram ofFig. 7.However, when the nickel powder fired above 500◦C theagglomeration was observed as can be seen in SEM (Fig. 6b).These particles were also identified and confirmed as purenickel by EDX analysis.

Table 1Thermal property of several Ni powders prepared using MH

Sample Crystallitesize (nm)a

Weight gain on oxidation (%)

At 500◦C At 800◦C

A 39 9.6 27.5B 23 16.9 26.2C 46 4.1 27.5D 31 12.1 27.1

a Estimated from XRD using Sherrer’s formula.

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Fig. 6. SEM images of nickel powder obtained after fired at the temperatureof (a) 400◦C and (b) 500◦C (bar= 1�m).

Fig. 7. Particle size distribution of nickel powder.

Fig. 8. XRD patterns of powders of different compositions: (a) Ni, (b)Ni4Co, (c) Ni3Co2, (d) Ni2Co3, (e) NiCo4, and (f) Co.

Fig. 9. Composition dependence of lattice parameter of the fcc phase inNinCo5−n.

3.2. Ni–Co alloy

NinCo5−n alloys forn = 1, 2, 3, 4 and 5 have been pre-pared from mixing of Ni- and Co acetate in isopropanol so-lution containing methyl hydrazine. In this study, The alloypowders produced under N2 atmosphere in the temperaturerange of 400–700◦C were highly crystalline with fcc struc-ture.Fig. 8shows composition dependence of XRD patternsof NinCo5−n obtained at 600◦C. As shown in he insert ofFig. 8, the diffraction angel of〈1 1 1〉 plane shifted depend-ing on alloy composition. Using the data, the variation ofthe lattice parameters of NinCo5−n alloy composition wasevaluated, and the results are illustrated inFig. 9. having agood linear relationship. Such a trend indicates the mutualalloying of nickel and cobalt: the formation of perfect solidsolution for any compositions NinCo5−n.

4. Conclusion

The most interesting aspect of this work is the fact thatvery fine particles of pure metals and alloys can be produced

Syukri et al. / Materials Chemistry and Physics 78 (2003) 645–649 649

with a rather simple and inexpensive method. The forma-tion phases of nickel are well manipulated by heat treatmentconditions. And selecting the compositions of starting so-lutions easily controls the compositions of the alloy. Fromthis work can be made the following conclusions:

1. Hcp Ni-powders can be obtained at low temperature(300◦C) and above 400◦C the hcp phase transformed tofcc phase.

2. Fcc nickel powders with mean particle sizes ranging from0.2 to 0.7�m of high purity were obtained in this way.

3. Using the mixtures containing different ratios of nickelto cobalt, corresponding NinCo5−n alloys with fcc phasecan be obtained.

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