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Materials Chemistry and Physics 131 (2011) 102–107

Contents lists available at ScienceDirect

Materials Chemistry and Physics

journa l homepage: www.e lsev ier .com/ locate /matchemphys

ormation mechanism of fibrous cobalt oxalate precipitated from alkalineo2+–NH3–C2O4

2−–H2O system

hiyong Liu, Zhihong Liu ∗, Qihou Li, Tianzu Yang, Duomo Zhangchool of Metallurgical Science and Engineering, Central South University, Changsha, Hunan 410083, China

r t i c l e i n f o

rticle history:eceived 6 November 2010eceived in revised form 16 June 2011ccepted 26 July 2011

a b s t r a c t

The cobalt oxalate particles with fibrous morphology were obtained by precipitating CoCl2 and mixed(NH4)2C2O4, NH3·H2O solutions in a double jets process. The samples were characterized by SEM,XRD, FTIR, TG/DTA and GCMS. Thermodynamic calculations and experiments revealed that the forma-

2+

eywords:. Composite materials. Chemical synthesis. Precipitation. Nucleation

tion of Co(NH3)n (n = 1,2,. . .,6) results in species variation of cobalt oxalate precipitated in alkalineCo2+–NH3–C2O4

2−–H2O system. It was found that there are two kinds of cobalt oxalates precipitated inCo2+–NH3–C2O4

2−–H2O system with the change of synthetic conditions. One is �-CoC2O4·2H2O with asquare columnar morphology. The other is Co(NH3)1.5C2O4·2H2O with a fibrous morphology, a species ofcobalt oxalate without being recorded in newest JCPDS files. The formation mechanism of fibrous cobaltoxalate powders is that Co(NH3)1.5C2O4·2H2O precipitates in alkaline Co2+–NH3–C2O4

2−–H2O system.

. Introduction

The powders of cobalt and its oxides, owing to their excel-ent mechanical, electrical, magnetic and catalytic properties, are

idely used in different industries, accounting for about 65% ofotal cobalt consumption. Metallic cobalt powder is mainly used inhe production of carbide and diamond tools [1,2]. The most impor-ant application of Co3O4 is used as precursor to synthesize LiCoO2,kind of cathode materials for lithium-ion battery, while CoO isainly used in the areas like battery, pottery, catalyst, pigment,

lass and so on [3]. Till now, it is also a persisting goal to synthesizeowders of cobalt and its chemicals with special morphologies andne sizes to develop new applications [4,5].

Thermal decomposition of cobalt oxalate is a main method ofroducing powders of cobalt and its oxides commercially [6–11].t present, cobalt oxalate is manufactured industrially by precip-

tating CoC2O4·2H2O in mixed solutions of cobalt chloride andmmonium oxalate under weakly acidic or neutral conditions.tudies have been carried out in control of the crystal structure,orphology and particle size of the cobalt oxalate. Yuan Ping’s

nvestigation showed that in the process of precipitating cobaltxalate, doping results in the crystal structures of the precipitateshange from � to � type. Factors, such as temperature, solution

oncentration and feeding rate, affect the morphology and particleize of cobalt oxalate precipitated due to the change of the superaturations in nucleation and growth processes, where lower super

∗ Corresponding author. Tel.: +86 731 88830478; fax: +86 731 88830478.E-mail address: zhliu@mail.csu.edu.cn (Z. Liu).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.07.070

© 2011 Elsevier B.V. All rights reserved.

saturations is helpful for producing larger particles [12]. Gao Jin’sresearch revealed that doping can change the morphology of cobaltoxalate from dendrite to spheroid, in favor of producing metalliccobalt powders with spherical morphology and good fluidity [13].

Many studies on preparing nanoparticles of nickel, cobalt,iron, zinc, copper and manganese oxalates by the microemulsion(reverse micellar) routes were carried out in recent years [14–19].In these studies, the oxalates are synthesized with ammoniumoxalate as the oxalate resource under a near neutral condition, andthere is no ammonia present in the precipitates. The particles tendto join together to form bundles, and their aspect ratio is changedwith the variation of synthetic conditions, such as surfactant, sol-vent, temperature, concentration and so on.

Up to now, it has been investigated extensively on precipitatingnickel oxalate powders in alkaline Ni2+–NH3–C2O4

2−–H2O system,as well as the control of their morphologies and particle sizes.Results show that at higher concentration of free ammonia, nickeloxalate precipitates in the form of NiC2O4·xNH3·yH2O. The intro-duction of ammonia into the precipitate results in the formationof nickel oxalate particles in fibrous morphology with high aspectratio [20]. Cobalt and nickel are the same group elements withsimilar chemical properties. Therefore, it can be deduced that inalkaline Co2+–NH3–C2O4

2−–H2O system, cobalt oxalate will pre-cipitate like nickel oxalate. This guess has been verified primarilyin Zhang Chuanfu’s study, where nickel and cobalt alloy powdershad been prepared [20,21]. However, There is little research on

the crystal structure, formation mechanism, as well as the controlof morphologies and particle sizes of cobalt oxalate precipitatedin alkaline Co2+–NH3–C2O4

2−–H2O system. Therefore, the studyon precipitating cobalt oxalate in alkaline Co2+–NH3–C2O4

2−–H2O

ry and Physics 131 (2011) 102–107 103

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Fig. 2. XRD pattern of the sample prepared under the control conditions.

Z. Liu et al. / Materials Chemist

ystem is of great importance to understand profoundly hydro-hemical process of preparing powder materials, as well as toevelop new applications of cobalt and its oxide powders.

The main finding in our work is that there are two kindsf cobalt oxalates precipitated in Co2+–NH3–C2O4

2−–H2O systemith the change of synthesis conditions, one is precipitated under

cidic or neutral condition and identified as �-CoC2O4·2H2O withsquare columnar morphology, while the other is precipitated

nder alkaline condition and identified as Co(NH3)1.5C2O4·2H2O,new species of cobalt oxalate, which has not being recorded in

he newest JCPDS files. In alkaline Co2+–NH3–C2O42−–H2O system,

obalt(II) ions exist in the form of Co(NH3)n2+ (n = 1,2,. . .,6), which

esults in cobalt oxalate precipitated as Co(NH3)1.5C2O4·2H2O withbrous morphology.

. Materials and methods

.1. Preparations of cobalt oxalate particles

Reagent grade of CoCl2·H2O, Na2C2O4, NH3·H2O (28%), NaOH, HCl and (C6H9NO)n

PVP25) (Naclai Tesque Company, Japan) were used as received.The preparation procedure and control conditions were as follows: (1) 50 mL

oCl2 solution (0.2 mol L−1, pH298K 5.75, nominated as solution A) and 50 mL mixedNH4)2C2O4 and ammonia solution((NH4)2C2O4 0.2 mol L−1, NH3·H2O 0.8 mol L−1,H298K 9.82, nominated as solution B) were prepared and 1 mol L−1 NaOH or

ig. 1. SEM images of the sample prepared under the control conditions (a) 300×;b) 10,000×.

Fig. 3. FTIR spectra of the sample prepared under the control conditions.

Fig. 4. TG/DTA results of the sample prepared under the control conditions in heliumat heating rate 20 K min−1.

104 Z. Liu et al. / Materials Chemistry and Physics 131 (2011) 102–107

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ig. 5. XRD patterns of Co(NH3)1.5C2O4·2H2O as well as the intermediates and thenal products of its decomposition in helium (a) 303 K, (b) 388 K, (c) 448 K, (d) 773 K.

Cl solution was used to adjust pH of the solutions prepared when needed. (2)5 mL deionized water containing 0.155 g PVP25 was added into 250 mL spher-

cal glass flask with four neck and stirred by a magnetic mixer. The flask wasmmersed in a thermostatic water bath kept at 353 K. (3) Two solutions pre-ared in step (1) were added into the flask by a double jets process with twoonstant flow pumps at the same rate of 1 mL min−1. (4) The slurry was thentirred continuously for 1 h and filtrated. The particles synthesized were washedtimes with deionized water and dried in a vacuum drying oven for 24 h at room

emperature.

.2. Characterization

The morphology of dried powders was determined with a scanning electronicroscopy (Hitachi S-800). The phase analysis and crystallinity have been deter-ined by a Shimadzu XRD-600 X-ray diffractometer, using Cu K� radiation source.

he structure of the samples was estimated by infrared spectroscopy (FTIR, Nicolet,EXUS470, KBr). Thermal gravimetric and differential thermal (TGA–DTA) analysisf cobalt oxalate precursor, as well as the chromatography and mass spectrom-try analysis of thermal decomposition gas (GCMS) were carried out in heliumtmosphere with the heating rate at 20k min−1 (TG–DTA/GCMS; TG8120/GCMS-P5050A, Rigaki/Shimadzu). Sample composition was analyzed by automaticlemental analyzer (2400II, Japan Perkin Elmer Company). The NH3 content of theample was determined by Kieldahl method. According to the NH3 content in theample, appropriate amounts of samples (0.2000–1 g) were placed in a 100 mLieldhl flask. The flask was immersed in a thermostatic oil bath kept at 393 K forh. The amount of released ammonia (the amount of nitrogen present in the sam-le) was absorbed by the solution of standard sulfuric acid (C1/2H2SO4

= 0.1 mol L−1).he ammonia reacts with the acid and the remainder of the acid is then titrated usingstandard sodium hydroxide solution (CNaOH = 0.01 mol L−1) with a methyl orangeH indicator.

The morphology of fibrous particles prepared was described in length (l), diam-ter (d) (the cross section of fibrous cobalt oxalate powder is not circular, but treatedimilarly as circular here) and aspect ratio (l/d) quantitatively.

Fig. 6. The equilibrium calculation results of Co2+–NH3–H2O system (numbers 0–6as shown within the square box denote coordination number (n) of Co[(NH3)]n

2+).

3. Results and discussion

3.1. Characterizations of cobalt oxalate powders prepared at thecontrol conditions

Cobalt oxalate powders prepared under the control condi-tions described in Section 2.1 were characterized by SEM, XRD,FTIR, TGA–DTA and chemical composition analysis. The results areshown in Figs. 1–4, Table 1.

The SEM images shown in Fig. 1 reveal that the powders pre-pared under the control conditions possess fibrous morphologywith high aspect ratio, sharp ends and irregularly polygonal tran-sect. The formation mechanism of the tropism aggregations alonglong axis among particles can be observed. The measurement byusing image tool software indicates that the lengths of the parti-cles are 20–160 �m, with the widths 0.4–6.5 �m, and aspect ratios24–50.

The XRD pattern of the sample is shown in Fig. 2 and thecorresponding XRD data are listed in Table 1. According to XRDanalysis, the sample prepared under the control conditions is a

new species that has not been recorded in JCPDS files. Its pattern isalmost the same as that of Ni(NH3)1.5C2O4·2H2O [20], which meansthat they have the same crystal structure. Indeed, several studies

Z. Liu et al. / Materials Chemistry and Physics 131 (2011) 102–107 105

ed at

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Fig. 7. SEM images of samples prepar

howed that crystallization of ferrous oxalate can lead to differ-nt allotropic forms depending on the precipitation conditions22,23]. According to Deyrieux and Peneloux’s studies, the stablend metastable forms, respectively noted � and �, have different

able 1RD data of samples prepared under the control conditions.

2� (◦) 15.45 19.61 20.89 23.96 29.39

d ( ´̊A) 5.729 4.523 4.250 3.712 3.036Int 100 55 3 24 7

different pH and ammonia additions.

crystallographic structures and differ from each other by the rela-tive positions of the metal-oxalate chains. Deyrieux also consideredthat the precipitations with ammonium oxalate are helpful forthe formation of �-CoC2O4·2H2O. Cobalt oxalate precipitated by

31.71 32.18 35.03 39.79 42.40

2.820 2.779 2.560 2.263 2.13023 11 5 7 4

106 Z. Liu et al. / Materials Chemistry and Physics 131 (2011) 102–107

Table 2Chemical composition of sample prepared under control condition.

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Mass (%) 28.23 11.56 52.07 9.96 4.05

. Baco with ammonium oxalate displays a fiber-like morphologyith �-CoC2O4·2H2O structure [24]. In our case, the obtained phase

s influenced by the nature of the oxalate source and precipitationonditions. It was found that in Co2+–NH3–C2O4

2−–H2O system,wo species of cobalt oxalates can be precipitated with the changesf synthetic conditions, one is �-CoC2O4·2H2O precipitated undercidic or neutral conditions, the other is Co(NH3)1.5C2O4·2H2O pre-ipitated under alkaline conditions, a new species of cobalt oxalate,ithout being recorded in JCPDS files at present.

Fig. 3 presents the FTIR spectra of the sample prepared under theontrol conditions. The strong complex absorption bands exist inhe range 3500–3200 cm−1 which can be assigned to the stretch-ng vibrations of NH3 (�a(NH) = 3290 cm−1 and �s(NH) = 3180 cm−1)nd to H2O groups (�s(OH) and �a(OH) = 3300 cm−1). An intensiveand at 1628.74 cm−1 is due to the deformation vibration of NH3ıa(HNH)), crystallization water (ıa(HOH) ∼ 1660 cm−1) and/or sometretching vibrations C2O4

2− ions (�a(CO)). The vibrations at 1367.53nd 1319.95 belong to stretch vibrations of (�a(CO)). The band at232.31 cm−1 is assigned to deformation vibration of (ıa(HNH)). Twoands at 810.03 cm−1 and 494.37 cm−1 are attributed to defor-ation vibrations of (ıa(O–C O)), while the band at 580.71 cm−1 is

ttached to stretch vibration of (�a(CC)) [25]. According to FTIR anal-sis, it can be concluded that there is ammonia molecule presentn the sample. The FTIR spectra is almost the same as that ofi(NH3)1.5C2O4·2H2O, indicating that they possess the same crystal

tructures.Fig. 4 shows TG/DTA results of thermal decomposition of the

ample obtained in helium. There are three endothermic peaksresent in DTA curve at 377.3 K, 432.8 K and 673.1 K, correspondingo the release of NH3, H2O, the decomposition of CoC2O4 with massoss (mass %) of 14.68, 10.91 and 43.74, respectively [20]. Gas prod-cts of decomposition at 377.3 K and 432.8 K had not been detectedy MS, probably due to the lower contents and a mass spectrum of/z equal to 44 was recorded, which belongs to CO2. Furthermore,e used Kieldahl method to determine NH3 content in the sam-le. After thermal decomposition, the content of released NH3 fromhe sample is determined to be 14.58%, in agreement with TG/DTAesults.

The chemical composition of the sample prepared under theontrol conditions, listed in Table 2, reveals that the chemical for-ula is Co(NH3)1.5C2O4·2H2O, in agreement with the results ofRD, FTIR, TG/DTA and Kieldahl method.

In order to determined the mechanism of thermal decomposi-ion and the structure of the Co(NH3)1.5C2O4·2H2O. The sample waseated at different temperatures of 303 K, 388 K, 448 K, and 773 Kith helium atmosphere respectively for 2 h. The solid products

f the decomposition were submitted to XRD analysis. As shownn Fig. 5, dehydrate cobalt oxalate is obtained at 388 K, anhydrousobalt oxalate is obtained at 448 K, and metallic cobalt is obtainedt 773 K.

.2. Formation mechanism of fibrous cobalt oxalate

.2.1. The present form of cobalt(II) in Co2+–NH3–H2O systemAccording to simultaneous equilibrium principle, thermody-

amic calculations were carried out to study the effects of pH on

he forms of Co(NH3)n

2+ (n = 0–6) ions in Co2+–NH3–H2O system.he data in calculations were taken from related references [26].

The results of equilibrium calculations, as shown in Fig. 6, indi-ate that the present forms and concentrations of Co(II) ions will

Fig. 8. XRD patterns of samples prepared at different pH and ammonia additions.

change with the variation of pH and molar ratio of [NH3]T/[Co]T inCo2+–NH3–H2O system. According to the results, it can be deducedthat in alkaline area, Co(II) ions mainly present in the form ofCo[(NH3)]n

2+ (n = 1–6), resulting in the change of cobalt oxalatespecies precipitated by adding oxalate into the system.

3.2.2. The role of ammonia in the formation of fibrous cobaltoxalate

In order to study the role of ammonia in the formation of fibrouscobalt oxalate, a group of experiments were designed by chang-ing solution B as follows: (a) 0.2 mol L−1 Na2C2O4, pH298K 7.1;(b) 0.2 mol L−1 Na2C2O4, pH298K 9.82; (c) 0.2 mol L−1 (NH4)2C2O4,pH298K 9.82; (d) 0.2 mol L−1 (NH4)2C2O4, 0.4 mol L−1 NH3·H2O,pH298K 9.82; (e) 0.2 mol L−1 (NH4)2C2O4, 0.8 mol L−1 NH3·H2O,pH298K of 9.82; (f) 0.2 mol L−1 (NH4)2C2O4, 1.2 mol L−1 NH3·H2O,pH298K 9.82. Experimental procedures and the other conditionswere the same as those described in Section 2.1. Correspondingly,the samples prepared in this group of experiments were named asa, b, c, d, e and f, respectively. Their results of SEM and XRD areshown in Figs. 7 and 8.

From SEM photographs of the samples, it can be seen that sam-ples a and b prepared in an ammonia-free system possess squarecolumnar morphologies (Fig. 7(a) and (b)), and correspondingly,the XRD results, as shown in Fig. 8, reveals that they are in theform of �-Co(C2O4)2·2H2O. Comparing with the preparing condi-tions of samples a and b, it also can be concluded that pH alonewill not result in the changes of structure and morphology of theparticles. Samples c, d, e, f were prepared by changing the addingamount of ammonia of solution B. From SEM and XRD results,as shown in Figs. 7 and 8, it can be seen that with the addition

of ammonia, the precipitate transforms from �-Co(C2O4)2·2H2Oto Co(NH3)1.5C2O4·2H2O, and correspondingly, their morphologiestransfer from square columnar to fiber-like. The results shown inFigs. 7 and 8 reveal clearly the formation mechanism of fibrous

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obalt oxalate precipitated from alkaline Co2+–NH3–C2O42−–H2O

ystem.

. Conclusions

Fibrous cobalt oxalate particles were prepared by blendingoCl2 solution with mixed (NH4)2C2O4 and NH3·H2O solutionhrough a double jets process. The sample prepared under the con-rol conditions was characterized by SEM, XRD, FTIR, TG/DTA, GCMSnd chemical composition analysis. The particles prepared pos-ess fiber-like morphology, with lengths of 20–160 �m, widths of.4–6.5 �m, and aspect ratios of 24–50. The molecular formula ofamples prepared was determined to be Co(NH3)1.5·C2O4·2H2O, aew cobalt oxalate chemical without record in newest JCPDF files.

The immanent cause of cobalt oxalate presenting fibrous forms owing to variations of the species and concentration of Co(II) inlkali Co2+–NH3–C2O4

2−–H2O system, which results in variationsf the species and supersaturation of cobalt oxalate precipitated.

Cobalt oxalate can be precipitated in two chemical forms ino2+–NH3–C2O4

2−–H2O system. One is �-CoC2O4·2H2O, a wellnown cobalt oxalate chemical (JCPDS 25-0250) with squareolumnar morphology, which precipitates when free ammonia isnsufficient in the system; while the other is Co(NH3)1.5·C2O4·2H2O,cobalt oxalate chemical with fiber-like morphology found in this

tudy, which precipitates from alkaline Co2+–NH3–C2O42−–H2O

ystem containing enough free ammonia.

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