Advanced Powder Technology 22 (2011) 715–721

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Advanced Powder Technology

journal homepage: www.elsevier .com/locate /apt

Original Research Paper

Synthesis and characterization of uniformly coated particles (cobalt compoundson copper compounds)

Ikram ul Haq ⇑, Farzana HaiderNational Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, NWFP, Pakistan

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 March 2010Received in revised form 10 October 2010Accepted 13 October 2010Available online 26 October 2010

Keywords:Copper oxalateCopper oxideCoated particlesCobalt basic carbonateCobalt oxideUrea

0921-8831/$ - see front matter � 2010 The Society ofdoi:10.1016/j.apt.2010.10.009

⇑ Corresponding author. Tel.: +92 91 9216766; fax:E-mail address: ikramhq@yahoo.com (I. ul Haq).

Spherical particles (�3 lm) of copper(II) oxalate were produced in the form of precipitated solids bygently mixing aqueous solutions of oxalic acid and copper nitrate with predetermined concentrationsat room temperature. These particles were isolated from the mother liquor and then coated with cobaltbasic carbonate. The coating trials involved heating of the aqueous dispersions, containing knownamounts of the dispersed copper oxalate particles (cores), urea, and cobalt nitrate, at 70–85 �C for variousperiods of time with constant stirring. The heating process decomposed urea, increased pH, liberated car-bonate ions, which resulted in the precipitation of the dissolved cobalt ions in the form of shells of cobaltbasic carbonate around each core particle. The coating process was sensitive to the applied experimentalparameters, since uniformly coated particles were obtained under a narrow range of coating mixturecomposition. In the absence of the cores, the same reactants solutions produced coating precursor parti-cles (cobalt basic carbonate), when subjected to similar heating conditions. Physical and chemical anal-yses indicated that the coating material of the coated particles and the coating precursor particles had thesame chemical compositions. The as-prepared core, coating precursor, and coated particles were con-verted into oxide forms by heating their dry powders at elevated temperatures under controlled heatingconditions. The heat treatment produced obvious changes in the surface morphology of these particlesdue to loss of material. Moreover, the heat-treated particles preserved shape integrity to a maximumextent, showing their thermal stability. Selected batches of the as-prepared and heat-treated productswere characterized by various physical methods.� 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder

Technology Japan. All rights reserved.

1. Introduction

Powders of inorganic compounds, such as metal oxides, carbon-ate, phosphate, oxalate, etc. are generally employed in the drystate, pastes, or dispersions in the wide range of technological pro-cesses, such as heterogeneous catalysis [1,2], conductive adhesives[3], adsorption [4], etc. These powders are produced either by theball milling of the coarse solids, or chemical precipitation methods.It has been reported that ball milling [5] can generate powders,composed of particles in the submicron range; however, this tech-nique has no control over achieving uniformity in the particleshape and size. In contrast, the latter could be controlled to a sig-nificant by using chemical precipitation methods. For example,Matijevic et al. have produced different types powder materialsby the chemical precipitation, such as forced hydrolysis [6], andhomogeneous precipitation method [7,8]. These systems werecomposed of either single or coated systems of uniform fine parti-cles of various morphologies, size ranges, and chemical composi-

Powder Technology Japan. Publish

+92 91 9216671.

tions. Similarly, some of the material scientist produced metaloxalate particles by gently mixing the precipitant (oxalic acid, orsodium oxalate) with the metal ions solutions in appropriate ratiosand ended up with particles, having reasonable uniformity with re-spect of the particle morphology and size [9,10]. Since uniformityin particle shape, size, and chemical composition of the powdermaterial is considered [11,12] essential for their reproducible char-acteristic in applications; therefore, a number research groups indifferent parts of the world, including our group, have producedsome novel systems of fine powders [13–15].

In continuation of earlier reported work, it was planned toproduce coated particles, composed of core and shell materialsof copper oxalate and cobalt basic carbonate, respectively. At-tempts would be made to convert them into copper/cobalt com-posite oxide by controlled heat-treatment processes. It ismentioned here that copper oxide and cobalt oxide have beenused individually [16,17] as well as in the form of copper–cobaltmixed oxide system [18] for various catalytic reactions. It hasbeen reported in the literature [19] that the catalytic performanceof the mixed catalysts generally depends on their preparationconditions. Since, Co3O4–CuO catalyst has never been produced

ed by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

716 I. ul Haq, F. Haider / Advanced Powder Technology 22 (2011) 715–721

in shell–core configuration. Therefore, we believe that our coatedpowder system, in which each individual particle is a compositeoxide system, may prove to be a material of reproducible perfor-mance in various catalytic processes and other relatedapplications.

In the present work, we describe the synthesis of spherical par-ticles of copper oxalate (cores), their coating with cobalt basic car-bonate, and conversion of core, and coated particles into metaloxides. The core particles are produced by mixing copper nitrateand oxalic acid solutions. The coatings on cores are developed inaqueous dispersion by the urea based homogeneous precipitationmethod and conversion of the synthesized products to oxide formby the heat treatment process.

2. Experimental procedures

2.1. Material

Analytical grade chemicals, that is, copper nitrate, oxalic acid,cobalt nitrate, and urea were purchased from Merck and employedin this work in the as-received form. Deionized water was used formaking stock and working solutions, using Pyrex glass vessels. Be-fore use, all the glass vessels were cleaned with nitric acid solutionin the ultrasonic bath and then extensively washed with demon-ized water. The insoluble undesirable contaminants were isolatedfrom the working solutions by filtration through micro-porousmembrane filters.

2.2. Synthesis of copper oxalate (core particles)

The core particles were synthesized essentially by the method,described in our earlier article [14]. In this method, equal volumes(50 ml each) of aqueous solutions of copper nitrate (0.04 moldm�3) and oxalic acid (0.04 mol dm�3) were gently mixed at roomtemperature and the resultant reactant mixture was kept unper-turbed for 1 h. During this time period, the precipitated particlesgot settled at the bottom of the reaction vessel. The particles wereseparated from the mother liquor by filtration through micro-por-ous membrane filter, washed several times with deionized water,dried in air, and then stored in a stoppered glass bottle.

2.3. Synthesis of cobalt basic carbonate/copper oxalate (coatedparticles)

For this purpose, dispersions of the core particles were pre-pared, containing 0.5–1.0 g/L cores, 0.01–0.06 mol/L cobalt nitrate,and 0.4–0.9 mol/L urea and were heated at 85 �C for various peri-ods (5–20 min) of time with constant stirring in a 400 ml double-walled Pyrex glass container. In each case, volume of the dispersionwas kept 300 ml and its temperature was maintained at the men-tioned temperature by circulating hot water from the circulatingwater bath through the double-walled jacket of the reaction vessel.After this process, the reactant mixture was cool down by switch-ing the water circulation system from hot circulating water to ice-cooled circulating water. The reactant mixture was then filteredthrough membrane filter, and the residue was washed with suffi-cient amount of deionized waters. It was dried in air beforecharacterization.

2.4. Synthesis of cobalt basic carbonate (coating precursor particles)

Coating precursor particles were prepared by heating a selectedcoating mixture in the absence of the dispersed cores under thesame conditions, as employed for the synthesis of the coatedparticles. The particles obtained in these experiments were sepa-

rated from the liquid phase, washed, and then dried in the samemanner as used for the core and coated particles, described above.

2.5. Heat-treatment

Samples of the above mentioned synthesized dry powders(cores, coated and coating precursor) were heated in a tube furnaceup to 350 �C at the heating rate of 5 �C/min and then kept them atthis temperature for 1 h. The samples were cool down to roomtemperature inside the furnace by turning it off and then storedin a desiccator.

2.6. Characterization

Shape and size of the particles of interest were inspected with ascanning electron microscope (SEM; JEOL, JSM-5910). For thisstudy, SEM samples were prepared in which a small quantity ofthe desired powder was mounted on a standard stub with the helpof a double stick conducting carbon tape and coated with a thinlayer of gold in a auto fine coater (JEOL, JFC-1600). SEM imagingof the particles on the stub was carried out at 15 keV. The coatedparticles were also analyzed with energy-dispersive X-ray analyzer(Inca-200), attached with the mentioned electron microscope, andatomic absorption spectrometer (Pye-Unicam SP-190) for the qual-itative and quantitative estimation of their metal content, respec-tively. The crystallinity of the desired powder samples wasdetermined by X-ray diffractometer (XRD, JEOL JDX-3532) withCuKa radiations, holding the voltage and current of the diffractom-eter at 40 kV and 20 mA, respectively. The scanning of every samplewas performed in the 2h range of 5–80�, with a step angle of 0.03�.Selected samples of core, coated, and coating precursor powderswere subjected to thermogravimetric analysis by using TGA/DTAanalyzer (Diamond TG/DTA, Perkin Elmer). In all the three cases,the samples were heated in the temperature range of 30–600 �Cin the dynamic nitrogen atmosphere, while keeping the rate ofheating of the thermal analyzer at 5 �C/min. Similarly, the Fouriertransform infrared spectrometer (Schimadzue, IR Prestige-21,FTIR-8400S) was used for recording the IR spectra of these powders.

3. Results and discussion

3.1. Core particles

The particles shown in Fig. 1A were employed as cores. The lat-ter were precipitated by mixing aqueous solutions of copper ni-trate and oxalic acid under the described experimentalconditions. These particles had sky blue color and the X-ray diffrac-tometric analysis (Fig. 2A) confirmed its chemical composition ascopper oxalate (CuC2O4). Infrared spectrometric analysis indicatedthat IR spectrum (Fig. 3A) of the particles in Fig. 1A was composedof absorption bands at various locations, corresponding to the –OHstretching vibrations (3650–3500 cm�1); –OH bending vibrations(1600–1650 cm�1); –C@O stretching and bending vibrations(1365 and 1300 cm�1); Cu–O bond vibrations (810–830 and500 cm�1).

Thermogravimetric analysis (Fig. 4A) revealed that the particlesin Fig. 1A lost 47.2% of its weight at �295 �C which was ascribed tothe phase transformation/thermal decomposition of copper oxa-late to copper oxide according to the following reaction:

CuC2O4 )47:51 wt:%

350 �CCuOþ COþ CO2 ð1Þ

The indicated weight loss in Eq. (1) agreed well with the weightloss observed in the TGA plot, shown in Fig. 4A.

Fig. 1. Scanning electron micrographs (SEM) of particles obtained, when (A) aqueous solution, containing 0.04 mol/L each of copper nitrate and oxalic acid, was aged for 1 h at85 �C, (B) particles in A were heated at 350 �C.

20 40 60 80

Inte

nsit

y (a

.u.)

2θ (degree)

bbb

bb

aaa

a

a

a, CuC2O

4

B

Aa

b, CuO

Fig. 2. X-ray diffraction (XRD) patterns of the particles shown in Fig. 1A (A) andFig. 1B (B).

4000 3500 3000 2500 2000 1500 1000 500

C

B

Tra

nmit

tanc

e (%

)

Wave number (cm-1)

A

Fig. 3. FT-IR spectra of the particles in Fig. 1A (A), Fig. 5C (B) and Fig. 5A (C).

100 200 300 400 500 60030

40

50

60

70

80

90

100

Wei

ght

loss

(%

)

Temperature ( oC)

B

C

A

Fig. 4. Thermogravimetric analysis (TGA) curves obtained with particles shown inFig. 1A (A), Fig. 5C (B) and Fig. 5A (C).

I. ul Haq, F. Haider / Advanced Powder Technology 22 (2011) 715–721 717

Following the results of the thermogravimetric analysis, a sep-arate experiment was also conducted in which a known amountof the copper oxalate particles, shown in Fig. 1A, were given heattreatment at 350 �C in air atmosphere for 1 h. The weight losswas noted, which was in close agreement with the weight lossesindicated in the TGA plot (Fig. 4A) and that in Eq. (1).

The heat treated powder had brownish color and composed ofCuO (XRD, Fig. 2B). In contrast, we observed in our earlier study

[14] that on heat-treatment at 350 �C, the submicron particles ofcopper oxalate transformed into powder, composed of CuO andCu2O. This observation clearly indicated that particle size of copperoxalate had significant effect on the composition of the heatedproduct.

Similarly, IR analysis (Fig. 7A) demonstrated that the heat trea-ted solids lost its thermally decomposable components, since thementioned IR spectrum was composed of a major absorption bandat 500 cm�1, which was attributed to the vibration of Cu–O bond.

Scanning electron microscopic observation indicated that theaverage size of the heat-treated particles (SEM, Fig. 1B) was�45% smaller than that of the as-prepared particles of copper oxa-late. This observation suggested the fact that reduction in particlessize was clearly due to the material loss, which took place accord-ing to Eq. (1). Moreover, inspection of the SEM image in Fig. 1B re-vealed that each particle was composed of uniform nanosizedsubunits. The latter observation pointed to the fact that the copperoxalate particles in Fig. 1A were in fact aggregates of closely packedsmaller particles, which made their appearance in the heated par-ticles by loosing the thermally decomposable components. It isadded that to our knowledge, the displayed morphology of copperoxide particles (SEM, Fig. 1B) is being reported for the first time.

3.2. Coated particles

Attempts were made to develop shell material (cobalt basic car-bonate) on the dispersed core particles of copper oxalate (SEM,Fig. 1A) by the precipitation process, initiated by the thermal

Fig. 5. Scanning electron micrographs (SEM) of the particles obtained, when (A) aqueous dispersion, containing 0.8 mol/L of urea, 0.04 mol/L of cobalt nitrate and 0.7 g/L ofcopper oxalate (cores) were aged for 11 min at 85 �C, (B) particles in A were heated at 350 �C, (C) coating precursor particles, (D) particles in C were heated at 250 �C.

718 I. ul Haq, F. Haider / Advanced Powder Technology 22 (2011) 715–721

decomposition of urea. The latter generally takes place at hightemperatures which increases pH of the medium, liberate carbon-ate ions [12], and precipitate the dissolved metal ions [cobalt ionsin the present case] in the form of metal basic carbonate.

For these purpose aqueous dispersions of known compositions,containing urea, cobalt nitrate, and dispersed cores were heated at70–85 �C for different intervals of time (5–20 min). It was notedthat the dispersion composition (coating mixture) and tempera-ture had significant effect on coating the dispersed cores withthe shell material. SEM analysis revealed that the products, ob-tained in most of the coating trials, were composed of mixturesof coated or partially coated and coating precursor particles. Assuch, many experiments had to be performed in order to establishrange of dispersion compositions, and temperature under whichuniformly coated particles could be obtained.

For example, the coated particles shown in Fig. 5A were ob-tained by heating at 85 �C the coating mixture, containing 0.7 g/L

0.5 0.6 0.7 0.8 0.9 1.015

20

25

30

35

40

Coa

ting

mat

eria

l (w

t %

)

Copper oxalate (g/L)

Fig. 6. Variation in deposited coating on cores as a function of the core contents,when aqueous dispersions, 0.8 mol/L in urea, 0.04 mol/L in cobalt nitrate and 0.5–1.0 g/L in cores (copper oxalate), were heated at 85 �C for 11 min with constantstirring.

cores, 0.8 mol/L urea, and 0.04 mol/L cobalt nitrate for 11 min. Inthis case, the cores gained �30% of weight in the coating process,which was estimated from the gain in weight of the core particlesduring the coating process. The same mixture when heated forlonger period than 11 min resulted in solids composed of mixtureof coated and coating precursor particles. This observation indi-cated that after the indicated aging time of 11 min, the shell forma-tion on the surface of the dispersed cores might have exhaustedand the on going thermal decomposition of urea led to the precip-itation of the coating precursor material in the bulk of the coatingmixture. In contrast, when the heating time was kept shorter than11 min, negligible difference was observed in the morphology andchemical composition of the dispersed cores. The latter results sug-gested the fact that in the period, shorter than 11 min, the coatingmixture had not achieved the status, which could have initiatedthe formation of the precipitated shells on the surface of the coreparticles.

4000 3500 3000 2500 2000 1500 1000 500

C

B

A

Tra

nmit

tanc

e (%

)

Wave number (cm-1)

Fig. 7. FT-IR spectra of the particles in Fig. 1B (A), Fig. 5B (B) and Fig. 5D (C).

I. ul Haq, F. Haider / Advanced Powder Technology 22 (2011) 715–721 719

Following the established of recipe for the coated particles inFig. 5A, it was of interest to study the effect of the amount of coresin the coating mixture on the fate of the coated particles. For thispurpose, coating experiments were performed under the sameconditions, as mentioned in the legend of Fig. 5A, except theamount of the cores in the coating mixture was kept in the range<0.7 g/L>. The results are depicted in Fig. 6. It can be seen from thisfigure that the ratio (Shellwt.%/Corewt.%) in the coated particles de-creased with the increase in the cores content of the coating mix-tures. This was obviously due to the fact that for the constantamount of the shell material precursors, increasing the availablesurface for the precipitation process led the formation of thinnercoatings on the dispersed cores.

It is mentioned here that the ‘‘coated particles” in the rest of thetext would mean the particles shown in Fig. 5A.

In order to determine the composition of the shell material inthe coated particles, the former was precipitated from the coatingmixture, described in Fig. 5A, in the absence of the dispersed cores.The product obtained is displayed in Fig. 5C. Inspection of this fig-ure showed that the obtained morphology of these particles wasdifferent than that of the shell material in the coated particles,

20 30 40 50 60 70 80

Inte

nsit

y (a

.u.)

2θ (degree)

a aa

a

B

A

a

a, Co3O

4

Fig. 8. X-ray diffraction (XRD) patterns of the particles shown in Fig. 5C (A) andFig. 5D (B).

Fig. 9. Energy-dispersive X-ray analysis (EDX)

which indicated that the presence of dispersed cores did affectthe particles growth during the precipitation process. The particlesshown in Fig. 5C were found to be amorphous in nature (XRD,Fig. 8A). IR analysis of these particles showed prominent absorp-tion bands corresponding to –OH (3500 cm�1), –C@O (1370–1500 cm�1), and cynate (2250 cm�1) (see Fig. 3B). It is added thatthe cynate was one of the decomposition product of the thermaldecomposition of urea [8], which might have incorporated in theprecipitated solid during the particles growth.

Thermogravimetric analysis demonstrated (TGA, Fig. 4B) thatthe particles in Fig. 5C experienced 34.4% loss in their weight at200–250 �C, which most probably took place according to the fol-lowing phase transition reaction (Eq. (2)):

CoCO3 � CoðOHÞ2 � 2H2O )�250 �C

35:24%Co3O4 þ 9H2Oþ 2CO2 ð2Þ

Based on the agreement between the weight losses in Eq. (2) andthe TGA trace in Fig. 4B, the particles in Fig. 5C were formulatedas CoCO3�Co(OH)2�2H2O.

The thermally initiated weight loss in CoCO3�Co(OH)2�2H2O alsoappeared in the form of some damage to the particle morphology(SEM, Fig. 5D), obviously due to the loss of material.

XRD analysis confirmed the composition of the heat-treatedparticles as crystalline Co3O4 (see, Fig. 8B). Similarly, the twoabsorption bands (doublets) at 550, and 680 cm�1 in the IR spec-trum (Fig. 7B) of the particles in Fig. 5D may be ascribed to the dif-ferent vibration modes of Co–O bonds.

Energy-dispersive X-ray analysis was performed with thecoated particles and the resulting EDX pattern is shown in Fig. 9.This figure showed the presence of copper and cobalt in the coatedparticles, which were obviously part of the core and shell materi-als, respectively. Chemical analysis indicated that the coated parti-cles contained 5 and 30 wt.%, of cobalt and copper, respectively. Itis further added that the chemically estimated amounts of cobaltand copper in the coated particles were close enough with theamount of these metals, calculated on the basis of the experimen-tally determined weight fractions of the shell (CoCO3�Co(OH)2�2H2O) and core (CuC2O4) materials in the coated solids.

X-ray diffractometric analysis demonstrated that the XRD pat-tern (Fig. 10A) of the coated particles was identical with that of

pattern of the particles shown in Fig. 5A.

20 40 60 80

A

BInte

nsit

y (a

.u.)

2θ (degree)

aaaa

a

c

c

bbb

bb

a = CuC2O

4

b = CuOc = Co

3O

4

Fig. 10. X-ray diffraction (XRD) patterns of the particles shown in Fig. 5A (A) andFig. 5B (B).

720 I. ul Haq, F. Haider / Advanced Powder Technology 22 (2011) 715–721

the core particles (XRD, Fig. 2A). This showed that the shellmaterial was amorphous in nature. Moreover, the absence of anypeak in the XRD pattern of the coated particles other than thosefor copper oxalate (cores) demonstrated that the cores acted asdormant substrates for the deposition of the shell material andno chemical interaction took place between the core and shellmaterials during the coating process which might have led to thepossible formation of some crystalline composite. It was also notedthat the IR spectrum (Fig. 3C) of the coated particles was in fact acomposite spectrum, composed of all the absorption bands, pres-ent in the IR spectrum of CuC2O4 (Fig. 3A) and CoCO3�Co(OH)2�2H2O (Fig. 3B).

Furthermore, it was of interest to see the response of the shelland core materials in the coated particles towards the heat treat-ment. In this regard, the thermogravimetric analysis was carriedout with the latter particles in the temperature range 30–600 �C.The weight loss registered in the coated particles during heat treat-ment was plotted as a function of temperature in Fig. 4C. This fig-ure showed that the coated particles lost 11% and 33% of materialat �200 and �290 �C, respectively. These losses in materials wereattributed to the presence of shell and core materials, whichdecomposed into thermally stable products at the mentioned tem-peratures and ended up with particles, showing slightly differenttextures (SEM, Fig. 5B).

Moreover, the observed weight losses in the coated particles(TGA, Fig. 4C) agreed well with the experimentally determinedweight fractions of the shell (�30%) and core (�70%) materials inthe coated particles, believing that the core (CuC2O4) and shell (Co-CO3�Co(OH)2�2H2O) transformed independently into CuO and Co3O4

according to Eqs. (1) and (2), respectively. This was confirmed bythe X-ray diffractometric analysis (XRD, Fig. 10B) of the heat treatedcoated particles (SEM, Fig. 5B), which indicated that the latter werecomposed of only Co3O4 and CuO. Similarly, IR spectrum (Fig. 7C) ofthe particles in Fig. 5B appeared to be a composite spectrum of theIR spectra, displayed in Fig. 7A and B, which also supported theindependent transformation of the shell and core materials to theirrespective oxides during heat treatment.

It is worth mentioning here that the mutual inertness of theshell and core material during heat treatment was also observedin some of our earlier studies [20–22], concerning with other typesof coated systems. However, it contradicted with the results re-ported elsewhere [15,23]. These findings suggested the facts thatthe association status (chemical or physical) of shell and corematerials in the coated particles, their chemical compositions,

and heating temperatures may be held responsible for the incep-tion of solid state reaction at high temperatures.

4. Conclusions

(1) Gentle mixing of equal volumes of the aqueous solutions ofoxalic acid and copper nitrate at room temperature producesmicrosize uniform spherical particles of copper oxalate,when concentration of both the reactants are kept at0.04 mol/L. Change in concentration of either of the reac-tants had obvious effect on the morphology of the precipi-tated particles. This observation suggests that excessamount of either of the reactants interact with the stochio-metrically formed particles and thereby affect their growthprocess.

(2) Heating aqueous dispersions, containing the optimizedamounts of the spherical particles of copper oxalate (cores),urea, and cobalt nitrate at 85 �C, generates coated particles,comprised of shells of cobalt basic carbonate around the dis-persed cores. Uniformly coated particles were obtainedunder limited range of the coating mixture compositionand the applied temperature. These findings points to theinvolvement of complex chemistry in the precipitation pro-cess of the shells on the suspended substrates (cores). More-over, on heating in the absence of the cores, the samesolutions produced nearly flower shaped particles of cobaltbasic carbonate (coating precursors), which clearly indicatedthat the presence of cores in the reactant mixtures had obvi-ous affect on the precipitation mechanism.

(3) On heat-treatment at 340 �C, dry powders of core, coated,and coating precursor particles converts into oxide formwith obvious change in the surface morphology of the parti-cles due to thermally initiated loss of material. However, allthe particles maintain their shape integrity to a maximumextent, which indicated thermal stability of the particles atthe mentioned temperature.

Acknowledgements

We are thankful to the Higher Education Commission of Paki-stan and the National Centre of Excellence in Physical Chemistry,University of Peshawar, NWFP, Pakistan for supporting this work.

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