R ESEARCH ARTICLE

doi: 10.2306/scienceasia1513-1874.2012.38.102

ScienceAsia 38 (2012): 102–107

Synthesis of perovskite-type lanthanum cobalt oxidepowders by mechanochemical activation methodSupachai Sompecha, Autcharaporn Srionb, Apinon Nuntiyac,∗

a Department of Physics and Materials Science, Faculty of Science, Chiang Mai University,Chiang Mai 50200 Thailand

b National Metal and Materials Technology Centre, Klong Luang, Pathum Thani 12120 Thailandc Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200 Thailand

∗Corresponding author, e-mail: nuntiyaapinon@gmail.comReceived 13 Aug 2011Accepted 5 Mar 2012

ABSTRACT: Lanthanum cobalt oxide (LaCoO3) powders were prepared from mixtures of LaCl3 · 7 H2O, CoCl2, andNa2CO3 by grinding, heating, and washing operations. The reagents were mixed in a molar ratio of 1:1:2.5 in a planetaryball mill and milled at 300 rpm for 2 h. The milled samples were heated at various calcination temperatures and washedwith distilled water. Thermogravimetric and differential thermal analysis were used to evaluate the optimum conditions forcalcination. Phase formation was determined by X-ray diffraction (XRD) while specific surface area was measured by theBET method. The average particle size distribution was determined by a particle size analyser and morphology studiedby scanning electron microscopy (SEM). The TG and DTA curves of the milled samples indicated that the formation ofLaCoO3 occurred at temperatures in the range of 600 °C to 800 °C. XRD patterns showed clearly the formation of theLaCoO3 phase with perovskite-type structure at those temperatures. In addition, the results showed that the specific surfaceareas of the products decreased with increasing calcination temperature, while the average particle size D[4,3] increased.Furthermore, SEM micrographs demonstrated that the particles were in an agglomerated form with mean primary particlesizes in the range of 0.3–0.6 µm.

KEYWORDS: planetary ball mill, specific surface area, particle size, XRD, SEM

INTRODUCTION

The recent enforcement of severe emission controlsin auto exhaust and of environmental standards indeveloping countries have prompted the developmentof catalytic converters for automobile and industrialemission control during the past decade. However,the most effective catalysts used to date are expensiveconventional platinum/rhodium metals. Replacingthese costly metal catalysts with cheaper materials hasbeen an interesting and challenging research problemfor technologists and material scientists1. Therefore,many researchers have been searching for alternativematerials as the active catalytic phase. One of the mostpromising active phase for environmental applicationsis the perovskite group of materials (ABO3) whereA and B are cations of different sizes. The catalyticproperties of perovskite-type oxides depend mainly onthe nature of the A and B ions and on their valencestates2. LaCoO3 has many practical applicationsowing to its excellent physical and chemical propertiesand has been shown to have high catalytic activity forthe oxidation of carbon monoxide, methane, propane,

hexane, and toluene. Thus, it can be used as a catalystfor combustion, automobile exhaust, and waste gaspurification. Besides, it can be used as an electrodematerial for solid-electrode fuel cells and gas sen-sors3.

As the properties of the final materials obtainedare strongly dependent on the method of preparation,it is essential to control the synthesis of homogeneous,highly pure, and high surface area LaCoO3 materials.Since the activity of a catalyst strongly depends on itssurface area, the development of a convenient methodfor the synthesis of LaCoO3 with a sufficiently highspecific surface area is also essential. The synthesesof LaCoO3 powders have been achieved by severalmethods such as the conventional method based onsolid-state reactions at high temperature. The mainadvantages of this method are its simplicity and theuse of inexpensive oxides as starting materials, butit also has some drawbacks such as high reactiontemperature, large particle size, and limited degreeof chemical homogeneity. Other methods of prepa-ration include the sol-gel process4, solid-state thermaldecomposition of precipitated precursors5, flame hy-

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drolysis of an aqueous solution6, Pechini and Schiffbased methods7, amorphous heteronuclear complexdecomposition synthesis8, the polymerizable complexmethod9, the molten chloride flux method10, the al-kaline co-precipitation method11, and electrochemicaloxidation12. These methods can produce fine andhomogeneous particles with a high specific surfacearea but the processes are generally complicated andthe reagents used are very expensive. To overcomethese drawbacks, some researchers have developedanother route based on mechanochemical activationfor the synthesis of LaCoO3 powders to producenanoparticles and homogeneous particles. Recently,it has been reported that nanoparticle materials canbe synthesized by this mechanochemical activationmethod13–15. In this method, the chemical precursorsundergo reactions, either during milling or duringsubsequent low temperature heat treatment, to forma composite powder consisting of fully dispersednanoparticles embedded within a soluble salt by-product. The nanoparticles are then recovered byremoving the salt by-product by a simple washingprocedure.

Zhang et al16–18 have successfully synthesized asingle perovskite phase of ABO3 (A = La, Nd, Pr,Sm; B = Mn, Co, Fe, Ni) by grinding the mixture ofconstituent oxides using a planetary ball mill at roomtemperature. In this direct method for the synthesisof nanoparticle materials, it has been found that thereactivities of the starting oxides plays a significantrole in the completion of the mechanochemical solidreaction. The reactivities of the starting oxides isfound to be dependent on the preparation conditionsand the reaction proceeds with the grinding time. Theground samples were formed with a single perovskitephase after a milling time of about 120–180 min butthey were agglomerated giving large particles withlow specific surface areas. Consequently, attemptshave been made to increase the specific surface areaof the final product by a combination method19.This method, which is termed the indirect methodfor the synthesis of nanoparticle materials, producedpowders consisting of weak agglomerates which werewell dispersed and which had high specific surfaceareas. Moreover, this combination method has beenrecommended as a novel mechanochemical approachfor preparing nanoparticle materials.

The purpose of this present study was to synthe-size LaCoO3 powders from mixtures of LaCl3 · 7 H2O,CoCl2, and Na2CO3 by the mechanochemical activa-tion method. The microstructure, average particle sizedistribution, specific surface area, and morphology ofthe nanoparticles obtained have been investigated.

MATERIALS AND METHODS

Materials and preparation

The starting materials were LaCl3 · 7 H2O (Fluka,99.0%), CoCl2 (Sigma-Aldrich, 97%), and Na2CO3(Fluka, 99.0%). The powders were dried at 70 °C inair for 2 h prior to use. The starting materials weremixed in a molar ratio of 1:1:2.5 before milling. Aplanetary ball mill (Retsch, PM 100) was used to millthe starting mixture at 300 rpm under atmosphericconditions. A 20 g portion of the mixture was placedin a stainless steel pot of 250 cm3 inner volume with14 stainless steel balls of 20 mm diameter and milledfor 2 h. The milled sample was calcined at 600, 700,and 800 °C at a heating rate of 10 °C/min for 90 min.After that, the calcined samples were washed withdistilled water several times to remove the NaCl phasefrom the by-product. Then, 15 g of each calcinedpowder were dispersed in 300 ml of distilled waterand stirred continuously with a magnetic stirrer for1 h at room temperature. Subsequently, the sampleswere filtered and dried at 110 °C for 3 h and labelledas LaCoO3 600, LaCoO3 700 and LaCoO3 800, re-spectively.

Characterization

The thermal behaviour of the milled samples wasinvestigated by thermogravimetry from room tem-perature to 1000 °C in static air with α-Al2O3 as areference. A simultaneous thermogravimetric and dif-ferential thermal analysis (TG-DTA) apparatus (Net-zsch, STA 409 C/CD) at a heating rate of 10 °C/minwas used. The milled, calcined, and washed sam-ples were characterized by X-ray diffraction (XRD)analysis (Philips, X’Pert Pro MPD) using Cu-Kαradiation over the angular range of 2θ = 5° to 80°to identify the various phases present in the product.The average particle size distribution was determinedusing the laser diffraction method fitted with a wetsampling system (Malvern, Mastersizer S). The parti-cle diameters reported were calculated using volumedistribution (D[4,3]). The specific surface areas ofthe samples were measured by the BET nitrogenadsorption method (Quantachrome, Version 1.11) atthe liquid nitrogen temperature using nitrogen gas asan absorbent. The samples were degassed at 120 °Cfor 4 h before BET analysis. The morphologiesof the samples were observed by scanning electronmicroscopy (Jeol, JSM-6301F).

RESULTS AND DISCUSSION

Examples of the TG and DTA curves of the milledsamples are shown in Fig. 1. In the TG curve, the

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100 200 300 400 500 600 700 800 900

60

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80

90

100

En

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Fig. 1 TG-DTA curves of a sample after milling for 2 h:(a) TG curve and (b) DTA curve.

weight loss step of about 5.98% in the range of 90 °Cto 160 °C accompanied by the endothermic peak at114.1 °C in the DTA curve that can be ascribed tothe loss of water (H2O). In addition, the weight lossstep of about 12.54% in the range of 250 °C to 570 °Caccompanied by the exothermic peak at 275.3 °C inthe DTA curve is due to the decomposition of thecarbonates, and the endothermic peaks at 477.0 °C and565.4 °C are attributed to the release of CO2. Theevaporation of H2O and CO2 are consistent with theoverall reaction given below. In addition, the DTAcurve also shows an endothermic peak at 633.5 °C,without accompanying weight loss, indicating the for-mation of a perovskite-type LaCoO3 phase at this tem-perature19. The sharp endothermic peak at 792.3 °Ccorresponds to the melting of NaCl (the referencemelting point of NaCl is about 801 °C)20.

The XRD patterns of samples before and aftermilling for 2 h at a speed of 300 rpm are shown inFig. 2. The XRD pattern in Fig. 2a shows only thepeaks of the three starting materials: LaCl3 · 7 H2O,CoCl2, and Na2CO3. After milling, the intensitiesof these peaks decreased gradually until they dis-appeared completely after a milling time of 2 h.After milling, Fig. 2b clearly shows only the newpeaks of NaCl which correspond to the standardpowder diffraction pattern of NaCl (JCPDS file No.05-0628). The appearance of this NaCl phase indi-cated that displacement reactions between chloridesand Na2CO3 occurred during milling and that otherphases, such as various types of carbonate and possi-bly unreacted starting compounds, are present in theform of amorphous or minor phases in the groundproduct20. The NaCl phase formed in the groundproduct plays an important role in the dispersion

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Fig. 2 XRD patterns of samples before and after milling:(a) starting mixture before milling and (b) sample milled for2 h.

of the fine LaCoO3 particles after subsequent heattreatment13, 14, 21. This occurrence of this reaction in-dicated that the mechanochemical solid-state reactioncan proceed during milling of the powder mixtureof starting materials using a high-energy ball mill atroom temperature22–24. Moreover, the XRD patternsin Fig. 3 reveal new peaks of the LaCoO3 phase afterheat treatment at 600, 700, and 800 °C for 90 minwhich are associated with the NaCl phase. Thisindicates that a reaction between the starting materialsoccurred during calcination which can be representedby the following equation:

LaCl3 · 7 H2O(s) + CoCl2 (s) +52 Na2CO3 (s) +

14 O2 (g)

−−→ LaCoO3 (s) + 5 NaCl(s) +52 CO2 (g) + 7 H2O(g)

Normally, the LaCoO3 phase is synthesized fromthe constituent oxides as starting materials by heatingat around 1000 °C25. In comparison, the temperatureof about 600 °C used in this present work is consider-ably lower, which may be due to the mechanochemicaleffect induced by the milling. These results havetherefore demonstrated that the LaCoO3 phase can besynthesized successfully through this process.

Fig. 4 shows the XRD patterns of samples af-ter washing with distilled water. It was found thatonly the main peaks of the LaCoO3 phase togetherwith minor impurities of Co3O4 are present for allconditions, indicating that the NaCl phase in theproduct was successfully removed by the washing andfiltration operations. The XRD patterns also showsharper peaks with increasing calcination temperature,

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20 30 40 50 60 70

NaCl

LaCoO3

(c)

(b)

(a)

Inte

nsi

ty [

a.u

.]

2 (degrees)

Fig. 3 XRD patterns of a milled sample after heat treatmentat different temperatures: (a) at 600 °C, (b) at 700 °C, and(c) at 800 °C.

20 30 40 50 60 70

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Fig. 4 XRD patterns of samples after heat treatment andwashing with distilled water: (a) at 600 °C, (b) at 700 °C,and (c) at 800 °C.

suggesting that the powders were well crystallizedafter heat treatment. In addition, the peak positionsand their relative intensities correspond to the standardpowder diffraction pattern of a LaCoO3 phase (JCPDSfile No. 25-1060) with perovskite-type structure3.

The average particle size (D[4,3]) and specificsurface area of a LaCoO3 powder depend on thecalcination temperature, (Fig. 5). The results showthat the average particle sizes of the LaCoO3 powdersincreased rapidly with increasing calcination temper-ature from 3.14 to 7.96 and 9.87 µm after calcina-tion at 600, 700, and 800 °C, respectively (Fig. 5a).This is in accordance with the XRD patterns which

600 700 8002

3

4

5

6

7

8

9

10

11

12

(b)

Av

era

ge

pa

rtic

le s

ize

[ m

]

Su

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ce a

rea

[m2

/g]

Temperature [oC]

(a)

6

9

12

15

18

21

24

27

30

Fig. 5 Average particle size and specific surface area ofsamples after heat treatment and washing with distilledwater: (a) average particle size and (b) specific surface area.

show slightly sharper peaks with increasing calcina-tion temperatures, indicative of larger particles sizes.In contrast, the specific surface area decreased withincreasing calcination temperature, decreasing from25.67 down to 11.02 and 8.61 m2/g after calcination at600, 700, and 800 °C, respectively (Fig. 5b). The in-creasing average particle size and decreasing specificsurface area are also confirmed by the SEM micro-graphs showing grain growth and increasing degree ofparticle agglomeration at higher temperatures.

The morphology of the samples was investigatedby scanning electron microscopy (SEM). Fig. 6 showsthe SEM images of the microstructures of the LaCoO3powders obtained by calcination at 600, 700, and800 °C and after washing with distilled water. TheseSEM micrographs clearly demonstrate that the par-ticles have different shapes from different calcina-tion temperatures but are generally quite sphericaland clustered together. The SEM micrographs alsodemonstrate what are obviously polycrystalline par-ticles composed of many crystallites with a uniformgrain size distribution and homogeneous microstruc-ture. In addition, it was found that both the meanprimary particle size and degree of agglomeration ofthe particles tended to increase with the calcinationtemperature. In Fig. 6a, the SEM micrograph of theLaCoO3 600 powder shows a mean primary particlesize in the range of 0.3–0.4 µm with a low degree ofagglomeration of the particles. However, Fig. 6b andc show the LaCoO3 700 and LaCoO3 800 powdersto have mean primary particle sizes in the ranges of0.4–0.5 µm and 0.5–0.6 µm, respectively, with higherdegrees of agglomeration. The increases in primaryparticle size and degree of agglomeration were a result

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106 ScienceAsia 38 (2012)

Fig. 6a

Fig. 6b

Fig. 6c Fig. 6 SEM micrographs of samples after heat treatmentat different temperatures and washing with distilled water:(a) at 600 °C, (b) at 700 °C, and (c) at 800 °C.

of the grain growth of the particles after calcination athigh temperature.

CONCLUSIONS

LaCoO3 powders can be synthesized fromLaCl3 · 7 H2O, CoCl2, and Na2CO3 by milling,heating and washing operations (involving high-

energy ball milling and subsequent heat treatment atlow temperatures). The XRD patterns of the samplesclearly demonstrated the LaCoO3 phase formationafter heat treatment at 600, 700, and 800 °C, resultingin LaCoO3 phases with perovskite-type structure.In addition, the LaCoO3 powders can also bedispersed in a NaCl matrix after heat treatment,while the average particle size and specific surfacearea of the samples depended on the calcinationtemperature. Whereas the average particle sizeincreased with increasing calcination temperature, thespecific surface area decreased. Furthermore, SEMmicrographs revealed spherical particles clusteredtogether with an agglomerate size depending on thecalcination temperature.

Acknowledgements: This study was supported by agrant from under the program Strategic Scholarships forFrontier Research Network for the Ph.D. Program ThaiDoctoral degree from the Commission on Higher Education,Thailand. The authors are also grateful to the Faculty ofScience and the Graduate School of Chiang Mai Universityfor partial financial support.

REFERENCES1. Vaz T, Salker AV (2007) Preparation, characterization

and catalytic CO oxidation studies on LaNi1-xCoxO3

system. Mater Sci Eng B 143, 81–4.2. Seyfi B, Baghalha M, Kazemian H (2009) Modified

LaCoO3 nano-perovskite catalysts for the environmen-tal application of automotive CO oxidation. Chem EngJ 148, 306–11.

3. Yang Z, Huang Y, Dong B, Li HL, Shi SQ (2006) Sol-gel template synthesis and characterization of LaCoO3

nanowires. Appl Phys Mater Sci Process 84, 117–22.4. Simonot L, Garin F, Maire G (1997) A compara-

tive study of LaCoO3, Co3O4 and LaCoO3–Co3O4: I.Preparation, characterisation and catalytic propertiesfor the oxidation of CO. Appl Catal B Environ 11,167–79.

5. Traversa E, Sakamoto M, Sadaoka Y (1998) A chem-ical route for the preparation of nanosized rare earthperovskite-type oxides for electroceramic applications.Particul Sci Technol 16, 185–214.

6. Giacomuzzi RAM, Portinari M, Rossetti I, Formi L(2000) A new method for preparing nanometer-sizeperovskitic catalysts for CH4 flameless combustion.Stud Surf Sci Catal 130, 197–202.

7. Worayingyong A, Kangvansura P, Ausadasuk S,Praserthdam P (2008) The effect of preparation: Pe-chini and Schiff base methods, on adsorbed oxygen ofLaCoO3 perovskite oxidation catalysts. Colloid SurfaceA 315, 217–25.

8. Zhu Y, Tan R, Yi T, Ji S, Ye X, Cao L (2000) Prepa-ration of nanosized LaCoO3 perovskite oxide using

www.scienceasia.org

ScienceAsia 38 (2012) 107

amorphous heteronuclear complex as a precursor at lowtemperature. J Mater Sci 35, 5415-20.

9. Popa M, Frantti J, Kakihana M (2002) Characterizationof LaMeO3 (Me: Mn, Co, Fe) perovskite powders ob-tained by polymerizable complex method. Solid StateIonics 154-155, 135–41.

10. Armelao L, Bandoli G, Barreca D, Bettinelli M, BottaroG, Caneschi A (2002) Synthesis and characterization ofnanophasic LaCoO3 powders. Surf Interface Anal 34,112–5.

11. Liang JJ, Weng H-S (1993) Catalytic propertiesof lanthanum strontium transition metal oxides(La1-xSrxBO3; B = Mn, Fe, Co, Ni) for toluene oxi-dation. Ind Eng Chem Res 32, 2563–72.

12. Matsumoto Y, Sasaki T, Hombo J (1992) A new prepa-ration method of LaCoO3, perovskite using electro-chemical oxidation. Inorg Chem 31, 738–41.

13. Tsuzuki T, McCormick PG (2004) Mechanochemicalsynthesis of nanoparticles. J Mater Sci 39, 5143–6.

14. Dodd AC, McCormick PG (2001) Synthesis of nano-particulate zirconia by mechanochemical processing.Scripta Mater 44, 1725–9.

15. Stojanovic BD (2003) Mechanochemical synthesis ofceramic powders with perovskite structure. J MaterProcess Tech 143-144, 78–81.

16. Zhang Q, Saito F (2000) Mechanochemical synthesisof LaMnO3 from La2O3 and Mn2O3 powders. J AlloyComp 297, 99–103.

17. Zhang Q, Saito F (2000) Mechanochemical synthesisof lanthanum aluminate by grinding lanthanum oxidewith transition alumina. J Am Ceram Soc 83, 439–41.

18. Zhang Q, Lu J, Saito F (2002) Mechanochemicalsynthesis of LaCrO3 by grinding constituent oxides.Powder Tech 122, 145–9.

19. Saito F, Zhang Q, Kano J (2004) Mechanochemical ap-proach for preparing nanostructural materials. J MaterSci 39, 5051–5.

20. Muroi M, Street R, McCormick PG (2000) Enhance-ment of critical temperature in fine La0.7Ca0.3MnO3

particles prepared by mechanochemical processing.J Appl Phys 87, 3424–31.

21. Dodd AC, Raviprasad K, McCormick PG (2001) Syn-thesis of ultrafine zirconia powders by mechanochemi-cal processing. Scripta Mater 44, 689–94.

22. Tsuzuki T, McCormick PG (2000) Synthesis of Cr2O3

nanoparticles by mechanochemical processing. ActaMater 48, 2795–801.

23. Dodd AC, McCormick PG (2001) Solid-state chemicalsynthesis of nanoparticulate zirconia. Acta Mater 49,4215–20.

24. Tsuzuki T, McCormick PG (2001) ZnO nanoparticlessynthesised by mechanochemical processing. ScriptaMater 44, 1731–4.

25. Nakayama S, Okazaki M, Aung YL, Sakamoto M(2003) Preparations of perovskite-type oxides LaCoO3

from three different methods and their evaluation byhomogeneity, sinterability and conductivity. Solid State

Ionics 158, 133–9.

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