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Copyright © 2011 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 11, 1–9, 2011

Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles

Katalin Sinkó1�∗, Géza Szabó1, and Miklós Zrínyi21Institute of Chemistry, L. Eötvös University, H-1117 Budapest, Hungary

2Department of Biophysics, Semmelweis University, H-1085 Budapest, Hungary

Various liquid-phase syntheses of CoO and Co3O4 nanoparticles have been studied. The experi-ments focus on two synthesis routes: the coprecipitation and the sol–gel methods combined withthermal decomposition. The effect of synthesis route, the type of precursors (cobalt nitrate/chloride)and precipitation agent (carbonate, hydroxide, oxalic acid, and ammonia), the chemical composi-tions, pH, application of surfactants (PDMS, Triton X-100, NaDS, NaDBS, TTAB, ethyl acetate, citricacid), and the heat treatments on the properties of particles were investigated. The particle sizeand distribution have been determined by dynamic light scattering (DLS). The phases and the mor-phology of products have been analysed by XRD and SEM. The coprecipitation technique is lessable to shape the particles than sol–gel technique. PDMS can be applied efficiently as surfactant inpreparation methods. The finest particles (around 85 nm) with narrow polydispersity (70–100 nm)and spherical shape could be achieved by using sol–gel technique in medium of 1-propanol andethyl acetate.

Keywords: Cobalt Oxide, Nanoparticles, Coprecipitation, Sol–Gel Method.

1. INTRODUCTION

Recently, nanostructured transition metal oxides haveattracted a lot of attention due to their technological appli-cations and outstanding properties. The properties (such asmagnetic, optic, catalytic, and electronic) of nanomaterialsdepend strongly on their size, structure, and shape. Co3O4

nanoparticles exhibit weak ferromagnetic behavior.1 CoOnanocrystals display superparamagnetism or weak ferro-magnetism, whereas bulk CoO is antiferromagnetic.2�3

Co3O4 is a magnetic p-type semiconductor. Co3O4 hasa cubic spinel crystal structure in which the Co2+ ionsoccupy the tetrahedral sites and the Co3+ ions the octa-hedral sites.4 The Co3+ ions at the octahedral sites arediamagnetic in the octahedral crystal field. The Co2+

ions at the tetrahedral sites form an antiferromagneticsublattice with a diamond structure. The cobalt spinel com-pounds can act as efficient catalysts in a lot of heteroge-neous chemical processes.5–8 Nanoparticles of Co3O4 arepromising materials for electronic devices,9 gas sensors,10

magnetic materials,11 electrochromic devices,12 electro-chemical systems,13 and high-temperature solar selectiveabsorbers.14 CoO also shows interesting properties and has

∗Author to whom correspondence should be addressed.

applications as gas sensors15 and as anodes of lithium-ionbatteries.16�17

Many different synthesis techniques give access to nano-materials with a wide range of compositions, well-definedand uniform crystallite sizes. The liquid-phase synthe-ses offer a good technique and control for tailoring thestructures, the compositions, and the morphological fea-tures of nanomaterials. The liquid-phase routes include thecoprecipitation, the hydrolytic as well as the nonhydrolyticsol–gel processes, the hydrothermal or solvothermalmethods, the template synthesis and microemulsion-basedprocesses.The coprecipitation method offers some advantages.

Applying this simple and rapid technique, the control ofparticle size and composition is easy. The precipitationmethod provides several possibilities to modify the parti-cle surface and shape. Precipitation of various cobalt salts(nitrate,18–20 chloride,21�22 acetate20�23) from aqueous oralcoholic-aqueous solutions yields cobalt oxide nanopar-ticles. The particle size of the coprecipitated materials isstrongly dependent on the pH of medium and the concen-tration of the initial materials. The precipitation agent maybe oxalic acid or oxalate,19�20�24 carbonate,19�25 ammonia,26

and sodium hydroxide.25 Organic surfactants (e.g., sodiumdodecyl sulphate19) can be used during the precipitation in

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order to tailor the size and shape of nanoparticles and tohinder the aggregation.In sol–gel synthesis, a soluble precursor molecule is

hydrolyzed to form a dispersion of nano-sized particles.Inorganic cobalt salts are subjected to hydrolysis andcondensation generally in ethanol at 50–80 �C.20�27–29

Monodisperse tetrapod-shaped CoO nanocrystals wereprepared via sol–gel alcoholysis of cobalt(III) oleate indodecanol at high temperature. The dodecanol solvent actsas a reducing and a morphology controller reagent.30 Theheat treatment of wet gels has a strong effect on the sur-face area, pore volume, crystallinity, particle structure, andcorresponding electrochemical properties of the resultingxerogels.31

Nanoparticles can be also formed on liquid–liquid inter-faces. Among the chemical methods the microemulsionprocess involving reverse micelles has been demon-strated as a versatile method for obtaining a wide vari-ety of nanocrystalline oxides.32–35 Functionalized reversemicelles may be used to control the size and the poly-dispersity. The size of particles can be affected eitherby the volume of water, or by the solvent used toform reverse micelles, or by adding surfactants.32 In themicroemulsion (reverse micellar) route, the first step isthe synthesis of cobalt salt (e.g., oxalate23) nanoparti-cles from precursors in the presence of a surfactant (e.g.,cetyltrimethylammonium bromide23) and a cosurfactant(e.g., n-butanol23).Among all the kinds of synthetic techniques, hydrother-

mal/solvothermal method has been achieved some successdue to its simplicity, low-cost and different morpholo-gies of products.35–40 The hydrothermal/solvothermal meth-ods can be applied to form crystalline materials (films,nanoparticles, single-crystals etc.) from aqueous solu-tions, under high vapor pressure. The control of solution-recrystallization and growth of particles yields all kindsof nanostructured materials. Co3O4 crystals with variousmorphologies, such as nanospheres,41 nanorods,42�43 nano-crystalline,44 nanocubes,45 hollow spheres,46�47 nanorodsbunches48 and urchin-like nanocrystals49 can be success-fully prepared by a hydrothermal or solvothermal process.The aim of the present study was to synthesize CoO

and Co3O4 nanoparticles by various liquid-phase synthe-ses. The experiments were mostly carried out by twosynthesis routes; by coprecipitation and sol–gel methods.These techniques were combined with thermal decompo-sition. We have monitored the effect of synthesis route,the type of precursors, the chemical compositions, theconcentration of initial materials, the pH, the applicationof surfactants, and the heat treatments on the proper-ties of particles. The particle size and distribution weredetermined by dynamic light scattering (DLS). The iden-tification and characterization of the phases and the mor-phology of products were performed by X-ray diffraction(XRD) and scanning electron microscope (SEM). The

processes and the weight loss occurred by heat treat-ments were recorded by thermal analysis in a controlledatmosphere.

2. EXPERIMENTAL DETAILS

2.1. Preparation Methods

In the experiments of the coprecipitation method, cobaltnitrate and chloride were provided as cobalt precursors.The aqueous solution of precipitation agents (sodium car-bonate, oxalic acid, and ammonia) was dropped to theaqueous solution of cobalt precursors. The concentrationof cobalt precursor varied from 0.01 to 5 mol dm−3, theratio of precipitator/Co(II) ions from 1.0 to 5.0 (Table I).The mixtures were stirred for 2 h at room temperaturewith or without a surfactant. Pink (Co oxalate), violet(Co carbonate), or green (Co hydroxide) precipitates wereobtained. The dried precipitates were subjected to heattreatment at various temperatures resulting in CoO inreductive atmosphere and Co3O4 in oxidative atmosphere.Polydimethylsiloxane (PDMS); polyethylene glycol tert-octylphenyl ether (Triton X-100); sodium dodecyl sulfate(NaDS); sodium dodecylbenzene sulfonate (NaDBS); andtetradodecylammonium bromide (TTAB) were used as sur-factants in the concentration of 0, 1, 5, and 10 w/w%(Table I).In the experiments of the sol–gel method, only

Co(NO3�2 could be employed as initial material, becauseCoCl2 has poor solubility in alcohol solvents. The Co2+

ions were allowed to hydrolyze for 2 h at 50 �C in first

Table I. Summary of preparation experiments.

SurfactantsMethods Solvent Precipitator Surfactants (w/w%)

Coprecipitationa Water Na2CO3 — —Coprecipitation Water Na2CO3 Triton X-100 1, 5, 10Coprecipitation Water Na2CO3 NaDS 1, 5, 10Coprecipitation Water Na2CO3 NaDBS 1, 5, 10Coprecipitation Water Na2CO3 TTAB 1, 5Coprecipitationa Water Na2CO3 PDMS (550)b 1, 5, 10Coprecipitationa Water Na2CO3 PDMS (5600)b 1, 5, 10Coprecipitation Water Na2CO3 PDMS (5180)b 5Coprecipitation Water Na2CO3 PDMS (42500)b 5Coprecipitation Water Na2CO3 PDMS (83000)b 5

Coprecipitationa Water Oxalic acid — —Coprecipitation Water Oxalic acid Triton X-100 1Coprecipitation Water Oxalic acid NaDS 1Coprecipitation Water Oxalic acid NaDBS 1Coprecipitation Water Oxalic acid TTAB 1Coprecipitationa Water Oxalic acid PDMS (5600)2 1, 5, 10

Sol–gelc Ethanol — — —Sol–gelc Ethanol — Citric acid 0.1–1.0Sol–gelc Ethanol — PDMS (550)2 0.1–2.5Sol–gelc 1-propanol — Ethyl acetate 10, 20, 40

aConcentration of Co nitrate varied from 0.01 to 2 mol dm−3. bMolecular weight,g mol−1. cConcentration of Co nitrate was 0.4 mol dm−3.

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step and for 2 h at 85 �C in the second step in ethanolor 1-propanol solution with (citric acid, PDMS, or ethylacetate) or without a surfactant (Table I). The sol–geltechnique produced different basic Co nitrate precipitatesdepending on the used solvent and surfactant. The precip-itates were dried at 80 �C and heated at different temper-atures under oxidative atmosphere. In order to obtain alsoCoO, the dried samples were heat treated at 1000 �C underreductive (N2� atmosphere.

2.2. Characterisation Methods

Dynamic light scattering measurements (DLS) were per-formed by means of dynamic light scattering equip-ment (Brookhaven) consisting of a BI-200SM goniometerand a BI-9000AT digital correlator. An argon-ion laser(Omnichrome, model 543AP) operating at 488 nm wave-length and emitting vertically polarized light was used asthe light source. The signal analyzer was used in real-time“multi tau” mode. In this mode the time axis was loga-rithmically spaced over an appropriate time interval andthe correlator used 218 time channels. The pinhole was100 �m. The particles were dispersed in ethanol for DLSmeasurements instead of water in order to avoid the aggre-gation of the particles in water.The X-ray Diffraction (XRD) measurements were car-

ried out by using a Philips (PW1130) X-ray generator setup with a Guinier-chamber. The chamber has a diameter of100 mm and the patterns were recorded on FUJI ImagingPlates (BAS MS2025). The XRD data were collected overthe 2� range of 9–90� with a step size 0.005�. Identifica-tion of phases was carried out by comparing the diffractionpatterns with the standard PDF cards.The morphology was studied by scanning electron

microscopy (SEM) using a HITACHI S-4300 fieldemission scanning electron microscope. All samples werecovered with a thin gold layer. The microstructural char-acterization studies were conducted to determine the sizeof aggregates and examine the homogeneity and sizedistribution.Thermogravimetric analysis (thermogravimetry, TG and

differential thermal analysis, DTA) was used to investigatethe processes occurred during the heat treatment. TG andDTA curves were recorded with Derivatograph-C System(MOM, Hungary) under air or nitrogen flow at a heatingrate of 6 �C min−1 on crushed bulk specimens from roomtemperature to 1000 �C.

3. RESULTS AND DISCUSSION

3.1. Surfactant-Assisted Precipitation Method

The cobalt nitrate is able to provide the Co precursor ratherthan cobalt chloride due to its much better solubility inaqueous or alcoholic solutions. The solubility of Co nitrateis ≈7 mol in 1 l water and that of Co chloride is ≈3 mol/l.

Regarding the results of investigations with initial solu-tions of different concentrations, the precipitation from theconcentrated solutions leads to the finest particles. Theconcentrated solutions promote the nucleation. The favor-able concentration for Co nitrate solution is 5 mol dm−3

(close to the saturated value) and 2 mol dm−3 for Co chlo-ride solution, respectively. The use of oxidative (air forCo3O4� or inert (N2 for CoO) atmospheres does not resultin a remarkable difference in the size and distribution,rather the effect of heat treatment at 1000 �C is dominant.

In the study on the surfactant-assisted precipitationtechniques, the precipitation agents, the surfactants, theirconcentration, and the temperature of heat treatmentwere varied. The precipitations with sodium hydroxideor ammonia yield coarse and large particles (>1 �� inaqueous solutions. Thus the experiments concentrated onthe application of carbonate and oxalate precipitators. Thesizes of particles obtained by oxalate precipitators arelarger and more polydisperse (100–2000 nm) than thatof carbonate precipitates (60–<1000 nm). This differenceis more expressive if TTAB or PDMS surfactants areapplied (Table II). The precipitator ratio has only slightinfluence on the size and distribution above 1 mole ofprecipitator/Co precursor.The PDMS proved to be the most effective surfactant

considering the size and distribution of the particles syn-thesized with assistance of several surfactants (Table II).The use of non-ionic PDMS surfactant resulted in smalland less polydisperse (125–210 nm) particles. However,the ionic surfactants produced the finest precipitates (partlywith diameters of 60–100 nm) but with wide distribu-tion (60–700 nm). The ionic surfactants hinder the aggre-gation less than PDMS. The experiments with PDMSof various molecular weights support the application of

Table II. Effect of surfactants on the size and distribution of theprecipitates.

Particle size anddistributiona

Size-range Maximumof 90% of of sizeparticles distribution

Precipitateb Surfactantc (nm) (nm)

CoCO3 Triton X-100 345–1600 580±40CoCO3 NaDS 58–664 234±20CoCO3 NaDBS 60–700 174±15CoCO3 TTAB 62–702 175±15CoCO3 PDMSd 125–210 148±10

Co(COO)2 Triton X-100 300–1200 560±40Co(COO)2 NaDS 266–663 430±25Co(COO)2 NaDBS 89–848 212±20Co(COO)2 TTAB 380–2600 175±20Co(COO)2 PDMSc 270–2000 553±40

aThe particle size and distribution were measured by DLS. bThe precipitates wereheated at 300 �C. cThe concentration of surfactants was 1 w/w%. dPDMS of5600 g mol−1.

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Table III. Effect of amount of PDMS on the size and distribution ofCo3O4 powdersa.

Particle size anddistributionb

Molecular Size-range Maximumweight of Concentration of 90% of of sizePDMS of PDMS particles distribution(g mol−1� (w/w%) (nm) (nm)

— — 250–800 325±40550 1 130–220 150±10550 5 60–400 203±15550 10 73–500 408±20

5180 5 230–540 330±20

5600 1 30–340 130±55600 5 200–400 300±155600 10 500–900 543±40

42500 5 260–4700 1530±8083000 5 100–4500 1770±120

aCaCO3 precipitates were heated at 700 �C under air. bThe particle size and distri-bution were measured by DLS.

smaller (<6000 g mol−1� PDMS (Table III). The bestresults can be achieved by using PDMS of 550 g mol−1

(Table III). The nanoparticles can be adjusted even bythe concentration of surfactants. It is a surprise, that theparticles prepared with PDMS of 1.0 w/w% provide thesmallest size and the least distribution in both cases ofusing PDMS of 550 and 5600 g mol−1 (Tables III, IV andFig. 1). Considering the results of heat treatment at dif-ferent temperatures, 700 �C proved to be the optimal tem-perature (Table IV). Between 300 and 700 �C, the gasesreleased during the decompositions comminute the parti-cles. Above 700 �C, the aggregation process becomes moreintense, especially around 900 �C.

3.2. Sol–Gel Method

In the sol–gel method, the cobalt nitrate hydrolysis andproduces OH groups, which are capable for condensa-tion. The sol–gel method supports the nucleation of crys-talline particles by condensation reactions and guaranteesthe homogeneity due to the solution technique. A part of

Table IV. Effect of heat treatment on the size and distribution of Co3O4

powdersa.

Maximum of sizedistribution (nm)b

Concentration ofPDMS (w/w%) 300 �C 700 �C 900 �C

0 690±40 325±40 350±201 170±10 108±5 260±205 600±20 310±15 2500±20010 1000±50 543±40 9000±200

aThe Co3O4 was produced from CoCO3 precipitates in the presence of PDMS of5600 g mol−1. bThe particle size and distribution were measured by DLS.

0 1 2 3 4 5 6 7 8 9 100

200

400

600

800

1000

300 ºC5600 g mol–1

700 ºC

Mea

n si

ze o

f par

ticle

s

Concentration of PDMS (w/w%)

5600 g mol–1

550 g mol–1

Siz

e di

strib

utio

n

700 ºC

Fig. 1. Particle size of CaCO3 precipitates versus concentration ofPDMS.

nitrate content escapes as nitrous gases during the reac-tions increasing the pH that also promotes the condensa-tion. The decomposition of nitrate ions depends on thepolarity of the medium. The lower the polarity, the moreefficient the decomposition is.The application of sol–gel method also needs any sur-

factants in order to obtain nanoparticles. Citric acid,PDMS of various molecular weights, and ethyl acetatewere applied as surfactants in the sol–gel procedures. Theparticle size is continually reducing by increasing volumeof citric acid (Table V). The largest amount (1 w/w%) ofcitric acid yielded the finest powders with mean diame-ter of 120 nm. Citric acid in concentrations of >1 w/w%

Table V. Effect of surfactants on the size and distribution of sol–gelderived CoO powdersa.

Particle size anddistributionb

Size-range MaximumConcentration of 90% of of size

Surfactant of surfactant (w/w%) particles (nm) distribution (nm)

— — 324–2500 1520±120Citric acid 0,1 400–1550 750±50Citric acid 0,2 300–1500 400±30Citric acid 0,5 85–1200 140±20Citric acid 1 77–520 120±20

PDMS (550) 0,1 275–480 330±20PDMS (550) 0,2 55–1400 505±30PDMS (550) 0,5 100–210 150±10PDMS (550) 1 70–340 180±10PDMS (550) 2 76–636 200±10PDMS (550) 2,5 95–400 200±20

Ethyl acetate 20 85–140 100±20Ethyl acetate 40 70–100 85±10

aThe sol–gel derived precipitates were heated at 1000 �C under inert atmosphere.bThe particle size and distribution were measured by DLS.

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Table VI. Effect of temperature of heat treatment on the size and dis-tribution of sol–gel derived Co3O4 powdersa.

Particle size anddistributionb

Size-range of 90% Maximum of sizeTemperature (�C) of particles (nm) distribution (nm)

200 130–220 160300 230–400 310500 70–100 85700 85–180 120900 90–240 1701000 80–210 90

aCo3O4 powders were prepared by sol–gel method with ethyl acetate of 40 w/w%in propanol. bThe particle size and distribution were measured by DLS.

could not be used because it produced felt, “clammy” par-ticles. Drying these felt particles yielded blocks. PDMSof 550 g mol−1 (PDMS-550) is the most suitable amongthe PDMS of various molecular weights similarly to theexperiences of precipitation methods. The particle size inthe function of PDMS concentration shows also a mini-mum. The smallest particles (150 nm) with narrow disper-sity (100–210 nm) could be achieved in the solution forPDMS-550 of 0.5 w/w% in this series (Table V). The sizeminimum was at PDMS-550 of 1.0 w/w% in the series ofcoprecipitations. Although the particle diameters could bereduced below 100 nm with higher PDMS concentrationthan 0.5 w/w% but the particles became more polydisperseand the average size increased from 150 nm to 200 nm(Table V).The best results were obtained by ethyl acetate

employed in large amount in 1-propanol (Table V). Appli-cation of ethyl acetate in concentration of 40 w/w%yielded particles of average diameter of 85 nm with nar-row polydispersity (70–100 nm). The interesting results

0 200 400 600 800 1000

Temperature/ºC

0

5050

100

200

250

300

TG

Particle size

Tra

nsfo

rmat

ion

of

Co 3

O4

Wei

ght l

oss

(%)/

size

(nm

)

Co 3

O4

to C

oO

For

mat

ion

of

1500

Fig. 2. Weight loss and change of the particle size during the heat treat-ment of sol–gel derived Co-containing precipitate.

of experiments with heat treatments are summarized inTable VI. The particle size shows two maximums ataround 300 and 900 �C in the function of heating tem-perature (Table VI and Fig. 2). The alteration in the par-ticle size is closely followed by the weight loss recordedby thermogravimetry (Fig. 2). The process accompaniedwith weight loss at around 300 �C is attributed to thetransformation of basic Co salt to Co3O4. The changeat 900 �C is derived from the conversion of Co3O4 toCoO. This study confirms that the particle size increasesby effect of structural transformations (Fig. 2). Thus, theheat treatment of Co-containing particles is not allowedat around 300 and 900 �C. The optimal heating tem-perature proved to be 700 �C for Co3O4 under air and1000 �C for CoO under inert atmosphere (Table VI). In theexperiments with coprecipitation method, also 700 �C was

–80200 400 600 800 1000

200 400 600 800 1000

–60

–40

–20

0

Wei

ght l

oss/

%

Temperature/ºC

Co carbonateSol–gel derived with ethyl acetateCo oxalateSol–gel derived with citric acid Co nitrate

Precipitate

TG

endo

Co carbonateSol–gel derivedwith ethyl acetateCo oxalateSol–gel derivedwith citric acidCo nitrate

Temperature/ºC

Precipitates

exo∆T

DTA

Fig. 3. Thermogravimetric analysis (TG, DTA) of precipitates obtainedby various methods.

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Table VII. Summary of thermogravimetric analysis.

TG DTA

Temperature- Weight loss Peak maximumPrecipitates range (�C) (w/w%) (�C) Process

Co oxalate 150–240 17,4 214 Dehydration and escape of nitrous gases240–405 37,4 258 Escape of CO and CO2

851–893 3,0 866 Transformation of Co3O4 to CoO

Co carbonate 180–240 14,9 191 Dehydration and escape of nitrous gases535–623 10,1 600 Decarbonisation827–870 2,6 847 Transformation of Co3O4 to CoO

Sol–gel derived with citric acid 140–190 10,8 173 Dehydration, escape of nitrous gases, anddecomposition of organic content190–257 41,3 202

912–936 2,0 922 Transformation of Co3O4 to CoO

Sol–gel derived with ethyl acetate 140–172 9,3 157 Dehydration and escape of nitrous gases172–260 29,7 248 Decomposition of organic content790–864 3,4 846 Transformation of Co3O4 to CoO

found as ideal temperature for heat treatment of Co3O4.Comparing the last row of Tables V and VI, the param-eters of CoO and Co3O4 particles prepared with ethylacetate in 1-propanol and dried at 1000 �C show verygood correspondence (e.g., mean sizes of 85 and 90 nm).The reported particle sizes cover a wide range. The mostdata are around 100–200 nm20�28�50 in the literature, butthere are some examples for sizes of ≤50 nm,29�31�51 and0.5–5.0 �m.27�52�53 According to few authors, the fine andsmall particles produced via sol–gel route may be mechan-ically unstable.52�53

3.3. Processes of Heat Treatments

The processes performed during the heating can berecorded by thermogravimetry analysis (TG and DTAcurves in Fig. 3). The reactions taking place in theprecipitates of various methods by heating and the tem-perature ranges of these processes are summarized inTable VII. Regarding the data of the weight losses, theheating at 300 �C is necessary to obtain cobalt oxide(Co3O4� from sol–gel precipitates. However, Co3O4 may

CoCO3 CoCO3 + PDMS CoC2O4

Fig. 4. SEM images of products synthesized by precipitation techniques and dried at 1000 �C.

appear already after a heating at 80 �C in some samples.(See XRD data!) The products of the surfactant-assistedprecipitation techniques require higher temperatures forcomplete transformation, 400–420 �C in the case ofoxalate particles and 600–630 �C at carbonate particles.Every sample has a slight weight loss at around 900 �Crepresenting the conversion of Co3O4 to CoO. Exother-mic changes can be observed in the samples with organiccontent due to the combustion of organic moleculesin the temperature range of 250–450 �C (DTA curvesin Fig. 3).The thermogravimetry analysis can also be used for the

determination of the chemical composition of the materialsprecipitated from initial solutions. The compositions werecalculated from the data of the weight losses. The chem-ical composition of the precipitate formed by oxalic acidagent may be 3 CoC2O4 · 5H2O after a drying at 80 �C.The carbonate precipitator yields particles with composi-tion of CoCO3 ·2Co(OH)2 ·H2O after drying at 80 �C. Thesol–gel derived precipitates dried at 80 �C are basic Conitrate salts; the likely composition is Co(NO3� ·2Co(OH)2by ethyl acetate and Co(NO3� ·Co(OH)2 ·H2O by PDMS of

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CoO Co3O4

Fig. 5. SEM images of products synthesized by sol–gel technique and dried at 1000 �C.

0.5 w/w%. The similar run of the DTA curves for sol–gelderived precipitates and cobalt nitrate indicates the for-mation of basic nitrate salts (Fig. 3). The basic salt ofCo nitrate precipitated from propanol solution with ethylacetate consists of fewer nitrate ions than basic salt formedfrom ethanol solution with PDMS proving that the decom-position of nitrate ions is more efficient in the medium oflower polarity i.e., in the propanol.

3.4. Shape and Morphology of Particle

The scanning electron microscopy could be applied tocontrol the shape of powders rather than to determinethe exact size of primary particles. The primary particlescan lightly aggregate upon standing in air. The particlesobtained by carbonate precipitator without any surfac-tant are polydisperse and form plate-like aggregates. Thisplate-like shape is preserved in the case of using PDMS,too (Fig. 4). The particle dispersion is reduced by addi-tion of PDMS. The particles derived from oxalate precip-itate possess amorphous shape and broad polydispersity(Fig. 4). Figure 5 represents the finest and least polydis-perse nanoparticles synthesized by sol–gel technique usingethyl acetate. These particles are isotropic, they have moreor less spherical shape. There is no significant differencein the particle size and distribution between CoO heatedunder inert atmosphere at 1000 �C and Co3O4 heated inair at 1000 �C (Fig. 5).The phase compositions of sol–gel derived precipitates

dried at 80 �C were determined by XRD measurementsproving their poor crystallinity (Fig. 6). The particles syn-thesized by citric acid in ethanol can be characterized bycrystalline structure of Co(NO3�2 ·4H2O. Thermogravime-try made a chemical composition of Co(NO3� ·Co(OH)2 ·H2O probable for that. Thus, the precipitate consists of

even amorphous basic component. XRD detected severalcrystalline phases in the participate prepared by ethylacetate in propanol; Co(NO3�2 ·H2O, Co(OH)2, as well asCo3O4 (Fig. 6). Both XRD and TA measurements (TG:Co(NO3� ·Co(OH)2 ·H2O) indicate much lower nitrate con-tent for sample derived from propanol solvent. The XRDspectra also identify the crystalline structures of CoO andCo3O4 (Fig. 7). CoO can be characterized by “Fm3m”cubic lattice; Co3O4 particles orders into a “Fd3m” cubiclattice. In the samples heat treated at 1000 �C in air, onlyCo3O4 crystals can be observed, however, Co3O4 turns intoCoO around 900 �C. CoO always turns back into Co3O4

during the cooling to room temperature in air.

10 20 30 40 50 60 70

ov v v

v

v

x x x x

x

v: Co(OH)2

x x x x

x x

Ethanol

x x x

Inte

nsity

(a.

u.)

2θ

Co(NO3)2 6H2O

o

o

o: Co3O4

o

o

o

x: Co(NO3)2 4H2O

Propanol

Fig. 6. XRD spectra of sol–gel derived precipitates from Propanol orEthanol solutions and dried at 80 �C.

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Cobalt carbonate precipitates

1000 ºC

^ oo

o

o

o

o

^

o

2θ10 20 30 40 50 60 70 80

2θ10 20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

o : Co3O4

^: NaCO3

oo

ooooo

o

^

400 ºC

0

200

400

600

800

1000

1000 ºC

^: NaCO3

+: CoO

o: Co3O4

^o

++

+

+

Inte

nsity +

Air

N2

Sol–gel derived precipitates2θ

10 20 30 40 50 60 70 802θ

10 20 30 40 50 60 70 80

o: Co3O4+: CoOo: Co3O4

–200

0

200

400

600

800

1000

o oo

o

o

o

o

Inte

nsity

o

0

200

400

600

800

++

++

+

+

+

o

Inte

nsity

+

1000 ºCAir

1000 ºCN2

Fig. 7. XRD spectra of cobalt oxide powders prepared by various synthesis techniques.

4. CONCLUSIONS

In the present work, CoO and Co3O4 nanoparticles weresynthesized by various liquid-phase methods, namely,coprecipitation and sol–gel techniques combined with ther-mal decomposition. In the study on surfactant-assisted pre-cipitation techniques, the cobalt nitrate solution with highconcentration (5 mol dm−3� proved to be the most efficientinitial solution. The concentrated solutions support thenucleation of Co-containing particles. The basic precip-itation agents (sodium hydroxide or ammonia) producedcoarse and large particles (>1 �� in aqueous solutions.The oxalate precipitates are polydisperse (100–2000 nm).The carbonate agent yields fine and less polydisperseparticles (60–<1000 nm) with composition of CoCO3 ·2Co(OH)2 ·H2O after drying at 80 �C. The particles pre-pared without any surfactant form lightly plate-like aggre-gates. Application of several surfactants (PDMS of variousmolecular weights, Triton X-100, NaDS, NaDBS, andTTAB) was investigated in the concentration of 0, 1, 5, and10 w/w%. The best results (130–230 nm) can be achievedby PDMS of 550 g mol−1 used in 1.0 w/w%. PDMS effi-ciently reduces the polydispersity. In both cases of PDMS(550 and 5600 g mol−1�, the use of 1 m/m% concentrationresults in the finest precipitate (100–150 nm) and the best

dispersion. The formation of Co3O4 needs a heat treatmentat 700 �C in air; CoO requires 1000 �C and inert (N2�

atmosphere. The particles become smaller by a heat treat-ment at 700 �C. Heating at >700 �C increases again thesize.In the sol–gel method, the cobalt precursor (Co nitrate)

hydrolysis in alcoholic (1-propanol or ethanol) solutionand produces OH groups required for condensation. Theseprocesses provide the nucleation and growth of the crys-talline particles. In the sol–gel procedures, different surfac-tants (citric acid, PDMS of various molecular weights, andethyl acetate) have been also applied in order to improvethe dispersion of powders. Application of ethyl acetatein concentration of 40 w/w% yielded the finest particleswith mean diameter of 85 nm and narrow polydisper-sity (70–100 nm). The lower polarity of propanol andethyl acetate supports the decomposition of nitrate ions.The escape of nitrous gases increases the pH, which pro-motes the hydrolysis and condensation reactions of cobaltsalt. The sol–gel derived precipitate synthesized by ethylacetate contains less nitrate ions than that obtained byother surfactants in ethanol. The structural transformations(basic Co salt to Co3O4 and Co3O4 to CoO) induce anincrease of particle sizes, hence, the temperature of the

8 J. Nanosci. Nanotechnol. 11, 1–9, 2011

RESEARCH

ARTIC

LE

Sinkó et al. Liquid-Phase Synthesis of Cobalt Oxide Nanoparticles

heat treatment must be set between or above the tempera-tures of the transformations. The sol–gel precipitates con-sist of more spherical shaped and uniform particles, thanthose produced by coprecipitation.

Acknowledgments: This study has been supported byOTKA NK 68750 funds and I-04-009 EU in HASYLAB,DESY. The European Union and the European SocialFund have provided financial support to the project underthe grant agreement no. TÁMOP 4.2.1./B-09/KMR-2010-0003.

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Received: 24 July 2010. Accepted: 5 October 2010.

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