Effect of Solvents on Synthesis and Characterization of Cobalt Oxide

(Co3O4) Nanoparticles

Settakorn Upasen , Teerawat Nongpromma, Sirisak Trikamol

Chemical Engineering Department, Faculty of Engineering, Burapha University, 169 Longhad Bangsean

Road, T. Seansook, A. Mueng, Chonburi 20131

Abstract. We report characteristic of cobalt oxide (Co3O4) nanoparticles prepared by thermal

decomposition of Co (Ac)2·4H2O using oleic acid capping agent and NaBH4 reducing agent. Variable in this

study is types of solvent: acetone (S1), hexane (S2), and xylene (S3). Physical and chemical properties of the

as-prepared Co3O4 samples were investigated by TGA, TEM, BET, XRD, and FT-IR. The form of colloidal

suspension using xylene and hexane were well observed. Adding NaBH4 solution, consequently, the pink-

violet solution using these two solvents turned into black due to the oxidation reaction. From XRD result,

crystalline structure of Co3O4 nanoparticles prepared by using the three solvent types show the same result

indicated as cubic structure with Fd-3m symmetry group and space lattice of 8.06 Å. The particle size of

82.29, 26.83 and 20.59 nm and specific surface area of 7.04, 21.28, and 32.07 m2/g were observed for S1, S2,

and S3 sample, respectively. In addition, secondary phases were also detected.

Keywords: cobalt oxide, nanoparticle, synthesis, physical and chemical characterization.

1. Introduction

Cobalt oxide (Co3O4) have been widely used for an electrode of battery and p-type semiconductor. It is

due to its high theory current density of 890 mAh/g with direct optical band gap of 3.95-2.13 eV [1]-[3]. The

structural of Co3O4 belongs to spinel crystal structure which is composed of Co(II) ions occupy the

tetrahedral 8a sites and Co(III) ions occupy the octahedral 16d sites [2]. Intensive studies of this material

have been devoting to nanoparticle synthesis, its physical and chemical characterization, and its applications.

Various synthesis techniques have been utilized to prepare Co3O4 NPs such as solvothermal and

surfactant-free method [4]-[6], precipitation-oxidation method [7], soft chemistry method [8], and thermal

decomposition method [2]. Among the many methods, thermal decomposition method using organic solvent

and surfactants has been reported to produce significant particle size and shape. The use of surfactant,

especially oleic acid, is necessary to avoid aggregation in the colloidal dispersion and the carboxylic group in

oleic acid binds strongly to the particle surface and forces the long hydrocarbon chain out into the nonpolar

solution [9]-[11]. Furthermore, the long hydrocarbon chain might have an effect on the oxygen diffusion as

presented as a barrier for the nanocrystal, and a protection against oxidation. The effect of the oleic acid

concentration on the particle size, morphology, and agglomeration has been investigated [10], [12]. In the

case of Co nanoparticles, irregularly shaped black precipitates were commonly obtained for an oleic

acid/cobalt ratio of 0.15 [10]. In addition, the combination of surfactant-solvent has also been significantly

effected on reduction of cobalt-surfactant precursor. It consequently leads to effect the particle diameter. A

report of Costanzo and co-workers [5] show the mean diameters of Co nanocrystal of 3.9 nm was obtained

by xylene dried solution. Further, they reported that the higher solubility parameter (12) value is, the smaller

nanoparticle size was obtained.

Corresponding author. Tel.: +66 38102222 dial 3353; fax: +66 38102222 dial 3350.

E-mail address: settakorn@eng.buu.ac.th.

International Proceedings of Chemical, Biological and Environmental Engineering, V0l. 101 (2017)

DOI: 10.7763/IPCBEE. 2017. V101. 12

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In this work, we will prepare Co3O4 nanoparticles using thermal decomposition method. In order to

avoid aggregation in the colloidal dispersion, the oleic acid is used. And the reduction of Co-(OA) precursor

will be occurred using NaBH4 under air atmospheric. The main objective of this report is to compare the

effect of three different solvent agents on structural/physical as well as chemical properties.

2. Experimental

2.1. Synthesis

Cobalt oxide (Co3O4) nanoparticles were synthesized by thermal decomposition of cobalt acetate

tetrahydrate (Co(Ac)2·4H2O) as shown in Fig. 1. Capping reagent of 0.68 g oleic acid was first mixed with

40 mL of solvent and stirred for 30 minutes. Note: we used three different solvents: acetone (S1), hexane

(S2), and xylene (S3). Then, Co(Ac)2·4H2O of 0.5 g were slowly added into the solution while it was

sonicated for 6 minutes in order to avoid aggregation of particles. The three colloidal solutions in pink-violet

color were then reduced by sodium borohydride (10 mL NaBH4). By few minutes, the solution was turned

into black as its complete oxidation reaction. Precursor powders were separated by evaporation, and finally

calcined at 400 C under air atmosphere for 4 hrs.

Fig. 1: Protocol of facile Co3O4 synthesis experiment using thermal decomposition of Co(Ac)2·4H2O with oleic acid

capping agent and NaBH4 reducing agent.

2.2. Characterization

2.2.1 TEM analysis

Transmission electron micrographs of the as-synthesized Co3O4 samples were obtained using a

transmission electron microscope, Phillip-Tecnai20, at 100 kV with resolution of 13,5000x – 13,000x. In

order to disperse particle, the powder samples were dissolved in ethanol solution, and sonicated for few

minutes.

2.2.2 BET analysis

In order to analyze specific surface area of as-prepared Co3O4 sample, Quantachrome NOVA 1200 was

employed. The N2 physisorption at 77 K (liquid nitrogen temperature) was carried out to obtain the N2

adsorption results at different relative pressures (P/P0) in the range of 0.05-0.35.

2.2.3 TGA study

Thermalgravimetric measurement of precursor and as-prepared Co3O4 sample were performed using

Mettler Toledo 850 instrument. The TGA curves were recorded at temperature range of 25-900 C, and

heating rate of 15 C/min. The precursor samples were performed under O2 atmosphere with flow rate of 15

ml/L, while the as-synthesized Co3O4 samples were taken under N2 atmosphere with flow rate of 15 ml/L.

2.2.4 X-ray diffraction 85

X-ray power diffraction (XRD) were examined using Bruker D8ADVANCE, X-ray diffractometer with

high density Cu-K radiation (λ = 1.54056 Å). X-ray patterns were recorded over an angular range of 2 =

10-80 degree with 0.5 sec/step.

2.2.5 FT-IR study

The FT-IR spectra of as-synthesized samples were obtained using Fourier transform infrared

spectroscope (FT-IR), Perkin Elmer system 2000. The dry powder were blended with KBr, grounded and

pressed into disk. The spectra were recorded at 400-4000 cm-1

.

3. Result and Discussion

3.1. Structural and Physical Characterization

The crystalline nature and phases of as-synthesized Co3O4 NPs has examined by taking the XRD patterns,

as shown in Fig. 2. The diffraction peaks for the samples using xylene, hexane, and acetone can be readily

indexed to a face centered cubic (fcc) structure of Co3O4 cubic spinel, with symmetry group of Fd-3m [2 2 7],

and lattice constant a = 8.06 Å (JCPDS No.42-1467 [13]). The average crystalline size was calculated using

the Debye–Scherer equation.

(1)

Where k is a constant equal to 1.0, is the wavelength of Cu-K radiation, is the full width at half

maximum (FWHM) of the diffraction peak in the radiant and is the Bragg angles of the main planes. The

examined C.S. of 23.31, 23.50 and 18.02 nm (Table 1) were reported for as-prepared Co3O4 NPs of xylene,

hexane, and acetone, respectively.

According to the analysis of particle size and shape, there are two techniques. First, direct measurement

performed by the means of TEM as shown in Fig. 3, the TEM images clearly show that the particles were

obtained nearly spherical shape. Regarding to numerous reports [8,] [14]-[16], the as-synthesized Co3O4

samples using xylene and hexane show the most common nanoparticle size, 20.59 and 26.83 nm,

respectively. However, synthesis using acetone leads the Co3O4 particles to be bigger (82.59 nm). It is due to

Co(Ac)2·4H2O was not well dissolved in acetone, occurred aggregation, and unreacted with the NaBH4

reducing reagent. Consequently, the extracted Co(Ac)2·4H2O was just decomposed into pure Co3O4 [17].

Fig. 2: XRD pattern of as-prepared Co3O4 sample: a) acetone (S1), b) hexane (S2), and c) xylene (S3).

Table 1: Physical and structural properties of as-prepared Co3O4 sample

Sample Symmetry

group

Space lactic

(Å)

crystalline size

(nm)

particle size (nm) Surface area

(m2/g) TEM BET

S1 Fd-3m 8.06 18.02 82.59±58.42 140.76 7.04

S2 Fd-3m 8.06 23.50 26.83±5.67 46.97 21.28

S3 Fd-3m 8.06 23.31 20.59±5.04 30.90 32.07

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Fig. 3: TEM images with 135,000x illustrating shape and size of Co3O4 nanoparticle using a) acetone (S1), b) hexane

(S2), and c) xylene (S3).

Another particle size analysis techniques, indirect means, was obtained by the surface area (SBET)

analysis, using the cold N2 adsorption/desorption technique. And the surface area was calculated by the

Brunauer–Emmet–Teller (BET) equation. The average spherical particle size (dBET) was then estimated from

the Eq. (2) [18].

dBET = 6/(SBET*th) (2)

where ρth is the theoretical density for Co3O4 (6.055 g/cm3) and SBET is the surface area reported by BET

measurement.

The BET measurement (Table 1) indicate that the highest surface area of 32.07 m2/g was observed for

the as-synthesized Co3O4 using xylene. Consequently, it allows to predict particle size obtaining 30.90 nm.

This predicted value was yet on the range of size distribution obtained by TEM [18].

3.2. Thermogravimetric Analysis

The results of themogravimetric measurements for precursor and as-synthesized samples are reported in

Fig. 4a and 4b, respectively. The TG of precursor samples were performed under O2 atmosphere. It clearly

show three steps of weight decomposition. First, temperature below 180oC may assigned to the departure of

water and solvent. Secondly, weight loss detected at 350 C is probably due to the free or loosely bond of

oleate functional group which were reported by Jadhav et al. [19]. Then, a continuous curve of weight loss

detected around 450 C should be the decomposition of acetate functional group [17], [20].

Fig. 4: TGA curves of a) precursor samples and b) as-prepared Co3O4 samples.

As the TG measurement of precursor samples (Fig. 4a) shown an accomplished reaction at temperature

above 500 C, this temperature was then chosen to be a calcined temperature. However, in order to ensure

that the calcination was completed, the TG measurement of the calcined samples or as-synthesized Co3O4

samples (Fig. 4b) were also investigated. In Fig. 4b, the weight loss occurred in four steps. First,

approximately 6 wt.% of weight loss appeared for S2 and S3 sample were detected at temperature below 300

C. It could be attributed to the phase transformation of CoO(OH) to Co3O4 (eq. 3) reported by number of

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studies [7], [8]. The following weight loss was detected at 450 C, especially acetone sample (S1)

decreased for 4 wt.%. It is probably due to anhydrous cobalt to cobalt oxide [15].

12CoO (OH) 4Co3O4 + 6H2O (3)

2Co3O4 6CoO + O2 (4)

Interestingly, the third weight loss, traces of 1 wt.% for hexane sample (S2), was detected around 600-

650 C, contributed to the first induced loss of oxygen. And it leads to a phase distortion of CoO [21]. For

the last step, decomposition of Co3O4 to CoO (eq. 4) for each sample was found at around 850 C in amount

of 4 wt.% [7],[ 8].

3.3. Chemical Characterization

Fig. 2 shows XRD patterns of as-synthesized cobalt oxide powder. The diffraction profile of the sample

prepared by acetone (Fig. 2a) was identified nearly pure spinel cobalt oxide, Co3O4 [7], while the other two

samples (Fig. 2b, 2c) were detected traces of secondary phases i.e. cobaltous oxide (CoO), cobaltic oxide

(Co2O3), cobalt oxyhydroxide (CoOOH), cobalt hydroxide (Co(OH)2).

The chemical phases revealed by XRD is accompanied simultaneous variation in IR transmittance

spectra (Fig. 5). It shows that the IR spectra of the three samples displays two distinct bands due to the

stretching vibrations of metal-oxygen bonds [7[, [22], [23]. The first band (1) at 553 cm-1

is attributed with

the OB3 vibration in spinel lattice, where B denotes Co3+

in an octahedral hole. The second intense peak (2)

at 654 cm-1

is associated to ABO3 vibration, where A denotes the Co2+

in a tetrahedral hole.

Never the less, the IR spectra of S2 and S3 sample (Fig. 5b, 5c) show secondary compositions with

agreement of XRD result. According to Yao et al. [24], the IR spectra of cobalt oxyhydroxide were reported.

The band at 1658 cm-1

is attributed to Co-O vibration, while the broad band at 3,215 cm-1

is assigned to

stretching mode of hydroxide group. A small peak at 3,612 cm-1

strongly support the Co(OH)2 assignment

contributed to the O-H stretching mode [22], [25].

Fig. 5: FT-IR spectra of as-prepared Co3O4 sample using a) acetone (S1), b) hexane (S2), and c) xylene (S3).

4. Conclusion

The synthesized cobalt oxide was obtained by thermal decomposition method. The particle size was

controlled by using oleic acid and various organic solvents. A combination of techniques was used to study

chemical and structural of as-synthesized materials, including thermal analysis, BET, TEM, XRD, and FT-IR.

The synthetic cobalt oxide was identified as cubic structure with Fd-3m symmetry group and space lattice of

8.06 Å. Using xylene as an organic solvent allowed the smallest particle size (20 nm), as well as the highest

surface area (32 m2/g). In the case of acetone, it show none of reaction with NaBH4 reducing agent, which

resulted the largest particle size (83 nm), and the lowest surface area (7 m2/g).

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Moreover, chemical characteristic of as-synthesized sample from acetone was reported pure spinel

Co3O4 phase, while the samples from xylene and hexane gave trances of secondary phases. These phases

were identified as cobaltous hydroxide (Co (OH)2), cobalt oxyhydroxide (CoO (OH)), cobaltic oxide (Co2O3),

and cobalt monoxide (CoO).

5. Acknowledgments

The financial support of the fiscal year’s project for undergraduate program in chemical engineering,

faculty of engineer is gratefully acknowledged.

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