Materials Chemistry and Physics 74 (2002) 234–237

Short communication

Moderate temperature synthesis of nanocrystallineCo3O4 via gel hydrothermal oxidation

Yang Jianga, Yue Wua, Bo Xiea, Yi Xie a,b, Yitai Qiana,b,∗a Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China

b Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received 28 December 2000; received in revised form 26 May 2001; accepted 4 June 2001

Abstract

A novel gel hydrothermal oxidation route has been developed for preparation of nanocrystalline Co3O4 through the reaction betweenCo(OH)2 gel and hydroperoxide in a hydrothermal system at 180◦C. X-ray powder diffraction (XRD) studies indicated that the productwas well-crystallized cubic phase of Co3O4 with a cell constant ofa = 8.0722 Å. The transmission electron micrograph (TEM) showedthat the product consisted of monodispersive nanoparticles below 4 nm. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Moderate temperature; Nanocrystalline Co3O4; Gel; Hydrothermal oxidation

1. Introduction

Transition metal oxides have been the subjects of sci-entific and technological attention due to their electricalproperties [1–4]. Among these oxides, the tricobalt tetraox-ide Co3O4, of spinel structure is also known as a promisingmaterial that exhibits a gas-sensing behavior [5] and solarenergy reflecting properties [6]. Recently, this compoundhas been widely used as an effective catalysis in environ-mental protection and chemical engineering because of itscatalytic ability in the reduction of SO2 by CO [7,8], am-monia oxidation [9], and the reduction of NO by methane[10]. Additionally, Co3O4 is a traditional precursor of anodematerial in Li-ion rechargeable battery [11], whose electro-chemical properties have been extensively studied [12] andmost of all, it is a important magnetic material.

The performance of Co3O4 in its applications such ascatalysis and magnetic materials could be optimized by highsurface areas and a narrow particle size distribution. There-fore, the synthesis of nanocrystalline Co3O4 powder hasbeen the target of material chemists [13,14]. In this field,various synthesis routes have been proposed, such as thethermal decomposition of a solid cobalt nitrate (380◦C) [15],chemical spray pyrolysis (350–400◦C) [16,17], chemicalvapor deposition (CVD, 550◦C) [18], and the traditional

∗ Corresponding author.E-mail address: djy@mail.ustc.edu.cn (Y. Qian).

sol-gel method (above 260◦C) [19]. However, all of theabove methods need relatively high reaction temperaturesand the production of nanocrystalline Co3O4 is difficult andinconvenient to obtain. The other limitation of the traditionalsynthesis method of Co3O4 is the annealing treatment of upto 2–4 h needed to crystallize the oxide [20]. Hydrothermaloxidation was an efficient technique for preparing fine oxidepowders from metals by reaction with high temperature andhigh pressure water and solution during the years from 1980sto 1990s. Oxides such as Fe3O4, ZrO2, and HfO2 were pre-pared at 600–700◦C [21–23]. Herein we report a novel gelhydrothermal oxidation route that is more moderate and ef-fective to synthesize monodispersive nanocrystalline Co3O4.

2. Experimental

The experimental process consists of two steps: (1) prepa-ration of Co(OH)2 gel by conventional process; (2) gel ox-idation by hydroperoxide under hydrothermal treatment.

Analytical grade CoSO4·7H2O (99.5% pure), NH3·H2O(25.0–28.0 wt.% content of NH3), H2O2 (30 wt.% con-tent of H2O2) were used as starting materials. In a typicalsynthesis, 0.04 mol CoSO4·7H2O was dissolved in 100 mldistilled water (l.5 M� cm), then, excessive amounts ofNH3·H2O was added with electromagnetic stirring duringthe formation of Co(OH)2 gel. The gel was filtered usingvacuum filtration and washed by distilled water for several

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Y. Jiang et al. / Materials Chemistry and Physics 74 (2002) 234–237 235

times until no SO42− and NH4+ remained, which could be

indicated by chemical analysis method [24]. Then the freshwet gel particles were moved into a Teflon-lined stainlesssteel autoclave, finally a certain volume of H2O2 solution(volume ratio of H2O2 and H2O was 1:1) was added intothe autoclave very slowly until 80% of the total capacity(25 ml). The sealed autoclave was heated to 180◦C in 2 hand maintained for 24 h, then cooled to room temperature inair naturally. Precipitates were filtered and washed with dis-tilled water and absolute ethanol for five times respectively,and lastly dried in vacuum at 70◦C for 6 h.

The obtained samples were characterized by X-ray pow-der diffraction (XRD) scanned at a rate of 0.02 S−1 over the2θ range from 10 to 70◦, using a Japan Rigaku Dmax r-AX-ray diffractometer with graphite monochromatic Cu K�radiation (λ = 1.54178 Å). TEM images and selected-areaelectron diffraction (SAED) were taken with a HitachiModel II -800 transmission electron microscope, using anaccelerating voltage of 200 KV. The sample for TEM wasprepared by 2 h ultrasonic dispersion of 0.2 g of product in50 ml ethanol. Then, a drop of the solution was placed ona copper microgrid, and dried in air before performance.X-ray photoelectron spectra (XPS) were recorded on aVGESCALAB MK II X-Ray photoelectron spectrometer,using non-monochromatic Mg K� radiation as the excita-tion source.

3. Results and discussion

The XRD patterns for the samples obtained in differentconditions are shown in Fig. 1. All peaks shown in Fig. 1d–fcan be indexed to the cubic spinel structure Co3O4 withlattice parametera = 8.0722 Å, which is consistent withthe reported data [25]. The considerable broadening of the

Fig. 1. XRD patterns of the samples obtained using gel hydrothermaloxidation technique under different reaction conditions: (a) at 80◦C for24 h; (b) at 100◦C for 24 h; (c) at 150◦C for 24 h; (d) at 180◦C for 12 h;(e) at 180◦C for 24 h; and (f) at 180◦C for 48 h.

Table 1Comparison of Miller indexes (h k l), interplanar spacings,d (Å) andrelative intensitiesI/Imax of diffracted X-rays beams of Co3O4

h k l JCPDS No. 43-1003 As-obtained sample

d I/Imax d I/Imax

1 1 1 4.667 16 4.6381 28.92 2 0 2.858 33 2.8343 38.63 3 1 2.4374 100 2.4289 1004 0 0 2.021 20 2.0129 334 2 2 1.6501 9 1.6478 17.45 1 1 1.5558 32 1.5557 39.74 4 0 1.429 38 1.4301 54.6

diffraction peaks demonstrates the nanocrystalline characterof the Co3O4 powders. The diffraction width of the peak wascorrected by subtracting the diffractometer width obtainedfrom the diffractometer width working curve. Then usingthe well-known Debye–Scherrer’s equation [26], the averagegrain size of the sample is about 3.2 nm, calculated from thebroadening peaks 311, 220, and 440 shown in Fig. 1e.

Table 1 shows the Miller indexes (h k l), interplanar spac-ings,d (Å), and relative intensitiesI/Imax of diffracted X-raybeams of the oxide obtained by the gel hydrothermal ox-idation according to Fig. 1e and of the JCPDS card No.43-1003. From the table, we can see that the degree of crys-tallization of our sample without annealing is similar to thatof the JCPDS data, and no diffraction peaks other than thoseof Co3O4 are present.

The XPS spectra (Fig. 2) of Co3O4 shows that the Co 2pand O 1s binding energies are 779.9 and 529.8 eV respec-tively, which are corrected for specimen charging by refer-encing the C 1s to 284.60 eV. The molar ratio of Co:O isquantified by Co 2p and O 1s peak areas and an averagecomposition of Co3O4.09 is given, approaching to the stoi-chiometry of Co3O4.

The morphology of crystallized powder shown in TEMimages is very homogeneous in all the specimens with re-spect to particle sizes and shape. The TEM images (Fig. 3a)indicates that Co3O4 powders consist of monodispersivenanoparticles with the size ranged from 3 to 4 nm, which isgenerally consistent with the XRD analysis. The diffraction

Fig. 2. XPS spectra of Co3O4 obtained using the gel hydrothermal oxi-dation technique at 180◦C for 24 h.

236 Y. Jiang et al. / Materials Chemistry and Physics 74 (2002) 234–237

Fig. 3. TEM images of nanocrystalline Co3O4 synthesized using the gel hydrothermal oxidation technique under different conditions at 180◦C (a) for24 h; (b) SAED image of (a); (c) for 48 h.

rings (Fig. 3b) recorded from the sample reveals the char-acteristic of nanocrystalline cubic phase Co3O4.

The reactions involved in the process can be formulatedas follows:

CoSO4 + 2NH3 · H2Ostirring→ Co(OH)2 (gel) + (NH4)2SO4

3Co(OH)2 (gel) + H2O2hydrothermal→ Co3O4 + 4H2O

The gel hydrothermal oxidation could be influenced by reac-tion temperature and time. Table 2 outlines the effects of re-action temperature on the products. Reactions below 100◦Cwas incomplete and the products contained amorphous pre-cipitates (see Fig. 1a); when the reaction temperature wasincreased from 100 to 150◦C, CoO and Co3O4 appeared inturns as revealed in Fig. 1b and c. At temperatures above180◦C, the single phase of spinel structure Co3O4 formedas shown in Fig. 1d. This was because higher temperatureresulted in increases the oxidation power of hydroperoxide.

It is well known that the hydrothermal oxidation reactiontime influences the crystallized process. When the reactiontime was selected at 180◦C, at least 24 h were required toobtain single phase with well-crystallized Co3O4. Whenthe reaction time was increased to 48 h, particles with sizeslarger than 12 nm were obtained. The difference in the mor-phology was the formation of powder agglomerates andof irregular dendrites (Fig. 3c), compared with the former

Table 2Effect of reaction temperature on the product (hydrothermal treatment for24 h)

Sample Reactiontemperature (◦C)

Products

1 80 Amorphous precipitate2 100 CoO+ Co(OH)23 120 CoO+ Co(OH)2 + Co3O4

4 150 CoO+ Co3O4

5 180 Crystalline Co3O4

Y. Jiang et al. / Materials Chemistry and Physics 74 (2002) 234–237 237

powders (Fig. 3a). The effects of reaction time on the de-gree of crystallization can be illustrated by XRD patternsas shown in Fig. 1d–f. With increasing of reaction time,the crystallization of products rises obviously. When reac-tion time varied from 24 to 48 h, all the Co3O4 diffractionpeaks were strengthened, meanwhile, the FWHM of the3 1 1 peak was decreased from 0.5600 to 0.5000, which in-dicated the enlargement of the gain sizes of the samples. Inorder to obtain monodispersive nanocrystalline Co3O4 withsmall sizes, the optimum condition for the gel hydrothermaloxidation could be chosen at 180◦C for 24 h.

4. Conclusion

In summary, a gel hydrothermal oxidation route hasbeen successfully developed to synthesize monodispersivenanocrystalline Co3O4 at 180◦C, which is a moderate tem-perature compared with traditional methods to our knowl-edge. The XRD pattern indicated that the powders possessthe spinel structure of the well-crystallized cubic phase ofCo3O4. XRD pattern and TEM images showed that theaverage particle size was about 3–4 nm. The further investi-gation of their properties is in progress. Our results suggestthat the gel hydrothermal oxidation could, in principle, beused to synthesize other nanosized transition metal oxides,such as Fe3O4, which may offer opportunities for manytechnological applications.

Acknowledgements

Financial supports from the National Natural Funds ofChina and the 973 Projects of China are appreciated.

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