J. Cent. South Univ. Technol. (2011) 18: 993-997

DOI: 10.1007/s11771-011-0792-5

Preparation of microsized single-crystalline Co₃O₄ by high-temperature hydrolysis

LI Qi-hou(李启厚), LIU Zhi-yong(刘智勇), LIU Zhi-hong(刘志宏), HU Lei(胡雷)

School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: Microsized single-crystalline Co₃O₄ has been synthesized by high-temperature hydrolysis of CoCl₂·2H₂O at 600 °C. The samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results reveal that the as-prepared powders are microsized single-crystalline Co₃O₄ with cubic spinel structure. An increase in the high-temperature hydrolysis time results in the evolution of particle shapes from cube to quasi-sphere, and then to octahedron. The effect of NaCl additive on the surface morphologies of Co₃O₄ particles was experimentally investigated. The results indicate that the NaCl additive acts as an inert disperse phase in the high-temperature hydrolysis, and prevents the aggregation of Co₃O₄ particles effectively.

Key words: Co₃O₄; crystal structure; hydrolysis; surface morphology

1 Introduction

Co₃O₄ is an important functional material and has been widely studied for applications in solid-state sensors [1], magnetic materials [2], optical devices [3], heterogeneous catalysts [4], lithium ion batteries [5], etc. Various methods have been proposed to prepare Co₃O₄ materials, including precipitation-calcination process, sol-gel method [6], hydrothermal synthesis [7], spray pyrolysis [8] and chemical vapor deposition [9]. The traditional precipitation-calcination route is simple, but the powders made by this method often appear as porous aggregates with poor homogeneity. Other methods are very complicated with respect to the process, materials and equipment.

Recently, much interest has been focused on Co₃O₄ powders with controlled size and novel morphology, because many fundamental properties and applications of the materials depend on their shape, size and specific orientation. ZHAO et al [10] synthesized octahedral Co₃O₄ powders with good homogeneity in a non-aqueous alcohol and chloride reaction system. Single-crystalline nanocubes [11] and flower-shaped Co₃O₄ [12] were made from cobalt acetate by hydrothermal method. TENG et al [13] prepared cubic Co₃O₄ nanocrystals by a one pot hydrothermal reaction in the presence of the oxidant KClO₃ and the capping reagent polyvinyl- pyrrolidone (PVP). MASOUD et al [14] synthesized Co₃O₄ nanoparticles from a solid organometallic molecular precursor N-N′-bis(salicylaldehyde)-1,2- phenylenediimino cobalt(II).

In this work, we report a novel synthesis method of microsized single-crystalline Co₃O₄ by the high- temperature hydrolysis of CoCl₂·2H₂O. Both the starting materials and the process are simple and facile. So it will be expected that the method has promising prospect for using as the precursor of LiCoO₂ cathode.

2 Experimental

Co₃O₄ powders were synthesized by a high- temperature hydrolysis method. Commercial analytical grade CoCl₂·2H₂O and NaCl were used as the starting materials. In a typical process, 2 g CoCl₂·2H₂O powders were placed in a ceramic boat which was laid in the center of a furnace tube. Deionized water was instilled into a steel pipe at a feed rate of 2 mL/min, where the water was evaporated and then introduced into the furnace tube. The powders were heated up to 600 °C at a heating rate of 10 °C/min, then soaked at 600 °C for 3 h, and finally cooled to ambient temperature. The obtained powders were washed with water for three times, and then washed with alcohol for two times. To investigate the effects of inert disperse phases on the morphology of Co₃O₄, another sample was prepared by similar method except that a well-ground mixture of 2 g CoCl₂·2H₂O and 10 g NaCl was used as the starting material instead of the pure CoCl₂·2H₂O powders. Figure 1 shows the schematic diagram of the apparatus. In the experimental process, the equation of hydrolysis reaction of CoCl₂·2H₂O is as follows:

6CoCl₂·2H₂O+4H₂O+O₂→2Co₃O₄+12HCl           (1)

To characterize the structure and morphologies of the samples, X-ray diffraction (XRD) patterns were recorded by a Siemens D5000 diffractometer using   Cu K_α radiation. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-6360LV spectrometer. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM2000 EX transmission electron microscope with an accelerating voltage of 100 kV.

Fig.1 Schematic diagram of gas-solid hydrolysis at high temperature: 1—Temperature controller; 2—Deionized water; 3—Liquid flow meter; 4—Control valve of carry gas; 5—Quartz reaction tube; 6—Resistance furnace; 7—Quartz boat;  8—Absorption device of tail gas

3 Results and discussion

Co₃O₄ powders were prepared by high-temperature hydrolysis of CoCl₂·2H₂O at 600 °C for 3 h. XRD pattern and SEM images of the powders are shown in Fig.2. All the diffraction peaks in Fig.2(a) are perfectly indexed to the cubic spinel Co₃O₄ structure with the lattice parameters of a=0.808 4 nm, Z=8 and S.G=Fd3m which are in JCPDS card file No.43-1003. No characteristic peaks of impurities are detected on the XRD pattern. As shown in Fig.2(b), the Co₃O₄ powders present octahedral morphology. The sketch map drawn from the observation of the Co₃O₄ powders is given at the top left corner. The SEM image also reveals that the microsized octahedral Co₃O₄ particles have a wide particle size distribution and some of them aggregate into Co₃O₄ clusters.

Co₃O₄ powders prepared by different methods may have diverse morphologies such as cube [15], rod-like [16], and octahedron [17]. The difference in particle morphology can be attributed to the different growth rates of crystal facets, which greatly depends on the synthesis conditions and the nature of the material. The {100} planes of spinel oxides, which have cubic crystallographic symmetry, are predicted to have the lowest surface energy. The growth of spinel crystals tends to be terminated at their {100} planes under thermodynamic control. As a result, the crystals will have cube-like shapes [15]. However, the adsorption of organic or inorganic substance on different crystal facets may change the growth rates of the facets. It has been found that the preferential adsorption of ions in molten salt, such as Cl^-, to different crystal planes directs the crystal growth by controlling the growth rates along different crystal planes, and the growing planes of the Co₃O₄ nanorods are {111} planes [18]. Recent report has also demonstrated the importance of Cl^- ions, which are selectively adsorbed to the {100) and {111} planes of copper nanocrystals, in effectively controlling the crystal shapes. In this work, we prepared octahedral Co₃O₄ powder by the high temperature hydrolysis method. The novel octahedral morphology may be resulted from the characteristic adsorption of Cl^- ions.

Fig.2 XRD pattern (a) and SEM image (b) of Co₃O₄ powders prepared by high-temperature hydrolysis of CoCl₂·2H₂O at  600 °C for 3 h

The growth mechanism of Co₃O₄ crystal plays important roles in understanding and predicting its morphology. As to the crystals with geometrically symmetrical morphology and revealable crystal planes, their appearance depends on the growth rates of different crystal facets [19]. As far as the cubic spinel crystals are concerned, they exhibit various morphologies based on different ratios (R) of the growth rate in the á100? direction to that in the á111? direction [20]. Figure 3 shows the geometrical shapes of cubo-octahedral crystals as a function of the ratio R. With the increase of R from 0.58 to 1.73, the geometrical shapes of the crystals evolve from cube to sphere-like, and then to octahedron. Figure 4 gives the TEM images of Co₃O₄ powders synthesized at 600 °C for 30 min, 2 h, 3 h and the electron diffraction image of Co₃O₄ powder synthesized at 600 °C for 3 h. An increase in the high-temperature hydrolysis time results in the evolution of particle shapes from cube to quasi-sphere, and then to octahedron, indicating the ratio of the growth rate in the á100? direction to that in the á111? direction increases. The corresponding selected area electron diffraction (SEAD) (Fig.4(g)) reveals the single-crystalline nature of the Co₃O₄ octahedron.

As shown in Fig.2(b), the Co₃O₄ powders aggregate severely. In order to obtain well dispersed Co₃O₄ powders, an inert disperse phase was added into the starting material to separate the CoCl₂·2H₂O reactants and prevent the aggregation of Co₃O₄ powders. In this work, NaCl was selected as the inert disperse phase because of its high melting point (801 °C), large solubility in water and enough chemical stability. It keeps solid and inert during the hydrolysis process of CoCl₂·2H₂O at 600 °C and is easy to be removed from the products by washing with deionized water.

Fig.3 Geometrical shapes of cubo-octahedral crystals as function of ratio, R, of growth rate in á100? direction to that in á111? direction [20]: (a) R=0.58; (b) R=0.7; (c) R=0.87; (d) R=1.0; (e) R=1.15; (f) R=1.73

Fig.4 TEM images of Co₃O₄ powders synthesized at 600 °C for 30 min (a, b); 2 h (c, d); 3 h (e, f), and  electron diffraction image of Co₃O₄ powder synthesized at 600 °C for 3 h (g)

Figure 5 shows the SEM images of Co₃O₄ powders prepared by the high-temperature hydrolysis of CoCl₂·2H₂O in a NaCl inert disperse phase at 600 °C for 3 h, where the mass ratios of CoCl₂·2H₂O to NaCl are 1:3, 1:5 and 1:10, respectively. The Co₃O₄ powders are better dispersed in comparison to those in Fig.2(b), suggesting that the NaCl disperse phase has prevented the aggregation of Co₃O₄ powders. Increasing the concentration of NaCl can increase the dispersibility of Co₃O₄ powders.

Fig.5 SEM image of Co₃O₄ powders prepared by high- temperature hydrolysis of CoCl₂·2H₂O in a NaCl inert disperse phase at 600 °C for 3 h with different mass ratios of CoCl₂? 2H₂O to NaCl: (a)1:3; (b)1:5; (c)1:10

4 Conclusions

1) Microsized single-crystalline powders of Co₃O₄ are synthesized by a high-temperature hydrolysis method using CoCl₂·2H₂O as the starting material.

2) The Co₃O₄ particle evolves from cube to quasi-sphere, and then to octahedron with the increase of high-temperature hydrolysis time.

3) The NaCl inert disperse phase in the starting materials can effectively prevent the aggregation of Co₃O₄ powders during the synthesis.

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(Edited by PENG Chao-qun)

Foundation item: Project(50704038) supported by the National Natural Science Foundation of China; Project(108170) supported by the Key Foundation of Ministry of Education, China

Received date: 2010-09-28; Accepted date: 2011-02-23

Corresponding author: LIU Zhi-yong, PhD Candidate, Tel: +86-731-88830478; Fax: +86-731-88830478; E-mail: csuliuzhiyong@163.com