Self-assembled Co3O4 porous nanostructures and their photocatalytic activity

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Abstract

Co3O4 hollow microspheres were synthesized by a two-step method in ethylene glycol (EG) system. First, the cobalt oxide precursor was produced in the presence of ethylene glycol (EG) by using cobalt chloride hexahydrate (CoCl2·6H2O) and sodium acetate (NaAc) as reactants. Second, the precursor was calcined to obtain the products of crystalline cobalt oxide. The products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and scanning electron microscopy (SEM). The Brunauer–Emmett–Teller (BET) gas adsorption measurement shows that the products were porous structures with two narrow porous volume distributions. Because of the unique nanostructured hollow structure composed of nanoslices, the photocatalytic activity of the products was examined by measuring the photodegradation of methyl orange solution with ultraviolet radiation. The result shows that our products have a good photocatalytic activity.

Introduction

Nanosized materials have been the subject of intensive research throughout the world for their exception physical properties and potential applications in technology [1], [2], [3], [4], [5], [6], [7]. Nowadays, there is an increasing interest in developing materials with better performance, which depend not only on the compositions but also on the morphologies of the materials [8], [9], [10], [11], [12], [13]. To our best knowledge, the nanomaterials with urchinlike and flowerlike architectures can be used in catalysis because of their high specific surface area [14], [15], [16], [17], [18], [19], [20], while the nanocages and hollow spheres can be utilized in controlling release of drugs and protection of biologically active agents [21], [22], [23], [24], [25], [26], [11]. However, although many advances have been made in “building” the curved structure, developing facile and simple methods is still highly desired for fully understanding and exploiting the self-assembly process [27].

Mesoporous materials with pore structure and tailored framework composition are of great interest for broad applications ranging from adsorbent, catalyst, energy storage and conversion, to biological uses. Among this material family, Co3O4 porous hollow microspheres are of particular due to its potential applications as heterogeneous catalysts and electrochromical devices. There were some reports about the hard templating pathways for the synthesis of nanostructured porous Co3O4 [28] and surfactant-assisted solvothermal synthesis of Co3O4 hollow microspheres [29], but there were few reports on the synthesis of Co3O4 porous hollow microspheres without any template or surfactant. Our group has synthesized Co3O4 nanorods in the presence of surfactant and nanoporous Co3O4 nanoflowers without any template or surfactant by a solvothermal method [30], [31], respectively. In this paper, Co3O4 hollow microspheres were successfully synthesized by a two-step method. In the first step, cobalt oxide precursors were produced in the presence of ethylene glycol (EG) by using cobalt chloride hexahydrate (CoCl2·6H2O) and sodium acetate (NaAc) as main materials at 200 °C for different times. Then, the precursors were calcined to produce Co3O4 hollow microspheres at 500 °C for 3 h.

Section snippets

Experimental

All of the reagents used were of analytical purity and used without further purification. In a typical synthesis process, CoCl2·6H2O (0.7 g) was dissolved in ethylene glycol to form a clear solution, followed by the addition of NaAc (2.1 g). After being stirred vigorously for 10 min, the mixture was put into a 50-ml Teflon-lined stainless steel autoclave, and then the autoclave was filled with ethylene glycol up to 75% of the total volume. The autoclave was treated at 200 °C and maintained for

Results and discussion

There are a number of reports on flower-like cobalt hydroxide microspheres and their decomposition to Co3O4 microspheres of the same morphology. The precursor phase formed on solvothermal reaction of cobalt chloride was characterized by X-ray powder diffractometer. However, the precursor phase is not cobalt hydroxide. Fig. 1a shows the crystalline of the precursor, which is similar to the XRD patterns of Mn–EG and Fe–EG [32], [33]. In addition, the precursor was characterized by FTIR and

Conclusions

Porous hollow microspheres of Co3O4 have been synthesized in EG system without template or surfactant by a two-step method. Their probable formation mechanism has been proposed. The surface area and N2 adsorption/desorption measurements show that Co3O4 hollow microspheres have surface area of 22 m2/g and display two narrow porous volume distributions. They have a good photocatalytic activity to methyl orange.

Acknowledgements

The present work was supported by the special funding support from the National Science Foundation of China (NSFC) (Grants nos. 20371002 and 20771006) and Natural Science Foundation of Anhui Province (no. 2006kj042A).

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