Cobalt oxide (Co3O4) nanorings prepared from hexagonal β-Co(OH)2 nanosheets
Graphical abstract
Highlights
► Hexagonal cobalt oxide (Co3O4) nanorings can be obtained via a simple hydrothermal method using acetic acid and precursors (as-prepared hexagonal β-Co(OH)2 nanosheets). ► The hexagonal Co3O4 nanorings can be controlled by adjusting the size of the precursor. ► Etching and oxidation are responsible for the formation of the Co3O4 nanorings.
Introduction
In recent years, research on the fundamental properties and practical applications of nanomaterials has attracted considerable attention [1]. In particular, the size effect of nanocrystals has been studied extensively. The shape of nanomaterials has a considerable effect on their physical properties and is important in many potential applications [2], [3], [4], [5], [6], [7], [8]. Thus, much effort has been exerted on exploring various approaches for preparing nanoscale materials with different sizes and shapes. Of these, the preparation of sheet-like or ring-like nanocrystals is a new and interesting research focus. For example, sheet-like CuO or SnO2 nanostructures, NiFe2O4 nanosheet, ZnO ring-like nanostructures, and iron oxide magnetic nanorings have been synthesized [9], [10], [11], [12], [13].
Cobalt hydroxide is attracting increasing attention because of its novel electric and catalytic properties and important technological applications [14], [15], [16]. For example, cobalt hydroxide enhances the electrochemical performance of nickel oxyhydroxide electrodes by enhancing the electrode conductivity and chargeability [17], [18]. Because electric double-layer capacitance and pseudo-capacitance due to the redox reaction are both interfacial phenomena [19], materials with a sheet-like structure, which tend to form a layered structure that can provide large inter-sheet spacing for transferring ions rapidly and increase the electroactive material–electrolyte interface area, will improve the electrochemical performance. Several chemical methods have been used to prepare cobalt hydroxide with a sheet-like structure. Sampanthar and Zeng synthesized β-Co(OH)2 hexagonal sheets under flowing N2 with an ethylenediamine ligand [20]. By exfoliating as-synthesized layered double hydroxide (LDH) in formamide, hexagonal Co(OH)2 nanosheets with an average lateral size of 3–4 μm and a average thickness of 60–80 nm were obtained [21]. Recently, Liu et al. have synthesized nanosheets of α- and β-Co(OH)2 using hexamethylenetetramine as a hydrolysis agent [22]. Hou et al. also synthesized β-Co(OH)2 nanosheets by homogeneous precipitation with sodium hydroxide as the alkaline reagent in the presence of poly(vinylpyrrolidone) [23]. Despite these successes, there is a continuing need for simple, high-yield, environmentally benign methods for synthesizing β-Co(OH)2 nanosheets.
Spinel cobalt oxide (Co3O4) is an important magnetic p-type semiconductor that has many applications in solid-state sensors, ceramic pigments, heterogeneous catalysts, rotatable magnets, electrochromic devices, lithium-ion batteries, and energy storages [24], [25], [26], [27]. Furthermore, Co3O4 nanoparticles have interesting magnetic and field-emission properties [28], [29]. Several methods have been used to synthesize nanoparticles, such as spray pyrolysis, chemical vapor deposition, sol–gel techniques, and forced hydrolysis [30], [31], [32], [33]. A variety of novel shapes have been reported, such as Co3O4 hollow nanospheres [34], nanowalls [35], nanoboxes [36], nanocubes [37], nanofibers [38], nanorods [39], and nanotubes [40]. Recently, Co3O4 nanorings have been obtained by a thermal-decomposition method with β-Co(OH)2 nanosheets as the precursors [41]. To the best of our knowledge, however, there is no previous report on the synthesis of Co3O4 hexagonal nanorings consisting of cubic nanocrystals via a hydrothermal route.
Here, we demonstrate that hexagonal β-Co(OH)2 nanosheets can be synthesized in large quantities by a facile hydrothermal synthetic method with cobalt naphthenate as the cobalt source under mild conditions. Our group successfully prepared hollow hematite (α-Fe2O3) microspheres via a template-free hydrothermal reaction, in which acetic acid was used for chemical etching [42]. Here, hexagonal cobalt oxide (Co3O4) nanorings are also obtained via a simple hydrothermal method using acetic acid and as-prepared hexagonal β-Co(OH)2 nanosheets as the precursors. The mechanism of formation of the hexagonal nanorings of Co3O4 is discussed on the basis of time-dependent experimental results.
Section snippets
Synthesis
15 mL of 6% cobalt naphthenate ([(R(CH2)nCOO−)2Co2+, where R is a cyclopentyl or cyclohexyl group], DIC Co. Ltd.) in xylene solution was added to 15 mL of LiOH solution (15 mol dm−3) where distilled water was used as solvent at room temperature. Then, the pink mixed solution was put in a 50 mL teflon-lined autoclave. The autoclave was sealed, and maintained at 160 °C for 1–40 h, and then cooled to room temperature naturally. After centrifugation, the products were obtained and washed with distilled
Results and discussion
Fig. 1d shows a typical X-ray powder diffraction (XRD) pattern of the β-Co(OH)2 produced at 160 °C for 20 h. All the peaks in the XRD pattern can be indexed to the hexagonal cell of β-Co(OH)2, and are consistent with reported values (JCPDS 30-0443). No impurity phase was observed, indicating the high purity of the final products synthesized under these experimental conditions. SEM images (Fig. 2a) indicate that a large quantity of hexagonal β-Co(OH)2 nanosheets with good uniformity was obtained
Conclusions
We developed a simple method of synthesizing hexagonal Co3O4 nanorings under mild conditions using as-prepared β-Co(OH)2 nanosheets as the precursors. The hexagonal Co3O4 nanorings consisted of cubic nanocrystals and the size of the hexagonal Co3O4 nanorings could be controlled by adjusting the size of the precursor. A possible mechanism of formation of the hexagonal Co3O4 nanorings was proposed on the basis of time-dependent experiments that demonstrated that etching and oxidation were
Acknowledgements
This work was funded by the Sasakawa Scientific Research Grant from the Japan Science Society. We thank DIC Co. Ltd. for providing 6% cobalt naphthenate in xylene solution. Also this work was supported partially by Grant-in-Aid Scientific Research (C) (22560667) from the Ministry of Education, Science and Culture, Japan.
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