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J. Phys. Chem. C 2008, 112, 5322-5327
Controlling the Synthesis of CoO Nanocrystals with Various Morphologies Yuliang Zhang,† Jian Zhu,‡ Xin Song,‡ and Xinhua Zhong*,† Laboratory for AdVanced Materials, Department of Chemistry, East China UniVersity of Science and Technology, Shanghai, 200237, China, and School of Life Science and Technology, Tongji UniVersity, Shanghai 200092, China ReceiVed: October 12, 2007; In Final Form: January 22, 2008
A series of cubic CoO nanocrystals with various morphologies and sizes were obtained via the decomposition of cobalt(II) oleate complex at 280-320 °C in noncoordinating solvent octadecene containing dodecanol/ oleic acid. The morphology of CoO nanocrystals could be conveniently tuned by manipulating the decomposition rate of cobalt oleate with the introduction of activating reagent dodecanol or inhibiting reagent oleic acid into the reaction system. More specifically, the morphology of CoO nanostructures can be tuned from the simple isolated tetrahedral shape to the complex 3D flowerlike shape by increasing the concentration of oleic acid, while with increasing concentration of dodecanol, the morphology of the CoO structures can be tuned from the 3D nanoflower to isolated spheres. The structure and morphology of the obtained CoO nanocrystals were characterized by X-ray diffraction (XRD) and by standard and high-resolution transmission electron microscopy (TEM and HRTEM), and the structural evolution and formation mechanism were also illustrated.
Introduction Morphology-controlled synthesis of nanostructures is a great challenge in materials chemistry because the morphology (including dimensionality and shape) of most nano-objects can effectively tune their intrinsic chemical and physical properties.1,2 The interest in fabricating nano-objects with complex three-dimensional (3D) structures has been steadily growing since 3D nanostructures have been aimed at designing novel nanodevices and nanomachines.3 Porous or flowerlike 3D nanomaterials of metals and metal oxides have been recently synthesized for their potential applications in many fields such as catalysts, sensors, magnetic materials, optical hosts, etc.4-6 Even though gas-phase approaches have been successful in the fabrication of nanostructures with a variety of morphologies, harsh synthetic conditions and low yield prevent these approaches from being used for large-scale production and wide application.7 The colloidal solution-phase chemical routes using capping agents or micelles as regulating agents or templates to facilitate anisotropic crystal growth appear to be of particular interest in the preparation of nanocrystals with complex morphologies, because they offer the potential of facile scale-up and of flexible processing chemistry.4 High-temperature approaches in organic solvents have often been regarded as mainstream synthetic chemistry in the morphology-controlled synthesis of nanostructures.8-11 Thermodecomposition, aminolysis, and alcoholysis of organometallics assisted by organic ligands for morphology-controlled synthesis of metal oxide nanoparticles have been successfully realized, and orientation alignment has been found as the main growth mode for 3D porous or flowerlike nanostructures.12-14 CoO nanocrystals have attracted much attention due to their potential applications based on magnetic, catalytic, and gas* Corresponding author: fax: +86 21 6425 2485; e-mail: zhongxh@ ecust.edu.cn. † East China University of Science and Technology. ‡ Tongji University.
sensing properties.15 Cobalt monoxide typically crystallizes in one of the two phases: rock-salt phase (space group Fm3hm) and wurtzite phase (space group P63mc).16 In recent years monodisperse CoO nanoparticles have been obtained mainly by thermal decomposition of cobalt surfactant complexes in long-chain hydrocarbon solvents.17-20 Tetrahedral CoO nanocrystals with 4-5 nm size were synthesized by Yin and Wang17a,b via the oxidation of Co2(CO)8 in toluene in the presence of the surfactant Na(AOT) at 130 °C. Pure CoO nanoparticles with size ranging in 4.5-18 nm were produced by the decomposition of Co(II) cupferronate in decalin under solvothermal route.17c Peng and co-workers17d have recently prepared Co3O4 nanocrystals by the pyrolysis of cobalt carboxylate salts in a hydrocarbon solvent. Rod-shaped and cubic CoO nanocrystals were obtained by the decomposition of cobalt(III) acetylacetonate in oleylamine.18b Pencil-shaped CoO were obtained by Hyeon and co-workers18c,d by the thermal decomposition of cobalt oleate complex in octadecene. Recently, uniformed tetrapodal CoO nanocrystals were synthesized in our group by the dissolution of Co2O3 in oleic acid and then alcoholysis of the formed Co(III)-oleate complex.20 Many cobalt oxide nanocrystals with 0D and 1D morphologies have been synthesized; however, a few 3D CoO nanostructures were formed, which are of special interest for the surface/shape-related properties and applications.4a,19 Knowing that the reactivity of precursors in noncoordinating solvents can be fine-tuned by varying the concentration of both the ligands and the activating reagents, herein we extend the reaction of the decomposition of cobalt(II) oleate in octadecene containing dodecanol/oleic acid at high temperature and obtain CoO nanostructures with a variety of morphologies from isolated sphere, tetrahedron, dendrite, to flowerlike by varying the amount of activating reagent (dodecanol) and/or the inhibiting reagent (oleic acid).
10.1021/jp709943x CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008
CoO Nanocrystals with Various Morphologies Experimental Section Potassium oleate (95%), CoCl2‚2H2O (Shanghai Chemical Reagents Company, 99%), 1-octadecene (ODE, Alfa Aesar, 90%), oleic acid (OA, Acros, 97%), and 1-dodecanol (DDL, Lancaster, 97%) were used as received, without further purification. All experiments were carried out with standard oxygenfree techniques under nitrogen flow. Synthesis of Cobalt(II) Oleate Complex [Co(OL)2)]. Precursor Co(OL)2 was prepared according to a literature method.18c Typically, 4.76 g (20 mmol) of CoCl2‚2H2O and 12.82 g (40 mmol) of potassium oleate were added into a solvent system containing 20.0 mL of distilled water, 15.0 mL of C2H5OH, and 35.0 mL of hexanes. The resulting solution was incubated at 60 °C for 2 h. Then, the reaction solution was cooled to room temperature. The upper organic phase was separated and solvent was distilled off in a rotary evaporator, and the product was further dried in a vacuum oven. Purple Co(OL)2 was obtained with product yield more than 95%. Synthesis of CoO Nanostructures. In a typical preparation of 3D flowerlike CoO nanostructures, a 0.2 mmol (125.6 mg) portion of Co(OL)2 was loaded in a 50 mL three-neck flask together with 1.6 mL of ODE and 0.4 mL of OA. The resulting reaction mixture was degassed at 110 °C for 45 min under vacuum to remove the moisture and oxygen. The reaction system was then heated to reflux temperature (about 315 °C) under N2 flow with vigorous stirring. After the temperature stabilized, 1.0 mL of DDL was injected quickly (less than 0.5 s) into the reaction system. With the injection of DDL, the temperature of the reaction system dropped to about 280 °C immediately, but recovered and stabilized at 290 °C in about 1 min. The color of the reaction system changed to gray from the original dark blue, indicating the decomposition of Co(OL)2 and the formation of CoO nanostructures. After 5 min, the nanostructure growth was stopped by removal of the heating apparatus and the reaction mixture was allowed to cool to about 50 °C. The nanoparticles were precipitated by adding 5 mL of acetone to the reaction mixture. The crude product was recovered by centrifugation, dispersed in toluene, and subjected to a second round of purification. The obtained organic-surfactant-coated CoO nanostructures were able to redisperse in nonpolar solvents such as CHCl3 or toluene and used for further characterization without any size selection. By keeping the amount of Co(OL)2 and the total volume of solvent constant, with variation of the amount of OA and DDL, CoO nanostructures with diverse morphologies can be conveniently obtained. In each experiment, OA was loaded together with Co(OL)2 prior to heating, while DDL was injected at the reflux temperature of the reaction mixture except that the reaction solvent was composed of DDL solely, where DDL was added together with Co(OL)2 prior to heating. Characterization. The structures of the obtained CoO nanostructures were characterized by transmission electron microscopy (TEM), high-resolution TEM (HRTEM), electron diffraction (ED), and powder X-ray diffraction (XRD). The TEM and HRTEM images and ED patterns were recorded on a JEOL JEM2010 electron microscope using accelerating voltages of 200 kV. The XRD patterns were obtained using a Philips PW 1820 diffractometer equipped with a rotation anode and a Cu KR radiation source (λ ) 0.154 18 nm). Electron microscope specimens were prepared by depositing a drop of toluene dispersion of nanoparticles onto a carbon-film-coated copper grid. XRD samples were prepared by depositing nanostructure powder on a piece of Si (100) wafer.
J. Phys. Chem. C, Vol. 112, No. 14, 2008 5323 TABLE 1: Size and Shape of CoO Nanostructures Obtained by Decomposition of Co(OL)2 in ODE under Different Amounts of DDL Injected size sample
amount of DDL (mL)
diameter (nm)
size of branch (nm)
A B C D E
0 0.09 0.225 1.0 3.0
∼76 ∼31 ∼27 6 ( 1.1 2.8 ( 0.3
16 ∼3 × 7 ∼3.6 × 14
morphology flower dendron polypod tetrahedron sphere
Results and Discussion Co(OL)2 Decomposed under Different Amounts of DDL. In the well-studied experiment of the decomposition of Co(OL)2 in noncoordinating solvent ODE, we tuned the reactivity of the precursor by varying the concentration of inhibiting reagent (free OA) or the concentration of activating reagent (DDL), and thus the morphology of the CoO nanostructures obtained was conveniently tuned. When Co(OL)2 decomposed in a solvent mixture composed of DDL and ODE, with a fixed total volume of solvent and precursor amount and variable amounts of DDL, CoO nanostructures with morphologies of sphere, tetrahedron, polypod, branched multipod, and flower were obtained (detailed experimental conditions and the corresponding results are collected in Table 1). When Co(OL)2 decomposed in ODE without DDL, agglomerated nanoparticles formed after flash heating of the reaction system to the reflux temperature (∼320 °C). From the TEM image (Figure 1A), it can be clearly seen that the asprepared nanoparticles are flowerlike, ranging from 72 to 104 nm, and are aggregated by many irregular primary nanoparticles with average dimension of ∼16 nm. This means that the nanoflowers did not grow from a single seed. It should be noted that, in previous reports, when Co(OL)2 in ODE was heated to the decomposition temperature (320 °C) at a very slow rate (
