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Materials Letters 62 (20

Shape-controlled synthesis of single-crystalline cupric oxide by microwaveheating using an ionic liquid

Xiaodong Xu ⁎, Meng Zhang, Jing Feng, Milin Zhang

Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 15001, China

Received 30 July 2007; accepted 18 January 2008Available online 26 January 2008

Abstract

Cupric oxide (CuO) with leaf-like, chrysanthemum-like and rod shapes have been synthesized by microwave-assisted approach using an ionicliquid 1-n-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM]BF4). By controlling the concentration of [BMIM]BF4 and reaction time, shapetransformation of CuO nanostructures could be achieved in a short period of time. The crystal structure and morphology of products werecharacterized by XRD, TG, FESEM/EDS, and TEM/SAED. A possible mechanism for the shape transformation of CuO nanostructures wasproposed. In addition, UV–vis spectra were employed to estimate the band gap energies of the nanosized semiconductors.© 2008 Elsevier B.V. All rights reserved.

Keywords: Cupric oxide; Microwave heating; Ionic liquid; Crystal structure; Semiconductor

1. Introduction

As a p-type semiconductor with a narrow band gap (1.2 eV),CuO has been widely used in applications such as gas sensors [1],magnetic storage media [2], solar cells [3], and heterogeneouscatalysis [4]. Many efforts have been directed toward the fab-rication of CuO nanomaterials to enhance its performance incurrently existing application by thermal water baths [5], hydro-thermal methods [6], double-jet precipitation technique [7], etc.However, high temperature or long reaction time was usuallyinvolved in these methods. Microwave heating opened up thepossibility of realizing reactions in a very short time due to itsunique effects [8]. To date, CuO with shapes of nanoparticles [9],nanoleaves [10], whiskers and cubes [11] have been fabricated bymicrowave heating. However, few studies have been reported onshape transformation of CuO by microwave heating.

Ionic liquids (ILs) have been widely studied as a new kind ofreactionmedia owing to their unique properties such as extremelylow volatility, wide liquid temperature range, good thermal sta-

⁎ Corresponding author. Tel.: +86 451 82519696; fax: +86 451 82533026.E-mail addresses: xuxiaodong@hrbeu.edu.cn (X. Xu),

happymeng1980@126.com (M. Zhang), fengjing@hrbeu.edu.cn (J. Feng),zhangmilin@hrbeu.edu.cn (M. Zhang).

0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2008.01.046

bility, good dissolving ability, designable structures, high ionicconductivity, and wide electrochemical window, etc. Until now,several nanomaterials have been successfully synthesized throughemploying ILs reaction systems, for example, nanometal Al, Fe,and Co–Al alloys by electrodeposition [12], hollow TiO2

microspheres by Sol–Gel method [13]. Meanwhile, ILs wereexcellent microwave-absorbing agents due to their high ionicconductivity and polarizability, thus leading to a high heating rateand a significantly shortened reaction time. By combining ad-vantages of both ILs and microwave heating, Te nanorod [14],flower-like ZnO [15], CuO nanowires [16] and Cu/Ni nanopar-ticles [17] have been prepared. Here, we demonstrated thesynthesis of CuO nanostructure with different morphologiesusing [BMIM]BF4.

2. Experimental

Cupric acetate (Cu(CH3COO)2 ·2H2O), sodium hydroxide(NaOH) and ethanol were all analytical reagents. [BMIM]BF4was synthesized according to the procedures in the literature[18]. In a typical reaction, 50 mg Cu(CH3COO)2 ·2H2O wasdissolved in the mixture of 0.75 mL distilled water and 1.0 mL[BMIM]BF4 (samples A and B), 1.5 mL [BMIM]BF4 (samplesC) or 2.0 mL [BMIM]BF4 (sample D). Then, 0.75 mL NaOH

Fig. 2. TG curve of sample A.

2788 X. Xu et al. / Materials Letters 62 (2008) 2787–2790

aqueous solution (5 mol/L) was added into the above solutionand stirred for 3 min. Then the solution was transferred into a 10-mL Teflon tube and heated for 3.5 min (sample A) or 6 min(sample B, C and D) by a domestic microwave oven at a powersetting of 120 W (2.45 GHz, Haier MK-2270 M). And thetemperature of the solution reached to 74 °C (sample A), 86 °C(sample B), 90 °C (sample C) and 96 °C (sample D), respec-tively. The black product was separated by centrifugation,washed with distilled water and absolute ethanol, and driedunder vacuum overnight at 70 °C.

The morphologies of products were determined by Field-Emission Scanning Electron Microscopy (FESEM, FEI XL-30),equipped with EDS (Energy Dispersive Spectroscopy), andTransmission Electron Microscopy (TEM, JEM-2010) as well asSAED (SelectedArea ElectronDiffraction). The crystal structuresof products were characterized by X-ray Diffractometer (XRD,Rigaku D/max-IIB with Cu Kα radiation, λ=0.15406 nm). TheTGcurve of productswas performed on aNETZSCHSTA409PCthermal analyzer with a heating rate of 10 °C/min in Ar atmo-sphere. The UV–visible absorption spectrum of products wasrecorded by a Shimadzu UV-1601 spectrophotometer.

3. Results and discussion

A typical XRD pattern of sample A was shown in Fig. 1. Thecharacteristic peaks in the diffraction diagram could be indexed as themonoclinic phase of CuO with lattice constants a=0.4686, b=0.3435,and c=0.5129 nm, which were consistent with the literature values(JCPDS 05-0661). No impurities were detected by XRD. In our ex-perimental system, the concentration of NaOHwas considerably high, soCu(II) existed mainly as Cu(OH)4

2− [19]. The overall reaction might besimplified as follows:

Cu OHð Þ4� �2�Y

Microwave

CuOþ H2Oþ 2OH� ð1Þ

The thermal behavior of CuO structure was investigated by thermalanalyzer with a heating rate of 10 °C/min in Ar atmosphere, whichconfirmed the adsorption of [BMIM]BF4 on CuO nanostructure. Asshown in Fig. 2, the TG curve of sample A showed a total weight lossof 4.2% before 420 °C, which could be divided into two weight lossstage. One was before 150 °C, the other was between 150 °C and

Fig. 1. XRD patterns of sample A.

420 °C, which could be assigned to the loss of water and [BMIM]BF4,respectively.

The morphologies of sample Awere shown in Fig. 3. Leaf-like CuOnanosheets with uniform shape and size were obtained on a large scale(Fig. 3(a)). The enlarged image (Fig. 3(b)) showed CuO nanoleaveswith widths in the range of 160–280 nm, lengths in the range of 520–800 nm and thicknesses in the range of 25–35 nm. In addition, the EDSspectrum revealed that atomic ratio of Cu to O was equal to 1:1, whichwas consistent with stoichiometric CuO (Au was sprayed when pre-pared the sample for FESEM observation). A typical TEM image of anindividual CuO nanoleaf was presented in Fig. 3(d). It could be seenclearly that the nanoleaves was composed of layered nanoribbons. Thewidth of the ribbons was in the range of 20–40 nm. The SAED patternindicated that the assembled nanoleaves were formed through “orientedattachment” of small nanoribbons along the [010] direction. Among theplanes (001), (010), and (100), the most thermodynamically low-energyplane was (001), while the high-energy one was (010) [20]. In principle,the attachment of tiny primary crystals at their high-energy surfaces wasenergetically favored, because the formation of larger crystals couldgreatly reduce the interfacial energy [21]. Thus, it was reasonable thatthe final assembled leaf-like CuO nanosheets could be attained by thearrangement of oriented nanoribbons. The orientations along the length,width, and thickness of CuO nanoleaves could therefore be determinedto be [010], [100], and [001], respectively.

By controlling the concentration of [BMIM]BF4 and reaction time,shape transformation of CuO could be achieved in a short period oftime. When the reaction time was prolonged to 6 min (sample B),chrysanthemum-like CuO were formed in large quantities (Fig. 4(a)). Ifthe amount of [BMIM]BF4 was increased gradually at the same reactiontime as Sample B, chrysanthemum-like CuO would change to a inter-gradation (Sample C) as shown in Fig. 4(b), at last, numbers of nanorods(sample D) were formed (Fig. 4(c)).

With the prolongation of reaction time, the shape of CuO wastransformed from leaf-like to chrysanthemum-like, as shown in Fig. 4(a).The enlarged image demonstrated that the chrysanthemum-like CuOwascomprised of nanoleaves which overlapped each other and bundled at acommon center. The SAED pattern (Fig. 4(d)) taken from a single CuOpetal could be indexed to the monoclinic CuO, indicating that the surfaceof the nanosheet was (001) plane of CuO and the direction along thethickness of the obtained CuO was [010]. Thus the chrysanthemum-likeCuO was not a simple aggregation of small crystallites, but wascomposed of single-crystalline petals grown concentrically.

When the amount of [BMIM]BF4was increased, someCuO nanorods(sample C) grew from nanoribbons and broke from nanoleaves gradually,

Fig. 3. FESEM images of sample A (a–b), EDS results of sample A (c), and TEM image of sample A (d). The inset showed the SAED pattern taken from d.

2789X. Xu et al. / Materials Letters 62 (2008) 2787–2790

as shown in Fig. 4(b). Aswas pointed out in previouswork, the high ionicconductivity and polarizability of [BMIM]BF4 made it an excellentmicrowave-absorbing agent, thus leading to a high heating rate and asignificantly shortened reaction time [8]. Accordingly, the reinforced

Fig. 4. FESEM images of sample B (a), sample C (b) and sample D (c), TEM images oFESEM images (the scale bar=200 nm), (d) and (e): the corresponding SAED.

volatilization of the H2O and CH3COOH produced from the hydrolyza-tion of Cu(CH3COO)2 brought a blast to break the force between nano-ribbons, leading to nanorods split from nanoleaves. When the amountof [BMIM]BF4 was increased further, this effect would be reinforced

f sample B (d) and sample D (e). The insets of (a) and (c): the high-magnification

Fig. 5. (a) UV–vis spectrum of sample A and (b) (αEphoton)2 vs Ephoton curves of

the products.

2790 X. Xu et al. / Materials Letters 62 (2008) 2787–2790

markedly, as a result, numbers of nanorods (sample D) with the diameterof 20 to 40 nm formed (Fig. 4(c)). Fig. 4(e) showed that the diameter of arod was about 30 nm, and the preferential growth direction of the rod wasalong the [010] direction.

The optical absorption properties of leaf-like CuO dispersed in watersolution were investigated by UV–visible spectroscopy, as shown inFig. 5. There was an absorption peak at ~375 nm shown in the Fig. 5(a).A classical Tauc approach was further employed to estimate the Eg

value of CuO nanoleaves according to the following equation:

aEphoton ¼ K Ephoton � Eg

� �1=2 ð2Þ(where α was the absorption coefficient, Ephoton was the discrete photoenergy,Kwas a constant, andEg was the band gap energy) [22]. The plotsof (αEphoton)

2 vs Ephoton based on the direct transition were shown inFig. 4(b), exhibiting linear relationship at 2.97–3.18 eV. The band gap ofCuOnanoleaveswas estimated to be 2.51 eVaccording to prolongation oflinear section, which was larger than the reported value for the bulk CuO(1.20 eV), due to quantum confinement effects [23]. In addition, the bandgap of CuO nanoleaves was slight lower than the reported value ofnanoplatelets by hydrothermal conditions (2.74 eV) [24] but larger thanthe result of nanosheets by heating at reflux (2.05 eV) [21].

4. Conclusions

In summary, we have successfully synthesized leaf-like,chrysanthemum-like and rod CuO nanostructures by micro-wave-assisted method using an ionic liquid [BMIM]BF4. With

the prolongation of reaction time, leaf-like CuO would trans-form to chrysanthemum-like CuO, which was composed ofleaf-like CuO. Moreover, the rod CuO was split from leaf-likeCuO by increasing the concentration of [BMIM]BF4. [BMIM]BF4 served as an excellent microwave-absorbing agent duringthis process. The band gap of CuO nanoleaves modified with[BMIM]BF4 was estimated to be 2.51 eV, which showed sig-nificant blue-shift compared with that of the bulk CuO. Thissimple method might also be extended to synthesize other metaloxides with controlled morphologies in a short time.

Acknowledgments

This work was supported by the Specialized ResearchFund for the Doctoral Program of Higher Education of China(No. 20050217019) and Basic Research Foundation of HarbinEngineering University, China (No. HEUFT05018).

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