Microwave-Assisted Synthesis of Various ZnO Hierarchical

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CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 9 3148–3153

Articles Microwave-Assisted Synthesis of Various ZnO Hierarchical Nanostructures: Effects of Heating Parameters of Microwave Oven Pengli Zhu, Jingwei Zhang,* Zhishen Wu,* and Zhijun Zhang Key Laboratory for Special Functional Materials, Ministry of Education, Henan UniVersity, Kaifeng 475004, People’s Republic of China ReceiVed May 15, 2007; ReVised Manuscript ReceiVed May 16, 2008

ABSTRACT: Several novel hierarchical ZnO nanostructures have been successfully prepared in mixed solvents of ethylene glycol (EG)-water via a facile microwave-assisted method. By only change of the heating parameters of the microwave oven, ZnO nanostructures with straw-bundle-like, wide chrysanthemum-like, and oat-arista-like morphologies and microspheres were obtained. The products were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and room-temperature photoluminescence spectrometry (PLS). The possible mechanisms for the growth of these hierarchical ZnO nanostructures were tentatively proposed. Introduction Since the properties of inorganic materials are highly dependent on their size, shape, and crystalline structure, many efforts have been devoted to manipulating the shape and size of various inorganic materials in a controllable way. Lately, drawing inspiration from the hierarchy and assembly strategies found in Nature, many simple shapes (dot, wire, tube etc.), as well as low-dimensional structure-based morphologies, such as comb-like,1 dendron-like,2,3 snowflake-like,4,5 urchin-like,6 and flower-like7,8 patterns and structures, have been obtained. This offers opportunities to explore advanced materials with promising applications in optics, electronics, catalysis, biosensors, and magnetic properties. However, it still remains a big challenge to understand factors governing the creation of nanocrystal assemblies and develop simple and reliable self-assembly methods for synthesizing materials with designed chemical components and controlled morphologies in nanoscience and nanotechnology. Zinc oxide, as a versatile smart semiconductor, has been attracting extensive attention due to its wide range of technological applications.9–11 So far, a wealth of synthetic strategies has been developed for the synthesis of ZnO nanostructures. However, owing to its highly anisotropic growth rate along the c-axis, most reports are mainly focused on one-dimensional (1D) ZnO nanostructures, such as wires12–14 or rods,15–18 tubes,19–21 and belts22 or ribbons.23 Design of 2D or 3D ordered complex architectures based on these 1D structures is now increasingly attracting attention, because these new superstructures may * Corresponding author. E-mail: [email protected] or jwzhang@ henu.edu.cn.

provide opportunities to exploit novel properties due to unique 1D structure and explore possible new phenomena arising from hierarchical structures.26 Among various synthetic routes to fabricate these 3D ordered ZnO nanostructures, physical methods1,22–25 together with electrodeposition27,28 and solution based chemical methods are demonstrated to be effective approaches. With use of physical methods under rigid conditions, it is feasible to control the size and morphology of target products by properly adjusting temperature, catalyst, and pressure. While for the mildness and simplicity, solution-based chemical methods, long-chain or high molecular weight surfactants, regular or inverse micelles, biopolymers, (diblock) polymers, and precursor-induced thermal or ionothermal processes are often used to direct crystal growth.29–41 However, the introduction of organic or complexing agents as templates introduces many impurities, increases cost, and also leads to difficulty for scale-up of production.42 From this point of view, it is imperative to further exploit simple synthetic routes for preparation of 3D ZnO nanostructures. The application of microwave heating in synthetic chemistry began only in the late 1980s.43,44 It has the advantages of homogeneous volumetric heating, high reaction rate and selectivity, and energy savings as compared with conventional heating methods, making it promising for the synthesis of nanosized materials. Generally speaking, the power, heating frequency, and on/off irradiation cycles are the main heating parameters of a microwave oven, and each of them may have a great effect on the structure and properties of the products. To the best of our knowledge, although such a heating method has been a focus of research, most of previous reports were limited to fixed working conditions of the microwave oven,45–53 and there is

10.1021/cg0704504 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008

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Table 1. Experimental Conditions for Typical Samples and Their Morphologiesa sample

heating parameters of the microwave oven

1 2 3 4 5 6

H (3 min) + ML (1 h) M (5 min) + ML (1 h) L (30 min) + ML (1 h) 200 W (1 h) 400 W (1 h) 600 W (1 h)

7

ultrasonic (15 min) + ML (1 h)

morphology straw-bundle-like wide chrysanthemum-like nanorod-based microspheres microspheres (irregular) nanorod-based microspheres mixture of straw-bundle-like and wide chrysanthemum-like oat-arista-like

a [Zn2+] ) 0.05 mol/L for all samples. Samples 1-3 and 7 were prepared on a modified domestic microwave oven (Haier, HR-7751M, 750 W), while samples 4-6 were prepared on a special experimental microwave oven (NJL07-2). Microwave cycling modes: H ) high (continuous irradiation), HM ) high-mid (irradiation 17 s, stop 5 s), M ) mid (irradiation 12 s, stop 10 s), ML ) midlow (irradiation 10 s, stop12 s), and L ) low (irradiation 4 s, stop 12 s).

no comprehensive report addressing the effects of these heating parameters on the synthesis of nanomaterials. It has been proven that the polyol process is a convenient, versatile, and low-cost route for the synthesis of metals,54,55 alloys,56 and metal oxides (chalcogenide, carbonate),57–59 where the polyol involved in the reaction can act as a solvent, stabilizer, and reducing agent and limit particle growth and prevent agglomeration. Besides, polyols or diols, for example, ethylene glycol (EG), possess a high cohesive energy and fairly high dielectric constant and hence are excellent acceptors of microwave irradiation. Therefore, as reported by Gedanken et al.,51 the polyol process can be greatly facilitated by microwave irradiation. In this paper, we report the effects of heating parameters of the microwave oven (output work power and different on/off irradiation cycles) on the controllable synthesis of several novel hierarchical ZnO nanostructures in a mixture of EG and water as the solvent. Experimental Section Sample Preparation. Zinc acetate dihydrate (Zn(CH3COO)2 · H2O) and EG were of analytical grade and used without further purification. In a mixture of 50 mL of EG and 10 mL of deionized water in a roundbottomed flask was dissolved 0.66 g of zinc acetate dihydrate. The solution was vigorously stirred to give a transparent mixture, followed by microwave irradiation under specific heating parameters. After the reaction was finished, the white products were collected by centrifugation, washed with absolute ethanol three times, and finally dried at 60 °C in air. The details about the experimental conditions for typical samples and their morphologies are listed in Table 1. For comparison, sample 7 was prepared by combining ultrasonic irradiation with microwave irradiation, where the reaction solution, that is, the mixture of EG and water containing zinc acetate dihydrate, was first ultrasonically irradiated for about 15 min until it turned blue-white and then immediately transferred into a 250 mL reactor and irradiated with microwave for 1 h. Instruments and Characterization. In our experiment, two kinds of microwave ovens were used. Both of them were equipped with an in situ mechanical stirrer and a water-cooled condenser. One is a modified domestic microwave oven (Haier, HR-7751M, 750 W) working at a fixed power of 750 W but with different on/off time irradiation cycles. It can be set to work in the cycling modes of high (H, continuous irradiation), high-mid (HM, irradiation 17 s, stop 5 s), mid (M, irradiation 12 s, stop 10 s), midlow (ML, irradiation 10 s, stop12 s), and low (L, irradiation 4 s, stop 12 s). With this microwave oven, the reactant solution was first irradiated in “solicitation” cycling mode for a few seconds and then irradiated in “ML” cycling mode for 1 h, aiming at efficient control of the reaction and reduction of the risk of superheating the solvents. The other is a special experimental

Figure 1. XRD patterns of (a) samples 1, (b) 5, and (c) 7. microwave oven (NJL07-2) working under different output work power but with continuous irradiation, with which the ultrasonic irradiation was accomplished with a high-intensity ultrasonic probe (Xinzhi, JY922D, Ti-horn, 25 kHz). The products were characterized by means of powder X-ray diffraction (XRD, Philips X’Pert Pro MPD X-ray diffractometer, Cu KR radiation, λ ) 1.5418 Å), transmission electron microscopy (TEM, JEOL-JEM 100CX II type, accelerating voltage ) 100 kV), and scanning electron microscopy (SEM, JEOL JSM-5600LV type). The room-temperature photoluminescence spectra were recorded using a SPEX-F212 Xe laser device (Xe lamp, excitation wavelength ) 330 nm).

Results and Discussion X-ray Diffraction Study. Figure 1 shows the XRD patterns of three typical as-prepared products (samples 1, 5, and 7). All the diffraction peaks match well with those of standard hexagonal wurtzite structure of ZnO (JCPDS 36-1451, a ) 3.249 Å, c ) 5.206 Å). The sharpness of the peaks implies the high crystallinity of these as-prepared samples. Morphological Study. The morphology and size of samples 1-3 were studied by means of SEM. Figure 2a shows the SEM image of sample 1 prepared by first irradiating with microwave in “H” cycling mode for 3 min and then irradiating in “ML” cycling mode for 1 h (see Table 1). The image reveals that the product exhibits uniform straw-bundle-like hierarchical structures with a length of 5.2-5.4 µm. The high-magnification SEM image clearly shows that the straw-bundle-like architecture is built from ZnO nanorods (Figure 2b), rather like sheafs of straw tied in the middle but with a joint boundary dividing the whole structure into two parts (the size of which is also noted in Figure 2b). These nanorods have such a high packing density that some of them shell off from the mother; hence a small fraction of dispersed nanorods are visible in Figure 2b. Changing the irradiation cycling mode to “M” at the beginning (sample 2 in Table 1) results in wide chrysanthemum structures with average length or height of about 2.5 µm (Figure 2c). This kind of architecture can be considered as a half-part of the straw-bundle-like structures, but the primary nanorods grew with more actinomorphy and larger length. Figure 2d is a magnified SEM image seen from the bottom of a wide chrysanthemum like structure, revealing that the shape of the bottom is quasi-circular. Interestingly, microspheres with a diameter of 3-5 µm (sample 3 in Table 1) were prepared when “L” cycling mode was chosen in the first stage, and they have a solid interior structure and are also built from nanorods (see the high magnification SEM image of a broken microsphere, Figure 2f). These nanorods self-align and emanate from the center, and they are aligned so densely that no individual rod can be distinguished. From Figure 2, it can be seen that with

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Figure 2. (a, b) Low- and high-magnification SEM images of the straw-bundle-like ZnO in sample 1 prepared by microwave irradiating in “H + ML” cycling mode, (c, d) low-and high-magnification SEM images of the wide chrysanthemum-like ZnO in sample 2 prepared by microwave irradiating in “M + ML” cycling mode, (e) low-magnification SEM image of the nanorod-based microspheres in sample 3 prepared by microwave irradiating in “L + ML” cycling mode, and (f) high-magnification SEM image of a typical broken microsphere in sample 3.

Figure 3. SEM micrographs of samples 4-6 prepared under continuous irradiation but at different output work power of the microwave oven: (a) 200 W; (b) 400W; (f) 600 W. Images c-e show high-magnification SEM images of several typical structures present in sample 5.

variation of the initial cycling mode of the microwave oven, that is, the interval time of microwave irradiation, the morphology of the as-prepared products changed dramatically. The preceding experimental results imply that the morphology and size of the as-obtained ZnO nanostructures are very sensitive to the cycling mode of the microwave oven. Thus in the next section, we move our interest to the work power of the microwave oven. Figure 3a presents the SEM micrograph of sample 4 prepared under continuous irradiation at a fixed work power of 200 W. It is seen that sample 4 has a somewhat irregular morphology and contains microspheres with a diameter of 3-5 µm, and some of the microspheres have a joint boundary symmetrically or asymmetrically dividing the particles into two parts (indicated by arrows). This kind of microsphere structure, previously assigned to crystal twinning, has been reported by several groups who used different copolymers or capping molecules as additives in the preparation process.29,34,37,59 Increasing the work power of the microwave oven to 400 W (sample 5 in Table 1, Figure 3b) results in the microparticles

no longer retaining spherical structure but possessing a concave central part (Figure 3c) and having a mean diameter of 2.7 µm and fairly uniform size distribution. At the same time, the nanorods as the architectural units of the microspheres were clearly visible in this case (Figure 3c), and some of them even separately grew to a larger length (Figure 3d,e). When the work power was further increased to 600 W (sample 6 in Table 1), the straw-bundle-like nanostructure, combined with a fraction of wide chrysanthemum-like nanostructures, was obtained (Figure 3f), similar to that of sample 1. Mechanism. On the basis of the information obtained, it can be seen that the heating parameters of the microwave oven played important roles in the fabrication of ZnO samples with different hierarchical structures in our experiment. In aqueous solution, the growth of ZnO crystals is usually divided into nucleation and growth processes. Among them, the initial seeds during the nucleation process often are shape-determinant. The microwave irradiation with high heating rate and homogeneous

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Figure 4. Schematic illustration of the formation process of ZnO hierarchical nanostructures with straw-bundle-like and wide chrysanthemum-like morphologies and nanorod-based microspheres.

volumetric heating felicitously provides a favorite condition for fast and uniform nucleation. During the initial nucleation stage, we adopted different irradiation cycles, that is, different interval times of the microwave oven, but used fixed output work power of 750 W. When first irradiated in “M” mode, the temperature of the reaction system rose rapidly and got to the boiling point of the mixed solvent in 2 min. At the same time, the transparent solvent turned turbid quickly, indicating that many ZnO nuclei had formed and aggregated together due to instantaneous saturation. The resulting aggregates have concave spherical morphology, possibly owing to two reasons. One may arise from the wurtzite structure of ZnO itself. Namely, ZnO with wurtzite structure has a basal polar O (0001j) face and a top tetrahedroncorner-exposed polar Zn (0001) face, and they have different charge and reactivity, favoring formation of concave spherical morphology. Another, we suppose, could be closely related to the coordination between EG molecules and Zn2+ or selective anchoring of EG molecules on ZnO crystals, favoring generation of concave spherical morphology, although the exact role of EG in the reaction system remains unknown at this stage. When “H” cycling mode was used, powerful continuous irradiation would induce speedy nucleation, and the solvent would be boiling and turn turbid within only 1 min. Subsequently, in the presence of too vigorous reaction, two different concave spheres would attract each other and be fused or coalesced into one twinned crystal for decreasing surface energy. When “L” mode was used, the system would need as long as 20 min for the solvent to get turbid, implying that the contribution by fast heating of microwave irradiation became insignificant at an extended intermission time (on 4 s, stop 12 s). And in this case, more small nuclei would readily aggregate and grow into larger ones. In other words, with different irradiation cycling mode at the beginning of reaction, initially nucleated particles with different morphologies would be harvested. These primary particles are kinetically favored and unstable and will be redissolved into the reaction solution if kept silent for several minutes. After the initial nucleation stage, the precursor had greatly decreased concentration or was consumed completely; the nuclei on the primary particles would then grow only driven by the dissolution of unstable primary particles, owing to “Ostwald ripening”. The rapidly changing electromagnetic field of microwave irradiation can cause molecular-level heating and result in transient anisotropic microdomains and localized high temperatures, also facilitating the anisotropic growth of ZnO.

In this way, the nuclei on primary particles anisotropically grew along the c-axis, generating rod-like, straw-bundle-like, and wide chrysanthemum-like nanostructures or nanorod-based microspheres. If the secondary nanorods had a large enough density, most part of the primary particles would be consumed, leaving etched or entirely broken bottoms of some hierarchical architectures (see Supporting Information, Figure S1). The results of the experiments operated under different output work power gave evidence to the above-mentioned growth mechanism of ZnO nanostructures. A schematic representation of the development of these ZnO hierarchical architectures is depicted in Figure 4. The preceding discussion demonstrates again that the nucleation stage determines the final morphology of ZnO crystals. Another experiment was also conducted for comparison, where the reaction solution was first ultrasonically irradiated (Xinzhi JY 92-2D, Ti-horn, 25 kHz) for 15 min until it became bluewhite, indicating the formation of a few small ZnO nuclei, and then the solution was quickly transferred into a round-bottomed flask and irradiated in “ML” cycling mode for 1 h, generating sample 7. It is interesting that oat-arista-like structures were obtained in this case (Figure 5a). Further insight from Figure 5b shows that the oat-arista-like structure consists of a central wide and long rod and many side rods with different length laterally initiating from the central rod. The corresponding electron diffraction pattern of one single nanorod branch indicates that it is single-crystalline (Figure 5b, inset). By taking into account the time-dependent TEM image of this sample (see Supporting Information, Figure S2), its defect-based growth mechanism, somewhat different from the preceding, is proposed as follows. In the presence of acoustic cavitation by ultrasonic irradiation, a small number of primary nuclei were generated, and they were unstable because the ultrasonic irradiation was immediately stopped just when the reaction solution became turbid. When the reaction solution was transferred to microwave oven and irradiated in “ML” cycling mode for 1 h, those primary nuclei would anisotropically grow along the c-axis generating short nanorods with many defects, owing to fast growth rate (see Supporting Information, Figure S2b). Subsequently, abundant Zn2+ would remain in the solution and selectively absorb on the defects at one end of the shaping nanorods with polarity, acting as new growth sites for the nanorod branches at the expense of surrounding nuclei formed afterward under microwave irradiation.

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Figure 5. TEM images of sample 7 prepared by ultrasonically irradiating the reaction solution for 15 min and successively irradiating in “ML” cycling mode for 1 h: (a) low-magnification image and (b) high-magnification image. Supporting Information Available: SEM images of “etched” wide chrysanthemum-like architectures and TEM images of the oat-aristalike structure with respect to reaction time. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Room-temperature photoluminescence spectra of (a) strawbundle-like sample 1 and (b) oat-arista-like sample 7.

Figure 6 shows the room-temperature photoluminescence (PL) spectra of two typical hierarchical ZnO nanostructures measured using 330 nm Xe laser as the excitation source. ZnO typically exhibits UV band-edge emission and broad visible emissions at green and yellow bands. Strong emission at 382 nm was observed for oat-arista-like ZnO, while a slight red shift and an increase in peak intensity as well were observed for strawbundle-like ZnO, indicating that these two types of ZnO assemblies might have relatively large size. At the same time, green and yellow emissions were observed around 530-600 nm for the two types of ZnO samples, which might be referred to transition in defect states, in particular, oxygen vacancies. Conclusions Several novel hierarchical ZnO nanostructures have been synthesized via a simple microwave-assisted route, using a mixture of EG and water as the solvent. The products were characterized by means of XRD, SEM, TEM, and PLS, and the possible growth mechanisms of these ZnO nanocrystals with novel morphologies were proposed. It was found that the heating parameters of the microwave oven, in particular, the cycling mode used in the nucleation stage, played key roles in controlling the shape of ZnO microcrystals. By proper adjustment of the cycling mode of microwave irradiation, strawbundle-like and wide chrysanthemum-like ZnO nanostructures and nanorod-based microspheres were obtained, while an oatarista-like ZnO nanostructure was prepared by properly combining ultrasonic irradiation with microwave irradiation. The microwave-assisted polyol process is easily controllable and feasible to scaling-up. And hopefully, it might be extended to quick synthesis of other nanostructures with unique morphologies.

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