Controllable Synthesis of Hexagonal, Bullet-Like ZnO Microstructures

Mar 11, 2012 - Hexagonal, bullet-like ZnO microstructures and nanorod arrays have been prepared successfully by a simple hydrothermal method without ...
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Controllable Synthesis of Hexagonal, Bullet-Like ZnO Microstructures and Nanorod Arrays and Their Photocatalytic Property Limin Song,*,†,∥ Shujuan Zhang,‡,∥ Xiaoqing Wu,*,§ and Qingwu Wei† †

College of Environment and Chemical Engineering and State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, P. R. China ‡ College of Science, Tianjin University of Science and Technology, Tianjin, 300457, P.R. China § Institute of Composite Materials and Ministry of Education Key Laboratory of Advanced Textile Composite Materials, Tianjin Polytechnic University, Tianjin 300387, P. R. China ABSTRACT: Hexagonal, bullet-like ZnO microstructures and nanorod arrays have been prepared successfully by a simple hydrothermal method without surfactants. The structure and composition of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), infrared spectrum (IR), and UV−vis diffusive reflectance spectrum (UV−vis). The as-synthesized bullet-like ZnO has hexagonal microstructures with an average diameter of about 10 μm and a length of 30 μm. The obtained ZnO nanorod arrays are highly oriented. Their average diameter is around 50−100 nm, and their length is about 1 μm. In the present work, the effect of concentration, heating temperature, and time on the morphology of ZnO structures was studied experimentally, and their formation was discussed in detail. The photocatalytic degradation of methylene blue (MB) was also investigated.

1. INTRODUCTION ZnO is a wide direct band gap (3.37 eV) semiconductor, which has a potential application in nano- and microscale electronic and opticalelectronic devices, self-powdered nanosystems, and sensors.1−6 Many physical properties of structures are well-known to be strongly dependent on morphology, for example electronic, optical, and magnetic performance.7−10 Therefore, various kinds of onedimensional ZnO structures were developed with the aim to improve their physical properties. A number of studies have been made on the synthesis of one-dimensional ZnO structures using vapor-phase transport, thermal evaporation, anodic alumina membrane templates, chemical vapor and metal organic chemical vapor deposition, and hydrothermal and solvothermal processes, among others.11−18 However, although many routes have been tried in the preparation of one-dimensional ZnO structures, the development of a facile and one-step route for the synthesis of large-scale one-dimensional ZnO structures remains a challenge. In the present work, we have successfully synthesized hexagonal, bulletlike ZnO microstructures and ZnO nanorod arrays using ZnCl2 and NaOH as raw materials under hydrothermal conditions without surfactants and organic solvents. The structure, morphology, and optical property of the ZnO microstructures were characterized. The possible growth mechanisms for the ZnO structures were proposed. The morphology-dependent photocatalytic activities of the as-prepared ZnO products were also examined and compared.

Figure 1. X-ray diffraction patterns of a typical hexagonal bullet-like morphology of the as-prepared products at 200 °C for 24 h (1.5 mol/ L NaOH).

adding 0.1 g of ZnCl2 (0.05 mol/L) and 0.9 g of NaOH (1.5 mol/L) to 15 mL of deionized H2O. The precursor solution was stirred continuously using a magnetic stirrer for 10 min to form a homogeneous solution. The precursor solution was then transferred to a 25-mL stainless Teflon-lined autoclave and then a Cd sheet was placed in the autoclave as substrate in order to obtain samples. The autoclave was aged at 200 °C for 24 h and then it was cooled to room temperature

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZnO Structures. In this work, ZnO structures were synthesized by a hydrothermal method. ZnCl2 was taken as Zn2+ source and NaOH was used as the precipitating agent to release hydroxyl ions during the reaction. In a typical experiment, the precursor solution was prepared by

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These two authors contributed equally to this work. © 2012 American Chemical Society

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Figure 3. FESEM images of the as-prepared products obtained at 180 (a) and 220 °C (b) for 24 h (1.5 mol/L NaOH).

Figure 2. FESEM images and EDX of the as-prepared products obtained at 200 °C for 24 h (1.5 mol/L NaOH).

naturally. The Cd sheet at the bottom of the container was collected and washed repeatedly with water and alcohol to remove the unreacted ions. Finally, the resulting sample was dried in air atmosphere at 120 °C for 3 h. 2.2. Characterization of Hexagonal and Bullet-Like ZnO Microstructures. The X-ray diffraction (XRD) patterns of samples were taken using a Rigaku D/max 2500 powder diffractometer with a Cu Kα radiation of wavelength of 1.5406 Å and were analyzed from 3° to 80° (2θ) with a graphite monochromator. The morphology and size of the as-prepared products were observed by a Hitachi S-4800 scanning electron microscope (FESEM). The group on the samples was studied by infrared absorption spectroscopy using a Bruker Tensor37 Fourier

Figure 4. FESEM images of the as-prepared products obtained at 200 °C for 24 h: (a) 0.75 mol/L NaOH and (b) 3 mol/L NaOH.

transform infrared spectrometer (FT-IR). UV/vis spectra were recorded on a HP8453 spectrophotometer at room temperature. 4923

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Figure 5. FESEM images of the as-prepared products obtained at 200 °C for 24 h (1.5 mol/L NaOH): (a) 3 h, (b) 6 h, (c) 12 h, and (d) 18 h.

2.3. Catalytic Activity Evaluation. The photocatalytic activity under UV light irradiation of the ZnO samples was evaluated by using methylene blue (MB) as the model substrate. In a typical process, 250 mL of MB (10 mg/L) aqueous solution and 200 mg of photocatalyst powder were mixed in a quartz photoreactor. Prior to a photocatalytic reaction, the photocatalyst suspension was sonicated to reach adsorption equilibrium with the photocatalyst in darkness. The above solution was photoirradiated by using a 300 W Hg lamp as light source under continuous stirring. At a defined time interval, the concentration of MB in the photocatalytic reaction was analyzed by using an UV−vis spectrophotometer at 665 nm.

indicating that the sample is a pure ZnO phase (Al, Cd, Au, and C elements are from the SEM sample holder). From the experiment, the reaction temperature and the concentration of NaOH were found to affect the morphology of the products. Figure 3 shows the FESEM images of the product obtained at 180 and 220 °C. As shown in Figure 3a, the product obtained at 180 °C consisted of mocro-sheets that were 1−2 μm in diameter. The product obtained at 220 °C showed a hexagonal, albeit imperfect, structure (Figure 3b). Figure 4a and 4b show the products obtained with about 0.75 and 3 mol/L NaOH, respectively. ZnO was composed of arrays of nanorods when the NaOH concentration was 0.75 mol/L (Figure 4a). As shown in Figure 4a, the ZnO nanorods in the arrays were about 50−100 nm in diameter and around 1 μm long. ZnO had a flower-like morphology when the concentration of NaOH was increased to 3 mol/L (Figure 4b). 3.2. Formation Mechanism. To study the formation of the ZnO microstructures with a hexagonal bullet-like morphology, the morphology of these microstructures obtained at different reaction times was investigated when the reaction temperature and the concentration were kept constant (Figure 5). Figure 5a−5d present the FESEM images of the products when the reaction times were 3, 9, 12, and 18 h. ZnO showed sheet structures of 100−500 nm in diameter when the reaction time was 3 h (Figure 5a). The thickness of the ZnO sheets increased significantly, and the shape was similar to hexagonal structures when the reaction time was extended to 9 h (Figure 5b). After 12 h, a small number of bullet-like shaped ZnO emerged, albeit imperfect (Figure 5c). A large number of hexagonal bullet-like ZnO structures appeared, and the morphology of ZnO became more and more regular (Figure 5d). The hexagonal bullet-like shape was perfect when the reaction time was 24 h (Figure 2). Based on the experimental results mentioned above and in reference 13, the possible growth mechanisms of the hexagonal

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure. XRD analysis was used to analyze the crystal structure of the as-synthesized products. The samples were treated at 200 °C for 24 h (1.5 mol/L NaOH). The XRD patterns of the as-prepared ZnO are shown in Figure 1. Except the diffraction peaks corresponding to the cadmium, all diffraction peaks in Figure 1 can be indexed perfectly to the hexagonal ZnO, space group: P63mc [186], with lattice constants a = 3.253 Å and c = 5.213 Å (JCPDS 89-1397). The XRD patterns indicate that the ZnO obtained via the current synthetic method consists of pure phases. Figure 2a and 2b show a typical hexagonal bullet-like morphology of the as-prepared products at 200 °C for 24 h (1.5 mol/L NaOH). As shown in the figure, a bullet-like microstructure was obtained at 200 °C, which consists of a hexagonal trunk and a hexagonal tip on one side. The trunk was around 20 μm long, whereas the side tip was about 10 μm. The result shows that ZnO with a bullet-like morphology can be prepared using our approach. To determine the composition of the bullet-like ZnO sample, EDS analysis was carried out. Figure 2c shows that the ZnO sample consists of Zn and O elements, 4924

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concentrations of NaOH at 200 °C for 24 h. From Figure 7, commercial ZnO powder did not facilitate absorption in the range of 400−800 nm. However, the absorption wavelength range of the as-prepared ZnO powder was shifted to long wavelength compared with the commercial ZnO powder. The absorption in the visible range can be attributed to its morphology-dependent surface. The broad and intense absorption in the visible region observed in Figure 7 may relate to various kinds of defects in ZnO crystal. These defects are generally related to singly ionized oxygen vacancies. The oxygen vacancies result in the increase in the active centers, which may be helpful for ZnO photocatalytic activity. Several samples in Figure 7 show different absorption intensity in the visible region, indicating dissimilar photocatalytic activity for them. As shown in Figure 8, the photocatalytic activities of flowerlike (Figure 4b), bullet-like (Figure 2a), and arrays of nanorods

bullet-like structures were proposed. They consist of three stages: (1) deposition of crystal seeds on the Cd substrate, (2) growth of randomly oriented crystals from the seeds, and (3) oriented growth of crystals after extended reactions. In the early stages of growth, the ZnO crystals grew along the fastest growth orientation, and they were not oriented. However, as the sheetlike randomly oriented crystals grew further, they began to overlap, and their growth became physically limited. As a result, there was preferred orientation of the hexagonal bullet-like structures. 3.3. Photocatalytic Activity. The Fourier transformed infrared spectra of the bullet-like and commercial ZnO samples are shown in Figure 6. Comparing the IR spectra of the two

Figure 6. Infrared (IR) spectra of samples.

samples, no significant difference can be found. In Figure 6, absorption peaks of about 3447 cm−1 and 1636 cm−1 can be attributed to the stretching and bending modes of −OH. The peaks at around 2357 cm−1 corresponded to the absorbance of CO2 in the process of sample preparation. The peaks of Zn−O were observed at around 1090, 795, and 443 cm−1.19 Figure 7 shows the UV−vis diffuse reflection spectra of flowerlike (Figure 4b), bullet-like (Figure 2a), arrays of nanorods

Figure 8. Photocatalytic activities of samples on reaction time under UV-light irradiation: (a) no catalyst, (b) ZnO arrays of nanorods (no UV-light irradiation), (c) bullet-like ZnO, (d) flower-like ZnO, and (e) ZnO arrays of nanorods.

ZnO samples were investigated on the photodegradation of MB under UV light irradiation. A blank experiment without the ZnO photocatalyst but under UV irradiation shows that only a small quantity of MB was degraded (Figure 8a). Another blank experiment using ZnO arrays of nanorods as the photocatalyst without UV irradiation also showed that slight MB adsorption occurred (Figure 8b). According to Figure 8c−e, the photodegradation of MB was more efficient by ZnO arrays of nanorods (Figure 8e) than by flower-like and bullet-like ZnO (Figure 8c and 8d). The MB in aqueous solutions can be eliminated 83%, 72%, and 60% by arrays of nanorods, flowerlike, and bullet-like ZnO, respectively, after illumination by UV light for 180 min.

4. CONCLUSION In general, the oxygen vacancy of the defect in ZnO crystallinity can act as an active center to capture photoinduced electrons, and the recombination of photoinduced electrons and holes can be inhibited effectively. As a result, ZnO crystallinity photocatalytic activity can be improved greatly. The higher photocatalytic activity of the ZnO arrays of nanorods may be due to the greater oxygen vacancy on the ZnO surface.

Figure 7. UV−vis diffuse reflectance spectra of samples.

(Figure 4a), and commercial ZnO samples. The ZnO powders with different morphologies were prepared with different 4925

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(18) Xie, W.; Li, Y. Z.; Sun, W.; Huang, J. C.; Xie, H.; Zhao, X. J. J. Surface modification of ZnO with Ag improves its photocatalytic efficiency and photostability. Photochem. Photobiol., A 2010, 216, 149. (19) Xie, J.; Li, Y. T.; Zhao, W.; Li, B.; Wei, Y. Simple fabrication and photocatalytic activity of ZnO particles with different morphologies. Powder Technol. 2011, 207, 140.

AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-22-83955458. E-mail: [email protected] (L.S.); [email protected] (X.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Tianjin Science & Technology Project of Research Fundation (Grant 20100206) and the National Natural Science Foundation of China (Grant 21103122).

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