pubs.acs.org/Langmuir © 2009 American Chemical Society
Formation and Photocatalytic Application of ZnO Nanotubes Using Aqueous Solution Dewei Chu,* Yoshitake Masuda, Tatsuki Ohji, and Kazumi Kato National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan Received August 4, 2009. Revised Manuscript Received October 20, 2009 Vertically aligned ZnO nanotubes were prepared by etching ZnO rod arrays in aqueous solution, which were previously developed by chemical bath deposition method. The morphological, structural, photoluminescence, as well as photocatalytic properties of the ZnO nanotubes were examined with respect to the pH values of chemical bath solution. The morphology of the products was found to be sensitive to the pH values and chemical bath temperatures. The nanotubes synthesized at a low pH value (5.82) exhibited a strong UV emission and a weak defect-related visible emission. The highest photocatalytic efficiency was also observed at pH = 5.82. The possible mechanism for the difference of photocatalytic efficiency was discussed.
Introduction Nowadays, environmental problems are becoming more and more serious with the development of industry and economy. Since Fujishima and Honda reported the evolution of oxygen and hydrogen from titanium oxide electrode in the electrolyte cell under irradiation of light, photocatalysis has been regarded as one of the most effective ways to solve these problems.1 Various photocatalysts, especially metal oxide photocatalysts such as titanium oxide, tin oxide, and zinc oxide are promising materials for degradation of organic pollutants by utilizing UV or solar light.2-9 Zinc oxide (ZnO), a typical n-type wide band gap semiconductor, plays an important role in many application fields from optoelectronics to energy conversion, photocatalysis, and gas sensing.10-14 As to photocatalysis, ZnO is more efficient than TiO2, the most intensively studied photocatalyst, in the photo*Corresponding Author, Tel.: þ81 52-736-7238; Fax: þ81 52-736-7234; E-mail address:
[email protected]. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Wang, Z.; Huang, B.; Dai, Y.; Qin, X.; Zhang, X.; Wang, P.; Liu, H.; Yu, J. J. Phys. Chem. C 2009, 113, 4612–4617. (3) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707–10711. (4) Quan, X.; Yang, S.; Ruan, X.; Zhao, H. Environ. Sci. Technol . 2005, 39, 3770–3775. (5) Usseglio, S.; Damin, A.; Scarano, D.; Bordiga, S.; Zecchina, A.; Lamberti, C. J. Am. Chem. Soc. 2007, 129, 2822–2828. (6) SoliCasados, D.; Vigueras-Santiago, E.; Hernadez-Loez, S.; Camacho-Loez, M. A. Ind. Eng. Chem. Res. 2009, 48, 1249–1252. (7) Hou, L.-R.; Yuan, C.-Z.; Peng, Y. J. Hazard. Mater. 2007, 139, 310–315. (8) El-Maghraby, E. M.; Nakamura, Y.; Rengakuji, S. Catal. Commun. 2008, 9, 2357–2360. (9) Ohsaki, H.; Kanai, N.; Fukunaga, Y.; Suzuki, M.; Watanabe, T.; Hashimoto, K. Thin Solid Films 2006, 502, 138–142. (10) Baxter, J. B.; Aydil, E. S. Sol. Energy Mater. Sol. Cells 2006, 90, 607–622. (11) Yuhas, B. D.; Yang, P. J. Am. Chem. Soc. 2009, 131, 3756–3761. (12) Yamazaki, T.; Wada, S.; Noma, T.; Suzuki, T. Sens. Actuators, B 1993, 13-14, 594–595. (13) Ye, C.; Bando, Y.; Shen, G.; Golberg, D. J. Phys. Chem. B 2006, 110, 15146–15151. (14) Han, X.-G.; He, H.-Z.; Kuang, Q.; Zhou, X.; Zhang, X.-H.; Xu, T.; Xie, Z.-X.; Zheng, L.-S. J. Phys. Chem. C 2009, 113, 584–589. (15) Sakthivel, S.; Neppolian, B.; Shankar, M. V.; Arabindoo, B.; Palanichamy, M.; Murugesan, V. Sol. Energy Mater. Sol. Cells 2003, 77, 65–82. (16) Yan, H.; Hou, J.; Fu, Z.; Yang, B.; Yang, P.; Liu, K.; Wen, M.; Chen, Y.; Fu, S.; Li, F. Mater. Res. Bull. 2009, 44, 1954–1958. (17) Pardeshi, S. K.; Patil, A. B. J. Hazard. Mater. 2009, 163, 403–409.
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degradation of some organic compounds.15-19 However, the application of ZnO as photocatalyst was limited due to its photoinstability in aqueous solution. Thus, improving the stability of ZnO without sacrificing its efficiency represents very attractive scientific and technological problems to be solved. Kislov et al. investigated the photostability and photocatalytic activity of different crystal surfaces of ZnO.20 They showed that photolysis of ZnO was strongly orientation dependent, and the (000-1)-O surface exhibited lowest photolysis stability. Besides, photocatalytic reaction for ZnO (10-10) shows the highest activity followed by ZnO (0001)-Zn and ZnO (000-1)-O surface shows the lowest activity. Inspired by this investigation, it is worthwhile to design ZnO micro/nanostructures that exhibit photostable surfaces and find methods to block O-terminated polar surface, which would make ZnO a more feasible material for photocatalysis. According to above analysis, c-oriented tubular ZnO might be suitable candidates for photocatalysis application because such tubular structure is mainly composed of stable (10-10) surfaces instead of (000-1)-O and (0001)-Zn surfaces and it owns large surface area. The principal techniques used for growing ZnO nanotubes include template-assisted growth and electrodeposition.21-25 Although these methods can produce highquality ZnO nanotubes, they usually confront problems of templates removal, tedious operation procedures, or requirement of special substrates. Recently, chemical bath deposition (CBD) process was found to be attractive for growing various ZnO tubular structures because of the moderate temperature and (18) Tian, J.; Chen, L.; Yin, Y.; Wang, X.; Dai, J.; Zhu, Z.; Liu, X.; Wu, P. Surf. Coat. Technol. 2009, 204, 205–214. (19) Pardeshi, S. K.; Patil, A. B. J. Mol. Catal. A: Chem 2009, 308, 32–40. (20) Kislov, N.; Lahiri, J.; Verma, H.; Goswami, D. Y.; Stefanakos, E.; Batzill, M. Langmuir 2009, 25, 3310–3315. (21) Elias, J.; Tena-Zaera, R.; Wang, G.-Y.; Le-Cleent, C. Chem. Mater. 2008, 20, 6633–6637. (22) Guang-Wei, S.; Xiao-Hong, Z.; Wen-Sheng, S.; Xia, F.; Jack, C. C.; ChunSing, L.; Shuit-Tong, L.; Chang-Hong, L. Appl. Phys. Lett. 2008, 92, 053111. (23) She, G. W.; Zhang, X. H.; Shi, W. S.; Fan, X.; Chang, J. C.; Lee, C. S.; Lee, S. T.; Liu, C. H. Appl. Phys. Lett. 2008, 92, 053111. (24) Yu, L. G.; Zhang, G. M.; Li, S. Q.; Xi, Z. H.; Guo, D. Z. J. Cryst. Growth 2007, 299, 184–188. (25) Sun, Y.; Fuge, G. M.; Fox, N. A.; Riley, D. J.; Ashfold, M. N. R. Adv. Mater. 2005, 17, 2477–2481.
Published on Web 11/09/2009
DOI: 10.1021/la902866a
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Figure 1. SEM images of ZnO nanotubes, chemical bath solution pH = 5.82, temperature=75 °C.
simple manipulations on different substrates.26 In this paper, we report the shape controlled synthesis of ZnO nanotubes by a simple CBD method and corresponding photocatalytic properties. By tuning reaction parameters, such as pH values and growth temperatures, ZnO nanotubes with different morphologies are successfully synthesized. The relationship between their structure, optical, and photocatalytic properties are discussed.
Experimental Section All chemicals (Wako Pure Chemical Industries, Ltd., Japan) were used as received without further purification. FTO glass (FTO, SnO2: F, Asahi glass Co., Ltd., Japan, 26 50 1.1 mm) were used as substrates. ZnO seed was prepared by a sol-gel method. Basically, 1.1 g Zn(CH3COO)2 3 2H2O and 0.29 g LiOH 3 H2O were dissolved into 50 mL ethanol. They were mixed rapidly, and stirred at 80 °C for 10 min, then cooled to room temperature. The resultant sol was transparent with ZnO nanoparticles. The solution was then spincoated on the substrate at 500 rpm for 5 s, and 3000 rpm for 30 s. After processing, the substrate was heated at a certain temperature to remove the solvent. Growth of ZnO nanotube was carried out by suspending the substrate in a 40 mL beaker filled with an equimolar aqueous solution of 0.1 M zinc nitrate hexahydrate (Zn(NO3)2 3 6H2O), and 0.1 M methenamine (C6H12N4, HMT) at 75 °C for 3 h, the initial pH value of the solution was tuned by adding CH3COOH. Subsequently, the substrate was removed from the solution, rinsed with deionized water and immersed into 0.3 M KOH aqueous solution at 80 °C for 1 h. Finally, the substrate was washed by deionized water and dried at 100 °C for 1 h. The phase composition of the samples was characterized by X-ray powder diffraction (XRD, RINT-2100 V, Rigaku, Cu KR,). The morphologies of the samples were observed by field emission scanning electron microscopy (FESEM; JSM-6335FM, JEOL, with an accelerating voltage of 5 kV). Specific surface area measurement was carried out using Autosorb-1(Quantachrome Instruments). Samples were cut and outgassed at 110 °C under 10-2 mmHg for 6 h prior to measurement. Specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method using desorption isotherms. The photocatalytic activity of as-prepared ZnO for decomposing methyl blue (MB) in aqueous solution was investigated by the bleaching of dye solvated. In a typical measurement, the obtained ZnO film was put into a quartz reactor with 60 mL of MB aqueous solution, and the initial concentration was 20 mg L-1. The reactor was then kept in the dark with agitation for 30 min to obtain adsorption equilibrium, prior to light irradiation by a 110W UV lamp (SUV110GS-36, SEN Light Corp). The efficiency of the degradation processes was evaluated by monitoring the dye decolorization at the maximum absorption around λ=663 nm (26) Vayssieres, L.; Keis, K.; Hagfeldt, A.; Lindquist, S. E. Chem. Mater. 2001, 13, 4395–4398.
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as a function of irradiation time in the separated MB solution with a UV-vis spectrometer (Shimadzu UV 2550). To estimate the photostability of nanotubes, 5 cycles of cyclic photodegradation experiments were carried out before measuring, and each cycle was 1 h.
Results and Discussion Figure 1 shows the typical SEM images of the as-grown arrays of ZnO nanotubes. The arrays of nanotube exhibit high orientation perpendicular to the substrates, and hexgonal nanotubes with wall thickness of several tens nanometers and external diameters of ∼650 nm were formed. Since the nanotubes were evolved from the nanorods, the size of nanotubes are determined by the original ZnO nanorods, which can be readily controlled by tuning the reaction parameters such as Zn2þ ions concentration and growth time. Therefore, it is possible to fabricate ZnO nanotubes with various sizes for different application by this method. The inset of figure 1 (b) shows enlarged SEM image of single nanotube, where completely tubular structure is found. The pH values have been found to be critical parameters for tuning the morphology of ZnO nanostructures grown from CBD process.27 In general, ZnO growth is directly affected by the decomposition of HMT in the zinc nitrate hexahydrate-HMT system. HMT provided the hydroxide ions (OH-) to the solution, and usually Zn2þ cations form the hydroxyl complex of Zn(OH)42- anions, becoming the precursors of ZnO.28 Therefore, it is reasonable to deduce that pH value has a great influence on the morphology of ZnO nanotubes. The growth process with a lower pH value ( 6.13 solution, it is (31) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2007, 129, 12380–12381. (32) Xu, L.; Hu, Y.-L.; Pelligra, C.; Chen, C.-H.; Jin, L.; Huang, H.; Sithambaram, S.; Aindow, M.; Joesten, R.; Suib, S. L. Chem. Mater. 2009, 21, 2875–2885. (33) Li, G. R.; Hu, T.; Pan, G. L.; Yan, T. Y.; Gao, X. P.; Zhu, H. Y. J. Phys. Chem. C 2008, 112, 11859–11864. (34) Li, H.; Bian, Z.; Zhu, J.; Zhang, D.; Li, G.; Huo, Y.; Li, H.; Lu, Y. J. Am. Chem. Soc. 2007, 129, 8406–8407.
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Figure 6. Concentration changes of MB dyes as a function of irradiation time: (a)-(d) ZnO nanotubes prepared under different pH values, (a) pH = 5.82; (b) pH = 6.05; (c) pH = 6.20; (d) pH = 6.13. (e) ZnO nanotubes (from pH = 5.82 solution) after 5 cycles of photodegradation. (f) ZnO nanorods (from pH =6.7 solution) after 5 cycles of photodegradation.
Figure 7. Atomic stacking and morphology models of ZnO.
composed of hexagonal rods. The atomic stacking and morphology models of ZnO hexagonal rods are shown in Figure 7 (a) and (b), respectively. In the structure of wurtzite ZnO, O2- and Zn2þ ions stack alternatively along the c-axis, resulting in the top surface of (000-1)-O and bottom surface of (0001)-Zn. As to ZnO obtained from pH = 5.82 solution, the top and bottom Langmuir 2010, 26(4), 2811–2815
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surfaces almost disappear, remaining (10-10) surfaces, which is shown in Figure 7 (d). The (10-10) planes are composed of equivalent O2- and Zn2þ ions at the same planes, and thus they are nonpolar with the lowest surface energy. The difference of surface atomic structures should result in a distinct chemical ability and the reported photocatalytic activity for various ZnO planes is contradicted. Basically, it has been proved that the stability of ZnO planes can be described as (10-10) > (0001)-Zn > (000-1)O. The instability of (000-1)-O is caused by its high surface energy, and in the solution environment of photodegradation the following reactions would occur: 2hþ þ ZnO f Zn2þ þ O/
ð1Þ
O/ þ O/ f O2
ð2Þ
where hþ is hole and O* is intermediate atomic oxygen. The holes play an important role in photocatalytic reactions and they are consumed by the above photolysis reactions. For ZnO rods, (000-1)-O and (0001)-Zn surface are easier to be attacked by holes thus show lower quantum yield than that of nanotubes. On the basis of the discussion above, it can be concluded in principle that the (10-10) surface of ZnO shows the best photocatalytic
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activity, which agrees well with our experimental results and other reports.
Conclusion ZnO nanotubes were fabricated by a facial two-step chemical method, and the relationship between surface structures, photoluminescence and photocatalytic properties have been studied. The results indicate that the surface structures are greatly affected by pH value of chemical bath solution, and well-defined hexagonal ZnO nanotubes composed of (10-10) surfaces was obtained from pH = 5.82 precursor solution. The UV-visible emission ratio of the nanotubes was significantly improved by decreasing pH values of chemical bath solution. Besides, the nanotubes prepared from lower pH values shows better photocatalytic activity for MB degradation. The observations suggest that the properties of ZnO are strongly dependent on the surface structure, and ZnO nanotubes might be potential materials for photocatalytic applications because of their stability and activity. This study should be helpful for comprehending the anisotropy of metal oxide nanostructures and improve their photocatalytic efficiency. Acknowledgment. This work is supported by the Ministry of Economy, Trade and Industry (METI), Japan, as a part of the Environmentally Friendly Sensor Project.
DOI: 10.1021/la902866a
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