18846
J. Phys. Chem. C 2008, 112, 18846–18848
Enhanced Photocatalytic Water Splitting Properties of KNbO3 Nanowires Synthesized through Hydrothermal Method Qing-Ping Ding,*,†,§ Yu-Peng Yuan,†,‡ Xiang Xiong,† Rui-Pu Li,†,‡ Hong-Bo Huang,† Zhao-Sheng Li,†,‡ Tao Yu,†,‡ Zhi-Gang Zou,†,‡ and Shao-Guang Yang*,† Nanjing National Laboratory of Microstructures and Eco-Materials and Renewable Energy Research Center (ERERC), Nanjing UniVersity, Nanjing 210093, P. R. China, and School of Physical Electronics, UniVersity of Electronic Science and Technology of China, Chengdu 610054, P. R. China ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: October 13, 2008
Potassium niobate (KNbO3) nanowires were synthesized by a hydrothermal method. X-ray diffraction and Raman spectroscopy indicated the nanowires are well-crystallized with orthorhombic structure. High resolution transmission electron microscopy showed the KNbO3 nanowires grew along the [110] direction. The photocatalytic water splitting results over KNbO3 nanowires showed that the rate for H2 evolution from aqueous CH3OH solutions was up to 5.17 mmol/h/gcat. To the best of our knowledge, the photocatalytic water splitting activity of the prepared nanowires is the highest in all KNbO3 materials. Introduction As inspired by the finding of carbon nanotubes in 1991, onedimensional (1D) and quasi-one-dimensional nanostructures, such as nanotubes, nanowires and nanobelts have been the hottest topics in the research of nanoscience and nanotechnology due to their unique physical and chemical properties.1-3 During the past decades, interest on KNbO3 has been intensified for its acousticoptic, electro-optic, nonlinear optic, and piezoelectric properties. KNbO3 is often used as a frequency converting material and also as optical waveguides and a holographic storage medium.4 Besides, for applications in nanoelectromechanical systems (NEMS), KNbO3 is playing an important role, it has potential applicaions as a lead-free and biocompatible transducer with tunable piezoelectric response.5 Since Honda and Fujishima discovered photocatalytic H2 evolution over TiO2 in 1972,6 more and more researchers have joined in the researches on photocatalytic splitting of water because of its potential application for the direct production of clean H2 energy.7-10 One of the biggest bottlenecks in photocatalytic water splitting is low quantum efficiency. Now a lot of research work is focusing on enhancing the photocatalytic activities of photocatalysts.11-14 KNbO3 has also been reported as a photocatalyst for H2 production.15-18 Many publications have discussed the photocatalytic properties of KNbO3, but the quantum efficiency is too low for industrial production. It is generally believed that the morphology and structure are important factors which will influence the photocatalytic properties of a photocatalyst,19 and there are a lot of papers concerning different morphologies, but research on the photocatalytic properties of KNbO3 nanowires has not been reported so far. In this work, we tried to investigate the photocatalytic properties of KNbO3 nanowires which were synthesized by a hydrothermal method. To the best of our knowledge, the photocatalytic water splitting activity of the prepared nanowires is the highest in all KNbO3 materials. * Corresponding author. E-mail:
[email protected]. Telephone: 86-25-83597483. Fax: 86-25-83595535. † Nanjing National Laboratory of Microstructures, Nanjing University. ‡ Eco-Materials and Renewable Energy Research Center (ERERC), Nanjing University. § School of Physical Electronics, University of Electronic Science and Technology of China.
Figure 1. XRD pattern of the orthorhombic KNbO3 nanowires.
Figure 2. Raman spectrum of the KNbO3 nanowires excited by laser with wavelength of 514.5 nm.
Experimental Section We synthesized KNbO3 nanowires using a hydrothermal method following the work reported by Magrez et al.20 In this
10.1021/jp8042768 CCC: $40.75 2008 American Chemical Society Published on Web 11/11/2008
KNbO3 Nanowires
J. Phys. Chem. C, Vol. 112, No. 48, 2008 18847
Figure 3. (a) SEM image of KNbO3 nanowires with the TEM image as the inset. (b) HRTEM image of the nanowires with the SAED pattern as the inset.
reaction, niobium pentoxide powder was added to distilled water in which potassium hydroxide was dissolved. 38.7 wt % KOH, 0.4 wt % Nb2O5, 60.9 wt % H2O mixture, was stirred for 2 h and then poured into Teflon vessel. After the autoclave was heated to 150 °C for 1 week, a white precipitate was obtained. The precipitate was filtered, washed with distilled water and ethanol, and dried at 120 °C for 10 h. The as-prepared samples were characterized by scanning electron microscope (SEM, Philips-XL30 operated at 20.0 kV), transmission electron microscope (TEM and HRTEM, Tecnai-F20 operated at 200 kV), X-ray diffraction (XRD) and Raman scattering (Jobin Yvon-HR800). Results and Discussion Figure 1 presents the XRD pattern recorded from the prepared nanowires, it is revealed that the nanowires are highly crystallized and exhibit a single-phase perovskite structure. The XRD pattern can be indexed to orthorhombic structured KNbO3 with lattice parameters of a ) 5.695 Å, b ) 5.721 Å, and c ) 3.973 Å (JCPDS 32-0822). Micro-Raman measurement was performed at room temperature using the 514.5nm line of an argon laser as the source. Figure 2 shows the Raman spectrum of the KNbO3 nanowires. All peaks in the spectrum can be indexed to orthorhombic (Amm2) structured KNbO3.21 The typical scanning electron microscopy (SEM) image of the KNbO3 nanowires is shown in Figure 3a, where the inset is a representative transmission electron microscopy (TEM) image. From the SEM image, we can see the nanowires are largequantity, which is the advantage of hydrothermal method. These nanowires are several µm in length and about 100 nm in diameter. To study the structural properties of the nanowires, high-resolution transmission electron microscopy (HRTEM) analysis was performed. As can be seen in Figure 3b, the nanowires are single-crystalline with edges parallel to a lowindexed plane of KNbO3. The selected area electron diffraction (SAED) pattern, which was obtained from an entire nanowire, was indexed by assuming the lattice constants refined from XRD measurements. According to this result, the nanowire axis grew along the [110] direction of KNbO3 referred to the orthorhombic unit cell, and the nanowire edges are parallel to the (010) and (100) planes. Figure 4 shows the absorption spectra of the as-prepared KNbO3 nanowires converted from the diffuse reflection spectra data by Kubelka-Munk (K-M) theory method.22 The band gap of KNbO3 is estimated to be 3.2 eV, which is a little higher than that of KNbO3 nanocubes.17 This difference might be due
Figure 4. UV-visible absorption spectra of KNbO3 nanowires at ambient temperature.
Figure 5. Production rates of H2 of KNbO3 nanowires, nanocubes, and powder.
to the different crystal sizes of the samples. The sample with smaller crystal size tended to have a blue shift of optical absorption or larger energy band gap, possibly caused by quantum-size effect. The photocatalytic water splitting reactions over KNbO3 nanowires were carried out in a gas-closed system.23 0.2 g KNbO3 nanowires were suspended in a methanol aqueous solution (370 mL distilled water, 50 mL methanol) under magnetic stirring in an inner irradiation quartz cell. 0.5 wt
18848 J. Phys. Chem. C, Vol. 112, No. 48, 2008 % platinum particles were deposited onto the photocatalyst surface from H2PtCl6 methanol aqueous solution by an in situ photodeposition method. The reactions were initiated under the irradiation of a 400 W high-pressure mercury lamp while the mixed solution was covered with a quartz jacket to keep the reactor temperature at 20 °C by the cooling water. Generated gas was analyzed through an online gas chromatograph (Shimadzu GC-8A, molecular sieve 5 A column, thermal conductivity detector, Ar carrier), which was connected to a gas circulating line. In order to compare the photocatalytic properties with other KNbO3 materials. We synthesized KNbO3 powder through solid state reaction and KNbO3 nanocubes with hydrothermal method. The photocatalytic water splitting experiments of these two KNbO3 materials were carried out under the same conditions of the nanowires. Figure 5 shows the profile of H2 evolution versus time over KNbO3 nanowires, nanocubes and powder. The H2 production rate for nanowires nanocubes and powder are 1.03 mmol/h, 0.42 mmol/h and 0.79 mmol/h respectively. It is well-known that the surface area is a key factor which affects the photocatalytic activity of a photocatalyst. The BET (Brunauer-Emmett-Teller) measurement showed that the surface area of the KNbO3 nanowires was 8.50 m2/g while that of nanocubes was just 6.13 and 2.31 m2/g for powder. Therefore, the higher surface area of the KNbO3 nanowires might be responsible for the higher efficiency. Besides the surface area, high crystallinity and fineness of KNbO3 nanowires are probably the other reasons for the high activity of nanowire photocatalyst.24,25 Conclusions In summary, we synthesized a large quantity of KNbO3 nanowires under mild conditions (low temperature) with a hydrothermal method. The single-crystalline nanowires exhibit a single-phase perovskite structure. The photocatalytic activity of KNbO3 nanowires in water splitting is 5.17 mmol/h/gcat, which is much higher than the KNbO3 nanocubes and KNbO3 powder. This promising result may be ascribed to the higher surface area and high crystallinity and fineness of KNbO3 nanowires, and should be of significant value on improving the quantum efficiency of photocatalytic water splitting. The endeavor is being made to synthesize other materials with onedimensional morphology for photocatalysis application. Acknowledgment. The authors would like to acknowledge the financial support from the National High Technology Research Project of China (No. 2006AA052113), the National Natural Science Foundation of China (Nos. 60577002 and 20528302), the Natural Science Foundation of Jiangsu Province
Ding et al. (No. BK2006111), and the National Key Project for Basic Research (No. 2007CB936300). One of the authors (Prof. Yang) would acknowledge the financial support from the New Century Talent Project of Ministry of Education (Grant No. 07-0430). Supporting Information Available: Text giving the experimental details and figures showing the XRD patterns and SEM images of the KNbO3 nanocubes and KNbO3 powder synthesized through solid state reaction. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Iijima, S. Nature 1991, 354, 56. (2) Whitney, T. M. Science 1993, 261, 1336. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (4) Nakayama, Y.; Pauzauskie, P. J.; Radenovic, A.; Onorato, R. M.; Saykally, R. J.; Liphardt, J.; Yang, P. D. Nature 2007, 447, 1098. (5) Saito, Y.; Takao, H.; Tani, T.; Nonoyama, T.; Takatori, K.; Homma, T.; Nagaya, T.; Nakamura, M. Nature 2004, 432, 84. (6) Honda, K.; Fujishima, A. Nature 1972, 238, 37. (7) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, O. K.; Taga, Y. Science 2001, 293, 269. (8) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Nature 2002, 414, 625. (9) Tsuji, I.; Kato, H.; Kudo, A. Angew. Chem., Int. Ed. 2005, 44, 3565. (10) Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Nature 2006, 440, 295. (11) Kato, H.; Asakura, K.; Kudo, A. J. Am. Chem. Soc. 2003, 125, 3082. (12) Tang, J. W.; Zou, Z. G.; Ye, J. H. J. Phys. Chem. B 2003, 107, 14265. (13) Hara, M.; Nunoshige, J.; Takata, T.; Kondo, J. N.; Domen, K. Chem. Commun. 2003, 24, 3000. (14) Liu, M. Y.; You, W. S.; Lei, Z. B.; Zhou, G. H.; Yang, J. J.; Wu, G. P.; Ma, G. J.; Luan, G. Y.; Takata, T.; Hara, M.; Domen, K.; Can, L. Chem. Commun. 2004, 19, 2192. (15) Uchida, S.; Inoue, Y.; Fujishiro, Y.; Sato, T. J. Mater. Sci. 1998, 33, 5125. (16) Hayashi, H.; Hakuta, Y.; Kurata, Y. J. Mater. Sci. 2004, 14, 2046. (17) Liu, J. W.; Chen, G.; Li, Z. H.; Zhang, Z. G. Int. J. Hydrogen Energy 2007, 32, 2269. (18) Ryu, S. Y.; Choi, J.; Balcerski, W.; Lee, T. K.; Hoffmann, M. R. Ind. Eng. Chem. Res. 2007, 46, 7476. (19) Shangguan, W.; Yoshida, A. Int. J. Hydrogen Energy 1999, 24, 425. (20) Magrez, A.; Vasco, E.; Seo, J. W.; Dieker, C.; Setter, N.; Forro’, L. J. Phys. Chem. B 2006, 110, 58. (21) Shen, Z. X.; Hu, Z. P.; Chong, T. C.; Beh, C. Y.; Tang, S. H.; Kuok, M. H. Phys. ReV. B 1995, 52, 3976. (22) Kubelka, P.; Munk, F. Tech. Z. Phys. 1931, 12, 593. (23) Zou, Z. G.; Ye, J. H.; Arakawa, H. Chem. Phys. Lett. 2000, 332, 271. (24) Li, Z. S.; Yu, T.; Zou, Z. G.; Ye, J. H. Appl. Phys. Lett. 2006, 88, 071917. (25) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 45.
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