CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3521–3525
Articles Controlled Growth of Lead Oxide Nanosheets, Scrolled Nanotubes, and Nanorods Liang Shi,*,† Yeming Xu,‡ and Quan Li‡ Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, P. R. China, and Department of Physics, The Chinese UniVersity of Hong Kong, Shatin, New Territory, Hong Kong, P. R. China ReceiVed September 20, 2007; ReVised Manuscript ReceiVed May 31, 2008
ABSTRACT: PbO2 nanosheets, scrolled nanotubes, and nanorods have been selectively synthesized on lead foil with a convenient hydrothermal approach by simply modifying the reaction conditions. The presence of ammonia plays a key role in the preparation of PbO2 nanosheets, while the volume ratio of NH3 to H2O and reaction temperature have a significant influence on the shapes and sizes of the PbO2 product. The formation mechanism of the PbO2 nanostructures was discussed. The samples were characterized using X-ray powder diffraction, energy dispersive X-ray spectrometry, transmission electron microscopy, and scanning electron microscopy.
1. Introduction During the past decades, low dimensional nanostructured materials such as nanosheets, nanotubes, and nanorods have triggered a worldwide interest because of their unique electronic, optical, and mechanic properties and potential application in constructing nanoscale electric and optoelectronic devices. It has been reported that the morphology and dimensions exert a significant influence on the physical and chemical properties of nanomaterials.1-3 Therefore, controlled growth or fabrication of nanostructrues with desired morphology, shape, and sizes is very important for both the nanomaterials science and technology.4-6 Up to now, various nanostructures of many oxides such as Ga2O3, MgO, GeO2, In2O3, SiO2, and ZnO have been synthesized successfully.7-14 However, there have been relatively few research reports about the controlled growth of lead oxide nanostructures.15,16 As the basic material of the electrode active mass in lead-acid batteries, lead oxide has wide industrial applications. It is well-known that lead oxide has four basic types including PbO, PbO2, Pb2O3, and Pb3O4. Among them, lead dioxide (PbO2) is an attractive material and has found application in many electrochemical and industrial fields. For example, it has been used as electrodes in electrochemical devices17 in the oxidation of organic compounds in wastewater,18 in the evolution of ozone,19 and as an electrocatalyst for 2-napthol.20 Lead dioxide exists as a tetragonal, orthorhombic, or cubic crystal structure. The orthorhombic R-phase PbO2 was * To whom correspondence should be addressed. Phone: 86-551-3606447. Fax: 86-551-3607402. E-mail:
[email protected]. † University of Science and Technology of China. ‡ The Chinese University of Hong Kong.
found on an electrode of a lead battery by Zaslavsky et al.21 The tetragonal β-phase of lead dioxide crystallizes with the rutile structure type.22 The cubic fluorite structure type PbO2 is obtained by Syono and Akimoto23 at high pressure. Recent studies have shown that the morphology and size of the electrode components play an important role in the improvement of operating electrochemical activity of electrodes in batteries.24-27 Consequently, controlled growth of lead oxide nanostructured materials is very valuable and remains still a challenge so far. Here, we report a novel simple one-step hydrothermal method to control the shape and morphology of a series of PbO2 nanostructures by direct oxidation of a lead foil in an ammonia solution. This method provides a controllable, mild, and convenient approach for the preparation of PbO2 nanostructures without the sophisticated technique and catalysts. PbO2 nanosheets, scrolled nanotubes, and nanorods can be selectively synthesized on a large scale with the modification of the volume ratio of ammonia solution to H2O and reaction temperature.
2. Experimental Section 2.1. Preparation of PbO2 Micrometer Columns Composed of Stacked Nanosheets. In a typical procedure, 8 mL of concentrated ammonia solution (about 30%), 8 mL of distilled water, and a piece of lead foil (1 × 1 cm2) were added to a 20 mL Teflon-lined autoclave. The autoclave was sealed and maintained at 150 °C for 24 h and then air cooled to room temperature. The lead foil was then collected from the solution and rinsed with distilled water and ethanol and air-dried for characterization.
10.1021/cg700909v CCC: $40.75 2008 American Chemical Society Published on Web 08/28/2008
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Figure 1. XRD pattern of the as-prepared PbO2 micrometer columns composed of stacked nanosheets.
2.2. Preparation of PbO2 Scrolled Nanotubes and Nanorods. The procedure for the preparation of PbO2 scrolled nanotubes was similar to that for the preparation of the PbO2 column of stacked nanosheets except that 5 mL of concentrated ammonia solution (about 30%), 10 mL of distilled water, and a piece of lead foil (1 × 1 cm2) were added to a 20 mL Teflon-lined autoclave. For preparation of nanotubes, the autoclave was sealed and maintained at 180 °C for 24 h. For preparation of nanorods, the autoclave was sealed and maintained at 200 °C for 24 h The overall crystallinity of the product is examined by X-ray diffraction (XRD, Rigakau RU-300 with Cu KR radiation). The general morphology of the products was examined by scanning electron microscopy (SEM LEO 1450VP). Detailed microstructure analysis was carried out using transmission electron microscopy (TEM, PhilipsCM120). The chemical composition analysis was obtained by energy dispersive X-ray spectrometry (EDX) using an EDX spectrometer attached to the TEM.
3. Results and Discussion 3.1. Controlled Growth of PbO2 Nanosheets: Effect of Amount of Ammonia. A typical XRD pattern of the asprepared columns of stacked nanosheets is shown in Figure 1, and all the diffraction peaks can be indexed to those of face-centered-cubic PbO2, except those marked with Pb arising from the lead foil. After refinement, the lattice constant, a ) 5.344 Å, is obtained, which is very close to the reported value for PbO2, a ) 5.349 Å (JCPDS card, No. 22-0389). The broadening of the XRD peaks suggests that the grain sizes of the sample are on a nanometer scale. The SEM image in Figure 2a shows an overview of the asprepared products. It is apparent the products consist of a large quantity of sheets and columns structure. The side of a fallen column in the magnified SEM image (Figure 2b) indicated clearly that the column is actually made up of many sheets, which were stacked one by one. As can be seen from Figure 2a,b, these sheets are nearly round in shape with a typical thickness of about 100 nm and diameters in the range of 2-4 µm. A TEM image of the sample shown in Figure 2c confirms its nanosheet structure. The corresponding selected-area electron diffraction (SAED) pattern along the [1j11] zone axis in the inset of Figure 2c reveals that the sheets are single crystalline. The EDX spectrum (Figure 2d) taken from the sample shows intense peaks of Pb and O, suggesting the composition as Pb and O only. The copper signals come from the copper grid. In the present case, no PbO2 sample can be obtained in the absence of ammonia. If the NaOH solution (0.01 M) is used instead of ammonia, only particles of PbO2 can be produced.
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Therefore, ammonia plays a key role in the preparation of PbO2 nanosheets. Detailed experimental analysis indicates that the volume ratio of ammonia solution to distilled water (V-NH3/ V-H2O) has a significant influence on the morphology of final products. If only concentrated ammonia solution is used, very thin PbO2 sheets (thickness is about 10 nm) can be obtained (Figure 3a). The addition of a small amount of water (V-NH3/ V-H2O ) 4/1) resulted in still sheets, but the thickness increases a little to about 50 nm (Figure 3b). A further decrease of the ratio of V-NH3/V-H2O to 1/1 produced a large amount of columns of stacked sheets with an average thickness of about 100 nm (Figure 3c). When the ratio of V-NH3/V-H2O was decreased to 1/4, a lot of hexagonal plates with an average thickness of about 500 nm appeared in the sample (Figure 3d). So, it is obvious that the thickness of produced sheets increases with the decrease of the ratio of V-NH3/V-H2O. These results suggest that the shapes and sizes of the PbO2 sheets are strongly dependent on the amount of ammonia and the reaction kinetics. In the present case of formation of PbO2 nanosheets, we believe that the ammonia acts as both an etching agent and a surface-passivating agent.28-30 Under the basic condition of the ammonia solution, a thin layer of PbO2 may first grow due to the etching effect of ammonia. Meanwhile, the NH3 molecules perhaps can control the growth rates of different crystalline faces of PbO2 by interacting with these faces through selective absorption and desorption. It may be presumed that the wide surfaces of PbO2 sheets are completely passivated by ammonia while the side surfaces are partially passivated by NH3. Therefore, the ammonia absorbs strongly on the wide surface, hindering the growth along the direction that is perpendicular to the wide plane. So, the lamellar shape is kept. The decrease of ammonia solution NH3 will reduce the passivation effect, which induces the thickening of the PbO2 sheet. The formation of columns of stacked sheets under the suitable condition (V-NH3/V-H2O ) 1/1) can be explained as follows. When the first PbO2 layer grows larger and thicker to some degree, it will gradually be away from the lead foil and stop growth due to the absence of the lead source. Then, the lead underneath the first formed PbO2 layer is continuously selectively etched by NH3 and grows into the second PbO2 sheet. Once the second sheet is off from the lead surface, the third one under the second one will begin to grow. These sheets may connect through hydrogen bonds between NH3, which absorbed on the wide plane surface. As a result of this growth process, a column of stacked sheets formed. As for the formation of the hexagonal plates, this may be determined by the crystal growth kinetics. When the ratio of V-NH3/V-H2O was 1/4, the PbO2 crystal would grow mainly along the six directions of {220} crystalline facets with the same growth speed, while the growth along the [1j 11] direction is hindered. This led to the formation of hexagonal plates. 3.2. Controlled Growth of PbO2 One-Dimensional Nanostructues: Effect of Reaction Temperature. It is found that the reaction temperature also has a significant influence on the final morphology of the product. When the ratio of V-NH3/V-H2O is 1/2, the lower temperature (below 150 °C) can only produce PbO2 nanosheets with a shape similar to that of Figure 3b. When the temperature was increased to 180 °C, a lot of scrolled nanotubes could be obtained. The XRD pattern of the as-prepared product is similar to Figure 1 and shows all the diffraction peaks can be indexed to those of face-centered-cubic PbO2 (JCPDS card, No. 22-
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Figure 2. (a), (b) SEM image of the as-prepared columns of the stacked nanosheets sample. (c) TEM image of the nanosheets. Inset: selected area electron diffraction pattern of the nanosheets sample. (d) The EDX spectrum of the nanosheets sample.
Figure 3. SEM image of PbO2 sheets prepared by using different volume ratios of NH3 to H2O, showing the influence of the amount of ammonia on the shapes and sizes of the PbO2 sheets: (a) only concentrated ammonia; (b) V-NH3/V-H2O ) 4/1; (c) V-NH3/V-H2O ) 1/21/1; (d) V-NH3/ V-H2O ) 1/4.
0389). The SEM image in Figure 4a shows the products are rodlike. However, the contrast at the tips reveals clearly their hollow structure. The tip openings are usually sharp, irregular, and forklike, which suggests the tubes have scrolled edges.
These scrolled nanotubes are all circular in cross section with an average diameter of about 150 nm and length of about 5 µm. The TEM image (as shown in Figure 4b) of the product shows two scrolled nanotubes marked with “Tube A” and
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Figure 4. (a) SEM image of PbO2 scrolled nanotubes. (b) TEM image of the PbO2 scrolled nanotubes. (c) A magnified TEM image of tube A’s end in Figure 4b. Inset: selected-area electron diffraction pattern of PbO2 scrolled nanotube. (d) HRTEM image recorded from the wall of a PbO2 scrolled nanotube.
Figure 5. (a) SEM image of PbO2 nanorods. (b) TEM image of the PbO2 nanorods. Inset: selected area electron diffraction pattern. (c) HRTEM image of the PbO2 nanorods.
“Tube B”. They all display also open tips, confirming further the tube structure. A scrolled tubular structure can be clearly observed from the magnified image of tube A’s end in Figure 4c. Tube A may be formed by curling the two opposite sides of the sheets until tubular closure and partly overlapping lead to different contrast on the side of the tube. The two edges of tube B in Figure 4b reveal that it has at least two or three scrolled layers. The corresponding selected-area electron diffraction (SAED) in the inset of Figure 4c was obtained by focusing the electron beam along the [1j 12] axis. An HRTEM image in Figure 4d was taken from the wall of tube A, showing lattice spacing of ca. 0.309 nm, consistent with the spacing for the (111) planes of cubic PbO2. These results demonstrate that the PbO2 scrolled nanotubes are single crystalline and grow preferentially along the [220] direction.
On the basis of the above SEM and TEM results of the PbO2 scrolled nanotubes, we believe that the nanotubes are formed by the rolling-up mechanism.31-35 The PbO2 nanosheets were first produced on lead foil under the present hydrothermal ammonia-water conditions. Then these sheets turned to scroll in some degree to relieve the stress due to the surface tension that could be modified by the reaction condition including temperature and ratio of V-NH3/V-H2O. Here the ratio (V-NH3/V-H2O ) 1/2) and temperature (180 °C) provide suitable conditions for the PbO2 nanosheet to roll up and finally form the tubular structure. When the reaction temperature was increased further to 200 °C with the ratio of V-NH3/V-H2O being 1/2, a lot of PbO2 nanorods would be obtained. The SEM image in Figure 5a shows that the product was composed of many short rods with
Growth of Nanosheets, Nanotubes, and Nanorods
a length of about 1-2 µm and diameter of 100-200 nm. Figure 5b displays a TEM image of the product, which reveals clearly the rod morphology, consistent with the SEM observation. The TEM image also shows that some edges of nanosheets can be seen from the rod’s sides, which suggests that the PbO2 nanorods are formed also based on a rolling-up mechanism. The SAED pattern of the sample is displayed in the inset of Figure 5b, confirming that the PbO2 nanorod is single crystalline. A welllatticed-resolved HRTEM image (as shown in Figure 5c) shows that the lattice spacing of 0.267, corresponding to that of the (020) plane of face-centered-cubic PbO2. So, the growth direction of the as-prepared nanorods is along the (200) plane. Compared with the formation of PbO2 scrolled nanotubes, in the present case, the higher temperature may lead to higher surface tension on the surfaces of PbO2 nanosheets, which formed earlier. Therefore, the sheets rolled up tightly onto themselves with several scrolled layers, and finally the rods were formed under long time hydrothermal conditions.
4. Conclusions In summary, a convenient ammonia-assisted hydrothermal process can be employed for the controlled growth of PbO2 nanostructured materials on Pb foil. It has been found that the shapes and sizes of the PbO2 sheets are strongly dependent on the volume ratio of NH3 to H2O. The columns of stacked sheets can be formed under the suitable condition (V-NH3/V-H2O ) 1/1). Ammonia may act as both an etching agent and a surface-passivating agent for the formation of PbO2 nanosheets. With the ratio of V-NH3/V-H2O being 1/2, PbO2 scrolled nanotubes and nanorods can be prepared by setting reaction temperature at 180 and 200 °C, respectively. PbO2 scrolled nanotubes and nanorods may be formed based on a rolling-up mechanism. These results demonstrate that the PbO2 nanostructures including nanosheets, scrolled nanotubes, and nanorods can be selectively prepared by simply modifying the reaction conditions through a mild hydrothermal solution approach, which could be further extended as a facile synthetic route to access other desired inorganic oxide nanostructures. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20771096) and the Research Grants Council of the Hong Kong Special Administrative Region.
References (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208.
Crystal Growth & Design, Vol. 8, No. 10, 2008 3525 (3) Chen, J. Y.; Herricks, T.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 2589. (4) Zhang, J. H.; Liu, H. Y.; Wang, Z. L.; Ming, N. B.; Li, Z. R.; Biris, A. S. AdV. Funct. Mater. 2007, 17, 3897. (5) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P. X.; Hughes, W. L.; Yang, R. S.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 944. (6) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (7) Ma, R. Z.; Bando, Y.; Sasaki, T. J. Phys. Chem. B 2004, 108, 2115. (8) Thangala, J.; Vaddiraju, S.; Bogale, R.; Thurman, R.; Powers, T.; Deb, B.; Sunkara, M. K. Small 2007, 3, 890. (9) Wang, Y. W.; Liang, C. H.; Meng, G. W.; Peng, X. S.; Zhang, L. D. J. Mater. Chem. 2002, 12, 651. (10) Huang, M. H.; Wu, Y. Y.; Feick, H. N.; Tran, N.; Weber, E.; Yang, P. D. AdV. Mater. 2001, 13, 113. (11) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Wang, Y. W.; Zhang, L. D.; Zhang, S. Y. Appl. Phys. Lett. 2001, 783032. (12) Dev, A.; Chaudhuri, S. Nanotechnology 2007, 18, 175607. (13) Peng, X. S.; Meng, G. W.; Zhang, J.; Wang, X. F.; Wang, Y. W.; Wang, C. Z.; Zhang, L. D. J. Mater. Chem. 2002, 12, 1602. (14) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (15) Cao, M. H.; Hu, C. W.; Peng, G.; Qi, Y. J.; Wang, E. B. J. Am. Chem. Soc. 2003, 125, 4982. (16) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Appl. Phys. Lett. 2002, 80, 309. (17) Barriaga, C.; Maffi, S.; Bicelli, L. P.; Malitesta, C. J. Power Sources 1991, 34, 353. (18) Johnson, D. C.; Feng, J.; Houk, L. L. Electrochim. Acta 2000, 46, 323. (19) Amadelli, R.; Armelao, L.; Velichenko, A. B.; Nikolenko, N. V.; Grienko, D. V.; Kovalyov, S. V.; Danilov, F. I. Electrochim. Acta 1999, 45, 713. (20) Panizza, M.; Cerisola, C. Electrochim. Acta 2003, 48, 3491. (21) Zaslavsky, A. I.; Kondrashev, Y. D.; Tolkachev, S. S. Dokl. Akad. Nauk SSSR 1950, 75, 559. (22) Antonio, P. D.; Santoro, A. Acta Crystallogr. B 1980, 36, 2394. (23) Syono, Y.; Akimoto, S. Mater. Res. Bull. 1968, 3, 153. (24) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (25) Shen, P. K.; Wei, X. L. Electrochim. Acta 2003, 48, 1743. (26) Andersen, T. N. Prog. Batteries Battery Mater. 1992, 11, 105. (27) Preisler, E. J. J. Appl. Electrochem. 1989, 19, 540. (28) Wen, X. G.; Zhang, W. X.; Yang, S. H. Langmuir 2003, 19, 5898. (29) Wang, J. X.; Sun, X. W.; Huang, H.; Lee, Y. C.; Tan, O. K.; Yu, M. B.; Lo, G. Q.; Kwong, D. L. Appl. Phys. A: Mater. Sci. Process. 2007, 88, 611. (30) Sˇimurda, M.; Nejmec, P.; Preclı´kova´, J.; Troja´nek, F.; Miyoshi, T.; Kasatani, K.; Maly´, P. Thin Solid Films 2006, 503, 64. (31) Xiong, Y. J.; Xie, Y. Z.; Li, Q.; Li, X. X.; Gao, S. M. Chem.-Eur. J. 2004, 10, 654. (32) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411. (33) Chen, X.; Sun, X.; Li, Y. Inorg. Chem. 2002, 41, 4524. (34) Tsai, C. C.; Teng, H. S. Chem. Mater. 2004, 16, 4352. (35) Schmidt, O. G.; Eberl, K. Nature 2001, 410, 168.
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