- Email: [email protected]
One-pot synthesis of single-crystalline Cu2O hollow nanocubes
ARTICLE IN PRESS Journal of Physics and Chemistry of Solids 70 (2009) 719–722
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-pot synthesis of single-crystalline Cu2O hollow nanocubes Zhenghua Wang , Hui Wang, Lingling Wang, Ling Pan Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e in fo
abstract
Article history: Received 8 December 2008 Received in revised form 18 February 2009 Accepted 19 February 2009
Single-crystalline Cu2O hollow nanocubes have been successfully synthesized via a simple wet chemical route in the absence of any surfactants or templates. By studying the growth process of the Cu2O hollow nanocubes, we found that the Cu2O hollow nanocubes were formed through a reducing and simultaneously etching process. The speed of reducing Cu(OH)2 into Cu2O was much faster than the speed of etching Cu2O. As a result, Cu2O solid nanocubes were firstly formed, and then the solid nanocubes were gradually etched into hollow nanocubes. & 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides A. Semiconductors B. Chemical synthesis
1. Introduction It is well-known that the size and morphology of inorganic nanomaterials have much influence on their physical properties [1–5]. Nanomaterials with hollow interiors such as nanotubes [6,7], hollow nanocubes [8,9], and hollow nanospheres [10,11] are of great interest in the past decades owing to their high specific surface area, low density, and well permeation, and potential applications in nanoscale encapsulation, drug delivery, plasmon photonics, and calorimetric sensing. Cuprous oxide (Cu2O), an important p-type semiconducting material, has attracted much interest due to its applications in many fields such as solar energy conversion, gas sensor, and negative electrode material for lithium ion batteries [12–15]. Besides, Cu2O has great potential applications in photocatalytic degradation of organic pollutants, and decomposition of water into O2 and H2 under visible light [16–18]. Cu2O nanocrystals with different shapes have been synthesized. Recently, the synthesis of Cu2O hollow nanocrystals has also attracted much interest. For example, Chen and coworkers [19] have prepared Cu2O hollow nanospheres through a soft template route. Zeng and coworkers [8] have synthesized Cu2O hollow nanocubes from the selfassembly of reductive CuO nanocrystals. We have previously prepared Cu2O nanoboxes using Cu2O nanocubes as sacrificial template through acidic etching [20]. However, how to obtain hollow nanocrystals conveniently and effectively is still a great challenge. Herein, we report a simple and convenient solution-phase approach for the synthesis of Cu2O hollow nanocubes at room
Corresponding author. Tel.: +86 553 3883513; fax: +86 553 3869302.
E-mail address: [email protected] (Z. Wang). 0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.02.011
temperature. The Cu2O hollow nanocubes are formed through a reducing and simultaneously etching process, and no surfactants or templates are needed. The photocatalytic property of the asprepared Cu2O hollow nanocubes is examined, which indicates the potential application of the Cu2O hollow nanocubes in environmental protection.
2. Experimental section All chemical reagents were of analytical grade and used as received without purification. In a typical procedure, 1.0 mL of a 0.1 M CuCl2 solution and 1.0 mL of a 0.2 M NaOH solution were added to 50 mL distilled water in sequence under constant magnetic stirring. After stirring for about 5 min, 1.0 mL of a 0.1 M L-ascorbic acid solution was added. The solution was further stirred for 30 min. Then, the solution was centrifuged at 4000 rpm for 10 min, the precipitates were washed with distilled water and absolute ethanol for several times each, and dried in vacuum at 50 1C for 2 h. The photocatalytic degradation was conducted as follows: 0.020 g of as-prepared Cu2O sample was dispersed into an aqueous solution of Pyronine B (0.01 g/L, 50 mL). A 300 W column-like low-pressure mercury lamp was placed 20 cm above the solution, and the solution was irradiated with the lamp under constant magnetic stirring. At intervals of 30 min, 3 mL of the suspension was taken out, centrifuged, and then the supernatant fluid was collected, and analyzed with a UV–vis absorption spectrometer. X-ray powder diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 X-ray diffractometer equipped with graphite-monochromatized Cu Ka radiation (l ¼ 0.15406 nm). The scanning electron microscopy (SEM) images were taken with a
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Intensity (a.u.)
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Hitachi S-4800 scanning electron microscope. The high-resolution transmitting electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were recorded on a JEOL2011 high-resolution transmission electron microscope performed at an acceleration voltage of 200 kV. UV–vis absorption spectra were measured with a Shimadzu UV-3010 spectrometer.
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50
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2 (Degree) Fig. 1. XRD pattern of the as-prepared Cu2O sample.
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3. Results and discussion The as-obtained sample was firstly examined by XRD. Fig. 1 shows a typical XRD pattern of the sample. All of the reflection peaks in this pattern can be readily indexed to cubic phase Cu2O, and the lattice constant calculated from this pattern is a ¼ 0.4297 nm, which is in good agreement with the reported value of a ¼ 0.4269 nm (JCPDS Card no. 5-667). No impurities, such as CuO and Cu(OH)2, can be detected in this pattern. The strong and sharp reflection peaks indicate good crystallization of the sample. Fig. 2A shows a SEM image of the Cu2O sample, which reveals that the sample is composed of many nanocubes with holes on their faces. Fig. 2B gives a clear view of the nanocubes. It can be seen that the average edge length of these nanocubes is about 250 nm. There are holes on one or more faces of these nanocubes, and the interiors of these nanocubes are hollow. Fig. 2C gives a TEM image of the Cu2O hollow nanocubes, which further demonstrated the hollow nature of these nanocubes. HRTEM image of the Cu2O hollow nanocube is shown in Fig. 2D, the observed interplanar spacings are about 0.21 and 0.30 nm, which correspond to the separation between (2 0 0) and (11 0) lattice planes of cubic Cu2O, respectively. The corresponding SAED pattern (inset in Fig. 2D) indicates the single-crystalline nature of the Cu2O hollow nanocube. Fig. 3A gives a schematic illustration of the structure of L-ascorbic acid, which shows two enolic hydroxyl groups in L-ascorbic acid. It is known that enolic structure is unstable. With the existence of oxidizer, L-ascorbic acid may easily lose the two hydrogen atoms on the enolic hydroxyl groups and turn into
Fig. 2. (A, B) SEM images of the Cu2O hollow nanocubes with different magnifications. (C) TEM image of the Cu2O hollow nanocubes. (D) HRTEM image of the Cu2O hollow nanocube, inset is the corresponding SAED pattern.
ARTICLE IN PRESS Z. Wang et al. / Journal of Physics and Chemistry of Solids 70 (2009) 719–722
dehydroascorbic acid. This makes L-ascorbic acid exhibit strong reducing property. Recently, ascorbic acid and ascorbate have been applied as the reducer for the preparation of Cu2O or Cu(OH)2 [21,22]. In addition, nanocubes from Cu(OH)2 4 L-ascorbic acid is a weak dibasic acid, the hydrogen atoms on the enolic hydroxyl groups can be ionized in aqueous solution. The first dissociation constant of L-ascorbic acid (Ka1 ¼ 5.0 105) is little larger than that of acetic acid (Ka ¼ 1.8 105). Previously, we have employed acetic acid to etch Cu2O nanocubes into nanoboxes [20]. In this study, L-ascorbic acid which served as the reducer is excessive, so that the solution after the reduction is still acidic. Cu2O nanocubes can be etched in such an acidic solution. Fig. 3B gives a schematic illustration of the structure of Pyronine B. Pyronine B is a kind of organic dye. It is hard to degrade under natural conditions. Here we chose Pyronine B to study the photocatalytic activity of Cu2O nanoparticles.
Fig. 3. Schematic illustration of the structures of (A) L-ascorbic acid and (B) Pyronine B.
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The developing process of the Cu2O hollow nanocubes was studied by characterizing the products at different stages with SEM. The solution before adding L-ascorbic acid was light blue in color, which contained Cu(OH)2 precipitates. With the addition of L-ascorbic acid, the solution turned into bright yellow, and then yellow within 1 min. About 5 mL of the solution was taken out at this stage, the precipitates were quickly separated from the solution by centrifugation at 4000 rpm for 1 min. SEM image of the as-obtained sample (Fig. 4A) indicates that many solid nanocubes are obtained, and the faces of these nanocubes are smooth. With the reaction proceeding, the color of solution has no obvious change. However, SEM image of the sample obtained after the addition of L-ascorbic acid for 5 min (Fig. 4B) shows that small holes appear on the faces of the nanocubes. With the reaction further proceeding, the holes became larger and deeper, and finally the hollow nanocubes are obtained. The concentration of L-ascorbic acid is important for the formation of the Cu2O hollow nanocubes. When the concentration of L-ascorbic acid solution is decreased to 0.05 M, the reducing speed of Cu(OH)2 becomes much slower, as indicated by the slow color change of the solution. Fig. 4C shows a SEM image of the sample obtained after the addition of 0.05 M L-ascorbic acid for 30 min, which indicates that many cubic-like nanocrystals are obtained. These nanocrystals are solid, and their faces are very rough. When the concentration of L-ascorbic acid solution is changed to 0.075 M, many solid nanocubes were still obtained; however, the faces of the nanocubes become smooth, but the edges of the nanocubes are still rough. These results indicate that the decrease of the dosage of L-ascorbic acid can lead to slower
Fig. 4. (A, B) SEM images of the sample obtained after the addition of 0.1 M L-ascorbic acid for 1 and 5 min, respectively. (C, D) SEM images of the sample obtained after the addition of 0.05 and 0.2 M of L-ascorbic acid for 30 min, respectively.
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degraded by UV light alone, the presence of Cu2O nanocrystals can remarkably promote the degradation of Pyronine B. The degradation of Pyronine B in the presence of Cu2O hollow nanocubes is obviously faster than that with Cu2O solid nanocubes, which may be due to the higher specific surface area of Cu2O hollow nanocubes. This result indicates that the Cu2O hollow nanocubes may have potential application in environmental protection.
with Cu2O hollow nanocubes
Degradation rate (%)
with Cu2O solid nanocubes without Cu2O
60
40 4. Conclusions
20
0 0
30
60
90
120
150
180
Time (min) Fig. 5. Degradation rate of Pyronine B aqueous solution irradiated with UV light for different time intervals.
reducing speed of Cu(OH)2 and much more slower etching speed of Cu2O, and finally result in solid Cu2O nanocubes with incompletely developed shapes. If the concentration of L-ascorbic acid is higher than 0.1 M, it is obvious that the Cu2O nanocubes will be further etched. Fig. 4D shows the SEM images of the sample obtained after the addition of 0.2 M L-ascorbic acid solution for 30 min. It can be seen that most of the hollow nanocubes are broken. When a 1.0 M L-ascorbic acid solution is used, no Cu2O nanocrystals can be obtained because the Cu2O nanocrystals are totally dissolved. On the basis of the experimental results, we believe that the Cu2O hollow nanocubes are formed through the following process: first, Cu2+ reacts with OH to form Cu(OH)2 precipitates. Then, with the addition of L-ascorbic acid, Cu(OH)2 precipitate is quickly reduced to Cu2O little particles. On the one hand, these little particles quickly aggregate in order to reduce the overall energy of the system and grow into nanocubes. On the other hand, because L-ascorbic acid is in excess, the solution after the reduction is still acidic, and the freshly formed Cu2O little particles will be etched simultaneously. The etching may proceed as follows [23]: Cu2 O þ 2Hþ þ 4Cl ! 2CuCl2 þ H2 O However, the speed of the growth of Cu2O nanocubes is much faster than the speed of the etching of Cu2O nanocubes. As a result, Cu2O solid nanocubes with smooth faces can be obtained at the earlier stage. After Cu(OH)2 is fully consumed, the Cu2O nanocubes stop growing. Then, with the etching proceeds, holes begin to appear on the faces of the nanocubes. As the etching further proceeds, the holes become larger and deeper, and finally, Cu2O hollow nanocubes are obtained. The photocatalytic property of the Cu2O hollow nanocubes and solid nanocubes were studied. Fig. 5 shows the degradation rate of Pyronine B solution irradiated with UV light for different time intervals. The degradation rates were calculated by measuring the concentration of the Pyronine B solution with UV–vis absorption. From this figure we see that although Pyronine B can be slightly
In summary, single-crystalline Cu2O hollow nanocubes were successfully prepared through a one-pot wet chemical route. The study on the evolution of the Cu2O hollow nanocubes indicated that the Cu2O hollow nanocubes were formed through a reducing and simultaneously etching process. The difference of the reducing and etching speed led to the formation of Cu2O solid nanocubes at first, then, the solid nanocubes were gradually etched into hollow nanocubes. The Cu2O hollow nanocubes show better photocatalytic activity than the Cu2O solid nanocubes because of their higher specific surface area. This method may provide an alternative way to synthesize other hollow nanomaterials.
Acknowledgements Financial supports form the National Natural Science Foundation of China (20701001) and the school young teacher foundation of Anhui normal university (2007xqn70) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
R. Rosetti, S. Nakahare, L.E. Brus, J. Chem. Phys. 79 (1983) 1086–1088. F. Williams, A.J. Nozik, Nature 312 (1984) 21–27. A.P. Alivisatos, Science 271 (1996) 933–937. M.A. El-Sayed, Acc. Chem. Res. 34 (2001) 257–264. X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanish, A.P. Alivisato, Nature 404 (2000) 59–61. T.W. Ebbesen, P.M. Ajayan, Nature 358 (1992) 220–222. S. Iijima, Nature 354 (1991) 56–58. J.J. Teo, Y. Chang, H.C. Zeng, Langmuir 22 (2006) 7369–7377. L.F. Gou, C.J. Murphy, Nano Lett. 3 (2003) 231–234. X.C. Jiang, Y. Xie, J. Lu, L.Y. Zhu, W. He, X.M. Liu, Can. J. Chem. 80 (2002) 263–268. Y. Zhang, Y.D. Li, J. Phys. Chem. B 108 (2004) 17805–17811. D. Snoke, Science 298 (2002) 1368–1372. R.N. Briskman, Sol. Energy Mater. Sol. Cells 27 (1992) 361–368. O. Musa, T. Akomolafe, M.J. Carter, Sol. Energy Mater. Sol. Cells 51 (1998) 305–316. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Taracon, Nature 407 (2000) 496–499. J. Ramı´rez-Ortiz, T. Ogura, J. Medina-Valtierra, S.E. Acosta-Ortiz, P. Bosch, J.A. de los Reyes, V.H. Lara, Appl. Surf. Sci. 174 (2001) 177–184. H.M. Yang, J. Ouyang, A.D. Tang, Y. Xiao, X.W. Li, X.D. Dong, Y.M. Yu, Mater. Res. Bull. 41 (2006) 1310–1318. T. Takata, S. Ikeda, A. Tanaka, M. Hara, J.N. Kondo, K. Domen, Appl. Catal. A: Gen. 200 (2000) 255–262. L.S. Xu, X.H. Chen, Y.R. Wu, C.S. Chen, W.H. Li, W.Y. Pan, Y.G. Wang, Nanotechnology 17 (2006) 1501–1505. Z.H. Wang, X.Y. Chen, J.W. Liu, M.S. Mo, L. Yang, Y.T. Qian, Solid State Commun. 130 (2004) 585–589. L.F. Gou, C.J. Murphy, J. Mater. Chem. 14 (2004) 735–738. C.H. Kuo, C.H. Chen, M.H. Huang, Adv. Funct. Mater. 17 (2007) 3773–3780. C.H. Kuo, M.H. Huang, J. Am. Chem. Soc. 130 (2008) 12815–12820.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
One-pot synthesis of single-crystalline Cu2O hollow nanocubes Zhenghua Wang , Hui Wang, Lingling Wang, Ling Pan Anhui Key Laboratory of Functional Molecular Solids, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, PR China
a r t i c l e in fo
abstract
Article history: Received 8 December 2008 Received in revised form 18 February 2009 Accepted 19 February 2009
Single-crystalline Cu2O hollow nanocubes have been successfully synthesized via a simple wet chemical route in the absence of any surfactants or templates. By studying the growth process of the Cu2O hollow nanocubes, we found that the Cu2O hollow nanocubes were formed through a reducing and simultaneously etching process. The speed of reducing Cu(OH)2 into Cu2O was much faster than the speed of etching Cu2O. As a result, Cu2O solid nanocubes were firstly formed, and then the solid nanocubes were gradually etched into hollow nanocubes. & 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides A. Semiconductors B. Chemical synthesis
1. Introduction It is well-known that the size and morphology of inorganic nanomaterials have much influence on their physical properties [1–5]. Nanomaterials with hollow interiors such as nanotubes [6,7], hollow nanocubes [8,9], and hollow nanospheres [10,11] are of great interest in the past decades owing to their high specific surface area, low density, and well permeation, and potential applications in nanoscale encapsulation, drug delivery, plasmon photonics, and calorimetric sensing. Cuprous oxide (Cu2O), an important p-type semiconducting material, has attracted much interest due to its applications in many fields such as solar energy conversion, gas sensor, and negative electrode material for lithium ion batteries [12–15]. Besides, Cu2O has great potential applications in photocatalytic degradation of organic pollutants, and decomposition of water into O2 and H2 under visible light [16–18]. Cu2O nanocrystals with different shapes have been synthesized. Recently, the synthesis of Cu2O hollow nanocrystals has also attracted much interest. For example, Chen and coworkers [19] have prepared Cu2O hollow nanospheres through a soft template route. Zeng and coworkers [8] have synthesized Cu2O hollow nanocubes from the selfassembly of reductive CuO nanocrystals. We have previously prepared Cu2O nanoboxes using Cu2O nanocubes as sacrificial template through acidic etching [20]. However, how to obtain hollow nanocrystals conveniently and effectively is still a great challenge. Herein, we report a simple and convenient solution-phase approach for the synthesis of Cu2O hollow nanocubes at room
Corresponding author. Tel.: +86 553 3883513; fax: +86 553 3869302.
E-mail address: [email protected] (Z. Wang). 0022-3697/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2009.02.011
temperature. The Cu2O hollow nanocubes are formed through a reducing and simultaneously etching process, and no surfactants or templates are needed. The photocatalytic property of the asprepared Cu2O hollow nanocubes is examined, which indicates the potential application of the Cu2O hollow nanocubes in environmental protection.
2. Experimental section All chemical reagents were of analytical grade and used as received without purification. In a typical procedure, 1.0 mL of a 0.1 M CuCl2 solution and 1.0 mL of a 0.2 M NaOH solution were added to 50 mL distilled water in sequence under constant magnetic stirring. After stirring for about 5 min, 1.0 mL of a 0.1 M L-ascorbic acid solution was added. The solution was further stirred for 30 min. Then, the solution was centrifuged at 4000 rpm for 10 min, the precipitates were washed with distilled water and absolute ethanol for several times each, and dried in vacuum at 50 1C for 2 h. The photocatalytic degradation was conducted as follows: 0.020 g of as-prepared Cu2O sample was dispersed into an aqueous solution of Pyronine B (0.01 g/L, 50 mL). A 300 W column-like low-pressure mercury lamp was placed 20 cm above the solution, and the solution was irradiated with the lamp under constant magnetic stirring. At intervals of 30 min, 3 mL of the suspension was taken out, centrifuged, and then the supernatant fluid was collected, and analyzed with a UV–vis absorption spectrometer. X-ray powder diffraction (XRD) patterns were obtained on a Shimadzu XRD-6000 X-ray diffractometer equipped with graphite-monochromatized Cu Ka radiation (l ¼ 0.15406 nm). The scanning electron microscopy (SEM) images were taken with a
ARTICLE IN PRESS 720
Z. Wang et al. / Journal of Physics and Chemistry of Solids 70 (2009) 719–722
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30
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110
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220
200
Intensity (a.u.)
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Hitachi S-4800 scanning electron microscope. The high-resolution transmitting electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns were recorded on a JEOL2011 high-resolution transmission electron microscope performed at an acceleration voltage of 200 kV. UV–vis absorption spectra were measured with a Shimadzu UV-3010 spectrometer.
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50
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70
2 (Degree) Fig. 1. XRD pattern of the as-prepared Cu2O sample.
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3. Results and discussion The as-obtained sample was firstly examined by XRD. Fig. 1 shows a typical XRD pattern of the sample. All of the reflection peaks in this pattern can be readily indexed to cubic phase Cu2O, and the lattice constant calculated from this pattern is a ¼ 0.4297 nm, which is in good agreement with the reported value of a ¼ 0.4269 nm (JCPDS Card no. 5-667). No impurities, such as CuO and Cu(OH)2, can be detected in this pattern. The strong and sharp reflection peaks indicate good crystallization of the sample. Fig. 2A shows a SEM image of the Cu2O sample, which reveals that the sample is composed of many nanocubes with holes on their faces. Fig. 2B gives a clear view of the nanocubes. It can be seen that the average edge length of these nanocubes is about 250 nm. There are holes on one or more faces of these nanocubes, and the interiors of these nanocubes are hollow. Fig. 2C gives a TEM image of the Cu2O hollow nanocubes, which further demonstrated the hollow nature of these nanocubes. HRTEM image of the Cu2O hollow nanocube is shown in Fig. 2D, the observed interplanar spacings are about 0.21 and 0.30 nm, which correspond to the separation between (2 0 0) and (11 0) lattice planes of cubic Cu2O, respectively. The corresponding SAED pattern (inset in Fig. 2D) indicates the single-crystalline nature of the Cu2O hollow nanocube. Fig. 3A gives a schematic illustration of the structure of L-ascorbic acid, which shows two enolic hydroxyl groups in L-ascorbic acid. It is known that enolic structure is unstable. With the existence of oxidizer, L-ascorbic acid may easily lose the two hydrogen atoms on the enolic hydroxyl groups and turn into
Fig. 2. (A, B) SEM images of the Cu2O hollow nanocubes with different magnifications. (C) TEM image of the Cu2O hollow nanocubes. (D) HRTEM image of the Cu2O hollow nanocube, inset is the corresponding SAED pattern.
ARTICLE IN PRESS Z. Wang et al. / Journal of Physics and Chemistry of Solids 70 (2009) 719–722
dehydroascorbic acid. This makes L-ascorbic acid exhibit strong reducing property. Recently, ascorbic acid and ascorbate have been applied as the reducer for the preparation of Cu2O or Cu(OH)2 [21,22]. In addition, nanocubes from Cu(OH)2 4 L-ascorbic acid is a weak dibasic acid, the hydrogen atoms on the enolic hydroxyl groups can be ionized in aqueous solution. The first dissociation constant of L-ascorbic acid (Ka1 ¼ 5.0 105) is little larger than that of acetic acid (Ka ¼ 1.8 105). Previously, we have employed acetic acid to etch Cu2O nanocubes into nanoboxes [20]. In this study, L-ascorbic acid which served as the reducer is excessive, so that the solution after the reduction is still acidic. Cu2O nanocubes can be etched in such an acidic solution. Fig. 3B gives a schematic illustration of the structure of Pyronine B. Pyronine B is a kind of organic dye. It is hard to degrade under natural conditions. Here we chose Pyronine B to study the photocatalytic activity of Cu2O nanoparticles.
Fig. 3. Schematic illustration of the structures of (A) L-ascorbic acid and (B) Pyronine B.
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The developing process of the Cu2O hollow nanocubes was studied by characterizing the products at different stages with SEM. The solution before adding L-ascorbic acid was light blue in color, which contained Cu(OH)2 precipitates. With the addition of L-ascorbic acid, the solution turned into bright yellow, and then yellow within 1 min. About 5 mL of the solution was taken out at this stage, the precipitates were quickly separated from the solution by centrifugation at 4000 rpm for 1 min. SEM image of the as-obtained sample (Fig. 4A) indicates that many solid nanocubes are obtained, and the faces of these nanocubes are smooth. With the reaction proceeding, the color of solution has no obvious change. However, SEM image of the sample obtained after the addition of L-ascorbic acid for 5 min (Fig. 4B) shows that small holes appear on the faces of the nanocubes. With the reaction further proceeding, the holes became larger and deeper, and finally the hollow nanocubes are obtained. The concentration of L-ascorbic acid is important for the formation of the Cu2O hollow nanocubes. When the concentration of L-ascorbic acid solution is decreased to 0.05 M, the reducing speed of Cu(OH)2 becomes much slower, as indicated by the slow color change of the solution. Fig. 4C shows a SEM image of the sample obtained after the addition of 0.05 M L-ascorbic acid for 30 min, which indicates that many cubic-like nanocrystals are obtained. These nanocrystals are solid, and their faces are very rough. When the concentration of L-ascorbic acid solution is changed to 0.075 M, many solid nanocubes were still obtained; however, the faces of the nanocubes become smooth, but the edges of the nanocubes are still rough. These results indicate that the decrease of the dosage of L-ascorbic acid can lead to slower
Fig. 4. (A, B) SEM images of the sample obtained after the addition of 0.1 M L-ascorbic acid for 1 and 5 min, respectively. (C, D) SEM images of the sample obtained after the addition of 0.05 and 0.2 M of L-ascorbic acid for 30 min, respectively.
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degraded by UV light alone, the presence of Cu2O nanocrystals can remarkably promote the degradation of Pyronine B. The degradation of Pyronine B in the presence of Cu2O hollow nanocubes is obviously faster than that with Cu2O solid nanocubes, which may be due to the higher specific surface area of Cu2O hollow nanocubes. This result indicates that the Cu2O hollow nanocubes may have potential application in environmental protection.
with Cu2O hollow nanocubes
Degradation rate (%)
with Cu2O solid nanocubes without Cu2O
60
40 4. Conclusions
20
0 0
30
60
90
120
150
180
Time (min) Fig. 5. Degradation rate of Pyronine B aqueous solution irradiated with UV light for different time intervals.
reducing speed of Cu(OH)2 and much more slower etching speed of Cu2O, and finally result in solid Cu2O nanocubes with incompletely developed shapes. If the concentration of L-ascorbic acid is higher than 0.1 M, it is obvious that the Cu2O nanocubes will be further etched. Fig. 4D shows the SEM images of the sample obtained after the addition of 0.2 M L-ascorbic acid solution for 30 min. It can be seen that most of the hollow nanocubes are broken. When a 1.0 M L-ascorbic acid solution is used, no Cu2O nanocrystals can be obtained because the Cu2O nanocrystals are totally dissolved. On the basis of the experimental results, we believe that the Cu2O hollow nanocubes are formed through the following process: first, Cu2+ reacts with OH to form Cu(OH)2 precipitates. Then, with the addition of L-ascorbic acid, Cu(OH)2 precipitate is quickly reduced to Cu2O little particles. On the one hand, these little particles quickly aggregate in order to reduce the overall energy of the system and grow into nanocubes. On the other hand, because L-ascorbic acid is in excess, the solution after the reduction is still acidic, and the freshly formed Cu2O little particles will be etched simultaneously. The etching may proceed as follows [23]: Cu2 O þ 2Hþ þ 4Cl ! 2CuCl2 þ H2 O However, the speed of the growth of Cu2O nanocubes is much faster than the speed of the etching of Cu2O nanocubes. As a result, Cu2O solid nanocubes with smooth faces can be obtained at the earlier stage. After Cu(OH)2 is fully consumed, the Cu2O nanocubes stop growing. Then, with the etching proceeds, holes begin to appear on the faces of the nanocubes. As the etching further proceeds, the holes become larger and deeper, and finally, Cu2O hollow nanocubes are obtained. The photocatalytic property of the Cu2O hollow nanocubes and solid nanocubes were studied. Fig. 5 shows the degradation rate of Pyronine B solution irradiated with UV light for different time intervals. The degradation rates were calculated by measuring the concentration of the Pyronine B solution with UV–vis absorption. From this figure we see that although Pyronine B can be slightly
In summary, single-crystalline Cu2O hollow nanocubes were successfully prepared through a one-pot wet chemical route. The study on the evolution of the Cu2O hollow nanocubes indicated that the Cu2O hollow nanocubes were formed through a reducing and simultaneously etching process. The difference of the reducing and etching speed led to the formation of Cu2O solid nanocubes at first, then, the solid nanocubes were gradually etched into hollow nanocubes. The Cu2O hollow nanocubes show better photocatalytic activity than the Cu2O solid nanocubes because of their higher specific surface area. This method may provide an alternative way to synthesize other hollow nanomaterials.
Acknowledgements Financial supports form the National Natural Science Foundation of China (20701001) and the school young teacher foundation of Anhui normal university (2007xqn70) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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