Anion-Controlled Construction of CuO Honeycombs and Flowerlike

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CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 3 467-470

Communications Anion-Controlled Construction of CuO Honeycombs and Flowerlike Assemblies on Copper Foils Yang Liu,† Ying Chu,*,† Yujiang Zhuo,† Meiye Li,‡ Lili Li,† and Lihong Dong† Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, P. R. China, and State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed July 24, 2006; ReVised Manuscript ReceiVed January 21, 2007

ABSTRACT: In this paper, novel honeycomb- and flowerlike CuO nanoarchitectures assembled from nanowires and nanoribbons were successively prepared on copper foils by an anion-controlled mild hydrothermal method. A series of control experiments confirm that the concentration of inorganic anions, the concentration of surfactant, and the kind of inorganic anions have direct influences on the morphology of the products. On the basis of these experimental results, possible influence mechanisms in the growth processes are proposed. The development of rational approaches for the multidimensional interconnection of nanoscaled building blocks (nanoclusters, nanowires, nanobelts, and nanotubes) into highly desired structures is a key step for nanofabrication techniques and the realization of advanced nanodevices.1 Self-assembly driven by various interactions, such as surface tension, capillary effects, electric and magnetic forces, and hydrophobic interactions, is an effective strategy for forming versatile “soft” nanocrystal-assembly motifs.2-8 As a result of rapid advancements in assembly strategies, various one-, two-, and three-dimensional (1D, 2D, 3D) and curved architectures have been obtained using various methods.9-15 From the viewpoint of both basic science and technology, understanding factors and mechanisms governing the formation of nanocrystal assembly would allow the design of desired nanostructures for optical, microelectronic, chemical, and biological applications.16 To date, however, it is still desirable to search for facile routes induced by different interactions for the accomplishment of synthesis and the highly assembly structures of nanobuilding blocks. Among various 3D transition metals and their derivatives, cupric oxide (CuO) is an important transition metal oxide with a narrow band gap (Eg ) 1.2 eV) and forms the basis of several hightemperature superconductors and giant magnetoresistance materials.17 Because of its photoconductive and photochemical properties, CuO is a promising material for fabricating solar cells and lithiumion batteries.18 Furthermore, because CuO has complex magnetic phases and forms the basis for several high-Tc superconductors and materials with giant magnetoresistance,19 it has been used in the preparation of a wide range of organic-inorganic nanostructured composites that possess unique characteristics such as high thermal and electrical conductivities as well as high mechanical strength and high-temperature durability.20 Therefore, on the basis of the fundamental and practical importance of CuO nanomaterials, welldefined CuO nanostructures with various morphologies have been * To whom correspondence should be addressed. E-mail: chuying@ nenu.edu.cn. † Northeast Normal University. ‡ Chinese Academy of Sciences.

fabricated, and many synthesis techniques have been developed to prepare different assemblies of CuO building blocks.21-23 In our lab, we have demonstrated the in situ preparation of different dimensional CuO nanostructures and their architectures could be achieved on copper foil via a one-step surfactant-assisted mild hydrothermal method.23a More recently, Choi and co-workers systematically studied the effect of additives (NaNO3, Na2SO4, NH4NO3, or (NH4)2SO4) on the growth of Cu2O crystals.24 This work together with our previous experimental results inspires us to explore a new method to fabricate other novel assemblies of CuO nanostructures through the addition of inorganic additives. In this communication, we first describe an anion-controlled mild hydrothermal method for the in situ preparation of CuO honeycomband flowerlike nanoarchitectures assembled from nanowires and nanoribbons on copper foils. In a typical procedure, the starting solution was prepared by mixing 6.4 g of NaOH (5 M), 1.0954 g of (NH4)2S2O8 (0.15 M), and a bit of Na2WO4 or Na2MoO4 (0.02 M) in 32 mL of water. Next, 0.4614 g of SDS (0.05 M) was introduced into the aqueous solution under stirring, resulting in a white aqueous solution. After SDS had completely dissolved, the solution was transferred into Teflon-lined stainless steel autoclaves, and the previously cleaned copper foil (1.5 cm × 1.5 cm × 0.2 mm) was immersed in the solution. The autoclaves were sealed and maintained at 160 °C for 24 h; after the autoclaves were cooled to room temperature, the copper foil was taken from the solution and rinsed with distilled water, and a sample of black film was obtained on the copper foil. The X-ray powder diffraction (XRD) pattern was obtained with a Rigaku D/max 2500 V PC diffractometer with Cu Ka radiation. The size and morphology of the CuO nanostructures were examined by field-emission scanning electron microscopy (FE-SEM; XL30 ESEM-FEG) and transmission electron microscopy (TEM; JEOL 2010). The composition of the as-prepared sample was examined by XRD, and all of the products obtained gave similar results. A typical XRD pattern of the sample is shown in Figure 1, and all the diffraction peaks can be indexed to monoclinic-phase CuO (space

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Figure 3. (a) TEM, (b) single-nanowire TEM, and (c) HRTEM images of the CuO nanowires building the honeycomblike structure. Inset in (c) is the SAED pattern of the single nanowire in (b). The section marked by the white block in (b) is the location of the HRTEM. Figure 1. XRD pattern of the as-prepared CuO sample.

Figure 2. SEM images of the honeycomblike CuO sample on a copper substrate obtained by reaction at 160 °C for 24 h with CSDS ) 0.05 M, CNa2WO4 ) 0.02 M. (a) Overview SEM image; (b, c) high-magnification SEM images.

group C2/c), except those marked with asterisks from the copper substrate. Compared with the standard diffraction patterns (JCPDS Card 45-0937), no characteristic peaks from impurities, such as Cu(OH)2, Cu2O, or CuMoO4, are detected, which further confirms that the growth of CuO nanostructures in the reaction system can be represented by the following reactions: Cu + 2NaOH + (NH4)2S2O8 f Cu(OH)2 + Na2SO4 + (NH4)2SO4, Cu(OH)2 + 2OH- f [Cu(OH)4]2-, [Cu(OH)4]2- f CuO + H2O + 2OH-. Consistent with the XRD results, the EDX result obtained from the diffraction intensities of integrated areas reveals that the atomic ratio of Cu to O in our samples is nearly equal to 1:1 (SI), which further proves that the products are pure CuO. Figure 2a is a typical SEM image of the CuO sample obtained with the addition of Na2WO4 into the reaction system, clearly showing a honeycomblike superstructure. Examining numerous SEM images of the sample, we found that almost all the products are honeycomblike, which indicates that well-defined CuO honeycomblike superstructures can be obtained under the present experimental conditions. As can be seen from a higher-magnification SEM image (Figure 2b), the entire honeycomb structure is based on numerous CuO nanowires. Although the nests of the honeycombs are not very uniform in size, their units (nanowires) are uniform in the diameter of 90-120 nm and length of tens of micrometers. Figure 2c is a high-magnification image that focuses

on a region at the brim of a honeycomb nest. As depicted in it, all of the neighboring nanowires have their ends assembled and knitted together from different orientations, forming the brims of the honeycombs; the middle section of the nanowires are all bent down, forming the nests of the honeycombs. Further structural characterization of the sample is performed using TEM after the black films on copper foil are carefully scraped off; Figure 3a shows the TEM image of the CuO nanowires building up the honeycombs. Figure 3c is the HRTEM image and SAED pattern taken on the specific nanowire shown in Figure 3b. The clear fringe spacing of 2.53 Å, running perpendicular to the long axis of the nanowire, accords well with the d value of the monoclinic CuO (1h11) crystal plane, which indicates that the CuO nanowire grows along the [1h11] direction. The SAED pattern, as shown in the inset, exhibits a single-crystalline CuO structure, which accords well with the XRD and HRTEM results. Here, we examine the samples prepared with different CNa2WO4, and find that the appropriated CNa2WO4 is crucial in ensuring the formation of the CuO honeycomblike structure. Decreasing CNa2WO4 to 0.014 M, we obtain only dense CuO nanowires, shown in Figure 4a. From this figure, we find that some nanowires assemble together, which is the hint of the trend for the formation of honeycombs. When CNa2WO4 was further lowered to 0.01 M, the products consist almost entirely of uniform CuO nanorods with large aspect ratios (Figure 4b). Keeping CNa2WO4 at 0.02 M and decreasing CSDS to 0.03 M, another kind of honeycomb structure is obtained (Figure 5). Although small quantities of nanowires still exist in the product, all the honeycombs are mainly constructed by subplates. The morphology of the CuO nanostructures was sensitive to the kind of salts added. Figure 6 illustrates the morphology and structural characterization results of the CuO architectures prepared by adding Na2MoO4 into the reaction system. In the present case, no honeycombs and instead flower-shaped CuO structures based on nanoribbons are obtained (images a and b of Figure 6). A low CNa2MoO4 of 0.01 M results in the formation of very loose CuO flowers structure made up of CuO nanobelts, as shown in Figure 6c. It is interesting that a kind of flower constructed from spindlelike particles was obtained when CSDS was decreased to 0.01 M (Figure 6d). In our previous work, we prepared 1D CuO nanowire, nanorod, and nanoribbon arrays on copper foils via the hydrothermal processes.17 It is can be concluded from the present experimental results that after the addition of Na2WO4 or Na2MoO4 into the same reaction system, the obtained CuO products have obviously different morphologies compared with our previous experiment results. Although the exact mechanism for the formation of these architectures has not been fully understood, the deduced reasons for this are 3-fold. First, according to the theory of Israelachvili,25 surfactants will spontaneously form aggregates when their concentration is above a certain “critical” value. The aggregate morphology depends on the well-known packing parameter P

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Figure 4. SEM images of CuO samples on a copper substrate obtained by reaction with CSDS ) 0.05 M and (a) CNa2WO4 ) 0.014 M; (b) CNa2WO4 ) 0.01 M.

Figure 5. (a) Overview and (b) high-magnification SEM images of CuO honeycomblike structure constructed by subplates obtained by reaction with CSDS ) 0.03 M, CNa2WO4 ) 0.02 M.

Figure 7. SEM images of the CuO samples on a copper substrate obtained by reaction reaction with CSDS ) 0.05 M and (a) CNaCl ) 0.02 M and (B) CNa2SO4 ) 0.02 M. Figure 6. SEM images of the CuO samples on copper substrate obtained by reaction with (a, b) CSDS ) 0.05 M, CNa2MoO4 ) 0.02 M; (c) CSDS ) 0.05 M, CNa2MoO4 ) 0.01 M; (d) CSDS ) 0.01 M, CNa2MoO4 ) 0.02 M.

P ) V/lca0 where V and lc are the volume and chain length of the hydrophobic group, respectively, and a0 is the cross-sectional area of the headgroup dictated by the electrostatic repulsion between adjacent headgroups in the associates. Alternatively, reducing the electrostatic repulsion between adjacent headgroups can increase the packing factor P. The pH, temperature, and salinity26 can all alter the electrostatic repulsion and thus the aggregation morphology. Therefore, if increasing the salinity of the aqueous solution of singletailed ionic surfactant (adding Na2WO4 or Na2MoO4 into our experiment system), the compressed headgroups may increase the value of P, which may result in the formation of the new aggregate structures.26 Second, the self-assembly of aligned carbon nanotubes to bundles or honeycomblike micropatterns has been achieved because of the capillary forces,27 and ultralong ZnO nanowire and

nanobelt arrays self-assembled into honeycomblike micropatterns via capillary forces during drying.15 Such a capillary-driven selfassembly represents a new type of coalescence process28 and may be the second reason for the formation of the honeycomblike CuO nanoarchitecture in this case. Here, an important question emerged: Does the volume of the inorganic anion influence the assembly structures of the CuO nanoproducts? To answer this question, we tested the inorganic anions with small volume by adding the same amount of NaCl or Na2SO4 into the reaction system. When NaCl exists, no special CuO architectures are formed, but some bundles of nanowires are formed (Figure 7a). In the case of Na2SO4, numerous nanoneedles are induced and unorderly dispersed on the copper surfaces, and among the samples, we find some urchinlike structures assembled from the nanoneedles (Figure 7b). These phenomena indicate that different inorganic anions will result in dissimilar morphology of the product, and anions with larger volumes play key roles in the formation of diverse and novel assemblies of the CuO nanostructures. So besides the above-mentioned 2-fold reasons, the steric

470 Crystal Growth & Design, Vol. 7, No. 3, 2007 hindrance effect caused by anions with larger volume should also influence the micelle aggregates, and these three effects together result in the novel assemblies of the products. More studies and work are underway to further research the mechanisms for the assembly process induced by this new driving action. Conclusion In summary, a facile anion-controlled self-assembly route was demonstrated for the construction of CuO nanoarchitectures on copper foils. In a simple chemical route, with introducing WO42and MoO42- into the reaction system, we successfully synthesized honeycomb- and flowerlike CuO assemblies built from nanowires and nanoribbons, respectively. It is expected that the novel CuO architectures may offer exciting opportunities for potential applications in catalysis, electrochemistry, superconductivity, and superhydrophobic coating. Although the detailed mechanism is not very clear and still needs more investigation, it is no doubt a pretty simple and easily controlled route for producing various novel superstructures. Further investigation may lead to an extension of this technique to the preparation of assemblies of other materials. Supporting Information Available: This work was financially supported by the National Natural Science Foundation of China (20573017) and the Science Foundation for Young Teachers of Northeast Normal University (20060306).

References (1) Huang, Y.; Duan, X. F.; Wei, Q. Q.; Lieber, C. M. Science 2001, 291, 630. (2) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (3) Thalladi, V. R.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 3520. (4) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (5) Yang, P. Nature 2003, 425, 243. (6) Shi, H.; Qi, L.; Ma, J.; Cheng, H. J. Am. Chem. Soc. 2003, 125, 3450. (7) Park, S.; Lim, J. H.; Chung, S. W.; Mirkin, C. A. Science 2004, 303, 348. (8) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124. (9) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289, 1170. (10) Yang, H. G.; Zeng, H. C. Angew. Chem., Int. Ed. 2004, 43, 5930. (11) Wu, H.; Thalladi, V. R.; Whitesides, S.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 14495. (12) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399.

Communications (13) Zhou, X. F.; Chen, S. Y.; Zhang, D. Y.; Guo, X. F.; Ding, W. P.; Chen, Y. Langmuir 2006, 22, 1383. (14) Mo, M. S.; Yu, J. C.; Zhang, L. Z.; Li, S. A. AdV. Mater. 2005, 17, 756. (15) Lu, C. H.; Qi, L. M.; Yang, J. H.; Tang, L.; Zhang, D. Y.; Ma, J. M. Chem. Commun. 2006, 3551. (16) (a) Kang, Z.; Wang, E.; Gao, L.; Lian, S.; Jiang, M.; Hu, C.; Xu, L. J. Am. Chem. Soc. 2003, 125, 13652. (b) Shipway, A. N.; Katz, E.; Willner, I. Chem. Phys. Chem. 2000, 1, 18. (17) (a) Musa, A. O.; Akomolafe, T.; Carter, M. J. Sol. Energy Mater. Sol. Cells 1998, 51, 305. (b) Zheng, X. G.; Xu, C. N.; Tomokiyo, Y.; Tanaka, E.; Yamada, H.; Soejima, Y. Phys. ReV. Lett. 2000, 85, 5170. (18) (a) Maruyama, T. Sol. Energy Mater. Sol. Cells 1998, 56, 85. (b) Lanza, F.; Feduzi, R.; Fuger, J. Mater. Res. 1990, 5, 1739. (19) Prabhakaran, D.; Subramanian, C.; Balakumar, S.; Ramasamy, P. Physica C 1999, 319, 99. (20) Kumar, R. V.; Elgamiel, R.; Diamant, Y.; Gedanken, A.; Norwig, J. Langmuir 2001, 17, 1406. (21) (a) Xu, Y. Y.; Chen, D. R.; Jiao, X. L. J. Phys. Chem. B 2005, 109, 13561. (b) Yao, W. T.; Yu, S. H.; Zhou, Y.; Jiang, J.; Wu, Q. S.; Zhang, L.; Jiang, J. J. Phys. Chem. B 2005, 109, 14011. (c) Cao, M. H.; Hu, C. W.; Wang, Y. H.; Guo, Y. H.; Guo, C. X.; Wang, E. B. Chem. Commun. 2003, 1884. (22) (a) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867. (b) Lu, C. H.; Qi, L. M.; Yang, J. H.; Zhang, D. Y.; Wu, N. Z.; Ma, J. M. J. Phys. Chem. B 2004, 108, 17825. (c) Du, G. H.; Van Tendeloo, G. Chem. Phys. Lett. 2004, 393, 64. (d) Gao, X. P.; Bao, J. L.; Pan, G. L.; Zhu, H. Y.; Huang, P. X.; Wu, F.; Song, D. Y. J. Phys. Chem. B 2004, 108, 5547. (23) (a) Liu, Y.; Chu, Y.; Li, M. Y.; Li, L. L.; Dong, L. H. J. Mater. Chem. 2006, 16, 192. (b) Hou, H. W.; Xie, Y.; Li, Q. Cryst. Growth Des. 2005, 5, 201. (c) Zhao, Y.; Zhu, J. J.; Hong, J. M.; Bian, N. S.; Chen, H. Y. Eur. J. Inorg. Chem. 2004, 4072. (d) Yang, R.; Gao, L. Chem. Lett. 2004, 33, 1194. (24) Siegfried, M. J.; Choi, K. S. J. Am. Chem. Soc. 2006, 128, 10356. (25) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992. (26) Zhai, L. M.; Zhao, M.; Sun, D. J.; Hao, J. C.; Zhang, L. J. J. Phys. Chem. B 2005, 109, 5627. (27) (a) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (b) Liu, H.; Li, S.; Zhai, J.; Li, H.; Zheng, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 1146. (28) Bico, J.; Roman, B.; Moulin, L.; Boudaoud, A. Nature 2004, 432, 690.

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