Fabrication of Copper Oxide Dumbbell-Like Architectures via the

Feb 3, 2009 - intermediates of basic copper salt with lamellar structures were observed at ... carbonate (CaCO3), the previously added organic ligands...
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Langmuir 2009, 25, 3152-3158

Fabrication of Copper Oxide Dumbbell-Like Architectures via the Hydrophobic Interaction of Adsorbed Hydrocarbon Chains Haihua Wang, Qiang Shen,* Xinping Li, and Fenglin Liu Key Laboratory for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, China ReceiVed October 6, 2008. ReVised Manuscript ReceiVed December 27, 2008 In this paper, the synthesized surfactant of copper dodecyl sulfate (Cu(DS)2) was used as a special metal-ion source for the morphological control of copper oxide (CuO) architectures. During the fabrication processes, the ribbon-shaped intermediates of basic copper salt with lamellar structures were observed at 60.0 °C for the first time. In the absence or presence of dodecanol (DOH), Cu(DS)2 could react with sodium hydroxide to form dumbbell-like architectures of CuO nanoparticles. The incorporation of DOH molecules into the adsorption monolayers of surfactant ions could greatly enlarge the dumbbell size in length, probably depending upon the formation of the DOH-DS complex. These indicated that the template effectiveness of the intermediate ribbons, together with the hydrophobic interactions of adsorbed hydrocarbon chains, should account for the formation process of CuO dumbbells. Interestingly, the addition of sodium chloride into the reaction systems could induce the morphological change of CuO dumbbells to the twinanchors and then to the twin-spheres with two holes in the center. This further suggests that the hydrophobic interaction of pendent hydrocarbon chains provides an important approach for material fabrication purposes.

1. Introduction Self-organized aggregation of organic molecules and/or inorganic particles is a common phenomenon in colloid chemistry. Recently, the secondary architectures composed of nanostructured building blocks have attracted significant interest in material syntheses and device fabrications.1 In the absence of organic additives, the two-stage mechanism was proposed and had been effectively used to describe the nucleation, growth, and aggregation process of nanosized primary particles.2 It has been well-known that the in vivo aggregation of nanocrystals controlled by the organism leads to the formation of well defined hybrids (for example, molluscous shells, bone, and teeth) with complex morphologies and superior properties. Similarly, in the biomimetic mineralization process the organic-inorganic binding affinity can dominate the self-assemblies of inorganic particles.3 The driven forces for the formation of mesocrystals probably involve the fusion of adjacent particles along the crystal plane with the lowest lattice-free energy, as well as the steric, van der Waals, and hydrophobic interactions among the pendent hydrocarbon chains of adsorbed organics.4 Generally, inorganic nanoparticles can be kinetically dispersed in solution without the assistance of organic ligands. The adsorption of counterions, as well as the formation of precursor salt, could be used to account for the oriented aggregation of naked primary particles.2,4a In the precipitation of calcium carbonate (CaCO3), the previously added organic ligands could stabilize the metastable phases of CaCO3 crystallites and control * Correspondingauthor.Fax:+86-531-88564750.E-mail:[email protected]. (1) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418–2421. (b) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576–5591. (c) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124–8125. (d) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237–240. (2) (a) Privman, V.; Goia, D. V.; Park, J.; Matijevic´, E. J. Colloid Interface Sci. 1999, 213, 36–45. (b) Libert, S.; Gorshkov, V.; Privman, V.; Goia, D.; Matijevic´, E. AdV. Colloid Interface Sci. 2003, 100-102, 169–183. (3) (a) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980–999. (b) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393–395. (c) Yu, S.-H.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2124–2147. (4) (a) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem., Int. Ed. 2002, 41, 1188–1191. (b) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971.

the oriented attachment of nanoparticles.5 When the doublechain surfactants of M(DS)2 (M ) Cu, Co, Fe, Ni, or Pb) were used as metal-ion sources in the material synthesis processes, the template effects of different M(DS)2 self-assemblies and/or the classic mechanism of crystal growth had been used to explain the formation of inorganic products.6 Nevertheless, it is also worthwhile to assay the hydrophobic interaction of M(DS)2 hydrocarbon chains in the aggregation processes of inorganic nanoparticles. As a p-type semiconductor with a narrow band gap (1.2 eV), copper oxide has been of considerable interest because of its technological importance.7 Therefore, CuO nanoparticles have successfully been prepared by using a number of methods, including the double-jet precipitation, the solid-state or wetchemical, the electrochemical, the sonochemical, and so on. Also, various CuO architectures, such as the nanorods, nanorings, nanoplatelets, and flowers, have been observed.8-21 In this paper, the synthesized surfactant of Cu(DS)2) was used both as the (5) Shen, Q.; Wang, L. C.; Huang, Y. P.; Sun, J. L.; Wang, H. H.; Zhou, Y.; Wang, D. J. J. Phys. Chem. B 2006, 110, 23148–23153. (6) (a) Liu, Q.; Liang, Y. Y.; Liu, H. J.; Hong, J. M.; Xu, Z. Mater. Chem. Phys. 2006, 98, 519–522. (b) Leontidis, E.; Orphanou, M.; Kyprianidou-Leodidou, T.; Krumeich, F.; Caseri, W. Nano Lett. 2003, 3, 569–572. (c) Liu, Q.; Liu, H. J.; Han, M.; Zhu, J. M.; Liang, Y. Y.; Xu, Z.; Song, Y. AdV. Mater. 2005, 17, 1995–1999. (d) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160–4166. (7) (a) Lu, C. H.; Qi, L. M.; Yang, J. H.; Zhang, D. Y.; Wu, N. Z.; Ma, J. M. J. Phys. Chem. B 2004, 108, 17825–17831. (b) Hou, H. W.; Xie, Y.; Li, Q. Cryst. Growth Des. 2005, 5, 201–205. (c) 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, 5547–5551. (d) Zhang, J. T.; Liu, J. F.; Peng, Q.; Wang, X.; Li, Y. D. Chem. Mater. 2006, 18, 867–871. (8) Lee, S.-H.; Her, Y.-S.; Matijevic´, E. J. Colloid Interface Sci. 1997, 186, 193–202. (9) (a) Wang, W.; Zhan, Y.; Wang, G. Chem. Commun. 2001, 727–728. (b) Wang, W.; Zhan, Y.; Wang, X.; Liu, Y.; Zheng, C.; Wang, G. Mater. Res. Bull. 2002, 37, 1093–1100. (c) Wang, W.; Liu, Z.; Liu, Y.; Xu, C.; Zheng, C.; Wang, G. Appl. Phys. A: Mater. Sci. Process. 2003, 76, 417–420. (10) Widmer, R.; Haug, F.-J.; Ruffieux, P.; Gro¨ning, O.; Bielmann, M.; Gro¨ning, P.; Fasel, R. J. Am. Chem. Soc. 2006, 128, 14103–14108. (11) (a) Vijaya Kumar, R.; Elgamiel, R.; Diamant, Y.; Gedanken, A.; Norwig, J. Langmuir 2001, 17, 1406–1410. (b) Zhu, H. T.; Zhang, C. Y.; Tang, Y. M.; Wang, J. X. J. Phys. Chem. C 2007, 111, 1646–1650. (12) (a) Keyson, D.; Volanti, D. P.; Cavalcante, L. S.; Simo˜es, A. Z.; Varela, J. A.; Longo, E. Mater. Res. Bull. 2008, 43, 771–775. (b) Wang, H.; Xu, J.-Z.; Zhu, J.-J.; Chen, H.-Y. J. Cryst. Growth 2002, 244, 88–94.

10.1021/la803276z CCC: $40.75  2009 American Chemical Society Published on Web 02/03/2009

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copper-ion source and as the structure-directed reagent to fabricate mesoscale architectures of CuO nanoparticles. Herein, the main subject is to illuminate the role of the hydrophobic interaction of pendent hydrocarbon chains in the formation of CuO dumbbelllike architectures.

2. Experimental Section Materials. All chemicals, sodium dodecyl sulfate (SDS), ndodecanol (DOH), copper chloride dihydrate, sodium hydroxide, ethanol, and sodium chloride, are of A. R. grade and were used without further purification. Deionized water was used throughout the solution preparations. The ionic surfactant of copper dodecyl sulfate, Cu(DS)2, was synthesized by the mixing of 2.5 M CuCl2 (40 mL) and 0.5 M SDS (100 mL) solutions at the low temperature of 15.0 ( 0.2 °C, according to the Krafft point (24 °C) and the solubility values (3.48 × 10-2 M, 15.0 °C; 6.53 × 10-2 M, 25.0 °C; 9.30 × 10-1 M, 25.8 °C) of Cu(DS)2 · 4H2O.22,23 The precipitate was filtered and washed with sufficient cold water (15.0 °C) to remove excess copper (Cu2+) and dodecyl sulfate (DS-) ions. The structural properties of Cu(DS)2 were also characterized, shown in the Supporting Information (Figure S1). CuO Synthesis. The aqueous solutions of 8.0 mM Cu(DS)2, 8.0 mM CuCl2 and 16.0 mM NaOH were freshly prepared and then preheated in a water bath at 60.0 °C prior to each experiment. NaCl, SDS, and/or dodecanol were only added into a Cu2+-source solution before its mixing with the NaOH solution. In a typical procedure, a 50 mL NaOH solution was rapidly poured into a 100 mL beaker containing the equal volume of Cu2+-source solutions; then the reaction systems were stirred at a constant rate of ∼1000 rpm by a Teflon-coated magnetic stirring bar for 10 min. Subsequently, the reaction systems were sealed and allowed to stand still in a desiccator cabinet at 60.0 °C for 3 days. Finally, the black precipitates of CuO were centrifugated, filtered off, extensively washed with deionized water then ethanol, and dried in a vacuum desiccator cabinet at 60.0 °C. It should be emphasized that, for the possibly complete transformation of blue-green basic copper salts to black CuO, the reaction time (i.e., 72 h) was fixed to overcome the influence of the adsorbed hydrocarbon chains on the stability of intermediates. Crystal Characterizations. The crystallographic information for all the samples was established with powder X-ray diffraction (D8 Advance X-ray diffractometer, Cu KR radiation of 40 kV/100 mA). The sample was Pt-coated prior to examination by a Hitachi S-4300 scanning electron microscopy (SEM), fitted with a field emission (13) (a) Wang, S.; Huang, Q.; Wen, X.; Li, X.-Y.; Yang, S. Phys. Chem. Chem. Phys. 2002, 4, 3425–3429. (b) Hsieh, C.-T.; Chen, J.-M.; Lin, H.-H.; Shih, H.-C. Appl. Phys. Lett. 2003, 82, 3316–3318. (c) Wu, H.-Q.; Wei, X.-W.; Shao, M.-W.; Gu, J.-S.; Qu, M.-Z. Chem. Phys. Lett. 2002, 364, 152–156. (14) (a) Du, G. H.; Van Tendeloo, G. Chem. Phys. Lett. 2004, 393, 64–69. (b) Chen, D.; Shen, G. Z.; Tang, K. B.; Qian, Y. T. J. Cryst. Growth 2003, 254, 225–228. (15) (a) Leo´n, A.; Glenn, J. S.; Farver, T. B. Small Ruminant Res. 2000, 35, 7–12. (b) Zhu, Y.; Sow, C.-H.; Yu, T.; Zhao, Q.; Li, P.; Shen, Z.; Yu, D.; Thong, J. T.-L. AdV. Funct. Mater. 2006, 16, 2415–2422. (c) Zhu, J. W.; Bi, H. P.; Wang, Y. P.; Wang, X.; Yang, X. J.; Lu, L. D. Mater. Lett. 2007, 61, 5236–5238. (d) Liu, Y.; Chu, Y.; Zhuo, Y. J.; Li, M. Y.; Li, L. L.; Dong, L. H. Cryst. Growth Des. 2007, 7, 467–470. (16) (a) Zhu, J.; Chen, H.; Liu, H.; Yang, X.; Lu, L.; Wang, X. Mater. Sci. Eng., A 2004, 384, 172–176. (b) Wen, X.; Zhang, W.; Yang, S. Langmuir 2003, 19, 5898–5903. (17) Yao, W.-T.; Yu, S.-H.; Zhou, Y.; Jiang, J.; Wu, Q.-S.; Zhang, L.; Jiang, J. J. Phys. Chem. B 2005, 109, 14011–14016. (18) Behr, G.; Lo¨ser, W.; Apostu, M.-O.; Gruner, W.; Hu¨cker, M.; Schramm, L.; Souptel, D.; Teresiak, A.; Werner, J. Cryst. Res. Technol. 2005, 40, 21–25. (19) Premkumar, T.; Geckeler, K. E. Small 2006, 2, 616–620. (20) Maruyama, T. Sol. Energy Mater. Sol. Cells 1998, 56, 85–92. (21) (a) Chang, Y.; Zeng, H. C. Cryst. Growth Des. 2004, 4, 397–402. (b) Zarate, R. A.; Hevia, F.; Fuentes, S.; Fuenzalida, V. M.; Zu´n˜iga, A. J. Solid State Chem. 2007, 180, 1464–1469. (c) Xu, Y.; Chen, D. R.; Jiao, X. L. J. Phys. Chem. B 2005, 109, 13561–13566. (d) Zhong, Z.; Ng, V.; Luo, J.; Teh, S.-P.; Teo, J.; Gedanken, A. Langmuir 2007, 23, 5971–5977. (e) Wang, X. Q.; Xi, G. C.; Xiong, S. L.; Liu, Y. K.; Xi, B. J.; Yu, W. C.; Qian, Y. T. Cryst. Growth Des. 2007, 7, 930–934. (22) (a) Moroi, Y.; Motomura, K.; Matuura, R. J. Colloid Interface Sci. 1974, 46, 111–117. (b) Miyamoto, S. Bull. Chem. Soc. Jpn. 1960, 33, 371–375. (23) Orphanou, M.; Leontidis, E.; Kyprianidou-Leodidou, T.; Koutsoukos, P.; Kyriacou, K. C. Langmuir 2004, 20, 5605–5612.

Figure 1. XRD profiles of the CuO products obtained from the Cu2+source solutions of 8.0 mM Cu(DS)2 (A), 8.0 mM CuCl2 + 16.0 mM SDS (B), and 8.0 mM CuCl2 (C), respectively. The marked peak belong to the [110]-face diffraction of cubic Cu2O (JCPDS 05-0667).

source, and operating at an accelerating voltage of 15 kV. For TEM (JEM-100CX11, 100 kV) and high-resolution TEM (JEM-2100, 200 kV) studies, one or more drops of the dispersion of precipitates in ethanol were deposited on an amorphous carbon film supported by a copper grid. Thermogravimetric analysis (TGA) was carried out on a METTLER to monitor the mass loss of products at a heating rate of 10 °C/min from room temperature to 900 °C under an air atmosphere.

3. Results and Discussion 3.1. Formation of CuO Dumbbell-Like Architectures. When NaOH solution (16.0 mM, 50 mL) was rapidly poured into the aqueous solution of organocopper surfactant Cu(DS)2 (8.0 mM, 50 mL), the admixing systems became opaque immediately, due to the formation of blue-green intermediates. At 60 °C the apparently blue-green color of reaction solutions could last ∼1.0 h. Under the same experimental conditions, the blue-green precipitates generated from the inorganic Cu2+-source solutions had relatively short lifetimes: ∼1.0 min for 8.0 mM CuCl2 and ∼10 min for 8.0 mM CuCl2 + 16.0 mM SDS. Therefore, organic anions (i.e., DS- ions) could greatly prolong the lifetime of basic copper salts. These also suggest that the affinity between SDS molecules and basic copper salts is weak, while that between DS- ions and the intermediate particles is strong. The XRD profiles for the final products sampled from different Cu2+-source solutions were shown in Figure 1. All the diffraction peaks can be readily indexed using the monoclinic-structured CuO (JCPDS 05-0661), except that a weak peak marked in Figure 1A at ∼29.44° corresponds to the diffraction peak of the [110]face diffraction of cubic Cu2O (JCPDS 05-0667). Therefore, it was the surfactant anions of Cu(DS)2, not the organic additive of SDS molecules, that exerted a special influence on the solutionmediated phase transformation of blue-green intermediates to black CuO. On the basis of the fwhm (i.e., full width at halfmaximum) value of the [1j 11]-face diffraction peak and Debye-Scherrer formula, the estimated sizes of the CuO building blocks were 16.2 (Figure 1A), 18.9 (Figure 1B), and 13.9 nm (Figure 1C), respectively, indicating the nanocrystalline nature of the products. For the precipitation reactions of CuX2 (X ) Cl-, NO3-, OAc-, or, n-CmH2m+1COO-) and NaOH, the chemical nature of X could determine the phase structure of basic intermediates (i.e.,

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Figure 3. TGA curves of CuO architectures obtained from the reactant systems of 8.0 mM Cu(DS)2 (A) and 8.0 mM CuCl2 + 16.0 mM SDS (B), respectively.

Figure 2. TEM images (left-hand side) and the correspondingly magnified pictures (right-hand side) of CuO architectures obtained from the Cu2+source solutions of 8.0 mM Cu(DS)2 (A, B), 8.0 mM CuCl2 + 16.0 mM SDS (C, D), and 8.0 mM CuCl2 (E, F), respectively.

Cu2(OH)3X) with different morphology,21d,24,25 then the resulting intermediates functionalized as the precursors of CuO in the bulk phase. However, the lamellar structures of Cu2(OH)3X were short-lived so that they could only be obtained at a relatively low temperature.25 Herein, the intensity ratio between the [1j11] and j /I111) was calculated to be 1.08, 0.83, [111] diffraction peaks (I111 and 1.04 for the CuO sampled from the Cu2+-source solutions of 8.0 mM Cu(DS)2 (Figure 1A), 8.0 mM CuCl2 + 16.0 mM SDS (Figure 1B), and 8.0 mM CuCl2 (Figure 1C), respectively. j /I111 value of 1.04, it was In comparison with the theoretical I111 deduced that DS- ions of Cu(DS)2 might take part in the formation of intermediates of basic copper salt at 60 °C, whereas these of SDS added into the stock solution of CuCl2 could not do so. Therefore, it was the participation of hydrocarbon chains into basic copper salts that prolongated the lifetime of blue-green intermediates and consequently promoted the formation of the minor byproduct Cu2O. The influence of different Cu2+ sources on the morphology of CuO products was shown in Figure 2. Figure 2A exhibits a TEM picture of the CuO dumbbell-like architectures obtained from the Cu2+-source solution of Cu(DS)2, and the magnified picture of a dumbbell (Figure 2B) clearly shows the aggregation behavior of numerous tiny nanoparticles. These primary nanoparticles interconnected with one another to form larger secondary dumbbell-like architectures with recognizable boundaries or voids between the component subunits. Time-dependent experiments exhibited that the average length of CuO dumbbells increased (24) Kratohvil, S.; Matijevic´, E. J. Mater. Res. 1991, 6, 766–777. (25) (a) Fujita, W.; Awaga, K. Inorg. Chem. 1996, 35, 1915–1917. (b) Fujita, W.; Awaga, K.; Yokoyama, T. Inorg. Chem. 1997, 36, 196–199.

with the increasing incubation time: 176 ( 30 and 350 ( 30 nm for these sampled at the time intervals of 6 and 72 h, respectively. As a control, Figure 2C presents the branchlike architectures of CuO sampled from the Cu2+-source solution of 8.0 mM CuCl2 + 16.0 mM SDS. Figure 2D clearly shows that the branchlike architectures are comprised of the nanosized particles. In fact, the branchlike nanostructures of CuO had been successfully fabricated through the hydrothermal reactions of CuSO4 and NaOH in the presence of sodium citrate or ethylene glycol.26 Herein, the added SDS molecules functionalized as flotation reagents and the relatively weak adsorption of SDS molecules onto the primarily formed particles also induced the formation of the branched CuO architectures. As the second control, CuO nanoparticles synthesized by the direct mixing of CuCl2 and NaOH solutions could self-organize into the ellipsoidal architectures (Figure 2E). This morphological control could be achieved by using different Cu2+ sources; nevertheless, the similar formation mechanism for these ellipsoids had been drawn.27 According to M. Han’s opinion,27a these ellipsoids are comprised of building blocks through the strong interaction between particles themselves (Figure 2F), because of the minimization of the interfacial energy. TGA measurements of the thoroughly washed CuO samples were conducted to determine the possible hydrocarbon-chain content in the final products. Prior to analysis of the TGA results (Figure 3), several related literatures should be cited: (1) for pure SDS powder, there was only one obvious weight loss step in the TGA curve between 200 and 400 °C, ascribed to the complete decomposition of the hydrocarbon chains;28 (2) the dehydration of solid-state Cu(OH)2 to CuO occurred between 190 and 220 °C,21d,29 while the conversion of CuO to Cu2O could not take place without the assistance of hydrogen gas;30 (3) in aqueous solution, Cu(OH)2 was difficult to be completely converted into CuO at a temperature lower than 100 °C;9c (4) during the decomposition process of copper oxalate, Cu2O was produced as the intermediate depending upon the calcination time and then it was oxidized to CuO at 240-300 °C.12a Herein, what we care about is the only weight loss aroused by the degradation of organic contamination. (26) (a) Xiao, H.-M.; Fu, S.-Y.; Zhu, L.-P.; Li, Y.-Q.; Yang, G. Eur. J. Inorg. Chem. 2007, 14, 1966–1971. (b) Li, S.; Zhang, H.; Ji, Y.; Yang, D. Nanotechnology 2004, 15, 1428–1432. (27) (a) Zhang, Z. P.; Sun, H. P.; Shao, X. Q.; Li, D. F.; Yu, H. D.; Han, M. Y. AdV. Mater. 2005, 17, 42–47. (b) Liu, J. P.; Huang, X. T.; Li, Y. Y.; Sulieman, K. M.; He, X.; Sun, F. L. Cryst. Growth Des. 2006, 6, 1690–1696. (28) Grossiord, N.; van der Schoot, P.; Meuldijk, J.; Koning, C. E. Langmuir 2007, 23, 3646–3653. (29) Carnes, C. L.; Stipp, J.; Klabunde, K. J. Langmuir 2002, 18, 1352–1359. (30) Kim, J. Y.; Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L. J. Am. Chem. Soc. 2003, 125, 10684–10692.

Copper Oxide Dumbbell-Like Architectures

In Figure 3, the temperature marked with an arrow in each TGA curves denoted the decomposition onset of adsorbed hydrocarbon chains, which were delayed owing to the strong association of surfactants with the collected samples.28,31 Prior to the marked temperatures, the weight losses could be attributed to the existence of the physically adsorbed moisture and the surfactant-stabilized basic copper salts.12a,28-32 Beyond the marked temperature, the mass loss of hydrocarbon-chain decomposition was about 5.8 and 3.6 wt % for the dumbbell-like (Figure 3A) and the branchlike architectures of CuO (Figure 3B), respectively. Interestingly, the TGA curves prior to the calcinations onset of adsorbed hydrocarbon chains were extremely different for the different CuO products. For the branchlike CuO, there was only one decomposing process before the marked temperature (i.e., 222 °C), possibly due to the presence of the Cu(OH)2 stabilized by adsorbed hydrocarbon chains (Figure 3B).21d,29 For the dumbbell-like architectures, there were two decomposing processes before 233 °C (Figure 3A). The first one was due to the decomposition of “hydrated” Cu(OH)2-DS and the removal of crystal water in the trace residue (shown in Figure S1 in the Supporting Information) during the low-temperature period. Then, the resulting Cu(OH)2, together with the Cu(OH)2 previously stabilized by adsorbed hydrocarbon chains, exhibited a relatively big mass loss in the temperature region of 162 and 233 °C (refer to Figure S2 in the Supporting Information).23 This coincided well with the apparent lifetimes of blue-green intermediates, further confirming the strong association of DSions with basic copper salts (i.e., Cu(OH)2 and Cu(OH)2-DS). By the above results listed, the DS- ions of Cu(DS)2 could not only stabilize the basic copper salts but also play an important role in the morphological control of CuO dumbbells. The bluegreen intermediates generated in the Cu2+-source solution of Cu(DS)2 were investigated by TEM observation, exhibiting the ribbonlike structure (Figure 4A). Also, these ribbons were proven to be the orthorhombic phase of Cu(OH)2 by the [002] diffraction peak marked in the corresponding XRD profile (Figure 4B). Interestingly, Figure 4B mainly showed the formation of the layered Cu(OH)2-DS complex with an interlayer spacing of ∼2.32 nm. The structure of the layered intermediates was possibly comprised of a monolayer of dodecyl sulfate ions and the two sheets of polymerlike copper hydroxide (Figure 4C), based on the previous reports.6b,33 Therefore, the dehydration of Cu(OH)2-DS ribbons, as well as the hydrophobic interaction among the hydrocarbon chains anchored onto the consequently resulting CuO nanoparticles, should account for the formation of CuO dumbbells (Figure 4D). 3.2. Effect of Inert Electrolyte. When 16.0 mM NaCl solution was added into the reactant solution of 8.0 mM Cu(DS)2, the stoichiometric reaction of Cu2+ and hydroxide ions resulted not only in the dumbbell-like architectures but also in the twinspheres with two holes in the center (Figure 5A-C). Interestingly, this reaction resulted in blue-green intermediates with a lifetime of ∼2 days. Figure 6A presents the magnified TEM picture for the end tip of a CuO dumbbell obtained from the Cu2+-source solution of 8.0 mM Cu(DS)2, while Figure 6B is the correspondingly filtered high-resolution TEM image. The broken curves in Figure 6A show the curving alignments of building blocks, implying the (31) Guo, Z.; Liang, X.; Pereira, T.; Scaffaro, R.; Hahn, H. T. Compos. Sci. Technol. 2007, 67, 2036–2044. (32) Kim, Y. K.; Riu, D.-H.; Kim, S.-R.; Kim, B.-I. Mater. Lett. 2002, 54, 229–237. (33) (a) Okazaki, M.; Toriyama, K.; Tomura, S.; Kodama, T.; Watanabe, E. Inorg. Chem. 2000, 39, 2855–2860. (b) Kopka, H.; Beneke, K.; Lagaly, G. J. Colloid Interface Sci. 1988, 123, 427–436.

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Figure 4. TEM image (A), powder XRD pattern (B), and the twodimensional structure model (C) of the blue-green precipitates sampled from the Cu2+-source solution of 8.0 mM Cu(DS)2. In panel B, the diffraction peak marked with a star represents the [002] crystal face of orthorhombic Cu(OH)2 (JCPDS 35-0505), and the others correspond to the [003], [004], [005], [006], [007], and [008] peaks for the lamellar structure of Cu(OH)2-DS intermediates. Panel D shows the schematic illustration for the formation process of CuO dumbbells.

self-coiling of linear aggregates of CuO nanoparticles.21e,34 Figure 6B displayed the CuO lattice planes of the [110] and [111] planes with the interplaner spacing of 0.279 and 0.238 nm, respectively. And the solid-broken-solid lines marked in it showed the dislocations of CuO crystals, suggesting the formation mechanism for the gradual widening of the dumbbell ends. When 16.0 mM NaCl was added into the reactant solution of 8.0 mM Cu(DS)2, the seemingly intermediate architectures between the dumbbells and the twin-spheres (i.e., the twin-anchor architectures of CuO nanoparticles) were fortunately observed (Figure 7A). Figure 7B indicated that each fluke in one twinanchor superstructure started to connect with the opposite one. Then, the further bending of the curving alignments of the CuO nanoparticles induced the formation of two holes in the final forms (Figure 5A,B). Therefore, these imply that the addition of 8.0 mM NaCl into the reaction system of 4.0 mM Cu(DS)2 + 8.0 mM NaOH could induce the morphological change of the CuO architectures from the dumbbells (Figure 2A,B) to the twinanchors (Figure 7A,B) and then to the twin-spheres with two holes in the spherical center (Figure 5A,B). Herein, we would like to cite G. Warr’s results: the adsorbed SDS molecular monolayer on mineral oxide surfaces could form the globular surface micelles on solid substrates, and this could be directly observed by using atomic force microscopy.35 So far as the topological structure of adsorbed micelles was concerned, the curvature of surfactant monolayers could be altered by the (34) Park, S.; Lim, J.-H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348–351. (35) Schlz, J. C.; Warr, G. G. Langmuir 2002, 18, 3191–3197.

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Figure 7. TEM images (A, B) of CuO architectures obtained from the Cu2+-source solution of 8.0 mM Cu(DS)2 + 16.0 mM NaCl. (C) Schematic presentation of the shape transformation of CuO architectures from the dumbbell to the twin-anchor and then to the spherical structures with two holes in the middle.

Figure 5. SEM (A) and TEM images (B, C) of the CuO architectures obtained from the Cu2+-source solution of 8.0 mM Cu(DS)2 + 16.0 mM NaCl.

Figure 6. TEM (A) and the filtered high-resolution TEM image (B) for the end tip of a CuO dumbbell obtained from the Cu2+-source solution of 8.0 mM Cu(DS)2. The alignments of building blocks were shown in panel A, while the lattice parameters and dislocations of CuO were shown in panel B.

addition of electrolytes, based on the thermodynamic models of bending energy.36 The morphological evolvement of CuO architectures with the addition of NaCl was schematically shown in Figure 7C, enclosed also was the curvature change of the pendent hydrocarbon chains. Therefore, if the addition of electrolytes reinforced the hydrophobic interaction between the surfactant hydrocarbon chains,36 the curvature change of pendent surfactant monolayers should reasonably answer for the morphological evolvement of CuO architectures from the dumbbelllike to the twin-spherical. Another phenomenon should also be mentioned that, for the Cu2+-source solution of 8.0 mM CuCl2 + 16.0 mM SDS, the reaction system has the exact same chemical components as that of the system using 8.0 mM Cu(DS)2 + 16.0 mM NaCl as the Cu2+-source solution. The reason why there is a huge morpho(36) (a) Acosta, E.; Szekeres, E.; Sabatini, D. A.; Harwell, J. H. Langmuir 2003, 19, 186–195. (b) Carlsson, I.; Wennerstro¨m, H. Langmuir 1999, 15, 1966– 1972. (c) Barneveld, P. A.; Hesselink, D. E.; Leermakers, F. A. M.; Lyklema, J.; Scheutjens, J. M. H. M. Langmuir 1994, 10, 1084–1092.

logical difference between the branchlike CuO architecture and the twin-spheres still deserves to be further conducted. The addition of NaCl into aqueous Cu(DS)2 solution could increase the ionization degree of Cu(DS)2 micelles (Figure S3 in the Supporting Information), while the addition of CuCl2 into the micellar solution of SDS caused the neutralization of micellar charges. Both of the reverse phenomena were due to the competitive counterion attraction with the charged spheres of surfactant micelles.22a,37,38 However, only the former situation could promote the adsorbed surfactants to form micelles on CuO architectures.35 3.3. Effect of Dodecanol. In analogy to the aqueous medium of 8.0 mM CuCl2 solution, the freshly prepared Cu(DS)2 solution (8.0 mM) is acidic (pH∼2.6). The stoichiometric reaction of Cu(DS)2 and NaOH (16.0 mM) caused a sharp increase and then a slow decrease for the pH value of the reaction system, reaching a plateau pH value of ∼8.0 at last. It should be emphasized that (1) SDS molecules dissolved in water can hydrolyze to produce dodecanol (DOH) under an acidic or basic condition,39 (2) there is a very strong attractive interaction between DOH and SDS molecules to form a complex of SDS-DOH,40 (3) the in situ generated DOH can serve both as a reductive reagent and as an nonionic surfactant to produce and to stabilize metal nanoparticles under reflux conditions.39b Considering the mild experimental conditions and the slow hydrolysis of aqueous Cu(DS)2 during the incubation period, DOH was used as an additive in the reaction systems to check the predominant role of hydrophobic interactions for the formation mechanism of CuO dumbbells. The XRD pattern of the blue-green intermediates sampled from the Cu2+-source solution of 8.0 mM Cu(DS)2 + 10.4 mM DOH (Figure 8A) indicated that the ribbonlike amorphous solids of basic copper salts (inset in Figure 8A, Figure S2 in the (37) (a) Kallay, N.; Pastuovic´, M.; Matijevic´, E. J. Colloid Interface Sci. 1985, 106, 452–458. (b) Kallay, N.; Fan, X.-J.; Matijevic´, E. Acta Chem. Scand., Ser. A 1986, 40, 257–260. (c) Caponetti, E.; Chillura Martino, D.; Pedone, L. Langmuir 2003, 19, 554–558. (d) Hall, D. G. Langmuir 1999, 15, 3483–3485. (38) Oko, M. U.; Venable, R. L. J. Colloid Interface Sci. 1971, 35, 53–59. (39) (a) Kurz, J. L. J. Phys. Chem. 1962, 66, 2239–2246. (b) Wang, W. L.; Wang, Y. Y.; Wan, C. C.; Lee, C. L. Colloids Surf., A 2006, 275, 11–16. (40) (a) Moroi, Y.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1971, 44, 2078–2082. (b) Moroi, Y.; Motomura, K.; Matuura, R. Bull. Chem. Soc. Jpn. 1972, 45, 2697–2702. (c) Lu, J. R.; Purcell, I. P.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A. R.; Penfold, J. J. Colloid Interface Sci. 1995, 174, 441–445. (d) Kralchevsky, P. A.; Danov, K. D.; Kolev, V. L.; Broze, G.; Mehreteab, A. Langmuir 2003, 19, 5004–5018.

Copper Oxide Dumbbell-Like Architectures

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Figure 9. TEM images of CuO architectures obtained from the Cu2+source solution of 8.0 mM Cu(DS)2 + 10.4 mM DOH (A) and 8.0 mM Cu(DS)2 + 10.4 mM DOH + 16.0 mM NaCl (B), respectively.

Figure 8. XRD pattern (A) and the two-dimensional structure model (B) of the blue-green intermediates sampled from the Cu2+-source solution of 8.0 mM Cu(DS)2 + 10.4 mM DOH. Inset shows the corresponding TEM image of precipitated ribbons.

Supporting Information) possessed the layered structure with an interlayer spacing of ∼3.07 nm. The interlayer spacing is bigger than that of the Cu(OH)2-DS complex (i.e., 2.32 nm), but it is smaller than the double value of the surfactant monolayer. On the basis of the incorporation of DOH molecules into the adsorbed monolayer of surfactant ions and the strong interaction between SDS and DOH molecules,33b,40 the structure of the layered intermediates was postulated to adopt the formula of Cu(OH)2-DS-DOH, which was schematically illustrated in Figure 8B. The comparison of parts A and B of Figure 4 also proved that Cu(OH)2-DS-DOH complexes exerted a relatively high prohibiting effect on the formation, crystal growth, and the consequent transformation of Cu(OH)2 than the Cu(OH)2-DS complexes, because no XRD characteristics for orthorhombic Cu(OH)2, cubic Cu2O, and monoclinic CuO were detected in the 2θ region of 10 -80°. According to Y. Moroi’s results,40a intermolecular hydrogen bonds could induce the formation of a complex between SDS and DOH at 60 °C, and the optimum molar ratio between them was 1.0. Herein, for the morphological control of adsorbed hydrocarbon chains in the formation of CuO dumbbells, there was also a DOH concentration-dependent relationship (Figure S4 in the Supporting Information). Surprisingly, the presence of a low concentration of DOH could cause the CuO “shape change” of dumbbells (Figure 2A) to rods (panel A of Figure S4 in the Supporting Information), indicating that DOH molecules did not functionalize as solubilized oils. The comparison of the average dumbbell size shown in Figure 2A (∼350 nm) with that shown in Figure S4 in the Supporting Information (∼1000 nm) could

further demonstrate the incorporation of DOH into the adsorbed monolayer of surfactant ions. Therefore, the main role of DOH in the formation of CuO dumbbells was possibly due to the formation of DS-DOH complexes on the surfaces of inorganic building blocks.35,40a Figure 9A and the panel C in Figure S4 in the Supporting Information were the TEM pictures of CuO architectures sampled from the Cu2+-source solution of 8.0 mM Cu(DS)2 + 10.4 mM DOH, which displayed the major dumbbells, with an average size of 0.9 ( 0.2 µm, the minor rods, and dumbbell ends. Then, the addition of inert electrolyte (i.e., 16.0 mM NaCl) into this Cu2+-source solution resulted in the twin-spherical architectures of CuO with two holes in the center (Figure 9B). Despite the influence of NaCl on the mole fraction of DOH in the adsorbed monolayer of surfactants,35,40d this could further prove the predominant role of hydrophobic interactions for the formation mechanism of CuO dumbbells.

4. Conclusion In the absence or presence of DOH molecules, the two-tail surfactant Cu(DS)2 could react with NaOH to form ribbonlike intermediates (i.e., Cu(OH)2-DS or Cu(OH)2-DS-DOH) with a lamellar structure. Then, the template effectiveness of these intermediate basic copper salts, as well as the hydrophobic interaction of the adsorbed hydrocarbon chains onto the consequently formed CuO nanoparticles, manipulated the morphological control and the final average size of dumbbelllike architectures. No matter whether DOH molecules were presented or not, the addition of NaCl into the reaction systems of Cu(DS)2 and NaOH could caused the formation of CuO twinspheres with two holes in the center. The morphological change of CuO architectures from the dumbbells to the twin-anchors and then to the twin-spheres agreed seemingly with the

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micellization behavior of adsorbed SDS molecules on solid substrates. These provide an effective approach to the morphological control of materials and to the hydrophobic modification of inorganic salts. Acknowledgment. The financial support from the National Natural Science Foundation of China (Grants 20773079 and 20833010), from the National Basic Research Program of China

Wang et al.

(Grant 2009CB930802), and from the Science and Technology DevelopmentPlanofShandongProvince(Grant2007GG10003004) is gratefully acknowledged. Supporting Information Available: Surface tension and TGA curves, specific conductance curves, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA803276Z