Aqueous Synthesis of Copper Nanocubes and Bimetallic Copper

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Langmuir 2006, 22, 5900-5903

Aqueous Synthesis of Copper Nanocubes and Bimetallic Copper/ Palladium Core-Shell Nanostructures Guangjun Zhou, Mengkai Lu,* and Zhongsen Yang State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed February 3, 2006. In Final Form: April 18, 2006 We have synthesized copper nanocubes with uniform shape and size and copper/palladium core-shell bimetallic nanostructures in high yield by a two-stage procedure in the presence of dodecyl benzene sulfonic acid sodium. The copper nanocubes with a slight hole in the centers of the six {100} surfaces was prepared at the first stage. Later, the bimetallic copper/palladium core-shell nanostructures formed on the basis of the successive reduction of H2PdCl4 and the Pd growth on the surfaces of the Cu seeds.

1. Introduction Metal nanostructures have been extensively studied for many decades because of their use in applications such as catalysis, photography, electronics, optics, optoelectronics, biological and chemical sensing, information storage, and surface-enhanced Raman scattering (SERS).1 In general, the intrinsic properties of a metal nanostructure can be affected by its shape as well as its structure. For example, hollow Pd nanospheres have recently been demonstrated as effective, recoverable catalysts for Suzuki coupling reactions,2 while solid Pd nanoparticles usually lose their catalytic activities or show lower catalytic activities after one cycle of operation.3 Colloidal silver nanoparticles with various morphologies, such as triangular, pentagonal, and spherical particles, are able to display red, green, and blue colors, respectively, under optical microscopy.4 Metallic nanorods such as gold5 and silver6 exhibit anisotropic optical properties directly related to their aspect ratios. In the same way, the optical properties of three-dimensional (3D) Pd spheres7 and nanocubes,8 the electronic and magnetic properties of ultrathin Fe-Co alloy nanowires,9 and the luminescence of CdSe nanorods10 are also markedly affected by their shape and structure. * To whom correspondence [email protected].

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(1) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Lewis, L. N. Chem. ReV. 1993, 93, 2693. (c) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. Mater. Res. Soc. Bull. 2001, 26, 985. (d) Novak, J. P.; Brousseau, L. C., III; Vance, F. W.; Johnson, R. C.; Lemon, B. I.; Hupp, J. T.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 12029. (e) Teng, X.; Black, D.; Watkins, N. J.; Gao, Y.; Yang, H. Nano Lett. 2003, 3, 261. (f) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (g) Tkachenko, A. G.; Xie, H.; Coleman, D.; Glomm, W.; Ryan, J.; Anderson, M. F.; Franzen, S.; Feldheim, D. L. J. Am. Chem. Soc. 2003, 125, 4700. (h) Jackson, J. B.; Westcott, S. L.; Hirsch, L. R.; West, J. L.; Halas, N. J. Appl. Phys. Lett. 2003, 82, 257. (i) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (j) Seto, T.; Akinaga, H.; Takano, F.; Koga, K.; Orii, T.; Hirasawa, M. J. Phys. Chem. B 2005, 109, 13403. (2) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (3) (a) Li, Y.; Hong, X. M.; Collard, D. M.; El-Sayed, M. A. Org. Lett. 2000, 2, 2385. (b) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340. (c) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 8572. (4) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755. (5) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (6) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 7, 617. (7) Zheng, L.; Li, J. J. Phys. Chem. B 2005, 109, 1108. (8) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z. Y. Nano Lett. 2005, 5, 1237. (9) Jo, C.; Lee, J. I.; Jang, Y. Chem. Mater. 2005, 17, 2667. (10) Hu, J.; Li, L.; Yang, W.; Manna, Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060.

Copper metal nanostructures have received considerable attention because of their unusual properties and potential applications in nanomaterials, thermal conducting, lubrication, nanofluids, and catalysts.11 Various preparation methods such as microemulsion,12 reverse micelles,13 reduction of aqueous copper salts,14 UV-light irradiation,15 and physical vapor deposition16 have been reported. Palladium nanoparticles have also received extensive attention because of their application in the primary catalyst for the low-temperature reduction of automobile pollutants17 and for organic reactions, such as Suzuk, Heck, and Stille coupling.2,3,18 Palladium nanoparticles with various morphologies have been prepared through the thermal decomposition of a Pd-surfactant complex,19 with the mediation of DNA,20 via the use of a coordinating ligand,21 by a seed etching process,22 and using a modified polyol process.23 Nevertheless, obtaining ordered metal spheres, particularly palladium microspheres, with a diameter larger than 100 nm can be difficult.24 As a result, these smaller particles are of limited value for nanometer-scale architecture. More recently, particular interest has been focused on bimetallic nanoparticles, which exhibit unique characteristics that are not just the addition of the two properties of the constituent metals. For example, Pd/Pt bimetallic nanoparticles have much higher catalytic activity than the mixture of the corresponding monometallic nanoparticles.25 Core-shell structured Au/Pd bimetallic (11) (a) Lu, L.; Sui, M. L.; Lu, K. Science 2000, 287, 1463. (b) Eastman, J. A.; Choi, S. U. S.; Li, S.; Yu, W.; Thompson, L. J. Appl. Phys. Lett. 2001, 78, 718. (12) Qi, L. M.; Ma, J. M.; Shen, J. L. J. Colloid Interface Sci. 1997, 186, 498. (13) Lisiecki, I.; Biorling, M.; Motte, L.; Ninham, B.; Pileni, M. P. Langmuir 1995, 11, 2385. (14) Ren, X.; Chen, D.; Tang, F. J. Phys. Chem. B 2005, 109, 15803. (15) Kapoor, S.; Palit, D. K.; Mukherjee, T. Chem. Phys. Lett. 2002, 355, 383. (16) Wang, J.; Huang, H.; Kesapragada, S. V.; Gall, D. Nano Lett. 2005, 5, 2505. (17) Nishihata, Y.; Mizuki, J.; Akao, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamada, N. Nature 2002, 418, 164. (18) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000, 39, 165. (19) Kim, S. W.; Park, J.; Jang, Y.; Chung, Y.; Hwang, S.; Hyeon, T. Nano Lett. 2003, 3, 1289. (20) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. Science 2004, 304, 850. (21) (a) Naka, K.; Itoh, H.; Chuji, Y. Nano Lett. 2002, 2, 1183. (b) Son, S. U.; Jang, Y.; Yoon, K. Y.; Kang, E.; Hyeon, T. Nano Lett. 2004, 4, 1147. (22) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z. Y. Nano Lett. 2005, 5, 1237. (23) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 7332. (24) (a) Goia, D. V.; Matijevic, E. Colloids Surf., A 1999, 146, 139. (b) Brown, K. R.; Walter, D. G.; Natan, M. J. Chem. Mater. 2000, 12, 306. (25) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 22, 1179.

10.1021/la060339k CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006

Synthesis of Cu Nanocubes and Cu/Pd Nanostructures

Langmuir, Vol. 22, No. 13, 2006 5901

nanoparticles show higher activities for the hydrogenation of 4-pentenoic acid than for those of the mixtures of monometallic nanoparticles with a corresponding gold/palladium ratio. When the gold/palladium ratio is 1:4, the activity of the bimetallic particles is about 3 times higher than that of palladium monometallic nanoparticles prepared under the same conditions.26 In this paper, we present a two-stage procedure for the preparation of copper nanocubes and copper/palladium coreshell bimetallic nanostructures in high yield via a simple surfactant-assisted route in the presence of anionic surfactant dodecyl benzene sulfonic acid sodium (DBS). At first, the uniform copper nanocubes with slight holes in the centers of the six {100} surfaces was synthesized at the first stage in the surfactantassisted process. Later, the bimetallic Cu/Pd nanoparticles with a confirmed core-shell structure were formed on the basis of the successive reduction of H2PdCl4 and the Pd growth on the surfaces of the Cu seeds. 2. Experimental Section 2.1. Preparation of Copper Nanocubes. All the regents were purchased from Shanghai Chemistry Co. with analytical grade purity and were used without further purification. The synthesis of copper nanocubes was carried out by using hydrazine as the reducing reagent in the presence of DBS at a suitable reaction temperature and time. In a typical experiment, 0.5 mL of 10 mM CuCl2 and 1 mL of 50 mM DBS solution were added into 45.5 mL of deionized water in a three-necked flask equipped with a condenser. The reaction mixture was heated to 100 °C under continuous stirring, and 3 mL of 50 mM hydrazine solution were then added dropwise to the above solution. The molar ratio of the N2H5OH and CuCl2 was 30. The reaction solution was refluxed for 20 min at 100 °C. The blue solution turned yellow, indicating the formation of copper nanocubes. The resulting yellow precipitate products were separated by centrifugation at 4000 rpm for 20 min, washed several times with water and ethanol, and then dried at 60 °C for 5 h in a vacuum dryer. The other part of the sample was used for the preparation of Cu/Pd core-shell nanoparticles without preliminary filtration. 2.2. Preparation of Cu/Pd Core-Shell Nanostructures. A H2PdCl4 aqueous solution (10 mM) was prepared by mixing 0.53 g of PdCl2, 6 mL of 0.5 M HCl, and 294 mL of deionized water. After the CuCl2-DBS-N2H5OH reaction solution was refluxed for 20 min at 100 °C, the copper nanocubes were formed. A 0.5 mL portion of 10 mM H2PdCl4 aqueous solution was then added dropwise to the above solution, and the mixtures were refluxed for another 10 min at 100 °C. The yellow solution turned black quickly, indicating that the Pd2+ was reduced by hydrazine to Pd(0). The solution was left to cool to room temperature naturally, and the Cu/Pd core-shell structures formed. The resulting black precipitate products were separated by centrifugation at 4000 rpm for 20 min, washed several times with water and ethanol, and then dried at 60 °C for 5 h in a vacuum dryer. The samples for transmission electron microscopy (TEM) and scanning electron microscopy (SEM) measurements were dispersed into water without drying in a vacuum, and then a drop was placed on copper grids and allowed to dry at room temperature. 2.3. Characterization of Samples. The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/Max 2200PC diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å). TEM images were carried out using a JEM-100CX2 transmission electron microscope. High-resolution transmission electron microscopy (HRTEM) images were carried out using a Philips Tecnai 20U-TWIN transmission electron microscope. SEM images were measured on a JEOL JSM-6700f scanning electron microscope. All the measurements were carried out at room temperature.

3. Results and Discussion The copper nanocubes were synthesized from the reduction of CuCl2 by using hydrazine as the reducing reagent under the (26) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028.

Figure 1. (A) SEM and (B) TEM images of copper nanocubes synthesized under the final concentration CCuCl2 ) 0.1 mM and CN2H4 ) 3 mM and refluxed for 20 min at 100 °C in the presence of 1 mM DBS. The inset shows the electron diffraction pattern obtained with the electron beam parallel to the 〈111〉 direction.

final concentration CCuCl2 ) 0.1 mM and CN2H4 ) 3 mM and refluxed for 20 min at 100 °C in the presence of 1 mM DBS. It is worth noting that DBS served only as a capping agent in the present study. Its concentration (1 mM) is lower than the critical concentration for the formation of spherical micelles (1.6 mM, 40 °C), especially at the reaction temperature of 100 °C, since the concentration of the micelle formation increases for DBS as the temperature rises. Therefore, DBS cannot form any micelles, including spheres, cubes, and rods, to serve as a soft template. The morphology and characterization of the product are shown in Figure 1. Figure 1A shows SEM images of a typical sample of copper nanocubes and indicates the large quantity and good uniformity that were achieved using this method. These copper nanocubes have a mean edge length of 50 ( 6 nm. It is also clear from Figure 1A that all the nanocubes have a slight hole in the center of the six surfaces of the {100} plane. Figure 1B shows a TEM image of the copper nanocubes, in which some of them self-assembled into ordered two-dimensional (2D) arrays on the surface of the TEM grid. The inset shows the electron diffraction pattern obtained by directing the electron beam parallel to the 〈111〉 direction. These diffraction spots suggest that each cube is a single crystal. Figure 6A shows an XRD pattern of the nanocubes. The peaks in our samples followed those observed in the standard material (JCPDS file no. 04-0836), which revealed that the sample was crystalline copper.

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Figure 2. TEM images of copper nanocubes synthesized under the same conditions as those described in Figure 1, except that the concentration of CuCl2 was changed from 0.1 mM to (A) 0.05, (B) 0.15, and (C) 0.30 mM.

Figure 4. TEM images of copper particles synthesized by equal molar CTAB-substituted DBS.

Figure 3. TEM images of copper nanocubes synthesized under the same conditions as those described in Figure 1, except that the refluxing time was changed from 20 min to (A) 5 and (B) 30 min.

The morphology and dimensions of the product were found to strongly depend on reaction conditions such as the concentration of CuCl2, refluxing time, and the type and concentration of surfactant. For example, when the concentration of CuCl2 was reduced to 0.05 mM, the nanocubes were the major product, and the morphology and dimensions have little change (Figure 2A). When the concentration of CuCl2 was increased to 0.15 mM, the surfaces of the nanocubes became rough (Figure 2B). When the concentration of CuCl2 was higher than 0.3 mM, the product was dominated by nanoparticles with irregular shapes (Figure 2C). This indicated that it was necessary for the formation of copper nanocubes that the concentration of CuCl2 be lower than 0.15 mM. Copper nanocubes of various dimensions could be obtained by controlling the refluxing time. Figure 3A,B shows TEM images for 5- and 30-min growth times, and the nanocubes had a mean edge length of 46 ( 4 and 66 ( 8 nm, respectively. It indicated that it is possible to tune the size of copper nanocubes by controlling the experimental conditions. The type and concentration of surfactant are two other important parameters. If no surfactant was present, a few nanocubes with rough surfaces and some irregular nanoparticles were the major products. When the concentration of DBS was higher than 10 mM, only conglomeration nanoparticles were obtained. When equal molar cetyltrimethylammonium bromide (CTAB)-substituted DBS was used, porous nanospheres were the major product (Figure 4). The bimetallic Cu/Pd particles were prepared by a two-stage procedure. Pure copper nanocubes with definite diameters were

formed at first by adding hydrazine to a CuCl2 aqueous solution in the presence of DBS. Later, an H2PdCl4 aqueous solution (Pd/Cu molar ratio was 1/1) was added dropwise to the colloid solution, and the Cu/Pd core-shell structures were achieved by the reduction of H2PdCl4 and Pd(0) growth on the Cu seeds. An SEM image (Figure 5A) shows the morphologies of the bimetallic Cu/Pd core-shell structures. The TEM image (Figure 5B) clearly shows that the as-prepared nanoparticles have a coreshell structure. An HRTEM image of the shell is shown in Figure 5C. The distance between the adjacent fringes is about 0.22 nm, which could be indexed to the {111} planes of the face-centered cubic (fcc) palladium (JCPDS file no. 46-1043; a ) 3.89 Å). It is strong evidence for the formation of a Pd shell on the Cu nanocube cores that seeded the growth process. The XRD pattern recorded from the metallic sample is also displayed in Figure 6B, and the peaks are assigned to diffraction from the (111), (200), and (220) planes of fcc copper and palladium. The selected area electron diffraction (SAED) image (the inset in Figure 5B) displays a set of diffraction spots that are indexed to the Cu core by the electron beam parallel to the 〈111〉 direction and a set of diffraction rings that identify the crystalline structure of the palladium shell. It is further confirmed that the as-prepared samples have a bimetallic core-shell structure. Usually, the surface energies associated with different crystallographic planes are different, and a general sequence may hold: γ{111} < γ{100} < γ{110}.27 On the nanometer scale, metals (most of them are fcc) tend to nucleate and grow into twinned and multiply twinned particles (MTPs) with their surfaces bounded by the lowest-energy {111} facets.28 As illustrated by Wang,27 the shape of an fcc nanocrystal was mainly determined by the ratio R between the growth rates along the 〈100〉 and 〈111〉 directions. Perfect cubes bounded by the less stable {100} planes could be achieved if R is reduced to 0.58. For the nanocubes with the slight holes illustrated in Figure 1, the ratio R should have a value lower than 0.58. If DBS was not present, the copper atoms generated by reducing CuCl2 nucleated and grew into (27) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (28) (a) Allpress, J. G.; Sanders, J. V. Surf. Sci. 1967, 7, 1. (b) Sun, Y.; Xia, Y. Science 2002, 298, 2176.

Synthesis of Cu Nanocubes and Cu/Pd Nanostructures

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Figure 5. (A) SEM (B) TEM images of the bimetallic Cu/Pd nanoparticles. The inset is the SAED image of the bimetallic Cu/Pd nanoparticles. (C) HRTEM images of the shell of the Cu/Pd nanoparticles.

plane and the nuclei, and growth of Pd on the nanocubes could decrease its surface energy. Second, palladium crystals with fcc structures tend to nucleate and grow into twinned particles and MTPs, with their surfaces bounded by the lowest-energy {111} facets. The {111} interplanar distance of Pd is about 0.22 nm, which is close to the value (d110 ) 0.256 nm and d111 ) 0.209 nm) of copper. That is an advantage to the growth of Pd on the nanocubes surface and the formation of the Cu/Pd core-shell structure.

4. Conclusion

Figure 6. XRD patterns of (a) pure copper nanocubes and (b) copper/ palladium core-shell nanostructures (the asterisk indicates Pd).

MTPs bounded by the most stable {111} facets. When DBS was introduced, it is believed that the selective interaction between DBS and various crystallographic planes of fcc copper could greatly reduce the growth rate along the 〈100〉 direction and/or enhance the growth rate along the 〈111〉 direction, and thus reduce R to less than 0.58. Copper nanocubes with slight holes were achieved. When H2PdCl4 was introduced into the reaction solution containing Cu nanocubes, Pd2+ was reduced to Pd(0) and grew on the Cu seed, and Cu/Pd core-shell structures were formed. By the analysis from the HRTEM, SAED, and XRD images, it was confirmed that the bimetallic Cu-Pd samples were Cu/Pd core-shell nanostructures, not the mixtures of Cu nanocubes and Pd nanoparticles or Cu-Pd alloys. The reason for the formation of Cu/Pd core-shell nanostructures was as follows: First, the Cu nanocubes have higher surface energy and are more active because of the existence of the slight hole on the {100}

In summary, copper nanocubes with uniform shape and size and copper/palladium core-shell bimetallic nanostructures have been synthesized in high yield via a simple surfactant-assisted route in the presence of anionic surfactant DBS. Uniform copper nanocubes with a slight hole in the centers of the six {100} surfaces was synthesized first in the surfactant-assisted process. Later, bimetallic Cu/Pd nanoparticles with a confirmed coreshell structure were formed on the basis of the successive reduction of H2PdCl4 and the Pd growth on the surfaces of the Cu seeds. Palladium nanoparticles have been extensively used in the catalyst area. Nevertheless, obtaining ordered metal structures, particularly palladium nanostructures with larger diameter, can be difficult. We present a simple approach for the preparation of Cu/Pd nanostructures with larger size. It is predicted that the Cu/Pd core-shell multifunctional nanomaterial may have important applications in catalytic reactions, energy storage, and the fabrication of sensors. Acknowledgment. This work is supported by the awarded funds of the excellent State Key Laboratory (No. 50323006) and the Natural Science Foundation of Shandong Province (No.Y2003F08). LA060339K