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Synthesis of Bimetallic Nanoshells by an Improved Electroless Plating Method J. B. Liu, W. Dong, P. Zhan, S. Z. Wang, J. H. Zhang, and Z. L. Wang* National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China Received September 24, 2004. In Final Form: January 5, 2005 In the Letter, we demonstrate an improved electroless plating method for the synthesis of bimetallic shell particles. The procedure involves a combination of surface reaction, seeding growth, and removal of supporting cores. We modified ammonical AgNO3 in ethanol with a controlled amount of HCHO in the seeding process and a uniform and relatively dense coverage of silver nanoparticle seeds on colloid cores was achieved. Following the second kind of metal plating, we extended this method to prepare continuous bimetallic core-shell and hollow particles with a submicrometer diameter. The morphologies of the bimetallic Cu/Ag and Pt/Ag particles were studied with transmission electron microscopy and scanning electron microscopy, and their crystallinity and chemical composition were confirmed by X-ray diffraction. The prepared materials may be of applied value in areas such as catalysis, optics, and plasmonics.
Introduction Metallodieletric composite particles with core-shell and hollow structure have been continually reported due to their fascinating optic, electric, magnet, and catalysis properties.1-8 For example, colloid core-metal shell particles display surface plasmon resonances that can be tuned over a wide range as a function of the core-to-shell ratio.1a In addition, hollow metal spheres exhibit enhanced catalytic activities with the advantages of low density and reduced cost.5,6 The extension of such structure to bimetallodieletric is of considerable interest, because bimetallic nanoparticles, either as alloys or as core-shell structures, exhibit unique electronic,9 optical,10 catalytic,11 and biological properties,12 which are usually different from those of either of their parent metals. However, synthesis of * To whom correspondence should be addressed. E-mail: zlwang@ nju.edu.cn. (1) (a) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (b) Oldenburg, S. J.; Hale, G. D.; Jackson, J. B.; Halas, N. J. J. Appl. Phys. Lett. 1999, 75, 1063. (c) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729. (d) Jackson, J. B.; Westcott, S. L.; Hirson, L. R.; West, J. L.; Halas, N. J. Appl. Phys. Lett. 2003, 82, 257. (2) (a) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (b) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (3) (a) Caruso, F.; Spasova, M.; Salgueirin˜o-Maccria, V.; Liz-Marza´n, L. M. Adv. Mater. 2001, 13, 1090. (b) Cassagneau, T.; Caruso, F. Adv. Mater. 2002, 14, 732. (c) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (4) Pol, V. G.; Srivastava, D. N.; Palchik, O.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.; Gedanken, A. Langmuir 2002, 18, 3352. (5) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon,.T. J. Am. Chem. Soc. 2002, 124, 7642. (6) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wang, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (7) Bao, J.; Liang, Y.; Xu, Z.; Si, L. Adv. Mater. 2003, 15, 1832. (8) Wang, Z. L.; Chan, C. T.; Zhang, W. Y.; Ming, N. B.; Sheng, P. Phys. Rev. B 2001, 64, 113108. (9) Schmid, G. Clusters and Colloids; VCH: Weinheim, 1994. (10) (a) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 11, 1179. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (c) Henglein, A. Langmuir 2001, 17, 2329. (d) Mallin, M. P.; Murphy, C. J. Nano. Lett. 2002, 2, 1235. (11) (a) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (b) Mizukoshi, Y.; Fujimoto, T.; Nagata, Y.; Oshima, R.; Maeda, Y. J. Phys. Chem. B 2000, 104, 6028. (c) Harpeness, R.; Gedanken, A. Langmuir 2004, 20, 3431. (12) (a) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (b) Mandal, S.; Selvakannan, PR.; Pasricha, R.; Sastry, M. J. Am. Chem. Soc. 2003, 125, 8440.
bimetallodieletric composite spheres with core-shell and hollow structure was relatively limited.13 In the Letter, we demonstrate a modified electroless plating method for synthesis of bimetallic shells with a submicrometer diameter. In previous reports on pretreatment in electroless plating, deposition of silver particles on colloid surface with Sn2+ ions was performed in an aqueous solution of ammonical AgNO3.14 Uneven distribution and low coverage of silver nanoparticles on the surfaces of colloids were usually observed.14 Herein, we modified ammonical AgNO3 in ethanol solution with a controlled amount of HCHO in the seeding process. Uniform and relatively dense coverage of silver nanoparticles on each colloid core was achieved. Following the second kind of metal plating, we extend this method to preparation of continuous bimetallic core-shell and hollow particles with a submicrometer diameter. The strategies to fabricate such composite materials are similar to that in ref 13. First, the surfaces of polystyrene (PS) beads are functionalized with tin salt solution. This step is followed with silver ion reaction in an improved plating solution that leads to uniformly deposited silver nanoparticles on the surfaces of PS cores. Then these silver-deposited PS particles are added to a second plating solution to deposit a second kind of metal on the Ag-coated PS beads to form a bimetallic shell structure. Finally, the PS core bimetallic shell composite particles are treated with toluene solution to remove PS cores resulting in hollow bimetallic nanoshells. Experimental Section (I) Materials. In our experiment, the following materials purchased from Shanghai Chemicals have been used: silver nitrate (AgNO3 g99.8%), formaldehyde solution (methanal 37.0%), ammonia water (NH3 g29.3%), stannous(II) chloride dihydrate (SnCl2‚H2O g99.0%), hydrochloric acid (HCl 39.0%), cupric sulfate (CuSO4 g99.0%), sodium carbonate (NaCO3), sodium hydroxide (NaOH), nickel chloride (NiCl2 g98.0%), potassium sodium tartrate (C4H4O6KNa g99.0%), ascorbic acid (C6H8O6 g99.7%), chloroplatinate acid (H2PtCl6 g99.0%). Ab(13) (a) Kobayashi, Y.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M. Chem. Mater. 2001, 13, 1630. (b) Schierhorn, M.; Liz-Marza´n, L. M. Nano. Lett. 2002, 2, 13. (c) Chen, Z.; Wang, Z. L.; Zhan, P.; Zhang, J. H.; Zhang, W. Y.; Wang, H. T.; Ming, N. B. Langmuir 2004, 20, 3042 (14) Lu, L.; Sun, G.; Xi, S.; Wang, H.; Zhang, H.; Wang, T.; Zhou, X. Langmuir 2003, 19, 3074.
10.1021/la047616c CCC: $30.25 © 2005 American Chemical Society Published on Web 02/01/2005
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solute ethanol (g99.7%), and distilled water were used in all experiments. (II) Syntheses. The bimetallic core-shell and hollow particles were synthesized in a multiple step reaction. All particle diameters and shell thicknesses mentioned in the Experimental Section were measured by transmission electron microscopy (TEM). PS Particles Coated with Silver Seeds. Monodisperse PS cores with a diameter of 700 nm were synthesized by emulsion polymerization.15 The PS colloidal particles (20 mg) were added under magnetic stirring into a freshly sensitizing solution (10 mL) that was prepared by mixing 0.2 g of SnCl2 and 1 mL of HCl with 9 mL of distilled water. After 10 min of stirring, the functionalized PS particles were separated from the solution by centrifugation and flushed with distilled water two times, then redispersed in ethanol. Subsequently, these beads covered with bivalent stannous ions were added to 10 mL of freshly prepared ethanol solution that consists of silver nitrate (0.2 g), formaldehyde solution (0.5 mL), and a controlled amount of ammonia water under magnetic stirring. Consequently, silver ions were reduced and a layer of silver nanoparticles was formed on the surfaces of PS cores. The obtained PS particles deposited with silver seeds were separated through centrifugation and redispersion in water several times. Finally, they were redispersed in 10 mL of water. Cu/Ag-Coated PS Cores and Hollow Cu/Ag Shells. To synthesize Cu/Ag shell particles, 1 mL of solution of PS colloidal dispersion coated with a layer of silver nanoparticles was added to the solution containing A (4 mL) and B (4 mL) under magnetic stirring. Solution A was a mixture of cupric sulfate (0.7 g), nickel chloride (0.2 g), formaldehyde solution (2 mL), and distilled water (50 mL); solution B was prepared from potassium sodium tartrate (2.25 g), sodium carbonate (0.21 g), sodium hydroxide (0.45 g), and distilled water (50 mL). Here, nickel chloride was used as an accelerating agent for the reaction of cupric sulfate and formaldehyde. The time for plating was about 10 min. After this step, continuous Cu/Ag layers on the PS surfaces were formed. Hollow Cu/Ag shells were produced by dissolving the PS cores in toluene solution. Hollow Pt/Ag Shells. The preparation procedure for hollow Pt/Ag shells is similar to that for hollow Cu/Ag shell particles. First, 1.5 mL of 0.1 M freshly prepared ascorbic acid and 10 mL of 0.8 mM chloroplatinate acid were mixed with 1 mL of silver surface-seeded PS particle dispersion under magnetic stirring. After 10 min, a procedure of centrifugation and redispersion was carried out several times. The obtained Pt/Ag shells containing PS cores were redispersed in water. Again, PS cores were dissolved with toluene solution and hollow Pt/Ag shells were obtained. (III) Characterization. Several methods were used to analyze the morphology and composition of the prepared bimetallic shell particles. Transmission Electron Microscopy (TEM). Samples for TEM analysis were prepared by dropping a drop of suspension on a copper grid. The coverage of silver nanoparticles on the surfaces of PS colloids and the morphology of bimetallic coreshell and hollow particles were observed with JEOL 2000-EX transmission electron microscopy. Scanning Electron Microscopy (SEM). SEM (JSM 5610LV) was also used for obtaining the surface morphology of particles prepared above. The samples were prepared by dropping several drops of suspension onto a glass substrate. X-ray Diffraction (XRD). As a way to identify the composition of these composite particles, XRD analysis was carried out on a Japan Rigake D/max-RA X-ray diffraction meter using Cu KR radiation (λ ) 1.5418 Å). Samples were prepared by adding several drops of condensed solution to a glass substrate and allowing the substrate to dry in air.
Letters
Figure 1. TEM (a) and SEM (b) images of the PS particles used as supporting cores. The diameter of the PS particles is about 700 nm and polydispersity is about 5%.
TEM and SEM images of the bare PS particles are shown in panels a and b of Figure 1, respectively. The mono-
dispersed PS particles have a diameter of about 700 nm with a polydispersity of about 5% and were prepared by emulsion polymerization. It has been demonstrated that a uniform distribution of silver seeds on the surfaces of colloid cores was crucial for the formation of continuous metallic shell particles. We found that using a plating solution in ethanol with a suitable amount of HCHO can lead to much improvement of the attachment of silver nanoparticles on the surfaces of colloid cores, as compared to the use of aqueous solution of ammonical AgNO3.14 This may be due to a better affinity of HCHO for PS cores than that of inorganic reductants, such as hydrazine and tannins.16 In addition, substituting ethanol media for water can reduce the reaction velocity of HCHO and AgNO3, which allows “silver seeds” to grow evenly. Figure 2a shows TEM images of PS beads coated with silver nanoparticles after the seeding process. The inset of Figure 2a presents a higher magnification (100000×) TEM of the particles. As is seen from TEM image, a uniform and relatively dense coverage of silver nanoparticles on each PS core was achieved and silver nanoparticles have a typical size of less than 10 nm. The SEM analysis of the particles also confirmed this improvement (see Figure 2b). It was also observed that it was difficult to organize these composite particles into an ordered structure after metal coating, which could be due to the increase of the van der Waals forces between PS spheres due to the presence of silver nanoparticles. Similar phenomena were also observed in ref 2a. The silver nanoparticles on the surfaces of PS cores act as catalysts for further reduction of Cu ions. Reaction of cupric sulfate and formaldehyde solution resulted in a continuous coverage of Cu nanoparticles on the surfaces of silver-coated PS particles. The Cu/Ag weight ratio can be adjusted by controlling the amount of PS particles and the plating time. Panels a and b of Figure 3 show the TEM and SEM of the obtained PS core-Cu/Ag shell particles,
(15) Zhang, J. H.; Chen, Z.; Wang, Z. L.; Zhang, W. Y.; Ming, N. B. Mater. Lett. 2003, 57, 4466.
(16) Mayer, A. B. R.; Grebner, W.; Wannemacher, R.J. Phys. Chem. B 2000, 104, 7278.
Results and Discussions
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Figure 2. TEM (a) and SEM (b) of PS beads deposited with silver nanoparticles. The inset in Figure 2a is the TEM image under a higher magnification (100000×). The silver nanoparticles are uniformly distributed with typical size of less than 10 nm.
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Figure 4. TEM (a) and SEM (b) of hollow Cu/Ag shells. Both confirm that the PS cores have been removed and the obtained particles are hollow.
Figure 5. Figure 3.
Figure 3. TEM (a) and SEM (b) of PS-core Cu/Ag-shell particles. The inset shown in Figure 3a is the TEM image under a higher magnification (60000×). The obtained bimetallic particles have a continuous shell structure. The shell thickness was measured to be about 40 ( 5 nm.
respectively. From both TEM and SEM images, it is seen clearly that the PS beads are covered by a continuous Cu/Ag layer. The outer Cu/Ag shell has a thickness of about 40 ( 5 nm. We found that silver seeds directly influence the coverage of Cu nanoparticles on PS particles. A uniform and relatively dense distribution of silver nanoparticles is crucial for high-quality Cu/Ag shell particles. Nonuni-
The XRD analysis of the sample shown in
form or partial Cu/Ag structure resulted when PS beads with a nonuniform or low coverage of silver nanoparticles were used as the starting material for electroless plating of the second metal. Aggregation of PS-core Cu/Ag-shell particles became serious when thicker metallic shells were formed, due to much increase in the van der Waals force between the composite particles. The TEM and SEM images of hollow Cu/Ag shells by dissolution of the PS cores are shown in panels a and b of Figure 4, respectively. The strong contrast between the plate edges and dark centers in the TEM image indicates the hollow nature of the Cu/Ag shells. The existence of a hollow nature can also be confirmed through a broken shell in the SEM image. Again, it is seen that these bimetallic shells are continuous and relatively uniform. Figure 5 presents the XRD analysis of the synthesized Cu/Ag bimetallic hollow particles. In the pattern, five diffraction peaks were observed at 2θ ) 36.46°, 38.18°, 43.40°, 50.56°, and 74.16°. Among them, the three diffraction peaks at 2θ ) 43.40°, 50.56°, and 74.16° are
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We also adopted this method to the preparation of other bimetallic shell particles using metals such as Pt and Au. Figure 6a shows the representative TEM image of Pt/Ag shell particles by using a similar procedure. A broken sphere due to the removal of the supporting core in the TEM image indicates the hollow structure of the particles. Figure 6b shows the corresponding XRD pattern of the sample. These diffraction peaks correspond to the facecentered cubic structure of metallic Pt. The relatively broad X-ray diffraction lines suggest a nanometer size of the Pt particles within the shell. It is noted that no detectable Ag diffraction peaks were observed in the XRD pattern of Pt/Ag shell particles. We explain this as a result of the extremely low weight fraction of Ag in the nanoshell. Conclusion In summary, by using ammonical AgNO3 in ethanol solution with a controllable amount of HCHO, a uniform seeding of silver nanoparticle on the surface of PS cores was achieved. Subsequent electroless plating was performed for the formation of a second kind of metal layer on the PS supporting cores. Hollow Cu/Ag and Pt/Ag bimetallic nanoshells were obtained by dissolving the PS cores in toluene. The present method could be extended to prepare other kinds of bimetallic hollow particles, as standard recipes exist for the deposition of virtually any metal.17 It is expected that such materials may have various applications in catalysis, plasmonic devices, sensors, and many other fields. Figure 6. (a) TEM image of hollow Pt/Ag shells. (b) XRD analysis of the sample in (a).
characteristic of cubic metallic Cu and correspond to the (111), (200), and (220) reflections, respectively. The diffraction peak at 2θ ) 38.18° corresponds to the (111) reflection of the face-centered-cubic structure of metallic Ag. The diffraction peak at 2θ ) 36.46° corresponds to the (111) reflection of cubic Cu2O that could be formed from the oxidation of copper during the sampling procedure.
Acknowledgment. This work is supported by a grant for the State Key Program for Basic Research of China and by NSFC under Grant No. 10174031 and 90101030 and Excellent Youth Foundation. LA047616C (17) Mallory, G. O.; Hajdu, J. B. Electroless Plating: Fundamentals and Applications; American Electroplaters and Surface Finishers Society: Orlando, FL, 1990.
