NANO LETTERS
Template-Engaged Replacement Reaction: A One-Step Approach to the Large-Scale Synthesis of Metal Nanostructures with Hollow Interiors
2002 Vol. 2, No. 5 481-485
Yugang Sun, Brian T. Mayers, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received February 21, 2002; Revised Manuscript Received March 16, 2002
ABSTRACT This paper describes a general approach that generates nanoscale hollow structures of metals by reacting solutions of appropriate salt solutions with solid templates of a more reactive metal. Typical examples include Au3+, Pt2+, and Pd2+ salts and nanoparticles or nanowires of silver. The morphology, void space, and wall thickness of these hollow structures are all determined by the solid templates, which are completely converted into soluble species during the replacement reaction. Both electron microscopy and diffraction studies indicate that single crystalline hollow structures of metals can also be obtained when the templates are single crystals. These metallic hollow structures, having well-controlled sizes and shapes, are expected to find use in a number of applications that involve nanoscale encapsulation, drug delivery, plasmon photonics, and calorimetric sensing.
Nanostructures with hollow interiors are useful in many areas.1 For example, they can serve as extremely small containers for encapsulationsa process that has been extensively explored in applications related to catalysis, drug delivery, and protection of environment-sensitive materials such as enzymes.2 When used as fillers in making composite materials, hollow structures also offer some advantages over their solid counterparts as a result of their relatively low densities.3 Hollow nanostructures of noble metals are particularly interesting to synthesize and investigate because they exhibit plasmonic properties completely different from those of solid nanoparticles (even made of the same metal).4 Recent studies by Halas and co-workers, for example, suggest that the plasmon peak of spherical gold nanoshells could be conveniently tuned to cover the spectral regime from 600 to 1200 nm, whereas it is difficult to shift the plasmon peak of spherical gold (or silver) nanoparticles by more than 20 nm.5,6 The strong absorption of gold nanoshells in the near-infrared region (800-1200 nm, the transparent window of tissues) makes these materials ideal candidates for photothermally trigged drug releasing in tissues.7 Hollow spheres of various materials have been extensively investigated in the literature. They were usually prepared by templating against solid particles that include nanoscale gold, silver, or cadmium sulfide colloids; and mesoscale silica beads or polymer latexes.8 In a typical procedure, the surfaces * To whom correspondence should be addressed. E-mail: xia@ chem.washington.edu. 10.1021/nl025531v CCC: $22.00 Published on Web 03/30/2002
© 2002 American Chemical Society
of these templates are coated with thin layers of the desired material (or its precursor) to form core-shell structures; subsequent removal of the templates (by calcination at elevated temperatures in air or selective etching in an appropriate solvent) generates hollow spheres whose void sizes are determined by the diameters of templates. Most materials that have been processed into hollow spheres are ceramic or polymeric materials.8,9 These materials are amorphous and thus able to form conformal coatings on spherical templates. More recently, several effective methods have been demonstrated to coat particulate templates with metal shells. In one instance, Au2S/Au core-shell nanostructures with diameters of ∼50 nm were synthesized by mixing aqueous solutions of Na2S and HAuCl4 with appropriate molar ratios.10 Another method involved the formation of gold shells on the surfaces of silica nanoparticles through nanoscale self-assembly.11 In this case, 3-aminopropyl)triethoxysilane was adsorbed onto the silica templates, whose amine groups could covalently bind to gold nanoparticles of 1-2 nm in diameter. These chemically immobilized gold colloids could serve as catalysts and nucleation sites for the subsequent electroless deposition of gold (often induced through a redox process). In a third approach, layer-by-layer adsorption of polyelectrolytes and charged gold colloids were used to build a shell structure around the template particle whose surface had been derivatized with appropriately charged groups.12 This method has been successfully applied to the formation of homogeneous and
Figure 1. Schematic illustration of the experimental procedure that generates nanoscale shells of gold from silver templates with various morphologies. The reaction is illustrated in the schematic as follows: (A) Addition of HAuCl4 to a dispersion of silver nanoparticles and initiation of the replacement reaction; (B) The continued replacement reaction of HAuCl4 with the silver nanoparticles; (C) Depletion of silver and annealing of the resultant shells to generate smooth hollow structures. Note that the shape of each silver nanoparticle is essentially preserved in this template-engaged reaction.
dense coatings of metal colloids on the surfaces of a number of spherical templates. Selective removal of the solid cores in the following steps yielded hollow spheres consisting of both metal colloids and polyelectrolyte materials. A major problem associated with all of these methods is that the metal nanoshells are polycrystalline in structure, and in some cases, are made of discrete gold colloids (or domains) characterized by poor connections among them. Here, we describe a new approach, based on template-engaged replacement reactions (TERR), that was able to generate hollow nanostructures of noble metals with well-defined void spaces and homogeneous, highly crystalline walls. Figure 1 shows the possible steps involved in the formation of highly crystalline nanoshells of metals, with gold as an example.13 Because the standard reduction potential of AuCl4-/Au pair (0.99 V, vs SHE) is higher than that of Ag+/ Ag pair (0.80 V, vs SHE), silver nanostructures suspended in solution can be oxidized by HAuCl4 according to the following replacement reaction 3Ag(s) + AuCl4-(aq) f Au(s) + 3Ag+(aq) + 4Cl-(aq) (1) The elemental gold produced in this reaction is confined to the vicinity of the template surface. It nucleates and grows into very small particles, and eventually evolves into a thin shell around the silver template. This shell seems to have an incomplete structure in the initial stages because both HAuCl4 and AgCl can continuously diffuse across this layer until the silver template has been completely consumed. When the reaction is continued with refluxing at 100 °C, the gold shells can reconstruct their walls into highly crystalline structures via processes such as Ostwald ripening.14 At the same time, the surfaces of these hollow 482
Figure 2. UV-vis absorption spectra of an aqueous dispersion of silver nanoparticles (∼50 nm in diameter) before and after various volumes of 1 mM HAuCl4 aqueous solution had been added. There existed an isosbestic point at ∼530 nm.
structures are smoothened, and any openings in the incomplete shells should be closed to form seamless shells. These gold shells have a morphology similar to that of the silver templates, with their void sizes mainly determined by the dimensions of templates. On the basis of the stoichiometric relationship shown in eq 1, the wall thickness of each gold nanoshell should be approximately one tenth of the lateral dimension of the corresponding silver template. The silver chloride formed in this replacement reaction is completely soluble in water under our experimental conditions. The solubility product of AgCl is ∼1.8 × 10-10 in cold water at 20 °C.15 When the temperature is raised, it should increase due to a negative enthalpy for the dissolution process. Theoretically, the solubility product of AgCl can reach a value of ∼1.2 × 10-6 in hot water refluxed at 100 °C.16 In our synthesis, the concentrations of chloride and silver ions in the final solution were ∼0.50 mM and ∼1.6 mM, respectively. The product of these two concentrations was 8 × 10-7, which fell between the solubility products of AgCl at 20 and 100 °C. As a result, no AgCl precipitate formed in the solution when the reaction was carried out around 100 °C. When the reaction mixture was cooled to room temperature, white solid was observed at the bottom of the container, indicating the formation of AgCl precipitate. The formation of gold and AgCl solids in two separate steps helped prevent the gold nanoshells from being contaminated by AgCl. We first followed the replacement reaction between HAuCl4 and silver nanoparticles using the UV-vis spectroscopic method. The plasmon absorption of Ag and Au nanoparticles has been a subject of intensive research for many decades.17 It is well-known that their aqueous suspensions exhibit strong plasmon resonance peaks at ∼420 and ∼530 nm, respectively. Figure 2 shows a series of absorption spectra when an aqueous solution of silver nanoparticles (with a mean size of ∼50 nm) was reacted with different amounts of HAuCl4 at 100 °C. The plasmon peak (∼422 nm) of silver nanoparticles continued to decrease in intensity as the volume of added aqueous HAuCl4 solution (1 mM) was increased. This peak disappeared after 750 µL HAuCl4 solution had been introduced into the reaction system, Nano Lett., Vol. 2, No. 5, 2002
Figure 3. (A) TEM image of silver nanoparticles synthesized using the polyol process. (B, C) TEM and SEM images of gold nanoshells formed by reacting these silver nanoparticles with an aqueous HAuCl4 solution. (D) The selected-area electron diffraction (SAED) pattern obtained by focusing the electron beam on a random assembly of gold nanoshells. (E) A TEM image of gold nanotubes formed by reacting silver nanowires with an aqueous HAuCl4 solution. (F) A HRTEM image of the edge of an individual gold nanotube, indicating its uniform wall thickness and single crystalline structure.
indicating the complete consumption of all silver templates. At the same time, a new absorption peak evolved with increasing intensity at 634 nm, having a full width at halfmaximum (fwhm) of ∼227 nm. The position of this new peak was consistent with the plasmon resonance absorption of gold nanoshells calculated with a mean diameter of 50 nm and wall thickness of 6.5 nm.18 The appearance of an isosbestic point at ∼530 nm implied that no measurable intermediate was involved in this TERR process. The absence of absorption peaks around 530 nm also indicated that almost no free gold nanoparticles were formed in the dispersion medium. The final products obtained from this TERR route were further characterized using electron microscopy. Figure 3A shows a TEM image of the silver nanoparticles that were used as templates in Figure 2. These nanoparticles exhibited a range of different morphologies, with an average size of ∼50 nm. The central portions of these particles were darker than their edges due to different thickness of silver along the path of electron beam. Thinner regions (edges) were less effective in scattering electrons. Figure 3B gives a TEM image of these silver nanoparticles after they have completely reacted with the aqueous HAuCl4 solution. In this case, the center portion of each particle was lighter than its edge, indicating the formation of a shell-type nanostructure. Figure Nano Lett., Vol. 2, No. 5, 2002
3C shows an SEM image of these gold nanoshells. Note that the morphology of these gold nanoshells was similar to that of the silver templates, and the surfaces of these gold nanoshells were essentially free of defects such as pinholes. These gold nanoshells were also strong enough to survive the capillary forces involved in solvent evaporation process. Figure 3D shows the selected-area electron diffraction (SAED) pattern obtained from a random assembly of gold nanoshells. All diffraction rings could be indexed to facecenter-cubic gold with a lattice constant of ∼4.08 Å. Figure 3E shows the TEM image of gold nanotubes formed by templating against bicrystalline silver nanowires. These nanotubes also had a uniform wall thickness of approximately one tenth of the diameter of the silver nanowires. The ends of gold nanotubes generated using this method exhibited a morphology similar to that of the silver templates. Figure 3F shows a HRTEM image taken from the edge of an individual gold nanotube, indicating the formation of a highly crystalline structure for the metallic wall. The wall thickness measured from this HRTEM image was ∼4 nm, which agreed well with the value estimated from the diameter (∼40 nm) of silver nanowires. We note that polycrystalline nanowires (rather than nanotubes) of metals have recently been synthesized by Yang et al. through the reaction of metal salt solutions with molybdenum selenide nanowires.19 In analyzing the crystallinities of the hollow gold structures by electron diffraction and HRTEM, we noticed that single crystalline templates of silver often led to the formation of gold nanoshells with single crystalline walls. Figure 4A and 4B gives SEM and TEM images of some single crystalline cubes of silver synthesized using a solution-phase method.20 The microdiffraction patterns (the inset shows a typical example) obtained from individual cubes indicated that they were single crystals, with {100} planes as their faces. Figure 4C shows the TEM image of a cubic nanobox of gold formed by reacting aqueous HAuCl4 with these silver cubes. The inset gives a typical electron diffraction pattern obtained by aligning the electron beam perpendicular to one of the faces of this cubic box. This diffraction pattern indicated that these gold nanoboxes had a symmetry similar to the silver template, with their faces being {100} planes. Figure 4D4F show HRTEM images taken from different regions (labeled as I, II, and III in Figure 4C) of this gold nanobox. These images show well-resolved, continuous fringes with the same orientation, confirming the single crystallinity of the gold nanoshell. The fringe spacing measured from all these HRTEM images was ∼0.20 nm, a value that corresponds well with the separation between (200) planes of facecenter-cubic gold (0.204 nm). On the basis of these observations, we believe that there exists an epitaxial relationship between the silver templates and gold shell structures. The templating procedure and silver templates have also been extended to generate shell-like nanostructures from other metals. The only requirement seemed to be that these metal ions could be reduced by silver. For example, the standard reduction potentials of Pd2+/Pd pair (0.83 V, vs SHE) and Pt2+/Pt pair (1.2 V, vs. SHE) are higher than that of Ag+/Ag pair, and both Pd and Pt hollow structures could 483
should be immediately useful in fabricating plasmonic devices and calorimetric sensors, and as near-infrared absorbers to control the releasing of drugs from a polymer matrix.7 Acknowledgment. This work has been supported in part by a DARPA-DURINT subcontract from Harvard University, a Fellowship from the David and Lucile Packard Foundation, and a Career Award from the NSF (DMR-9983893). Y.X. is a Research Fellow of the Alfred P. Sloan Foundation (2000-2002). B.T.M. thanks the Center for Nanotechnology at UW for an IGERT Fellowship funded by the NSF (DGE9987620). References
Figure 4. (A, B) SEM and TEM images of cubic silver nanoparticles synthesized using a solution-phase approach. The insert shows the microdiffraction pattern recorded by directing the electron beam perpendicular to the top surface of an individual silver nanocube. (C) The TEM image of a cubic gold nanoshell formed by reacting these silver nanocubes with HAuCl4. The inset shows an electron microdiffraction pattern recorded by focusing the electron beam on the top surface of this nanobox. (D-F) HRTEM images taken from three different portions of this nanoshell that are labeled as I, II, and III in (C). These HRTEM images also confirm the single crystallinity of this gold nanobox.
be generated by reacting their salts with silver templates Pd(NO3)2(aq) + 2Ag(s) f Pd(s) + 2AgNO3(aq)
(2)
Pt(CH3COO)2(aq) + 2Ag(s) f Pt(s) + 2Ag(CH3COO)(aq) (3) The Pd and Pt nanoshells and nanotubes obtained using this method also exhibit morphologies similar to those of gold hollow structures shown in Figures 3 and 4. In summary, we have demonstrated a simple and versatile route to the large-scale synthesis of hollow nanostructures of noble metals whose voids and wall thicknesses could be controlled by varying the templates. The major requirement of this approach is the availability of templates that can reduce the salt form of a desired material. Given the fact that most metals have already been synthesized into nanoparticles or nanowires, the number of metals that can be processed as hollow nanostructures using this approach is potentially very large. Due to their well-controlled morphology and void size, these hollow nanostructures will find use in a number of applications that involve nanoscale encapsulation. Those made of coinage metals such as gold or silver 484
(1) See, for example: (a) Caruso, F. AdV. Mater. 2001, 13, 11. (b) Sauer, M.; Streich, D.; Meier, W. AdV. Mater. 2001, 13, 1649. (c) Bergbreiter, D. E. Angew. Chem., Int. Ed. Engl. 1999, 38, 2870. (d) Hollow and Solid Spheres and Microspheres: Science and Technology Associated with Their Fabrication and Application; Wilcox, D. L., Sr.; Berg, M.; Bernat, T.; Kelleman, D.; Cochran Jr., J. K.; Eds.; MRS Proceedings Vol. 372; Materials Research Society: Pittsburgh, PA, 1994. (e) Wooley, K. L. Chem. Eur. J. 1997, 3, 1397. (2) (a) Mathlowitz, E.; Jacob, J. S.; Jong, Y. S.; Carino, G. P.; Chickering, D. E.; Chaturvedl, P.; Santos, C. A.; Vijayaraghavan, K.; Montgomery, S.; Bassett, M.; Morrell, C. Nature 1997, 386, 410. (b) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (c) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (3) Ohmori, M.; Matijevic, E. J. Coll. Interface Sci. 1992, 150, 594. (4) (a) Sarkar, D.; Halas, N. J. Phys. ReV. E 1997, 56, 1102. (b) Neeves, A. E.; Birnboim, M. H. J. Opt. Soc. Am. B 1989, 6, 787. (5) (a) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2473. (b) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Chem. Phys. Lett. 1999, 300, 651. (6) The plasmon peaks of silver or gold nanoparticles could also be tuned in the visible region by increasing their aspect ratios or changing their morphological shapes. See, for example, (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (c) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (d) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (e) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (7) (a) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. J. Biomed. Mater. Res. 2000, 51, 293. (b) West, J. L.; Halas, N. J. Curr. Opin. Biotech. 2000, 11, 215. (8) (a) Philipse, A. P.; van Bruggen, M. P. B.; Pathmananoharan, C. Langmuir 1994, 10, 92. (b) Chang, S. Y.; Liu, L.; Asher, S. A. J. Am. Chem. Soc. 1994, 116, 6739. (c) Giersig, M.; Ung, T.; LizMarza´n, L. M.; Mulvaney, P. AdV. Mater. 1997, 9, 570. (d) Yao, H.; Takada, Y.; Kitamura, N. Langmuir 1998, 14, 595. (e) Ung, T.; LizMarza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. (9) (a) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (b) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (c) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 8523. (d) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (e) Marinakos, S. M.; Novak, J. P.; Brousseau III, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (f) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (10) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (11) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (12) (a) Caruso, F.; Spasova, M.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M. AdV. Mater. 2001, 13, 1090. (b) Ji, T.; Lirtsman, V. G.; Avny, Y.; Davidov, D. AdV. Mater. 2001, 13, 1253. (13) Silver nanoparticles were synthesized using the polyol process (Silvert, P.-Y.; Herrera-Urbina, R.; Duvauchelle, N.; Vijayakrishnan, V.; Elhsissen, K. T. J. Mater. Chem., 1996, 6, 573). Silver nanowires were synthesized using a solution-phase approach recently demonstrated by our group (Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Nano Lett., Vol. 2, No. 5, 2002
Lett. 2002, 2, 165). By changing the experimental conditions, cubic silver nanoparticles were also obtained with relatively high yields (unpublished results). In a typical replacement reaction, a 250-µL aliquot of the as-synthesized silver nanoparticles or silver nanowires was diluted with 5 mL water and then refluxed at boiling temperature for 10 min. Aliquots of 1 mM HAuCl4 (99.9%, Aldrich) aqueous solution were added dropwise to the refluxing solution. This mixture was continuously refluxed until its color became stable. Vigorous stirring was maintained throughout all syntheses. A similar procedure was applied to form nanoshells of palladium and platinum using Pd(NO3)2‚xH2O (99.9%, Alfa Aesar) and platinum acetate (98%, City Chemical LLC) as precursors. The UV-vis absorption spectra were recorded on a Hewlett-Packard 8452A spectrometer (Palo Alto, CA). The transmission electron microscopy (TEM) samples were prepared by placing small drops of reaction solutions (diluted by ∼5 times with water) on copper grids, and allowing the solvent to slowly evaporate at room temperature in a fume hood. The TEM images and electron diffraction patterns were taken using a JEOL microscope (1200EX II) operated at 80 kV. The electron beam spot was ∼100 nm in diameter. The high-resolution TEM (HRTEM) images were
Nano Lett., Vol. 2, No. 5, 2002
captured using a TOPCON 002B microscope operated at 200 kV. The scanning electron microscopy (SEM) images were obtained using a JEOL field-emission microscope (6300F) operated at 15 kV. (14) Roosen, A. R.; Carter, W. C. Physica A 1998, 261, 232. (15) CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, R., Eds.; CRC Press: Florida, 1981. (16) The dependence of solubility product on temperature is:
ln
k1sp k2sp
)-
(
∆H0 1 1 R T2 T1
)
(17) See, for example, Kerker, M. J. Colloid and Interface Sci. 1985, 105, 297. (18) (a) Bohren, C. F.; Huffman, D. R. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983. (b) Kreibig, U.; Gartz, M.; Hilger, A. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1593. (c) Mulvaney, P. Langmuir 1996, 12, 788. (19) Song, J. H.; Wu, Y.; Messer, B.; Kind, H.; Yang, P. J. Am. Chem. Soc. 2001, 123, 10 397. (20) Sun, Y.; Xia, Y., unpublished results.
NL025531V
485
