Metallic Double Shell Hollow Nanocages: The Challenges of Their

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Metallic Double Shell Hollow Nanocages: The Challenges of Their Synthetic Techniques M. A. Mahmoud* and M. A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States ABSTRACT: Hollow metallic nanoparticles have been attracting the attention of many researchers in the past five years due to their new properties and potential applications. The unique structure of the hollow nanoparticles; presence of two surfaces (internal and external), and the presence of both cavities and pores in the wall surfaces of these nanoparticles are responsible for their unique properties and applications. Here the galvanic replacement technique is used to prepare nanocages made of gold, platinum, and palladium. In addition, hollow double shell nanoparticles are made of two metal shells like Au−Pt, Pt−Au, Au−Pd, Pd−Au, Pd−Pt, and Pt−Pd. Silver nanocubes are used as templates during the synthesis of hollow nanoparticles with single metal shell or double shell nanocages. Most of the problems that could affect the synthesis of solid Silver nanocubes used as template as well as the double shell nanocages and their possible solutions are discussed in a detail. The sizes and shapes of the single-shell and double-shell nanocages were characterized by a regular and high-resolution TEM. A SEM mapping technique is also used to image the surface atoms for the double shell hollow nanoparticles in order to determine the thickness of the two metal shells. In addition, optical studies are used to monitor the effect of the dielectric properties of the other metals on the plasmonic properties of the gold nanoshell in these mixed nanoparticles. scattering, absorption, and fluorescence processes.21 Plasmonic hollow nanoparticles have two surfaces (and thus have two plasmon fields inside and outside of the nanocage).22 The coupling between these two fields is responsible for the tunability of their surface plasmon resonance spectra in the visible and the NIR regions18,22 and leads to the enhancement of the overall plasmon field.18,23 Due to the small nanovolume of the cavity, these hollow nanoparticles are efficient nanocatalysts.24,25 Recently, we have shown that the new nanoreactor phenomenon in which the nanocage catalyst confines the reacting material inside the cavity of the nanocatalyst and as a result greatly enhancing the efficiency of the catalyst.26,27 The first method that has been used to synthesize the hollow nanoparticles was a template-mediated method.5 This method is based on the use of a template nanoparticle made up of polymer beads or silica and then coating their surface with the metal layer of the cage. The inside template is etched away and an empty metallic nanocage is formed. The limitation of this method is to synthesize hollow nanoparticles of sizes smaller than 100 nm, which is hard to make.28 In addition, the surface of the nanocages prepared by this method is not smooth as well as post treatment was required to remove the internal template material. Yin et al. prepared hollow nanoparticles by different

1. INTRODUCTION In the last few decades, a wide range of different sizes and shapes of metallic nanoparticles have been prepared;1 such as nanospheres,2 nanorods,3,4 nanocubes,5−7 nanostars,8 and so forth. The nanoscale size and shape of the nanoparticle are responsible for the unique optical and physical properties.9 Because of the small size of the nanoparticle; it has high surface to volume ratio and this makes the nanoparticle an efficient nanocatalyst and a valuable shuttle for transferring biological materials and drugs.10 In catalysis, the large surface to volume ratio increases the area exposed to the reacting materials and results in increased efficiency of the catalyst.2,11 In addition, atoms on edges or corners are chemically unsaturated and become very chemically active centers. In drug delivery, the high surface area of the nanoparticles leads to the increase in the number of bound drug or biological molecules on their surface.12 Moreover, the conjugated size of the nanoparticles increases the rate of their uptake by the cells (endocytosis). All of this increases the rate of drug action in destroying the cancer cells in chemotherapy.13 The most attractive properties of gold and silver metallic nanoparticles are a result of their surface plasmon resonance phenomena, which involves the collective excitation of the free conduction band electrons in resonance9,14−17 with an optical photon. The coherent resultant oscillation of their conduction band electrons gives rise to a very strong surface electromagnetic field. This strong electromagnetic plasmon field leads to the enhancement of both Raman18−20 and Rayleigh © 2012 American Chemical Society

Received: October 11, 2011 Revised: January 7, 2012 Published: January 12, 2012 4051

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methods based on the Kirkendall effect.29,30 The Kirkendall effect involves a nonequilibrium mutual diffusion of two touching metals through the interface between them. A resulting vacancy diffusion occurs in order to compensate for the difference in diffusion flow rates between the two metals. The disadvantage of this method was that the hollow nanoparticles were polycrystalline. Currently, the most common method used for the synthesis of the hollow nanocages is the galvanic replacement method, which was developed by Sun and Xia.5 Different sizes and shapes of hollow nanostructures have been prepared by this method.31−33 This method is based on the use of a sacrificial nanoparticle template and then adding a metal ion to it that is reduced at the expense of more than one atom of the template resulting in a hole inside the template. In this work, we have discussed the method of synthesis of cubic shaped hollow nanoparticles made up of one metal shell or by two different metal shells prepared by the galvanic replacement method. The different problems that researchers could face during the synthesis and their practical solutions are discussed and the prepared hollow materials are characterized by optical and imaging electronic microscopy.

with a stream of cold water. The particle size of AgNCs can also be controlled by adjusting the weight of the PVP capping polymer from 0.35 g to be in the range of 0.3 g to 0.45 g (as the amount of PVP increases the particle size decreases). 2.2. Cleaning of the AgNCs Template. In order to clean the AgNCs from the solvent and the extra PVP; the AgNCs solution was diluted with twice its original volume with water−acetone mixture and centrifuged at 13 000 rpm for 5 min. The supernatant AgNCs were precipitated down and redispersed in 20 mL of deionized water (DI). The acetone−water ratio required for precipitation increases as the size of the nanoparticle decreases. The reason for increasing the amount of acetone as the particle size decreases is that; acetone decreases the stability of AgNCs and so facilitates their precipitation during the centrifugation. If the amount of acetone is higher than what it should be, then AgNCs will aggregate. The stability of the nanoparticles (during the centrifugation) decreases as their size increases because the probability of aggregation increases with increasing the size of the nanoparticles. Therefore, as the size of AgNCs decreases, more acetone was required. 2.3. Synthesis of Gold Nanocages. Gold nanocages (AuNCs) with different wall lengths and wall thicknesses have been prepared from a silver nanocube template by the galvanic replacement technique.5,35,36 In order to tune the size of the nanocage, the particle size of the template should vary accordingly. The synthesis procedure of the AuNCs from AgNCs is valid for all different particle sizes. Ten mL from the 20 mL cleaned AgNCs was diluted to 200 mL with DI water and brought to boiling and stirred with stirring rate of 300 rpm. Hydrogen tetrachloroaurate (HAuCl4, Sigma-Aldrich) solution (0.1 g/ L) was injected slowly into the hot AgNCs solution until the SPR spectrum peak of the solution shifted to 600 nm.36 In order to increase the wall thickness of the AuNCs and shift the SPR peak position to the red, HAuCl4 solution with the concentration of 0.01 g/L was then injected into the solution of AuNCs. The AuNCs solutions were then continuously refluxed until their absorption spectrum became stable. Then the solution was cooled down to room temperature and left for 2 days to allow the AgCl byproduct to settle down and precipitate. The AgCl precipitate was removed by decantation of the AuNCs solution. In order to clean the AuNCs cavity from the AgCl salt, if any are left, the solution of AuNCs was sonicated for 30 min, and then left for a day until all of the AgCl precipitates settled to the bottom of the solution. The solution was centrifuged at 10 000 rpm for 10 min after separation of AgCl by decantation. Finally, the AuNCs precipitate was dispersed in DI water. 2.4. Synthesis of Platinum and Palladium Nanocages. In order to prepare platinum nanocages (PtNCs), 2 mL of the aqueous solution of the cleaned AgNCs was diluted with 20 mL of DI water in a 30 mL vial. Potassium tetrachloroplatinate (II) (K2PtCl4) with a concentration of 0.05 g/10 mL DI water was added to the AgNCs solution with the rate of 0.5 mL per 5 min, with constant shaking until the SPR peak of the AgNCs shifted to red (∼550 nm) and the solution became reddish. Afterward, a dilute solution of K2PtCl4 (0.005 g/10 mL DI water) was added (0.5 mL every 5 min) until the solution turned black. The same procedure was used to prepare palladium nanocages (PdNCs) but sodium tetrachloropalladate (II) (Na2PdCl4, Sigma-Aldrich) with a concentration of 0.02 g/10 mL of DI water was used instead of the platinum salt. The palladium salt was added until the color of the solution turned black, no need to use two different concentrations of the palladium salt as in the case of platinum. The synthesis of both PtNCs and PdNCs takes place in a cold solution. In order to clean PtNCs and PdNCs, the same cleaning procedure of AuNCs was used. The main difference between PtNCs and PdNCs is that latter becomes unstable and settles down if the excess palladium salt solution is left in the solution, therefore addition of palladium salt should be carefully monitored. In order to increase the stability of the PdNCs after cleaning from the byproduct, 0.05 g of PVP (MW ≈ 55 000 g) was added and the solution shook vigorously until the polymer dissolved completely. Different sizes of PtNCs and PdNCs can be prepared by this procedure by changing the size of the AgNCs template.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Silver Nanocube Template. In a 100 mL round-bottomed flask, 35 mL of anhydrous ethylene glycol SigmaAldrich (EG) was stirred at 400 rpm and heated at 140 °C for 1 h in an oil bath (the oil bath is for uniform heat distribution) in order to remove residual water in the EG. The EG has to be highly pure and the flask well-cleaned with both piranha solution (30% H2O2 and 70% H2SO4) and then aqua regia (small contamination leads to negative results). The flask was covered with vacuum adaptor with opening of 5 mm diameter. After 1 h heating of the EG, 0.35 g polyvinyl pyrrolidone (PVP) (molecular weight of ∼55 000 g, Sigma-Aldrich) dissolved in 5 mL EG was added at once to the reaction mixture. The temperature of the reaction mixture was then raised gradually until it reached 155 °C. At this temperature, 0.4 mL of a 3 mM solution of sodium sulfide in EG was added after 5 min from the addition of PVP. The solution of sodium sulfide must be prepared 1 h before injection into the reaction mixture. However, the dissolution of Na2S in EG took 1 h to be ionized into HS− and the CH2OH−CH2O− ions. While NaHS salt can be used as a source of sulfide ion, in this case the dissolution and ionization time are decreased 5 min only. The size of the AgNCs can be tuned by adding a AgNO3 solution with different concentrations. For example, a stock AgNO3 solution (0.48 g AgNO3 dissolved in 10 mL EG) was prepared and 3 different volumes were added 2, 2.5, and 3 mL in order to prepare AgNCs with wall length of 50, 60, and 80 nm, respectively. After adding the AgNO3 solution, the stirring rate was reduced to 100 rpm and the reaction mixture turned into dark brownish-yellow. This dark color is due to Ag2S resulting from the reaction of sulfide ions with silver ions. Ag2S then acts as a catalyst for the reduction of Ag+ into silver metal.34 When the color becomes light yellow (the color of single crystals silver small seeds), the stirring was stopped and the reaction flask was opened until the solution became slightly turbid (the reason of stopping the stirring is to let the seeds to grow). Then the reaction mixture was stirred again with a speed of 400 rpm until the entire solution becomes non transparent. The size of the AgNCs can also be controlled by increasing the time of heating after the solution becomes turbid and the SPR peak position was measured with time, as the SPR peak position red-shifts the nanoparticle size increases (the size of AgNCs increases by 5 nm every 30 s). This is why controlling the size by this method is a little risky because if the AgNCs are overheated, then different shapes would be formed with the AgNCs such as wires which appear after 7 min. If the reaction is occurring rapidly (as observed from the rapid color change and the shift of the SPR band), then the growth of the AgNCs can be stopped by cooling the reaction mixture 4052

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Figure 1. (A) SEM of silver nanocubes with a size of 86 nm (the scale bar is 100 nm). (B) SPR of AgNCs with a size of 25, 39, 52, and 86 nm. AgNCs have three plasmon peaks and a shoulder, the peaks shift to higher energy as the nanoparticle size decreases.

nm,39−42 and this allows synthesis of hollow nanostructures with different sizes. (3) AgNCs are prepared with a high concentration yield compared with any other nanoparticles, and thus facilitate their cleaning and handling.41 (4) The AgNCs are stable and can be stored over a year without shape change in the EG solvent; the possible reason for the stability of AgNCs would be attributed to the EG solvent, the PVP capping polymer wraps around the AgNCs as well as the EG is a reducing agent so the surface oxidation is not possible. (5) The PVP polymer that is capping the AgNCs template is compatible with most of the metallic nanoparticles, therefore during the synthesis of the nanocages, the capping of the AgNCs template also works with the newly prepared nanocage preventing their aggregation. (6) Silver atoms in AgNCs can be easily oxidized into monovalent silver ions by any metal ion with the higher or comparable oxidation potential of silver. AgNCs have been prepared from the reduction of silver nitrate by EG in the presence of PVP as a capping agent; as mentioned in the Experimental Section.37 Some important observations during the synthesis of AgNCs are as follows: (1) During the synthesis of the AgNCs, a certain volume of EG solvent was heated at 140 °C for one hour, but if the temperature rises above 150 °C, a dark turbid solution is produced instead of AgNCs (big undefinable shape of microscale size silver particle). (2) Covering the solution tightly after adding the silver salt solution prevents the removal of water byproduct, so the reaction will not finish in the right time required for the reaction. Also, the water oxidizes silver into a white precipitate of silver oxide that particulate settles down. The container of the reaction mixture has to be opened once in a while in order to allow the water resulting from the reduction of silver ions by EG43 to escape outside the container of the reaction. (3) Stirring vigorously will not allow for the small nuclei of silver to grow and form the cubic shape nanoparticles, rather the reaction ends up with small sized, asymmetric shaped nanoparticles. Therefore, it is recommended to stir the reaction mixture after injection of the silver ions solution gently (100 rpm). (4) Silver nitrate salt solution used in the synthesis of AgNCs has to be freshly prepared (less than 10 min after dissolving AgNO3 into EG), because EG reduces silver ions into a brownish color silver nanoparticles with

2.5. Shell−Shell Nanocages. Shell−shell nanocages can be prepared with different sizes as well as thicknesses of the two shells, e.g., Au−Pt, Au−Pd, Pt−Pd, Pt−Au, Pd−Pt, and Pd−Au (the outer shell made of the first metal). The outer shell of these shell−shell nanocages can be synthesized using the same procedure as that of a pure nanocage. In the case of the outer shell being made of gold, the thickness of the gold nanolayer can be controlled by increasing the amount of the gold salt added to the AgNCs template, as in the case of AuNCs synthesis. The SPR peak position gives an indication of the thickness of the gold layer, as the thickness of the gold layer increases the SPR shifted to red. In order to synthesize Au−Pd and Au−Pt shell−shell nanocages solutions of K2PtCl4 (0.05 g/10 mL) and Na2PdCl4 (0.02 g/10 mL) were added to the AuNCs solution, after cleaning as shown above. The SPR peak of AuNCs shifted to red as the amount of K2PtCl4 or Na2PdCl4 increased due to the replacement of the remaining silver inside the AuNCs with Pt or Pd inner shell. The SPR peak position was measured as a function of the volume added from the salt solution until the SPR peak position became constant and was not shifting any more. For the synthesis of Pt−Pd, Pt−Au shell−shell nanocages, K2PtCl4 (0.05 g/10 mL) solution was added to the AgNCs until the SPR peak shifted to ∼550 nm, then the nanoparticles were cleaned as in the case of pure AuNCs. Finally, 2 mL of HAuCl4 (0.01 g/L) or Na2PdCl4 (0.02 g/10 mL) was added to the PtNCs shell. The same procedure was carried out to prepare Pd−Pt and Pd−Au shell−shell nanoparticles. However, 2 mL of HAuCl4 (0.01 g/L) and H2PtCl4 (0.02 g/10 mL) were added to the PdNCs sample with an SPR peak spectrum at ∼550 nm. Shell−shell nanocages are cleaned by the same method as the pure nanocages. A JEOL 100C Transmission Electron Microscope (TEM) was used to image the prepared nanoparticles, while JEOL 4000EX was used for the high resolution TEM measurements. Zeiss Ultra60 was used for the scanning transmission electron microscopy and X-ray energy dispersive (STEM-XEDS) measurement.

3. RESULTS AND DISCUSSION 3.1. Silver Nanocube Template. Silver nanocube has been used as a template for synthesis of many cubic hollow materials by galvanic replacement,23,26,37 due to the following reasons: (1) Since AgNCs are prepared at high temperature, they have been proven to be a single crystalline structure,38 a single crystalline template produces single crystalline hollow nanoparticles, especially if the lattice parameters of silver and hollow metal material matching one another. (2) The wall length of AgNCs can be easily tuned from 20 nm up to 250 4053

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Figure 2. TEM images of AuNCs I (A), AuNCs II (B), and AuNCs III (C) prepare from the same AgNCs template by adding different amounts of gold salt solution, the gold concentration increases in the order of AuNCs (I, II, and III). The scale bar is 100 nm.

3.2.1. Synthesis of Nanocages. Pure Nanocages. Galvanic replacement method is the most convenient method used to synthesize nanocages.5,26 Six requirements should be considered in order to use this synthetic technique: (1) The nanocage takes the same shape as the template, so the suitable template shape should be chosen. (2) The capping material of the template should be compatible with the metal that makes the nanocage to prevent the nanocage aggregation. (3) The lattice parameters of the template should be close to the lattice parameters of the nanocage material, to produce a single crystalline nanocage. (4) The valency of the nanocage metal must be higher than the valency of the soluble ionic form of the template metal. This ensures that one atom of the nanocage material replaces more than one atom of the template, leading to the generation of a space inside the nanocage (nanocage cavity). (5) The oxidation potential of the template atoms must be higher than that of the nanocage material. (6) The surface of the template should be cleaned as much as possible without aggregating the nanoparticles. 3.2.1.1. Gold Nanocages. In order to make cubic gold nanocages (AuNCs), silver nanocubes were used as a template because they satisfy all of the requirements mentioned above. In principle, the gold nanocages are obtained by adding gold ions (Au3+) to AgNCs; however, the optical, crystal, and mechanical properties of the AuNCs depends on three factors: (1) The rate of injection of gold ions to the silver template. Three silver atoms are oxidized and replaced by one gold atom; if the gold salt added with low concentrations as compared with the concentration of AgNCs template, a thin layer of gold and gold−silver nanoalloy will be built on the surface of AgNC template. With increasing the concentration of gold salt, a porous AuNC will be produced that has pores on its edges and corners, not on the walls as it should be. This kind of AuNC is not thermally and mechanically stable. In order to overcome this problem; two solutions of gold salt with different concentrations are used; one with a high concentration added first and the other is with low concentration added afterward. The higher concentration of gold solution was added first, which reacts with the edges and corners forming a poreless corners and edges of gold (gold ions attack corners and edge because it is more thermodynamically active as mentioned before), then the lower concentration of gold ions solution added, which replaces the silver inside the nanocage with gold. (2) The temperature of the solution of the AgNC template and the rate of stirring should also be controlled. In fact, the reaction of gold ions and silver atoms takes place even at room temperature, but at low temperature, the reaction will be slow, which causes the formation of most likely Au−Ag alloys. Low rate stirring also has a bad effect since the gold salt will concentrate at a certain spot in the solution, finishing the entire silver template on that area, while the rest of the solution will

different sizes (because there is no capping material to control the growth). Therefore, it is favorable to ground AgNO3 salt with a mortar before dissolving into EG solvent in order to decrease the time required for the dissolution of the silver salt. (5) Sulfide salt should be dissolved in EG one hour before its use for dissociation and ionization as mentioned in the Experimental Section. (6) During the synthesis of AgNCs, after the reaction mixture solution turns opaque yellowish (end of the reaction), the reaction container should be removed from the oil bath immediately, uncovered, and shaken for a minute to prevent the formation of octahedral and wire-shaped silver nanoparticles. Figure 1A shows the SEM image of AgNCs prepared by using 3.2 mL of AgNO3 solution, procedure and composition of the other reagents used mentioned in the Experimental Section. The wall length of the AgNCs is 86 nm. Figure 1B shows the SPR spectrum of AgNCs with different size, the size was controlled as shown in the Experimental Section. AgNCs of 86 nm have four plasmon peaks, three at 470 and 390 nm 350 in addition to a shoulder peak at 446 nm. As the AgNCs particle size decreases, the SPR peaks blue shift, e.g., the sharpest peak shifts from 470 for 86 nm AgNCs to 450, 440, and 425 nm when the AgNCs particle size decreases to 52, 39, and 25 nm, respectively. Cleaning of AgNCs is a critical step during the synthesis of nanocages; since AgNCs are prepared in EG solution combined with different byproduct ions and excess capping material.41 Before using silver nanocube as a template for synthesis of nanocages or shell−shell nanocages, it has to be cleaned from the contaminating materials and then transferred into an aqueous solvent.18,37 Not only should the excess capping material be removed from the solution of AgNCs template, but also the capping material on its surface has to be removed as much as possible without inducing aggregation. As mentioned in the Experimental Section, the surface of AgNCs covered with PVP, the density of PVP increases at the edges and corners of the nanocube more than at the faces. The reason for the high density of PVP at the corners is that the atoms of the corners and edges surrounded with less number of atoms compared to the atoms in the faces and internal atoms. Therefore, the atoms present in the corners and the edges of the AgNC template are more thermodynamically active, and this makes PVP adsorb on the corners and edges more than the faces to nullify the potential charge on the corners and edges. The shape and morphology of the resulting nanocage depend on the degree of cleanness of the AgNCs. AgNCs are stable in EG solvent for a year, while the shape of the cleaned nanoparticles in water solvent will change after a few days, so it is recommended to use the clean AgNCs right after cleaning to avoid the shape change which starts with truncation of the corners due to oxidation. 4054

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platinum nanocage synthesis, each platinum ion displaces two silver atoms, whereas for gold nanocages, each of the three silver atoms are replaced by one gold atom. On the basis of these differences, if the same procedure used for the synthesis of AuNCs were used for the PtNCs; a core−shell of silver− platinum was obtained with an unsmooth surface, as reported.44 In order to prepare PtNCs from the AgNC template, the following requirements should be satisfied: (1) The surface of the AgNCs template should be cleaned (as in case of AuNC synthesis). (2) Two solutions of Pt salt should be prepared and added to the solution of AgNCs, the first is concentrated and the second is diluted. The concentrated solution was added first and the diluted solution was added after the SPR of AgNCs was red-shifted to ∼500 nm. However, when the concentrated solution was added to the AgNCs template; a porous continuous layer of platinum on the top of the AgNCs template was formed. If a diluted solution was added first, then islands of platinum were formed on the surface of the template, in further addition of platinum salt solution, the islands were grown more and more, forming a highly rough surface. (3) The galvanic replacement should take place in the cold solutions; because if the solution of Pt salt added to a hot solution of a template, the reaction will be fast, and due to the less lattice matching between Pt and Ag, Pt will build on the surface of AgNCs randomly and the surface will be highly rough.44 Figure 4 shows the SPR spectra of the AgNCs template after mixing with platinum salt as a function of time. The

not be affected by the gold ions. Therefore, the formation of different sorts of AuNCs with different SPR peaks and thus the SPR spectrum will be broad. In order to obtain a uniform AuNCs; it is preferable to add Au salt solution drop by drop or by injection inside the AgNCs solution with vigorous stirring. (3) Degree of cleaning of the surface of the template, since AgNCs are prepared at high temperature (∼150 °C) the surface is well-capped with PVP; and the density of PVP is concentrated at the corners and edges of the AgNCs more than the faces. If gold ions add to the less clean template surface, then gold will react with the faces of the template first and build up a gold layer on the faces and then on the edges resulting in corner-less AuNCs production. The reason could be that after gold atoms start building up on the edges and on the faces of AgNCs; silver atoms present in the corners become isolated and surrounded by the gold atoms present at the edges and faces. Each three silver atoms in the corner replaces with one gold atom, this leads to a size reduction of the corners and a corner-less AuNC will produce.5,38 Different gold nanocages with different SPR spectra peak position can be prepared by this method from the same AgNC template.26,33 Figure 2A−C shows the TEM images of AuNCs prepared from the same AgNCs template, the amount of gold salt solution added to AgNCs increases in the order of AuNCs I, AuNCs II, and AuNCs III, respectively. In addition to the morphological change, the SPR spectra peak position of the prepared AuNCs shifts to red by increasing the amount of gold salt added to the AgNCs template. Figure 3A shows the SPR spectrum of AgNCs template and AuNCs

Figure 3. (A) SPR of AgNCs and AuNCs, the SPR spectrum red shifts as the amount of gold salt added to the AgNCs template increases. (B) STEM-XEDS elemental map image for AuNCs I, the gold atoms present outside the nanocage and some silver atoms appear on the surface. The scale bar is 100 nm.

Figure 4. SPR spectra of AgNCs, Ag−Pt core−shell, and PtNCs, the SPR shifts to red and become much broader as the amount of Pt salt increases.

with increasing the amount of gold ions. Figure 3B shows the STEM-XEDS elemental map imaging for AuNCs I, based on this map AuNCs outer surface made of gold atoms and the holes in the wall has some silver atoms. 3.2.1.2. Platinum Nanocages. Despite the theory that synthesis of platinum nanocages (PtNCs) is similar to that of the AuNCs (galvanic replacement method), the synthesis procedure is partially different for the following reasons: (1) On using AgNCs as a template for PtNC synthesis, the rate of the reaction of the platinum ions and silver atoms in AgNCs template is slower than that for reaction of gold ions with silver atoms. However, the reduction potential of Au (1.5 V) is higher than Pt (0.73 V). (2) The lattice matching between Ag and Pt is not good as in the case of Ag and Au. (3) During the

concentrated platinum salt solution is added first and then the solution is shaken and the shift in the SPR spectrum peak of AgNCs is monitored with time. The SPR red-shifts and the peaks become broader with time until a broadband spectrum corresponding to the formation of PtNCs is obtained at the end of the galvanic replacement reaction. Figure 5A shows the TEM image of PtNCs with a wall length of 89 nm; however, AgNCs with a wall length of 86 nm is used as a template during the galvanic replacement method. The size distribution of the PtNCs greatly depends on the size distribution of the AgNCs template. Figure 5B shows the high resolution TEM of the wall of PtNCs, the inset panel is the electron diffraction pattern of PtNCs, and it seems that the PtNCs is a single crystalline material. 4055

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Figure 5. (A) TEM image of 89 nm PtNCs prepared with the galvanic replacement method from 86 nm AgNCs template. (B) HR-TEM of a PtNCs and the electron diffraction.

3.2.1.3. Palladium Nanocages. The galvanic replacement technique is also applicable for the synthesis of palladium nanocages (PdNCs) from AgNCs in which the silver atoms in AgNCs are replaced with palladium atoms.26,45 The method of synthesis of PdNCs is similar to PtNCs with some modification. The Xia group has prepared PdNCs by a galvanic replacement method using a hot solution of AgNCs and palladium salt, our procedure is based on carrying out the galvanic replacement at room temperature, in this case continuous wall of PdNCs nanocages with some pores can be prepared. The difference between the synthesis procedure of PdNCs and PtNCs is that: (1) The Pd salt adds during the galvanic replacement (replacing Ag atoms with Pd atoms) should be used in one concentration (diluted Pd solution) not as in case of PtNCs method which requires two different concentrations of Pt salt. (2) An external amount of PVP capping material has been added to the PdNCs after the galvanic replacement to improve the stability of the nanoparticles since in some cases the PdNCs become unstable after one or two days. The reason for changing the synthesis method is that the reaction of Pd ions with AgNCs is more active as compared to the Pt ions. Figure 6 shows the SPR spectrum of the AgNC template solution before and after adding Pd salt, the SPR spectrum peak

shifts to red by increasing the amount Pd salt added, the reaction between the AgNCs and Pd ions is fast enough, so any amount of Pd ions added to the AgNCs reacts immediately. PdNCs have an undefinable spectrum (only a background) due to the d to d electron transition (an interband transition spectrum). The shift of the SPR peak spectrum is due to the change in shape of AgNCs as well as the change in the dielectric of the surrounding, which results from the deposition of Pd on Ag. Figure 7A shows the TEM of PdNCs with particle size of 57 nm. The nanoparticles seem to be greatly uniform. The high resolution TEM image in Figure 7B showed that it is also a single crystalline and the wall looks smooth. 3.2.2. Shell−Shell Nanocages. Nanocages have exciting optical,23 biological,35 and catalytic26 applications. How about, if the empty nanocage made of two different metals arranged into two touch shell layers (shell−shell nanocages)? The optical and catalytic properties of these shell−shell nanoparticles should be addressed carefully, especially if one of the two shells has SPR and the other not, or if the two metals have different electrochemical potential. Recently, we prepared different shell−shell nanoparticles such as Pd−Pt, Pt−Pd, and Au− Pd.26 The idea of the synthesis method is also based on the galvanic replacement theory. AgNCs were used as a template for the shell−shell nanocage synthesis and the salt of the metal that is forming the outer shell was added first until a smooth continuous porous layer built on the surface of the AgNCs template. The interior shell was formed inside the cage by adding its metal ion salt solution of the interior metal shell. The interior shell forms inside by penetration of the ions inside the nanocage (the nanocage made of the exterior metal) through the pores in its wall replacing the remaining silver atoms left inside the nanocage and forming the interior layer. In order to prepare shell−shell nanocage the following requirements should be considered: (1) The surface of the template should be cleaned as much as possible before depositing the exterior layer, in order to obtain a continuous layer of the outer layer metal shell with some pores, as mentioned before in the case of the nanocages synthesis. (2) Some silver atoms should be left inside the nanocage, which is replaced by the interior layer of metal to form the interior shell. (3) The metal ions of the interior layer should not affect the exterior layer, e.g., aggregation or replacement. (4) The lattice parameters of both the metals forming the two shells have to match each other and also match with the lattice parameter of the template. (5) The capping material of the template should be suitable for both the metals forming interior and exterior shell.

Figure 6. SPR spectra of AgNCs, Ag−Pd core−shell, and PdNCs. The SPR peak of AgNCs shifts to red and becomes broader as the amount of Pd salt increases. PdNCs have no SPR peak but interband transition. 4056

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Figure 7. (A) TEM image of 57 nm PdNCs prepared with the galvanic replacement method from 52 nm AgNCs template. (B) HR-TEM of a PdNC and the electron diffraction.

3.2.2.1. Gold−Platinum and Gold−Palladium Shell−Shell Nanocages. The combination of plasmonic nanomaterials, e.g., gold with the catalytically active material such as Pt increases its efficiency especially if these two metals present in the same nanoparticle. In gold−platinum shell−shell nanocages (AuPtNCs), the combination between Au and Pt is through two touching shells, the outer-shell is gold and inner-shell is Pt. The AuPtNCs both have a plasmonic property that comes from gold and a catalytic property that comes from Pt (in some reactions gold can act as a catalyst too). In the catalysis reaction that involves electron transfer, the combination of gold and Pt could vary the electron transfer efficiency since gold and Pt have different electrochemical potentials. The plasmonic effect of gold can also improve the catalytic properties of the AuPtNCs if the SPR of gold in AuPtNCs excites during the reaction, because a local thermal heat generated by the gold can increase the rate of the reaction. Recently, we have prepared gold−platinum shell−shell nanocages (AuPtNCs) and used them in catalysis.27 In order to synthesize AuPtNCs, AuNCs should be prepared first and then the platinum salt should be added to it. In contradiction to the pure PtNCs synthesis, a platinum salt with a low concentration can be used. Figure 8A

thinner. Figure 8B shows the STEM-XEDS elemental map imaging of gold−platinum shell−shell nanocages with a wall length of 55 nm, the surface made of pure gold layer and the Pt atoms appeared in some points which is probably the holes in the wall. Different sizes and shapes of palladium nanoparticles have been prepared for catalysis purposes.1,26,46 One of these shapes is the palladium nanocage.26 A gold−palladium shell−shell nanocage (AuPdNCs) is prepared by the same procedure as AuPtNCs. AuPdNCs also have both plasmonic and catalytic properties. The technical concern that should be considered during the synthesis of AuPdNC is that PVP has to be added after the synthesis of AuPdNCs, because it is not highly stable compared with AuPtNCs which can be stored for over a year with no change in SPR. 3.2.2.2. Palladium−Platinum and Platinum−Palladium Shell−Shell Nanocages. The main purpose of making alloys is to improve the mechanical and physical properties of the materials. Alloying of two metals having catalytic properties could increase the catalytic efficiency of them. Both Pt and Pd catalyze many chemical and electrochemical reactions, e.g. the fuel cell reaction47 and Suzuki reaction in organic synthesis.46 The main problem that faces the Pt nanocatalyst is poisoning of its surface with CO which is present as an impurity with hydrogen fuel or which is produced as a byproduct (as in the case of methanol fuel cell). Possible solutions for the poisoning of the Pt are either covering its surface with a shell of other metal,48 or by depositing the few layers of Pt on the surface of other metals.49 Palladium−platinum shell−shell nanocage PdPtNCs offers a platinum surface with two advantages: (1) The two touching metal shells increase the efficiency of the nanocatalysts. (2) Confinement of the reacting materials in a small area, therefore increasing the catalysis efficiency due to the accumulation of the reacting materials in a nanoscale area. Our recent studies have shown that in catalysis with shell−shell nanocages, the metal forming the inside layer of the nanocage catalyst is the one that drives the reaction.26,27 Synthesis of PdPtNCs is similar to AuPtNCs, but the PdNCs prepared first and then the Pt salt is added. The Pt atoms replace the silver atoms left inside the PdNCs. Figure 9 shows the high resolution TEM of PdPtNCs shell−shell nanoparticles. Two layers are observed and in-between the two layers there is a layer of Pt−Pd alloy. Platinum−palladium shell−shell nanocages (PtPdNCs) have catalytic properties that differ from that PdPtNCs.23,24 Figure 10 shows the high resolution TEM of PtPdNCs similar to the PdPtNCs the nanoparticle has an outer layer of platinum and inner layer of palladium and Pt−Pd alloy layer in-between.

Figure 8. (A) SPR spectra of gold nanocages and gold−platinum shell−shell nanocages. (B) STEM-XEDS elemental map imaging of gold−platinum shell−shell nanocages.

shows the SPR spectrum of AuNCs with different wall thicknesses and pore sizes. Similar to the synthesis of pure AuNCs; as the amount of gold ions added to the silver template increases the wall thickness and the pore size of AuNCs change and therefore the SPR peak position shifts to red. After the Pt salt solution added to the AuNCs, the SPR of the AuNCs shifts to red and becomes broader due to the formation of the inside Pt shell. Moreover, the amount of red shift and the SPR bandwidth increases as the outer gold nanoshell becomes 4057

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Figure 9. (A) Magnified TEM image of a palladium−platinum shell− shell nanocage. (B) High-resolution TEM mage of PdPtNCs, the inset is the diffraction pattern for the PdPtNCs.

Figure 10. (A) Magnified TEM image of platinum−palladium shell− shell nanocage. (B) High-resolution TEM image of PtPdNCs, the inset is the diffraction pattern for the PtPdNCs.

4. CONCLUSIONS The synthetic procedures of pure nanocages made of gold, platinum, and palladium utilizing the galvanic replacement technique, were discussed. The PtNCs were prepared for the first time and PdNCs with smooth surface were prepared with a new procedure as well. The possible problems that could face the researchers during the synthesis of those nanocages and practical solutions to those problems have been reported in detail. The shell−shell nanoparticles, made of two touching metals, have been prepared for the first time. Moreover, the theory behind the optimization of the synthetic procedure of the shell−shell hollow nanoparticles has been discussed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.A.M.); melsayed@ gatech.edu (M.A.E.).

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ACKNOWLEDGMENTS This work was supported by DMR-NSF under Grant No. (DMR-0844082). We thank Mr. Justin Bradley for his proofreading of the manuscript.

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REFERENCES

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