Synthesis of Core-shell (Pd-Au) bimetallic ...

Synthesis of Core-shell (Pd-Au) bimetallic nanoparticles in microemulsions Eduardo Lariosa,c, Lilián Calderónc, Karen Guerreroc, Emanuel Pinedoc, Amir Maldonadob and Judith Tanoria* a

Depto. de Investigación en Polímeros y Materiales, Universidad de Sonora. Apdo.

Postal 130, 83 000, Hermosillo, Sonora, México. b

Depto. de Fisica, Universidad de Sonora, Apdo. Postal 1626, 83000, Hermosillo,

Sonora, México. c

Departamento de Ingeniería Química y Metalurgia, Universidad de Sonora. Apdo.

Postal 130, 83000, Hermosillo, Sonora, México. ∗ Corresponding author. Tel.: 52 + (662) 259-2161; fax: 52 + 662 259-2216 E-mail address: [email protected]

Abstract Palladium-gold core-shell nanoparticles were synthesized in the aqueous domains of water in oil microemulsions by the sequential reduction of H2PdCl4 and HAuCl4. The nanoparticles were characterized by UV-Vis spectroscopy and Transmission Electron Microscopy (TEM). The UV-Vis spectra confirm the presence of palladium nanoparticles after reducing H2PdCl4. These particles have been used as seeds for the core-shell particles. UV-Vis spectra show that, after reducing HAuCl4, the surface plasmon absorption of the nanoparticles is dominated by gold, revealing the encapsulation of the palladium seeds. These results agree with crystallographic analysis performed with High-Resolution TEM pictures, as well as with Selected Area Electron Diffraction. The TEM pictures show the core-shell nanoparticles with an average diameter of 9.1 nm, as compared with 5 nm for the palladium seeds, in good agreement with the used Pd:Au molar ratio.

Keywords: microemulsions, palladium-gold, nanoparticles, core-shell.

I. Introduction In the present days, the synthesis and the manipulation of nanometric entities has special relevancy in the so-called nanotechnology revolution1. The interest in nanometric-scale materials stems from their size and from the different physical properties they have, as compared to the same bulk materials. In this context, the synthesis of nanoparticles of different materials (metallic, semiconductor, dielectrics) has attracted a lot of attention due to their applications in catalysis, medicine, electronics, etc. Material scientists continuously look for methods that allow the control of the size and/or shape of the nanoparticles, as required by their applications. In this respect, the preparation of core-shell nanoparticles2-6 is of significance in nanotechnology due to the fact that their physical and chemical properties can be tuned by controlling parameters such as the size of the core, the thickness of the shell, as well as the overall shape of the particle. In some applications, it is desirable to have at the same time properties given by the material in the core (magnetism, density, etc) as well as properties provided by the material in the external surface, the shell (catalytic ability, optical properties, conductivity, etc). In this way, nanotechnology has the potential to create many new materials and devices with a wide spectrum of applications in fields like medicine7,8, catalysis9,11, photo-electronics12,13, energy production, and so forth. A number of methods have been used to prepare metallic nanoparticles. Physical methods involve processes such as vapor deposition14,15 and laser ablation16. The chemical methods start with the reduction of the metal ions to metal atoms. They are followed by the controlled aggregation of the material9. These methods perform the synthesis in different environments: microemulsions17-22, solution10,13,23, a solid matrix24, etc. They use a number of reducing agents, including alcohols, hydrazine, polyols, borohydrides, light, sound waves, etc. Particularly, methods of preparation of bimetallic nanoparticles can be divided into two categories: sequential and simultaneous reduction. The first one involves the consecutive addition of two different metallic precursors in the reaction system3,22. Nanoparticles of one material are created in the initial times of the reaction to form the core of the nanoparticle. Over this core, a shell of a different material is deposited in the second part of the reaction. In the case of a simultaneous reduction, the two metallic precursors are present in the reactor at the same time2,9.

In this work, we study the preparation of palladium-gold (Pd-Au) core-shell nanoparticles in water-in-oil microemulsions. We chose these materials because palladium has applications in catalysis and gold has a lot of applications in nanotechnology. For instance, it has been shown that Pd displays catalytic activity towards trichloroethene (TCE), a common hazardous organic contaminant of ground water.25 Furthermore, this activity increases as much as 70 times when palladium is supported on gold nanoparticles.10,25 Pd-on-Au nanoparticles have been successfully applied to the aqueous phase catalysis of TCE. This is a promising technology for ground water treatment.26 It is worth noting that gold-palladium alloy nanoparticles have been used in the direct synthesis of hydrogen peroxide (H2O2). The nanoparticles switch off the decomposition of H2O2, an undesirable effect of other catalysts.27 On the other hand, gold nanoparticles on their own have a lot of applications. For instance, in biology and nanomedicine, they are used as contrast agents for tissue visualization, for immunostaining, single particle tracking, gene delivery, etc. Recent reviews of these applications can be found in references.28,29 For all these reasons, it is interesting to device a system with the properties of gold and some, even if residual, catalytic activity given by palladium. As a first step towards this goal, in this paper we use the microemulsion method to synthesize Pd-Au core-shell nanoparticles. Our aims are to assess if this method is suitable for the preparation of this kind of material, and to characterize the obtained nanoparticles. The microemulsions are created in the ternary system water/Aerosol OT/isooctane. The nanoparticles are prepared by the sequential reduction of the tetrachloropalladate (II) and tetrachloroaurate (III) ions with hydrazine. The size, optical properties and core-shell formation were characterized by transmission electron microscopy (TEM), high resolution TEM (HRTEM), selected area electro diffraction (SAED) and UV/Vis spectroscopy. The remainder of the manuscript is divided as follows. In the next section we give the details of our experiments. In section III we present and discuss our results. Finally, in section IV we draw some conclusions.

II. Experimental section

Microemulsions

The core-shell nanoparticles have been synthesized in the aqueous domains of microemulsions of the ternary system water-aerosol OT-isooctane. The phase diagram of the system can be found elsewhere.30 Aerosol OT (AOT or sodium bis(2-ethylhexyl) sulfosuccinate) is a commercially available surfactant. It was purchased from SigmaAldrich. The oil phase in the microemulsion is isooctane, from Fluka. In addition, we used ultrapure water from a Milli Q system (Millipore). The nanoparticles were prepared in the reactor environment defined by the aqueous domains of reverse micelles. These micelles are water droplets suspended in isooctane, stabilized by an AOT monolayer. We performed the synthesis in micelles with a surfactant concentration of 0.1 M (in isooctane). Water was added to form the microemulsion, until a water-surfactant concentration ratio, w ≡ [H2O]/[AOT] = 5 was obtained. Preparation of Palladium seeds (metallic core). For the synthesis of Palladium seeds, metallic ions were introduced as an aqueous solution (0.2 M) of H2PdCl4, prepared by dissolving PdCl2 in a 0.2 N HCl solution

. 1 µL of this solution was

31,32

added per 1 mL of microemulsion. The reducing agent was hydrazine (1 µL per 3 mL of microemulsion). The reaction proceeded in the water pool after hydrazyne addition. The metallic precursor was reduced to its zero-valence state, and then the palladium nanoparticles formed by aggregation. These palladium particles have served as seeds for the core-shell nanostructures. Addition of a Gold layer (metallic shell). The obtained palladium nanoparticles were used as preformed cores for the preparation of core-shell bimetallic Pd-Au nanoparticles. An aqueous 0.2 M solution of HAuCl4 (2 µl per 1 ml of microemulsion) was added into the reaction system containing the Pd seeds under stirring. Afterwards, hydrazine was added to the system under vigorous stirring. The bimetallic nanoparticles were obtained as a purple-brown suspension. The Pd:Au molar ratio was 1:2. Preparation

of

Passivated

Core-shell

Bimetallic

Palladium-Gold

Nanoparticles. We passivated the surface of the Pd-Au nanoparticles in order to stabilize them against coagulation driven by attractive van der Waals forces. For this, we adsorbed the functional -SH thiol group of dodecanethiol onto the gold surface of the nanoparticles. When the palladium-gold particles were obtained, we added to the solution 9 µL of dodecanethiol per mL of microemulsion. Characterization of Pd and Pd-Au nanoparticles. The nanoparticles were characterized in a JEOL 2010F Transmission Electron Microscope. A drop of each

sample was placed on a carbon film supported by a copper grid in order to obtain TEM micrographs. We have further analyzed the core-shell structures by means of High Resolution TEM images and Selected Area Electron Diffraction (SAED). The optical properties of the samples were investigated by UV-Vis spectroscopy. The UV-Vis spectra were obtained with a Perkin- Elmer Lambda 2 spectrophotometer with the samples placed in 1x1x3 cm rectangular quartz cells.

Results The

prepared

H2O/AOT/Isooctane

microemulsions

are

homogeneous,

transparent liquids. Their UV-Vis spectra do not show any absorption band. These features allow to monitoring the nanoparticle growth by the change in color and by the UV-Vis spectra of the microemulsions used as chemical reactors. In these microemulsions we performed the sequential reduction of palladium and gold in order to obtain the core-shell nanoparticles.

Palladium Nanoparticles For the synthesis of the palladium nanoparticles, which served as seeds for the core-shell structures, Pd ions were reduced to metallic Pd in the presence of hydrazine. As the reaction proceeded, the color of the solution slowly turned from a subtle brown to a dark brown. We let the reaction proceed for a week. The UV-Vis spectrum of the Pd nanoparticles in suspension is shown in figure 1. The strong surface plasmon resonance band around 280 nm is indicative of the presence of palladium nanoparticles.33 A representative TEM image of the Pd particles is shown in figure 2. The reaction resulted in well dispersed nanoparticles with a relatively narrow size distribution. Their average diameter was 5.3 nm (standard deviation of 2.5 nm, with a coefficient of variation of 47 %). In order to characterize the palladium structure, we performed electron diffraction experiments. In figure 3(b), we present an electron diffraction pattern obtained for these nanoparticles. The diffractogram corresponds to the crystallographic structure of palladium nanoparticles oriented along different planes of the Pd atomic lattice. The diffractogram displays several reflections. The spots on rings 1, 2 and 3 are related to the lattice spacings 2.24 Å, 1.945 Å, and 1.375 Å, which correspond to the (111), (200), and (220) planes of the fcc structure of palladium (JCPDS file no. 46-1043). In figure 3(a) we present a HRTEM image where the lattice

spacing is 2.24 Å, corresponding to the (111) plane. As we explained before, these small Pd nanoparticles where used as seeds in order to deposit on them a gold shell. It is worth noting that Pd nanoparticles remained in suspension, without aggregation between particles. This behavior is probably due to the adsorption of the surfactant molecules on the surface of the nanoparticles.

Core-shell Palladium-Gold Nanoparticles The microemulsions containing the suspended Pd nanoparticles were used as reactors for the reduction of gold. When this new reaction took place, the color of the suspension changed from the previously attained brown to a purple-brown color. This modification was almost instantaneous. The UV-Vis spectrum of the resulting nanoparticle suspension is shown in figure 4. The observed plasmon resonance band around 530 nm is indicative of the presence of metallic gold. That is, it indicates the formation of a gold shell around the palladium nanoparticles. Even if the palladium seeds were obtained as isolated nanoparticles and had a relatively narrow size distribution, the particles obtained after adding gold, had different characteristics. As can be observed in figure 5a, the bimetallic nanoparticles formed large agglomerates. The coalescence is due to attractive Van der Waals forces between the gold-covered surfaces. The average diameter of the observed particles is 9 nm (standard deviation of 3.6 nm, with a coefficient of variation = 40 %). The coalescence observed in figure 5a is also very clearly revealed in the HRTEM images. In figure 6a we show two fused nanoparticles. The diffractograms obtained for the particles of figure 6b exhibit the Debye-Scherrer rings that correspond to the fcc structure of gold. The rings in the diffractogram can be indexed and correspond to what is expected for gold. In fact, spots on rings 1, 2, 3, 4, and 5 are related to the lattice spacings 2.35 Å, 2.039 Å, 1.442 Å, 1.230 Å, and 1.177 Å which correspond to the (111), (002), (022), (113) and (004) planes of the fcc structure of gold (JCPDS file no. 4-784). It is worth noting that some spots of diffraction in the vicinity of the first ring are rather related to the lattice spacing 2.24 Å, which corresponds to the (111) planes of the fcc structure of palladium. This means that the diffraction experiment is evidencing the presence of gold, with a residual signal for palladium, as expected for core-shell particles of these materials.

Passivated Core-shell Bimetallic Palladium-Gold Nanoparticles.

As shown in figure 5a, the gold-covered particles coalesce, due to van der Waals forces. In order to stabilize a suspension of isolated, individual nanoparticles, we covered their surface with thiol groups. Figure 8a shows TEM image of passivated bimetallic Pd-Au nanoparticles with an average diameter of 9.1 nm (standard deviation of 3.03 nm with a coefficient of variation = 33 %). This image shows that the particles disperse better as compared to the case of the bimetallic particles with the bare gold surface (figure 5a). Thus, the strategy of covering the particle surface with thiol groups prevents agglomeration and coalescence of the particles. The UV-Vis spectra for this passivated Pd-Au bimetallic system, figure 7, display a slight shift and a strong damping in the surface plasmon resonance band, due to the presence of the thiol groups in the surface. In figure 9a we show an HRTEM image for a passivated nanoparticle. We can appreciate one of the lattices spacings associated with gold: 2.039 Å.

Discussion. Our experiments confirm that the sequential reduction of Pd and Au in the aqueous domains of the AOT-isooctane-water microemulsions effectively leads to the formation of core (Pd) – shell (Au) nanoparticles. The first synthesized palladium seeds are successfully covered with a gold shell. From the UV-Vis spectroscopy experiments we confirm the modification occurring in the surface of the palladium seeds. It is known that Pd and Au nanoparticles have surface plasmon resonance bands around 280 and 520 nm, respectively. The formation of core-shell Pd-Au nanoparticles is deduced from the position of the adsorption band in the spectrum. The plasmon maximum is shifted towards the red region. This band was initially located at 280 nm, as expected for palladium, but after the addition of HAuCl4 and hydrazine, it shifted to 530 nm. This reveals the encapsulation of the palladium seeds in gold shells, since the first absorption is expected for Pd while the latter is near of that of gold. This result is in agreement with the prediction of Mulvaney et al5. It is expected that even one monolayer of gold is sufficient to mask the palladium plasmon resonance band completely. On the other hand, the absorption spectra of the passivated Pd-Au nanoparticles shows a drastic damping of the plasmon band. This damping is associated with the SH- chemisorption onto the gold surface, which involves changes in the free electron concentration.12 This implies demetalization of the metal nanoparticles, which causes the damping of the plasmon band.

Note that the electron diffraction patterns support the conclusions obtained with UV-Vis spectroscopy. The electron diffraction experiments performed with the Pd-Au nanoparticles yielded results compatible mostly with the gold structure. However, the patterns show some spots related with the palladium diffraction. In figure 9b, we superimpose and compare the spectrum of the palladium seeds (left) with that of the PdAu nanoparticles (right). In the second case we can distinguish the diffraction from both materials: gold and palladium. And this diffraction pattern is in agreement with what one would expect of a core-shell nanoparticle. The diffraction from gold is clear, but the palladium diffraction is less intense due to the fact that it is masked by the gold shell. In figure 10 we present a schematic representation of the core-shell nanoparticle, based on the actual sizes of the Pd and Pd-Au nanoparticles, as well as on the Pd and Au atomic radii. In this idealistic atomic model, the core-shell nanoparticle is composed of 7 atomic layers of palladium and 13 layers of gold.

Conclusions Core-shell Pd-Au nanoparticles have been synthesized by sequential reduction of H2PdCl4 and HAuCl4 in the presence of hydrazine. Both UV-Vis spectroscopy and Transmission Electron Microscopy revealed the formation of core-shell bimetallic nanoparticles. For the used Pd:Au molar proportion, the core-shell nanoparticles display a diameter around 8 – 10 nm. The electron diffraction patterns display the characteristics of gold, with a residual signal for palladium, in agreement with the UVVis results.

Acknowledgments.- E. R.-L. acknowledges a fellowship from Conacyt (Mexico). The TEM experiments were performed in the Laboratorio de Microscopía Electrónica of Universidad de Sonora. References 1. Wang, Z. L. J. Phys. Chem. B, (2000) 104:1153-1175. 2. Ferrer, D., Torres-Castro, A., Gao, X., Sepúlveda-Guzman, S., Ortiz-Mendez, U., Yacaman, M.J., Nano Letters (2007) 7:1701-1705. 3. Lee, W.R., Kim, M.G., Choi, J.R., Park, J.I., Ko, S.J., Oh, S.J., Cheon, J. J. Am. Chem. Soc. (2005) 127:16090-16097. 4. Wang, Y., Toshima, N. J. Phys. Chem. B (1997) 101:5301-5306.

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Figure captions Figure 1.- UV-Vis spectra of the palladium nanoparticles. AOT-water/H2PdCl4isooctane system, [AOT=0.1 M], [H2PdCl4] = 2x10-4 M, W= 5. The adsorption band around 280 nm corresponds to metallic palladium nanoparticles. Figure 2.- TEM micrograph (a) and particle size distribution (b) of the palladium nanoparticles. AOT-water/H2PdCl4-isooctane system, [AOT=0.1 M], [H2PdCl4] = 2x104 M, W= 5. These particles are used as seeds for the core-shell nanoparticles. Figure 3.- High Resolution TEM image of a palladium nanoparticle (a) and Selected Area Electron Diffraction pattern of one palladium particle. The crystallographic data obtained from the analysis of both pictures agrees with the fcc structure of palladium. Figure 4.- UV-Vis spectra of the core-shell palladium-gold nanoparticles Pd:Au 1:2. The curve displays a shoulder around 530 nm, very close to the expected value for gold (520 nm). The inset shows and amplification of the region around the adsorption maximum. Figure 5.- TEM micrograph (a) and particle size distribution (b) of the core-shell palladium-gold nanoparticles. Pd:Au 1:2. Note the strong tendency to agglomeration, due to the presence of gold in the external surface of the particle. Figure 6.- High Resolution TEM image of a core-shell nanoparticle (a) and corresponding Selected Area Electron Diffraction (b). The crystallographic data obtained from the analysis of both pictures agrees with the fcc structure of gold, except for some spots in the SAED picture corresponding to palladium. Figure 7.- UV-Vis spectra of the passivated core-shell nanoparticles. Note that the adsorption around 530 nm is damped (see figure 4) due to the presence of the thiol molecules in the external surface of the particles. Figure 8.- TEM micrograph (a) and particle size distribution (b) of the passivated coreshell nanoparticles. The picture shows that the thiol molecules effectively protect the particles against aggregation. Figure 9.- High Resolution TEM image of a passivated core-shell nanoparticle (a) and corresponding Selected Area Electron Diffraction pattern. As in the case of nonpassivated core-shell nanoparticles, the crystallographic data correspond to the fcc structure of gold, with a residual signal for palladium. Figure 10.-. Schematic rendering of the structure of a core-shell palladium-gold nanoparticle.

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