ag@pd core-shell nanoparticles - niscairnopr.niscair.res.in/bitstream/123456789/12777/1/ijca...

Indian Journal of Chemistry Vol. 50A, Sept-Oct 2011, pp. 1308-1317

Ag@Pd core-shell nanoparticles

Deepa Jose & Balaji R Jagirdar*

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India

Email: [email protected]

Received 13 May 2011; accepted 1 July 2011

Colloids of silver and palladium nanoparticles have been prepared by the Solvated Metal Atom Dispersion method. The as-prepared Ag colloid consisting of polydisperse nanoparticles is transformed into a monodisperse colloid by the digestive ripening process which involves refluxing the as-prepared colloid in the presence of a surfactant. In addition to the monodisperse nanoparticles, a small amount of an Ag-thiolate complex is also formed. Refluxing a mixture of the as-prepared Ag and Pd colloids results in Ag@Pd core–shell nanoparticles. The core-shell structure has been established using a combination of techniques such as UV-visible spectroscopy, high resolution electron microscopy, energy filtered electron microscopy, energy dispersive X-ray analysis, high angle annular dark field imaging and powder X-ray diffraction.

Keywords: Nanoparticles, Bimetallic nanoparticles, Colloids, Solvated metal atom dispersion, Digestive ripening, Core shell nanoparticles, Silver nanoparticles, Palladium nanoparticles, Silver colloid, Palladium colloid

Bimetallic nanoparticles have been attracting immense attention due to their novel physical and chemical properties arising from their size and cooperative effects1. The properties of a bimetallic system could be tuned by the kind of arrangement of individual atoms resulting in the formation of a random alloy, core–shell, or a cluster–in–cluster. Control of the architecture of a bimetallic system could be achieved by a judicious choice of starting materials, surfactants, and synthetic techniques2. Various synthetic strategies such as co-reduction of two metal ions, successive reduction, reduction of co-complexes have been employed for the preparation of bimetallic nanoparticles2. Recently, we showed that bimetallic nanoparticles can be prepared by refluxing together a mixture of the colloids of the respective metal nanoparticles3.

The Ag/Pd bimetallic system (random alloy or core–shell structures) has been attracting immense attention4. Ag and Pd are known to form solid solutions over a range of compositions5. The beauty of the Ag@Pd core–shell system is that the catalytic activity of Pd in this system could be monitored by means of the SERS activity of Ag4a. The partially formed Pd shell ensures the catalytic activity, whereas Ag ensures the SERS activity. Ag/Pd bimetallic systems were found to act as selective hydrogenation catalysts6. Pd-Ag alloys are important with regard to hydrogen diffusion because the Pd0.77Ag0.23 alloy is

employed industrially as a H2 purification membrane7. Ag/Pd nanoparticles also show promising application as a novel activator for electroless copper deposition4e. Motivated by these aspects, we became interested to study the Ag/Pd bimetallic system. Although various synthetic methods have been reported for the Ag/Pd system, they suffer from certain problems related to the scale up, reproducibility, and high monodispersity of particle size. Therefore, we undertook a study to develop a relatively simple synthetic methodology for the Ag/Pd bimetallic system.

We have been using the Solvated Metal Atom Dispersion (SMAD), a top-down synthetic approach of broad appeal in combination with the digestive ripening process to obtain highly monodisperse colloids of metal nanoparticles8. The SMAD method involves growth of metal atom clusters in organic solvents as matrix host material at low temperatures. In this method, co-deposition of metal atoms and a stabilizing solvent on the walls of a reactor maintained at –196 ºC followed by warm up of the matrix results in a slurry comprised of metal particles that grow in size. Addition of an organic ligand/surfactant halts the growth process. Digestive ripening of this as-prepared colloid consisting of polydisperse nanoparticles by heating it at or near the boiling point of the solvent renders it highly monodisperse9. This process is still not well

JOSE & JAGIRDAR: Ag@Pd CORE-SHELL NANOPARTICLES

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understood. The SMAD process offers advantages compared to the other methods for the preparation of colloids of metal nanoparticles such as easy scale up, high reproducibility, and avoidance of tedious purification procedures. Taking advantage of the SMAD method in combination with digestive ripening, highly monodisperse monometallic, bimetallic core–shell, and composite nanomaterials have been realized3,10. Hodak et al.

11 showed that irradiation of Au@Ag core–shell structures with laser light induces melting and the subsequent erasing of the core-shell structure via interparticle diffusion of atoms resulting in AuAg alloys. Annealing of α-brass@γ-brass core–shell particles at 260 °C results in β-brass as reported by Suzuki and Ito12. We showed that kinetically stable Cu@ZnO and Au@Pd core–shell nanoparticles could be obtained via digestive ripening of a mixture of separately prepared colloids of the two metals at fairly low temperatures3a,10e. Herein, we report the synthesis and characterization of monodisperse Ag@Pd core–shell nanoparticles obtained by the refluxing of dodecanethiol capped Ag and Pd colloids prepared by the SMAD method. Materials and Methods

Silver foil (0.3 mm thickness) (99.99 %) and palladium foil (0.3 mm thickness) (99.95 %) were purchased from Arora-Matthey, Kolkata, India. Tungsten crucibles were obtained from RD Mathis Company, California. 2-Butanone (HPLC grade), dodecanethiol, and 4-tert-butyltoluene were purchased from Aldrich. 2-Butanone was dried over K2CO3 and degassed by several freeze-pump-thaw cycles. Dodecanethiol and 4-tert-butyltoluene were used after purging with Ar for 30 min before each experiment. Pd-2-butanone colloid was prepared as described by us previously3a.

UV-visible spectra were recorded using a Perkin-Elmer Lambda 35 UV/vis spectrometer. The TEM bright field images, electron diffraction patterns, EDX analysis, HRTEM, EF-TEM, and HAADF images were obtained using TECNAI F30 transmission electron microscope. The powder X-ray diffraction measurements were carried out using a Philips powder X-ray diffractometer. For annealing the samples, the powders were filled in 0.5 mm glass capillaries, flame sealed, and annealed for 12 h. Sample preparation for TEM

To 0.5 mL of the colloid, 10 mL of absolute ethanol was added and then it was centrifuged at

2500 rpm for 1 h. The supernatant liquid was poured out and the residue collected at the bottom of the centrifuge tube was washed with ethanol several times to remove the excess thiol. Finally, the residue was dried under vacuo. It was then redispersed in ethanol by sonication (90 W, 1 min). A 1 µL drop of the colloid was casted on a carbon-coated copper grid and the grid was dried under a table lamp for 12 h. Preparation of Ag-2-butanone colloid by the Solvated Metal

Atom Dispersion (SMAD) method

The details of the SMAD technique are given elsewhere8. The crucible was charged with about 100 mg of Ag foil. The reactor was evacuated to 10-3 torr. The walls of the reactor were maintained at 77 K using liquid N2 and 2-butanone (~ 20 mL) was slowly condensed on the walls of the reactor. At the same time the crucible was heated resistively in vacuo

slowly, ensuring that no change in pressure took place. Once the vaporization of the metal started, further heating was stopped and the metal vapor was allowed to co-condense with butanone vapor. The matrix was further coated with butanone (~20 mL) once the metal vaporization was completed. The matrix was allowed to melt down and warmed upto room temperature slowly under Ar atmosphere. The Ag-2-butanone colloid was stirred at room temperature for 1 h and the colloid was siphoned into a Schlenk tube and kept under Ar atmosphere. Preparation of M-2-butanone-4-tert-butyl toluene-

dodecanethiol colloid (M = Ag, Pd) by the SMAD method

These colloids were prepared in a manner similar to that of the Ag-butanone colloid except that 4-tert-butyl toluene (20 mL) and dodecanethiol were taken initially at the bottom of the reactor before the co-condensation procedure. The M:dodecanethiol molar ratio was maintained at 1:20. Preparation of Ag@Pd core-shell nanoparticles

The as-prepared Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid (~6 × 10–4 g of Ag/mL) and the as-prepared Pd-2-butanone-4-tert-butyl toluene- dodecanethiol colloid (~6 × 10–4 g of Pd/mL) were mixed in 3:1, 1:1, and 1:3 molar ratio (Ag:Pd) and 2-butanone was removed in vacuo. Then the mixture was diluted with 4-tert-butyltoluene to keep the total volume of 10 mL. The metal to dodecanethiol concentration was 1:20. The reaction mixture was refluxed under Ar for 6 h during which time the colloid became transparent and turned brownish

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yellow. The progress of the reaction was monitored by UV-visible spectroscopy. Precipitation of the Ag@Pd core–shell nanopowder was accomplished by addition of ethanol.

Results and Discussion Ag-2-butanone colloid

Silver colloids have been prepared by the SMAD process. The as-prepared Ag-2-butanone colloid was red in color and the UV-visible spectrum of this colloid showed a broad absorption band with a maximum at 448 nm corresponding to the surface plasmon resonance (SPR) of Ag (Fig. 1a)13. The TEM bright field image (Fig. 1b) of this as-prepared colloid showed polygonal-shaped particles with agglomerated regions. The HRTEM image (Fig. 1c) showed lattice fringes indicating the crystalline nature of the sample. The interplanar distance calculated from the HRTEM image matched that of the (111) plane of FCC Ag. The EDX analysis also showed the presence of Ag (Fig. 2).

The as-prepared Ag-2-butanone colloid was found to be very stable towards oxidation and precipitation, if kept under Ar atmosphere. 2-Butanone acts as the capping agent as well as the solvent. A greyish black powder of Ag nanopowder was obtained upon

removal of the weak capping agent 2-butanone from the Ag-2-butanone colloid. The powder XRD pattern (Fig. 3) of the Ag nanopowder showed reflections corresponding to that of FCC Ag. The powder XRD pattern could be indexed to the (111), (200), (220), (311), and (222) planes of FCC Ag (JCPDS No. 040783). The average crystallite size calculated from the powder XRD pattern using the Debye-Scherrer equation is 15 nm. Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid

The Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid was prepared in a similar manner to that of Ag-2-butanone colloid. The as-prepared colloid was brownish red in color and the UV-visible spectrum showed a broad absorption band with a maximum at 465 nm corresponding to the surface plasmon resonance band of Ag nanoparticles13 (Fig. 4a). As in

Fig. 1 − As-prepared Ag-2-butanone colloid. [(a) UV-visible spectrum; (b) TEM bright field image; (c) HRTEM image].

Fig. 2 − EDX analysis of as-prepared Ag-2-butanone colloid.

Fig. 3 − Powder XRD pattern of Ag nanoparticles obtained from the Ag-2-butanone colloid.


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the case of Ag-2-butanone colloid, the TEM bright field image (Fig. 4b) of the as-prepared sample of Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid showed polygonal shaped particles with agglomerated regions. The SAED pattern (Fig. 4c) could be indexed to that of FCC Ag. The HRTEM image (Fig. 4d) showed lattice fringes corresponding to the (111) plane of Ag. The EDX analysis (Fig. 5) also showed the presence of Ag.

The powder XRD pattern (Fig. 6) showed reflections corresponding to that of FCC Ag. The average crystallite size calculated from the powder XRD data was about 18 nm. The as-prepared Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid was found to be stable under Ar atmosphere. 2-Butanone acts as the initial capping agent and prevents the extensive agglomeration of the Ag

Fig. 5 − EDX analysis of as-prepared Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid.

Fig. 4 − As-prepared Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid. [(a) UV-visible spectrum; (b) TEM bright field image; (c) SAED pattern; (d) HRTEM image].


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nanoclusters during the melting of the matrix. Once the partially melted Ag-2-butanone matrix comes into contact with dodecanethiol, 2-butanone gets substituted by the strong capping agent, dodecanethiol via ligand exchange.

Removal of the initial weak capping agent, 2-butanone, ensures the complete capping of surface cluster atoms by the final capping agent, dodecanethiol. A blue shift in the SPR band maximum from 465 nm to 445 nm was noted in the UV-visible spectrum (Fig. 7) upon removal of 2-butanone from the as-prepared Ag colloid. The blue shift is indicative of some ripening of Ag nanoparticles in such a manner that the average particle size decreased10a. The color of the colloid changed from brownish red to yellow and became transparent after 3 h of reflux indicating that further ripening took place. The SPR band of the colloid after 3 h of reflux became narrow with a maximum at 430 nm (Fig. 7) indicating that the average particle size has decreased with a narrowing down of the particle size distribution10a.

The TEM bright field image (Fig. 8a) of a sample of Ag-4-tert-butyl toluene-dodecanethiol colloid after 3 h of reflux showed spherical particles of a narrow size distribution of about 6.8 ± 0.5 nm (Fig. 8b). The SAED pattern (Fig. 8c) could be indexed to that of FCC Ag. The HRTEM image (Fig. 8d) showed lattice fringes corresponding to that of the (111) plane of FCC Ag.

The XRD pattern (Fig. 9) of the powder obtained by precipitation from the Ag-4-tert-butyl toluene-

dodecanethiol colloid showed peaks at low 2θ values like 7.6°, 10.2°, 12.7°, 15.3°, and 17.8°, in addition to the FCC Ag reflections at 38.2°, 44.4°, 64.6°, and 77.4°. Based on our earlier observations in the case of digestive ripening of Pd-4-tert-butyl toluene-dodecanethiol colloid wherein, a Pd-thiolate complex of the form [Pd(SC12H25)2]6 was formed [3a], we assign the low 2θ reflections to an Ag-thiolate complex. In the case of Pd, digestive ripening process did not result in a colloid of monodisperse Pd nanoparticles; only the Pd-thiolate complex was obtained. However, in the present Ag case, we obtained Ag nanoparticles together with the Ag-thiolate complex as is evident from the TEM and powder XRD studies.

We isolated the Ag-thiolate complex as a grey colored powder. The UV-visible spectrum (Supple-mentary Data) of Ag-thiolate in 4-tert-butyl toluene showed a maximum at 344 nm. The powder XRD pattern of the Ag-thiolate sample showed reflections at 7.6°, 10.1°, 12.6°, 15.2°, and 17.7° (Supplementary Data). The Ag-thiolate is formed by the leaching of surface cluster atoms by dodecanethiol. We do not know the exact formulation of this species.

Ag@Pd core–shell nanoparticles

The as-prepared dodecanethiol-capped Ag and Pd colloids were mixed in 1:3, 1:1, and 3:1 molar ratios and refluxed together. The course of the reaction (1:1) was monitored using UV-visible spectroscopy (Fig. 10). The behaviour was the same in all the cases. The UV-visible spectral data for the 1:3 and 3:1

Fig. 6 − Powder XRD pattern of Ag nanoparticles obtained from the Ag-2-butanone-4-tert-butyl toluene-dodecanethiol colloid.

Fig. 7 − Progress of the digestive ripening of the as-prepared Ag colloid [1, as-prepared Ag colloid; 2, after removal of butanone; 3, after 15 min reflux; 4, 1 h reflux; 5, after 2 h reflux].


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Fig. 8 − Ag-4-tert-butyl toluene-dodecanethiol colloid (after digestive ripening). [(a) TEM bright field image; (b) histogram showing the particle size distribution; (c) SAED pattern; (d) HRTEM image].

Fig. 9 − Powder XRD pattern of Ag nanoparticles and Ag-thiolate complex.

Fig. 10 − Progress of the digestive ripening of the as-prepared Ag, Pd colloids (1:1) as monitored by UV-visible spectroscopy. [1, Ag digestive ripening; 2, Pd digestive ripening; 3, 0 h; 4, 0.5 h; 5, 3 h; 6, 6 h].


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(Ag:Pd) reactions have been given as Supplementary Data. Removal of the initial capping agent, 2-butanone from a mixture of the two colloids resulted in a dampening of the Ag Plasmon. The Ag SPR band disappeared completely after 6 h of reflux in all the cases and the UV-visible absorption spectrum showed a Mie scattering profile corres-ponding to that of Pd nanoparticles14. Klabunde and co-workers10d observed new bands during the formation of Au/Cu and Au/Ag alloy nanoparticles by refluxing a mixture of the two colloids. Koga and his co-workers4d noted the disappearance of the Ag SPR in the formation of Pd shell around an Ag core.

In order to get an insight into the structural features of this sample, we obtained more data in the form of UV-visible spectra, TEM images, and powder XRD pattern. The powder XRD pattern (Fig. 11) of the Ag:Pd/1:1 sample after 6 h of reflux showed reflections corresponding to that of FCC Ag. The Pd reflections were not apparent in the powder XRD pattern. Mizukoshi et al.15 noted that 0.8 nm thick Pd shell is insufficient to give significant diffraction intensities in the powder XRD pattern of Au@Pd core–shell nanoparticles synthesized by sonochemical route.

The TEM bright field image (Fig. 12a) of a sample of Ag:Pd/1:1 after 6 h of reflux showed spherical particles of average particle size of about 6.7 ± 0.7 nm (Fig. 12b). The SAED pattern (Fig. 12c) showed a ring pattern which matched with that of only Ag but not with that of Pd. This could be again due to a very small amount of Pd. The HRTEM image (Fig. 12d) showed lattice fringes corroborating the crystalline

Fig. 12 − Ag/Pd bimetallic nanoparticles [(a) TEM bright field image (obtained by digestive ripening); (b) histogram showing the particle size distribution; (c) SAED pattern; (d) HRTEM image].

Fig. 11 − XRD pattern of Ag/Pd bimetallic nanoparticles.


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nature of the sample. Not much difference in the lattice fringes between the core and the peripheral part of the cluster, which is expected for a core–shell structure, was apparent from the HRTEM image. However, the bright field image (Fig. 12a) showed a difference in contrast within each particle between the core and the periphery. From the HRTEM image, we calculated the interplanar distance in the core part of the cluster to be about 2.35 Å, which matches with that of the (111) plane of Ag (JCPDS file number 040783). The interplanar distance in the peripheral part of the cluster was about 2.32 Å. The corresponding distance in Pd is 2.24 Å (JCPDS file number 461043). The Pd shell formed may be too thin to detect the Pd (111) interplanar distance from the HRTEM image which is in accordance with our powder XRD observations (No Pd reflections were noted in the powder XRD pattern). From all the above available data, the assignment of the presence of both Ag and Pd was inconclusive.

Since we could not conclude about the nature of the sample (Ag:Pd::1:1) from the TEM bright field

and the HRTEM images and the SAED patterns and also the powder XRD data, we carried out EDX analysis (Fig. 13) on several isolated clusters. The EDX analysis gave atomic compositions of 83 % for Ag and 17 % for Pd. Further characterization of the 1:1 molar ratio Ag:Pd sample was accomplished using the compositional image mode of TEM. The energy-filtered (EF)-TEM images obtained using the Ag, Pd, and S edges are shown in Fig. 14. The bright contrast in the images represents Ag (a), Pd (b), and S (c), respectively. The contrast is diffuse in the case of sulfur. The EF-TEM images confirm the presence of both Ag and Pd in a single cluster thus ruling out a physical mixture of the monometallic components. An alloy of Ag/Pd is also ruled out based on our UV-visible spectroscopic studies wherein, excepting dampening of the Ag SPR band no other change took place, and, powder XRD data in which only peaks due to Ag were observed. In addition, the interplanar distance (from HRTEM image) in the case of an alloy structure would be expected to be in between 2.35 Å and 2.24 Å, which, however was not the case.

All of the above observations point to another possible structural feature, a core–shell structure. The TEM bright field image revealed a slight difference in contrast between the core and the periphery of each particle, which is expected for a core–shell structure. The HRTEM image, however, did not show much difference in the lattice fringes anywhere in the cluster. The problem is compounded because the lattice parameters of both Ag and Pd are very close to one another (Ag: 4.090 Å; Pd: 3.898 Å). Conclusive evidence for a core–shell structure was obtained from the High Angle Annular Dark Field (HAADF) image, which is based on Z-contrast16. The HAADF image of the sample is shown in Fig. 15. As is evident from the figure, there is a clear difference in contrast between the core and the shell confirming a core–shell structure. From the HAADF image, the core diameter

Fig. 14 − EF-TEM images of Ag@Pd core–shell nanoparticles. [(a) Ag spectral map; (b) Pd spectral map; (c) oxygen spectral map].

Fig. 13 − EDX analysis of Ag/Pd bimetallic (1:1) nanoparticles.


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was found to be ~9.8 nm whereas, the shell thickness was ~0.8 nm.

The Ag@Pd core–shell structure was found to be quite stable. Annealing a sample of Ag@Pd core–shell structure to realize an Ag/Pd alloy at 400 ºC for 12 h under Ar did not result in any change in the powder XRD pattern, except that the peaks sharpened (Supplementary Data). The sharpening of the peaks is due to sintering of the particles at high temperature. No alloy formation was evident. This could be due to the melting of the surface layer of clusters (surface melting) before the complete melting of the nano-cluster17.

Thus, reluxing a mixture of dodecanethiol-capped as-prepared Ag and Pd colloids gave the Ag@Pd core–shell system. The mechanism of formation of the core–shell structure involves first the digestive ripening of the as-prepared Ag dodecanethiol colloid to give largely, monodisperse Ag seeds together with a small amount of the Ag-thiolate complex. During the reflux process, the Pd clusters in the as-prepared Pd dodecanethiol colloid in the presence of Ag seeds could either be transformed into the Pd-thiolate complex or broken down into smaller particles that stick to the surface of Ag seeds. Both of these processes are likely to take place; especially when Ag seeds are present in the medium they favor the disintegration of Pd clusters to smaller particles to a great extent which limits the formation of the Pd-thiolate complex. Thus, the Pd nanoparticles present in the medium form an overlayer on Ag seeds. Once the entire Ag surface atoms are covered by

Pd particles, no further Pd particles are formed since no more Ag seeds are available. Instead, the rest of the Pd nanoparticles are transformed into the Pd-thiolate complex.

In order to determine if smaller Pd nanoparticles are formed only from the larger Pd clusters in the presence of Ag seeds for the formation of Ag@Pd core–shell bimetallic nanoparticles, we refluxed a mixture of the already ripened Ag and Pd colloids for several hours and the course of the reaction was followed by UV-visible spectroscopy (See Supple-mentary Data). The mixture of the ripened Ag and Pd colloids consists of Ag nanoparticles and the Pd-thiolate complex. No separate signals corres-ponding to the SPR band of Ag nanoparticles and absorption maximum of Pd-thiolate complex were noted because of the small difference in the position of the SPR band of Ag nanoparticles (430 nm) and the λmax of the Pd-thiolate complex (415 nm). Instead, a broad absorption in the visible region corresponding to that of the Ag SPR band and the Pd-thiolate absorption spectrum was obtained. The broad absorption band did not disappear even after 6 h of reflux. This indicates that disintegration of the Pd cluster in the presence of Ag seeds (formed during the reflux process–digestive ripening) is essential for the formation of Ag@Pd core–shell nanoparticles. Conclusions

In conclusion, we have prepared Ag- and Pd-2-butanone-4-tert-butyl toluene-dodecanethiol colloids by the Solvated Metal Atom Dispersion method. On digestive ripening, the as-prepared polydisperse Ag colloid under reflux conditions got transformed into a highly monodisperse colloid. Under similar conditions, the Pd colloid gave a Pd-thiolate complex. A mixture of the as-prepared Ag- and Pd colloids under reflux conditions resulted in Ag@Pd core–shell nanoparticles. The core–shell structure could not be transformed into alloy phase even upon heating to a high temperature demonstrating its high stability. The core–shell structure could be established with certainty using HAADF imaging.

Supplementary Data

Supplementary data associated with this article, viz., the UV-visible spectral data and powder X-ray diffractograms of the samples, may be obtained from the authors on request.

Fig. 15 − HAADF image of Ag@Pd core–shell nanoparticles.


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Acknowledgement

We are grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support. We thank the IISc Institute Nanoscience Initiative and the Solid State and Structural Chemistry Unit, IISc, Bangalore, India, for allowing us to access the microscopic facilities and the powder X-ray diffractometer, respectively. DJ thanks the CSIR for a fellowship. References

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