J Nanopart Res (2010) 12:1489–1494 DOI 10.1007/s11051-009-9721-z
RESEARCH PAPER
Synthesis of palladium nanoshell using a layer-by-layer technique Roya Ashayer Æ Mark Green Æ Samjid H. Mannan
Received: 10 February 2009 / Accepted: 15 July 2009 / Published online: 30 July 2009 Ó Springer Science+Business Media B.V. 2009
Abstract Complete palladium nanoshells were prepared by reducing palladium ions in a one-step reaction onto preformed silica cores of ca. 90 nm, which had been coated with successive layers of poly(diallyldimethyl ammonium chloride), poly(sodium styrenesulfonate) and finally poly(diallyldimethyl ammonium chloride) to reverse the zeta potential of the silica cores. This constitutes the first reported method for complete palladium nanoshell formation without the use of other metals as nucleation sites. The morphology of the nanoshell is of the rough discrete particle type rather than the smooth continuous type. Keywords Palladium Nanoshell Layer-by-layer Silica core It has been shown that alcohols can act as both solvent and reducing agent for preparation of polymer-stabilised metal particles (Hirai et al. 1979; Bradley et al. 1992, 1993; Toshima and Liu 1992; Rampino and Nord 1941; Hirai et al. 1986). Nanoscale palladium particles have drawn particular attention due to their
R. Ashayer S. H. Mannan (&) Department of Mechanical Engineering, Kings College London, Strand, London WC2R 2LS, UK e-mail:
[email protected] M. Green Department of Physics, Kings College London, Strand, London WC2R 2LS, UK
magnetic and catalytic properties (Esumi et al. 2004; Son et al. 2004) and previously particles 6 nm in diameter have been prepared from palladium acetate using polyvinylpyrrolidone (PVP) in refluxing methanol where methanol acts as both solvent and reducing agent (Bradley et al. 1992, 1993). Nanosized palladium particles have also been synthesised from palladium complexes in aliphatic alcohols of different carbon chain lengths in the presence of PVP (Esumi et al. 1994; Teranishi et al. 1997; Boonekamp et al. 1994). Besides the pure metal particles, bimetallic core/shell structures involving palladium have also been prepared (Bradley et al. 1993; Henglein 2000; Tong et al. 1996; Lee et al. 1995; Mizukoshi et al. 2000). Fabrication of nanostructures consisting of a dielectric core, surrounded by a thin metal shell, termed ‘‘nanoshells’’ (Bradley et al. 1992), is a subject of extensive research due to their unique application in many areas such as nonlinear optics, catalysis and surface-enhanced Raman scattering (SERS) (Bradley et al. 1993; Toshima and Liu 1992; Rampino and Nord 1941; Hirai et al. 1986). These types of nanoparticles have received particular attention because of their stability and their ease of preparation (Esumi et al. 2004). One of the most used materials in nanotechnology is silica due to its availability and ease of production. Silica has a wide range of applications in microelectronics, optical communications and thin-film technology. The preparation of nanoshells, where a shell of a metal, normally gold is controllably deposited on
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Polyanion Adsorption
Pd
PSS (polyanion)
Adsorption
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Polycation
SiO2 Adsorption
Polycation
a silica sphere has attracted immense interest (Lal et al. 2002; Oldenburg et al. 1999; Jackson et al. 2003; Jackson and Halas 2004; Oldenburg et al. 1998), as the surface plasmon can be easily tuned and such particles are promising candidates for use in biology, notably in cancer therapy (O’Neal et al. 2004). Palladium nanoshells with silica cores have been prepared using deposited gold particles as nucleation sites (Kim et al. 2006), and despite attempts at the fabrication of palladium shells by the direct reduction of palladium salts onto the surface of amine-functionalised silica particles, this approach has only led to incomplete, aggregated, and/or small palladium particles attached to the silica-core particles (Kim et al. 2006). However, the use of ethanol as the sole reducing agent for the core/shell synthesis has not been reported previously, and we have also found that poly(diallyldimethyl ammonium chloride) (PDADMAC) can also act as a stabiliser (to be reported elsewhere). In this study, we report on layer-by-layer (LBL) assembly for complete nanostructured palladium shell formation. In this study, PDADMAC and poly (sodium styrenesulfonate) (PSS) were used to construct multilayer assemblies. The LBL technique developed by Decher et al. is one of the most effective used methods of thin film formation and has often been used for the deposition of oppositely charged polymers (Decher and Hong 1991; Decher 1997). Stable multifunctional super-hydrophilic coatings can easily be created from LBL-assembled films of negatively charged colloidal SiO2 nanoparticles and a suitable polycation (Cebeci et al. 2006). The LBL assembly of multilayer thin films containing nanoparticles of SiO2 was first reported in detail by Lvov et al. (1997). However, to our knowledge, no other studies have been reported on the preparation of palladium/silica nanoshell consisting of only silica core and palladium shell as shown in Fig. 1. In this work, tetraethylorthosilicate (TEOS) (99.9%, 3aminopropyltrimethoxysilane (APTMS), poly (diallyldimethyl ammonium chloride) (PDADMAC) (low molecular weight), poly (sodium-4styrenesulfonate) (PSS) (Mw = 70,000), ammonium hydroxide solution (33% NH3), palladium(II) acetate (99.9%) and sodium chloride were obtained from Aldrich Chemical. Ethanol ([99.5%) was purchased from Merck. All chemicals were used as received. Deionised water was used when needed. TEM was performed with, a FEI TecnaiT20 electron microscope
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Reduction
PDADMAC (polycation)
Fig. 1 Summary of the palladium nanoshell synthesis process
operating at a bias voltage of 200 kV. TEM sample preparation involved deposition of the nanoparticles in their original solution onto a carbon-coated copper grid. The excess solvent was absorbed with the means of filter paper. The grid was then set aside to allow drying before analysis. The size of the particles was determined with the measurement tools of the analysis software (Digital Micrograph, Gatoan Ltd., USA). Zeta potential was determined by injecting 0.7 ml of the solution into a clear disposable zeta cell. The zeta potential was calculated using the Huckle relationship. The choice of model will only influence the magnitude of the zeta potential value and will not influence the trend in the zeta value. The zeta potential measurements were performed either in pure water after centrifuge or in ethanol prior to addition of palladium salt. Therefore, the Huckel relationship is the most appropriate model for this study due to the low ionic strength of the dispersant and the particle size. In a typical synthesis, a colloidal suspension of 90 nm SiO2 particles in ethanol (ca. 7 9 1012 particles/ ml) was synthesized as described elsewhere (Ashayer et al. 2008). Next, 70.5 ll of PDADMAC (0.101 g) and PSS (0.115 g) were each added to separate NaCl solutions (80 ml of 0.7 M). Then, 2 ml of the negatively charged SiO2 colloid was added to 20 ml of the PDADMAC/NaCl solution and stirred vigorously by magnetic stirrer. After 30 min, 20 ml of first PSS and then finally PDADMAC salt solutions were added. Therefore, a total of three polymer layers (PE3) were deposited yielding a positive nanosilica particle surface. After each addition, a sample was taken for zeta potential analysis. In order to remove non-adsorbed polyelectrolytes which can lead to formation of complexes unattached to the silica particles, this mixture
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Fig. 2 TEM micrographs of Pd reduction at a PDADMAC layer and b PDADMAC/PSS layer. Scale bar = 100 nm
oppositely charged components (Lvov et al. 1993; Caruso et al. 1999). The electrostatic attraction between positively charged PDADMAC and negatively charged SiO2 caused the adsorption of the polymer, which resulted in the reverse net charge of the SiO2 surface (i.e. from negative to positive). The positive net charge in turn attracted the negatively charged PSS, resulting in change of surface charge from positive to negative. Hence, positively charged cationic polymer (i.e. PDADMAC) was adsorbed on the surface of the negatively charged silica. The zeta potential changed as a function of polyelectrolyte layer number for the PDADMAC/PSS/PDADMAC system. Measuring the surface electric potential revealed that the surface charge of the multilayer coated silica particles alternated between positive and negative values after addition of polymers (Fig. 3). 60 40
zeta Potential (mV)
was centrifuged twice at 4,100 rpm for 10 min in deionised water. Polyelectrolyte adsorption times of 30 min were sufficient for adsorption saturation. Longer adsorption time had no effect on the zeta potential values. Experiments were also undertaken varying the concentration of the NaCl solution. To deposit the palladium shell, a pellet of silica prepared as described above ca. 0.04 g was dissolved in 50 ml ethanol. The solution was then heated to 75 °C, after which 24.5 mg of palladium acetate was added. The solution changed from opaque to yellow then black in 2–3 s, indicating the formation of palladium nanoparticles, approximately 6 nm in diameter, on the silica shell. The volume of ethanol was kept constant and a sample was taken for TEM analysis after approximately 1 h of boiling. In order to investigate the effectiveness of the polyelectrolyte coating after each layer deposition, an aliquot was taken after each coating and the mixture was centrifuged twice at 4,100 rpm for 10 min using deionised water as the medium. From Fig. 2, palladium particle attachment can be seen after coating the core silica particles with only one layer of PDADMAC, indicating that the polymer has been adsorbed on the surface of silica nanoparticles (Fig. 2a). However when silica particles were coated with PDADMAC/PSS, there is hardly any Pd attachment on the surface of silica particles proving that free cationic sites of PDADMAC no longer exist (Fig. 2b). This method of investigation can be used as a novel way of examining the effect of polyelectrolyte adsorption layers on the existing cores. The LBL assembly process is driven by electrostatic attraction and complex formation between the
20 0 -20 -40 -60 0
1
2
3
Layer Number Fig. 3 Zeta potential as a function of PE layer number for nanosilica particles coated with PDADMAC/PSS/PDADMAC layer. The odd numbers correspond to PDADMAC deposition and the even numbers to PSS layer
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LBL Zeta potential (mV)
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55
50
45 0
0.2
0.4
0.6
0.8
1
Nacl concentration( M) Fig. 4 Zeta potential of LBL as a function of NaCl concentration
The zeta potential of uncoated silica nanoparticle showed a negatively surface charge of -59 mV. The presence of PDADMAC, which is cationic, caused a reversal in zeta potential to 48.7 mV. Subsequent deposition of PSS onto the PDADMAC-coated silica nanoparticles once again reversed the zeta potential to a value of -43.7 mV. The deposition of an additional layer of PDADMAC again caused a reverse in the zeta Fig. 5 TEM micrographs of attachment of palladium nanoshell formation at NaCl concentration of a 0.2 M, b0.5, c 0.7 M, and d 0.9 M. The scale bars on the main pictures and inset are 100 and 20 nm, respectively
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potential. This qualitatively demonstrates that the SiO2-PDADMAC/PSS/PDADMAC layers are formed. In this study the LBL layer had a surface charge that yields a potential of around 36.6 mV. The adsorption of palladium particles was found to be affected by the amount of salt used; a complete palladium nanoshell was produced only when the salt concentration was increased to 0.7 M. Figure 4 shows the measured Zeta potential of LBL as a function of salt concentration after the pellet was redispersed in ethanol prior to the addition of palladium. It was noticed that the maximum reduction of palladium particles occurred when the zeta potential reached the lowest value of 45.9 mV at 0.7 M salt concentration. Figure 5 shows electron micrographs of the palladium nanoshell on silica particles. It was also observed that when water was solely used as a solvent, the palladium particles agglomerate (data not shown). The dependence of polyelectrolyte multilayer thickness on salt concentration, salt type, solvent
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quality, deposition time, and polymer concentration has been studied before (Dubas and Schlenoff 1999; Caruso et al. 1998). The thickness and roughness of the layers is also expected to increase as the concentration of NaCl increases (Lvov et al. 1993). We observed that in the absence of NaCl, no shell formation takes place. In this case, the adsorbed layers were thin and should overcompensate the surface charge only slightly, leading to minimal polymer buildup when the process was repeated as has been observed previously (van de Steeg et al. 1992). At a salt concentration of 0.7 M, a full palladium shell was formed. However, beyond this concentration of salt (i.e. 0.9 M), the palladium nanoparticles started aggregating. This could be due to the fact that at high salt concentration electrostatic interactions within the polyelectrolyte might be sufficiently screened for the polymer to behave as though it were neutral (Dubas and Schlenoff 1999). It should also be noted that the alcoholic reduction achieved the core/shell structure when the ratio of polyelectrolyte in salt solution to silica was at 1:10. This optimum ratio was found after series of experiments with differing ratios between 1:1 and 40:1. In conclusion, the process of layer by layer assembly depends on the amount of salt, polyelectrolyte, and the mixing ratio of the silica nanoparticles and the polyelectrolyte solutions. We have successfully made a palladium shell using the LBL technique. Acknowledgments We thank Dr. Tony Brain for TEM measurements. This work was funded by Innovative electronics Manufacturing Research Centre grant SP/06/03/01.
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