ARTICLE pubs.acs.org/JPCC
Co-reduced Ag/Pd Bimetallic Nanoparticles: Surface Enrichment of Pd Revealed by Raman Spectroscopy Kwan Kim,†,* Kyung Lock Kim,† and Kuan Soo Shin‡,* † ‡
Department of Chemistry, Seoul National University, Seoul 151-742, Korea and Department of Chemistry, Soongsil University, Seoul 156-743, Korea
bS Supporting Information ABSTRACT: In the hope of developing metal nanoparticles that are expected to possess simultaneously higher surface-enhanced Raman scattering (SERS) activity and also catalytic activity, we have prepared Ag/Pd alloy nanoparticles by coreduction of Ag and Pd ions using citrate and then examined their characteristics in terms of their composition. Ag/Pd(2% in atomic percent) particles, about 53 nm in diameter, are alloy-like, with their surfaces being composed of Ag and Pd atoms. Ag/Pd(5%) particles, about 29 nm in diameter, are also alloy-like, but their surfaces appear to be covered fully with Pd atoms of ∼2 nm thickness. The surface enrichment of Pd atoms were clearly identified by the SERS spectra of 2,6-dimethylphenylisocyanide, 4-nitrobenzenethiol, and 4-aminobenzenethiol, as well as by high-angle annular dark-field scanning transmission electron microscopy analysis, even though the presence of Pd atoms was hardly detectable by X-ray diffraction. Owing to the borrowing effect from Ag, the SERS activity of Ag/Pd(5%) particles was at least an order of magnitude higher than that of pure Pd particles: the SERS activity deteriorated rapidly when the Pd content in Ag/Pd was increased above 5%. However, the catalytic activity of Ag/Pd(5%) nanoparticles was the highest among Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles in the reduction reaction of 4-nitrophenol to 4-aminophenol by NaBH4, suggesting the presence of a synergistic interaction between the Ag and Pd atoms. Ag/Pd(5%) particles appeared thus to be the optimum type, possessing both higher SERS activity and also higher catalytic activity simultaneously.
1. INTRODUCTION Surface-enhanced Raman scattering (SERS) is a phenomenon in which the scattering cross sections of molecules adsorbed on certain metal surfaces are dramatically enhanced.13 In recent years, it has been reported that even single-molecule detection is possible by SERS, suggesting that the enhancement factor (EF) can reach as much as 1014∼1015; the effective Raman cross sections are then comparable to the usual fluorescence cross sections.49 SERS has thus been used in many areas of science and technology, including chemical analysis, corrosion, lubrication, catalysis, sensor, and molecular electronics, etc.1012 One of the weak points of SERS is that only noble metals such as Ag and Au can usually provide large enhancement effects, which severely limits wider applications involving other metallic materials of both fundamental and practical importance.1315 In recent years, even transition metals have been proven to be SERS active when they are subjected to a proper roughening process.1619 However, it is still difficult to obtain Raman spectra of molecules adsorbed on transition metals like platinum and palladium, especially in nonelectrochemical environments. For instance, the EF of a Pt film made of ∼7 nm sized Pt particles is at best 1.5 102 for benzenethiol at 514.5 nm excitation.20,21 r 2011 American Chemical Society
Palladium is an important transition metal with high catalytic activity, although it is not used as widely as Pt at the present time. However, a great deal of attention has recently been focused on the preparation of Pd nanoparticles and their application in the fields of catalysis, hydrogen storage, and chemical sensors due to their large surface area-to-volume ratio, relatively lower price than Pt, and especially their unique function in absorbing hydrogen.2228 Unfortunately, Pd has intrinsically weak SERS activity even among the transition metals, such as Pt, Rh, Ru, Fe, Co, and Ni, so that it is difficult to follow catalytic reactions by detecting the SERS spectra of the surface-adsorbed reagents and products on Pd. Nonetheless, many attempts have been made to improve the SERS activity of Pd nanoparticles. Chen et al.29 reported that Pd nanoparticles of about 60 nm show higher SERS enhancement than those of either 30 or 110 nm; any comparison with the SERS activity of Ag or Au nanoparticles was not made, however. On the other hand, Tian and his colleagues30 have synthesized the coreshell nanoparticles in order to utilize the Received: April 5, 2011 Revised: May 17, 2011 Published: July 01, 2011 14844
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The Journal of Physical Chemistry C strategy of “borrowing SERS activity”. The EF for 135-nm Au@Pd(0.7 nm) was reported to be as large as 5 104 , but the SERS signal decreased exponentially with increasing thickness.30 Pergolese et al.11 have therefore deposited Pd clusters onto Ag nanoparticles in order to combine the SERS enhancement of the noble metal with the catalytic efficiency of Pd. Ag and Pd are able to form an alloy over the entire composition range, as stable even at room temperature as the alloy of Ag and Au. It is nonetheless difficult to prepare alloy nanoparticles homogeneously when mixed on the atomic scale. Even for the case of Ag and Au, alloy nanoparticles highly enriched with Ag atoms at the surface sites are unequivocally produced in the coreduction of their ions.3133 It is thus not surprising to find that success in the preparation of Ag/Pd alloy nanoparticles is very rare. In principle, homogeneously mixed alloy nanoparticles should be formed in simultaneous reduction conditions, but coreshell type nanoparticles are usually produced, due to the kinetics associated with the respective, size-dependent reduction potentials of different metal nanoparticles.34 However, a variety of SERS active nanoparticles of specific shape and size could be synthesized, but polymeric capping agents or surfactants, which are inevitably present in most practical systems, would hinder the adsorption of the probe molecules of interest onto the SERS active sites.35 Taking all these facts into consideration, we have examined the SERS characteristics of Ag/Pd nanoaggregates fabricated using corresponding particles that were produced by the coreduction of Ag and Pd ions by citrates. First, the shape and size of Ag/Pd nanoparticles, as well as their UVvisible (UVvis) absorption characteristics and crosssectional composition of elements, were examined primarily to ascertain the formation of alloy or coreshell structures. Second, the SERS activity of the prepared Ag/Pd nanoaggregates, as well as the surface composition versus the bulk composition, were analyzed by referring to the SERS of 2,6-dimethylphenylisocyanide (2,6-DMPI) in ambient and electrochemical environments. Third, any surface photoreaction capability and the possibility of the metal-to-adsorbate or adsorbate-to-metal electron transition were examined by referring to the SERS of 4-nitrobenzenethiol (4-NBT) and 4-aminobenzenethiol (4-ABT). Fourthly, the reductive catalytic activity was examined using 4-nitrophenol, in expectation that Ag/Pd nanoparticles would show higher selectivity and activity than pure Pd nanoparticles. To our knowledge, this is the first study reporting the surface enrichment of Pd atoms in Ag/Pd alloy nanoparticles that is specifically useful in the development of Pd-based catalysts.
2. EXPERIMENTAL SECTION Palladium(II) nitrate (Pd(NO3)2, 99%), silver nitrate (AgNO3, 99+%), sodium citrate trihydrate, 3-aminopropyltrimethoxysilane (3-APS), 2,6-DMPI (96%), 4-NBT (80%), 4-ABT (97%), 4-nitrophenol (99%), 4-aminophenol (98+%), sodium perchlorate (NaClO4, 98%), and Pt wire (99.99%) were purchased from Aldrich and used as received. Other chemicals unless specified were reagent grade, and triply distilled water of resistivity greater than 18.0 MΩ 3 cm was used in making the aqueous solutions. Both the pure Ag and Pd sols were prepared by following the recipes of Lee and Meisel.36 Initially, 45 mg of AgNO3 or Pd(NO3)2 was dissolved in 250 mL of water, and the solution brought to a boil. A solution of 1% sodium citrate (5 mL) was added therein under vigorous stirring and boiling was continued for 30 min. The aqueous Ag/Pd alloy sols were also prepared by
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the citrate reduction method. Initially, an aqueous solution (100 mL) of AgNO3 (1 mM) and Pd(NO3)2 (1 mM), mixed in a molar ratio of either 0.98:0.02 or 0.95:0.05, was brought to a boil. Subsequently, a solution of 1% sodium citrate (2 mL) was added therein under vigorous stirring and boiling was continued for 30 min. (In the subsequent descriptions, Ag/Pd nanoparticles produced from a mixture of 98% Ag and 2% Pd in atomic percent, for instance, are denoted by Ag/Pd(2%)). When the Ag/Pd alloy sols, as well as pure Ag and Pd sols, finally cooled down to room temperature, they were centrifuged at 3000 rpm for 20 min, and then the collected products were washed and redispersed in distilled water. The centrifugation and redispersion in water was repeated three times to remove excess citrates on the metal nanoparticles. Ag/Pd nanoaggregate films were prepared by dropping the above sol solutions onto indium tin oxide (ITO) substrates. Initially, the ITO substrates were subjected to ozonolysis to render them hydrophilic at a water contact angle of Pd > Ag/Pd(2%) > Ag is of particular significance. It suggests that the catalytic activity of both Ag and Pd can be enhanced by forming Ag/Pd alloys. However, the catalytic synergic effect must be precise and specific since the optimum is found for Ag/ Pd(5%) nanoparticles.
Figure 7. (a) UVvis absorption spectra of 4-nitrophenol taken before and after immediate addition of NaBH4 and that of 4-aminophenol shown for comparison. Successive UVvis absorption spectra taken after adding NaBH4 into 4-nitrophenol solution in the presence of (b) Ag, (c) Ag/Pd(2%), (d) Ag/Pd(5%), and (e) Pd nanoparticles, and (f) plots of ln(At/A0) versus time (t) corresponding to the reduction of 4-nitrophenol in (b)(e) at 25 °C.
For a more precise experiment, we maintained the concentration of the borohydride ion, used as a reductant, to exceed 70 times of that of 4-nitrophenol, so that it remained essentially constant throughout the reaction. On the other hand, the amount of metal nanoparticles, Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd, added into the reaction vessel as a catalyst was converted into the equivalent surface area by taking into account their size and concentration. In this way, as soon as we added the NaBH4, all of these nanoparticles started the catalytic reduction by relaying electrons from the donor BH4 to the acceptor 4-nitrophenol right after the adsorption of both onto the particle surfaces. Figure 7(b)(e) shows the successive UVvis absorption spectra taken after adding NaBH4 into 4-nitrophenol solution in the presence of Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles, respectively. The absorbance at 400 nm is seen to decrease. These decreases in absorbance were analyzed by assuming pseudofirst-order kinetics to hold. The ratio of absorbance At of 4-nitrophenol at time t to its value A0 measured at t = 0 must then be equal to the concentration ratio Ct/C0 of 4-nitrophenol, therefore the kinetic equation for the reduction can be written as dCt/dt = kappCt or ln(Ct/C0) = ln (At/A0) = kappt, in which Ct is the concentration of 4-nitrophenol at time t and kapp is the apparent rate constant, which can be obtained from the decrease of the peak intensity at 400 nm with time.55 Figure 7(f) shows the plots of ln(At/A0) versus time (t) for the data gathered using Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles as a catalyst.
4. CONCLUSIONS In principle, Ag and Pd can form an alloy irrespective of their composition, but in reality Ag/Pd alloy core-Pd shell nanoparticles are exclusively formed when Ag and Pd ions in moderate molar ratios are reduced simultaneously by citrate, presumably due to their different size-dependent reduction potentials. Nonetheless, alloy-like Ag/Pd nanoparticles appear to form, however, when the Pd content is less than 5% of the atomic composition. The formation of an alloy and thus the homogeneous distribution of Ag and Pd atoms were evident for Ag/Pd(2%) particles by XRD and HAADF-STEM analyses; the copresence of Ag and Pd atoms on their surfaces was also confirmed from the SERS of 2, 6-DMPI, 4-NBT, and 4-ABT. Ag/Pd(5%) particles, about 29 nm in diameter, looked very attractive in the sense that their surfaces were covered fully with Pd atoms of ∼2 nm thickness. The surface enrichment of Pd atoms were clearly identified by the SERS spectra of 2,6-DMPI, 4-NBT, and 4-ABT, as well as by the HAADF-STEM analysis, even though the presence of Pd atoms was hardly detectable by XRD. Owing to the borrowing effect from Ag, the SERS activity of Ag/Pd(5%) particles was found to be at least an order of magnitude higher than that of pure Pd particles: the SERS activity deteriorated rapidly when the content of Pd in Ag/ Pd was increased above 5%. Another noteworthy point was that the catalytic activity of Ag/Pd(5%) nanoparticles was the highest among Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles in the reduction reaction of 4-nitrophenol to 4-aminophenol by NaBH4, suggesting the presence of a synergistic interaction between the Ag and Pd atoms. Ag/Pd(5%) particles are thus presumed to be the optimum type, possessing both higher SERS activity and also higher catalytic activity at the same time. ’ ASSOCIATED CONTENT
bS
Supporting Information. FE-SEM images of Ag, Ag/Pd(2%), Ag/Pd(5%), and Pd nanoparticles films; elemental maps of Pd and Ag for Ag/Pd(5%) nanoparticles; SERS spectra of 2, 6-DMPI adsorbed on Ag/Pd(2%), Ag/Pd(5%), Ag/Pd(10%), Ag/Pd(30%), and pure Pd nanoparticles films; and normalized SERS intensity of the CC stretching band drawn versus the mole percent of Pd contained in Ag/Pd nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Authors
*Tel: +82-2-8806651; Fax: +82-2-8891568; E-mail: kwankim@snu. ac.kr (K.K.). Tel: +82-2-8200436; Fax: +82-2-8244383; E-mail:
[email protected] (K.S.S.).
’ ACKNOWLEDGMENT This work was supported by National Research Foundation of Korea Grant funded by the Korean Government (Grant 20110001218, M10703001067-08M0300-06711-Nano2007-02943, and KRF-2008-313-C00390). 14850
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