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Novel Synthesis, Structure, and Oxidation Catalysis of Ag/Au Bimetallic Nanoparticles Shiho Tokonami,†,§ Nobuyasu Morita,† Kanako Takasaki,† and Naoki Toshima*,†,‡ Department of Applied Chemistry, Tokyo UniVersity of Science Yamaguchi, SanyoOnoda, Yamaguchi 756-0884, CREST, Japan, and Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan ReceiVed: December 17, 2009; ReVised Manuscript ReceiVed: February 21, 2010

The surface plasmon resonance peak due to Au nanoparticles dramatically increased after addition of AgClO4 into an aqueous dispersion of Au nanoparticles without appearance of Ag plasmon absorbance near 400 nm. The addition of AgClO4 into aqueous dispersions of Au nanoparticles has been found to produce bimetallic silver-gold nanoparticles with a core-shell type structure. The Ag-core/Au-shell type configuration was suggested by the data from energy dispersive X-ray spectra of prepared nanoparticles on a high-resolution electron microscope. Moreover, the catalytic activity for glucose oxidation was investigated for thus-prepared Ag/Au bimetallic nanoparticles with various compositions. The highest activity (4079 mol-glucose h-1 molM-1) was observed for the Ag/Au nanoparticle prepared with Ag/Au ratio of 0.25/1, the activity being 18 times higher than that of original monometallic Au nanoparticles under the same conditions. Introduction Metal nanoparticles (NPs) have extensively been studied not only from the point of unique catalysts but also of their unique optical properties. Especially the NPs of coinage metals such as gold (Au) and silver (Ag) have a broad absorption band in the visible region of the electromagnetic spectrum. These characteristics arise from the collective oscillation of free conduction electrons induced by an interacting electromagnetic field, and their resonances are noted as surface plasmon.1 The surface plasmon resonance has been widely applied to monitor a broad range of samples (e.g., DNA, proteins, and antigenantibody).2-4 The specific resonance frequency depends on a number of parameters such as composition, morphology, concentration, and surface charge of NPs as well as refractive index and temperature of the medium.5-12 Therefore, the composition changes from monometallic NPs to bimetallic ones can be followed up by the UV-vis spectrum easily. For example, the maximum optical absorption peak for dispersion of random alloy occurs in between each plasmon peak of constitutive elements.5,13 As for catalysis in metal NPs, Au had been neglected as a catalytic metal in a heterogeneous catalysis for a long time because it is indeed one of the most stable metals and is resistant to oxidation. However, Haruta et al. discovered in 1987 that Au NPs supported on Co3O4, Fe2O3, or TiO2 were highly active catalysts, under high dispersion, for CO and H2 oxidation, NO reduction, water-gas shift reaction, CO2 hydrogenation, and catalytic combination of methanol.14-19 Since their findings, Au NPs for the catalysis have widely been studied in the fields of applied chemistry, surface science, and computational science. In consequence, it has been dealt with a viable catalyst for preparation of propylene oxide,20 hydrogen peroxide,21,22 hydrodechlorination,23 and vinyl acetate synthesis.20 On the contrary, bimetallization of Au NPs has also been expected to * Corresponding author: e-mail [email protected]. † Tokyo University of Science Yamaguchi. ‡ CREST, Japan Science and Technology Agency. § Present address: Research Institute for the 21st Century, Osaka Prefecture University, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8570, Japan.

enhance the catalytic activity and selectivity.25,26 Bimetallic NPs, which are composed of two metal elements in a particle, exhibit interesting electronic, optical, chemical, and biological properties due to new bifunctional or synergistic effects.27-36 Ag/Au NPs with certain bimetallic compositions have been synthesized using two-phase (toluene-water)13a and one-phase (water)13b methods. Although there are several methods,5,37-45 it has been believed that controlling the composition and size of Ag/Au particles is difficult because Ag+ ions easily form precipitates with halogen ions in aqueous solutions when Ag+ ions were used as a starting material in combination with a HAuCl4 precursor. Among various structures of the bimetallic NPs, the core-shell structure could be scientifically interesting especially from a viewpoint of catalysis. We have already reported the formation of core-shell structured bimetallic NPs prepared by simultaneous reduction,41,42,46sacrificial hydrogen reduction,40 and selforganization by mixing two colloidal dispersions at room temperature.48,49 From a viewpoint of catalytic property, core-shell bimetallic NPs have higher activity than monometallic ones because the catalytic activity of shell atoms can be electronically affected by the core atoms for the better.41,50,51 Here, we report on a novel synthetic method of Ag/Au bimetallic NPs by a simple procedure of mixing Ag+ ions with Au NPs dispersed in an aqueous solution at room temperature. In this system, the above-mentioned precipitate problem can be solved since the mixing of Ag+ and Au NPs is performed after the Au precursor is completely reduced to form Au NPs, and the ionic impurities such as chloride are removed by the ultrafiltration in advance. Thus, this method can lead a wellcontrolled composition. The prepared bimetallic NPs have been analyzed by UV-vis spectra, TEM images, and XPS spectra. The catalytic activity of thus prepared bimetallic NPs with various composition ratios has also been investigated for a glucose oxidation. Experimental Section Materials. All the chemicals used in this study were of reagent grade. Tetrachloroauric acid (HAuCl4), poly(N-vinyl-2-pyrrolidone) (PVP, K-30, average molecular weight 40 000), trisodium citrate,

10.1021/jp9119149  2010 American Chemical Society Published on Web 05/18/2010

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Figure 1. (a) UV-vis spectra of PVP-protected Au NP dispersion solution before (blue) and after addition of AgClO4 (after 4 min; purple, 30 min; green and 60 min; red). (b) Absorbance increasing ratio as a function of time regarding the absorbance of aqueous dispersion of Au NPs (before Ag+ ion addition) as 1.0 as measured peak height (closed diamonds), after baseline correction (open triangles), and in integrated absorbance (open squares).

silver acetate, silver perchlorate, silver nitrate, sodium perchlorate, ammonium perchlorate, NaOH solution (0.1 M), ethanol, and NaCl were purchased from Wako Pure Chemical Industries, Ltd., Japan. Poly(sodium acrylate) (PAANa) was purchased from Aldrich. Preparation of Nanoparticles. PAANa-Protected Gold Nanoparticles (PAANa-Au NPs). PAANa-Au NPs was prepared by the light irradiation reduction of HAuCl4 as follows: A 30 mL aqueous solution containing PAANa (1.16 mM in monomer unit concentration) and HAuCl4 (0.452 mM) was added into a Schlenk flask, and then the degassing of the solution was carried out by freeze-thaw cycles of the solution. Light irradiation of the solution with an ultrahigh-pressure mercury lamp (UI-502Q, Ushio, Japan) under stirring for 3 h in nitrogen resulted in a color change from pale yellow to deep red. PVP-Protected Gold Nanoparticles (PVP-Au NPs). A typical procedure to prepare PVP-Au NPs is as follows: A solution of HAuCl4 (0.66 mM) and NaOH (2.65 mM) in 500 mL of ethanol/ water (1/9, v/v) was heated with refluxing for 3 h under nitrogen in the presence of PVP (13.2 mmol in monomer unit) as a protecting polymer. Ethanol was served as a reducing agent. Thus-prepared Au NPs were purified by filtration with an ultrafilter (Advantec, Q0100076E) and by washing with ethanol to remove byproduced ions and then dried under vacuum. Preparation of Ag/Au Bimetallic Nanoparticles. A 2 mL of 0.83 mM AgClO4 aqueous solution was added to the stirring 50 mL of 0.33 mM Au NP dispersed solution, and the mixtures were stirred for 60 min. Time course of the ultraviolet and visible (UV-vis) spectra was obtained at room temperature using a Shimadzu 2500PC recording spectrometer equipped with a 10 mm quartz cell. Characterization of Ag/Au Bimetallic Nanoparticles. The characterization of the monometallic and bimetallic NPs was performed by a transmission electron microscope (TEM, JEM1230, JEOL, Japan), operating at 80 kV. The detailed analysis of the bimetallic NPs was carried out at UBE Scientific Analysis Laboratory, Inc., with the high-resolution TEM (HR-TEM) and energy dispersive X-ray spectrometer (EDS) on a JEM-2010F high-resolution electron microscope operated at 200 kV. To confirm the valence of Au and Ag containing in the bimetallic NPs, we performed X-ray photoelectron spectroscopy with a KRATOS AXIS-HS spectroscope (Shimadzu) using Mg KR X-radiation. The Mg anode operated at 450 W was used to generate X-rays, and a hemispherical analyzer was operated at a band-pass energy of 40 eV. To reveal the composition and structure of the bimetallic NPs, powder X-ray diffraction (XRD)

experiments were carried out using a Rigaku Rint 2400 X-ray diffractometer incorporating Cu KR radiation with applied voltage and current at 40 kV and 30 mA, respectively. The elemental analysis of bimetallic NPs was conducted with an atomic absorption spectrophotometer (Z-6100, Hitachi, Japan). Glucose Oxidation Catalytic Property of Ag/Au Nanoparticles. The catalytic performance of Ag/Au NP was examined by glucose oxidation as a model reaction. The reactions were carried out at 60 °C in a thermostated water bath. During the experiment, the pH of the reaction suspension was kept constant at 9.5 ( 0.1 by addition of NaOH (1.0 M) using an automatic potentiometric titrator (AT-510, KEM, Japan) for 2 h. Oxygen was bubbled through the suspension with a flow rate of 100 mL min-1 at an atmospheric pressure. The suspension was stirred with a magnetic stirrer while measuring. The starting concentration of glucose was 813 mM. The catalytic activity was obtained by dividing the linear slope of the summarized amount of dropped NaOH against time by the metal amount of gold. Results and Discussion Plasmon Absorption Enhancement of Au Nanoparticles by Addition of Ag Ions. The optical properties of metal NPs are determined by the interaction of incoming light with the free conduction electrons. When there is a coupling between the frequency of alternating electric field of the electromagnetic radiation and the oscillation of conduction electrons, the plasmon resonance condition is fulfilled and the absorption occurs. Moreover, the surface plasmon resonance bands are strongly dependent on the size, shape, composition, and dielectric properties of NPs and their local environment.52 We have found by chance that Au plasmon absorption intensity dramatically increases by addition of Ag+ ions into the dispersion of Au NPs. Figure 1a shows the UV-vis absorbance spectra as a function of time when AgClO4 is added into Au NP-dispersed aqueous solution. The 517 nm surface plasmon peak due to Au NPs was observed before the addition of AgClO4. The peak intensity at 517 nm was dramatically increased after addition of Ag+ ion without appearance of Ag NP surface plasmon resonance peak expected at ca. 400 nm. Figure 1b represents the relationship between time and the normalized absorbance of the plasmon by regarding to the absorbance of original Au NP dispersions (before Ag+ ion addition) as 1.0. The drastic increase in absorbance occurred within 4 min, suggesting some

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kinds of conformation change of Au NPs. To improve the accuracy, the same data are plotted by corrected height after the baseline subtraction (shown with open triangles in Figure 1b) and integrated absorbance (shown with open squares in Figure 1b) in addition to measured peak height (closed diamonds). They have the similar tendency with each other. So, we decided to use measured peak heights hereafter to show the time course of the plasmon peaks. In general, the optical absorption spectrum shows only one plasmon band at a specific wavelength at which the maximum absorbance occurs in between each plasmon peak of constitutive element for a random alloy structure in a linear fashion.5,13 In the present case, the peak shift was not observed at all after addition of Ag+ ion. There are two possibilities for the increase of surface plasmon absorbance of gold dispersions. One is the increase in the concentration of gold NPs. The other is the increase in the size of gold NPs, keeping the concentration of NPs constant. The first possibility is not acceptable without addition of gold resources. The second possibility will be acceptable if the added Ag+ ions can be reduced to Ag atoms and form alloys with gold NPs. In this case, the absence of the shift in absorption peak suggests that the structure of thusprepared NPs could not be a random alloy, but a core/shell type structure having Au surfaces. However, it might not be easy to conclude that the particles would have a core/shell structure based only on the UV spectra because no plasmon peak attributed to Ag was observed after the Ag+ ion addition. The plasmon enhancement was enlarged with the additional amount of AgClO4 (Figure 2a), which indicated that the Ag+ ion plays an important role in the surface plasmon resonance enhancement. The influence of other cationic species on the plasmon enhancement was examined using NH4ClO4 and NaClO4 (Figure 2b). The enhancement of the plasmon was hardly observed by addition of NH4ClO4 and NaClO4, suggesting that the plasmon enhancement observed by addition of AgClO4 is not a common effect of cation species but a specific effect of Ag+ ion. The effect of variant metal ions such as Pt2+ and Rh3+ was also investigated, as shown in Figure 2c. The plasmon enhancement rate was the highest in the addition of Ag+. On the other hand, it was hardly observed in Rh3+ and PtCl62-. Since the interaction strength between PVP, being used as a protectant for Au NP, and clusters such as Rh and Pt is strong, Rh and Pt may adsorb on PVP rather than attacking Au NP core. As for the addition of PtCl62-, it would not so easy to get into a Au NP because of its large size in the ionic radius and electronegative charge. Therefore, the addition of RhCl3 and H2PtCl6 had little effect on the absorption peak due to Au NPs. On the contrary, Ag hardly interacts with PVP. Therefore, Ag might be able to approach the Au NP core directly and aggregate Au NPs as speculatively illustrated in Figure 3. Furthermore, the reason why the enhancement occurred only for Ag+ ion might be the easy reduction of Ag+ ion by natural light because plasmon enhancement was hardly observed in the dark room. Consequently, Ag+ ions might be reduced by the natural light to metal atoms or tiny particles first, and then they could barge into the lattice of Au NP, causing the aggregation of Au NPs. As wellknown, some changes in the wavelength of plasmon absorption should be observed at which time Ag particles appears in this mechanism. However, neither peak shift nor absorbance peak attributed to Ag particles has not been observed. If the Au/Ag alloy structure is formed, the plasmon band must be shifted to the blue as the molar ratio of Ag increases.53 However, no peak shift was observed even if the Ag ratio was increased. Therefore,

Tokonami et al.

Figure 2. Time function of absorbance increasing ratio of PVPprotected Au NP dispersed aqueous solution. Effect of (a) addition amount of AgClO4, (b) various kinds of cationic ions, and (c) several metal ions.

Figure 3. Speculative presentation of the formation of Ag/Au bimetallic NPs by the addition of AgClO4 aqueous solution into Au NP dispersion.

it appears most likely that NPs obtained by this method have a Ag-core/Au-shell type structure or, at least, a Au-rich surface structure. Diameter Change of Au Nanoparticles by Addition of Ag Ions. TEM images of the Au NPs before and after addition of a AgClO4 aqueous solution into PAANa-Au colloidal dispersion are shown in Figure 4. The particles were well dispersed (a), but they turned to be aggregated a little after the AgClO4 addition (b). Moreover, the average diameter of the particle (3.3 nm) became larger (4.3 nm) with addition of AgClO4. This phenomenon was also observed in the Au NPs prepared with different protectants and Ag+ ion species in a reproducible fashion as shown in Table 1. The increment in the diameter might be caused by an ingression of Ag into Au NPs after

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Figure 4. TEM micrographs of PAANa-protected Au NPs (a) before and (b) after addition of AgClO4 aqueous solution.

TABLE 1: Mean Diameter of Au NPs Prepared with Various Protectants before and after Ag+ Ion Addition and Their Ag/Au ratio Measured by an Atomic Absorption Spectrophotometer protectant of Au NPs

Ag ion

PAANa PAANa PVP citrate

AgClO4 AgOAca AgClO4 AgClO4

a

particle diameter (nm) before and after Ag ion addtion 3.3 f 4.3 4.2 f 4.6 10.4 f 10.9 14.0 f 15.0

Au/Ag ratio of produced NPs 1:0.8 1:0.7 1:0.5

Silver acetate.

reduction of Ag+ ions, which could be considered the reason for the surface plasmon enhancement. The present experimental method in which Ag+ ions are added to the Au NP-dispersed solution can increase the quantity of Ag NPs but never change that of Au NPs. Nevertheless, the plasmon peak attributed to Ag NPs has never been observed. Instead, enhancement of the plasmon peak attributed to Au NPs has been obtained as shown in Figure 1. This phenomenon cannot be explained by a simple mechanism of the continued reduction of Ag+ ions, which results in increase of Ag NPs. Therefore, it is speculated that such a plasmon enhancement may be caused by enlargement of the grain diameter of Au NPs as observed in Figure 4. Structure Analysis of Ag/Au Bimetallic Nanoparticles. Figure 5 represents the HR-TEM (a) and STEM-EDS (b and c) images of Ag/Au bimetallic NPs prepared by addition of AgClO4 aqueous solution into PAANa-Au colloidal dispersion. The EDS analysis indicated that Ag contents in a NP were 19% (edge) and 39% (center), respectively, as shown in table b of Figure 5 (spots 6-1 and 6-2). It suggests that the prepared NP has a Ag-core/Au-shell type structure. Although difference in the contents of the Ag atom between edge and center of the particle was not so large, it should be considered as a significant difference under the condition of ca. 1 nm of the electron beam size. On the contrary, a small NPs (∼2 nm) with a 100% of the Ag atomic content were observed (Figure 5b, particle 7-1), indicating the formation of pure Ag NPs. Since the particle size of Ag NPs was too small to show the plasmon absorption, only the surface plasmon enhancement of Au NPs was observed

Figure 5. (a) HR-TEM and (b) small and (c) large scale STEM-EDS images of Ag/Au NPs prepared by the addition of AgClO4 aqueous solution into PAANa-Au colloidal dispersion. Inset table represents the EDS analysis results of each position.

TABLE 2: XPS Peak Positions (BE/eV) for Ag/Au Nanoparticles binding energy (eV) reference metal

bulk

ionic

sample

Au 4f7/2 Ag 3d/1

83.8 367.9

86.1 (AuCl) 367.1 (Ag2O)

83.8 367.8

without appearing the surface plasmon resonance of Ag NPs in UV-vis absorption spectra. The EDS analysis for randomly chosen NPs (Figure 5c) suggested that most of particles included Ag atoms. Moreover, the Ag/Au atomic ratio measured by the atomic absorption spectrophotometer (Au:Ag ) 1:0.6) was different from that (Au:Ag ) 1:0.2) taken as an average of EDS analysis data of particle 2-10 in table c of Figure 5. It might be caused by the presence of a few Ag NPs as observed in (particle 7-1) (b) and (particle 1) (c) in Figure 5. The XPS technique was used for further analysis of the composition of Ag/Au NPs. The peak binding energy of Au 4f7/2 and Ag 3d/1 with bulk and ionic references are shown in Table 2. The peak positions of Au and Ag for prepared Ag/Au NPs well accord with those of bulk references. It indicates that Au and Ag exist as zero valency in Ag/Au NP. Further conformation analysis was carried out by X-ray diffraction measurement of Au and prepared Ag/Au NPs as shown in Figure 6. The diffraction pattern of prepared Ag/Au NPs is almost the same as that of Au NPs. It is conceivable that the Au element is mainly located near the surface of Ag/Au NP, which is also supported by the EDS results in Figure 5. In HR-TEM a clear contrast difference is often observed for the core-shell structured bimetallic NPs with 20-30 nm in diameter.44 In the present experiment, however, there is no contrast change observed between Au and Ag in HR-TEM image (Figure 5a). It is probably because the observed particle

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Tokonami et al. Conclusion

Figure 6. XRD spectra of PVP-protected NPs of monometallic Au and those of Ag/Au bimetallic prepared by adding the AgClO4 aqueous solution into PVP-capped Au NP dispersed solution.

The colloidal dispersion of Ag/Au bimetallic NPs can be prepared by adding the aqueous AgClO4 solution into the Au NP dispersion. It was observed that the surface plasmon resonance of Au NP was enhanced after the addition of AgClO4, which could be caused by the increment of the particle mean diameter according to the TEM observation before and after Ag+ ion supplementation. In the traditional method for observation of Au NPs in biological organisms by TEM, Ag+ solution is often added to the TEM samples to clearly observe the Au NPs.57 The addition of Ag+ solution usually increase the Au particle sizes. The present research results can clearly explore the validity of the traditional method. The bimetallic NP thus prepared may have a core/shell type structure (Ag-core/Aushell), which is suggested by HR-TEM, STEM, and XRD. In other words, this procedure provides a novel method to synthesize Ag-Au bimetallic NPs with a Ag-core/Au-shell type structure at various Ag/Au ratios. The bimetallic NP with the composition of Au/Ag (1/0.25) had the highest catalytic activity for glucose oxidation among those with various composition ratios and monometallic NPs. The high catalytic activity of the Ag/Au NP may be due to the electronic effect between Au and Ag element in a particle. References and Notes

Figure 7. Glucose oxidation catalytic activity of Ag/Au NPs at various feed composition ratios and Au and Ag monometallic NPs. Each measurement was carried out at least three times, and the average is presented.

diameter is too small (ca. 5 nm) to get such a clear contrast difference. Thus, it may be difficult to conclude the particle structure as a complete core-shell. However, it has been confirmed that the particle has at least a Au-rich surface structure from the EDS analysis (table b in Figure 5). In addition, no Ag plasmon peak was observed in the present bimetallic NPs, which implies that the bimetallic NP has not a random alloy structure. Although it cannot be denied the possibility that a few Ag clusters are formed as an island inside the Au NP, they conceivably do not distribute in the whole particle but rather gather near the center of the particle. So, we can regard the obtained NP as a kind of Ag-core/Au-shell type structure, which may include not only a “complete” Ag-core/Au-shell structure but also the structure with Au-rich surface and Ag-rich center. Glucose Oxidation Catalytic Property of Ag/Au Nanoparticles. It has been known that glucose is easily oxidized to gluconic acid by oxygen, being activated not only by the monometallic NPs such as gold and platinum but also by bimetallic NPs.25,54-56 The catalytic activity for the glucose oxidation of the Ag/Au bimetallic NPs prepared in the present experiments with various composition ratios was investigated (Figure 7). All Ag/Au NPs examined here had higher activity than that of the monometallic Au or Ag NPs. The highest activity among the NPs examined here (4079 mol-glucose h-1 mol-M-1) was observed for the Ag/Au NPs prepared with the metallic Au/Ag ratio of 1/0.25, which was 18 times higher than that of monometallic Au NPs examined in the present experiments. It can be attributed to the electronic (ligand) effect between the core and shell atoms.41,51 The ionization potential of Au and Ag is 9.22 and 7.58 eV, respectively. Consequently, electronic charge could transfer from Ag in the core to Au in the shell, leading an increase in the electron density on the NP surface, which may activate the dissolved oxygen to attack the aldehyde group in the glucose.51

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