Fabrication of catalytically active AgAu bimetallic nanoparticles by physical mixture of small Au clusters with Ag ions

Applied Catalysis A: General 447–448 (2012) 81–88

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Fabrication of catalytically active AgAu bimetallic nanoparticles by physical mixture of small Au clusters with Ag ions Haijun Zhang a , Naoki Toshima b,c,∗ a b c

College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan, Hubei Province 430081, China Department of Applied Chemistry, Tokyo University of Science Yamaguchi, SanyoOnoda-shi, Yamaguchi 756-0884, Japan CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan

a r t i c l e

i n f o

Article history: Received 13 June 2012 Received in revised form 3 September 2012 Accepted 6 September 2012 Available online 9 October 2012 Keywords: AgAu Bimetallic nanoparticles Aerobic glucose oxidation Gold catalyst Physical mixtures

a b s t r a c t Catalytically highly active PVP-protected AgAu bimetallic nanoparticles (BNPs) less than 2 nm in diameter were fabricated by simultaneous physical mixture of aqueous dispersions of Au clusters with Ag+ ions. The prepared AgAu BNPs, the dispersion of which was stably kept for more than 2 months under ambient conditions, were characterized by UV–vis, ICP, HR-TEM, and EDS in HR-STEM. The prepared BNP colloidal catalysts possessed a high activity for aerobic glucose oxidation. The highest activity of 3.77 mol-glucose s−1 mol-metal−1 was observed for the BNPs prepared with Ag/Au atomic ratio of 2/8, which was more than two times higher than that of Au nanoparticles with nearly the same particle sizes.

1. Introduction Bimetallic nanoparticles (BNPs) are important catalysts because they usually possess enhanced catalytic activity and selectivity compared with those of the corresponding monometallic nanoparticles (MNPs) [1–9]. AgAu BNPs having various structures and varying Ag/Au ratios are one of the most widely studied bimetallic systems in the literatures [10]. These materials show interesting optical properties that are dependent not only on the composition but also on the geometrical structure, specially a random alloy or has a core/shell structure. AgAu alloy BNPs are usually synthesized by the simultaneous reduction of both salts in solutions, while core/shell-structured AgAu BNPs are traditionally synthesized by controlled deposition of the shell metal onto a seed of the core metal. Core/shell BNPs with apparent well-defined interfaces between the two metals have been reported by several authors via successive reduction of the different metals [11–16]. Mulvaney et al. deposited gold onto radiolyticaly prepared silver seeds by irradiation of KAu(CN)2 solutions [11]. Treguer et al. prepared layered nanoparticles (NPs) by radiolysis of mixed Au(III)/Ag(I) solutions [13]. Silver colloids covered with gold in the surface layer were prepared by mixing a solution of HAuCl4 with a silver colloid and addition of

∗ Corresponding author. Tel.: +81 836 88 4561; fax: +81 836 88 4567. E-mail address: [email protected] (N. Toshima). 0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2012.09.040

© 2012 Elsevier B.V. All rights reserved.

a reductant (p-phenylenediamine) in the second step [14]. Layered core/shell bimetallic AgAu colloids have been prepared by the seed-growth method and analyzed with transmission electron microscopy (TEM) images and electron diffraction patterns [15]. Additionally, bimetallic Au and Ag particles with a core/shell type structure have been prepared by a UV-photoactivation technique [16]. Although there are several methods reported for the preparation of AgAu BNPs [11–31], it has been believed that synthesis of AgAu BNPs with controlled compositions and sizes on a large scale is difficult because Ag+ ions easily form precipitates with halide ions in aqueous solutions when Ag+ ions were used as a starting material in combination with a HAuCl4 precursor (a most widely used starting materials for Au). Moreover, it is still a challenge to prepare AgAu BNPs with sizes of less than 2 nm. We have already reported the formation of core/shell structured BNPs with various combinations of metal elements by simultaneous reduction [21–23], sacrificial hydrogen reduction [24], and self-organization by mixing two colloidal dispersions in solutions at room temperature [25,26]. In a previous paper [27], we report on a novel synthetic method of AgAu BNPs by a simple procedure of mixing Ag+ ions with Au NPs dispersed in an aqueous solution at room temperature. In this system, the problem of precipitation of silver halide (such as AgCl) can be well solved since the mixing of Ag+ ions 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 ultra-filtration in advance. However, the prepared AgAu BNPs

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Table 1 Detail preparation conditions of Ag/Au BNPs using physical mixtures of Ag+ ions and Au NPs (in light). Code

AgAu-I1 AgAu-I3 AgAu-I4 AgAu-I5 AgAu-I6 AgAu-I7 AgAu-I8

Preparation conditions of Au NPs

Mixing conditions

Compositions

T (◦ C)

Time (h)

RPVP

RNaBH4

T (◦ C)

Time (h)

0 0 0 0 0 0 0

1 1 1 1 1 1 1

100 100 100 100 100 100 100

5 5 5 5 5 5 5

RT RT RT RT RT RT RT

2 2 2 2 2 2 2

Au Au/Ag = 9.5/0.5 Au/Ag = 9/1 Au/Ag = 8/2 Au/Ag = 7/3 Au/Ag = 5/5 Au/Ag = 2/8

RPVP : the molar ratio of PVP monomer units to the Au3+ ions; RNaBH4 : the molar ratio of NaBH4 to the Au3+ ions.

have a large size of about 4.3 nm and then a low catalytic activity of 1.13 mol-glucose s−1 mol-metal−1 for aerobic glucose oxidation was observed for the AgAu BNPs prepared with the metallic Ag/Au ratio of 2/8. It is well known that the metal catalytic activity is strongly dependent on the particle shapes, sizes and the particle size distributions, and that BNPs with sizes less than 2 nm is the favorite size range for the application of catalysis in general. In this paper, we prepared the AgAu BNPs with sizes less than 2 nm using the physical mixture method. The prepared BNPs have been analyzed by UV–vis spectra, TEM images, ICP, HR-TEM images, and EDS in HR-STEM. The catalytic activity of thus prepared AgAu BNPs with various composition ratios has also been investigated for aerobic glucose oxidation. 2. Experimental

2.3. Preparation of AgAu BNPs by simultaneous physical mixture of Au NP aqueous dispersions and Ag+ ions We tried to prepare AgAu BNPs by simultaneous physical mixtures of Au NP aqueous dispersions and Ag+ ions [27]. Aqueous solutions of AgClO4 were added to the as-synthesized Au NP aqueous colloidal dispersions with continuous stirring at room temperature for 2 h in natural light to get a series of catalysts. The detail compositions and preparation conditions are shown in Table 1. For example, Ag50 Au50 BNPs were prepared as follows (Hereafter, the numbers in the subscript stand for the synthetic feeding ratios of the two metals.): An aqueous solution of AgClO4 (25 mL, 0.44 mmol L−1 ) were added into an as-prepared Au NP colloidal dispersion (25 mL, 0.44 mmol L−1 ) with continuous stirring at room temperature for 2 h in a N2 atmosphere. The AgAu BNPs were obtained using the same washing, evaporation and drying procedures as that of Au NPs.

2.1. Materials 2.4. Characterization of BNPs Hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl4 ·4H2 O, 99.9%) purchased from Tokyo Kasei Kogyo, Ltd., and silver perchlorate (AgClO4 , 99.99%), sodium borohydride (NaBH4 , 99.0%), and PVP (poly(N-vinyl-2-pyrrolidone, K30, molecular weight about 40,000) purchased from Wako Pure Chemical Industries Ltd. were used without further purification. All glasswares and Teflon-coated magnetic stirring bars were cleaned with aqua regia, followed by copious rinsing with purified water. Water was purified with a Millipore Milli-RX 12 plus water system. 2.2. Preparation of PVP-protected Au NPs by rapid injection of NaBH4 The monodispersed and PVP-protected Au NPs were prepared by Tsukuda’s protocol [28], i.e., rapid injection of NaBH4 into the AuCl4 − /PVP aqueous solutions as follows: An aqueous solution of HAuCl4 ·4H2 O (25 mL, 0.44 mmol L−1 ) were added into an aqueous PVP (50 mL, 44 mmol L−1 in monomer unit) solution and then stirred at 0 ◦ C for 30 min. Then, an aqueous solution of NaBH4 (6.67 mL, 16.5 mmol L−1 , 0 ◦ C) was rapidly injected into the mixtures under vigorous stirring. The molar ratio of PVP monomer units to the total metal ions (RPVP ) was 100, and the molar ratio of NaBH4 to the total metal ions (RNaBH4 ) was 5. The addition time of rapid injection of NaBH4 into AuCl4 − /PVP was within 5 s. In the last, the transparent and brownish colloidal dispersions obtained were filtered by an ultrafilter membrane with a cutoff molecular-weight of 10,000 (Toyo Roshi Kaisha LTD.) and washed two times with water and then once with ethanol under nitrogen to remove extra agents and byproducts. The residual ethanol of PVP-protected Au colloids was removed by using a rotary evaporator at 40 ◦ C. PVP-protected Au NPs were finally obtained by vacuum drying at 40 ◦ C for 48 h. The average diameter based on the size distribution is 1.4 ± 0.5 nm for the Au NPs [29].

UV–vis (ultraviolet and visible light) absorption spectra were measured over a range of 200–800 nm with a Shimadzu UV-2500PC recording spectrophotometer using a quartz cell with 10 mm of optical path length. Transmission electron microscopy (TEM) images were observed with a JEOL TEM 1230 at accelerated voltage of 80 keV. The specimens were obtained by placing one or two drops of the Ag/Au colloidal ethanol solutions onto a thin amorphous carbon filmcovered copper microgrid and evaporating ethanol in air at room temperature. Prior to specimen preparation, the colloidal ethanol solutions were sonicated for 10 min to obtain a better particle dispersion on the microgrid. Image analysis was performed with iTEM software (Olympus Soft Imaging Solution GmbH). For each sample, generally at least 200 particles from different parts of the grid were used to estimate the mean diameter and size distribution of particles. High resolution TEM (HR-TEM) and dark field scanning transmission electron microscopy (DF-STEM) were observed with a JEOL TEM 2010F microscopy at accelerated voltage of 200 keV at UBE Scientific Analysis Laboratory. Energy dispersion X-ray spectroscopy (EDS) measurements was carried out with a NORAN UTW type Si(Li) semiconducting detector with about 1 nm beam diameter attached to the HR-TEM equipment. The metal contents of the PVP-protected AgAu BNPs were determined by optical emission spectroscopy with inductive coupled plasma (ICP-OES, Varian 720-ES). For this purpose the samples were solubilized in aqua regia (HCl/HNO3 ) in advance. 2.5. Glucose oxidation at controlled pH The catalytic performances of all catalysts were evaluated by glucose oxidation as a model reaction. The reactions were carried out at 60 ◦ C in a 50-mL glass beaker settled in a thermostat (about

H. Zhang, N. Toshima / Applied Catalysis A: General 447–448 (2012) 81–88

3.1. Plasmon absorption enhancements of Au NPs by addition of Ag ions The colloidal dispersions of Au NPs were prepared by rapid injection of NaBH4 , the UV–vis spectra, the TEM micrographs and size distribution histograms of the dispersion of the Au NPs are shown in a previous report of our group [29]. The spectrum of the starting Au NPs exhibits a very weak plasmon absorbance at 520 nm, indicating the formation of enough small NPs. The average diameter based on the size distribution is 1.4 ± 0.5 nm for the Au NPs. The colloidal dispersions of Ag/Au BNPs were prepared by mixing the colloidal dispersions of Au NPs and Ag+ ions. The UV–vis absorbance spectra were recorded as a function of time to monitor the formation of Ag/Au BNPs. According to Mie theory, metallic NPs with a radius much smaller than the incident wavelength of light will absorb strongly at certain wavelengths because of the resonant excitation of the surface plasmons. Furthermore, the position and intensity of the absorption bands are strongly influenced by particle sizes and shapes, the surrounding mediums, and the boundary conditions imposed by adjacent metallic particles [30]. Fig. 1 shows the UV–vis absorbance spectra as a function of time of the mixed dispersions of the Au NPs and Ag+ ions. The results show that only one surface plasmon peak is observed in all the UV–vis spectra of the Ag/Au colloidal dispersions mixed under various time in the range of 0–2 h. Moreover, the peak intensity at 520 nm was dramatically increased after addition of Ag+ ion without appearance of Ag NP surface plasmon resonance peak expected at ca. 400 nm. Fig. 1b represents the relationship between the mixing time and the normalized absorbance of the plasmon by regarding to the absorbance of original mixture of Au NP dispersion and Ag+ ion solution (the mixing dispersion after 1 min addition of Ag+ ion solution was approximately regarding as the original mixture) as 0. The drastic increase in absorbance occurred within 3 min, suggesting some kinds of conformation change of Au NPs. Other UV–vis spectra of the AgAu BNPs colloidal dispersions with various Ag/Au ratios as a function of mixing time are shown in Figs. S2–S6, revealing the similar phenomena. A series of AgAu BNPs with various contents of Ag were prepared by mixing colloidal dispersions of PVP-protected Au NPs and Ag+ ion solution at room temperature for 2 h. Only one plasmon absorption at about 520 nm was observed for all the UV–vis spectra of the final BNPs (Fig. S7). In addition, the plasmon peaks have various

40

Absorbance increment ratio / %

3. Results and discussion

(a) UV-Vis spectra In natrual light

30 20 10 0

0

20

40

60 80 Time / min

100

120

(b) Absorbance increment ratio Fig. 1. (a) Normalized UV–vis spectra of the dispersions of original Au NPs and mixture of Au NPs and Ag+ ions at various mixing time (the spectra of Ag+ /Au mixture were normalized to compensate the dilution effect of the addition of Ag+ ions). (b) Absorbance increment ratios as a function of mixing time regarding the absorbance of the original mixture of Au NPs and Ag+ ions (after 1 min addition of Ag+ ion solution) as a standard. When an aqueous dispersion of Au NPs (25 mL, 0.44 mmol L−1 ) and an aqueous solution of AgClO4 (25 mL, 0.44 mmol L−1 ) were mixed at room temperature under light (Ag+ /Au = 50/50, atomic ratio; The absorbance intensities of 520 nm were used for calculation.).

relative intensities depending on the feeding atomic ratios of Ag/Au. If all the spectra were normalized to compensate the dilution of the addition of Ag+ ions (Fig. 2), as the Ag contents increase from 5 to 80 at%, the peak intensity at around 520 nm was dramatically increased after addition of Ag+ ion. Fig. 3 represents the

3

In natural light

Ag increasing

Absorbance (a.u.)

2000 mL). During the experiment, the pH of the reaction suspensions was kept constant at 9.5 by addition of a 1 mol L−1 NaOH solution using an automatic potentiometric titrator (Kyoto Electronics MFG. CO. Ltd., Japan). Oxygen was bubbled through the suspensions with a flow rate of 100 mL min−1 at an atmospheric pressure. The suspensions were vigorously stirred with a magnetic stirrer. The starting concentration and volume of glucose solution were 0.264 mol L−1 and 30 mL, respectively, and the charged weight of catalyst was about 2 mg. The catalytic reactions were automatically carried out for 2 h. The activity (mol-glucose s−1 mol-metal−1 ) for glucose oxidation of the BNPs was calculated from the slope of a straight line fitted with the NaOH amount vs reaction time curves. The specific activity related to the metal content of the catalysts was used to compare catalysts with different gold contents, and the maximum activity was calculated for comparison. A typical NaOH amount vs time diagram with fit line is shown in Fig. S1. The slope of the fit line reflects the maximum activity of the catalyst. Catalytic activities of all the samples were measured at least twice at the same conditions and the mean value of the measuring results was used for comparsion.

83

520 nm

2

1

0 200

300

400

500

600

Au + Ag /Au=5/95 + Ag /Au=10/90 + Ag /Au=20/80 + Ag /Au=30/70 + Ag /Au=50/50 + Ag /Au=80/20 Ag

700

800

Wavelength / nm Fig. 2. Normalized UV–vis spectra of dispersion of Au NPs, Ag NPs and AgAu BNPs at various compositions prepared by physical mixture of Au NPs and AgClO4 solution at room temperature for 2 h under light. (The spectra of AgAu BNPs were normalized to compensate the dilution effect of the addition of Ag+ ions.)

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relationship between mixing time and the normalized plasmon absorbance of all prepared AgAu BNPs with Ag contents ranging in 5–80 at% by regarding to the absorbance of original mixture of Au NP dispersion and Ag+ ion solution as 0 (the mixing dispersion after 1 min addition of Ag+ ion solution was approximately regarding as the original mixture), clearly showing that the absorbance intensities of the BNPs increase with increasing mixing time. In addition, Ag50 Au50 BNPs show the highest absorbance intensities in all range of mixing time among all prepared BNPs. In case of Ag80 Au20 BNPs, however, its surface plasmon peaks indicate much lower intensity compared with other samples but Ag5 Au95 BNPs. We think the much lower intensity is arisen from the dilution of the addition of a large amount of Ag+ solution (Ag+ /Au = 4/1, v/v). It is well known that the Aucore /Agshell BNPs usually have two SPR bands in the UV–vis profile near the Ag and Au SPR band [31,32], while AgAu alloy BNPs has only one plasmon band and the wavelength at the maximum absorbance shifts in a linear fashion depending on the composition [33]. In present Ag/Au BNPs, it is worth mentioning that only one plasmon peak was observed and that clear red shift or blue shift of the plasmon peak cannot be observed for all the BNPs with various contents of Ag in Fig. 2. This result suggests that the prepared BNPs should have a Agcore /Aushell structure. Otherwise, the SPR band near about 520 nm in Fig. 2 will shift to low wavelength with increasing of the Ag contents. Similar results were also observed in a previous report of our group [27]. Even though the UV–vis spectra in Fig. 2 suggest the formation of the Ag/Au core/shell-structured BNPs by using present physical mixture method, it should be pointed that, in Fig. S4, the plasmon peak of Ag20 Au80 BNPs (AgAu-I5) is unnoticeable blue shifted with the reaction time, implying that some random alloy-structured AgAu BNPs may be formed during the mixing step for the sample. Thus, according to our analysis and interpretation above, it can be expected that AgAu core/shell-structured BNPs as well as a few alloy-structured AgAu BNPs were formed in present experiments. UV–vis spectra of the prepared AgAu BNPs suggest that the added Ag+ ions were reduced to Ag atoms by light in the presence of Au NPs in the present experiments. In order to further confirm the presence of Ag in the final BNPs, the chemical compositions of the prepared AgAu BNPs were obtained from ICP analysis as shown in Table 2, indicating a representative set of results by comparing Ag at% in the final BNPs and that in the feeding metal precursors. The feeding atomic Ag ratios in the synthetic solutions are approximately proportional to the resulting Ag atomic ratios in the final BNPs. When the Ag contents are less than 50 at% (or Au atoms are higher than 50 at%), the final Ag contents in the prepared BNPs are little larger than that of feeding Ag contents (Table 2), suggesting

Fig. 3. Absorbance intensity increasing ratio as a function of mixing time of PVPprotected AgAu series BNP dispersed aqueous solutions prepared by physical mixture. (Intensity at 520 nm was used for calculation, regarding the absorbance of the starting mixture of Au NPs and Ag+ ions (after 1 min addition of Ag+ ion solution) as 0. In light.)

Table 2 Comparison between theoretical and ICP results of Au and Ag content for the final AgAu series BNPs prepared at room temperature for 2 h in light by physical mixtures. Ag/Au BNPs

AgAu-I3 AgAu-I4 AgAu-I5 AgAu-I6 AgAu-I7 AgAu-I8

Au (at%)

Ag (at%)

Theoretical

ICP results

Theoretical

ICP results

95 90 80 70 50 20

91.1 59.6 75.5 64.9 56.0 46.3

5 10 20 30 50 80

8.9 13.4 24.5 35.1 44.0 54.7

that the added Ag+ ions were completely reduced to Ag atoms in these samples. When the Ag contents are higher than 50 at%, however, the final Ag contents in the prepared BNPs are much less than the feeding Ag contents in the synthetic solutions. For example, for the sample of AgAu-I7 with 50 at% Ag in theoretical value, the final content of Ag in the prepared BNPs is as low as 44.0 at%, suggesting that the reduction of the added Ag+ ions cannot completely finished in this case. The higher the feeding Ag contents are, the larger the differences between the final and the feeding Ag contents are. These results suggest the maximum reduced contents of Ag in the present case may be about 50 at%. The excessive Ag+ ions cannot be reduced by light and will be removed during the washing process when the Ag+ contents are more than 50 at%. This could be the reason why the absorbance intensities of Ag50 Au50 BNPs are the highest, and the absorbance intensities of Ag80 Au20 BNPs are much lower than that of most of other BNPs during all mixing time (Fig. 3). It should be pointed that, in the absence of Au NPs, however, this reduction of Ag cannot occur at all. Our results show that the aqueous solutions of PVP/Ag+ ions can be stably kept in light for several days. This means that the presence of Au NPs play an important role in preparation of AgAu BNPs using the present physical mixtures. The roles of the Au NPs in the preparation process can be explained as follows: (1) the photoexcited state of Au NPs might be involved in the process. The lifetime of the photoexcited state of sufficiently small Au NPs (about 1.4 nm) may be long enough to reduce the Ag+ ions into Ag atoms. Since the luminescences of Au clusters [34,35] and AuAg alloy clusters [36] in polymer matrices were reported recently, we infer that the photoemission effect of the small Au NPs might play an important role for the synthesis of AgAu BNPs by the present physical mixture method even though we have no strict evidence on it. (2) The Au NPs act as substrates for heterogeneous nucleation of Ag atoms reduced by UV-light in the natural light [37]. Based on the theory of heterogeneous nucleation, we can suggest that the Au NPs acting as nucleation seeds could decrease the potential barrier for nucleation and

Fig. 4. UV–vis spectra of colloidal dispersions of mixed Ag+ ions and Au NPs as a function of mixing time during 0–2 h (Ag+ /Au = 20/80, atomic, in dark).

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Fig. 5. TEM images of the final AgAu BNPs prepared at room temperature for 2 h in light by physical mixture.

therefore accelerate nucleation and growth of the Ag atoms, and then make it easy to synthesize Ag atoms by light reduction with lower energy barrier. Moreover, PVP used as a protective medium for the colloids in this experiment may also play a positive effect on the photoreduction of Ag+ ions in the present physical mixing process. (1) PVP will affect the particle motion of prepared BNPs and subsequently depress the aggregation of colloids. (2) On the other hand, the participation of PVP in silver reduction should be also considered. PVP contains a functional C O group. The excited state of PVP can reduce Ag+ to Ag in the solution [38]. In addition, since the metal–metal bond lengths in elemental Ag and Au are very similar ˚ respectively), so Ag+ ion would be to each other (2.889 and 2.884 A, easily reduced by light on the surface of the Au nucleation seeds. The Au NPs may work as a catalyst for the reduction of Ag+ ions. Moreover, it should be pointed that the photoreduction of Ag+ ions

in the presence of Au NPs in natural light seems to be a unique phenomenon. The similar photoreduction of various metal ions such as Pt2+ and Rh3+ was also investigated. The plasmon enhancement rate of these metal ions is much lower than that of Ag+ ions. In other words, the photoreduction of Rh3+ and PtCl6 2− in presence of Au NPs can be hardly observed under similar experimental conditions. It is well known that Ag NPs can be easily synthesized from Ag+ ions in solution as starting materials by photoirradiation with UV light [38–40]. In present paper, preparation of AgAu BNPs was carried out in natural light. As is well-known that the majority of the natural light consists of the visible and infrared light and about 3–5% of UV light, we think the reduction of the Ag+ ions can be ascribed to the presence of the UV light in the natural light. However, to investigate whether the light influences the plasmon enhancement, Au NP dispersion was mixed with Ag+ ion solution (Ag+ /Au = 20/80,

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at%) in dark for 2 h, and the UV–vis absorbance spectra (Fig. 4) were recorded as a function of time to monitor the plasmon enhancement phenomenon. Surprisingly the peak intensity at 520 nm of the mixture was also increased as that of the samples in natural light after addition of Ag+ ion. In addition, the TEM results showed that the particle size of the mixture increased with the mixing time (starting Au NPs, 1.4 nm; mixture of Ag+ /Au after 5 min, 2.0 nm; mixture of Ag+ /Au after 60 min, 2.4 nm). At present time, the reason for the plasmon enhancement of the mixture of Au NP dispersion and Ag+ ion solution in dark is still unclear and need further investigation. Work along these lines is now in progress.

3.2. Structure analysis of AgAu BNPs TEM images of the prepared AgAu BNPs are shown in Fig. 5. The particles were well dispersed, but the average diameters of the particles became larger with additions of AgClO4 . The size distribution analysis (Fig. S8) based on the TEM images yields the average diameter of 1.5 ± 0.5 nm for Ag5 Au95 , 1.6 ± 0.6 nm for Ag10 Au90 , 1.7 ± 0.7 nm for Ag20 Au80 , 1.8 ± 0.5 nm for Ag30 Au70 , 2.0 ± 0.8 nm for Ag50 Au50 , and 2.1 ± 0.8 nm for Ag20 Au80 BNPs. These results indicating that the final average particle sizes of the prepared AgAu BNPs increase with increasing the Ag contents, suggest that Ag+ ions are indeed reduced by the present method. The increment in the diameters may be caused by an ingression of Ag atoms into Au nanoparticles after reduction of Ag+ ions by light, which could be considered as one of reasons for the surface plasmon enhancement. UV–vis spectra in Figs. 1 and 2 suggest that the prepared BNPs have a Agcore /Aushell structure. In order to further confirm the result, HR-TEM and EDS in HR-STEM were carried out to investigate the structure and composition of the prepared BNPs. Fig. 6 represents the HR-TEM (a) and STEM-EDS (b) images of AgAu BNPs prepared by addition of AgClO4 aqueous solutions into PVP-protected Au NP colloidal dispersions. The HR-TEM images in Fig. 6(a) clearly illustrate again that the prepared BNPs have a uniform size. Since the size of EDS electron beam is 1 nm in diameter, the compositions of different parts of a nanoparticle can be examined independently. The EDS analysis of the randomly chosen NPs (Fig. 6(b)) confirms again that the prepared BNPs include Ag, in consistent with the ICP results. The dot EDS results also reveals that there are two kinds of nanoparticles formed in the dispersion of Ag20 Au80 BNPs. The first one is composed of Ag and Au (the selected particle 1 and particle 2 in Fig. 6(b)). For example, the composition of particle 1 was Ag (23%) and Au (77%) in the edge, and Ag (3%) and Au (97%) in the central part of the nanoparticle as shown in the insert table of Fig. 6. This suggests that the two BNPs have an Au-rich core/Agrich shell structure. Another kind of particles also contain both Ag and Au components (the selected particles 3 in Fig. 6(b)), and the contents of Au in the shell are higher than that in the center. It could possess an Ag-rich core/Au-rich shell structure. To the best knowledge of the authors, this is the first report on formation of PVP-protected Agcore /Aushell -like BNPs with sizes of less than 2 nm by physical mixture. Although it is still hard to accurately figure out the yield percentage of the Agcore /Aushell -like structured BNPs at the present time, judging from the UV–vis spectra, we can reasonably estimate that most of the prepared BNPs should have an Ag-rich core/Au-rich shell structure. It is worth emphasizing that the present method is simple, easily operated, and acceptable as a strategy for large-scale synthesis, and also that the prepared Agcore /Aushell BNPs have a quite small average particle size. In our previous paper [27], we had speculatively proposed the formation mechanism of core/shellstructured AgAu BNPs by the addition of AgClO4 aqueous solution into Au NP dispersion based on the results of HRTEM, STEM-EDS, UV–vis, XPS, and so on.

Fig. 6. HR-TEM (a) and HR-STEM (b) image, and EDS results (c) of the final Ag20 Au80 BNPs at room temperature for 2 h in light by physical mixture.

3.3. Glucose oxidation catalytic property of Ag/Au BNPs In order to get more information on the effects of compositions upon the catalytic activities, all the AgAu BNPs with various atomic ratios were used as the colloidal dispersion catalysts for aerobic glucose oxidation in solution. The catalytic activities vary with the compositions of the BNPs as shown in Fig. 7. The highest catalytic activity is achieved for the samples of Ag20 Au80 BNPs, whose activity is 3.77 mol-glucose s−1 mol-metal−1 . The activity of the Ag20 Au80 BNPs is more than two times higher than that of pure Au NPs (1.73 mol-glucose s−1 mol-metal−1 ) even though the Ag20 Au80 BNPs have a larger particle size than that of Au NPs. In addition the actvity of present Ag20 Au80 BNPs is more than three times higher than that of Ag20 Au80 BNPs with an average particle size of 4.3 nm (1.13 mol-glucose s−1 mol-metal−1 ) reported in our previous paper [27]. The results show a significant dependence of the catalyst activity on the particle sizes of AgAu BNPs prepared by physical mixture. The higher catalytic activity of the AgAu BNPs than that of Au NPs can be explained by electronic charge transfer effects between the various kinds of adjacent elements which were reported for the reasons of high catalytic activities of several cases of BNPs and TNPs [41–43]. In the case of the AuAg BNPs, the ionization potential of Au and Ag is 9.22 and 7.58 eV, respectively. The electronic charges

H. Zhang, N. Toshima / Applied Catalysis A: General 447–448 (2012) 81–88

Fig. 7. Catalytic activities toward glucose oxidation of the final AgAu BNPs with various contents of Ag.

could transfer from Ag atoms to Au atoms, leading an increase in the electron density on the Au, which may act as catalytically active sites and activate molecular oxygen by donating excess electronic charge to the antibonding orbital and resulting superoxo- or peroxo-like oxygen and then promotes glucose oxidation [44]. The electronic charge transfer effect in AgAu BNPs has been proposed in our previous paper [29]. 4. Conclusion The colloidal dispersions of AgAu BNPs with particle sizes of less than 2 nm were prepared by adding the aqueous AgClO4 solutions into the Au NP dispersions. This procedure provides a novel method to synthesize AgAu BNPs with small size at various Ag/Au ratios on a large scale. The prepared BNPs with structures of Agrich core/Au-rich shell, and Au-rich core/Ag-rich shell, as well as random alloy showed high catalytically activities for aerobic glucose oxidation. The prepared AgAu BNPs with the composition of Ag/Au (2/8) had the highest catalytic activity for glucose oxidation among those with various composition ratios and MNPs. The activity of the Ag20 Au80 BNPs is more than two times higher than that of pure Au NPs, even though the Ag20 Au80 BNPs have a larger particle size than that of Au NPs. The high catalytic activity of the AgAu BNPs may be due to the electronic charge transfer effects between Ag and Au atoms in a particle. Acknowledgement This work was financially supported by Grants-in-Aid from Core Research for Evolutional Science and Technology (CREST) program sponsored by Japan Science and Technology Agency (JST), Japan. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apcata. 2012.09.040. References [1] O.S. Alexeev, B.C. Gates, Supported bimetallic cluster catalysts, Ind. Eng. Chem. Res. 42 (2003) 1571–1587. [2] A. Takao, T. Yamamoto, S. Nakagawa, H. Seino, Nitani, Bimetallic nanoparticles of PtM (M = Au, Cu, Ni) supported on iron oxide: radiolytic synthesis and CO oxidation catalysis, Appl. Catal. A: Gen. 387 (2010) 195–202. [3] H. Nitani, T. Nakagawa, H. Daimon, Y. Kurobe, T. Ono, Y. Honda, A. Koizumi, S. Seino, A. Takao, Yamamoto, Methanol oxidation catalysis and substructure of PtRu bimetallic nanoparticles, Appl. Catal. A: Gen. 326 (2007) 194–201.

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